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It was previously demonstrated that the controlling crack growth criterion in stress corrosion cracking is that derived from fracture mechanics concepts and not a criterion based on gross or net stress. This demonstration was accomplished by testing two types of cracked specimens and showing that crack arrest and crack initiation stress intensities are the same while gross and net stresses are different. However, these tests were performed on thick section material. The work now reported was intended to check the validity of the correlation between arrest and initiation stress intensities using thin section material having higher thoughness and lower yield strength than the material previously tested. The results showed crack arrest stress intensities were substantially higher than those for crack initiation when testing thin materials. It was also observed from the test results that when testing titanium materials testing must be accomplished as soon as possible after fatigue cracking to eliminate any possible effects of crack blunting.
Fretting fatigue crack initiation behavior of titanium alloy, Ti-6Al-4V was investigated by using different geometries of the fretting pad. Specimens were examined to determine the crack initiation location and the crack angle orientation along the contact surface. Fretting fatigue experiments were analyzed with the finite element analysis. Several critical plane based parameters were used to predict the number of cycles to fretting fatigue crack initiation, crack initiation location and crack orientation angle along the contact surface. These predictions were compared with the experimental counterparts to study the role of normal and shear stresses on the critical plane in order to characterize fretting fatigue crack initiation behavior. From these comparisons, the fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by the shear stress on the critical plane. However, the role of normal stress on the critical plane at present appears to be unclear.