ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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lapped shear plates that are tack welded back to back, as shown in Fig. 2. These geometries are chosen to avoid

rotation during loading. The longitudinal specimen requires machining of grooves after the fillet welds are

made. The base plate is cut under the center of the lapped plate. The lapped plate is cut near each end so that

each length of weld connecting the base plate to the lapped plate across the gap in the base plate is 38 mm (1

in.).

Fig. 1 Transverse fillet weld shear test specimen. Source: Ref 9

Fig. 2 Longitudinal fillet weld shear test specimen. Source: Ref 8

Fillet-weld strength tests are sensitive to surface contour of the welds and to the condition of the weld root.

Excessive gaps between the lapped plates should be avoided, because these tend to magnify stresses at the weld

root. The specimens are also sensitive to underbead cracking and undercut.

Fillet size is most accurately measured after failure in the test. The stress is calculated based on assuming

uniform stress across the entire weld throat.

Bending Strength and Ductility

Bend tests are commonly used to evaluate the acceptability of weld procedures for providing sound welds (Ref

10). They allow rapid determination of strength and ductility on a specimen substantially simpler than the

standard tensile specimen. Bend tests tend to provide vivid demonstrations of difference between welds with

surface or near-surface flaws and welds without flaws adjacent to the convex surface of the bend. Bend tests are

further described in the article “Bend Testing” in this Volume.

Bending ductility can be calculated by determining the radius of the outer surface of the bend specimen at the

completion of the test. This ductility is usually smaller than that measured in a uniaxial tensile test. The bend

ductility is localized at the outer surface of the specimen, and the constraint is more severe because of the shear

stresses generated through the thickness of the bend specimen.

The thickness of the specimens and the size of the plunger or mandrel determine the outer surface ductility

requirement. Table 1 provides a summary of the radii of plungers or mandrels and the maximum test specimen

thicknesses for several groups of materials as required by the American Society of Mechanical Engineers

(ASME) Boiler and Pressure Vessel Code (Ref 11).

Table 1 Bend test geometry for testing based on material thickness and material type

Radius of

plunger or

mandrel

Maximum test

thickness

Materials

in.

With >20% elongation

(3 + )t

in.

Alloy steels with <20% elongation

High strength Al alloys

in.

Ti or Ti alloy with strength <65 ksi

in.

Ti or Ti alloy with strength ≥65 ksi

Zr or Zr alloy

(8 + )t in.

4000 series Al alloy

Al alloy welded with 4000 series electrodes

Cu base alloys with Al and <20% elongation

(a)

Other alloys with <20% elongation

t, material thickness.

(a) Radius of plunger or mandrel and maximum test thickness chosen to achieve the required minimum

elongation from a base metal tensile test at the outer fiber of the convex surface of the bent specimen

Increasing specimen width can increase the constraint and reduce the likelihood of achieving the required bend

test radius without cracking. Face bends for welds in plates of greater than 38 mm (1 in.) thickness may

require multiple specimens.

Root, Side, and Face Bends. The primary geometries for bend test specimens place the butt weld so that the

bending stress is transverse to the weld axis. Different areas of the welds reach the highest bending stress in

transverse root bends, transverse face bends, and transverse side bends. Root bends put the weld-root side of the

tested butt weld on the convex side of the bend specimen. Face bends put the weld-cap side of the tested butt

weld on the convex side of the bend specimen. Side bends put a cross section of the weld on the convex side of

the bend specimen.

Longitudinal bend tests may sometimes be used to replace transverse tests, particularly when the strengths of

the regions within the specimen differ greatly. However, longitudinal side bends are not possible since the weld

cross section does not include the longitudinal direction. Longitudinal root bends, with the convex side of the

bent specimen on the weld-root side, and longitudinal face bends, with the convex side of the bent specimen on

the weld-cap side, can both be made and tested.

Longitudinal tests are less likely than transverse tests to fail from flaws that are long in the same direction as the

weld.

