Fatigue crack and evolution prediction of compacted graphite iron under thermal loading with variable amplitude

摘要

With the development of engine and environmental protection, thermal cracking has become one of the main failure modes of components and materials working under complex thermal conditions. The fatigue microcrack and evolution prediction model of compacted graphite iron under thermal fatigue with variable amplitude is studied in this paper, which is induced by pulsed laser. According to the different length, microcracks shorter than 0.1 mm and longer than 0.3 mm are defined as "secondary microcrack" and "main microcrack", respectively. Results show that, the secondary microcrack caused by the superimposed high-cycle thermal loading shows different behavior of initiation and propagation with the main microcrack. The propagation of secondary microcrack usually limits in a eutectic cell, because that the energy provided in the high-cycle stage is not enough to break across the eutectic boundary. However, the main microcrack can propagate across the eutectic boundary because of enough energy provided in the low-cycle stage. With consideration of both plastic strain energy density and elastic strain energy density, a modified Paris-type model is proposed based on fractal theory. The threshold Delta W-th is introduced to evaluate the damage contribution of elastic strain energy density. The proposed model is helpful for lifetime prediction of components and materials working under thermal fatigue with variable amplitude.