The Structural Integrity of Cast Aluminium Automotive Components Subjected to Fatigue Loads

Abstract

In moves towards lighter, more energy-efficient vehicles, the automotive industry is showing interest in space-frame aluminium structures for car bodies. One concept design features the use of hollow extrusions which slot over suitably shaped cast nodes, which will be subjected in service to bending stresses and to shear stresses. Given the presence of bending stresses and the sensitivity of fatigue life to defects, particular attention has been paid in the present research to the surface defects produced by casting and to effects of polishing or shot-peening the as-cast surface. The alloy studied is LM25 TF and two main casting processes have been employed: sand-casting and gravity-die-casting. For sand-castings, the defects of significance were found to be surface hollows and pores, but fatigue cracks in the die-cast material were associated mainly with entrained aluminium oxide films. Surface machining removes the surface hollows but leaves pores and films unaffected, The fatigue S-N curves can be derived from a consideration of fatigue-crack growth-rate data and distributions of initial defect size, with an allowance for an initiation contribution. The calculations are made on a probabilistic basis. The paper also discusses results obtained on shot-peened material, the residual stresses generated, and ways in which these may be incorporated into crack-growth models.