School of Mechanical Engineering Seminar
Wednesday, November 2, 2022 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Fatigue delamination propagation in a multi-directional woven composite–mixed mode deformation

Snir Aizen

M.S.c student of Prof. Leslie Banks-Sills

School of Mechanical Engineering

Composite materials are used in a variety of industries including automotive, medical, and aerospace. Advantages of composite include high strength to weight and high toughness to weight ratios. Thus, lighter aircraft may be produced, for example, which reduces fuel consumption. In particular, composites are used as laminates which consist of a number of plies, either unidirectional or multi-directional. One of the common types of damage which occurs in a laminate structure is the separation of two plies which is called a delamination. This investigation focuses on a delamination between two plain woven carbon fiber-epoxy plies fabricated from the prepreg G0814/913. The upper ply of the interface has yarn in the 0°/90° - directions and the lower ply has yarn in the +45°/-45° - directions.

Nearly pure mode I and mixed-mode delamination propagation tests were carried out using double cantilever beam (DCB) and mixed mode end loaded split (MMELS) specimens, respectively. Both quasi-static and fatigue delamination propagation tests were performed.  The mode mixity, taken as the energy release rate of mode II, G_II, to the total energy release rate, G_T, was about 0.43 for the MMELS specimens. Delamination propagation resistance curves, for both mode I and one mixed mode ratio, G_iR, where i represents interface, were determined using the quasi-static test results and finite element analyses. Each ply in the finite element model was assumed to be linear elastic, anisotropic and homogenous. The mechanical properties of each ply were homogenized. The energy release rate, G, was calculated by means of the virtual crack closure technique (VCCT) using the finite element results. The fracture resistance toughness curves were compared with results from previous studies and showed good correlations.

The rate of delamination propagation in fatigue was found using the Paris equation and DCB beam specimens for nearly pure mode I with a cyclic displacement ratio R_d = 0.1. For a mode mixity of about 0.43, the rate of delamination propagation was found using MMELS beam specimens with R_d = 0.1 and 0.5. Using the fatigue data from the DCB and MMELS specimens, three master curves were obtained: a master curve for the DCB specimens, a master curve for the MMELS specimens, and a master curve for all the specimens together. It was found that for the DCB specimens, the slope of the Paris equation, as well as the delamination propagation rate, were the highest. Finally, the delamination propagation rate was back-calculated from the master curves for each specimen. Those values were then compared to the values calculated from the Paris equation for each specimen. The back-calculation was found to be more accurate when the master curve has only one mode mixity i.e. nearly pure mode I or mixed mode.

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