PSI - Issue 14

C.Jayarami Reddy et al. / Procedia Structural Integrity 14 (2019) 634–641 C.Jayarami Reddy / Structural Integrity Procedia 00 (2018) 000–000

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3.1 Pressure Measurements From Fig. 3(a) and 3(b), it is observed that the incident pressure profiles (P i1 , P i2 ) measured by both the pencil probes at their corresponding SoDs are similar in both pattern and magnitude, which indicates the spherical form of the wave generated by detonation of spherically shaped explosive. In both the figures, the sharp rise in pressure is due to the arrival of shock wave to the pencil probes as well as to the target plate. The fast drop in the pressure after reaching the peak value is due to the propagation of shock wave to the surroundings. From the figures, it is also observed that, as the SoD is reduced, the incident peak over pressure is marginally increasing. This can be attributed to the poor dissipation of shock energy within the small distance from explosive. The secondary peaks appeared in both the graphs may be due to the reflected wave which has travelled back after hitting the target plate and sensor plate to the pencil probes. From Table 2, maximum incident pressures recorded on all the target samples are around 9 bars. As mentioned in table, the maximum reflected pressures recorded at 400mm and 300mm SoD are around 76 bars and 94 bars, respectively. The reflected pressures are approximately 8 and 9.5 times higher than their corresponding incident pressures at 400mm and 300mm SoD. As explained by Gilbert F. Kinney and Graham (1985), the reflective pressures are 2 to 8 times higher than their incident pressures, depending on the incident pressure and angle of attack. The reflected pressures shown in Fig. 4(a) and 4(b) reached instantaneously to the peak and dropped to 20 bars immediately. This sudden fall in pressure is due to the rapid movement of shock wave from the plate or sensor position, which is due to the increased shock velocity of the reflected wave. The slow drop of pressure from 20 bars to the ambient pressure is due to the slow transfer of shock wave to the surroundings and approaching the equilibrium conditions. The two peak pattern observed in Fig. 4(b), at smaller SoD of 300mm is due to the severe vibration of sensor plate when impacted with the shock wave. 3.2 Deformation of target plates The deformation of the flat target plates after subjecting to the blast loads was investigated by observing the front and back side of the composite plates visually. The front and back side of the H-glass and E-glass composites subjected to 142 grams of explosive loading at 400mm SoD and 300mm SoD are shown in Fig.5 and Fig.6, respectively. The final displacements on front and back side of the composites were measured physically and tabulated in Table 2. From the table 2 and Fig. 5, it is observed that, at 400mm SoD, the final displacement of E glass composites on both sides is slightly lower than the H-glass/composites. This is due to the higher flexural rigidity or strength (properties in table 1) and less ductility of E-glass/composites than H-glass composites. However, due to their more rigidity, E-glass/epoxy composites exhibited severe matrix cracking and fibre breakage over the exposed area on the both sides. As the impact energy dissipates or absorbed through the thickness of the sample, the matrix cracking and the fibre breakage were found to be less on the back side of the panels. Due to the restraint offered by the clamping, more matrix cracking and fibre breakage were observed at boundary on both sides of the composites. Though the final displacements of H-glass composites are higher than their E-glass counterparts, no matrix cracking and fibre breakage were observed on both sides of exposed area. The reason for this behavior is the lower flexural rigidity or strength (properties in table 1) of H-glass composites, due to which the composite on impact of blast wave, offer no resistance and deforms to dissipate all the energy and leaving no energy for activation of matrix cracking and fibre breakage. As shown in Fig. 6 and table 2, as the SoD decreased from 400mm to 300mm, the final displacements of composites on both front and back sides have increased due to the higher loading pressures. Even at reduced SoD, H-glass composites did not exhibit any matrix cracking and fibre breakage. On the other hand, in E-glass composites, the reduction in the extent of matrix cracking and fibre breakage was observed at the reduced SoD. From this, it is inferred that, in E-glass/composites the higher energy of shock wave at reduced SoD, was consumed more in displacing the composite by overcoming its flexural rigidity. As not much energy was available for initiating the damage modes, the severity of matrix cracking and fibre breakage was reduced. However, the final displacements of H-glass composites are marginally higher than their E-glass counterparts.