About the heat sources generated during fatigue crack growth: What consequences on the stress intensity factor?

Abstract

During cyclic loading of a cracked metallic alloy at room temperature, heat sources are generated and produce a heterogeneous temperature field around the crack tip. Those heat sources are: (i) the thermo-elastic coupling source, (ii) the intrinsic dissipation due to microplasticity in the material, and (iii) the cyclic plasticity dissipated into heat in the reverse cyclic plastic zone (RCPZ) ahead of the crack tip. The thermoelastic source is computed by finite element analysis in agreement with classic linear thermoelasticity theory. The intrinsic dissipation due to microplasticity is experimentally estimated by carrying out self-heating fatigue tests on uncracked specimens, and then approximating its values in the cracked specimens by using self-heating curves. The cyclic plastic strain energy dissipated into heat in the RCPZ is also experimentally quantified by carrying out fatigue crack growth tests and using infrared measurements. The temperature fields, generated by the three types of heat sources, are separately computed by using the linearity of the heat diffusion equation. Afterward, the stress fields, associated with each thermal effect and induced by the material thermal expansion, are computed by considering the hypothesis of the linear elastic fracture mechanics (LEFM). Thus, the mode I stress intensity factor is calculated by taking into account the thermal effect associated with each heat source. The consequences on K,ΔK and RK=Kmin/Kmax are discussed. It is shown that the heat sources do not modify significantly ΔK, but the modification of RK can be significant since the effects are proportional to the loading frequency.