Ultrasonic Testing of materials :

Bats use ultrasonic echo location to find its flying path, search and catch insects. Bats operate in the frequency range of 50 150 KHZ.

Ultrasonic testing is the process of applying ultrasonic sound to a specimen and determining its soundness, thickness or some physical property. Sound is a vibration that transmits energy by a series of small material displacements. Vibrations above human hearing range are called ultrasonics vibrations. The energy is originated in a piezoelectric transducer which causes material displacement within the specimen. The transducer can also convert mechanical energy coming back from the specimen into electrical energy. Therefore, a transducer can both send and receive energy. The returning energy can be transformed in an electrical impulse which can be displayed on a monitor in form of echoes, thus allowing the identification of flaws within the specimen or directly showing the thickness in mm.

ultrasonic inspection Ultrasonic testing is the process of applying ultrasonic sound to a specimen and determining its soundness, thickness or some physical property. Sound is a vibration that transmits energy by a series of small material displacements. Vibrations above human hearing range are called ultrasonics vibrations. The energy is originated in a piezoelectric transducer which causes material displacement within the specimen. The transducer can also convert mechanical energy coming back from the specimen into electrical energy. Therefore, a transducer can both send and receive energy. The returning energy can be transformed in an electrical impulse which can be displayed on a monitor in form of echoes, thus allowing the identification of flaws within the specimen or directly showing the thickness in mm. Basic Applications of UltrasonicsUltrasonics is a versatile inspection technique, which is used to test a variety of both metallic and non metallic products such as welds, forgings, sheet, tubing, plastics and ceramics.Ultrasonics has an advantage of detecting subsurface discontinuities with access to only one side of the specimen.The objective of ultrasonics testing is to ensure product reliability by means of:obtaining information related to discontinuities disclosing the nature of the discontinuity without empairing the uselfuness of the part Separating acceptable and unacceptable materials in according with predetermined standards Ultrasonics units are portable, radiation free and has major advantages compared to other NDT: can inspect deeper that Penetrants and Magnetic Particles and may reveal flaws that cannot be seen with radiography (such as planar defects).Major disadvange of ultrasonic is to be too much inspector depending, as no trace is left.However, in the last few years, new technologies (TOFD) are making UT recordable and replacement of radiography by UT is now possible under circumstances stated in the ASME Code Case 2235- 6.

Ultrasonic Testing :

High frequency [ 0.5 to 15 MHz ] ultrasound waves [ mechanical vibrations ] are introduced into a material to detect changes in material properties. A piezoelectric transducer is excited with a pulsating voltage to generate ultrasound waves in the test material. The sound is reflected back from something--either the back side of the part or from a flaw--depending on what is in the material. When it reflects back, the signals are detected, displayed and interpreted to determine the thickness of the metal or the flaw thats inside the metal.

Common uses of Ultrasonic testing : Thickness measurements Corrosion mapping Metal cracking Bonding Plates Pipes Forged products Cast products Rolled products Welded joints Concrete

Advantages :Provides immediate informationGood penetration power. Allowing the inspection of thick sections Accurate determination of imperfection position and estimation ofimperfection severity. Fast response time, Permits high speed automatic testing. One surface access. Access is required to only one surface of the product being inspected. Very small imperfections can be detected Highly sensitive to planar defects like Crack and Lack of Fusion

Disadvantages :Provides indirect indication, Discontinuities can not be identified directly. Requires surface preparation. Requires full scanning of entire test area. Requires a coupling medium which makes recording difficulties. Conventional techniques does not provide permanent record of test signals. Less sensitive to smaller flaw like porosity and slag fragments. Discontinuities must be intercepted perpendicularly. Test reliability depends on operators skill and attention.

Ultrasound waves are high frequency mechanical vibrations traveling through a medium, which may be a solid, a liquid, or a gas. These waves will travel through a given medium at a specific speed or velocity, in a predictable direction, and when they encounter a boundary with a different medium they will be reflected or transmitted according to simple rules. This is the principle of physics that underlies ultrasonic flaw detection.

Ultrasonic testing : The reflections from discontinuities and the back wall are detected and displayed as information of the test object.Display

Frequency : All sound waves oscillate at a specific frequency, or number of vibrations or cycles per second, which we experience as pitch in the familiar range of audible sound. Human hearing extends to a maximum frequency of about 20,000 cycles per second [ 20 KHz ], while the majority of ultrasonic flaw detection applications utilize frequencies between 500,000 and 10,000,000 cycles per second [ 500 KHz to 10 MHz ]. At frequencies in the megahertz range, sound energy does not travel efficiently through air or other gasses, but it travels freely through most liquids and common engineering materials.Velocity : The speed of a sound wave varies depending on the medium through which it is traveling, affected by the medium's density and elastic properties. Different types of sound waves will travel at different velocities.

Wavelength: Any type of wave will have an associated wavelength, which is the distance between any two corresponding points in the wave cycle as it travels through a medium. Wavelength is related to frequency and velocity by the simple equation = c / fwhere = wavelengthc = sound velocityf = frequencyWavelength is a limiting factor that controls the amount of information that can be derived from the behavior of a wave. In ultrasonic flaw detection, the generally accepted lower limit of detection for a small flaw is one-half wavelength. Anything smaller than that will be undetectable. In ultrasonic thickness testing, the theoretical minimum measurable thickness is one wavelength.

Ultrasound parameters Wavelength : one complete oscillation of a vibrating particle. Frequency : No of complete oscillation per second. Velocity : The distance, sound energy travels in one second.

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Ultrasound behavior : Ultrasound gets Scattered and diffracted by very small reflectors [ larger than wavelength ] with a resultant loss of energy. Scattering is random reflection of sound energy from grain boundaries and similar microstructure. Diffraction at ends of a larger reflectors which may be detected and used for flaw measurements.

Steel grains under very high magnifications. Sound is scattered by these grain boundaries when their size approaches the wavelength. Sound is scattered with a resultant loss of energy.

Graphite noodles in cast iron scatter ultrasound which produce noise and loss of penetration.

Coarse grained material disperses ultrasound by random reflection from grain boundaries. This produces noise on the baseline, serious loss of back reflection and indistinguishable signal from smaller flaws.

Attenuation of ultrasound : The distance that a wave of a given frequency and energy level will travel depends on the material through which it is traveling. As a general rule, materials that are hard and homogeneous will transmit sound waves more efficiently than those that are soft and heterogeneous or granular. Three factors govern the distance a sound wave will travel in a given medium: beam spreading, attenuation, and scattering. As the beam travels, the leading edge becomes wider, the energy associated with the wave is spread over a larger area, and eventually the energy dissipates. Attenuation is energy loss associated with sound transmission through a medium, essentially the degree to which energy is absorbed as the wave front moves forward. As frequency decreases, beam spreading increases but the effects of attenuation and scattering are reduced. For a given application, transducer frequency should be selected to optimize these variables. The amplitude of ultrasonic wave decreases as the propagating distance increases. The amplitude of ultrasonic wave which has propagated the distance of x is represented as V(x)=V0 e -ax where, a is the attenuation coefficient of a material.

Materials that cannot be tested ultrasonically include anything that can not transmit ultrasound or scatter energy. Coarse grained material such as copper, cast iron, stainless steel disperses ultrasound by random reflection from grain boundaries.

A Scan testing [ time / distance amplitude display ] : A transducer generates ultrasound in the test part, which travels in a straight line through the base metal and gets reflected from the end of the metal and from locations where there is a change in the base metal located in the sound travel path. A CRT screen displays the signals for entry and the locations of reflectors. The screen is interpreted for the length of the test part and the locations of discontinuities.

