1. 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