Wrap-Around Bend Testing. While the plunger-type bend fixtures are by far the most widely used for guided

bend testing, some circumstances require a fixture that creates a different distribution of strain. The most

common is a wrap-around testing fixture. Both the plunger type and the wrap-around type force the material

into a specified radius. However, the wrap-around fixture moves the points of bending load application around

a mandrel rather than using a fixed location for the central force, as shown in Fig. 3.

Fig. 3 Wrap-around bend testing. Source: Ref 9

The wrap-around fixture is most commonly used for welds that have significant mismatch between base metal

strengths, between base metal and weld metal strengths, or where the HAZ strength differs greatly from the

weld or base metal. If such welds are tested in a plunger-type fixture, the strain can be concentrated into the

lower-strength material, leaving a sharper bend in that material and much less bending in the higher-strength

material. The wrap-around fixture forces a more uniform strain into the materials because the loading point in

the center of the specimen moves across the weld.

Wrap-around test fixtures are commonly used for aluminum alloys where the strength of the weld metal and

HAZ may be chosen to be lower than the base metal. For instance, 6061-T6 aluminum base metal that is

welded with 4043 electrodes has a minimum yield strength of 240 MPa (35 ksi) in the base metal but only 100

MPa (15 ksi) for cross-weld tensile specimens (Ref 12). Wrap-around testing can limit the localization of the

strain at the weld.

Hardness

Hardness testing of welded joints is widely used as a rapid measurement of mechanical properties across the

varying microstructures of the welded region. It allows local regions and individual microstructures to be

compared for strength, because strength is correlated to hardness. A further discussion of hardness testing can

be found in the Section “Hardness Testing” in this Volume.

Hardness has been primarily related to the tensile strength rather than to the yield strength or the ductility.

Standard conversion charts are available for conversion of one hardness measurement to another and from

hardness to tensile strength measurement. Such converted information should be used with caution, because the

variation of weld microstructure may cause the average hardness to correspond to values that cannot be

obtained in larger scale specimens.

Macrohardness testing of welds requires preparation of a small region of the surface. The major techniques are

Brinell testing, which uses a spherical indenter, and Rockwell testing, which uses a diamond penetrator or a

sphere. The Brinell indentation is typically 2 to 6 mm in diameter while the Rockwell indentation is much

smaller but still is visible, unaided. Rockwell methods use several different loads for different hardness scales,

so it is possible for a weld to require different hardness scales for different regions.

Macrohardness testing results can be limited by the microstructural gradients around the welds. A result of 240

HB may represent a hardness for one uniform microstructure or an average over the regions deformed by the

indenter. Welds and HAZs often have gradients of microstructure and chemistry that can cause variations in

hardness across the indentation. Interpretation of the hardness from the impression may be made more difficult

if there is a large gradient in the hardness of the material under the indenter. This can result in noncircular

Brinell impressions and Rockwell tests with the deepest point not under the deepest point of the indenter.

Microindentation hardness testing using an indenter requires an even smaller region of the surface to be used

than macrohardness testing, but the surface preparation requirements are more stringent. Thus the Knoop and

Vickers microindentation hardness tests are primarily applied to ground and polished cross sections or to

ground, polished, and etched cross sections. Microindentation hardness traverses are often used to determine

the variation of hardness within the weld, across the fusion line, and across the HAZ.

Impact Toughness

Several methods are available for measuring the material resistance to starting and growing cracks that can be

applied to welded joints. This section discusses test methods that cause a crack to grow from a notch under the

rapid load of an impact. Methods that use sharp crack tips and thus can apply the loading more slowly are

discussed in the next section on fracture toughness.

Charpy. The Charpy V-notch impact test is the most common measurement method for fracture toughness of

welded joints. Specifications for the test are given in ASTM E 23 (Ref 13) and AWS B4.0. The test uses a

pendulum hammer to rapidly fracture a notched bar with dimensions of 55 mm by 10 mm by 10 mm (2.165 in.

by 0.394 in. by 0.394 in.).

