0 50 100 150 200 250 300 350 100 120 140 160 180 200 Damage area, mm2 Time, µ sec 0 deg 15 deg 30 deg 45 deg 0 50 100 150 200 100 120 140 160 180 200 Damage area, mm2 Time, µ sec 0 deg 30 deg 45 deg (a) (b) Figure 3: Propagation of damage zone during the observation period (a) single layer (Vf = 2:4%) and (b) two layer (Vf =5:3%) composites Figures 3a and b show the variation of damage area with respect to time for the composites specimens studied.Table1alsopresentsthedamagegrowthratecalculatedfromtheabove gures.Fromthe gures and table, it is seen that for single layer composites, irrespective of the initial notch orientation, the damage zone grows at a constant rate. On the other hand, the damage has to overcome theresistancebytheincreased berandsothedamagezonegrowsataslowerrateduringtheend of the observation period in two layer composite specimens. Damage in Chopped Strand Mat Composites Figures4and5showsdamagesequencesinshort bercompositeswith Vf = 10% and 0 Æ notch. It is noted that the overall damage is perpendicular to the loading direction as observed in woven cloth composite specimens. In mat composites, the damage zone is formed due to multi-directional micro-cracksinthematrix,attheinterfaceanddueto berfracture.Thedamagezoneinitiates at the crack tip and grow perpendicular to the loading direction in a narrow band. Damage zone splitting, as seen in woven fabric composites, is not observed in mat composites. However, secondary damage, away from the notch tip and independent of main damage, has been noted. Some time this secondary damage is from the corner of the `V' notch (Figure5). The energy absorbing mechanisms inmatcompositesarethematrixcracksandinterfacedebondingattheendsoftheshort bers.As the stress waves passes over, the dynamic load increases, which leads to an increase in the stress intensity around the notch. This raise in stress level leads to the concentration of stresses at the endsofthe bers,ultimatelyleadingtotheinitiationofthedebonding. Table 1 also gives the the velocity of damage in chopped strand composites. It is observed that the initial propagation velocities are in the range of 250 500 m=sec. These damage velocities are higher than the crack velocity of 240 m=sec for glass-polyester composite model [3]. It is observed that in all the damage propagation velocity reduces and gets arrested. This reduction in velocity is analogous to the reduction in crack velocity with decrease in stress intensity factor in Homalite-100. Itmaybepointedoutherethattheabovede nitionofdamagepropagationvelocitymayprovidea method for analyzing the results of dynamic fracture experiments on composite materials. However, thedi erencebetweenthisdamagepropagationvelocityandthecrackvelocityinhomogeneous materials should be recognized and properly accounted for in developing the analysis procedures and interpreting results. Actual damage in composites consists of numerous 3D micro damages intheformof ber{matrixinterfacedebonding, ber,matrixcracks,etc.,whichisquitedi erent from through-the-thickness cracks. On the other hand, the Cranz-Schardin camera records the 2D
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