For complete examination, the probe is moved over the entire test surface. The back wall signal is monitored along with any new signal appearing between entry signal and the back wall. Any significant drop in back wall signal height or appearance of a new signal are to be interpreted for the possible presence of a discontinuity along the sound travel path.

DisplayUltrasonic display : Sound generated by the probe is reflected from the back wall, the CRT screen displays the Initial and the Back wall echoes. The distance of the back wall echo in a calibrated scale is the thickness of the test object.

DisplayUltrasonic display : Display shows a Flaw echo between the initial and back wall echoes. Distance of the flaw can be read, if the screen is calibrated Flaw reduces back wall amplitude.

DisplayUltrasonic display : Flaw being closer to the probe, the flaw signal shifts to the left of the screen. Back wall signal remains at the same location because the thickness is unchanged, with further reduction in signal amplitude.

DisplayIn a flawless object, energy reflected from the back wall is received by the probe if the front and the back surface are parallel and the back wall echo is produced. The red zone is the focus of the ultrasound beam where sound pressure is maximum.

DisplayA flaw reflects some of the energy thereby reducing the energy reflected by the back wall, with a resultant loss of amplitude of the back wall signal.

The CRT Screen divisions where test signals are displayed : The CRT screen is graduated in 50 small equal divisions, divided into10 major groups. By positioning known back wall echo signals at appropriate scale divisions, different test ranges are produced.

CRT Display : Natural test signals are radio frequency type and have a serrated look. The signals are rectified to smooth looking positive going signal for easy interpretation.

CRT Display : Natural serrated signals are rectified to smooth looking positive going signal.

1 2 3 Reading a CRT display : Test range : 100 mm [ 1 small division for a 100 mm range is, 100 / 50 divisions = 2 mm ] 1. Initial pulse [ scale zero ] 2. Flaw signal at 78 mm [ 39 X 2 ] 3. Back wall signal at 100 mm [ 50 X 2 ]

Sound Waves : are propagation of mechanical energy through a medium. Sound waves in solids can exist in various modes of propagation that are defined by the type of motion involved. Longitudinal waves and shear waves are the most common modes employed in ultrasonic flaw detection. Surface waves and Lamb waves are also used depending on applications.

Longitudinal [ or compressional ] Waves : Longitudinal wave is produced when mechanical force acts perpendicular to the test surface. Longitudinal wave propagates by pushing the particles toward the path of propagation. Wave propagates through compression and rarefaction of the particles. Particle displacement is parallel to the direction of the wave propagation. In solids, the particles do not move away from its original position, but oscillate around its rest position. Longitudinal wave has highest velocity among all waves and lower attenuation. All ultrasound waves for material testing are generated in the longitudinal mode. Straight beam probe generates these waves.

A longitudinal or compressional wave is characterized by particle motion in the same direction as wave propagation, as from a piston source. Audible sound exists as longitudinal waves only.

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Expansion of the compressed zone produces more compression zones. A series of compression and rarefaction transfer energy from one end to the other end of the object.

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DisplayParticle movements in longitudinal waves

When the mechanical force acts at an angle to the test surface, shear wave also known as transverse waves are produced if the material is solid in nature. A shear wave is characterized by particle motion perpendicular to the direction of wave propagation. The particles move up and down, with respect to its rest position.

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In a shear wave, the particles move up and down, pulling other particles with it. This is possible only in solids, where the particles are locked by inter atomic forces. Shear waves can not be generated in liquids and gasses.

DisplayParticle movements in shear waves.

Wave length of ultrasound depends on the frequency and velocity of sound, in the medium through which it is traveling. Velocity = frequency X wavelength

Reflection at an interface of two materials : When ultrasound hits an interface of two mediums, part of the incident energy is reflected back into the incident medium. The remaining energy will be transmitted through.

Reflection at an interface of two materials : When ultrasound hits an interface of two mediums, part of the incident energy is reflected back into the incident medium. The remaining energy will be transmitted through.

Reflection at an interface of two materials : The amount of energy reflected, or reflection coefficient, is related to the relative acoustic impedance of the two materials. Acoustic impedance is the resistance to sound propagation which is a material property. It is defined as density multiplied by the speed of sound in a given material. For any two materials, the reflection coefficient as a percentage of incident energy / pressure may be calculated through the formula Reflection % = [ (Z1 Z2) / (Z1 + Z2) ] 2 X 100. Where,Z1 = acoustic impedance of first materialZ2 = acoustic impedance of second materialFor the metal / air interface, the reflection coefficient approaches 100%. Virtually all of the sound energy is reflected from a crack or other discontinuity in the path of the wave. This is the fundamental principle that makes ultrasonic flaw detection possible.

Couplants: is a material [ usually liquid ] that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between transducer front face, air and the test specimen, is large and, therefore, nearly all of the energy is reflected and very little is transmitted into the test material. The couplant displaces the air and makes it possible to get more sound energy into the test specimen so that a usable ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of oil, glycerin or water is generally used between the transducer and the test surface. In immersion testing, water column or water bath conducts ultrasound into the test material.

Reflection of ultrasound : ultrasound is highly directional, and at test frequencies used for flaw detection, are well defined. A sound beam that hits an interface at perpendicular incidence will reflect straight back. When the sound beam hits the interface at an angle, will reflect forward at the same angle. The angle of reflection equals the angle of incidence.

Refraction : Sound energy that is transmitted from one material to another, bends in direction. A beam that is traveling straight will continue in a straight direction, but a beam that strikes an interface at an angle will be bent according to Snells Law :Sin 1 V1 -------- = ----- Sin 2 V2 Where,Sin 1 = incident angle in first materialSin 2 = refracted angle in second materialV1 = sound velocity in first materialV2 = sound velocity in second material

Reflection and Refraction of ultrasound : When ultrasound travels from one medium to another medium at an angle to the interface, both the reflected and refracted beams split into longitudinal and shear wave modes.

Reflection and Refraction of ultrasound : When sound travels from water to steel, 88% of the incident energy reflects back into water. The remaining 12% energy enters steel. Reflection % = [ (Z1 Z2) / (Z1 + Z2) ] 2 X 100. Only 1% of the generated energy finally returns to the probe.

Normal Incidence : For perpendicular incidence, the direction and wave mode in the second medium is the same as the first medium.Angular Incidence : the refracted beam splits into longitudinal and shear waves. The angle of the longitudinal component is larger than the shear waves.

First Critical angle : For a certain angle of incidence, the longitudinal wave is refracted along the surface and produces creeping waves which travel immediately below the surface. Second Critical angle : with further increase in incident angle, at double the first critical angle, the shear wave is also refracted along the surface and converted to surface waves traveling on the surface. .

Surface Waves in water : The floating object moves up and down at its own place, and the energy propagates in the form of waves.

Surface Waves in water : The particles movement is elliptical. The waves propagate, perpendicular to the particle vibration.

Surface Waves : Surface waves are known as Rayleigh waves which travel on and just beneath the surface of a material, penetrating up to a depth of approximately one wavelength. Below one wavelength, the energy drops to only 4% and there is no possibility to detect any defect at this depth.

DisplayParticle movement in surface waves

Testing for surface cracks with a surface wave probe. As the name suggests, surface waves [ or Rayleigh waves ] travel along the surface of components, penetrating to a depth in the order of one ultrasonic wavelength. These waves propagate along the surface, follows smooth curve, travel with low attenuation, and is reflected from defects at or very near the surface. Surface waves are sensitive to surface condition and will be attenuated by excess couplant left on the surface. Since energy is concentrated in the surface region, small blemishes on the surface can give rise to spurious indications. The inspection surface requires excess couplant or dirt to be removed.

