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The interaction between intersecting phonon beams in liquid 4He has been studied as a function of pressure using superconducting A1 tunnel junction detectors. For pressures < 13 bar we detect high frequency phonons which are created by the annihilation of two lower frequency phonons. The critical frequency for this upconversion is well defined and is in good agreement with the value found from phonon decay via the 3 phonon process

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The use of fibres to improve the work-of-fracture and strength of brittle matrices has gained interest in a number of fields, especially in the cement and plaster industry, and also in h igh-temperatu re applications of ceramics and glasses. Here a theoretical analysis is presented to account for the enhanced strain-to-failure values of a brittle matrix containing microcracks and reinforced with fibres. The theory assumes that the Griffith energy criterion for failure applies when a crack is present in the matrix. It predicts the stabilizing effect of a unidirectional uniform distribution of fibres which bridge a matrix crack by computing the rates of release of strain energy and of absorption energy with increasing length of a matrix crack. Published experimental data on the carbon fibre reinforced glass system is used to compare the predictions of the theory here with a version given by Aveston, Cooper and Kelly. Descriptions of the boundary conditions of single and multiple fracture are given. The theory is shown to further indicate the possible existence of an upper limit to the fibre volume fraction at which multiple fracture occurs, i.e. when the fibres stabilizing the crack cannot support the load. Applications of the analysis to the design of composite systems of technological importance are suggested.