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It is generally recognized that resistance to cutting consists of two aspects, one reflecting the intrinsic strength of the material, and the other a frictional contribution. In practice, cutting usually involves a normal force and a sliding movement. This work aims to analyze the effects of friction on cutting resistance of materials in the presence of a normal force and sliding movement of a sharp object. International Standard ISO 13997 is used as a reference to evaluate the cut resistance of some selected protective materials under practical conditions in service. The resistance to cutting of a material in the presence of both a normal force and sliding movement of a sharp object is strongly controlled by friction between the blade and the cut material. An increase in the friction coefficient can enhance or reduce the cut resistance, depending on the thickness, and the microstructure of the material to be cut. Thus the total energy required to propagate a cut strongly depends on the friction coefficient and consists of two components: The lost energy dissipated by the squeezing force exerted by the cut material, due to its elasticity, on the blade sides, and the essential cutting energy at the edge of the blade. These energies have opposite effects on the cutting resistance of materials. An increase in the energy dissipated in gripping frictional force increases the cut resistance, whereas an increase in friction at the blade’s edge reduces the energy required to cut the material.

In this work the effective properties of materials containing coated inclusions are analyzed. The analysis is based on a double-homogenization realized with the aid of Hill’s interfacial operators, Eshelby’s solution for ellipsoidal inclusion, and the Mori-Tanaka scheme. The system inclusion-coating is assumed to consist of two confocal spheroids. The proposed approach gives closed-form solutions and holds for both thin and thick coatings.

It is well known that the mechanical properties of thermoplastic composites depend upon processing parameters such as cooling rate, melting temperature, and residence time at the melt temperature. This is due to the change in matrix morphology, fiber/matrix interaction, and interphase structure with processing conditions. In previous works on carbon fiber/PEEK composites, it has been found that an increase in the melting temperature and the residence time improves the short-beam-shear strength of the composite, and increasing the cooling rate results in a decrease in this strength. Crystallinity at the interface appeared to be efficient only if there is a strong physico-chemical interaction between the matrix and the fiber. Moreover, transcrystallinity does not seem to be the primary factor responsible for a good fiber/matrix interaction in thermoplastic composite materials. The purpose of this work is to foster the understanding of the role of morphology in the bulk matrix and at the fiber/matrix interphase in the composite performance. The morphology of the matrix is observed by optical microscopy and compared with the analysis results from differential scanning calorimetry (DSC). The composite performance is investigated by Mode-I and Mode-Il delamination tests. The results show that the transcrystalline region strongly depends on the molding parameters and controls the fiber/matrix interfacial strength in the composite. A better-defined spherulite structure is observed with specific molding temperature and residence time at melt temperature. The cooling rate also strongly affects the fiber/matrix interaction and the transcrystalline phase. The Mode-I delamination performance is controlled mainly by the matrix morphology, whereas the Mode-Il delamination resistance is related to the fiber/matrix interfacial strength and the interphase structure in the composite.

The non-isothermal crystallization kinetics of PEEK in the presence of carbon fibers are investigated for PEEK/carbon composites made from both prepreg and commingled systems. The crystallization kinetics under non-isothermal conditions are studied over a wide range of cooling rates, varying from 0.5°C/min to 160'C/min and compared with the pure PEEK matrix resin. For the composite materials as well as for the PEEK resin, the mechanism of crystallization is found to undergo two transitions, at about 5°C/min and 40'C/min. While the presence of carbon fibers affects the temperature and the rate of crystallization of the PEEK matrix, it does not seem to alter the basic crystallization mechanisms. The results are in agreement with some reported morphological studies on PEEK/carbon composites, suggesting that the transcrystalline structure resulting from epitaxial growth of the PEEK matrix at the carbon fiber surface constitutes a small element in the global crystallinity of the PEEK matrix in the molded composite.

Effects of remolding on mechanical performance are particularly important in post-processing of thermoplastic resin composites. The effects of molding and remold ing temperatures on the Mode-I delamination toughness of APC-2 composites are in vestigated experimentally. It is found that both the initiation and the propagation energies of delamination are influenced by the remolding process, and the effect depends on the previous molding history. In particular, the extent of fiber bridging strongly depends on molding and remolding conditions. Enhanced fiber bridging results in a rising R-curve and unstable crack propagation. The observed behaviors are interpreted from the viewpoint of matrix crystalline morphology.

This paper deals with the fiber/matrix interface in carbon /PEEK compos ites. It is found that although interphase crystallinity is essential to ensure good interfacial strength in carbon /PEEK composites, it is not the primary factor responsible for the crea tion of fiber/matrix interaction in this composite. The results suggest that such interaction is a result of several complex mechanisms such as matrix adsorption on the fiber surface, fiber/matrix reaction leading to chemical bonding, and crystallization of the interphase. The concept of adsorption proposed by Brady and Porter for the carbon/polycarbonate system seems to be valid for interpreting the evolution of the short beam shear strength with residence time, as described by a Langmuir-type expression. The temperature de pendence of the formation of the fiber/matrix interaction, as measured by the short beam shear strength, is also found to follow an Arrhenius equation with an activation energy of about 324 kJ mol-1. The high value of the activation energy and the irreversibility of the interaction process suggest that chemisorption could be a dominant mechanism, since, in the range of processing temperatures for carbon / PEEK composites, there is effective modification of the matrix (chain scission and crosslinking) as well as thermal degradation leading to reactive species.

Although the processability of PEEK/carbon prepreg has received much attention, the effects of processing on the microstructure and performance of the co mingled fabric system is not well known. In this paper, the effect of molding temperature, molding time and cooling rate on the mechanical performance of PEEK/carbon compos ites made from both prepreg and comingled forms are discussed. To obtain good mechanical properties of PEEK/carbon fiber composites, processing must be carried out at a temperature sufficiently high to destroy the previous thermal history of the PEEK matrix. While the fiber/matrix interaction in the prepreg system does not seem to be af fected by the thermal processing conditions, this interaction in the comingled system must be created during the molding process. Fiber/matrix adhesion in the comingled system depends on the molding temperature, residence time at melt temperature as well as cooling rate. This is probably a result of several complex mechanisms such as matrix adsorption on the fiber surface, matrix degradation leading to chemical bonding and interfacial crystallization.