Several measures of toughness can be obtained from a Charpy test. Absorbed energy, measured in ft · lbf or

joules, is the most commonly reported, but the percent shear fracture and the lateral expansion in inches or

millimeters are also sometimes reported. Greater toughness material will have higher values of each of these

three parameters. Occasionally, percent fibrous fracture, which is 100% minus the percent shear fracture, is

reported.

Many metals, including carbon and alloy steels, have toughnesses that vary strongly with temperature. So tests

on welded joints are often conducted at several temperatures, and the absorbed energy or other parameter is

plotted as a function of temperature. Material specifications and weld qualifications that include Charpy V-

notch testing normally require a minimum absorbed energy at a particular temperature. In this case, testing is

routinely conducted only at the temperature of interest.

The choice of minimum absorbed energy and test temperature are often varied between standards or within a

standard, based on service conditions. For instance, welded joints on bridges to be used in cold climates are

qualified to lower temperatures than those used in warm climates.

The absorbed energy in a Charpy V-notch test includes both the energy to start the crack from the 0.25 mm

(0.010 in.) radius notch and the energy to propagate the crack across the Charpy specimen. For many cases,

including constructional steels, these two parts are of comparable magnitude. In fact, the popularity of the

Charpy V-notch test was originally based on its ability to predict both crack initiation and crack arrest in ship

steel plates. This means that both the metal microstructure at the notch tip and through the specimen thickness

contribute to the reported toughness. For welded joints with heterogeneous microstructures, the position of the

notch tip will be important in determining the measured absorbed energy. The absorbed energy, however, will

also depend on the microstructures through which the fracture passes.

The dependence of Charpy impact test results on microstructure for many metals causes weld joints with

heterogeneous microstructures to have a range of Charpy values depending on specimen orientation in the weld

and notch position. Often weld centerline values are reported or compared with standards. Sometimes the HAZ

is tested at a particular location, such as 1 mm from the fusion line. These tests cannot determine a toughness

appropriate to all microstructures in the weld or HAZ. Additional tests of a greater variety of specimens may

reveal zones of lower toughness, such as unrefined columnar weld metal or coarse-grained HAZ, or zones or

higher toughness, such as reheated weld metal or fine-grained HAZ.

Subsize Charpy specimens are sometimes taken from thin material or areas where the geometry prevents a full-

size specimen. Only one dimension is reduced, the distance from the notched face to the unnotched surface

opposite. Reductions of this dimension can be from 10 mm (0.394 in.) to 7.5 mm (0.296 in.), called three-

quarter size; to 5 mm (0.197 in.), called half-size; and to 2.5 mm (0.099 in.), called quarter-size. These are the

most common reductions. Reduced thickness Charpy tests can be used to test the toughness of the root or cap

regions of fillet welds.

Charpy toughness test specimens can be taken from welded joints in several orientations. These orientations can

be given two-letter designations to show the orientation. The first letter is the direction normal to the crack

plane (the long direction of the Charpy specimen), while the second letter is the direction in which the crack

will propagate. The letter designations are L, longitudinal direction; T, long transverse direction (the weld width

direction); and S, short transverse direction (the through thickness direction). The letter designations are shown

for compact tension specimens in Fig. 4. Care should be taken that the orientation letters describe the weld area,

because different combinations of these letters may apply to the same orientation of specimen in base metal.

For instance, in a girth weld in a pipe, the long direction of the weld is the hoop direction of the pipe, not the

longitudinal or axial direction of the pipe.

Fig. 4 Orientations of toughness specimens in relation to welds. L, longitudinal direction; T, long

transverse direction (weld width direction); S, short transverse direction (weld thickness direction). In

the two-letter code for specimen designation, the first letter designates the direction normal to the crack

plane, and the second letter designates the expected direction of the crack plane. Source: Ref 9

Nil-Ductility Temperature. Drop weight tests use a notched weld bead as the starting point for a crack. The test

determines the ability of the base metal to arrest the crack running from an overlay of brittle weld metal. The

test results do not describe the properties of the overlay weld metal, so the overlay weld metal is standardized.

