PSI - Issue 6

Laurence A. Coles et al. / Procedia Structural Integrity 6 (2017) 5–10 Coles et al. / Structural Integrity Procedia 00 (2017) 000–000

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1. Introduction

In many applications, including automotive, aerospace, naval, defence, energy and sport, a use of fibre-reinforced composites (FRCs) has increased considerably over the last few decades. For many of these applications dynamic loading is often an unavoidable part of in-service conditions, and can result in deformation, which may lead to visible or hidden damage. Typical examples may include blast loading resulting from close proximity of structures or components to explosions or sudden pressure increases. As a result, developing a full understanding of these types of dynamic loading conditions and their effect on the response of FRCs is very important. In particular, air-blast loading can cause a significant amount of wide-spread internal delamination, even though the only external indication of damage may be a small amount of rear-surface tensile failure in surface plies. A specific realisation of damage modes depends on the blast parameters, specimen’s dimensions and fixture/support conditions. Over many years there has been a large number of research efforts aimed at characterization of the response of various materials (Langdon et al., 2015; Silberschmidt, 2016) including fibre-reinforced composites (Langdon et al., 2014) under air blast loading conditions (LeBlanc et al., 2007; Tekalur et al., 2008); even the effect of curvature on the air blast response of composite specimens was included (Kumar et al., 2013). But, typically, the analysis of the resultant damage is limited to a visual inspection of external surfaces, or a use of invasive techniques to study internal damage, which could introduce additional unwanted damage making the analysis difficult. This study is focused on shock damage in a simply supported carbon fibre/epoxy composite plate induced by incident air-blast pressures ranging from 0.4 – 0.8 MPa with respective air speeds of 650 – 950 m/s. Both inter- and intra-ply damage were examined using a combination of non-invasive analysis techniques, including Digital Image Correlation and X-Ray computed tomography. 2. Experimental Setup The carbon fibre reinforced epoxy specimen were fabricated for our tests from 10 plies of carbon-fibre fabric, pre-impregnated with a toughened epoxy matrix (IMP530R). These 10 plies were formed to a laminate consisting of two surface plies (T300 3K) and eight central bulking plies (T300 12K), with a 0/90° layup configuration. Each specimen measured approximately 195 mm x 195 mm with a thickness of 5.6 mm, and a theoretical density of 1600 kg/m 3 . All specimens were manufacture using the autoclave process, cured at 120°C with a 1.5°C per minute ramp rate and a soak time of 160 minutes at a pressure of 90 PSI whilst under full vacuum. In tests, the specimens were positioned vertically on a 3-point-bend style fixture, which consisted of two slightly rounded knife edges located 152.4 mm apart; the remained part of the specimen was unsupported (as shown in Figure 2a). A rubber band was used to keep the specimen firmly against the knife edges, while positioned vertically on the fixture. The shock-tube apparatus (8 m in length, see Fig. 1), consisting of a driver, diaphragm and driven section, produced the air-blast shock wave by pressurising the driver section up to a critical pressure, at which the diaphragm ruptured, creating the dynamic pressure-wave profile. The muzzle of the shock tube, with its inner diameter of 76.2 mm, was then moved towards the specimen until there was only a paper-thin gap (approximately 0.1-0.2 mm) between the specimen and the muzzle. Pressure sensors located towards the end of the muzzle recorded the shockwave profile during the test. The deformation process was captured using three cameras (Photron SA1 Photron USA, Inc., CA, USA), two cameras recording at 28,800 fps viewed the rear surface of the specimen for Digital Image Correlation (DIC) using the VIC-3D (Correlated Solutions) system. The third camera, also recording at 28,800 fps, was placed perpendicular to the edge of the specimen to acquire side-view images and observe the mechanisms of failure for each specimen. 2.1. Materials and specimens 2.2. Shock-tube setup