Whena solid Material is subjected to external loading, is undergoing plasticdeformation and is in equilibrium, there forms internal stress distribution inthe material. When these external loadings are removed there is a residual stressin the material. Residual stresses are formed when there are mechanical loads,thermal loads or phase changes and the material is deforming plastically and isin equilibrium.
1Graph 1: Stress vs Strain after amaterial is plastically deformed, unloaded and elastically recovered.Asit is seen in graph 1 after a material is subjected to mechanical load and thenunloaded, the stress required to plastically deform the material is higher. Residualstresses can be generated by cold working the material, hammering, rolling,coining, grinding, milling, turning, welding, casting, forging, heat treatment.Residual stresses may be a requirement or they may be an unwanted side effect.Themethods of measurement for residual stresses are Hole Drilling, X-RayDiffraction, Synchrotron, Neutron Diffraction, Curvature and Layer Removal,Magnetic Methods, Ultrasonic Methods, Piezo-Spectroscope. The hole drillingtechnique is one of the most used methods for measuring residual stresses,because it’s cheaper than the other techniques and it can be applied to manymetals. X-Ray Diffraction measures internal stresses with spacings in latticeplanes.
Synchrotron is similar to X-Ray diffraction but has more intense beamsand can analyze the depths of material and provide 3D maps of straindistribution in the material. Neutron Diffraction depends on elasticdeformations in polycrystalline material that cause changes in spacing of thelattice planes. Curvature and Layer removal is mostly used on simplegeometries. In this technique when a layer of a flat plate which containsresidual stress is removed bends, because it becomes unbalanced, with simplecalculations residual stress can be calculated by relating it to curvature.
Magnetic methods are advantageous, because they are cheap, simple, fast and non-destructive.The ferromagnetic materials are sensitive to the internal stress state.Ultrasonic Methods use velocity of ultrasounds waves traveling through amaterial, these waves are sensitive to the stress levels within the material.Piezo-Spectroscopic utilizes the Raman Effect. Raman Effect is the interactionof light with a matter.
Incident laser light causes the bonds between atoms tovibrate. 234Inwelding processes the heat flow from the weld area causes the joint area toexpand, thermal expansion and contractions can leave permanent stress onmaterial, higher heat welds are more likely to generate residual stresses. 5Thereare two major types of stress removal, these are Thermal and Mechanical.
The biggestdifference between these two are that thermal treatment also effect metallurgicalproperties of the metal. As the metal heats up it becomes weaker, residualstress becomes equal to the yield strength, thus the material is relieved. The mostcommon method of Thermal stress removal is heat treatment.
Shot peening is a mechanicalstress relief method. Small metal balls are projected on the surface of the material,this induces compressive stress. When the compressive stress is equal to thetensile stress due to residual stresses the material is relieved.
Another mechanicalstress removal method is vibration. Mechanical vibrating device is attached tothe material. Depending on materials weight and type the time and the frequencyof the vibration changes. The vibration evens the stress distribution withinthe material. 34