Multi-scale model for fatigue in carbide rich tool steel

摘要

Carbide-rich tool steel is most commonly used not in the tooling industry, but also in engine parts, e.g. springs, bearings, diesel injections, connecting rods etc.. Components made from this kind of material are often subjected to cyclic mechanical stresses. Fatigue is important as it occupies the largest cause of failure in metal, aproximately estimated 90% of all metallic failures, tool steels are also susceptible to this type of failure. Fatigue resistance of this material strongly depends on the microstructural features including shapes, shape ratio, volume fractions, and distributions of primary and eutectic carbides. Thus, besides loading condition microstructural features are considered as the main factor which influences lifetime of tool components.It is known that the lifetime prediction of carbide-rich tool steel in alternating applied stress is not an easy task to perform. Therefore, gaining knowledge about the effects of microstructural features on the fatigue behavior of this material is necessary. Subsequently, the main objective of this research is to develop a simple model as well asa computational framework to quantify the influence of these microstructural features on the fatigue behavior of the material in the high cycle fatigue (HCF) regime.In general, fatigue crack mechanisms can be divided into 3 stages: initial crack formation (crack incubation or nucleation), short crack and long crack growth, which have successfully been established by McDowell, in a so-called multistage fatigue model (MSF). To model fatigue behavior of carbide rich tool steel, McDowell’s model was modified and developed at three length-scale levels, resulting in amulti-scale fatigue model. For fatigue crack formation and early growth, a hierarchical approach was used, and lifetime of this stage was estimated based on local cyclic micro plasticity within a representative volume element (RVE). The short crack stage consists of microstructurally short crack (MSC) and physically short crack (PSC) growth in which short crack drivingforce was determined from the process zone at the crack tip, so-called cyclic crack tip opening displacement (CTOD). From this relation, the effects of microstructural features on the cyclic short crack growth were explicitly identified. For long crack growth, an accumulated fatigue damage concept was implemented to calculate the lifetime of this stage. Based on that relation, the long crack growth rate was easily derived from low cycle fatigue (LCF) properties because it is believed that LCF test is easy to calibrate and it may be interpolated from monotonic tensile test, which results in saving time and cost for fatigue prediction.The most important contributions of this study are to simulate and model the influence of carbides on three different length scales of fatigue crack mechanisms in tool steels. The proposed model is considered as a powerful tool for lifetime prediction not only in tool steels, but also in particle reinforced composites and other heterogeneous materials. Moreover, optimization process on microstructural features can be done basedon the results of this study. Consequently, the in-service life of materials may be improved.