Why do Growing Cracks Branch? An Optical Investigation of Brittle Monolithic and Bilayer Transparencies Under Stress Wave Loading
Meenakshi Sundaram, Balamurugan
Type of DegreePhD Dissertation
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Understanding the fracture mechanics of materials due to stress wave (or impact) loading under combined tensile and shear (mixed-mode) conditions is important for two opposing reasons: (a) for creating impact resistant structures; e.g., aircraft canopy, (b) for inducing maximum damage with minimum effort; e.g., mining and pulverization. In transparent armor systems such as fighter aircraft canopies and automotive windshields, layering is often used to promote crack branching to create more surface area and increase energy absorption. In this context, the first part of this research deals with mixed-mode dynamic fracture mechanics investigation of relatively brittle poly(methyl methacrylate) (PMMA) and ductile polycarbonate (PC), the two most common structural polymers used for transparent armor construction. As these fracture events last only a few hundred microseconds, quantifying the mechanical fields in real-time and over the whole field of interest is rather challenging. A full-field, non-contacting, optical measurement technique called Digital Gradient Sensing (DGS) used in conjunction with ultrahigh-speed photography (up to 1 million frames per second) has been extended in this dissertation for measuring the mixed-mode fracture parameters for PMMA and PC. Next, the dynamic interactions of fast fracture fronts with weak interfaces oriented normally to incoming cracks in nominally homogeneous PMMA bilayers with discrete planes of weakness are studied. The interfaces drastically perturb crack growth behavior resulting in higher absorption of energy by exciting mixed-mode crack growth, crack branching, and fragmentation. Using measured fracture parameter histories, the energy release rate of growing cracks have been quantified in bilayers with different degrees of interfacial weakness. The mechanics of crack penetration vs. branching at a weak interface is further investigated by focusing on the effect of interface location in PMMA bilayers and the associated crack-tip parameters on ensuing fracture behaviors for constant impact velocity. The results show that the crack path selection at the interface and the second layer are affected by the location of the interface within the bilayer. Using fracture parameter histories, reasons for direct crack penetration instead of branching are mechanistically explained and visual evidence is offered. The explanation is further validated by increasing the impact velocity to promote crack branching in configurations that produced direct penetration at a lower impact velocity. The experimentally challenging problem of dynamic crack growth and branching in monolithic glass is explored in the next part of the dissertation. Plate glass being a stiff and extremely brittle material, deformations are highly localized to the crack-tip while crack speeds exceed 1600 m/s (or 3600 mph). A methodology to make full-field optical measurements using DGS in conjunction with ultrahigh-speed photography (> 1 million frames per second) is developed. The method has been first calibrated under both quasi-static and dynamic loading conditions for soda-lime glass. Subsequently, the crack branching problem in glass is tackled. A potential mechanism for crack branching phenomenon is proposed based on empirical evidence and elasto-dynamic fracture mechanics of brittle solids. Lastly, the fracture behavior of transparent graft Interpenetrating Polymer Networks (graft-IPNs) as alternative lightweight armor materials is examined. The graft-IPNs with a copolymer (CoP) made from PMMA as the stiff phase and polyurethane (PU) as the ductile phase with varying CoP:PU ratios and two molecular weights of poly(tetramethylene ether) glycol (PTMG), 650 g/mol and 1400g/mol, are synthesized and studied. Quasi-static tests provide an optimum range of CoP:PU ratio for enhanced fracture toughness (~130%) when compared to PMMA especially using the higher molecular weight PTMG. Under high strain rate loading conditions, the 80:20 composition of graft-IPNs with 650 g/mol PTMG showed crack initiation toughness improvement of approx. 190% and 38% over PMMA and PC, respectively.