حد تحمل فولاد متخلخل
Abstract
1-Introduction
2-Static properties
3-Bearing caps test rig
4-Numerical model of test rig
5-Fatigue experiment procedure and analysis
6-Haigh diagram
7-Conclusion
Acknowledgement
References
Abstract
Nowadays, 3D numerical and fatigue analysis have become common place to design parts in the automotive industry. A small investment in numerical simulation often enables to save a lot of time and avoid onerous validation tests. Nevertheless, the major challenge now is to obtain more predictive models. To do so, we need to develop tailored modelling for materials using probabilistic approaches in order to take into account the scatter of physical properties occurring in the manufacturing process or inherent to fatigue mechanisms. We illustrate here this theme in the context of introducing a new sintered steel material for main bearing caps of recent 3 cylinder gasoline engines. We have developed an innovative experimental and numerical methodology along with a probabilistic approach to determine endurance limit. A four point bending test has been developed and specimens machined from parts have been used to be as close as possible of actual material properties and loadings. A virtual numerical model of the test bed, tightly correlated with strain measures allowed to calibrate fatigue campaigns and also gave access to mechanical variables of high cycle fatigue tests. Finally probabilistic S-N models have been identified with the “likelihood maximization method” at two stress ratio higher than +0.1. Thus, a resultant fatigue domain related to the part’s reliability objective has been defined for the design department.
Introduction
This article deals with a numerical and experimental combined analysis to estimate the sintered steel endurance limit for main bearing caps. The main bearing caps hold in place the crankshaft in the cylinder block. Fig. 1 shows an example of bearing caps – with localization of the critical zones – associated with an open deck cylinder block without skirt. Cast iron is the usual choice for bearing caps but for a better compromise between costs, mass and material supply, sintered steel is preferred for a small gasoline engine. Main bearing caps have to withstand to static and dynamic loads. Static loads are due to main journal mounting, screw clamping and residual stresses. Dynamic loads come from combustion and inertia forces, applied by the piston through the conrod and crankshaft, with a very high number of cycles up to ten millions at maximum load over customer usage. Due to those two kinds of loads we have to design the part to overcome the risk of high cycle fatigue failure at positive load ratio. The purpose of this article is to present our method to get actual and robust fatigue properties. The main points of this method are firstly to extract specimens from production parts, secondly to use a dedicated test rig with appropriated load conditions and thirdly to apply statistical analysis in order to get a probabilistic fatigue model. First section is devoted to static measurements.