DisplayWhen a surface wave probe is used in pulse-echo mode, it is suitable for the detection of surface breaking flaws, provided that the beam direction is normal to the plane of the flaw, sound will be reflected back to the transducer. Surface wave probes can also be used in transmitter -receiver mode, such that when the signal detected by the receiver is weakened or totally disappears, it signifies that a surface breaking flaw lies between them.

Lamb wave : Plate waves, can be propagated only in very thin metals. Lamb waves are the most commonly used plate waves in NDT. Lamb waves are a complex vibrational wave that travels through the entire thickness of a material. Propagation of these waves depends on density, elastic, and material properties of a component, and they are influenced by a great deal by selected frequency and material thickness. With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves. This technique can detect crack and lamination in thin strips.

Testing for surface cracks with a surface wave probe. Surface wave propagate along the surface and is reflected from defects at surface.

DisplayCritically refracted longitudinal waves / Creeping Waves : The angle of incidence of ultrasonic beam at perspex / steel surface necessary for producing LCR waves in the specimens is estimated as sine inverse of the ratio of longitudinal ultrasonic velocity in perspex to that in steel specimens. This angle has been estimated to be about 27.23. study.Creeping waves are high angle compression waves, which propagate immediately beneath the inspection surface. They are used for a number of applications where surface-breaking or very near-surface planar flaws need to be detected. As creeping waves propagate, they interact with the inspection surface causing secondary shear waves to be emitted. This continuous transfer of energy, from one wave mode to another, means that creeping waves are attenuated rapidly and inspection is only effective over a relatively short range [ 40 - 50mm / 1.6 - 2.0 ]. For this reason they are normally used to inspect specific areas such as the toe of welds, where the probe can be placed in close contact with the area of interest. Unlike true surface waves, creeping waves are relatively insensitive to the condition of the inspection surface and do not require excess ultrasonic couplant or dirt to be removed.

DisplayCreeping wave probes are a special type of Transmitter Receiver probes, which generate longitudinal waves at angles between 70 and 90 in the test material. These waves, commonly known as creeping waves, propagate parallel to the surface of the test piece; a shear wave beam is also generated, which radiates at an angle of about 33. Creeping wave probes are suitable for detection and sizing of flaws close to the surface like deep IGSCC (intergranular stress-corrosion cracking). Creeping waves are unaffected by liquid drops, welding spatters or other materials on the surface. However, the working range is short because of the steep energy decay. Usually, the most sensitive point, the so-called "focus" is located just in front of the probe itself. Nominal focus distance ranges up to 20 mm and the maximum useful range is typically 45 mm.

DisplayAttenuation of ultrasound : The amplitude of ultrasonic wave decreases as the propagating distance increases. The amplitude of ultrasonic wave which has propagated the distance of x is represented as V(x)=V0 e -ax where, a is the attenuation coefficient of a material.

DisplayUltrasonic Probes / Transducers :There are two major types of transducers : contact and immersion. Contact : As the name implies, contact transducers are used in direct contact with the test surface. Contact transducers, utilize a coupling material, such as water, glycerin, grease, engine oil, wall paper paste, methyl cellulose etc to prevent air gaps from resisting ultrasound transmission in to the test material. The coupling medium must be non corrosive. Except immersion type, all other transducer operate in contact with the test object. Immersion : Immersion transducers are designed to couple sound energy into the test piece through a water column or water bath. They are used in automated scanning applications and also in situations where a sharply focused beam is needed to improve flaw resolution. These transducers are longitudinal wave type with normal incidence. The transducer is angulated to produce refracted angular beams inside the test object. It is not possible to transmit shear wave in water.

Ultrasonic Probes : Normal Incidence : They introduce sound energy perpendicular to the surface, and are typically used for locating voids, porosity, and cracks or delaminations parallel to the outside surface of a part, as well as for measuring thickness. Angular incidence

Piezoelectric Crystals : Piezoelectric materials are used for generation of ultrasound.Certain materials such as Quartz becomes electrically charged when mechanical force deforms its shape. This property of the disc is used for detection of waves reflected by the test part. Modern ultrasonic Probes use artificially produced ceramics which is polarized to develop piezoelectric properties. The ceramic material is non conductor, hence both the faces are coated with silver to make electrical connections.

Piezoelectric effect : piezoelectric materials becomes electrically charged when mechanical force acts on its surface. Piezoelectric disc is utilized for detection of flaws when reflected waves applies deforming force on the disc. Ultrasonic probes use artificially produced ceramics which generates ultrasonic waves in the test material with better efficiency than quartz.

Reverse Piezoelectric effect : The thickness of a piezoelectric disc changes when an electric field is applied on to its surface.

A Triggering high voltage electrical pulse of short duration is applied to the piezoelectric disc to force it into oscillation : The oscillating crystal, when in contact with a medium, produces mechanical vibrations in the medium.

Expansion and contraction of the front surface of the piezoelectric element, which is in contact with a material, produces successive compression and rarefaction in the medium which transfers mechanical energy from one end to the other end.Display

Generation and Reception of Ultrasound : Ultrasonic testing relies on the transducer to generate and receive ultrasound. The ceramic piezoelectric crystal produces mechanical vibrations that pass through the part and also change the returning pulse echo from mechanical vibrations back into electrical signal so that the detector can display these signals.

Composite elements : An array of active piezoelectric rods are embedded into a passive ceramic polymer structure known as the 1-3 piezo-composite structure. Their properties depend on the ceramic and polymer properties and on the microstructure itself . Composite materials have a high coupling coefficient that confers a high sensitivity and signal to noise ratio [ + 10 to 30 dB compared to conventional ceramics ]. The lower and adjustable acoustic impedance allows a higher energy transfer in water, and a lower reverberation level on the front face for immersion testing applications.

Composite crystals : The 1-3 structure of the composite avoids radial vibration modes. This performance directly benefits to the beam pattern and pulse shape.Composite materials can be mechanically focused. This property allows the manufacturing of cylindrical, spherical or curved transducers without using acoustic lens. Lens attenuation is avoided and allows a more predictable beam pattern. Composite materials also have a higher mechanical resistance, that confers to the transducers a higher resistance to mechanical shocks, vibrations, temperature constraints and pressure constraints.

Normal probe : Typical transducers for ultrasonic flaw detection utilize an active piezoelectric element ceramic, composite, or polymer. When this element is excited by a high voltage electrical pulse, it vibrates across a specific spectrum of frequencies and generates a burst of sound waves. When the element is vibrated by an incoming sound wave, it generates an electrical pulse. The front surface of the element is usually covered by a wear plate that protects it from damage during contact testing.

Damping the crystal vibrations : The back surface of the element is bonded to backing material [ usually tungsten powder in araldite ] .The damping material attached to the back of the crystal mechanically damps the vibration and shortens its ringing time. Sharper signals are produced with an increase in echo resolution. Because sound energy at ultrasonic frequencies does not travel efficiently through gasses, a thin layer of coupling liquid or gel is normally used between the transducer and the test piece.

Piezoelectric Ceramic elements : Lead Zirconate Titanate [ PZT ] and Barium Titanate are most common. Barium Titanate is the most efficient ultrasound generator. Lithium Sulphate is the best receiver but hygroscopic in nature. Lead Zirconate Titanate has the best overall generating receiving performance. Other elements are Lead Meta Niobate, Polivinilidine Chloride etc.

Wear Plates : Mostly Aluminium Oxide ceramic discs bonded to the front surface of the active element is used as rubbing face which protects the soft silver coated surface from wear during contact testing. The piezo element is protected as long as the wear plate is undamaged.

Normal probe with 24 mm diameter active element. A Lemo type connecting cable which connects the probe to the flaw detector and replaceable plastic front face protective membranes.

