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.