The Measurement of Residual Stresses Using Neutron Diffraction
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BULLETIN/NOVEMBER1990
and are extensive in nature. A plate with compressive residual stresses on the flat surfaces will deflect if the compressive région on one surface is removed. Destructive stress measurement techniques such as hole drilling or strain gauging and sectioning can be used to détermine residual macrostresses. Residual microstresses are short-range relative to the scale of the sample—that is, they vary from grain to grain or between phases. A good example is a SiC whisker-reinforced a l u m i n u m alloy matrix. The Al alloy matrix has a much greater coefficient of thermal expansion than the SiC whiskers. After cooling from the diffusion bonding températ u r e , the matrix s h r i n k s more t h a n the whiskers, placing the whiskers in compression and the matrix in tension. Such a stress state is intensive in nature; sectioning will not deflect or alter the stress state. The présence of residual stresses can affect the physical response of the material, particularly with regard to mechanical behavior. This is because such stresses interact vectorially with applied stresses, acting to raise or lower the effective yield or fracture stress; an important example is fatigue life. Since fatigue cracks commonly initiate at the surface, a compressive surface residual stress will extend fatigue life and, conversely, a tensile stress will reduce fatigue life. This is why critical load-bearing régions of structural members are often subjected to shot-peening prior to service in order to induce compressive surface residual stresses, leading to enhanced fatigue life. Another example occurs in weldments. Welds are often given stress-
relieving heat treatments to reduce or eliminate detrimental tensile residual stresses. Stress corrosion cracking can also resuit from the présence of tensile residual stresses in an unloaded part exposed to a corrosive environment. Accurate prédiction of a residual stress state is difficult. Therefore, although a ground steel surface is expected to be in compression, the magnitude, direction, and depth of the compressive stress will d é p e n d on the g r i n d i n g force, number of passes, angle of attack, direction in the surface, and yield strength of the steel. The latter parameter d é t e r m i n e s the m a x i m u m value because residual stresses are elastic. Because of the complex and uncertain nature of residual stresses, and their potential effect on mechanical behavior, there is a need to be able to characterize them. Diffraction has long been appreciated as the principal method of nondestructive measurement of residual stresses in engineering materials. To appreciate important aspects of diffraction stress measurements, consider a steel sample, in the shape of a bar, subjected to a uniaxial stress cr = 200 MPa. The résultant axial and transverse elastic strains are given by Hooke's law, eaxia, = a/E and e,ransvcrSL. = -vC0S2^
E E +
\ + V ,, . —=—(((Tl3)COS
E +
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