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Computer Aided Design Environment for Composites (CADEC) 14.1.6392.14245

Software Support for Introduction to Composite Materials Design-- 2nd Edition, CRC (2011) by Ever J. Barbero

Micromechanics-Fracture Toughness GIIc

The fracture toughness GIIc is a material property of the lamina. Mode II means shearing of the cracks. When the calculated Energy Release Rate (ERR) in mode II, namely GII, reaches the critical value GIIc, a shear crack appears. This fracture-mechanics criterion is used to predict damage initiation and accumulation. Since the fracture toughness GIIc is a material property of the lamina, it's value should be measured experimentally and entered as a material property for the lamina, in My Documents, My Laminas. Lacking experimental data for fracture toughness, one can calculate it from known experimental data for in-plane shear strength of a unidirectional lamina, by using the concept of transition thickness. As it is explained in the textbook, the in-situ shear strength decreases with the square root of ply thickness. For a certain ply thickness, called transition thickness, the in-situ strength does not decrease any more, but it becomes constant. A lamina thicker than this is caller a thick lamina; thinner than this is called a thin lamina. As it is explained in the textbook, the transition thickness can be measured by testing laminates with different ply thickness. It happens that the transition thickness is more or less constant for each material system. For glass-epoxy it is about 0.6 mm. For carbon-epoxy it is about 0.8 mm. Knowing this, one can estimate the fracture toughness in terms of experimental values of transverse tensile strength for a unidirectional lamina.

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