A study was undertaken to investigate the loading rate effect on delamination fracture in itiation toughness of a thermoplastic composite. For this purpose, double cantilever beam specimens of graphite/PEEK were tested in a displacement controlled mode using an Instron tensile machine. Specimens were loaded at various crosshead speeds ranging from 0.05 cm/min to 100 cm/min. The interlaminar fracture toughness was found to decrease with increasing loading rate, and this decrease was up to 65 percent over the five decades of loading rate employed.

Schapery's nonlinear formulation for viscoelastic materials has been successfully used by many investigators at room temperature [1-3]. Little work has been presented in the literature where the above approach is applied to viscoelastic materials at elevated temperature. In the present work, Schapery's constitutive equation is used to study the nonlinear viscoelastic creep response of neat and carbon fiber-reinforced Polyether-etherketone (PEEK) and epoxy resin at different temperatures. As reinforced materials, the laminates [904] s and [±454] s were investigated. Series of 10-hour isothermal tensile creep tests were conducted on each laminate at four temperatures (up to 140°C for the epoxy system and up to 120°C for the PEEK system) and different stress levels. For comparison reasons the same type of experiments was conducted on the respective neat polymers. Schapery's approach was used to characterize the nonlinear viscoelastic response of the above materials. The stress and temperature dependence of the nonlinearity factors was evaluated using a numerical procedure based on least squares techniques. The results show that the linear viscoelastic limit is shifted to lower values with increasing temperature. This was observed for both neat polymers as well as for the [±454] s laminates investigated. On the other hand, for the [904] s laminates the influence of the temperature on the linear viscoelastic limit seems to be relatively restricted. Moreover, for all resins and laminates studied it is shown that the influence of temperature on the nonlinearity of the instantaneous material response is significantly lower than that on the transient nonlinearity. For the investigated temperature range it can therefore be assumed that the instantaneous creep response is linear and independent of temperature over a stress range relevant in practical applications. On the other hand, the influence of temperature on the transient creep response was found to be nonlinear. The transient creep response of the composite materials subjected to intralaminar shear stress showed higher temperature sensitivity than that under normal stress.

The paper establishes the mechanical properties of a viscoelastic composite material reinforced with fibers, where the fiber is transverse isotropic and the matrix is isotropic (a common case met in engineering practice). A computation method using the Mori–Tanaka mean field method has been developed in order to apply on viscoelastic materials. Using this procedure, the time-dependent response of a viscoelastic composite material can be determined. Schapery’s nonlinear constitutive equation is also used in the compliance matrix determination of the composite material under investigation. Nonlinearity factors were determined by creep tests at different values of stresses and temperatures and for different materials, based on the least squares method. The results obtained experimentally and their comparison with the theoretically obtained values show a good agreement between experiment and calculation.

Usually, a polymer composite with a viscoelastic response matrix has a creep behavior. To predict this phenomenon, a good knowledge of the properties and mechanical constants of the material becomes important. Schapery’s equation represents a basic relation to study the nonlinear viscoelastic creep behavior of composite reinforced with carbon fiber (matrix made by polyethrtethrtketone (PEEK) and epoxy resin). The finite element method (FEM) is a classic, well known and powerful tool to determine the overall engineering constants. The method is applied to a fiber one-directional composite for two different applications: carbon fibers T800 reinforcing an epoxy matrix Fibredux 6376C and carbon fibers of the type IM6 reinforcing a thermoplastic material APC2. More cases have been considered. The experimental results provide a validation of the proposed method and a good agreement between theoretical and experimental results.

The homogenization theory, used for the study of differential equations with periodic coefficients, with a rapid variation, is used in the paper for the analysis of the creep phenomenon of composite materials, reinforced with fibers. Generally, a polymer composite having a matrix with a viscoelastic response manifests a creep behavior. A good knowledge of mechanical constants allows us to predict the time response under the action of a load, which is important in engineering. The homogenization method is used to determine the engineering constants for a composite reinforced with carbon fibers. The method is applied for the particular case of fiber-reinforced unidirectional composites to obtain the equations that finally offer the required values. The epoxy matrix Fibredux 6376C is reinforced with carbon fibers T800 and the thermoplastic specimens made by APC2 material is reinforced with carbon fibers of the type IM6. The experimental results give a good concordance with the theoretical predictions.