Ultrasonic measurement of wall thickness

M. Čuletić Čondrić a, V. Domanović a

a College of Slavonski Brod, dr. M. Budaka 1, 35 000 Slavonski Brod, Croatia, mccondric@vusb.hr; vjekoslavdomanovic@gmail.com

Space reserved for the Editorial Board please do not fill in, besides information given below

Abstract The paper describes the method of ultrasonic measurement of the thickness of the wall. The work consists of a theoretical and practical part. Theoretical part describes methods of non destructive control with an emphasis on the ultrasound method. The principle of ultrasonic measurement of wall thickness and the equipment used for ultrasonic testing are described. In the practical part, a wall thickness measurement was performed on the tank. The tank was already in operation and then changed for other purposes. After the restoration of the tank, we measured the thickness of the walls on each of the sanitized parts. Attention was drawn to the eligibility criteria and the tolerance prescribed by the norm. Based on the practical work, the results were analyzed and a conclusion was drawn on the advantages and disadvantages of this method.

Keywords: ultrasound methods, ultrasonic equipment, thickness measurement, tolerance

1. INTRODUCTION

Quality Control is a set of methods and procedures by which the quality criteria determine compliance and meet the requirements set. Quality control is a fully standardized approach.

Depending on the types of tests carried out on the material and the welded joints, the quality control is divided into destructive control and control.

Non destructive control contains all methods of test and measurement which, by their application, do not affect the functionality of the tested object. Destructive control methods provide data on the condition of the test object (surface quality, volume quality, grade of weld quality, etc.).

The basic division of non-destructive methods is on surface and volume. Surface methods give data on the surface state of the test object and are divided into visual, penetrant and magnetic methods. Volumetric methods provide data on the volume state, ie the interior of the test object, and are divided into ultrasound and radiographic methods. Of the other methods that do not belong to these groups, it is often used to extract acoustic emission, vortex currents, leakproofness control, isolation testing on improperness and hardness measurement.

In this paper an ultrasound method of wall thickness measurement will be analyzed. The aim of the paper is to describe the mode of operation and to demonstrate the use of ultrasound on a specific example. The work will be divided into theoretical and experimental part.

In the theoretical part will be described the principle of the ultrasonic method, the physical basis of ultrasound and ultrasonic sources. I will be dealing with ultrasonic waves and ultrasound parameters as well as reflexion and fracture of ultrasonic waves and parts of the beam. Special attention will be paid to ultrasonic probes, device and signal mode. When measuring the thickness of the wall, I will talk about the choice of ultrasonic equipment and measurement techniques, surface condition and test personnel.

The experimental section will show the wall thickness measurement on a tank that was already in use. Ultrasonic measurement will be used when assessing the actual condition of the object. According to the state of the surface of the test object, a suitable test technique and an ultrasound system will be selected. According to the analysis of the results of the experimental work, conclusions will be drawn on the advantages and the lack of this method.

The paper will use expert literature on ultrasound control, Croatian and European standards

2. Ultrasonic control

The principle of ultrasonic control is based on the transmitting of the ultrasonic wave to the material, it is repelled from the obstacle and the time required to revert the wave determines the distance and shape of the object or some irregularity in the material. Ultrasound is a sound whose frequency is above the upper limit of hearing for a normal human ear, which is 20 kHz. The average human sound is 16 Hz to 20 kHz.

Transmission and reception of the ultrasound into the test material is carried out by ultrasonic transducer and ultrasonic sensor. The ultrasonic wave pulses through the pass through the probe and the material we measure, and such a signal in the device is precisely measured as the time of passing through the metering object.

