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Metal Casting Technologies : March 2011
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28 www.metals.rala.com.au TECHNICAL FEATURE Introduction he failure of metallic components under alternating stress conditions is known as "fatigue". When stress is repeatedly applied or alternated such failure can occur at much lower stress levels than when the stress is continually applied. Very few service failures result from static stress overload, nearly all mechanical failures occur as a result of fatigue damage. Most foundries assess the mechanical property characteristics of their castings via routine tensile testing of specially cast test bars and hardness testing on the various casting sections with relatively few foundries becoming involved in fatigue or fracture toughness testing. The drive to lightweight components, especially in vehicle and aircraft parts, has resulted in ever increasing demands being made on the mechanical performance of all materials including metal castings. In recent years research, especially with regard to entrainment of oxide films [1], has highlighted how fatigue behaviour and reliability of castings can be seriously damaged by casting defects that can result from inadequate metal cleanliness, incorrect filling of moulds and insufficient feeding, etc. [e.g. 2-4]. This article is therefore intended to provide non-metallurgists with a brief overview of some aspects affecting fatigue behaviour in metal castings. Characteristics of fatigue failure Fatigue failures are most commonly observed in rotating parts such as drive shafts or gears and in parts in vehicle suspensions and internal combustion engines. Fatigue failure begins when a crack is nucleated, usually at or near the surface of the component. Localized plastic deformation (slip) at stress raisers during alternating stresses leads to the development of fatigue cracks. Although stress may be reversed the structural effect of slip on two or more slip systems is not and this can result in slip band intrusions and extrusions at the surface which nucleate cracking. During further service this crack then grows, normally Fatigue in metal castings By John Pearce T Figure 1. Example of a fatigue failure in a steel which was initiated at internal shrinkage porosity showing "beach" marks on the fracture surface. Figure 2. Example of striations on a fatigue fracture surface (a) schematic (b) SEM view [5]. a b
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