This paper presents a combined experimental‐numerical technique for the calculation of the J‐integral as an area integral in cracked specimens. The proposed technique is based on full‐field measurement using digital image correlation (DIC) and the finite element method. The J‐integral is probably the most generalised and widely used parameter to quantify the fracture behaviour of both elastic and elastoplastic materials. The proposed technique has the advantage that it does not require crack length measurements nor is it limited to elastic fracture mechanics, provided that only small scale yielding is present. Evaluated are three test geometries; compact tension, three‐point bend and the double torsion beam. Possible errors and their magnitude and the limitations of the method are considered.

A B S T R A C T It is generally believed that a lower bound on the fracture toughness of a material is obtained from a standard test, particularly in metals where yielding occurs prior to fracture. The understanding is that in such a test the material around the crack tip is highly constrained hence reducing the extent of yielding. In this paper, we report the results of fracture tests where a tensile load is applied to a biaxial aluminium alloy specimen in the direction parallel to the crack front in addition to the fracturing load normal to the crack surface. We show that in this case a lower fracture toughness is measured than that obtained from a standard test. Indeed, for the highest value of tensile load used in our tests the J-integral at fracture was half the value measured in a standard test. It is also shown that the volume of the plastic region can be used to measure the effect of constraint, irrespective of the manner in which the constraint arises. This approach suggests an even lower fracture toughness may be obtained than that measured here in certain loading conditions. a = crack length A = area of the plastic region A c = area of the plastic region at fracture A ref = area of the plastic region at fracture measured in a standard test A 2 = parameter quantifying second and third term of stress relative to the first term in a cracked elastic-plastic body B = biaxiality ratio (dimensionless T-stress) E = elastic modulus F c = corrected fracture load F c = fracture load F T = transverse load (side load)i = the number of tested specimen J = J-integral J c = J-integral at fracture J ref = J-integral at fracture measured in a standard test K I = mode I stress-intensity factors K Ic = mode I fracture toughness measured from a standard test N = total number of specimens tested P f = fracture probability Q = parameter quantifying the deviation of opening stress in a non-standard specimen from a standard specimen Correspondence: M. J. Pavier.

. In-situ X-ray computed tomography characterisation of 3D fracture evolution and image-based numerical homogenisation of concrete. Cement & Concrete Composites, 75,[74][75][76][77][78][79][80][81][82][83] University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms _________________________________________________________ * Corresponding author: Prof. Z Yang, Email: ac1098@coventry.ac.uk

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Abstract. Our observations of fracture are generally restricted to the surface of test specimens; yet the fracture process occurs within the material. X-ray computed tomography (CT) can provide valuable insights into the failure process inside the material: when X-ray CT is combined with digital volume correlation (DVC) the response to applied loads of the displacement field within the material can be measured with high precision. In this paper we study the fracture behaviour of a short-bar chevron notch specimen fabricated from polygranular nuclear graphite -a quasi-brittle material. Tomographic absorption contrast images were obtained from the specimen before and after crack propagation. The DVC-measured displacement field was used to visualise the crack, and also to measure and map its opening displacement in 3D. Three-dimensional finite element simulation of the specimen obtained the relations between crack length, opening displacement and stress intensity factor along the crack front. The experimentally calculated crack opening displacements were consistent with the FE-predicted values, and could be used to obtain the critical stress intensity factor for crack propagation.

To investigate the fracture behaviour of polygranular graphite (a quasi-brittle material), crack propagation in a short bar chevron notched specimen was studied by synchrotron X-ray computed tomography combined with digital volume correlation. Displacements were measured within the loaded test specimen, particularly the three-dimensional (3-D) profile of crack opening displacement. Analysis of the 3-D displacement field confirmed the existence of distributed damage in a fracture process zone, which significantly increased the effective crack length. Finite element simulations affirmed that the measured crack opening profiles could be reproduced using a cohesive zone model, but not with a linear elastic analysis. Comparing the simulation to the experimental results, it was deduced that the critical strain energy release rate varied across the crack front, i.e. the fracture toughness is constraint-dependent. This is proposed to be a general characteristic of quasi-brittle materials.

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