Certain failures, such as resin bound shear cracks in fiber-reinforced materials, give rise to little change in ultrasonic attenuation or radiographic transmission but can result in significant reductions in material properties. A vibration technique is being evaluated as a nondestructive test tool under such conditions. The work described forms the first part of this evaluation program and concerns unidirectional carbon and glass fiber reinforced plastic subjected to static and dynamic torsional loading. The results and techniques will eventually be applied to complex composite structures.
In the static torsion test program, ‘good,’ void-free specimens failed at shear stresses of between 52 and 60 MN/m2 (carbon fiber) and between 56 and 62 MN/m2 (glass fiber) in a very brittle manner. The cracks thus generated led to substantial changes in the dynamic (vibration) test data. Specimens containing voids failed at lower shear stresses and less catastrophically.
In torsional fatigue, all failures occurred by the propagation of resin-bound shear cracks at shear stresses below the static strength of the material. The dynamic (vibration) test parameters showed permanent and progressive changes due to the accumulation of fatigue damage. Crack development was observed visually in the glass fiber specimens and broadly related to the changes in the dynamic properties.