Spalling, multiple

The normal stress must be zero at the free surface, so a tension wave of a similar profile but opposite sign must start propagating in from the rear surfaces when the compressive front reaches this surface. The actual stress state shortly thereafter is shown in state 2 in Figure 21. When the tensile stress exceeds the tensile strength of the material, spall occurs on a plane parallel to the free surface. The normal stress then drops to zero again, and the process continues. In brittle materials weak in tension (such as concrete), it is possible for multiple spalls to occur before the reflected tensile waves decay enough to fall below the tensile strength. 

In the next chapter we will examine the properties and behavior of rarefaction, or relief, waves. We will solve problems, on the P-u plane, involving rarefactions and interactions of rarefactions. We will see how these interactions lead to material failure as in the cases of spall and scabbing (multiple spall). 

Interaction of a nonsquare shock pulse with a free surface we will see how this can lead to multiple spall or scabbing. 

Should tiles that could convert to cristobalite be used on the shuttle, multiple uses of the vehicle would not be possible. The amorphous tiles would convert to crystalline material from the heat of re-entry, but would likely remain intact until the temperature fell below the conversion temperature for cristobalite to convert to another form at about 700°C. This conversion and thermal shock could cause sufficient weakening of the material to cause the tiles to spall allowing the shuttle to overheat. 

Spalling. The IMC, initially formed at the substrate/liquid solder interface, sometimes starts to move away from the interface as a result of prolonged or multiple reflows. This phenomenon often seems related to Ni, especially electroless Ni(P) substrate. 

In this experiment, assuming that only main effects are important, the analysis can be carried out via multiple regression. The model for time to spall, ts is given by... 

It is not uncommon to see a network of cracks in rail cross sections, and multiple cracks may shield each other and reduce crack propagation rate. The cracks may also branch. If the crack branches upwards and reaches the surface then a piece may be separated from the rail and lost as wear debris. The resulting spall or pit leads to surface roughness and deterioration in ride quality. If however, the crack branches downwards and continues to propagate then potentially a more dangerous situation develops, and the rail may break. Since, the propagation due to contact stresses is expected to slow down as the crack extends there must be... 

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