Juan Carlos Figueroa

Interfacial studies on carbon/thermoplastic model composites using laser Raman spectroscopy

Interfacial studies were carried out on a model composite system consisting of a short carbon fibre embedded in a polycarbonate matrix. While the composite was being strained, the local strain along the fibre was monitored using a Raman spectroscopic technique. The residual compressive strain in the fibre due to fabrication was found to be −0.45%. Subsequent loading of the composite up to 0.55% in tension resulted in a complex stress field consisting of tension at the fibre ends and compression in the middle of the fibre. The fibre strain at different levels of applied load was converted to interfacial shear stress (ISS) distribution along the fibre by employing a simple equilibrium analysis. The shape of the ISS profiles indicated a predominantly frictional type of load transfer from the matrix to the fibre. Finally, the maximum ISS value of 15 MPa was found to be unaffected by the amount of strain experienced by the composite.

Micromechanics of single filament composites

Abstract The results presented are of work on carbon/thermoplastic systems aimed at identifying the relevant mechanisms of micromechanical interaction in single filament composites (SFC) and improving the understanding of the relative role of the strain and the fiber in these mechanisms. The experimental strategy has included the study of SFC specimens deformed in either axial or transverse tension, and the complementary nature of these testing modes is highlighted. Theoretical shortcomings of the Kelly-Tyson model, in its Weibull representation, are discussed on the basis of observed mechanisms for formation of saturated fragments. it is argued that failure conditions of bare fibers incompletely describe those of pre-saturated imbedded fragments. On this basis two procedures are proposed for estimating the interphase shear transmissibility which, as opposed to the Kelly-Tyson method, do not require information about the tensile strength of the filament. Interphase shear transmissibilities estimated by these methods are compared and differences are rationalized on the basis of the observed damage mechanisms. These mechanisms are discussed in terms of the interplay between matrix properties such as polymer chain mobility, toughness and stiffness and the shear transmissibility of the interphase. The short beam shear strength of J2/carbon composites correlates very well with interphase shear transmissibility estimated with SFCs. Methods are proposed for the used of filament fragmentation curves which

Interphase behaviour in graphite-thermoplastic monofilament composites: Part II cyclic behaviour

The fatigue properties of continuous filament composites are dependent on the fibre-matrix interphase behaviour, specifically the ability of the interphase to transfer load durably from the matrix to the fibre, and the energy dissipated in the interphase during crack growth. Although there has been a large effort to understand interphase behaviour in order to tailor the interphase for specific bulk composite properties, the basic role of the interphase in cyclic fatigue is not well understood. A monofilament composite system of a carbon fibre in a thermoplastic matrix was employed to study the behaviour of the fibre/matrix interphase during cycling because fibre/fibre interaction and processing variability are eliminated. The effects of cyclic loading direction, frequency and amplitude were studied with an optical fragmentation technique. Damage was characterized by fibre-matrix debonding, a reduction of the average interphase shear stress determined by the fragmentation test, and changes in the locus of failure as determined by SEM fractography. It was found that the integrity of the interphase was matrix dominated, and frequency dependent, and was not affected by the initial strength of the interphase when the cyclic loading was transverse to the fibre direction. When the composite was subjected to axial cyclic loading, the damage was dominated by the strength of the fibre until the fibre failed. After fibre failure, the subsequent damage was matrix dominated.

Interphase Behavior in Cyclic Fatigue of Monofilament Composites

Macroscopic properties of fiber reinforced composites are dependent on the micromechanics of the filament-matrix interphase. We present here some preliminary results of our studies aimed at understanding the behavior of the interphase when the composite is subjected to cyclic tensile loading. We have used carbon/polycarbonate mono-filament composites as a model system for studying the effects of loading direction (axial and transverse), frequency, and amplitude. The interphase was varied by etching the filament surface. Damage was characterized by fiber matrix debonding, reduction in interphase stress transfer efficiency, and changes in the locus of failure as determined by SEM fractography.