2.1. Physical basic of ultrasound

Acoustics is an area of physics dealing with the study of phenomena related to the formation, transmission and reception of sound or related tonsils in a wide spectrum of frequencies and applications. [1]

Sound is generated by vibration, mechanical titration, some matter. Titration is the form of periodic motion caused by elastic body traits, repetition of a series of states at certain time intervals (intervals). The parameters we use to describe the titration are:

· period T, time of one titre

· frequency f, number of titers per second

· elongation x(t), shift from equilibrium position at time t

· amplitude A, maximum elongation

Free titration is the periodic motion of the body caused by the action of elastic force proportional to the displacement of the body from the equilibrium position (Figure 1.). The total energy of the free-titrating body equals the sum of its kinetic energy and elastic potential energy. [2]

Fig. 1. Graphic view of free titration

2.2. Ultrasonic sources

The ultrasonic titration in the contact with the agent will transfer the titration to the surrounding substance. The particles in the ultrasound device excite titers with the same frequency as the source, but with a different phase. The titration phase depends on the rate of titration and the point in the space where the state of the titration is observed. [1] The wave velocity is calculated according to the expression:

(1)

where:

· v velocity of wave propagation, m / s

· λ wavelength, m

· f frequency, Hz

2.3. Creating ultrasound and ultrasonic waves

The ultrasound probe by pushing on the object transfers the titrating into the means they are in contact with. An integral part of the transducer is a titrator that generates ultrasonic waves due to various external stimuli.

Ultrasonic waves can only be spread in a homogeneous medium, and this is precisely the basis for detecting errors in the object being investigated. Waves at the boundary of the medium, whether it is the walls of the test object or the irregularities within the material, follow the legitimacy of the wave motion. The proper interpretation of the ultrasonic energy obtained by tracing the test material can be used to estimate the material condition and the parameters of the detected errors.

Ultrasonic energy is the most commonly used piezoelectric effect, and is based on the ability of some materials to generate electrical potential when subjected to mechanical pressure. When such material is processed appropriately, the active part of the inverter that can produce and receive ultrasound is obtained. Other ways of getting ultrasound are: magnetostrictive, mechanical acceleration, thermal stimulation, electrostatic, electrodynamic.

Ultrasonic waves transmit the energy of titration through the material. Depending on the type of material through which energy is transmitted and features such as the type of material, shape and dimensions, and the elastic properties of the material, various types of waves will occur.

There are two basic types of waves, which are longitudinal and transversal. Longitudinal waves are those waves in which the particles are titrated in the direction of wave propagation, causing dampening and dilution in the widening medium. Transversal waves are those waves in which the particles are titrated perpendicularly to the direction of wave propagation, and do not cause dampening and dilution in the medium being widened. Longitudinal waves can be spread in all three aggregate states, while transversals can only be spread in rigid media.

Of the other waves used in ultrasonic control, the most important surface and plate waves are the combination of longitudinal and transverse titers in the medium. Most used are Rayleighs, Lateral, Lambs and P-waves.

Ultrasonic waves spread in their original form only in infinite means. Since in practice the means are limited by dimensions, the infinite means is considered only as a means whose dimensions are considerably larger than the wave wavelength expanding. By analyzing response, ultrasonic waves returning from the boundary of the media, we can detect errors in the test object.

For all waves Snell's law is valid, which is calculated according to the expression:

(2)

where:

· α1 is the angle of propagation of the sound in the medium 1

· α2 is the angle of spreading sound in the medium 2

· v1 speed of sound in the media 1, km / s

· v2 speed of sound in the media 2, km /s

2.4. Ultrasonic probes

Ultrasonic test probes are a key part of the ultrasonic system. When constructing the probes, the following is taken into account: inverter material, electrode configuration silencer and its features, prism and / or mask for transmitting ultrasound to the material, housing, protective elements and masks.

In practice, the usual division of probes toward the direction of transmitting and receiving the ultrasonic beam in relation to the object of the test, the mode of excitation, the technique of work and the like. Standard probes used in manual ultrasonic control are: flat, double and angular probes.

The ultrasonic device consists of electronic components that allow the operator to detect the irregularities of the test object in structure and dimensions by ultrasonic waves. The ultrasound device produces high-voltage and short-life electrical pulses, which convert the piezoelectric transducer into the test probe into mechanical oscillations in the ultrasound field. These oscillations are in the subject of the test as sound waves. The sound waves are reflected on the boundary surfaces and the transducer of the test probe receives them and is displayed on the ultrasound device screen

3. MEASUREMENT OF THE THICKNESS OF THE WALL

Ultrasonic wall thickness measurement is used in the industry to carry out precise measurements on various types of materials, parts and components.

