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Neithalath, Spring 2006, CE 455/555 Structural Damage: Assessment, Repair, and Strengthening

NDT and Evaluation of SteelStructures

Lectures 21,22

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The Chemistry

The reaction between air and coke producescarbon monoxide (CO)

This gas reduces the iron ore to iron

At the high temperature, limestonedecomposes to calcium oxide and carbon

dioxide

The impurities react with this CaO to form slag

Called Blast furnace slag useful supplementary

cementitous material in concrete

2 3 2Fe O (s) + 3 CO (g) 2 Fe (s) + 3 CO (g)

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Continuous Casting of Steel

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Steel Finishing Final Shapes

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Which Iron is it?

Cast iron

Sandy texture (cast in molds filled with sand) Rounded internal corners, square external

corners

Typically hollow round shapes Wrought iron

Material delamination at edges corrosion

Members by riveting iron plates

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Cast Iron structures

Heavily used for columns

Widely available Pig iron Good corrosion and fire resistance

High compressive strength

Cheaper than wrought iron

Weak and unpredictable in tension and

bending Brittle failure

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Age as an indicator of strength

Before 1905 YS 25 ksi

1905-1932 YS 30 ksi 1933-1963 YS 33 ksi

After 1963 YS 36 ksi

YS of cast iron 20 ksi

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History of structural steel

Structural carbon steel 1856 (Bessemer

converter) 1884 1st steel I-beams, structural frame

(Home Insurance Co, Chicago)

1889 Rand McNally Bldg, Chicago 1st allsteel framed skyscraper

1

st

AISC specification - 1923

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Properties of Carbon steel

Carbon steel (

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Mechanical Behavior of Iron-Carbon

Alloys

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Reinforcing bars

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Visual Inspection of steel structures

Magnifying glass, borescope, weld gages

Detects member deterioration

Excessive sagging or buckling

Lack of bracing

Rust, cracking, missing / loose fasteners

Cannot detect subsurface problems Cheap and effective

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Ultrasonic testing of steel (ASTM E

164) Ultrasonic waves 0.1

to 25 MHz

Reflected by interiorvoids, changes indensity etc

Resulted transmitted toa screen or a meter

Can detect voids,lamellar tearing,

porosity, changes incomposition, inclusionsetc

Can check pieces ofthickness upto 60 ft

Small and portable Bad for complex

shapes, rough surfaces

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UP Testing

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Ultrasonics of steel

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Lamellar tearing

Occurs beneath theweld

In thick plates withpoor through-thicknessductility

Usually in large welds Welding heats the

steel and it expands

Cooling Tears thesteel

http://www.twi.co.uk/j32k/protected/band_3/jk47.html

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Radiography (ASTM E 94)

X-rays or gamma raysapplied

Identifies internal voids,changes in structure andother defects

X rays penetrate up to 30in, gamma rays 10 in

Used to detectundercutting and

incomplete penetration inwelds

Portable test, reliable

Expensive

Potentially dangerous,shielding needed

Large installations needcooling water, power

Orientation of the defects

influence the results

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Principle of radiography

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Magnetic particle method (ASTM E709)

Detection of surface and near surfacediscontinuities in a magnetic material

Generate magnetic flux in the article to beexamined

Flux lines should run along the surface at right

angles to the suspected defect When the flux lines approach a discontinuity, they

will stray out into the air at the mouth of the crack

The crack edge becomes magnetic Red or black oxide particles, or coated with a

fluorescent substance under UV

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Principle

Principle: magneticsusceptibility (degree of

magnetization of amaterial in response toa magnetic field) of a

defect is markedlypoorer (the magneticresistance is greater)

than that of thesurrounding material

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Finding cracks

www.simula.it

Seen

NotSeen

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Magnetizing the material

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Pros and Cons of Magnetic Particlemethod

Advantages of Magnetic Particle CrackDetection

Simplicity of operation and application.

Quantitative.

Can be automated, apart from viewing.

Disadvantages of Magnetic Particle CrackDetection

Restricted to ferromagnetic materials.

Restricted to surface or near surface flaws.

Not fail safe in that lack of indication could mean nodefects or process not carried out properly.

