PHOTOELASTIC STUDY OF A SINGLE-FIBER MODE L COMPOSITE IN THE COMPRESSIVE AND TENSILE MODES

Failure fiber phenomena of single carbon fiber embedded in an epoxy po lymer have been studied in the tensile and compressive modes. Experime nts have been conducted with a 7 mu m diameter carbon fiber but also w ith a greater diameter fiber (49 mu m) in order to characterize the in terfacial stress. The kinetics of fibre failure have been determined. The fragmentation gap is smaller in the compressive mode than in the t ensile mode. In the tensile mode: breaks with or without decohesion ha ve been observed and analysed. Iso-stress shear fringes and fiber/matr ix interfacial evolutions (decohesion length, fiber debonded length, s tability), of a fiber filament have been determined. The surface frict ion of the decohesion zone is too low to operate a charge transfert wh en the deformation is upper 3%. The critical length L(c), correspondin g to the minimal length fragment which can be loaded until the failure , is strongly influenced by the decohesion. In the compressive mode: o wing to thermal preparation conditions, microbuckling plays an importa nt role because the fractures of the fiber are not, as in the tensile mode, associated with the fiber defects, but seem to be initially deri ved from particular points generated by the microbuckling. The failure s occur preferentially at the extremities of the sinusoide where both tensile and compressive fibres are present. The interface is not the p lace where the strongest shear stress occurs.