Hiroko Mikado

Effect of stress ratio on fatigue lifetime and crack growth behavior of WC–Co cemented carbide

Two types of fatigue tests, a rotating bending fatigue test and a three- or four-point bending fatigue test, were carried out on a fine grained WC–Co cemented carbide to evaluate its fatigue crack growth behavior and fatigue lifetime. From successive observations of the specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC–Co cemented carbide was occupied with crack growth cycles. Using the basic equation of fracture mechanics, the relationship between the fatigue crack growth rate (da/dN) and the maximum stress intensity factor (Kmax) was derived. From this relation, both the values of the threshold intensity factor (Kth) and the fatigue fracture toughness (Kfc) of the material were determined. The fatigue lifetime of the WC–Co cemented carbide was estimated by analysis based on the modified linear elastic fracture mechanics approach. Good agreement between the estimated and experimental fatigue lifetimes was confirmed.

Fatigue Lifetime and Crack Growth Behavior of WC-Co Cemented Carbide

It is well known that WC-Co cemented carbides have excellent wear resistance. However, information about their fatigue crack growth behavior and fatigue properties is limited. In the present study, rotating bending fatigue tests were carried out on a fine grained WC-Co cemented carbide to evaluate its fatigue lifetime and crack growth behavior. From observations of the micro-notched specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC-Co cemented carbide is comprised of crack growth cycles. Using the basic equation of fracture mechanics, the relation between the rate of fatigue crack growth da/dN and the maximum stress intensity factor Kmax of the WC-Co cemented carbide was derived. From this relation, both the threshold intensity factor Kth and the fatigue fracture toughness Kfc of the material were determined. Fatigue lifetime of the WC-Co cemented carbide was estimated based on the fatigue crack growth law.

Crack Growth Behavior in Cemented Carbide by Repeated Thermal Shock

In this study, fatigue crack growth (FCG) behavior of cemented carbide under the repeated thermal shock (RTS) was experimentally evaluated by using the thermal-shock experiment method developed by the authors. Tests were carried out using cemented carbide having two different WC crystal grain sizes. In addition, FCG behavior under rotating bending fatigue (RBF) test was investigated using the same cemented carbides. Then the FCG results obtained by the RTS test and the results of the RBF test obtained at stress ratio, R = -1, were compared with each other. Here, the stress ratio R is defined as, R = σmin/σmax; σmin and σmax are the minimum and the maximum stresses, respectively. From this comparison, it was found that the relation between the rate of fatigue crack growth (FCG) and the maximum stress intensity factor in the RTS tests was equivalent to the one obtained under the RBF tests at stress ratio of -1. From a practical point of view, this result is important as it indicates that it is not necessary to purposely perform RTS experiments. In this research, the effect of WC grain size on the short surface FCG behavior of the cemented carbide was also studied and discussed.

Effect of Cobalt Content on Fatigue Lifetimes and Short Fatigue Crack Growth Behavior of Tungsten-Cobalt Cemented Carbides

Fatigue lifetimes and crack growth behavior of the cemented carbides with different Co content were investigated. Effect of the Co content on the fatigue lifetimes is found to be minimal within the Co content, 13 –18 wt% tested in the present study. At the high Kmax region, the crack growth resistance for the WC-18 wt%Co is higher than that for the WC-13 wt% Co. In the lower Kmax region, the reverse trend, i.e., the crack growth resistance for the WC-13 wt% Co is higher than that for the WC-18 wt%Co, was expected, but this point remain obscure due to the lack of the experimental data.