Acceleration techniques for the numerical simulation of the cyclic plasticity behaviour of mechanical components under thermal loads

Numerical simulations of components subjected to cyclic thermo-mechanical loads require an accurate modelling of their cyclic plasticity behaviour. Combined models permit to capture monotonic hardening as well as cyclic hardening/softening phenomena, that occur in reality. In principle the durability assessment of a component under thermal loads can be performed only if the cyclic behaviour is simulated until complete material stabilization. As materials stabilize approximately at half the number of cycles to failure, it follows that in case of small plastic strains a huge number of cycles must be considered and an unfeasible simulation time would be required. Accelerated models have thus been proposed in literature. The aim of this work is that of comparing the different acceleration techniques in the case a round mould for continuous casting loaded thermo-mechanically. It can be observed that the usual approach of using the stabilized stress-strain curve already from the first cycle could lead to relevant errors. An alternative method is that of increasing the value of the parameter that controls the speed of stabilization in the combined model. This approach permits the number of cycles to reach stabilization to be drastically reduced, without affecting the overall mechanical behaviour. Based on this approach, a simple design rule, that can be adopted, particularly when relatively small plastic strains occur, is finally proposed. 1 Numerical simulation of mechanical components under thermal loads The choice of a suitable cyclic plasticity model to be used in numerical simulation of components subjected to cyclic thermo-mechanical loads is a crucial aspect in design. Some of typical examples are discussed in [1, 2, 3]. In fact, to perform a reliable thermal fatigue analysis a model able to capture accurately the behaviour of the material observed during experimental testing is required. On the other hand, durability assessment of a component under thermal loads can be done only if complete material stabilization is reached [4]. As materials stabilize approximately at half number of cycles to failure, it follows that in case of small plastic strain a huge number of cycles must be considered and an unfeasible computational time would be required. In a recent work [5] dealing with a thermomechanical analysis of a squared mould showed that a huge number of cycles (around 60567) must be performed to reach the stabilized condition. Since the geometry of the squared mould requires a 3D finite element (FE) simulation, even taking into account symmetries and optimizing the mesh, ≈15 min is needed to compute 1 cycle or ≈630 day to obtain final results, according to the value (b≈4) of the speed of stabilization of the mould alloy, see Fig. 1. Therefore, to overcome this problem, some alternative models (accelerated, stabilized), proposed in [6], have been adopted and compared. It has been shown that accelerated models gave the most conservative results, as the lowest value of number of cycle to failure was obtained. On the other hand, in that work it was not possible to make a comparison with the combined (complete) model because a too long computational time was necessary to reach stabilization. Fig. 1. Possible strategy to reduce the computational time. Fig. 1 presents schematically the desirable diagram required to perform a correct design of the component. In principle the basic idea of the accelerated techniques ≈ 60567 (≈630 days) N (log) Sp ee d of st ab ili za tio n, b Combined model ≈ 4 Accelerated models N=100 (≈25 hours) UNREALISTIC Δεpl UNFEASIBLE 1500 A B MATEC Web of Conferences 165, 19010 (2018) https://doi.org/10.1051/matecconf/201816519010