Failure of epoxy resins C. Marotzke, T. Feldmann, BAM, Division 5.3, Mechanics of Polymers performance of epoxy resins epoxy resins are used as matrix material in fiber reinforced composites for high strength applications → aircraft-, automotive- and wind energy industry mechanical properties of matrix highly influence the performance of the composites → especially strength and toughness multiaxial fracture of epoxy resin occurs in composites fracture surface of a resin rich region in a carbon fiber reinforced epoxy 45° laminate failure of neat epoxy resins – tensile test failure of polymers is initiated at local faults → material inhomogeneities, micro cavities crack starts at local stress concentrations → multiaxial stress state after crack initiation → no strength but fracture mechanical problem measured overall failure stress significantly lower than actual local failure stress 1100µm 280µm 180µm fracture starts at edge, crack mirror origin of crack - apparent “particle” cliffy fracture surface at initial crack failure due to initial fault – cavity (bubble) failure starts at cavity → high strains arise at the equator of the cavity (total strain : 8.4%) material behaves highly plastically – multiaxial stress state FEM simulations – multilinear plastic material behaviour – measured in tensile tests 2200 µm micrograph of fracture surface axial stresses around cavity total axial strain – cavity, fracture mirror – high stress gradient around the cavity – 3 times higher than far field strain three point bending test – crack starting at a milled notch crack starts at notch tip – fracture mirror → unstable crack propagation outside fracture mirror → cliffy fracture surface – hackles → lower crack propagation speed complex local stress / strain field → conventional determination of failure strain → yields only mean value 140µm 140µm crack mirror with almost planar surface shear hackles, enlarged surface, height profile of hackles – high crack propagation speed – lower crack propagation speed different directions of stress vectors Acknowledgements Thanks to L. Buchta, F. Roth and O. Schwarze for specimen preparation and performing the tests