Cracked composite laminates least prone to delamination
- 8 January 1994
- journal article
- Published by The Royal Society in Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences
- Vol. 444 (1920), 17-35
- https://doi.org/10.1098/rspa.1994.0002
Abstract
In [($\pm \theta $)$_{n_{2}}$/(90 degrees)$_{n_{1}}$/($\mp \theta $)$_{n_{2}}$] fibre-reinforced composites, the outer ($\pm \theta $)$_{n_{2}}$ sublaminates are known to act as crack arrestors, i.e. to reduce the stress intensity factors at the tips of a crack in the central (90 degrees)$_{n_{1}}$ layer in all three modes of loading. The degree of reduction depends on the stiffness of the plies, the ply angle $\theta $ and the thickness of the outer sublaminates. However, although the stress intensity factor decreases, the crack-induced largest interfacial principal tensile stress increases. The situation is particularly severe under transverse mode II loading, inevitably resulting in interfacial delamination. The aim of this paper is to choose the design variables of the laminate, namely the ply angle $\theta $, relative ply stiffness and thickness, in such a way as to minimize the stress intensity factor at a crack tip in the (90 degrees)$_{n_{1}}$ layer without exceeding the interfacial bond strength. A constraint is also placed on the minimum flexural stiffness of the laminate. An alternative optimization problem in which the largest interfacial principal tensile stress is minimized subject to a limit on the stress intensity factor is also formulated and solved. The above optimization problem is solved in two stages. First, an analysis of the non-homogeneous, anisotropic medium containing a flaw is performed within the bounds of the classical lamination theory and fracture mechanics. This allows development of mathematical expressions relating the stress intensity factor in the central (90 degrees)$_{n_{1}}$ layer and the interfacial tensile stress to the sublaminate thickness and stiffness and the ply angle $\theta $. These expressions are highly complicated, precluding a completely analytical approach to the calculation of design sensitivities. In the next stage the optimization problem is reduced to a nonlinear mathematical programming one, whose solution is attempted by several techniques. The sensitivities with respect to the design variables required in these techniques are calculated by a mixed analytical/numerical approach.
Keywords
This publication has 21 references indexed in Scilit:
- Elimination/minimization of edge-induced stress singularities in fiber composite laminatesInternational Journal of Solids and Structures, 1992
- Determination of Interlaminar Stresses in Composite Laminates under Combined LoadsJournal of Reinforced Plastics and Composites, 1990
- Three-Dimensional Analysis of Combined Free-Edge and Transverse-Crack-Tip DelaminationPublished by ASTM International ,1990
- Stiffness reduction analysis of cracked cross-ply laminates by using integral equation approachEngineering Fracture Mechanics, 1989
- Experimental Observations of Free-Edge DelaminationPublished by Elsevier ,1989
- Structural optimization: A new dual method using mixed variablesInternational Journal for Numerical Methods in Engineering, 1986
- Experimental Determination of the In Situ Transverse Lamina Strength in Graphite/Epoxy LaminatesJournal of Composite Materials, 1982
- The Dependence of Transverse Cracking and Delamination on Ply Thickness in Graphite/Epoxy LaminatesPublished by ASTM International ,1982
- On the transverse cracking and longitudinal splitting behaviour of glass and carbon fibre reinforced epoxy cross ply laminates and the effect of Poisson and thermally generated strainProceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 1979
- Crack problem of transversely isotropic stripInternational Journal of Engineering Science, 1973