PSI - Issue 14

T Sreekantha Reddy et al. / Procedia Structural Integrity 14 (2019) 265–272 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Using the force-time data, impact parameters like absorbed energy, displacement and penetration velocity/deceleration were calculated using the software provided by the instrument supplier (ASTM D7136, 2007). The response of different thickness laminates in terms of force-time, force displacement and energy-time was compared at three distinct impact velocities viz. 4.34, 6.08 and 7.51 m/s, the corresponding energies are 50,100 and 150J respectively. Force-time history, Force-displacement behaviour and absorbed energy (E a ) due to elastic and/or plastic deformation of the laminate are reported. Minimum of three samples were tested at each energy level. 2.3.1. Visual observation As glass/epoxy laminates are translucent in nature it is possible to view the damage with naked eye with the help of back light reflectography (JN Baucom et al. 2006). After impact, test specimens were visually observed under normal laboratory conditions as well as under reflected light source in order to estimate the extent of damage and various failure modes at front and rear impact surfaces. Areas of fibre breakage and delamination regions at rear surface were separately measured and reported. Delamination regions were formed in the distorted circle shape. Hence the perimeter of this shape was measured and area was calculated as equivalent circle shape. 2.3.2. Infrared Thermography To have further insight information, damage analysis was carried out by using IR camera (Model: Therma CAM SC 3000) supplied by M/s. FLIR system AB. The equipment utilizes sterling cooled Quantum Well Infrared Photonic Detector (QWIP) which produces a resolution of 0.03 o C at 30 o C and a spectral response of 8-9µm. Specimens were heated for 5ms using two flash lamps mounted horizontally on each side of the IR camera with focal plane array pixel format of 320x240. Fig.1 shows the schematic view of IR thermography. The flash lamp impinges 9.6 kJ energy on specimen surface. The image acquisition was automatically executed by the system at a frame rate of 12.5 Hz for all the samples, immediately after pulse heating the sample. Further the acquired data was processed by using thermal wave imaging software with thermographic signal reconstruction (TSR) technique. 2.3. Impact damage characterization

Fig. 1. Schematic view of IR thermography

3. Results & Discussion 3.1. Force – displacement behaviour at different temperatures

Fig.2(a), Fig.2(b) Fig.2(c) and Fig. 2(d) illustrates Force- displacement curves of 3mm, 5mm, 7mm and 10mm thick laminates respectively impacted at three different impact energies. Closed loop and displacement in the negative direction indicates partial penetration and rebound of impactor. Oscillations in the curves represent the damage of the laminates either by matrix crack or fibre breakage. From the figure it is clear that, impactor has rebounded for all the thickness laminates at 50J energy. But close observation of curves indicates that, number of