PSI - Issue 14

P Rama Subba Reddy et al. / Procedia Structural Integrity 14 (2019) 676–683 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Damage area at different depth levels is calculated with the help of software provided along with the instrument and is given in Fig.6. In the case of 10 mm thick laminates, at projectile entry stage (i.e. 1-2 mm depth level) the damage area is found to be 50 mm 2 and 200 mm 2 for E-glass/epoxy and E-glass/phenolic laminates respectively. At 5mm depth level, the damage area is increased to 170mm 2 and 300 mm 2 respectively and further increase of depth there is no change in damage area. This may be due to limited interaction time between the laminate and the projectile. Hence resistance provided by the laminate may be limited due to the lower thickness. In the case of higher thickness laminate (i.e. 25mm) damage area has increased significantly with increase of depth level (i.e. from entry side of the projectile to the exit side). Damage area has increased upto 15mm depth level and became constant for both the laminates. The reason for more damage area at higher thickness is due to increased resistance offered by the laminate against the projectile penetration. At higher thickness also, E-glass/phenolic has shown more damage area than E-glass/epoxy. At lower thickness, laminates fail primarily through shear cutting of fibers rather than stretching and delamination. This is because under ballistic impact velocities, lower thickness laminates does not have sufficient time to propagate or dissipate the stress waves transversely and longitudinally across the laminate which promotes for delamination and fiber stretching. Whereas at higher thickness, laminate-projectile interaction time will be more relatively, therefore the laminate can undergo stretching and delamination in addition to the shear cutting of fibers due to the propagation of stress waves across the layers. The extent of stretching and delamiantion is depends on the matrix property and fiber/ matrix interface properties. Generally weak interface bond between the fiber and matrix helps for progressive delamination than the strong interface bond. The same is also confirmed through mechanical properties (Table.1, Fig.2) and DMA analysis.

Fig.6. Damage area at different depth levels (a) 10mm thick and (b) 25mm thick for E-glass/epoxy and E-glass/phenolic composite laminates

4.2. Failure behaviour at ballistic limit

Fig.7. Failure behaviour laminates at ballistic limit