A reliability analysis method in thermomechanical fatigue design

The design of structures against thermomechanical fatigue (TMF) is a relatively new concern and research has generally concentrated on deterministic methods to ensure the resistance of structures undergoing thermal–mechanical loadings. Many studies have thus been conducted to better represent the nonlinear behaviour of materials or to develop thermomechanical fatigue criteria. However, fatigue is a phenomenon which is random in nature: manufacturing processes, geometric tolerances and usage conditions can affect the lifetime of a structure. Typically, the use of a car by a customer is unique (type of roads, weather conditions, drivers behaviour, etc.) and thus thermomechanical loads for instance on a cylinder head become probabilistic. Similarly, the intrinsic strength of a structure is variable (casting and machining process, specific microstructures, etc.). It is therefore necessary to be able to guarantee the TMF resistance of a particular structure itself undergoing a particular load, whatever the structure is. The work presented here consists of the development of a complete protocol analysis of the risk of failure of a structure subjected to thermomechanical fatigue when either exact loading conditions or strength for a given structure are uncertain. The proposed method relies on the stress–strength interference analysis and also on numerical techniques based on finite element calculation and engine load analysis which enable us to compute a local damage from a global loading. The method is successfully applied to a cylinder head.