Strength prediction of random fiber composite beams using a phenomenological-mechanistic model

A phenomenological-mechanistic model, referred to as a lamini model and reported in a previous paper for homogeneous stress states, is used to predict random fiber composite beam strength in this paper. This model divides a random fiber composite panel into a stack of thin transversely isotropic layers (laminas) with each lamina constructed from a series of unidirectional layers (laminis). As the number of laminis and laminas approach infinity, the convergence of the extensional stiffness matrix [A] and the bending stiffness matrix [D] is proved. These two stiffness matrices also satisfy the relationship established for isotropic materials, thus ensuring the transverse isotropy typical of random fiber composite plates (and beams). Due to the difficulty in determining lamini post-peak stress-strain behavior, in this paper only bounds on the damage growth and ultimate strength of a random fiber composite beam have been determined. The lower bound is obtained by assuming the stress falls to zero immediately after it reaches its peak value, i.e., zero residual strength in the failed layers. The upper bound is based on the assumption that the stress remains constant after the peak value is reached. This is analogous to a perfectly plastic material. Both bound predictions follow the four point bend load-strain experimental curves closely until the failure process begins to dominate with the lower bound predicting failure load approximately 39% below the experimental results while the upper bound predictions are approximately 37% higher. The model also quantitatively predicts the failure initiation and damage growth behavior in the composite beam which were observed in the previous high sensitivity laser Moire interferometry experiments

Fatigue Issues of Polymeric Foam Sandwich Core Materials in Simple Shear

Obtaining the fatigue properties of sandwich core materials is a particularly difficult issue. This work evaluates the ASTM C 394 test procedure for determining the fatigue properties of core materials using compression-compression loading. The stress concentration at the ends of the specimen where the specimen meets the fixture has been suggested to lead to premature failures. A geometric optimization of specimen ends was attempted to minimize the effects of the stress concentration. This resulted in removing a circular arc of material from the specimen ends. While this allowed the stress concentration to be shifted away from the corner interface of the specimen and fixture, it did not seem to positively influence the fatigue life for the load levels tested. Fatigue results obtained using the compression-compression loading compared well with data obtained using other test methods, including four-point bending. This suggested that the current ASTM C 394 test standard, when used with compression-compression loading, allows acceptable fatigue data to be obtained for foam core materials. © 2001, Sage Publications. All rights reserved

Damage growth investigation in a random fiber composite beam by Moire interferometry

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