فرسودگی مواد و سازه های ارزیابی شده
Abstract
1-Introduction
2-Assumptions & Models
3-Standard load spectra
4-Comparison of chassis system load spectra
5-Conclusions & Outlooks
Acknowledgements
References
Abstract
High cycle fatigue behaviour of materials and structures is widely assessed with respect to constant amplitude loading, because of extreme simplification of experimental procedures and numerical computations. Nevertheless, fatigue strength is usually supposed to guarantee in-service integrity face to variable amplitude loading. As the fatigue response may be very sensitive to the load time history, the gap between the former two points of view results in a long-lasting debate among the fatigue community, implying the use of a damage model and the choice of a framework. The pioneering Wöhler model is the natural reference when dealing with constant amplitude loading, whereas standard load spectra are widely applied since the seminal work of Gassner. This paper takes advantage from fruitful discussions between German & French automotive researchers. It presents an appreciation of the Gassner model, usually applied by German engineers since at least 50 years, with the point of view of French engineers, more used to the Wöhler model. Neglecting any sequence effect of the load time history, thus accepting the Palmgren-Miner’s rule as an assumption, we focus on the dual representation of high cycle fatigue behaviour expressed by Wöhler & Gassner, respectively. This necessary calls for the use of load spectra (in terms of amplitude, not frequency), which are here discussed with particular attention to the automotive field.
Introduction
This paper addresses fatigue phenomenon applied to automotive passenger vehicles. In particular, we focus on the chassis system, i.e. front and rear axles as highlighted in figure 1. The chassis system is composed of safety parts, whose reliability with respect to fatigue is expected to be proven, by numerical computations and experimental tests, respectively. Fatigue is not the only design concern, nevertheless it is one of the most relevant indeed: in terms of orders of magnitude, considering a life target of about 105 km, the customer usage analysis leads to count up to 106 braking and cornering events, thus 106 cyclic load occurrences, thus calling for high cycle fatigue. For the sake of simplicity, within this framework we pay attention on usual driving situations only, coming from everyday life customer usage, thus neglecting any overload related to special driving conditions or misuse. On one hand, in-service loading of chassis parts leads to complex time histories, e.g. recorded by force transducers applied to the wheel hub and active during driving sessions, see figure 2, left side. On the other hand, it may be really useful to reduce a long time history into a single sinusoidal signal of given amplitude (and frequency), the simplest to be managed by a testing rig or a FE solver, see figure 2, right side.