Acoustic emission signals originating from interlaminar crack propagation in fiber reinforced composites were recorded during double cantilever beam testing. The acoustic emission signals detected during testing were analyzed by feature based pattern recognition techniques. In previous studies it was demonstrated that the presented approach for detection of distinct types of acoustic emission signals is suitable. The subsequent correlation of distinct acoustic emission signal types to microscopic failure mechanisms is based on two procedures. Firstly, the frequency of occurrence of the distinct signal types is correlated to different specimens' fracture surface microstructure. Secondly, a comparison is made between experimental signals and signals resulting from finite element simulations based on a validated model for simulation of acoustic emission signals of typical failure mechanisms in fiber reinforced plastics. A distinction is made between fiber breakage, matrix cracking and interface failure. It is demonstrated, that the feature values extracted from simulated signals coincide well with those of experimental signals. As a result the applicability of the acoustic emission signal classification method for analysis of failure in carbon fiber and glass fiber reinforced plastics under mode-I loading conditions has been demonstrated. The quantification of matrix cracking, interfacial failure and fiber breakage was evaluated by interpretation of the obtained distributions of acoustic emission signals types in terms of fracture mechanics. The accumulated acoustic emission signal amplitudes show strong correlation to the mechanical properties of the specimens. Moreover, the changes in contribution to the different failure types explain the observed variation in failure behavior of the individual specimens quantitatively.

Piezoresitivity describes the change in electrical resistance of a conductor due to an applied strain. Previous studies showed that certain ex-PAN carbon fibres exhibit an excellent linear piezoresistive behaviour. Besides the desired strain sensitivity, k l , in the longitudinal direction, carbon fibres show a strain sensitivity, k t , transverse to the fibre direction. An experimental method was developed to empirically characterize these electromechanical material properties. The strain sensitivities k l and k t of the investigated ex-PAN fibre (Torayca T300B) were determined by varying the ratio between longitudinal strain, " l , and transverse strain, " t , in uniaxial tensile tests and four-point bending tests. The results show a longitudinal strain sensitivity k l in the range of 1.72-1.78 and a transverse strain sensitivity k t in the range of 0.37-0.41. This significant transverse strain sensitivity must be considered in the case of strain measurements with carbon fibre sensors. An approach for dealing with the high transverse strain sensitivity of carbon fibre sensors is proposed, which is appropriate for orthotropic as well as isotropic materials. The approach is based on transfer functions, which includes both strain sensitivities of the carbon fibre k l and k t . The transfer function of a biaxial carbon fibre sensors element with orthogonally aligned carbon fibre sensors is given and experimentally checked for validity. Furthermore, the transfer function of a right-angled triangle carbon fibre sensor element is given. These triangle carbon fibre sensor element allows the determination of the complete state of strain of a structure including the shear strain.

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.

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