Normal probes, 24 mm and 10 mm element [ crystal ] size. The front ring holds the replaceable plastic membrane in place.

A normal beam [ Longitudinal wave ] probe being used for flaw detection : This method is called contact testing which uses a coupling medium between the probe and the test part .

Normal probes for immersion testing have a beam focusing lens attached to the face of the probe. Immersion transducers dont come into contact with the component under examination. Instead, they operate within a liquid. The watertight housing eliminates the chance of air pockets affecting results. The probe and the object is immersed in a water tank. This method is mainly used for thin objects, small objects with shapes and objects with complicates shapes. Immersion technique mostly uses C scan recording which records a plan view of the test object.

The Cathode Ray Tube [ CRT ] which displays the test signals on the front face, a phosphor coated screen. The heated filament in the electron gun emits electrons which are focused on the screen to produce an illuminated spot.

The X and Y plates deflects the electron spot across the screen. The fast moving spot display the electrical signals in X and Y plane.

The instruments which display the ultrasonic test information are known as Ultrasonic Flaw Detectors. Three major components of an ultrasonic system, are the transducer that generates ultrasound, the pulser -receiver which acts as communicator between the transducer and display and a screen to display test signals. The pulser provides excitation pulses to drive the transducer, and the receiver provides amplification and filtering for the returning signals coming from the part through the transducer. Pulse amplitude, shape, and damping can be controlled to optimize transducer performance, and receiver gain and bandwidth can be adjusted to optimize signal-to-noise ratios. The display may be a CRT, a liquid crystal, or an electro luminescent display. The screen will typically be calibrated in units of depth or distance. Multicolor displays can be used to provide interpretive assistance.

The basic controls which are used to setup an ultrasonic flaw detector for the examination are ; on off switch, focus, mode control, gain control, range and delay controls, reject control, gate controls. Battery operated machines also provide a battery charging socket and battery status indicator. The machine automatically switches off when the battery is drained.

In addition to on off, the switch selects between a low output high resolution or a high output low resolution mode. The mode switch selects single or double probe operation. In the single mode, both the probe sockets are identical. In the dual mode, one socket is only transmitter while the other is only receiver. Display

The coarse range selects the operating range of the flaw detector, which is normally 10 50, 50 250, 250 1000 and 1000 5000 mm, when using a longitudinal wave probe. When a shear wave probe is used the ranges become approximately half. The 20 dB control has 0, 20, 40, 60 dB settings. Each step above the 0 step, amplify the existing signals by 10 times. Display

The gate controls select a portion of the calibrated range to monitor ultrasonic signals. A signal located in the gated region triggers an alarm and a LED in the detector. The reject control can be used to suppress lower amplitude noise signals which interferes during the testing. Display

The focus control sharpens the CRT trace for better resolution. The 2 dB step gain control has 20 steps of 2 dB each. Each step amplifies the existing signals by 1.25 times. Display

Fine range and delay controls are multi turn controls used to adjust the calibration signals to appropriate scale divisions. The delay control is used to set the first calibration signal. The range control is used to set the second calibration signal. The delay control can be used to shift the signals across the CRT screen without disturbing a calibrated range. Display

Test Range Calibration with normal probe :Before actual testing, the machine is first set to a known distance range by calibrating the CRT screen using back wall echoes from test blocks accurately machined to a standard thickness.The test material and the material of the calibration block must be same.

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The International Institute of Welding calibration block, IIW - V1 is the standard block for setting up an ultrasonic flaw detector for testing applications.Display

These blocks are also produced with some difference in design features.Display

IIW V1 block major dimensions : The plastic insert is used for checking the sound generating power of the flaw detector. Display

Normal probe placed on the face [ 25 mm thk ] of the IIW block for the purpose of test range calibration. Display

Repetitive signal of the back reflection can be seen on the CRT screen.

Locations of echoes after 100 mm range calibration : Test range 100 mm, 1 small scale division equals 2 mm. Location of 1st back wall echo 25 / 2 = 12.5th division. Location of 2nd back wall echo 50 / 2 = 25th division.

By positioning the echoes as shown in the picture, 125 mm range can be calibrated :1st echo 25 / 2.5 = 10th division 2nd echo 50 / 2.5 = 20th division 3rd echo 75 / 2.5 = 30th division

Echo Signals as displayed on the CRT screen after calibration.

Signal locations on a 100 mm calibrated screen, when the probe is placed on a thickness of 25 mm : Echoes set to 12.5, 25, 37.5 and 50 divisions.

A block of material, which is accurately machined to a standard thickness can be used to calibrate different test ranges. The block produces a series of back wall signals at regular interval.

Modern Digital flaw detectors allow easy set up of test parameters. Internal data loggers can be used to record full waveform and setup information associated with each test. These flaw detectors can display selected information like echo amplitude, beam path, depth or distance readings.

Digital flaw detectors capture a waveform digitally and then perform various measurement and analysis function on it. A clock or timer will be used to synchronize transducer pulses and provide distance calibration. Signal processing may be as simple as generation of a waveform display that shows signal amplitude versus time on a calibrated scale, or as complex as sophisticated digital processing algorithms that incorporate distance / amplitude correction and trigonometric calculations for angled sound paths. Alarm gates are often employed to monitor signal levels at selected points in the wave train to flag echoes from flaws.

Pulse-echo method : This method uses short pulses of sound that travel through the part to either locate a crack or the back side of the part. Its suitable for flaw detection or thickness testing. The time it takes for the sound to travel through the part and bounce back is calculated using the simple equation, d = v t / 2 where d is the distance from the surface to the discontinuity, v is the velocity of sound waves and t is the round-trip transmit time. The user moves a transducer over the surface of the part, and the tester will record the echoes.

Pulse echo method

There are three ways to display information collected from an ultrasonic tester. Theyre known as A-scan, B-scan and C-scan. The A-scan presentation displays the relative amount of energy received on the vertical axis and elapsed time along the horizontal axis. The B-scan display is a cross-sectional view with travel time displayed along the vertical axis and linear position of the transducer displayed along the horizontal axis. C-scan presentations are used with automated data acquisition systems. The C-scan displays information along a plane of the image parallel to the scan pattern of the transducer. Gaps in the scan pattern represent defects within the material.

Pulse-Echo - One Transducer

Ultrasound reflected from the sample is used.Can determine which interface is delaminated. Requires scanning from both sides to inspect all interfaces.Provides images with high degree of spatial detail.Peak Amplitude, Time of Flight (TOF) and Phase Inversion measurementThrough Transmission - Two Transducers Ultrasound transmitted through the sample is used.One Scan reveals delamination at all interfaces. No way to determine which interface is delaminated.Less spatial resolution than pulse-echo.Commonly used to verify pulse-echo results. Pulse-EchoThrough TransmissionTransmit&ReceiveTransmit

Through TransmissionReceiveTransmit

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Initial PulseFront surfaceInterface of interestBack surfaceTransducerSample

A-Scans provide the following information:1. Amplitude / % of full screen height (FSH)2. Phase / positive or negative peak3. Time / DepthAmplitude %FSH0%100%-100%_+PhasePhaseTime / Depth

Front surfaceBack surfaceFront surfaceSignal from indicationBack surfaceThe blue line (B-scan gate) represents the depth of information recorded.Signal from indication

IPFront surfaceArea of interestBack surfaceThe red box (data gate) indicates the depth of information.

