EFFECTS OF A COMPOSITE-LIKE STRESS STATE ON THE FRACTURE OF EPOXIES

The strain to failure of a transversely loaded composite is much lower than for the pure matrix in uniaxial tension. Several studies of comp osites suggest the triaxial matrix stress state as one of the explanat ions. In order to investigate this experimentally, a triaxial tensile test previously used for rubbers (the poker-chip test) was successfull y applied to four epoxies in the glassy state. The chosen specimen geo metry mimicked the most severe stress state in the matrix as determine d by finite element analysis of a transversely loaded glass-fiber/epox y composite. The poker-chip strains to failure in the primary loading direction were 0.5-0.8%, whereas uniaxial strains to failure were 1.8- 7%. The triaxial stress state in composite matrices may therefore by i tself be a sufficient explanation for low values of transverse composi te strains to failure.