Abstract
Difficulties in obtaining satisfactory interface bonds in photoelastic models of fibre reinforced materials have focussed attention upon the importance of interface bond strength in determining the failure characteristics of a composite material. In order to simulate the complex stress conditions which exist at the interface in both two and three dimensional models, an apparatus was constructed in which cemented joints were tested to failure under predetermined combinations of tensile and shearing stresses, The strengths of a limited number of adhesives were investigated by making butt joints between the ends of cylindrical test pieces of dural and epoxy resin, these being the materials used to simulate the fibre and matrix in the model material. Initially a high scatter was observed in the results of strength tests and was only reduced to an acceptable level by adopting a chemical cleansing process for the bonding surfaces of the dural test pieces. The liability of unidirectionally reinforced composites to failure by the propagation of fractures in planes containing the fibre axes has drawn the attention to the study of stress concentrations associated with the diffusion of load around an array of high modulus circular inclusions embedded in an elastic matrix. As a preliminary to the array arrangements, the stresses around inclusion were investigated using both unbonded and bonded interference fit plug configurations. The elastic stress distribution associated with the differential thermal shrinkage may be shown to be exactly equivalent to that produced by a suitable initial interference between the fibre and the matrix. This equality has been used in fabricating two dimensional photoelastic models of a regular array of circular inclusions. The effect of initial shrinkage, and additional uniaxial and biaxial loads have been studied for a range of plug sizes representing the fibre volume fractions between 22 and 73%. Tests on cemented joints showed that chemical cleaning was required in order to obtain consistent strength values. An emperical curve defining the fracture envelope for joints subjected to combined tension and shear loading has been obtained. Tests on idealised two dimensional models indicate that the critical stresses produced in interference loading can be predicted by a simple theory. Additional tensile load in a plane perpendicular to the fibre axis induces interference slip. The actual load which initiate slip is proportional to the initial interference for a given combination of a fibre and matrix. material. The slip process is completed with a small increment in load. Subsequently the linear response to additional load is restored. Essentially similar behaviour was observed in models which incorporate bonded and unbonded inclusions.