Measurement of damage progress in fibre reinforced polymer materials

Prediction of damage progress based on measurements of the stiffness and damping changes has been suggested. A model is proposed based on experimental measurements of the rate of decrease of stiffness at given stress or strain levels. The fatigue damage was recorded by measuring the stiffness and the damping changes in the material under cyclic loading. A fatigue testing technique including full control of the test parameters was developed for online measurements of the initial values and the changes in the complex stiffness properties (elasticity and damping). The testing technique was based on a separate adaptive control of the test system using an A/D-D/A interface card in a personal computer setup as an external function generator to the test machine servo loop. The system is test machine independent and the setup and programmes can be used in any servo controlled test system. Further, by use of this control technique it is possible to increase the test frequency by a factor of 2-3 compared with the ability of a standard test system. The technique is described and its use is illustrated by tests on glasspolyester composites under fatigue loading.