Volume 6, Issue 10 (November 2009)
Definition of the Influence of Pore Size on the Fatigue Limit Using Short Crack Propagation Experiments
Aluminium high pressure die casting is used to reduce costs and weight of various components in the automotive industry. The main problem of components made by pressure die casting is connected with inherent flaws (porosity, oxide skins, etc.), which can hardly be avoided. Today the fatigue calculation of aluminium high pressure die cast parts is usually performed using two S/N curves—one account for the flawed basic material and one for the pore-free surface layer. This does not provide an accurate estimate for the computation of the lifetime or safety against failure of the component. In a cast component the number and size of pores increases towards the center. As a consequence the fatigue strength decreases. This inhomogeneous distribution throughout the component has to be taken into account for a realistic estimation of the fatigue strength. To do so, two models are required—one to compute the pore size distribution in the component [Oberwinkler, C., Leitner, H., and Eichlseder, W. “Improvement of an Existing Model to Estimate the Pore Distribution for A Fatigue Proof Design of Al HPDC Components,” TMS 2009, Shape Casting: 3rd Int. Symposium, 2009] and a material model [Oberwinkler, C., Leitner, H., and Eichlseder, W. “Computation of Fatigue Safety Factors for High-Pressure Die Cast (HPDC) Aluminium Components Taking into Account the Pore Size Distribution,” SAE World Congress 2009], which describes the influence of the defect size on the fatigue strength. This paper focuses on the definition of the correlation between the fatigue strength and the pore size. The lifetime of a component is defined by the crack growth if the crack initiation period can be neglected. Casting defects usually show high stress concentration factors due to their irregular shapes where cracks initiate. In this case the defect size is a part of the total crack length. Pores can be considered as physically short cracks because of their size of approximately 10–500 μm. The El-Haddad equation [El Haddad, M. H., Smith, K. N., and Topper, T. H., “Fatigue Crack Propagation of Short Cracks,” ASME Transactions, Vol. 101, 1979, pp. 42–46] has been used to describe the influence of the defect size on the fatigue strength. Short crack experiments have been performed to show that due to the shorter cracks the threshold stress intensity factor decreases below the threshold stress intensity factor of long cracks.