A brittle hard-facing alloy with good surface adhesion is used as the crack starter.

ASTM E 208 describes the test procedure (Ref 14). A standard weight is dropped onto test specimens at

different temperatures. The lowest temperature without full-section fracture is determined as the nil-ductility

temperature (NDT).

Weld metal can be tested for crack arrest by placing the notched weld overlay across a machined butt weld of

the weld metal of interest. The notch is typically placed so its long direction is above the longitudinal direction

of the butt weld.

Fracture Toughness

Fracture toughness testing of welded joints introduces several complications to standard fracture toughness

measurement as described in the Section “Impact Toughness Testing and Fracture Mechanics” in this Volume.

The weld and adjacent HAZ will have heterogeneous microstructures that can have widely varying strength and

toughness. In addition, welding residual stresses may be retained.

Fracture initiation testing, using slow loading and a crack tip sharpened by precracking, allows determination of

only the crack initiation portion of the fracture toughness. Charpy testing determines a combination of crack

initiation and arrest properties. Fracture initiation testing of welded joints thus is even more sensitive to the

local microstructure around the tip of the precrack than Charpy tests are to the microstructure at the notch.

Weld heterogeneity also causes welds to be particularly sensitive to the rules for data interpretation for fracture

initiation tests. Fatigue precracks are less likely to be straight in a heterogeneous material. Tests may have

multiple events of crack initiation and arrest in local areas (“pop-ins”). Varying strengths may cause validity

criteria based on yield strength and specimen size to give ambiguous results. Each of these issues must be

accounted for in a test protocol appropriate to welds.

Fracture toughness specimens can be taken from welded joints in several orientations. These orientations can be

given two-letter designations to show the orientation. The first letter is the direction normal to the crack plane,

while the second letter is the direction in which the crack will propagate. The letter choices are L, longitudinal

direction; T, long transverse direction (the weld width direction); and S, short transverse direction (the through

thickness direction). The letter designations are shown for compact tension specimens in Fig. 4, but the same

designations can be used for other shapes of test specimen. Care should be taken that the orientation letters

describe the weld area, because different combinations of these letters may apply to the same orientation of

specimen in base metal. For instance, in a girth weld in a pipe, the long direction of the weld is the hoop

direction of the pipe, not the longitudinal or axial direction of the pipe.

Fracture toughness testing to measure the fracture resistance of weld HAZs presents particular problems,

because several different microstructures can cluster within the HAZ, based on the different histories of heating

from the welding in different locations. A fracture toughness measured from the HAZ is likely to be affected

both by the properties of the several HAZ microstructures that the crack tip passes through and by the

properties of the adjacent weld metal and base material.

Fracture initiation toughness is measured most commonly using the stress intensity factor, K; the J-integral, J;

or the crack tip opening displacement (CTOD). All of these are regularly applied to welded joints. CTOD

measurements are somewhat more commonly specified in welded regions than in base metal, because the test

was originally developed for welded joints. Conversions between K, J, and CTOD are routinely performed but

should be noted, because the correlations have limited precision.

Test Modifications for Welds. Fracture toughness testing of welds may require precise positioning of the notch

to test the microstructure of interest. Testing of welds may also require modification of the test specimen. Two

methods of modification, local compression and gull-winging (Ref 15), are described in the following section.

Local compression counteracts the effects of welding residual stress. Gull-winging allows a curved piece with a

weld to be tested as a full-thickness specimen.

Local Compression. The sharp crack tip needed for a fracture toughness test to determine initiation toughness is

commonly provided by fatigue precracking at low levels of stress. In welds, fatigue precracking may produce a

crack front that is not straight across the specimen. This is particularly likely when the specimen is thick and

was removed from a weld with as-welded residual stresses. The crack tip of the fatigue precrack can deviate

from the average straight line so much that the fracture toughness test results are invalidated because the crack

depth is poorly described by a single average value.

