| Theodore von Kármán Professor of Aeronautics and Mechanical Engineering |
Research Interests Subsonically and Intersonically Moving Dynamic Cracks in Unidirectional Laminated Composites Collaborators: M. Ortiz, Caltech; A. Pandolfi, Polytechnico di Milano; Student: G. Lykotrafitis, Caltech
|
Here we study the phenomenon of highly dynamic crack growth events in unidirectional thick composite laminates. The crack growth events take place in graphite fiber, epoxy matrix composite plates containing an edge pre-notch in the fiber direction. The dynamic, in-plane loading is provided either by a high impact speed gas gun or a low speed drop weight tower. High speed photography and the optical method of CGS are used in a reflection arrangement to record dynamic crack initiation and growth. The different specimen geometries and loading configurations are designed to promote dynamic failure at different modes and crack tip velocity regimes. For Mode-I types of loading, the results reveal highly dynamic subsonic, crack growth processes, and predict decreasing dynamic toughness with crack tip speed. For Mode-II types of loading, the results however, reveal highly unstable and intersonic, shear-dominated crack growth along the fibers. The intersonic cracks propagate with phenomenal crack tip speeds as high as 10,000 m/s and feature a shock-wave (mach-cone) structure typically expected of disturbances traveling with speeds higher than some of the characteristic wave speeds in a material. These complex dynamic failure phenomena are modeled numerically by using the cohesive element methodology recently developed by M. Ortiz and his research group. Particular emphasis is given to the study of dynamic frictional sliding between the faces of the intersonically moving shear cracks and the resulting localized "hot spots." High speed infrared thermography is used to visualize the process of frictional heat dissipation and to guide numerical and analytical modelling of the phenomenon.
last
update:
05/31/2016