附录
Residual Stress
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 asmbling operations. Sometimes it is called initial stress, and the operations, prestressing. When the structure or machine is put into rvice, the rvice loads superimpo stress. If the residual stress add to the rvice-load stress, they are detrimental; if they subtract from the rvice-load stress they are beneficial.
In the plastic deformation the external force does the merit turns into outside the heat except the majority of extensions, but also some small part by the distortion can the form stores up in the deformation material. This part of energy named storage energy. The storage can the concrete manifestation way is: Macroscopic residual stress, microscopic residual stress and lattice distortion. According to the residual stress balance scope difference, usually may divide into it three kinds:
(1) First kind of internal stress, also called the macroscopic residual stress, it is caus by the work piece different part macroscopic distortion nonuniformity, therefore its stress balance scope including entire work piece. For example, rves with Jin Shubang the curving load, then above is pulled elongates, under receives the compression; The distortion surpass when the limit of elasticity has had the plastic deformation, after then the external force elimination by elongated one side on the existence compresd stress, the leg of right triangle is the tensile stress. This kind of residual stress corresponds the distortion can not be big, only accounts for always stores up can about 0.1%.
(2) Second kind of internal stress, also called the microscopic residual stress, it is produces by between the crystal grain or the subgrain distortion nonuniformity. Its sphere of action and the crystal grain size quite, namely maintain the balance between the crystal grain or the subgrain. Sometimes this kind of internal stress may achieve the very great value, even possibly creates the micro crack and caus the work piece destruction.
(3) Third kind of internal stress, also calls the lattice distortion. Its sphere of action is veral dozens to veral hundred nanometers, it is becau the work piece forms in the plastic deformation the massive lattice flaw (for example vacancy, interstitial atom, dislocation and so on) cau. In the distortion metal the storage can the major part (80%~90%) us in forming the lattice distortion. This part of energy enhanced the distortion crystal energy, caus it to be at the thermodynamics non-steady state, therefore it has one kind to make the distortion metal to restore to the free enthalpy lowest stable structure condition spontaneous tendency, and caus the plastic deformation metal in heating time reply and the recrystallization process.
Only a few examples of detrimental residual stress will be given here .One, in the asmbly of machinery, occurs when two shafts are not in line or are a few thousandths of an inch out of parallel, and they are forced into connection by rigid couplings. The resulting stress in the shafts become reversing stress when the shafts are rotated. The correction, when perfect alignment cannot be economically attained, as is frequently the ca, is to u flexible couplings of a type necessary for the degree of misalignment.
The preceding ca occurs with elastic stress only, and the residual stress are maintained by bearing constraints. In applications where mechanical work caus plastic yielding .stress remain when the constraints are removed. For example, the forging of shafts and crankshafts and the cooling after forging may induce residual stress, the equilibrium of which id changed in machining, causing some warping of the shafts. It is then common practice to straighten the shafts in a press before the final machining operation. Straightening requires a bending moment large enough to cau permanent t or yielding.
Detrimental residual stress 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 stress are t up along the weld.
In general, local or shallow heating which would expand the region or surface, if it were free, a distance well beyond that which the adjacent larger volume will allow caus yielding and uptting of the heated material, This readily occurs becau of the reduced yield strength at elevated temperatures. The same cooler volume prevents the upt, heated region from fully contracting during its cooling, and tensile general rule is that the “last to cool is in tension,” although there is an exception if certain transformations of microstructure occur. Methods for minimizing or reversing the stress include annealing for stress relief and hammer or shot peening of the weakened surface. Annealing requires heating mild steel to 1100~1200F, followed by slow cooling, Some preheating of the parts to be joined may minimize the tensile stress in welds.