It is useful to monitor the wall thickness of various types of pipes and pressurized vessels in exploitation, for comparison of loss of projected thickness, bearing capacity of corrosion-induced corrosion or erosion. The principle of ultrasonic wall thickness measurement is based on measuring the time required for a short ultrasonic pulse to pass through the material one or more times.

The thickness of the material is calculated according to the expression:

(3)

where:

· d the thickness of the material, m

· v known ultrasonic velocity in material, m / s

· t measured time pass through material, p

· n the total number of impulse passages through the material

If measurements are performed in steel, standard measurement methods can be measured in thicknesses of 1 mm to 10 mm.

The choice of probes depends on the type of equipment, the thickness of the material, the condition of the surface and the possible coating on the surface.

It is imperative to use a contact agent to compensate for roughness and roughness in the contact probe - material. The contact materials are most often viscous, non-toxic gels or oils which, by their structure and properties, do not substantially affect the ultrasonic pulse and at the same time ensure good contact between two different materials. The contact medium should be applied to the test surface in a thin layer. Layer thickness can affect test results and signal stability on the ultrasound device.

We use etalone and reference samples to check the characteristics of the equipment and to adjust the ultrasound system. When measuring the wall thickness, the measuring system should be calibrated on one or more reference samples, and the thickness of the material or the ultrasound velocity should be known for proper calibration.

The surface of the test object must be accessible and cleaned of dirt, grease, oil, without damaging the electrical arc or splash and without residues of corrosive products. Optimal measurement results are achieved on a clean, preferably wiped, surface.

3.1. Experimental part

The experimental part will explain the method of measuring the thickness of the wall on a tank that has been in use for a number of years, and is now overwhelmed by cement storage. Ultrasonic measurement of the wall thickness was used when assessing the actual condition of the object.

To measure the thickness of the wall, the handheld device PosiTector UTG C - Standard, manufactured by DeFelsko (Figure 2), was used. The device is designed to measure the thickness of metal and nonmetal, and uses the technique of single-sided measurement in its work.

Fig. 2. Test device and contact lenses

As a contact agent, a DeFelsko manufacturer's gel was used which is chemically neutral and can be used at temperatures from -15 ° C to 104 ° C.

In our case there was no technical documentation, it was not known from which material and the thickness of the test object was made. Due to experience with similar objects of similar use, it was assumed to be a structural steel, of S235JR or less, in a sheet thickness of 3 mm to 8 mm. Two-point calibration was accessed. For the purpose of the test, reference samples of 3 mm and 8 mm thick were cut. The two-point adjustment is based on calibration at the lower thickness and then on the upper. For proper reading of measured thickness results must be within these two limits.

The measurement results show that the first two sheaves, up to the funnel, were made from a thickness of 6 mm, the other two from a thickness of 5 mm and the last two from a thickness of 4 mm. After that, the device calibrated to the thickness to get the most accurate result and started with the test.

The test facility has been outdoors for a number of years, so the influence of atmospheric conditions left a trace on its surface. In such a situation the object was not suitable for measuring the thickness of the wall. The exact initial surface area assessment was performed by visual inspection according to HRN EN ISO 8501-2 [3] and the following was concluded:

• interior surface - the coating has been damaged with little or no rust

• external surface - the coating at the places began to separate, part of the surface began to rust, and a localized occurrence of deep corrosion

In both cases it was decided that the previous coatings and rusts were completely removed according to HRN EN ISO 8501-1 [4]:

• interior surface - thorough manual and machine cleaning, level of primary surface preparation by manual cleaning method St 2

• external surface - very thorough cleaning with abrasive blasting, level of primary surface preparation with abrasive cleaning method Sa 2½

After the surface preparation (Figure 3) there was another visual inspection of the object.

Fig. 3. External surface after cleaning

The tank bulkhead is worn out and the need for a different type of opening is replaced by a new one, and the roof is reinforced with a ribbed lime. At certain sites, a phenomenon of deep corrosion has been observed which could be the potential cause of loss of the basic thickness or design capacity of the facility. By grinding and polishing on one characteristic part and then by measuring the thickness of the wall, it has been found that remedies can be performed in this way and that loss of thickness is within the tolerance set. All other critical sites were remedied in a similar way. After all repairs, the areas on which the measurement will be performed are further polished for as accurate results and as such an object was ready for the test.

