Compression failure of angle-ply laminates
Peel, Larry D.
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The present work deals with modes and mechanisms of failure in compression of angleply laminates. Experimental results were obtained from 42 angle-ply IM7/8551-7a specimens with a lay-up of [ (±9) / (=F9)] 6s where 9, the off-axis angle, ranged from 0° to 90°. The results showed four failure modes, these modes being a function of off-axis angle. Failure modes include fiber compression, inplane transverse tension, inplane shear, and inplane transverse compression. Excessive interlaminar shear strain was also considered as an important mode of failure. At low off-axis angles, experimentally observed values were considerably lower than published strengths. It was determined that laminate imperfections in the form of layer waviness could be a major factor in reducing compression strength. Previously developed linear buckling and geometrically nonlinear theories were used, with modifications and enhancements, to examine the influence of layer waviness on compression response. The wavy layer is described by a wave amplitude and a wave length. Linear elastic stress-strain response is assumed. The geometrically nonlinear theory, in conjunction with the maximum stress failure criterion, was used to predict compression failure loads and failure modes for the angle-ply laminates. A range of wave lengths and amplitudes were used. It was found that for 0° S 9 S 15° failure was most likely due to fiber compression. For 15° < 9 oS 35° failure was most likely due to inplane transverse tension. For 35° < e ~ 70° failure was most likely due to inplane shear. For e > 70° failure was most likely due to inplane transverse compression. The fiber compression and transverse tension failure modes depended more heavily on wave length than on wave amplitude. Thus using a single parameter, such as a ratio of wave amplitude to wave length, to describe waviness in a laminate would be inaccurate. Throughout, results for AS4/3502, studied previously, are included for comparison. At low off-axis angles, the AS4/3502 material system was found to be less sensitive to layer waviness than IM7 /8551-7 a. Analytical predictions were also obtained for laminates with waviness in only some of the layers. For this type of waviness, laminate compressive strength could also be considered a function of which layers in the laminate were wavy, and where those wavy layers were. Overall, the geometrically nonlinear model correlates well with experimental results.
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