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Liquid Penetrant method (ASTM E165)

Reveals surface flaws by bleed out of acolored or fluorescent dye from the flaw

Liquid penetrant applied to the surface

Enters the defect by capillary action

Penetrant becomes visible when coated witha developing solution

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Penetrant glows in dark

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Uses and advantages

Locating surface cracks, areas of porosity,incomplete fusion in welds,

Complements the magnetic particle test

Simple and inexpensive

Shape independent

Can detect only small surface defects

Operator expertise, careful surface prepneeded

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Eddy current testing (ASTM E 566)

Eddy current

When alternating current is applied to the

conductor, a magnetic field develops in andaround the conductor

This magnetic field expands as the alternating

current rises to maximum and collapses as thecurrent is reduced to zero

If another electrical conductor is brought intothe close proximity to this changing magneticfield, current will be induced in this secondconductor

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What does it do?

The eddy current testing technique is based oninducing electron flow (eddy currents) in

electrically conductive material Any defect in the material .e., cracks, pitting, wall

loss, or other discontinuities - disrupts the flow of

the eddy currents Higher frequency signals, up to 8 MHz, are used to

detect near-surface flaws

Lower frequencies (down to 50 Hz) are used whendeeper, subsurface flaw detection is required

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Eddy current testing

Eddy Currents aregenerated when

alternating voltage orcurrent flows throughthe excitation coil

Surface and sub-

surface defects resultin perturbations ofthe field. Theseperturbations aredetected by thedetector coils.

http://www.ndt-ed.org

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Pros and Cons

Continuously done, easily automated

Moderate cost Only a comparative procedure

May not detect cracks in some directions

Applicable to relatively shallow objects only

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Hardness testing (ASTM E 10, E 18)

Hardness is the propertyof a material that enables

it to resist plasticdeformation, usually bypenetration

Tensile strength andeffects of cold workingcan be roughly estimated

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Portable hardness testing

A steel ball of 10 mm. diameter (B) is placedbetween two surfaces, one (S) of known Brinellhardness, and the other (X) of unknown hardness,(X) being the material under investigation. Anypressure exerted against the assemblage of thethree, presses the ball (B) with the same force intothe surface (S) as into (X), and the sizes of theindentations obtained are in the direct proportionto the Brinell hardness of the two metals.

http://visgage.com/meter_operation.html

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Hardness scales compared

http://www.zianet.com/ebear/metal/hardness.html

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Destructive Tests

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Chemical test (ASTM E 30)

Determine the chemical composition,including the carbon content

Helpful in the determination of weldability,ductility and corrosion resistance

Most common laboratory test Sample obtained using drilling

Portable spectrograph for in-situ testing

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Bend test (ASTM E 190, E 290)

To ensure that a metal has sufficientductility to stand bending without fracturing

A standard specimen is bent through aspecified arc (U-shape)

Direction of grain flow is noted and whetherthe bend is with or across the grain

Lacks quantifiable results

Coating imperfections from bend

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Coating imperfections from bendtest

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Tension testing

Specimen removed by flame cutting,machined, subjected to axial tension

Load-elongation curve, strength, elongation,% reduction in area measured

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Tension test (ASTM E 8)

pl

Y

f

u

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Steel failure

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Strength testing on coupons

Weiss 2001

Impact tests (ASTM E 23, E 208, A

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Impact tests (ASTM E 23, E 208, A673)

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Impact tests

Detects differences between materials which arcnot observable in a tension test

Charpy specimen (favored in US) square cross section (10x10 mm)

45 V notch, 2 mm deep with a 0.25 mm root radius

Specimen supported as a beam in a horizontal position

and loaded behind the notch by the impact of a heavyswinging pendulum

Specimen is forced to bend and fracture at a high strainrate on the order of 103 s-1

Izod specimen (favored in Europe) circular or square cross section and contains a V notch

near the clamped end

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Residual stress

Residual (locked-in) stresses in a structuralmaterial or component are those stresseswhich exist in the object without (andusually prior to) the application of anyservice or other external loads

From casting, welding, machining, molding,heat treatment, etc.

Detrimental cause of fatigue and otherfailures when service loads are superimposed

Hole drilling strain gage method

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g g g(ASTM E 837)

Hole of predetermined size drilled in a beamor column flange

Stresses around the hole are relieved andmeasured by strain gages

From these, principal surface stresses aredetermined

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Strain relief

http://www.npl.co.uk/materials/residualstress/

S i d i

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Strain gage design


St ti

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Stress equations


l d

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Potential errors and uncertainties

Hole dimensions diameter, concentricity, profile

Centre of drilled hole to coincide with centre of gauge

circle to 0.025 mm Hole depth (measure and control to < 1 mm !)

Surface roughness and flatness

Specimen preparation

Induced stresses from machining the hole

Material properties

Incorrect gauge selection

use small size where steep stress gradients


ce 555 - l21,22-ndt of steel

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