Immersion testing setup

Immersion testing machine

Immersion testing machine with 3 axis probe manipulator

Immersion testing machine

Immersion testing machine This is another name for a top (or plan) view image. C-Scans can be obtained from immersion testing systems (where a 0 compression wave probe is scanned across an area through a water path, i.e. non-contact scanning) or from direct 0 contact scans. Depending on the mode of operation selected, the colour coding levels on the image may represent signal amplitude or range. The latter case is used for automated corrosion mapping where on-screen cursors can be used to show the thickness at any point and sectional thickness plots

Immersion testing machine This is another name for a top (or plan) view image. C-Scans can be obtained from immersion testing systems (where a 0 compression wave probe is scanned across an area through a water path, i.e. non-contact scanning) or from direct 0 contact scans. Depending on the mode of operation selected, the colour coding levels on the image may represent signal amplitude or range. The latter case is used for automated corrosion mapping where on-screen cursors can be used to show the thickness at any point and sectional thickness plots

Type of test part which requires c scan recording.

Sound field [ intensity distribution ] of a probe : The near field is an area of space in which the sound waves are not uniform. The ultrasonic beam is more uniform in the far field, where the beam is spread out in a pattern originating from the center of the transducer. The variations that occur in the near field eventually change to a smooth and declining amplitude, at which point the far field begins.

Sound field, Near and Far zone of a normal probe : The near field is an area of space in which the sound waves are not uniform. The ultrasonic beam is more uniform in the far field, where the beam is spread out in a pattern originating from the center of the transducer. The variations that occur in the near field eventually change to a smooth and declining amplitude, at which point the far field begins.

Sound field of a normal beam probe Ultrasound spreads out from a true parallel beam and the intensity per unit area reduces with distance from the source.

Sound field of a probe :The sound field of a probe is divided into two zones. Near zone : Intensity in this zone vary because of interference effect. Signal from a constant reflector vary from place to place. This zone is not suitable for flaw measurements. Near zone length D2 / 4 , where D is element diameter and is effective wavelength. Far zone : is after near zone, and intensity is inversely proportional to square of distance. This zone is suitable for flaw measurements. Half Beam spread = 1.22 / D

Sound field of a probe :The sound field of a probe is divided into two zones. Near zone : Intensity in this zone vary because of interference effect. Signal from a constant reflector vary from place to place. This zone is not suitable for flaw measurements. Near zone length D2 / 4 , where D is element diameter and is effective wavelength. Far zone : is after near zone, and intensity is inversely proportional to square of distance. This zone is suitable for flaw measurements. Half Beam spread = 1.22 / D

Because of variation of sound intensity at different distances, the signal from a constant reflector vary with distance. Correction of :Display

Signal amplitude from a 2 mm dia FBH for a 2 MHz, 24 mm dia probe.Display

Signal amplitude comparison for a back wall and from a 2 mm dia FBH for a 2 MHz, 24 mm dia normal probe. Echo amplitude from a large reflector such as a back wall is inversely proportional to the distance. Echo amplitude from a small reflector such as a flaw is inversely proportional to the square of distance, i. e. signal of a small reflector becomes one-fourth if its distance is doubled. Display

Flat Bottom Holes which can be used for setting test sensitivity with normal probes and comparing disc equivalent reflectors in wrought products.

ASTM set of 10 Flat Bottom Hole blocks which can be used for checking dead zone, resolution and drawing Distance Amplitude Correction [ DAC ] curves for normal beam testing. :

Flat Bottom Hole blocks Distance Amplitude Correction [ DAC ] curves for normal beam testing. :

Flat Bottom Hole set of 19 blocks which can be used for checking amplitude linearity of signals and drawing DAC curves for normal beam testing. :

Probe placed on a FBH block, the larger echo is from the back of the block and the smaller one is from the hole bottom.

Side drilled hole block may also be used for setting up test sensitivity..

Display

Drawing DAC curve with Side drilled hole block. These method is normally used for weld testing.

Display

Drawing DAC with Flat Bottom Hole blocks.

Display

Drawing DAC with Side drilled hole block.

Display

Digital flaw detectors can draw DAC and the screen display can be saved for future use.Display

DAC curve is used for signal comparison.Display

Thickness Measurements : Thickness measurements are performed using a conventional flaw detector and a compression wave probe, which sends longitudinal waves into the component at normal incidence to the surface. Signals are displayed on the flaw detector screen in the form of an A-scan, in which the horizontal axis represents distance and the vertical axis represents signal amplitude. Since a 0 compression probe is being used, the horizontal axis is equivalent to depth from the scanning surface. When the probe is placed on the surface of the component, a reflection appears at a range corresponding to the thickness of the component at that point. The use of an A-scan display allows the operator to distinguish more easily between signals originating from embedded plate flaws and the nominal back wall response. Also, the dynamics of the back wall echo can be observed on the A-scan display to detect the presence of pitting. Conventional twin-crystal 0 compression probes are generally used to detect hidden corrosion. However, where pitted surfaces are being assessed for remaining thickness, pencil probes are used. These have a pointed tip which is designed to fit into the pits, so that the remaining thickness can be measured where the external pitting is at its most severe.

Display

Flaw Detection : Straight beam testing is used for examining bar stock for internal flaws.Display

Angle beam testing is used for examining welds for internal flaws.Display

DisplayThe shell of the mill roles are regularly monitored by ultrasonic testing.

DisplayIn a mill roll hard shell [ about 3 inches thick ] is bonded to a softer core. The bonding can be tested by straight beam examination. Cracking in shell material can be examined with angle beam probes. Depth of surface breaking cracks can be estimated using Surface wave probes.

DisplayFractured roll surface.

DisplayNormal probes are used for testing Ingots. Large ingots are forged to Blooms or Billets. Small ingots are rolled to bars. Efficiency of testing depends on the surface condition and the grain size of the ingot. Ingots are tested for piping and crack.

DisplayNormal probes are used for testing billets. Most Billets are produced by continuous casting. Billets are tested for crack, piping etc,

DisplaySlabs are tested with normal beam probes before they are rolled into plates. Slabs, produced by continuous casting may be rolled directly in to plates with out ultrasonic examination.

DisplayPlate testing is one of the major applications of normal beam probes. Plates are produced by rolling. Plates are tested for lamination, cracks and large inclusions which also produce laminar discontinuities. Surface breaking cracks are tested with a 450 angle beam probe.

DisplayPlate scanner : For scanning a large number of plates

Plate scanner : For scanning a large number of plates

DisplayNormal probes are used for testing Forgings. Forgings are tested for forging bursts, crack, flakes, piping, blowholes, inclusions, segregations and coarse grain structure.

DisplaySegregations and coarse grain structure.

DisplayRough forged bars limits the efficiency of testing. Forged bars are generally rough machined before ultrasonic testing.

DisplayRough forged blanks, are s limits the efficiency of testing. Forged bars are generally rough machined before ultrasonic testing.

DisplayRolled rings are ultrasonic tested for laminar defects.

DisplayFlanges are generally rough machined before ultrasonic testing.

DisplayBars and shapes are also tested by straight beam probes. For circular shapes, a matching curved shoe is usually fitted to front of the probe. Bars are tested for piping, lamination, chevrons and stringers

DisplayCasting are also tested by straight beam probes. Efficiency may be limited due to the material type, surface roughness and complicated shapes.

DisplayCasting are also tested by straight beam probes. Efficiency may be limited due to the material type, surface roughness and complicated shapes.

DisplayLarge Casting are also tested by straight beam probes. Efficiency may be limited due to the material type, surface roughness, material thickness, and complicated shapes.

DisplayPress forged parts, major defect, cracks at the outer surface.

DisplaySeamless tubes are tested using angle beam probes..

DisplaySeamless tubes are tested using angle beam probes..

DisplayNozzle welds

DisplayIf the sensitivity calibration block is different, correction for transfer loss is required.

