Application of localized hybrid methods of stress analysis to some problems in the mechanics of composites

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Virginia Tech


A new method of stress analysis which combines an experimental technique — moire interferometry, and a numerical method — finite element analysis, is presented. In this localized hybrid method, the displacement fields which the moire experiments provide in some local regions of interest are used as input data for finite element stress analyses.

Two important and controversial problems in the mechanics of composites are investigated using the localized hybrid method. One is a thermally loaded bimetal plate, and the other involves the Iosipescu shear specimen popularly used to determine the shear modulus and strength of a fiber reinforced composite. Before applying the localized hybrid analysis, the mechanics of the problems are discussed individually, through a numerical study and strength of materials analysis. Based on these fundamental studies, the localized hybrid method is applied to stress or strain analyses of moire experimental results, and special techniques of this method are developed to handle these data.

For the thermally loaded plate, several finite element models, simulating 2-D and 3-D mechanics, are used to assess the stress state at the interface near the free surface, and identify a boundary layer. It is shown that high gradients and stress turnaround are documented in stress component normal to the interface, along the surface line crossing the interface, and a boundary layer is identified in the small region near the interface around the free surfaces. These observations are also confirmed by the hybrid analysis of moire experimental results. However, additional variations of the localized hybrid method were needed to capture the three-dimensional nature of the problem. A comparison of numerical results with experimental data resolved an apparent anomaly between experiments and mechanics principles.

For the Iosipescu shear specimen, the 3-D mechanics associated with twisting is proposed for accounting for the inconsistent and variable results in the literatures. The results form the localized hybrid analysis indicate that uniformity and purity of shear stress state in the test section cannot be accomplished for each fiber orientation specimens; the 0° specimens suffer from a load proximity effect, the 90° specimens are affected by apparent in- and out-plane bending, and the 0°/90° specimens are midway between those.

Several variations of the localized hybrid method of stress analysis have been presented for three-dimensional problems in the mechanics of solids. It is showed that the approach developed not only provides a powerful and efficient technique for the reduction of experimental data, but also gives a good insight into the mechanics of the experimental observations.