Chapter 44 In-Situ Imaging of Flexure-Induced Fracture in Fiber-Reinforced Composites Using High-Resolution X-Ray Computed Tomography Brian P. Wingate and Michael W. Czabaj Abstract This work presents a new test method which allows for in situ high-resolution X-ray computed tomographic imaging of flexure-induced fracture in tape-laminate composites. Specimens with two distinct stacking sequences were tested to visualize, in 3D, the evolution of intralaminar ply cracks and delaminations as a function of the applied bending moments. The first laminate, which had small angle changes between adjacent plies, produced a fracture pattern that consisted solely of intralaminar cracks. The second laminate, having a more traditional quasi-isotropic stacking sequence, evolved a fracture surface that contained interacting intralaminar cracks and delaminations. The vastly different fracture surfaces obtained from both laminates provide invaluable, and previously unobtainable, 3D information for validation of existing and future progressive damage models. Keywords Computed tomography · Damage evolution · Tape-laminate composites · Model validation 44.1 Introduction Recent developments in finite-element (FE) based progressive-damage algorithms have enabled simulation of complex fracture patterns that can be observed in fiber-reinforced tape-laminate composites. FE codes based on the Floating Node Method (FNM) [1] or the Regularized eXtended-Finite Element Method (Rx-FEM) [2], are now capable of simulating initiation and evolution of intralaminar cracks and delaminations under a variety of loading conditions and laminate geometries. Despite these advances, there is a significant lack of experimental data that can be used to quantitatively evaluate the predictive nature of these codes. To date, most of the validation has been performed using in-plane uniaxial-tension static and fatigue tests, with progressive damage evolution imaged using 2D ultrasonic or X-ray radiography techniques. In this work, a new test is proposed which enables 3D imaging of flexure-induced progressive fracture in tape-laminate specimens using high-resolution X-ray computed tomography (CT). 44.2 Experimental Methodology The test geometry proposed in this study consists of a loading fixture that applies bending to a vertical composite beam using two eccentric compressive loads as shown in Fig. 44.1a. The test fixture is designed to ensure that the central portion of the composite beam is always exposed to the incoming X-ray beam, thus allowing for uninterrupted imaging of the gage region during flexure. Moreover, the horizontal loading arms are sized to ensure sufficiently high bending moments in the gage region, while keeping the asymmetric stress distribution across the specimen height to a minimum. The entire flexure apparatus is designed to interface within the hot-cell experiment available at the Advanced Light Source beamline 8.3.2 (Lawrence Berkeley National Laboratory, Berkeley, CA), and if necessary, can be used at temperatures in excess of 1000◦ C[3]. The preliminary evaluation of the test fixture was performed by considering two composite beam designs, each with a distinct tape-laminate stacking sequence. The first design, shown in Fig. 44.1b, contained a stacking sequence with 15◦ angle changes between adjacent plies, which promoted extensive evolution of through-thickness transverse cracks. The second design, shown in Fig. 44.1c, contained a more traditional, quasi-isotropic stacking sequence with 45◦ and90◦ angle changes B. P. Wingate · M. W. Czabaj ( ) Department of Mechanical Engineering, University of Utah, Salt Lake City, UT, USA e-mail: m.czabaj@utah.edu © The Society for Experimental Mechanics, Inc. 2019 P. R. Thakre et al. (eds.), Mechanics of Composite, Hybrid and Multifunctional Materials, Volume 5, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-319-95510-0_44 331
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