Strength in notched and impact damaged laminates

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1989
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Virginia Polytechnic Institute and State University
Abstract

The strength of notched and impact damaged laminates was studied. The solution for a plate containing an elliptic opening and inclusion was used as given by Lekhnitskii. The solution for the infinite plate, combined with laminate analysis to determine the ply stresses, and the average stress criterion proposed by Whitney and Nuismer were used to predict the notched strength. However, unlike Whitney and Nuismer, the average stress criterion was used at the ply level.

The strength of off-axis unidirectional laminates was predicted by using a matrix oriented failure criterion applied at a critical point on the boundary of the hole. A good agreement between the experimental and predicted data was obtained. On the other hand, an attempt made to predict the notched strength of angle-ply laminates was not as successful. This is believed to be due to the different failure modes existing among different [± 𝛉]s laminates.

The controversy on whether the characteristic dimension is a material or geometric property, together with the belief that the physics of fracture of composites is better represented at the ply level, have motivated the author to seek an invariant equation which describes the dependence of the characteristic dimension, D₀. A quantitative approach to determine the characteristic dimension in the average stress criterion was proposed. A good agreement between experimental and predicted data was found. It was also found that contrary to prior claims, the value of D₀ does not depend on the diameter of the hole, when used at the ply level. Moreover, the strength of quasi-isotropic laminates loaded at an angle ϕ with respect to the material x-axis was also studied. Again, excellent agreement between experiment and predictions was shown.

The tensile strength after impact (TSAI) was investigated. An approach based on modeling the delaminated area as an elliptic inclusion was used. The difference between the compliances of the plate and the inclusion was assumed to be proportional to the ratio of the delaminated areas. At low impact energy, a reference area was used. The results obtained using this approach gave good agreement with the experimental data.

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