Beware of the insidious efforts of residual stress which can have significant effect on fatigue performance!
In cyclic loading conditions, tensile residual stresses superimpose on the applied cyclic stresses and depending on the magnitude of the residual stress can promote failure at design stress levels.
Residual stresses are induced into components by metallurgical changes, chemical processes, thermal gradients (welding and localised heating), machining processes and mechanical deformation (cold bending). Although these stresses, which can be both tensile and compressive in nature, often reach yield magnitude they are often neglected as they are difficult to compute, quantify and measure.
Residual stresses are seldom uniform through the material and typically vary from the surface to the core of the component (e.g. in rolled plate the residual stress can be tensile on the surface and compressive in the core or, in the case of high per pass reduction compressive on the surface and tensile in the core). In many cases the magnitude and distribution of the stress profiles is dependent on multiple influences (e.g. during the quenching of martensitic steels the final residual stress profile is dependent on thermal stress induced plasticity that can occur during quenching as well as microstructural changes (and in the order in which these changes occur)). In welds residual stresses have a sigmoidal distribution that varies along the length and transverse to the weld.
Residual stresses can be measured using local sectioning, centre hole drilling, and diffraction techniques, but owing to the effects of local constraint often have to be measured on full sized components which precludes the use of diffraction techniques on large components (i.e. cannot section the component without effecting the residual stress profile). This makes centre hole drilling techniques, which are highly portable, well suited to measuring stresses in industrial components.
Residual stresses can and should be relieved by heat treatment. When properly applied thermal heat treatments typically reduce residual stresses to less thatn 20% of the material yield stress. However, poor support, local constraint and undue thermal gradients can have adverse results.
Besides for affecting component/machining stability, these manufacturing induced stresses, promote corrosion, play a significant role in stress corrosion cracking and superimpose on the applied loading and thereby influence fatigue performance. For example, in a reversed bending situation, tensile residual stresses (if of sufficient magnitude) can double the tensile component of the bending stresses. Assuming a material with typical fracture mechanics Paris equation parameters (C = 6.9 x10-12 and m=3) doubling of the tensile cyclic stress can increase the crack growth rate by a factor of up to eight and hence reduce the number of cycles to failure by a factor of 0.125!
Published in Technical Tips by Origen Engineering Solutions on 1 September 2015