Micromechanical modeling of time-dependent transverse failure in composite systems

The time-dependent failure behaviour of transversely loadedcomposites is investigated, assuming that fracture is matrix dominated.Since the stress and strain state of the matrix in composite structuresis complex, the yield and fracture behaviour of a neat epoxy system isinvestigated under various multi-axial loading conditions. A gooddescription of the multi-axial yielding behaviour of the matrix materialis obtained with the three-dimensional pressure modified Eyringequation. The parameters of this three-dimensional yield expression areimplemented into a constitutive model, which has been shown to correctly describe the deformation behaviour of polymers under complex loadings.By means of a micromechanical approach, the matrix dominated transversestrength of a unidirectional composite material was investigated.Numerical simulations show that a failure criterion based on maximumstrain provides a good description for the rate-dependent transversestrength of unidirectional glass/epoxy composites. Furthermore, such astrain criterion is also able to describe the durability (creep) oftransversely loaded unidirectional composites

Measurement of the interfacial normal strength in single fibre transverse tensile tests

A technique is presented which can be used to measure the interfacial normal strength in transversely loaded composites. The technique combines the measurement of local deformations during tests in a Scanning Electron Microscope and numerical simulations, in which the measurements are used as boundary conditions. Deformations are measured by placing a grid of markers on the surface of single fibre specimens. The displacements of these markers are measured during loading of the specimen. Numerical simulations show that the radial and tangential interface stresses increase towards the specimen surface, causing the initiation of debonding at the surface. This effect is supported by tests under an optical microscope. The interfacial normal strength is defined to be the maximum radial interface stress just before the initiation of debonding. Based on the presented results, it can be concluded that the presented technique is a promising tool for the measurement of the interfacial normal stress

Mechanical properties of natural-fibre-mat-reinforced thermoplastics based on flax fibres and polypropylene.

Thermoplastic composites based on flax fibres and a polypropylene (PP) matrix were manufactured using (i) a film-stacking method based on random fibre mats and (ii) a paper making process based on chopped fibres. The influence of fibre length and fibre content on stiffness, strength and impact strength of these so-called natural-fibre-mat-reinforced thermoplastics (NMTs) is reported and compared with data for glass-mat-reinforced thermoplastics (GMTs), including the influence of the use of maleic-anhydride grafted PP for improved interfacial adhesion. In addition some preliminary data on the influence of fibre diameter on composite stiffness and strength is reported. The data is compared with the existing micro-mechanical models for strength and stiffness. A good agreement was found between theory and experiment in case of stiffness whereas in the case of strength the experimental values fall well below the theoretical predictions. Results indicated that NMTs are of interest for low-cost engineering applications and can compete with commercial GMTs when a high stiffness per unit weight is desirable. Results also indicated that future research towards significant improvements in tensile and impact strength of these types of composites should focus on the optimisation of fibre strength rather than interfacial bond strength

Temperature and molecular-weight dependence of the strain hardening behavior of polycarbonate

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