FINITE ELEMENT ANALYSIS OF PLASTICITY-INDUCED CRACK CLOSURE FOR INCLINED CRACKS L.-W. Wei and M N James Department of Mechanical and Marine Engineering, University of Plymouth, Drake Circus, Plymouth PL4 8AA, Devon, UK ABSTRACT This paper presents work on developing a finite element model to assess fatigue crack closure taking place in cracks which are inclined to the loading axis. Crack closure measurements were also performed on singleedge-notch tension (SENT) fatigue specimens made of an aluminium alloy Al5083-O, using the strain gauge based compliance method. It is found that cracks in SENT specimens are prone to changing growth direction from initial notches. Small variations in predicted crack opening level (Sop) were found for different inclinations to the specimen width direction, whereas predicted crack closing level (Scl) demonstrated large change for the cases of α=450 and 600. Reasonable agreement was obtained between the model results and the experimental data in cases of low mode ratio KII/KI (α=0 0 and 150) which indicates (to some extent) the applicability of the numerical simulation of plasticity-induced crack closure for this class of inclined cracks. 1. INTRODUCTION Since the discovery by Elber [1] of the phenomenon of premature contact between two crack faces in a growing fatigue crack, even under tension-tension loading, a large number of studies have been undertaken to estimate the magnitude of crack closure, and correlate it with crack growth behavior [2]. The kind of closure observed by Elber is often termed plasticity-induced closure, as it arises from contact, prior to the minimum cyclic load, of the residual plastically deformed material along the crack flanks behind the crack tip. The significance of understanding fatigue crack closure lies in the fact that it can not only provide some rationalization for certain anomalous behaviors such as the growth of physically short fatigue cracks and the stress ratio effect on growth rates, but also is required for more accurate life prediction. If the contributions due to crack closure are neglected, the driving force for crack growth, solely based on nominal loads, may differ between specimen and component, thereby leading to an erroneous life prediction. The approaches to investigating plasticity-induced closure include experimental observations by means of various techniques such as the strain gauge based compliance method [1,3,4], analytical means evolved from the Dugdale strip-plasticity model [5,6], as well as numerical simulation using finite element analysis (FEA) [e.g. 7]. The pros and cons of these different methods have been critically reviewed by McClung and Sehitouglu [8]. In respect of FEA of plasticity-induced crack closure, efforts have been expended largely on two-dimensional FEA of fatigue crack closure assuming plane stress [7,8,9] or plane strain conditions [10,11], although some work has also been performed in a three-dimensional analysis of crack closure [12].
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