
MicromechanicsUnidirectionalInplane Shear Strength
The inplane shear strength refers to the strength of a composite lamina when subjected to shear stress that causes a shear deformation observable on the surface of the composite plate/shell. The inplane shear strength of a unidirectional lamina can be measured following one of several standard test procedures, which are mentioned in the textbook. However, when a lamina is part of a laminate, the inplane shear strength appears to increase just like the transverse tensile strength does. Because of this, the inplane shear strength of a lamina embedded in a laminate is called insitu shear strength. The value of insitu strength grows proportionally to the inverse of the square root of the thickness of the lamina; the thinner the lamina, the higher the insitu strength. Obviously, inplane shear strength is not an invariant material property if it's value increases inversely to the square root of the lamina thickness. As it is explained in the textbook, the invariant material property that controls the inplane shear strength is the fracture toughness of the material, from which the insitu strength can be predicted. Micromechanics formulas based on strength of the fiber and matrix are not accurate to predict inplane shear strength. This is due to the fact that this is not an invariant property. Instead, the value should be predicted from the value of the fracture toughness, which is invariant.
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