Determining valid or invalid precracks is part of the standard to which the testing is done, such as ASTM E

1290 for CTOD testing (Ref 16) or ASTM E 1737 for J-integral testing (Ref 17).

A local compression treatment can be applied before precracking to avoid excessive deviation of the precrack

from a straight line. Compression is applied in circular regions around the points where the notch tip reaches

the surface, as shown in Fig. 5. Local compression can be applied to the most common fracture test specimen

shapes, including compact tension specimens and single-edge notched bend bars.

Fig. 5 Local compression of fracture specimens before fatigue precracking, P, load. Source: Ref 15

Gull-Winging. Some geometries of weld may be difficult to test for fracture toughness because the shape of the

base metal around the weld limits the thickness or area of any standard geometry specimen, such as a compact

tension specimen or a three-point bend bar. Welds in curved shapes, such as spheres and cylinders, can be

particularly difficult.

As shown in Fig. 6(a), a flat specimen taken for fracture toughness testing from a curved part may be limited in

dimension by both the inside surface and the outside surface of the curved part. This could force the use of

small specimens preferentially determining the toughness near the inside or concave surface of the weld. Gull-

winging is a mechanical bending process that allows the full thickness of the curved part to be used for fracture

testing.

Fig. 6 Gull-winging of single-edge notched-bar weld fracture toughness specimen. (a) Small scale of

specimen that can be obtained from a longitudinal weld in a cylinder compared to full thickness. (b)

Gull-winged specimen at full cylinder thickness ready to be loaded. Source: Ref 15

Full-thickness transverse tests using a modified three-point bend bar geometry may be used f or geometries

where the weld is straight but the curvature is transverse to the weld by gull-winging. The gull wings are

introduced by plastically bending the base material away from the weld. These bends must allow the three

locations of loading to be the same as if the specimen were flat, as shown in Fig. 6(b). During gull-winging, the

area adjacent to the weld must be supported to prevent plastic deformation of the weld or the area directly

adjacent to it.

Gull-winged specimens should have limited deviation from the flat centerline of the equivalent flat specimen.

Greater deviation may allow plastic deformation during testing at the point of greatest deviation.

References cited in this section

5. “Standard Test Method for Young's Modulus, Tangent Modulus, and Chord Modulus,” E 111, Annual

Book of ASTM Standards, Vol 3.01, ASTM, 1999

6. “Standard Methods of Tension Testing of Metallic Materials,” E 8, Annual Book of ASTM Standards,

Vol 3.01, ASTM, 1999

7. “Standard Methods of Tension Testing Wrought and Cast Aluminum, and Magnesium Alloy Products,”

B 557, Annual Book of ASTM Standards, Vol 3.01, ASTM, 1999

8. B4 Committee on Mechanical Testing of Welds, Mechanical Testing of Welds, Part 1: Summary of

Tension Testing of Welds, Weld. J., Jan 1981, p 33–37

9. “Standard Methods for Mechanical Testing of Welds,” ANSI/AWS B4.0-98, American National

Standards Institute/American Welding Society, Miami, 1998

10. B4 Committee on Mechanical Testing of Welds, Mechanical Testing of Welds, Part 2: Bend Testing of

Welds. A Summary, Weld. J., Feb 1981, p 34–37

11. ASME Boiler and Pressure Vessel Code, Section IX, American Society of Mechanical Engineers

12. Aluminum Design Manual, The Aluminum Association, 1994

13. “Standard Test Methods for Notched Bar Impact Testing of Metallic Materials,” E 23, Annual Book of

ASTM Standards, Vol 3.01, ASTM

14. “Standard Method for Conducting Drop-Weight Test to Determine Nil-Ductility Transition Temperature

of Ferritic Steels,” E 208, Annual Book of ASTM Standards, Vol 3.01, ASTM

15. M.G. Dawes, H.G. Pisarski, and S. J. Squirrell, Fracture Mechanics Tests on Welded Joints, Nonlinear

Fracture Mechanics Vol II: Elastic-Plastic Fracture, STP 995, J.D. Landes, A. Saxena, and J.G.

