Abstract
In this paper, high sensitivity laser moire interferometry was used for observation and analysis of in-situ damage initiation and growth in random fiber composite beams. The technique was found to be very effective. The general approach was to load the beams to successively increasing load levels, and then unload, observing the residual field. Damage showed up as anomalies in the fringe patterns. In the four-point-bend tests, it was found that observable microscopic failure initiated in the tension region first at about 40% of the ultimate failure load, while failure initiation in the compression region started at about 50% of the ultimate failure load. To determine the effect of this early and asymmetric damage growth in the tension and compression regions, the neutral axis shift of the beam was determined as a function of increasing load. This was done by reloading the beams (after observing the residual field) to a small load well below any damage threshold. It was found that very little shift occurs indicating the stiffness of the beam was largely unaffected by the microstructural damage over a major range of loading including close to failure. This unexpected behavior is most likely caused by progressive failure of the chopped fibers in orientations at or near 90° to the length of the beam.
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