Mechanisms for Size Effects in Bending of Fiber Composites

The size of the specimen or structure has often been cited as having an effect on the apparent strength of fiber composites. Bend tests are often used to infer size effects, as the stressed volume is typically smaller than in uniform tension or compression test specimens. However bend tests also have a stress gradient through the thickness. In the present work a model is developed for compression failure of fiber composites that includes through-the-thickness effects. A primary feature of the model is that through-the-thickness shear stresses are predicted to occur because of the varying bending displacement of the wavy fibers. These shear stresses have a stabilizing effect on the fibers, and thus raise the apparent compressive strength. When applied to bend tests, the model predicts that the apparent compressive strength of fiber composites will depend on the specimen thickness, in accord with experimental evidence in the literature. The result is that through-the-thickness effects are predicted to affect the compressive strength of fiber composites, and this effect is distinct from a true size effect.