FE Calculation Methodology for the Thermodynamic Fatigue Analysis of an Engine Component

Thermo mechanical fatigue problem is treated to define an analysis methodology permitting the strength evaluation by reliability viewpoint. The main difficulty is the lack of both theoretical and experimental information; consequently the problem is treated verifying continually the validity and the limits of the developing solution method. The main task of the activity described in this paper was the development of a numerical methodology, based on FE analyses, for the evaluation of the structural behavior of engine components subjected to thermo mechanical fatigue phenomena. The chosen application was the exhaust manifold of an IC engine; FE analyses were executed following the standard methodology consolidated by this time in CRF. In particular a time transient thermal analysis was performed taking into account the exhaust gas fluid dynamics inside the manifold; the thermo structural analysis took into account both geometrical and material non linearities; the contact between the manifold and the cylinder head was simulated and the bolts pre load was applied. The methodology can be reassumed in the following fundamental points: 1. FE modeling of the component 2. Material modeling The evaluation of the thermal fatigue damage is executed 3. Thermal, thermo structural analyses (stress, strain) adapting methods developed to face isothermal fatigue, analysing also energetic criteria. A preliminary reliability evaluation was executed, on the base of numerical results and of experimental information. The results permitted the individuation of critical areas of the manifold and a preliminary evaluation of the number of cycles allowable before the rupture. Since the several criteria furnished different durations, a more accurate evaluation may be deferred to the execution of experimental tests.