The test object consisted of six sheaves with three different thicknesses of sheets, 4 mm, 5 mm and 6 mm. Each cloak was measured from the outside on 20 places, randomly selected on a clean surface (Figure 4) and additionally at any place where repairs were performed (Figure 5), which could result in up to 50 measurements per strap.

Fig. 4. Measurement of the thickness of the wall

Fig. 5. Measurements on brushed parts

For hot rolled steel sheet tolerances of basic requirements should be according to HRN EN 10029 [5], tolerance of thickness class A, unless otherwise agreed. In our case, tolerances for 4 mm thick sheets are from -0.4 to +0.8 mm, and for sheets of 5 mm and 6 mm are from -0.4 to +1.1 mm (Table 1).

Table 1. Tolerances to nominal thickness

Nominal thickness (mm)

Class A

Lover tolerance

Upper tolerance

3 ≤ t < 5

-0,4

+0,8

5 ≤ t < 8

-0,4

+1,1

When interpreting the measurement results, average measurements were taken into the broken parts (Table 2). Individual spots where measured less than the permitted -0.4 mm or -0.4 mm to -0.7 mm were less than 5% of the test surface. The highest measured deviation was measured on the sheath of 3 and -0.48 mm. Places where the measured values below -0.4 mm were few meters away so that multiple faults were switched off in one.

Table 2. Results of measurement

Measuring site

Nominal thickness t (mm)

Average of measurements

Cloak 1

6

5,87

Cloak 2

6

5,84

Cloak 3

5

4,81

Cloak 4

5

4,83

Cloak 5

4

3,87

Cloak 6

4

3,89

It is concluded that the thickness of the test object is within the eligibility criteria and that the facility meets the required requirements. After the tests, the grinding parts were further dampened for better adhesion and the surface was protected with a non-metallic coating (Figure 6).

Fig. 6. The final look of the tank

4. CONCLUSIONS

Ultrasonic control is part of non-destructive testing, and applying this method to the object of the test does not affect its functionality.The first part of the paper describes the physical basics of ultrasonic control and detailed parameters of the ultrasonic method as well as the parts of the ultrasonic system.

Particular attention is paid to the method of measuring the thickness of the wall. Ultrasonic Thickness Measurement is a lightweight and fast procedure that is often used for auxiliary measurement based on which other material parameters are calculated. The choice of measurement technique and ultrasonic system depends directly on the state of the test surface, the properties of the material and the required precision measurement. Prior to the measurement, the calibration procedure of the measuring device on the reference blocks made of materials of the same or similar properties must be carefully carried out.

The experimental section shows the wall thickness measurement on a tank already in use. According to the state of the test facility, the test technique and the ultrasonic system were selected. After surface treatment and restoration on the object, we measured the wall thickness. The criterion of eligibility was based on the tolerances laid down by European standards for hot rolled steel sheets.

By analyzing the measurement results, it was concluded that the reservoir meets the required conditions and that the use of the facility is allowed for the foreseen purposes. In this example, the importance of wall thickness measurements has been demonstrated since the measurement results can be used when deciding to continue working or rejecting the facility, so it requires seriousness and professionalism in the work and interpretation of norms.

Ultrasonic control is, in recent times, an extremely important method that is increasingly used in everyday work. In the future it is possible to further advance these methods and techniques.

5. REFERENCES

[1] V. Krstelj, Ultrasound control. FSB, Zagreb (2003).

[2] V. Domanović, Ultrasonic measurement of wall thickness. Final work of a student (2018)

[3] HRN EN ISO 8501-2: 2006 - Preparation of steel substrates prior to application of paints and related products - Visual assessment of surface cleanliness - Part 2: Degrees of preparation of previously protected steel surfaces after occasional removal of previous coatings

[4] HRN EN ISO 8501-1: 2007 - Preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness - Part 1: Degradation rates and steps for the preparation of unprotected steel surfaces and steel surfaces after complete removal of the previous coatings

[5] HRN EN 10029: 2010 - Hot rolled steel sheets of a thickness of 3 mm or more - Permissible deviations of dimensions and shape