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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pressure magnitude, after which both the amplitude and frequency of the oscillations changed due to the expected reduction in specimen stiffness. Observations of the vertical and horizontal out-of-plane displacement line demonstrated that the deformation and transitions in curvature of each specimen resulting from the air blasts with different magnitudes appeared to be very similar, with no obvious differences, except for the increasing out-of-plane displacement as excepted. With the applied air pressure distributed over a relatively large area, there was minimal localised indentation, with specimens undergoing global flexural bending. As the air blast magnitude increased, the specimens exhibited greater deformation of the rear surface at the centre of the free edges. This was due to the increasing damage and delamination at the rear surface due to growing bending stress causing larger out-of-plane displacements. Although this was more apparent for the major damage case, the progression of increased deformation at the free edge was found to increase at each damage level before resulting in a clear failure at pressure magnitude only slightly higher than that for the major damage case. Post-experiment visual inspection of the specimen demonstrated that the minor damage case showed no signs of external damage, whereas the medium and major cases resulted in clearly increasing levels of damage at the rear surface as the air-blast magnitude increased. The damage for all the studied cases can be compared to that of standard static three-point bending for this type of composite specimen. The first signs of damage appeared along a central line of symmetry between the supporting knifes (characterized by the highest stress and displacement levels) as tensile failure of the individual plies leading to substantial delamination. At the front surface for each specimen no signs of damage were observed until complete specimen failure, suggesting that the tensile failure and resultant damage propagated from the rear surface through the specimen’s thickness to its front surface. The results from the X-ray computed tomography confirmed this analysis of the damage, with increasing levels of tensile failure and de-lamination at the centre and free edges of the specimen with increased air-blast magnitude. The circular loading area was transformed into a central symmetrical horizontal band of damage; an example of the major damage case is shown in Figure 2(b). 3.2. Damage analysis

Figure 2. (a) CFRP sample during air-blast loading (0.00174 s; rear surface (left) and side view (right)); (b) example of X-ray computed tomography scan, demonstrating resultant damage clouds observed after air blast

The transparent 3D rendered CT views allowed the damage clouds to be accessed for any changes in area and greyscale intensity; the higher the intensity of the grey value, the greater the accumulation of damage through the thickness of the specimen. For example, in the major damage case shown in Fig. 2(b), the intensity is greater at the central area and near the free edges of the loaded plate, showing a larger amount of through-thickness damage in these locations.

Observations of 2D horizontal and vertical cross sections of each specimen made it possible to measure