1 Continuous vs. Discontinuous Crack Extension: Characterization of Ductile Tearing in Thin Plates Volodymyr P. Naumenko* Department of Structural Integrity, G. S. Pisarenko Institute for Problems of Strength, Kyiv 01014, Ukraine; E-mail: v.p.naumenko@ipp.kiev.ua _______________________________________________________________________________________ Abstract There are two alternative trends of thought concerning the mechanism of crack extension in an elastic-plastic material subjected to monotonically and slowly increasing tensile (Mode I) loading. The crack advances continuously, by infinitesimal growth steps, or discontinuously in finite growth steps. In this experimental study, slant cracks were extended in thin aluminium plates, depending on theirs geometry, with or without an intermittent attainment of the local instabilities displayed in test records. It means that the mechanism of ductile tearing in the test material may be changed simply by changing the specimen geometry. This observation is not in accord with the well-known energetic considerations that a mechanism of crack extension in a non-hardening or low-hardening elastic-plastic material must be only discontinuous. A wide consensus exists that a single-parameter characterization of crack extension is conceptually possible when fracture resistance is quantified by a critical, lower-limiting value ψc of the Crack Tip Opening Angle (CTOA-ψ). It is of scientific and practical interest to measure CTOA-ψ for slant cracks growing at different in-plane constraint states and in this way to clear up the above contradiction. In our specimens that were fractured under the highly constrained conditions of transverse plane strain, cracks were advancing in well-defined steps.The CTOA- ψ values related to different pairs of distinct fracture events strongly depend upon the choosing the neighboring pair. They were compared with the critical angles ψc determined with the use of ASTM/ ISO Standard test method and also with the CTOA- ψn values established from profiles of fully-developed cracks. Keywords Crack extension, Thin plates, Thin-sheet aluminium, Crack tip opening angle, In-plane constraint 1. Introduction There is an increasing interest in the development of valid fracture criteria and standard test methods for unified assessment of ductile tearing in thin-wall components. This paper deals with the characterisation of plane stress tearing under uncontained yielding in rectangular plates (Fig. 1) made from thin sheets of a high-strength low-hardening aluminium alloy. The principal obstacle to the development of an easy-to-use procedure for assessing the resistance to stable crack extension is placed by the need to correlate too many variables governing the fracture behaviour in ductile materials. These are the parameters of elasticity, including those of out-of-plane deformation (buckling); plasticity, including those of residual stress effects and anisotropy; diffused and localised necking; damage and cracking. It seems highly improbable to predict fracture using only the near crack-tip parameters in isolation from the global deformation pattern. Therefore, in collecting test data a purely mechanistic approach based on the minimum of assumptions was adopted. In this way, our concept of through-life fracture assessment [1-7] attempts to formalize the characterization of crack extension using only the interrelations between directly measurable quantities without taking into account the physical damage mechanisms in vicinity of a fracture process zone. Here, the term through-life assessment means that all measures of tear resistance can be determined continuously (from the nucleation of a tear crack and up to the complete separation) or in a point-by-point manner for test events of practical importance. We suggest that the global fracture behavior expressed in terms of averaged quantities might be assessed immediately from diagrams of loads vs. displacements vs. distances between the extreme
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