Dead zone : A single element normal probe has a dead zone starting immediately after the entry surface where flaws cannot be detected. The width of the initial pulse shows the dead zone during testing.

Delay Line Transducers Delay line transducers incorporate a short plastic wave guide or delay line between the active element and the test piece. They are used to improve near surface resolution and also in high temperature testing, where the delay line protects the active element from thermal damage.

Dead zone of a single element probe can be eliminated by dual element arrangement. This transducer uses a pitch-and-catch effect. It uses two elements. One element transmits the signal, while the other one receives it. The probe generates longitudinal waves into a delay line. The angled arrangement of the elements produce a pseudo focus where the detection sensitivity is maximum.Dual element probe :

Dual Element normal probes : are available in different element sizes and operating frequencies. Different probes are used for thickness measurements and flaw detection.

Dual Element Angle probes : are available in different element sizes and operating frequencies. Dual element angle probes are used for thin materials and coarse grained welds. To further improve signal to noise ratio, it is possible to use dual element, transverse wave probes (TRT probes) or lens focused transverse wave probes. As a consequence, there is a restricted range of maximum sensitivity, just as with TRL probes.

Dual element probe uses a long plastic delay line which eliminates the initial echo from the screen. A cross talk barrier [ cork ] separates the transmitter and receiver delay lines and does not allow detection of entry surface signal.The return signals from the transmitter delay line are not detected because the transmitting probe has no receiving function.

Sensitivity curve for twin probes : The arrangements of the elements produces a pseudo focus where the sensitivity of the probe is maximum and after this distance the sensitivity drops rapidly. For detecting small flaws the usable test range is around 50 mm.

A twin probe being used with an ultrasonic flaw detector. - Dual Element Transducers -- Dual element transducers utilize separate transmitter and receiver elements in a single assembly. They are often used in applications involving rough surfaces, coarse grained materials, detection of pitting or porosity, and they offer good high temperature tolerance as well.

Sheet metals : Dual Element Transducers -- Dual element transducers utilize separate transmitter and receiver elements in a single assembly. They are often used in applications involving rough surfaces, coarse grained materials, detection of pitting or porosity, and they offer good high temperature tolerance as well.

Examination of bonding is an important application of a twin probe. White metal lining on carbon steel are checked for bonding integrity. Titanium and stainless steel bonding to carbon steel plates are checked frequently.

Examination of bonding is an important application of a twin probe. White metal lining on carbon steel are checked for bonding integrity. Titanium and stainless steel bonding to carbon steel plates are checked frequently.

One of the important applications of Twin probe is in a Digital Thickness Gauge : The gauge has a built in thickness reference for calibration. Thickness is digitally displayed in mm or inches. Nominal accuracy 0.1 mm. The gauge can measure different materials with suitable calibration or correction.

Digital gauging is extensively used for measuring the remaining thickness of corroded plates in ships : Internal corrosion pitting and general erosion in most metals. A-Scan thickness surveys are also used for the inspection of parent material for inclusions and laminations. Generally used for thickness surveys on pressure vessels, pipelines, storage tanks and ship hulls.

A Digital gauge is used for measuring the remaining thickness of corroded pipes in chemical plants. Thickness measurements are performed using a conventional flaw detector and a compression wave probe, which sends longitudinal waves into the component at normal incidence to the surface. Signals are displayed on the flaw detector screen in the form of an A-scan, in which the horizontal axis represents distance and the vertical axis represents signal amplitude. Since a 0 compression probe is being used, the horizontal axis is equivalent to depth from the scanning surface.

When the probe is placed on the surface of the component, a reflection appears at a range corresponding to the thickness of the component at that point. Conventional twin-crystal 0 compression probes are generally used to detect hidden corrosion. However, where pitted surfaces are being assessed for remaining thickness, pencil probes are used. These have a pointed tip which is designed to fit into the pits, so that the remaining thickness can be measured where the external pitting is at its most severe.

Some Digital gauge can measure thickness of a part without removing the paint coating.They work on echo to echo measurement principle.

Echo to Echo measurement.

Echo to Echo thickness gauge with A - Scan display. The use of an A-scan display allows the operator to distinguish more easily between signals originating from embedded plate flaws and the nominal back wall response. Also, the dynamics of the back wall echo can be observed on the A-scan display to detect the presence of pitting. Conventional twin

A through paint test gauge in use.

A Step block is recommended for dual probe range calibration when an ultrasonic flaw detector is to be used.

Step thickness plates are used to check the accuracy of digital gauges.

Initial calibration is performed on the 5 mm block.

The minimum required accuracy is + / - .1 mm.

When measuring thickness below 2 mm, there may be an error, and the reading may be twice the actual thickness because the sound may bounce twice within the part before reaching the receiving element.

Various step blocks for thickness gauges.

DisplayFlaw Orientation :A normal probe detects flaws that are parallel to the test surface. Normal probe fails to detects flaws orientated at an angle to the test surface.

DisplayAn angle probe detects flaws orientated at an angle to the test surface. Angle probe fails to detects flaws that are parallel to the test surface such as laminations.

DisplayAngle beam reflects well from corners and surface flaws which produces corners. Angle beam transducer are used when looking for defects that are neither parallel nor perpendicular to the test surface.

DisplayAngle beam interception is the only way to examine welds with reinforcements, where the sound beam is to be directed to the weld body from the base material. Irregular contour of the weld surface does not allow suitable contact for straight beam probes. Flaws such as lack of fusion is inclined to the test surface and can be detected only by angle beam directed perpendicular to the major reflecting surface.

DisplayNormal beam often fails to detect surface breaking radial cracks.

DisplayThe narrow end of radial cracks reflect very little energy for detection. Delay line transducer. This contact transducer contains a plastic wedge between the transducer and the part being measured. Basically, it eliminates the near field. High-frequency transducer. Transducers use frequencies from 0.5 MHz all the way up to 25 MHz--and sometimes up to 50 MHz. The higher the frequency, the more sensitivity. Normal incidence shear wave transducer. This type of transducer emits shear waves directly into the material without having to use an angle-beam wedge.

DisplayA 700 or 600 angle probe detects surface breaking radial cracks reliably. The diverging beam reflects well from the corner formed by the crack at surface.

Angle probes : Angle beam transducers are used in conjunction with plastic wedges to introduce shear waves or longitudinal waves into a test piece at a designated angle with respect to the surface. They are commonly used in weld inspection. Longitudinal wave is refracted through the wedge to produce Refracted longitudinal waves, creeping waves, shear waves, surface waves as required. The incident angle in the plastic block controls the refracted beam angles and wave modes.

Display

Single crystal shear wave probes are the most commonly used probes for ultrasonic inspection, usually with 45, 60 and 70 beam angles designed for steel. For some inspections, however, it may be useful to optimize the angle and the crystal size to get the best sensitivity and signal to noise ratio. 30 and 80 probes are also used when required. By using elliptical or rectangular elements, the spot size in the material can be precisely defined and the beam can be directed to minimize beam spread.

Display

Incident angle in Perspex vs refracted angle in steel

Action of an angle beam probe.

Trigonometric flaw locating :Distance to reflector from probe exit point = beam path X Sin of beam angleDepth from test surface = beam path X Cos of beam angle

Range calibration and locating the exit point of an angle beam probe :

locating the exit point of an angle beam probe : The signal from the radius is maximized, the mark on the probe body which coincides with the center of the scale marked on the face of the IIW block is the exit point. Exit point shifts with probe wear.

Locating the exit point of an angle beam probe :

Angle probe Range calibration using reflection from the 100 mm radius :

200 mm calibrated screen Range 200 mm, 1 small scale division 200 / 50 = 4 mm

IIW V2 calibration block which can be used for range calibration with miniature angle probes. These blocks are made from Mild Steel, Stainless Steel and Aluminum.

