FATIGUE CRACK GROWTH AND R-CURVE OF SINTERED STEEL

Abstract

Fatigue crack growth and R-curve for Fe-1.9%Cu-0.8%C sintered steel have been studied experimentally and analytically. Focus has been placed on modelling the fatigue crack growth and R-curve for sintered steel based on experimental studies and theory of fracture mechanics. An extensive literature investigation has been conducted to understand the research background about fatigue and fracture of sintered steels. The investigation indicated that although sufficient experimental studies have been carried out and many phenomena have been observed on fatigue and fracture behaviour of sintered steels, no satisfactory theories have been forwarded to explain the experimental results. In the experimental study, fatigue crack growth rates of sintered steels with different porosity levels were measured. It was observed that the Paris law exponents for crack growth in sintered steels were over 4 and around 6 and that porosity had a significant influence on fatigue crack growth. Compliance variation along the crack length was monitored over a small cyclic interval. It was found that the compliance changed discontinuously and some times it dropped as the crack advanced. This is a strong evidence that fatigue crack growth inside the sintered materials was influenced by pore system. Repeated crack initiation/arrest mechanism appeared to dominate the growth of fatigue crack in sintered steels. R-curves at different porosity levels and notch sizes were measured. It was found that porosity had a significant influence on the initial and plateau values of the R-curve. R-curves for different notch sizes remained essentially the same. Microcracking around the crack tip were observed. Based on Coffin-Manson law and the present experimental evidence, a fatigue crack growth model for porous materials has been established. This model is able to give a clear explanation to the fatigue crack growth behaviour of materials with high Paris law exponent, like those in sintered steels. The Paris law exponent, m, in this model is a function of cyclic strain hardening exponent n'. The real load bearing area factor has been included in the model to describe the influence of porosity on fatigue crack growth rate. The proposed model could be applied to all porous materials. The predictions of the Paris law exponent of ( 4+ 12n')/(1 +n') and the influence of porosity on fatigue crack growth rate in sintered steels were shown to give good agreement with results from the experiment conducted in the present study and also with those obtained in the literature. Based on LEFM concepts, a new cohesive model has been established in the present study. Incorporating compliance function for standard specimen used in ASTM standard, this model could be used to investigate the influence of specimen configuration, the influence of cohesive law, maximum cohesive stress on R-curve behaviour of solid materials. R-curves for sintered steels with different porosity levels and notch sizes simulated by the cohesive model gave satisfactory agreement with the R-curve obtained from experiment. It was found that the resultant total fracture resistance is the combined result of microcracking effect at the front of crack tip and crack bridging effect behind the crack tip.