UBC Theses and Dissertations
Crack growth and damage modeling of fibre reinforced polymer composites McClennan, Scott Andrew
Notched tensile strength and the development of damage in composite laminates are studied in this thesis through experimental, numerical and analytical methods to develop simple models for predicting notched strength using completely physical input parameters. A series of experimental tensile fracture tests using the Overheight Compact Tension (OCT) specimen geometry, established by (Kongshavn and Poursartip, 1999), were conducted to study the development of the characteristic damage zone. The material used in the tests was a quasi-isotropic carbon fibre/epoxy. The specimens were modified so that the notch root radius varied while maintaining a constant crack width to specimen section ratio. Specimens with notch root radii less than 16 mm display stable crack growth with little notch sensitivity. Specimens with larger notch root radii are unstable and display more notch sensitivity. For all specimens, the height of the damage zone converges to between 6 mm. and 7 mm. The transition from stable to unstable behaviour is explained using fracture mechanics equations and a transition radius can be determined from three material parameters; elastic modulus, specific strain energy and tensile strength. A simple bilinear cohesive zone model called the simple damage model (SDM) was developed as a material model in the ABAQUS finite element code to be used as a demonstrator for modeling techniques that can be applied to all strain-softening material models. In particular, the relationship between input parameters and the element width was studied. Using the same input parameters as the analytical model and a sufficiently refined mesh, the numerical model predicts the peak loads from the series of experimental OCT tests well. A transition from stable to unstable behaviour is predicted at the same radius as seen experimentally. A method of determining an appropriate element width to ensure an accurate prediction is presented. A modified version of the SDM called the adaptive simple damage model (ASDM) was also developed, which automatically scales the input strength to account for the effect of element width. This modification allows larger elements to be used while still obtaining an accurate solution, a useful feature if large structures are being modeled.
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