Compressive surface residual strength

During the first two years of ORNL funding it was demonstrated that 1) three layer composites could be made with retention of compressive surface stresses to temperatures of 750°C[2,3] 2) residual stresses could be detected by strength testing[2,3J, strain gauge measurements[4], characterization of monoclinic content by x-ray diffraction[2 ], or indentation/strength measurements[5] 3) the three layer... 

The improved thermal shock resistance of the three-layer composites agrees with predictions based on compressive surface stress of 400 MPa. Th le results show that residual compressive stress of substanti. x depth in layered composites not only increases strength, apparent toughness and damage resistance, but also makes the materials more resistant to thermal shock. 

The failure of ceramic materials almost always results from a crack that is initiated at the surface by an applied tensile stress. To cause fracture of a tempered glass piece, the magnitude of an externally applied tensile stress must be great enough to first overcome the residual compressive surface stress and, in addition, to stress the surface in tension sufficient to initiate a crack, which may then propagate. For an untempered glass, a crack is introduced at a lower external stress level, and, consequently, the fracture strength is smaller. 

Stress Relieving. Residual stresses are induced in aluminum products when they are solution heat treated and quenched. Figure 5(a) shows the typical distribution and magnitude of residual stresses in thick high-strength material of constant cross section. Quenching places the surfaces in compression and the center in tension. If the compressive surface stresses are not disturbed by subsequent fabrication practices, the surface has an enhanced resistance to SCC because a sustained tensile stress is necessary to initiate and propagate this type of conosion. 

The gradient composition in FGMs not only results in a spatial variation in properties but will also generate residual stresses, which will affect the mechanical properties. One of the potential advantages of FG components is the positive influence of compressive residual surface stresses on the strength and wear resistance. A correct design of the gradient for an optimal distribution of the residual stresses is therefore important, as discussed in this chapter. 

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