Analysis of strain in fibre-reinforced materials

Abstract

Techniques such as birefringent coatings, moiré, brittle lacquers, strain gauge, and holography can all be used on composites since they enable surface strains to be measured. However, a correct and accurate interpretation of results obtained by these methods is essential. The significance of this is demonstrated by observing the effect of the differences between the directions of principal stresses and strains. In addition the suitability of the birefringent-coating technique for application to fibre-composite materials is examined by considering the three-point flexure of a narrow rectangular beam of span/depth ratio 6 in some detail. Within the framework of the assumptions that stresses will be distributed in accordance with elementary bending theory and that the appropriate stress-strain relations for orthotropic materials are applicable, isochromatic and isoclinic patterns are predicted and compared with experimental results. A more precise solution is also obtained by employing the finite-element displacement method. From a qualitative standpoint the comparison is found to be good although more research is required before the birefringent-coating technique can be used in situations demanding a reliable quantitative appreciation.