PSI - Issue 6

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

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Figure 1. Air-blast regime and three-point-bend style fixture

2.3. Air-blast pressures profiles and shock waves

The air blast and resultant pressure magnitudes were then chosen so to produce three levels of damage within the specimens: minor, medium and major (with the entire specimens still intact), and were determined during preliminary calibration experiments. For this experimental study, incident pressures of 0.4 MPa, 0.6 MPa and 0.8 MPa with respective reflected pressures of 1.35, 2.50 and 3.4 MPa were thus selected; the corresponding wave speeds ranged between 650 m/s and 950 m/s.

2.4. X-ray computed tomography

All the loaded specimens were inspected using a Metris 160 H-XT X-ray CT system to investigate the extent of the internal damage and its spatial distribution. Each scan was conducted at 140 kV and 130 µA using a tungsten target, with 2650 radiography projections taken over the 360° rotation for each specimen at an exposure of 500 ms. In order to reduce granular noise, 8 images where taken and averaged per projection. The total volume scanned for each specimen was 180 mm x 140 mm x 20 mm at a resolution of 97 µm. 3. Experimental Results and Discussion In dynamic loading, an initial oscillation of the specimen achieves its maximum displacement before the oscillations decay gradually due to the contact surface behind the shock wave and resultant back pressure. Differences in the oscillation peaks represent a transition of lateral oscillations to the free edges and back again, also offering insights into the initiation of delamination resulting in reduced local stiffness. This then leads to changing amplitude and frequency of the oscillations. In experimental case studies, the specimen underwent global flexural bending between the fixture supports with damage initiating at the centre of the rear surface. Double and triple peaks can be observed on the centre-point displacement oscillations, the double peaks represent the transition of lateral oscillations to the free edges and back again. The triple peaks are related to behavior similar to that of the double peaks, but the centre peak shows the delamination of a given number of plies that were free to move and displace further due to reduced local stiffness. Thus, delamination initiated at different times for each 3.1. Deformation analysis