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Q: What are residual stresses?
A: Residual stresses are the stresses that exist in a part without any loading. They can be introduced by any processes that involves permanent deformation or rapid change of temperature. Welding, heat treatment and machining are some of the typical processes that produce residual stresses through the thickness or near the surface of the part. In many situations the residual stresses have significant influence on the life, strength and deformation of a component.
Q: Can we estimate these stresses from numerical computations instead of measuring them experimentally?
A: Although numerical computations have been used to simulate some of the processes that lead to the formation of residual stresses, they currently applies only to some simple cases and their results have to be validated by experimental results.
Q: What are the techniques available for residual stress measurement?
A: A number of techniques have been developed in this century to measure residual stresses. Table 1 shows a comparison of these techniques.
Table 1 Comparison of techniques for residual stress measurements
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Techniques
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Features
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Strain gauge rosette with hole drilling
Operation: simple
Computation: simple.
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Stresses at the location of a hole are measured. Mostly used for near surface uniform stress. Not applicable to the inside wall of a tube or the toe of a fillet-weld. Error due to mis-alignment can not be corrected.
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Strain gauges with layer removal
Operation: complex and time consuming
Computation: moderate.
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Mainly used for through-the-thickness measurement. Not applicable to localized residual stresses or non-uniform geometries such as a fillet weld. Error may accumulate as the number of layers removed increases.
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X-ray diffraction with layer removal
Operation: complex and time consuming
Computation: complex
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Used for both near surface and through-the-thickness measurements. Not applicable to non-uniform geometries such as a fillet weld or the inside wall of a tube. Error may accumulate as the number of layers removed increases.
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Neutron Diffraction
Operation: complex and time consuming
Computation: complex
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Used for non-destructive through-the-thickness measurement. Resolution is currently lower than other techniques and its results in many cases need to be validated by other techniques.
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Strain gauges with crack compliance (or the slitting technique)
Operation: simple
Computation: complex
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Used for both near surface and through-the-thickness measurements of stress distributions on a plane of cut. Applicable to localized residual stresses and non-uniform geometries. Error accumulation reduced by least squares fit.
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