Study of the carbon fiber–Poly(Ether–Ether–Ketone) (PEEK) interfaces, 3: influence and properties of interphases

Abstract

The aim of this third part is to analyze the structure and properties of the interfacial region between carbon fibers and PEEK as a function of different thermal conditioning treatments. First, it is shown by means of optical microscopy that the interfacial zone is not different from the bulk matrix when standard cooling conditions are used. On the contrary, a transcrystalline interphase is formed near the carbon fiber surface in systems that have been subjected to isothermal treatments. By comparison with previous results concerning the mechanical properties of the fiber–matrix interface, it appears that the interfacial shear strength decreases in the presence of a transcrystalline interphase or when the crystallization rate of PEEK increases. Moreover, it seems that the “constraint state” of the amorphous phase of PEEK near the fiber surface could also play a role in the interfacial shear strength. Secondly, a method is proposed in order to estimate the elastic modulus of crystalline interphases. It seems that this modulus is strongly dependent on the crystallization rate of the polymer. Finally, the determination of the stress‐free temperature, defined as the temperature at which a longitudinal compressive stress just appears on the carbon fiber during the processing of the composites, is performed by recording the acoustic events corresponding to the fragmentation process in single‐fiber composites. The results confirm that the crystallization rate and the “constraint state” of the amorphous phase of the matrix play an important role in the mechanical behavior of carbon fiber–PEEK interfaces.

Study of the carbon fiber–Poly(Ether–Ether–Ketone) (PEEK) interfaces, 4: influence of fiber–matrix adhesion on the mechanical properties of unidirectional composites

“…Similar observations have been made on all the other failure propagates cohesively within the PEEK polymer. It must also be noted that, in the previous paper [3], both the orientation and the "constraint state" of the polymer chains near the fiber surface were directly related to the reversible work of adhesion, such phenomena being consistent with the approach of Carr6 and Schultz [9] and the assumption of a " molecular factor".…”

“…For unidirectional composites, these mechanical properties are mainly determined by the failure strain Ef of the carbon fibers. It has been seen in the previous paper [3] that Ef for T300 and AS4 are equal to 1.5% and 1.8% respectively. As a consequence, the measured values of tensile strength and strain of T300-based composites (Hoechst types 1 and 2) are smaller than for all the other materials involving AS4 fibers.…”

“…In order to correlate the mechanical properties of the unidirectional composites with the magnitude of physical interactions at the fiber-matrix interface, it is first necessary to define the values of the reversible energy of adhesion W corresponding to the different systems analyzed in this study. First, for the sake of clarity, only the values of W calculated from the surface properties of PEEK fibers [l] will be considered in this paper, since it appears [3] that these values seem to be the most appropriate for the description of all of the interfacial phenomena in single-carbon fiber-PEEK composites. Secondly, the surface properties of T300 (sized and then washed by HC1 solution) and AS4 carbon fibers involved, respectively, in Hoechst and BASF preforms were previously determined [l].…”

Materials for Construction and Civil Engineering

“…Although this theory does not help to understand how and why adhesion is produced, it is useful to understand why a bonded joint may not behave appropriately. If any, these weak boundary layers may originate in the substrate or in the adhesive, and may be formed before bonding, during application of the adhesive, during solidification or in service [2,14,22,43,[82][83][84][85][86][87][88].…”