Delamination Growth in Carbon Fiber Beams: Numerical and Experimental Study Using Computed Tomography

Thesis (Master's)--University of Washington, 2017-08This work focuses on delamination of composite laminates, particularly in the context of slender aerospace structures. Composite laminates are advantageous due to their relative low density and superior mechanical properties. A major disadvantage of composite laminates, however, is their predisposition to delamination which decreases their stiffness and strength. Understanding delamination is crucial to designers in order to enable them to take advantage of damage-tolerance design that is commonly used in aerospace engineering. Similarly, understanding delamination is critical to engineers designing and researching high performance aerospace structures as they are exposed to impact and extreme aerodynamic, thermal, and static loading conditions which may cause the initiation and propagation of delamination. End Notch Flexure (ENF) tests were done on carbon fiber reinforced polymer (CFRP) specimens with both a commercial load frame and an in-house load frame. The specimens tested were scanned with an X-ray computed tomography (CT) scanner. The use of CT scans to measure internal cracks (as opposed to measurement at the edge only) was one of the primary motivations for this study. The work focused also on obtaining 2D and 3D scans during testing. The scans were able to determine the through-the-width crack tip geometry, and serve as a proof of concept for detecting embedded delamination/delamination migration. Computed tomography scanning presents a few additional advantages, e.g. non-invasive, real-time measurements of delamination using 2D scans, and non-invasive spot checks using 3D CT models. The experimental results were used to characterize the material properties - namely the longitudinal modulus of elasticity and mode II critical strain energy release rate (SERR). Using the material parameters from the tests, finite element models of the ENF specimens were created with commercial software to predict delamination growth. The models used shell elements and implemented the Virtual Crack Closure Technique (VCCT) to simulate the crack propagation. The simulated critical displacement, critical load, crack growth, and post-crack growth stiffness were compared to the experimental results to evaluate the models’ accuracy