۱٫ Introduction

۲٫ Taira’s Damage Model

۳٫ Modification of Taira’s Damage Model

۴٫ Modified Taira’s Damage Model for TMF

۵٫ Conclusion

Acknowledgements

References

This study investigated a fatigue life prediction method based on extensive experiment results of cast aluminum alloy Al319-T7 subjected to repeated thermal and mechanical loading. Cyclic tests and fully reversed fatigue test results of the material were obtained from the specimens subjected to three different strain rates (5×10^-5, 5×10^-4 and 5×10^-3) and various temperature conditions. At each strain rate the specimens were subjected to room temperature (25°C), 150°C, 200°C, 250°C and 300°C. Thermo-mechanical fatigue (TMF) tests were also conducted for in-phase and out-of-phase conditions of the temperature and mechanical loading. During the thermo-mechanical fatigue tests, the effect of loading phases and dwell time on fatigue life of the specimens was also observed. This study modified Taira’s fatigue damage model for thermo-mechanical loading condition to include the strain rate effect on the fatigue damage. Taira assumed that fatigue damage per reversal is proportional to the damage factor, λ(T), and plastic strain range powered by n, (Δε_p)ⁿ. The relationship between the plastic strain range (Δε_p) and the number of cycles to failure (N_f) is presented as λ(T)•(Δε_p)ⁿ•N_f = C. Where C is a temperature independent material constant. The temperature effect is included in the damage factor, λ(T) that can be determined from the ratio of λ(T)/λ(T_o) for low cycle fatigue test results at various isothermal conditions. T_o is the reference temperature and can be determined by experiment. This study used stress range applied instead of plastic strain range in the original equation. Furthermore, the modified equation includes the effect of strain rate, phase, and dwell time. The new fatigue damage equation was well correlated with the experiment results.

Introduction

In automotive industry, the durability of engine components at high temperature is one of the major considerations in the design phase since they are usually running under the service environment of cyclic loading and high temperature. While most engine components were made of cast iron in the past, many cast iron components have been replaced by aluminum alloys nowadays to reduce the vehicle weight. Along with the use of light weight components in the vehicle, thermal loadings as up as to 300˚C can be applied to engine components [1]. With the strong demand to understand the thermos-mechanical behavior of aluminum alloy engine parts and predict their service life, many efforts have been made in the past decades. Thomas et al. [2] proposed a complete design approach and lifetime prediction method for aluminum alloy cylinder heads under thermos-mechanical fatigue (TMF) loading. Engler-Pinto et al. [3] investigated the thermosmechanical fatigue behavior of cast 319 aluminum alloys used in cylinder blocks. Kang et al. [4] introduced a fatigue life prediction method for thermo-mechanical fatigue damage under variable temperature and loading amplitudes using a rainflow cycle counting technique. Wei et al. [5, 6] reviewed the up-to-date methods for damage modelling and life assessment of components under thermal fatigue loading and developed a general life assessment procedure under variable amplitude thermal-mechanical loadings, emphasizing on hold-time effect. Santacreu et al. [7-9] proposed a TMF damage model of stainless steels based on the maximal temperature and plastic strain amplitude reached during a thermal cycle.