Matrix and interface effects on microcracking in polymer composites

Sammanfattning: An underlying industrial problem for the present work is leakage of pressure vessels. Fluid leakage was confirmed to take place along a path of connected microcracks and delaminations. The thesis is concerned with the micromechanisms for microcracking studied by in-situ optical microscopy of cross-ply laminates. The strain at onset of transverse cracking and K, a measure of the slope of crack density versus strain, are important parameters. The role of matrix and interface is emphasized although new light is shed also on the role of micro- and mesolevel residual stresses. Damage initiation and development behavior of glass bead/polypropylene (PP) composites is studied. Thermoelastic finite element analysis demonstrates strong effects on interfacial debond strains from residual thermal strains although the high relaxation rate reduces the problem for PP. Interfacial debonding is also important in the glass fiber composite studies. Previous studies have shown the transverse fatigue behavior in tension-compression to be substantially worse than the behavior in transverse tension. This was demonstrated to be due to debond extension in compression caused by Mode I loading at the crack tip of large debonds. Single fiber composites of different interfacial strengths were also studied. The strong interface system did not fail in any single fiber test (fragmentation test, transverse tension, pullout) which points to a basic problem with these tests for systems with strong adhesion. As this fiber was used in cross-ply laminate tests, substantial improvement was observed in c., whereas K decreased. Similar effects were observed with increased matrix fracture toughness and decreased fiber content. This is because debonding is delayed as well as the subcritical crack growth prior to formation of a crack of critical size. Residual stresses were studied on thin transverse sections of unidirectional glass fiber/vinylester. In transmitted polarized light, distinct optical patterns were observed in densely packed chains of fibers close to isolated matrix regions. In finite element calculation results, the optical patterns associated with these critical mesoscale configurations correspond to regions with large compressive stresses. The finite element calculations were performed on a scanned microstructure image containing 1410 fibers. Despite the complexity and size of this microstructure and short fiber-fiber distances, calculation results were virtually exact for a thermoelastic 3D case. Residual stresses were sufficiently large to result in compressive yielding which may cause the optical effects. Experimental results indicate that glass fiber/vinylester with poor interfacial adhesion (possible in commercial materials, as demonstrated in the thesis) can suffer interfacial debonding due to residual stresses only. With glass fiber/polyester, even short cracks are formed due to microlevel residual stresses. In the near future, high precision stress-state calculations in combination with detailed experimental studies will allow us to solve mesoscale problems of great complexity. This will change some of the current paradigms. For instance, increased confidence in stress-state calculations will lead to the need for improved accuracy in measurements of the constitutive behavior of fibers, matrices and interfaces.