Merkle, Ed., ASTM, 1989, p 191–213

16. “Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness

Measurement,” E 1290, Annual Book of ASTM Standards, Vol 3.01, ASTM

17. “Standard Test Method for J-Integral Characterization of Fracture Toughness,” E 1737, Annual Book of

ASTM Standards, Vol 3.01, ASTM

Mechanical Testing of Welded Joints

William Mohr, Edison Welding Institute

Residual Stress Measurement

Residual stress measurement on welded joints must take account of the characteristics of the welded joint.

Welds usually have gradients of residual stress through the weld area and have these residual stress gradients in

the same regions as gradients in microstructure. These features may limit the amount of information that can be

obtained from some techniques. Some residual stress measurement techniques, for instance, work by comparing

the distances between atoms in the crystal structure from an unstressed area to the stressed area of interest. If no

unstressed area is available with the same crystal structure as the weld, the zero stress level for that weld region

would be uncertain.

Two general types of measurements for residual stress in welds are most common: locally destructive

techniques and nondestructive techniques. The locally destructive techniques include hole drilling, chip

machining, groove machining, and block sectioning. These measure changes in strain as a new surface is

created and determine residual stresses most sensitively around the area where the machining took place. The

nondestructive techniques measure the local strain of the material by inputting a physical change, receiving a

signal based upon that change and the residual stress, and then decoding the part of the signal induced by the

residual stress. These techniques include x-ray diffraction, neutron diffraction, Barkhausen noise analysis, and

ultrasonic propagation analysis.

Both groups of techniques do not measure stress directly. Both measure strain. Both groups have limitations for

measuring rapidly varying residual stress fields because they need to sample a volume of material to get

sufficient signal to determine the residual stresses with precision.

Sectioning Methods

One extreme way of imagining a residual stress measurement of a welded region is that many tiny pieces of the

weldment could all be separated simultaneously from neighboring pieces and allowed to change shape to reach

zero residual stress. The shape change in each tiny piece could be measured and that change correlated using

the elastic properties of the material to the residual stress originally in each piece of the weldment.

Sectioning methods can come close to this extreme case, but each cut cannot happen simultaneously. So

analysis techniques for determining residual stress by multiple cuts need to include calculations of the effects of

previous cuts on the residual stress to find the original stress rather than the stress determined for an individual

cut.

Block sectioning and block layering and sectioning are significantly more destructive than hole drilling. The

part for which the residual stresses will be determined is cut into sections and layers while surface strain gages

are monitored. These techniques can provide distributions of residual stress in multiple dimensions.

Hole Drilling and Similar Local Measurements

Hole drilling techniques, often called center-hole drilling or blind-hole drilling, measure the change in strain on

the adjacent surface as the residual stress field is disturbed by the machining of a hole from that surface (Ref

18). The hole depth is generally between 1 and 2 mm (0.04 and 0.08 in.), although several organizations make

residual stress measurements as a function of hole depth as the hole is drilled.

The drilling techniques should cause as little as possible additional stress around the hole, so techniques are

commonly used that avoid contact of the walls of the hole as the bottom is drilled. Air turbine and air abrasion

systems can provide holes with close dimensional tolerance and little machining-induced surface stress on the

hole. Specialized strain gage rosettes are available for measurement, both with the hole placed in the center of

the rosette and with the rosette on one side of the hole. The rosette on one side of the hole is used for cases

where the surface is obstructed on one side of the measurement position, as may happen adjacent to a weld toe.

The measured strain must be converted to a local residual stress in the area where the hole was drilled. This

conversion can be done most simply by assuming a uniform residual stress distribution. Only components of

the residual stress that are parallel to the surface are measured by this technique, giving two directions of axial

stress and one of shear stress.

