Mechanism-based Thermomechanical Fatigue Life Assessment Method for High Temperature Engine Components with Gradient Effect Approximation 2019-01-0536

High temperature components in internal combustion engines and exhaust systems must withstand severe mechanical and thermal cyclic loads throughout their life time. The combination of thermal transients and mechanical load cycling results in a complex evolution of damage, leading to thermomechanical fatigue (TMF) of the material. Analytical tools are increasingly employed by designers and engineers for component durability assessment well before any hardware testing. In past publications, it has been shown that the mechanism-based TMF damage model (DTMF) is capable of providing reliable TMF life predictions for a wide range of high temperature components and materials in internal combustion engine. The standard DTMF model uses surface stresses, strains, and temperatures for estimating the number of cycles for a small initial defect or micro-crack to reach a critical length. In the presence of significant gradients of stresses, strains, and temperatures, the use of surface field values could lead to very conservative estimates of TMF life when compared to reported lives from hardware testing. As an approximation of gradient effects, the DTMF model is extended to use through-thickness fields, where the micro-crack growth law is integrated through the thickness taking into account these variable fields. With the help of software tools, this method is automated and applied to several components with complex geometries and fields. It is shown, in these applications, that gradient correction leads to more realistic life predictions and can distinguish between surface cracks that may arrest or propagate through the thickness and lead to component failure.