Ultrasonic calibration blocks : DC calibration block for angle probe range calibration. Produces echoes from both the radius simultaneously. calibration block for angle probe range calibration

Ultrasonic calibration blocks : AWS block for plotting vertical linearity of the flaw detector. AWS block for checking resolving power of angle probes.

IIW- V2 block is most popular for test range calibration in field testing because of its small size.

This block can be used to find exit point, beam angle determination and test range calibration. Beam exit point found from this block has a little error because of the near zone effect of the probe. The 50 mm radius reflection is recommended for this purpose.

Distance to echoes when the 25 mm radius is scanned.

Distance to echoes when the 50 mm radius is scanned.

A 100 mm calibrated screen :

A 125 mm calibrated screen :

A 125 mm calibrated screen :

Angle check using a V2 block : the hole indication is maximized and the angle is calculated using the equations given below.

Weld testingDisplay

Butt welds : A butt weld is made between two pieces of metal usually in the same plane, the weld metal maintaining continuity between the sections.

Fillet welds : These welds are roughly triangular in cross section and between two surfaces not in the same plane and the weld metal is substantially placed alongside the components being joined.

Weld joint preparation.

Weld joint preparation.

Weld joint fit up before welding. The weld groove which is to be filled by welding can be seen.

Weld joint fit up for a pipe to reducer joint, the root gap is clearly visible.

Double Vee Weld groove fit up for plates bend in to pipes

Welding sequence.Welding sequence :

Welding sequence

Welded layers

Weld and heat affected zone : The heat of fusion affects the base material adjacent to either side of the weld. Flaw detection should take into consideration of these zones.

Shielded Metal Arc Welding : Several welding processes are based on heating with an electric arc, the oldest and simple is the shielded metal arc welding [ SMAW ] or stick welding. In this process an electrical machine [ which may be DC or AC ] supplies current to an electrode holder which carries an electrode. An earth cable connects the work piece to the welding machine to provide a return path for the current. The weld is initiated by tapping [ striking ] the tip of the electrode against the work piece which initiates an electric arc. The high temperature generated [ about 6000oC ] almost instantly produces a molten pool and the end of the electrode. The electrode continuously melts into this pool and fills the groove. The operator needs to control the gap between the electrode tip and the work piece while moving the electrode along the joint.

In the shielded metal arc welding process [ SMAW ] the 'stick' electrode is covered with an extruded coating of flux. The heat of the arc melts the flux which generates a gaseous shield to keep air away from the molten pool and also flux ingredients react with unwanted impurities such as surface oxides, creating a slag which floats to the surface of the weld pool. This forms a crust which protects the weld while it is cooling. When the weld is cold the slag is chipped off. Major defects in this process are ; Undercutting, Incomplete penetration, incomplete fusion, Porosity, Slag Inclusions, Cracks, burn through.

Shown in the picture is the electrode and its holder. The cover on the electrode is flux. An old electrical power source can also be seen in the behind. The SMAW process can not be used on steel thinner than about 3mm and being a discontinuous process it is only suitable for manual operation. It is very widely used in fabrication shops and for on site steel construction work. A wide range of electrode materials and coatings are available enabling the process to be applied to most steels, heat resisting alloys and many types of cast iron.

A SMAW deposit and cover blanket of the slag. The flux covering the electrode melts during welding.This forms the gas and slag to shield the arc and molten weld pool.The slag must be chipped off the weld bead after welding. The flux also provides a method of adding scavengers, deoxidizers, and alloying elements to the weld metal.

The covering slag removed from the finished weld and the weld bead exposed.

Submerged arc welding [ SAW ] is a high quality, very high deposition rate welding process.The electric arc is submerged below the loose granular flux which is poured into the groove separately. This method is used for, fast, large scale welding of thicker plates in fabrication shops. Common defects in Submerged Arc Welding : Solidification Cracking, Hydrogen Cracking , Incomplete penetration, Incomplete fusion, Slag inclusion, Porosity.

The arc is formed between a continuously-fed wire electrode and the workpiece, and the weld is formed by the arc melting the workpiece and the wire. The layer of flux generates the gases and slag to protect the weld pool and hot weld metal from contamination. Flux plays an additional role in adding alloying elements to the weld pool.

Gas Tungsten Arc Welding : In this process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding, specially useful for thin joints. If filler wire is used, it is added to the weld pool separately. GTAW has played a major role in the acceptance of aluminium for high quality welding and structural applications.

The process is well suited to joining non - ferrous metals, including aluminum, magnesium, refractory and special metals and is effective for joining thin section metals. A high degree of skill is needed, but high quality welds can be produced. Helium is generally added to increase heat input. Hydrogen will result in cleaner looking welds and also increase heat input, however, Hydrogen may promote porosity or hydrogen cracking. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. Undercutting, Tungsten inclusions, Porosity, Weld metal cracks, Heat affected zone cracks are the common defects.

MIG welding : In MIG the arc is formed between the end of a small diameter wire electrode fed from a spool, and the work piece. The shielding gas, Argon or CO2 forms the arc plasma, stabilizes the arc on the metal being welded, shields the arc and molten weld pool, and allows smooth transfer of metal from the weld wire to the weld groove. Main equipment components are : power source Wire feed system Conduit Gun

In MIG welding, a shielding gas is fed into the welding torch and exits around the filler wire. The arc and the weldpool are protected from the atmosphere by this gas shield. This enables bare wire to be used without a flux coating. However, the absence of flux to 'mop up' surface oxide places greater demand on the welder to ensure that the joint area is cleaned immediately before welding. This can be done using either a wire brush for relatively clean parts, or a hand grinder to remove rust and scale. The other essential piece of equipment is a wire cutter to trim the end of the electrode wire. In this process a filler metal is stored on a spool and driven by rollers [ current is fed into the wire ] through a tube into a 'torch'. The large amount of filler wire on the spool means that the process can be considered to be continuous and long, uninterrupted welds can easily be made. In this process they key issues are selecting the correct shielding gas and flow rate and the welding wire speed and current. MIG process can readily be automated and MIG welding is now commonly carried out by robots. This welding process is widely used on steels and on aluminium. Although the inert gas shield keeps the weld clean, depending upon the process settings, there may be spatter of metal globules adjacent to the weld which detracts from its appearance unless they are removed.

Metal Inert Gas Welding machine :

Common defects in MIG welding are ; Undercutting, Excessive melt-through, Incomplete fusion, Incomplete joint penetration, Porosity, Weld metal cracks, Heat affected zone cracks. The primary shielding gasses used are ;Argon, Argon - 1 to 5% Oxygen, Argon - 3 to 25% CO2, Argon / Helium. CO2 is also used in its pure form in some MIG welding processes. However, in some applications the presence of CO2 in the shielding gas may adversely affect the mechanical properties of the weld.

Flux Cored Arc Welding : FCAW is a commonly used high deposition rate welding process that adds the benefits of flux to the welding simplicity of MIG welding.The welding wire is continuously fed from a spool.Flux cored welding is therefore referred to as a semiautomatic welding process. Self shielding flux cored arc welding wires are available or gas shielded welding wires may be used. Flux cored welding is generally more forgiving than MIG welding.Less precleaning may be necessary than MIG welding. However, the condition of the base metal can affect weld quality. Excessive contamination must be eliminated. Flux cored welding produces a flux that must be removed. Flux cored welding has good weld appearance (smooth, uniform welds having good contour). Common defects are Undercutting, Incomplete fusion ,Slag inclusions, Porosity, Cracks.