Chip machining has been used similarly to hole drilling to determine near-surface residual stresses. In this case,

a small region of the surface is removed with the strain gage rosette on the surface of the chip rather than

attached to the base material. The advantage is that the small chip can be assumed to reach essentially zero

residual stress, so the change of strain measured on the chip can be directly correlated to the stress in the chip

region. However, the chip must be larger than the most common size of drilled hole to carry the strain gage.

The chip machining method will average the strains over a larger volume of material.

Other shapes of local machining can be used, including deep holes and partial thickness slits (Ref 19). These

methods can measure residual strains either with surface strain gages or with measurements of post-cut

displacement. As with hole drilling, only part of the residual stress tensor is obtained by these methods, but the

direction of cutting can be oriented to find the residual stresses of most interest.

Nondestructive Techniques

Nondestructive techniques for measuring residual strains in welded joints use a variety of inputs, including x-

rays, neutrons, magnetic signals, and ultrasonic waves. Each of these inputs can induce different outputs,

depending on the residual strains in the welded joint area, but also depending on other parameters, such as the

crystal structure or grain size. Nondestructive techniques thus are best applied where a standard for comparison

without residual strains but with the same microstructure is available. The comparison of test material with an

unstrained standard may be easiest in base metal outside the HAZ and much more difficult for HAZ or weld

metal (Ref 20).

X-ray diffraction determines the residual strains by measuring the average distances between atoms. X-rays,

being limited in surface penetration in metallic materials, can be used for detection of residual strains only

within approximately 0.5 mm (0.02 in.) of the surface on which they impinge. The surfaces of welds may be

more difficult locations than smooth surfaces of base material for measurement of residual strains. The rough

surface of the weld cap and notches at weld toes may cause some of the area of interest for residual strain

measurement to be hidden from residual stress detection by x-rays.

Neutron diffraction uses an input that is much more difficult and costly to generate than x-rays but has the

advantage of penetrating much further into metallic materials. Neutrons can be used for detection of residual

strains in steel thicknesses beyond 25 mm (1 in.). Average values of the residual strains are determined in a

volume approximately 1 mm

(6 × 10

-4

in.

). The power of the source, the efficiency of the detector, and the

time of exposure all influence the volume required for residual stress detection. Neutron diffraction is easiest

when the grain size is significantly smaller than the detection volume and the grains are oriented randomly.

Large grains and highly oriented microstructures can eliminate the diffracted neuron signal.

Barkhausen noise analysis uses an external varying magnetic field as input and monitors the magnetic response

of the area of interest. The response comes from the jumps in magnetization as magnetic domain walls move

within the metal. This response is a surface response, so the surface condition of the part is important.

Ultrasonic propagation analysis inputs ultrasonic waves and then monitors the response of the time taken for the

waves to travel through the metal and reach the detector. Since the residual stress distribution can change the

speed of propagation all along the path of the ultrasound, the resulting effect is summed over the entire path.

Multiple path analysis can provide local results.

Barkhausen noise and ultrasonic propagation analysis both are limited by the microstructural variations around

welds. The combination of welding induced changes cannot be easily deconvoluted, because both techniques

are comparing unstressed areas to stressed areas and also are sensitive to microstructural variation.

References cited in this section

18. N.J. Rendler and I. Vigness, Hole-Drilling Strain-Gage Method of Measuring Residual Stresses,

Experimental Mechanics, Vol 6, 1966, p 577–586

19. W. Cheng and I. Finnie, A Method for Measurement of Axisymmetric Axial Residual Stresses in

Circumferentially Welded Thin-Walled Cylinders, J. Eng. Mater. Technol. (Trans. ASME), Vol 107,

July 1985, p 181–185

20. D.S. Kim and J.D. Smith, Residual Stress Measurements of Tension Leg Platform Tendon Welds, Proc.

of the Offshore Mechanics and Arctic Engineering Conference 1994, Vol 3, American Society of

Mechanical Engineers, 1994

Mechanical Testing of Welded Joints

William Mohr, Edison Welding Institute