Journal of Composite Materials

The paper describes a method for modelling progressive mixed-mode delamination in fibre composites. The procedure, which is incorporated within the non-linear finite element method, is based on the use of interface elements in conjunction with softening relationships between the stresses and the relative displacements. Fracture mechanics is indirectly introduced by relating the areas under the stress/displacement curves to the critical fracture energies.

The successful manufacture of thick-sectioned composites is challenging, since the highly exothermic nature of thermoset resins and limited temperature control make it difficult to avoid detrimental thermal and cure gradients within the composite. In order to make quality parts, it has been found experimentally that cure temperatures must be lowered as much as 50% from those suggested for thin parts. Differential Scanning Calorimetry (DSC) experiments of a vinyl-ester resin system at these lower temperatures revealed a significant dependence on temperature for the maximum extent of cure. If the resin is cured isothermally at 55°C, the final conversion of the resin was found to only reach 70%. When the maximum extent of cure parameter was incorporated into an empirical autocatalytic kinetic model, it was found to significantly improve the description of the cure kinetics. Inhibitors, added to the resin to improve shelf-life, disappear rapidly at higher cure temperatures but can double the time required to cure a thick composite processed at 55°C. A zeroth order kinetic relationship was developed to estimate the amount of inhibitor in the system during the resin's cure. The inhibitor relationship and the improved kinetic model were used in a finite difference cure simulation to successfully predict the thermal gradients during cure of a 2.54 cm thick composite manufactured by resin transfer molding (RTM).

The viscoelastic composites are being more widely used in the industry. While remaining light materials with good mechanical stiffness characteristics, the viscoelastic composite materials provide interesting absorption properties and can reduce the vibratory disturbances.

The paper is concerned with the behavior of a viscoelastic composite incorporating isotropic viscoelastic balls embedded in isotropic resin subjected to harmonic vibrations. The wavelength considered is large compared to one period of the medium, and therefore, the homogenization of the medium is substantiated allow to replace the highly heterogeneous material by a homogeneous material whose behavior depends on the mechanical properties and geometry of the various components. The homogenized coefficients are obtained by resolving elementary problems in which only complex quantities are present. These coefficients are numerically computed using a finite element method in 3-dimensional space of complex numbers.

Two examples are proposed allowing to trace the change in the various coefficients which are homogenized as a function of the pulse. These coefficients are only angular dependent frequencies and are approximated using a law of n parameters. This enables an important reduction in computational time.

Macro- and micro-observations of unidirectional thermoplastic composites under end-loaded quasi-static bending have been made. The presence of stable microbuckles has been identified on the compressive side of the specimen. The formation of the microbuckles has been videotaped. Frames of this film are examined and the failure process is identified. The stress rupture of these specimens at elevated temperatures in bending seems to be driven by the formation and propagation of microbuckles. Videotaping from a different angle also gives information on the change of shape of the bent specimen in the oven as a function of time. These unique data form the basis for an understanding of the stress rupture of unidirectional thermoplastic composites in bending that was not previously available.

Two phase particle reinforced heat conducting composites are considered. We treat the case when there is an interfacial thermal barrier between phases. We provide novel rules of thumb for selecting the particle size distribution and minimum particle size necessary for constructing composites with effective properties greater than that of the matrix. The rules are based on new energy dissipation inequalities obtained in the work of Lipton (1996, Journal of Applied Physics, 80:5583-5586).