Residual stresses reducing

The modeling of residual stress development during cure can be used to optimize the processing conditions to reduce or control residual stresses. The current process model is used next to assess the effects of several processing conditions on residual stresses. Reduced cure temperature, longer dwell times, slower cool down rate, and the use of novel cure cycles are all feasible for the reduction of residual stresses. 

The finite-element method applied in the elastic-plastic mode gives satisfactory predictions of the achievable improvements from autofrettage. As the residual stresses reduce efficiently the initiation and the growth of cracks, autofrettage is especially well useful for components... 

Recall that the fluid is a material in which the residual stress, reduced by the reversible part of the stress (i.e., the pressure), is dissipative and irreversible therefore, for an isotropic Newtonian fluid, we have... 

Basically, this process is accomplished by partially overstraining the irmer shape of the bore by a hydraulic or mechanical overloading. The apphed autofrettage pressure determines the amount of material at the bore that is yielded and thus determines the amount of residual compressive stresses when the pressure is removed. These residual stresses reduce the hoop stresses when the normal operation pressure is applied and improve the fatigue hfetime of the component tremendously. 

Crack Reflection. Crack deflection can result when particles transform ahead of a propagating crack. The crack can be deflected by the locali2ed residual stress field which develops as a result of phase transformation. The force is effectively reduced on the deflected portion of the propagating crack resulting in toughening of the part. 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Stress-relief-annealing cannot be expected to eliminate SCC in every case. Only residual stresses are reduced in stress-relief-annealing. Applied stresses, which may be responsible for the cracking, will remain. Inhibitors are not 100% effective in combating SCC. Complete coverage and inhibition is difficult to achieve, especially below deposits, in crevices, and in pits. 

Residual stresses caused by large temperature differences between the weld bead and adjacent metal can be reduced by preheating the metals to be welded, especially if the sections are thick, or by stress-relief-annealing following the welding procedure. Proper welding techniques, especially the use of appropriate weld filler metals to minimize weld-metal shrinkage, can minimize residual stresses. 

Contain tensile residual stresses which are usually as large as the yield stress. Weld liable to fatigue even when applied stress cycle is wholly compressive. Reduce residual stresses by stress relieving, hammering or shot peening. 

A metal s resistance to fatigue is markedly reduced in a corrosive environment. Many welded structures are subjected to fluctuating stresses which, with the superimposed tensile residual stress of the joint, can be dangerous. In addition to this a welded joint is a discontinuity in an engineering structure containing many possible sites of stress concentration, e.g. toe or root of the joint, weld ripple. 

Residual stress There is a condition that develops, particularly in products with thin walls. This is a frozen-in stress, a condition that results from the filling process. The TP flowing along the walls of the mold is chilled by heat transferring to the cold mold walls and the material is essentially set (approaching solidification). The material between the two chilled skins formed continues to flow and, as a result, it will stretch the chilled skins of plastics and subject them to tensile stresses. When the flow ceases, the skins of the product are in tension and the core material is in compression that results in a frozen-in stress condition. This stress level is added to any externally applied load so that a product with the frozen-in stress condition is subject to failure at reduced load levels. 

Arnold, S.M., Arya, V.K. and Melis, M.E. (1990). Elastic/plastic analysis of advanced composite investigating the use of the compliant layer concept in reducing residual stresses resulting from processing, NASA TM-103204. 

The ASME Codes recommend that hydrostatic tests be run at a temperature that is usually above the nil-ductility temperature of the material. This is, in effect, a pressure-temperature treatment of the vessel. When tested in the relatively ductile condition above the nil-ductility temperature, the material will yield at the tips of cracks and flaws and at points of high residual weld stress. This procedure will actually reduce the residual stresses and cause a redistribution at crack tips. The vessel will then be in a safer condition for subsequent operation. This procedure is sometimes referred to as notch nuUifrcation. 

The MRC cycle calls for a 182°C cure temperature. The effect of cure temperature on residual stress was investigated by curing specimens at four other cure temperatures (171, 165, 160, and 149°C) while holding the dwell time (4 hours) constant. In Figure 8.18 the dimensionless curvature for these specimens is plotted versus the cure temperature. The curvature is reduced as the cure temperature is decreased with significant reduction in curvature obtained for dwell temperatures of 165°C or less. The final curvature as predicted by the viscoelastic process model is overlaid with the experimental data in Figure 8.18 and is shown to capture the trend. 

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

---