Abstract
Fatigue crack growth tests were carried out on chopped strand mat polyester resin under biaxial tensile stress. Center-notch specimens were used to measure the effects of the load biaxiality factor, B, defined as the ratio of the load acting normally to the crack line and that acting parallel to it. A compliance calibration technique was used to measure the crack growth so as to overcome difficulties with visual measurement when using conventional stress-intensity factors in order to present the fatigue results on a fracture mechanics basis. An adaptation of the compliance technique was also used to extend the applicability of the stress-intensity factor concept to planar composite materials by proposing a K-equation that takes into account the effects of reinforcement geometry and biaxial stress. This allows a safe life crack growth criterion to be used more effectively to assess damage tolerance, taking account of a number of detrimental intrinsic effects due to the nature of the composite reinforcement. The Paris power relationship was found to be applicable to the results, the analysis of which shows that nonsingular stresses do affect the behavior of a crack subjected to plane stress cyclic loading. Biaxial stresses were found to produce a shift in the fatigue crack propagation rates, notably a decrease in the Paris exponent with increase in the load biaxiality factor, B. Analysis of the fatigue behavior indicates the failure mechanisms are influenced by the reinforcement geometry of the composite material, together with the Poisson's ratio of the material under biaxial stress. It would seem that the fatigue behavior is governed by the fiber-resin interface at low stress levels, while at higher stresses the Poisson's ratio of the composite determines the biaxial influence.
Published Version
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