Incomplete Penetration - A joint root condition in a groove weld in which weld metal does not extend through the joint thickness. Failure to bridge the root gap.

Lack of penetration and actually required penetration. Incomplete Fusion - A weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads.

Lack of fusion imperfections can occur when the weld metal fails to fuse completely with the sidewall of the joint (Fig. 1) to penetrate adequately the previous weld bead (Fig. 2). The principal causes are too narrow a joint preparation, incorrect welding parameter settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or scaled surfaces can also contribute to lack of fusion. These types of imperfection are more likely to happen when welding in the vertical position.

These types of imperfection are more likely in consumable electrode processes (MIG, MMA and submerged arc welding) where the weld metal is 'automatically' deposited as the arc consumes the electrode wire or rod. The welder has limited control of weld pool penetration independent of depositing weld metal. Thus, the non consumable electrode TIG process in which the welder controls the amount of filler material independent of penetration is less prone to this type of defect. Incomplete root fusion is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides of the joint are unfused. Typical imperfections can arise in the following situations: an excessively thick root face in a butt weld (Fig. 1a) too small a root gap (Fig. 1b) misplaced welds (Fig. 1c) failure to remove sufficient metal in cutting back to sound metal in a double sided weld (Fig. 1d) incomplete root fusion when using too low an arc energy (heat) input (Fig. 1e) too small a bevel angle, too large an electrode in MMA welding (Fig 2

Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms: distributed surface breaking pores wormhole crater pipesPorosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides (see Fig 2). Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result especially when using welding processes other than MMA.

Slag is normally seen as elongated lines either continuous or discontinuous along the length of the weld. This is readily identified in a radiograph, Fig 1. Slag inclusions are usually associated with the flux processes, ie MMA, FCA and submerged arc, but they can also occur in MIG welding. As slag is the residue of the flux coating, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs, Fig 2. As they both have an effect on the ease of slag removal, the risk of slag imperfections is influenced byType of flux Welder technique The type and configuration of the joint, welding position and access restrictions all have an influence on the risk of slag imperfections.

they occur only in the weld metal they normally appear as straight lines along the centreline of the weld bead, as shown in Fig. 1, but may occasionally appear as transverse cracking depending on the solidification structure solidification cracks in the final crater may have a branching appearance as the cracks are 'open', they are easily visible with the naked eye On breaking open the weld, the crack surface in steel and nickel alloys may have a blue oxidised appearance, showing that they were formed while the weld metal was still hotSegregation of impurities to the centre of the weld also encourages cracking. Concentration of impurities ahead of the solidifying front weld forms a liquid film of low freezing point which, on solidification, produces a weak zone. As solidification proceeds, the zone is likely to crack as the stresses through normal thermal contraction build up. An elliptically shaped weld pool is preferable to a tear drop shape. Welding with contaminants such as cutting oils on the surface of the parent metal will also increase the build up of impurities in the weld pool and the risk of cracking. The overriding cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include: insufficient weld bead size or shape welding under high restraint material properties such as a high impurity content or a relatively large amount of shrinkage on solidification. Joint design can have a significant influence on the level of residual stresses. Large gaps between component parts will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Therefore, weld beads with a small depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead, will be more susceptible to solidification cracking, as shown in Fig. 2. In this case, the centre of the weld which is the last part to solidify, is a narrow zone with negligible cracking resistance. of the plate and the filler determine the weld metal composition they will, therefore, have a substantial influence on the susceptibility of the material to cracking.

Under fill - A condition in which the weld face or root surface extends below the adjacent surface of the base metal.

Overlap - The protrusion of weld metal beyond the weld toe or weld root. There may be fusion problem.Undercut - A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal.

Misalignment -

Cracks : A crack is produced by a fracture which can arise from the stresses generated on cooling or acting on the structure. It is the most serious type of imperfection found in a weld and must be removed. Cracks not only reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature

T joint -

Excess weld cap - Excess penetration -

Weld probing : In a weld, discontinuities may be present anywhere within the weld and in the heat affected zone. A combination of normal and angle probes shall be selected such that the full body of the weld is intercepted. Angle probing by moving the probes perpendicular to the weld seam is performed to detect discontinuities which are along the length of the weld.

In addition, the weld shall be scanned by moving the probes nearly parallel to the weld seam such that the discontinuities which are transverse to the weld seam can also be detected.

The weld shall be scanned such that both longitudinal and transverse flaws can be detected. 10% overlapping with the previous scan path is required to ensure that complete length of the weld is examined.

DisplayProbe movement distance to scan the full body of the weld : Scanning Distance for weld testing is 2 X Thickness X Tan A from the edge of the weld.The weld must be scanned from both the sides of the weld.

The scanning path must overlap by 10% with the previous scan path.

The probe moves in a zig-zag path and parallel to the weld axis. The sound beam scans the weld perpendicular to the weld axis.

Locating flaw position within an angle beam probe.

Display Locating flaw position within Leg 1 during weld testing.

DisplayLocating flaw position within Leg 2 during weld testing.

Locating flaw position in Leg 1 [ before reflection from undersurface ] during weld testing.

Probe selection : Up to 6 mm : 800 up to 10 mm : 700 10 25 mm : 700 and 600 more than 25 mm : 600 and 450 Lower frequency [ 2 2.5 MHz ] probes have better detectability for disoriented flaws because of wider beam spread. Higher frequency misses disoriented defects.

Locating flaw position within Leg 2 [ after reflection from undersurface ] during weld testing.

Sensitivity setting for weld testing Side drilled hole blocks are generally used for weld discontinuity evaluation.

Notches shall be used for evaluating root discontinuities.

Notches shall be used for evaluating root discontinuities.

DisplayWhile plotting the DAC curve, the hole must be scanned such that the probe is at least 12.5 mm away from the edge of the block. The drilled hole should be preferably 37.5 mm deep.

DisplayDrawing DAC : up to 19 mm thk

DisplayDrawing DAC : above 19 mm thickness.

DAC plotting using Notches.

DisplayUsing DAC curve

A Side drilled hole block for examining curved surfaces such as pipe.

Notches on the base material and the weld surface can be used to set up test sensitivity and verify the efficiency of testing. Notches should be made on the root side also.

Notches shall be made along the longitudinal as well as transverse directions to establish a scanning technique.

Angle beam testing of weld, probe angle shall be selected such that the fusion line is intercepted perpendicularly. A strong specular reflection is required to resolve a flaw response from the background noise level with pulse echo ultrasonics. For planar flaws (cracks, lack of fusion, etc.) a specular reflection will only result if the ultrasonic beam is normal (or near normal) to the plane of the flaw. Angled beam shear wave probes are commonly used for the manual ultrasonic inspection of welds in ferritic steels, as these provide the only way of directing ultrasound into the weld body when the cap reinforcement is still present. Where a weld cap restricts probe movement, the sound can be reflected off the bottom surface and directed into the weld body under the cap.

For a typical girth weld, a 45 probe is used for inspecting the root region, and 60/70 probes for the sidewall fusion faces and weld body. The behaviour of the echo-dynamic pattern and shape of the flaw response (with respect to probe movement) can be used to identify the type of flaw, estimate the length and, in some cases, the through-wall height of the flaw.Vertically orientated planar flaws can be a particular problem for detection using an angle probe in pulse-echo mode. However, a variation of angled shear wave ultrasonics is the Tandem technique, which is normally used for the detection of vertical flaws in thick section components. Two 45 shear wave probes are positioned in a jig, one behind the other facing the area of interest. The rear probe is used to transmit ultrasound into the joint area and the front probe receives sound reflected from flaws within the insonified region. By moving the probes relative to each other, it is possible to obtain full-through thickness coverage.

The type of material to be inspected