Characterization of stable crack extension in aluminium sheet material using the crack tip opening angle determined optically and by the δ5 clip gauge technique

Abstract

Tests standards aimed at deriving fracture toughness data and crack resistance curves under low constraint condition have recently been finished by ASTM and are in the final stage within ISO. These standards cover various experimental methods for determining critical crack tip opening angles, CTOA, for characterising stable crack extension in sheet material. In this paper, some key items of these standard methods are validated, namely the experimental determination of the crack tip opening angle by optical observation and using the δ 5 -clip gauge method. When applying such standard methods to material characterization it is of particular interest to know how CTOA data derived by different methods compare to with each other. This paper compares CTOA data as derived by the optical method with that those derived by using the δ 5 -clip gauge method. In order to study possible specimen size and geometry effects the methods have been applied to a wide range of specimen geometries. The results demonstrate that CTOA data derived by the optical method are well suited to provide a specimen size and geometry independent characterization of stable crack extension where the thus obtained CTOA data are constant over a large amount of stable crack extension. In contrast to this result, CTOA-data obtained from the δ 5 clip gauge method revealed a complex pattern of size and geometry effects, and only in case of compact specimens with a selected size the two CTOA-methods provide nearly identical CTOA-data over a large amount of crack extension.

Review of fracture toughness (G, K, J, CTOD, CTOA) testing and standardization

“…The critical d 5 value and the d 5 -R curve are utilized as alternatives to describe the resistance of ductile sheet materials to stable crack extension under low-constraint conditions. More detailed discussions on this topic were given recently by Heerens and Schodel [223].…”

“…This newer standard uses C(T) and M(T) specimens made from thin-sheet materials in order to achieve low constraint conditions at the crack tip. The standard was validated by Heerens and Schodel [223] using a comprehensive data set on the stable crack extension in an aluminum sheet material with a thickness of 3 mm. The CTOA fracture criterion now becomes one of the most promising fracture criterion used for characterizing the stable tearing in thin metallic materials 224.…”

Crack-tip opening angle-based numerical implementation for fully plastic crack growth analyses

“…In the past, various mechanisms were studied and proposed to estimate CTOA for stable crack growth [1][2][3][4][5][6][7]. Especially for fully plastic crack growth in low strength materials, the sliding-off and shear-cracking model suggested by McClintock et al [1], among others, was found to best serve to estimate CTOA.…”

Fatigue Crack Growth Behaviour in Friction Stir Welded Aluminium-Lithium Alloy Subjected to Biaxial Loads

“…In previous projects, a slot design in the loading arms was successfully used [43]. An example specimen for static tensile load test is shown in Figure 4.…”

“…Photo of a HZG test sample for static tensile loading test with load-diffusion slots [43] For the purpose of this study, i.e. testing under cyclic loads, the basic geometry shown in Figure Specimen design was based on FEA [31].…”