UC San Diego

Polymeric materials, in particular composite materials, have seen an increase in use in naval applications. Under these conditions, structural materials need to be able to withstand dynamic loads such as those from underwater blast and wave slamming, as well as environmental factors like moisture absorption and ultraviolet radiation. Both conditions add challenges in the design process due to complex loading conditions and change in material behavior.

This dissertation presents an experimental study to understand the dynamic fracture behavior of poly(methyl methacrylate) (PMMA) and carbon fiber/epoxy. Dynamic loading was obtained by launching projectiles from a gas gun onto the specimen. A combination of ultra high-speed photography and digital image correlation was used to gather full-field experimental data.

First, a methodology was established using PMMA to obtain pure mode-II fracture loading conditions. This study generated valuable experimental results, which updated current data available to the fracture community. Sharp transitions from mode-II to mode-I fracture loading were observed in addition to the effect of crack-tip sharpness and loading conditions on the crack path.

Next, carbon fiber/epoxy specimens were studied after developing an experimental setup capable of obtaining dynamic mode-II loading conditions. The effect of moisture absorption on the fracture behavior of unidirectional carbon fiber/epoxy samples was studied. Two different methods were used to obtain significant moisture contents, first, samples were hygrothermally aged at 65°C for 28 days and second, submersion in room temperature distilled water for 400 days. Additionally, a control group of samples with no moisture content was used to compare results. This study determined that moisture absorption decreased the fracture toughness of the material, however no differences were observed between the different aging methods.

Last, unidirectional and woven carbon fiber samples were subject to mixed mode loading. The samples were hygrothermally aged at 65°C until they achieved moisture saturation. Mixed mode loading and woven composites allowed for the opportunity to create more realistic conditions compared to unidirectional composites under pure mode-II fracture. At the same time, these changes introduced complexities at the onset of crack growth. Unlike the previous experiment, the samples showed no sensitivity to moisture.