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Residual Stresses
May 17, 2018

A residual stress is  one that exists without external loading or internal temperature differences on a structure or machine. It is usually a result of manufacturing or assembling operations. When the sturcture or machine is put into service, the service loads superimpose stresses. If the residual stresses add to the service-load stresses, they are detrimental;if they subtract from the service-load stresses they are benfifical.

Only a few examples of detrimental residual stresses will be given here. One, in the assembly of machinery , occurs when two shafts are not in line, and they are forced into connection by rigid couplings. The resulting stresses in the shafts become reversing stresses when the shafts are rotated. The correction, when perfect alignment cannot be economically attained ,as is frequently the case,is to use flexible couplings of a type necessary for the degree of misalignment.

Detrimental residual stresses commonly result from differential heating or cooling. A weld is a common example. The weld metal and the areas immediately adjacent are ,after solidification ,at a much higher temperature than the main body of metal. The natural contraction of the metal along the length of the weld is partially prevented by the large adjacent body of cold metal. Hence residual tensile stresses are set up along the weld.

A general rule is that the “last to cool is in tension”,although there is an exception if certain transformations of miscrostructure occur. Methods for minizing or reversing these stresses includes annealing for stress relief and hammer or shot peening of the weakened surface. Annealing requires heating mild steel to   600~650°C ,some alloy steels to 870°C then holding for a period of time, flowed by slow cooling. Some preheating of

parts to be joined may minimize the tensile stresses in welds.

  A thin but lightly effective surface layer of compressive stress may be induced by roller burnishing ,ballizing ,and peening processes. It is seen that these processes work-harden on outer layer ,thus causing compressive stresses to remain,together with minor tensile stresses in adjacent interior layers. Since the compressive layer is readily obtained all around,these processes are suitable for reversing loads and rotating components where the stress varies between tension and compression. These processes must be carefully controlled in respect to roller pressures and feeds,shot size and speed.,ect, for which extensive information is available in engineering books and periodicals.

 In roller burnishing process ,the surface of the component is cold worked by a hard and highly polished roller or set of rollers. This process is used on various flat,cylindrical, or conical sufrace (figure 39.1). Roller burnishing reduces surface roughntess by removing scratches and tool marks and induces beneficial compressive surface residual stresses for improved fatigue life. Thread rolling of bolts and screws has long been part of a forming processthat not only forms but strengthens then threads by deformation and grain flow around the roots and by inducing compressive residual stresses.( continued)

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