Tensile strength of notched composite materials

A series of tensile strength tests were performed on a wide variety of composite materials. All tested materials contained some form of discontinuous fiber reinforcement; several also contained continuous reinforcement. Strength results are presented for unnotched material and for material with three sizes of ultrasonically machined center through-notches. The point stress theory is shown to give good agreement with experiment for both one- and two-parameter models, provided that the parameters are material constants and not universal constants. Linear elastic fracture mechanics (LEFM) is shown to provide poor predictions, with the critical stress intensity factor (fracture toughness) increasing with crack size. However, LEFM with a small-scale yielding or damage parameter is shown to give good predictions. There is some preference for the single-parameter damage-zone-corrected LEFM model over the two-parameter point stress model. In general, the fracture toughness increased with increasing proportion of continuous fiber reinforcement. A simple linear relationship between unnotched tensile strength and fracture toughness with a damage parameter is demonstrated, with a correlation coefficient of 99.2%.