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• W2740726650 abstract "Impact damage may significantly reduce the tensile and compressive strength of composite laminates. Elementary models for prediction of strength after impact replace the damage zone by a hole [1], while other models have assumed a soft inclusion [2-4]. The main deficiency of the latter approach is the lack of knowledge of the stiffness of impact damage zones. Tests on coupons cut from the damage region have indicated significant stiffness reductions [5] but such tests are destructive and may be misleading, due to stiffness reductions and premature failure caused by the free edges. This effect is particularly evident in compressive buckling, but is also present in tension. Full field optical methods have earlier been applied to determine the stiffness of uniform soft inclusions [6]. In this paper such methods are used in an initial study of impact damage [7]. Quasi-isotropic HSC/SE84LV carbon/epoxy laminates of the dimension 100 x 150 mm simply supported on a 75 x 130 mm frame were impacted by a drop weight impactor with 6.35 mm tup radius. Impact energies of 5, 10 and 20 J were used on 2.5 mm laminates, and 10 and 28 J on 4.8 mm laminates. The resulting damage was described by use of ultrasonic C-scan for detection of delaminations and by deplying for detection of fibre damage. The impacted specimens were tested in tension and compression using knife edge supports along the unloaded edges. Strain and displacement fields were measured by use of full field digital speckle photogrammetry (DSP) with an Aramis 3D system from GOM, using a second pair of cameras to monitor the back face in compression. The damage consisted of an approximately circular region with delaminations and a central zone with fibre damage, with a diameter of less than half of the delaminated area. For the lowest impact energy (5 J) no fibre damage was observed. The magnitude of in-plane strain concentrations is closely linked to the in-plane stiffness gradients in the laminate. Uniform inclusions produce a discrete stiffness change and a more severe strain concentration than regions with gradually varying stiffness. Thus, the local stiffness in a region with gradually decreasing stiffness must be lower than the stiffness of a uniform soft inclusion to produce the same strain concentration. An upper bound for Young’s modulus in a circular damage region is obtained by considering the strain concentration factor [6] and the finite width correction factor for a uniform inclusion suggested in [2]. Figure 1 shows strain distributions over the width in tension, and demonstrates that strain concentrations are closely linked to fibre damage. Assuming uniform properties in the fibre damage zone, its tensile modulus would be about 50% of the undamaged material for the cases where strain concentrations were observed. Figure 2 shows strain distributions over the width in compression. In spite of edge supports and moderate specimen widths compression invariably caused some buckling and measurements from both faces of the specimen were required to separate bending and membrane strains. The membrane strains and bending strains are of the same order and remain at a relatively constant ratio. Significant strain peaks are observed in the region with fibre fracture. The membrane strain concentrations in the first two cases would require a compressive modulus below zero, which is unphysical. The relative modulus in the 4.8 mm 28 J case would be 0.3 if considering the delamination zone and 0.12 if considering the fibre damage zone, which is too low to be explained by the reduced in-plane stiffness, as the effect of fibre fracture should be larger in tension. This initial study demonstrates that impact damage in tension causes fairly moderate strain concentrations confined to the small central zone with fibre damage. Thus, designs replacing the entire delaminated region by an open hole are overly conservative. Strain concentrations under compression cannot be explained by reduced in-plane stiffness and are clearly induced by buckling, which seems to be enhanced in the region with fibre fracture. Thus, predictive models assuming a “notch type” in-plane compressive failure appear to be incorrect. Earlier experiments have validated the theoretical prediction of a uniform strain field in uniform inclusions [6]. The significant strain variations within the current damage zones demonstrate that assumptions of a uniform soft inclusion are inapplicable, and further work is planned to study the spatial stiffness variation in the damage zone." @default.
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• W2740726650 date "2006-01-01" @default.
• W2740726650 modified "2023-09-26" @default.
• W2740726650 title "INITIAL STUDY OF IMPACT DAMAGE STIFFNESS BY FULL FIELD OPTICAL METHOD" @default.
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