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Residual Stresses

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. Sometimes it is called initial stress, and the operations, prestressing. When the structure 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 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 causes by the work piece different part macroscopic distortion nonuniformity, therefore its stress balance scope including entire work piece. For example, serves with Jin Shubang the curving load, then above is pulled elongates, under receives the compression; The distortion surpasses when the limit of elasticity has had the plastic deformation, after then the external force elimination by elongated one side on the existence compressed 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 causes the work piece destruction.

(3) Third kind of internal stress, also calls the lattice distortion. Its sphere of action is several dozens to several hundred nanometers, it is because the work piece forms in the plastic deformation the massive lattice flaw (for example vacancy, interstitial atom, dislocation and so on) cause. In the distortion metal the storage can the major part (80%~90%) uses in forming the lattice distortion. This part of energy enhanced the distortion crystal energy, causes 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 causes the plastic deformation metal in heating time reply and the recrystallization process.

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 or are a few thousandths of an inch out of parallel, 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.

The preceding case occurs with elastic stresses only, and the residual stresses are maintained by bearing constraints. In applications where mechanical work causes plastic yielding .stresses remain when the constraints are removed. For example, the forging of shafts and crankshafts and the cooling after forging may induce residual stresses, 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 cause permanent set or yielding.

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.

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 causes yielding and upsetting of the heated material, This readily occurs because of the reduced yield strength at elevated temperatures. The same cooler volume prevents the upset, 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 these stresses 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 stresses in welds.

A thin but highly effective surface layer of compressive stress may be induced by cold-rolling, coining, and peening processes. It is seen that these processes work-harden an 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. The processes must be carefully controlled in respect to roller pressures and feeds, shot size and speed, etc., for which extensive information is available in engineering books and periodicals.

Cold-rolling is applied primarily to cylindrical and other shapes that

can be rotated, such as threads and shaft fillets. The shape, size, and pressure of the roller and the yield strength of the shaft determine the depth of penetration, which can be calculated. A special fixture may be attached to the carriages of a lathe and made to slowly traverse the desired rolling of bolts and screws has long been part of a forming process that not only forms but strengthens the threads by deformation and grain flow around the roots and by inducing compressive residual stresses.

Coining of holes, also called ball drifting, is a manufacturing process of forcing a hard, tungsten carbide or AIDI 52100 steel, slightly oversize ball through a hole in a plate, bushing, or tubing to give the holes final size and a fine finish. The length of the hole may be from 1/20 to 10 times its diameter. The machine is often set up for a high production of small parts with unskilled labor. An incidental result is that the process increases hardness, hence wear resistance, and induces around the hole a compressive residual stress that is usually advantageous, as in roller-chain links. The links ate highly stressed in pulsating tension with a concentration of the stress at and near the hole surfaces. With the compressive stress from ball drifting, the net tensile stress in service is decreased, and failure is minimized.

Peening is the most widely used method for prestressing by mechanically induced yielding. By the impact of rounded striking objects, the surface is deformed in a multitude of shallow dimples, which in trying to expand put the surface under compression. Hammer peening, usually by air-driven tool with a rounded end, is useful on limited areas, such as a weld in shaft or on areas found weakened by corrosion, decarburization, or minor fatigue damage. With a hard spherical end to the tool, the depth of the compressed layer, which occurs below the surface, is about half the strain-hardened region.

Shot peening is done on steels by the high-velocity impingement of small, round, steel or chilled cast-iron shot with diameters from 0.007to

0.175 in.. The compressed layer has a depth from a few thousandths to a few hundredths of an inch, less than with hammer peening, but roughly proportioned to the shot size used and its velocity. Again the residual stress produced is about half of the strain-hardened yield strength. Shot peening is extensively used because it may be applied with minimum cost to most metals and shapes, except some interior ones. On soft metals, glass beads may be used. Helical springs are commonly shot peened, with up to a 60%increase in allowable stress under pulsating loads. Part of the improvement may be due to the removal of the weakening longitudinal scratches left from the wire-drawing operation. Similarly, coarse-machined and coarse-ground surfaces are smoothed and improved by shot peening, which may be a more economical method than producing a final finish by machining or grinding. Peening is not used on bearing and other closely fitting surfaces where high precision is required. A final grinding for accuracy after peening would remove part or all of the residual stress. Machines are available for the automatic and continuous peening of small-and medium-size parts moving on a conveyor or turntable through the blast.

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