The problem of determining whether a crack impinging on an interface will penetrate into the substrate or deflect along the interface is vital to the effective design of layered and composite material systems. Of particular interest is the transition between crack propagation by penetration through an interface and deflection along an interface. There has been a great deal of work done on this problem to determine what parameters and formulations are necessary to accurately determine under what conditions penetration-deflection transition will occur. Previous work has studied this problem using stress-based, energy-based, and combined stress-energy-based approaches. Most recently, a combined stress-energy-based approach was implemented via a cohesive-zone formulation; this work showed the conceptual basis and correctness of the cohesive-zone approach, however only presented limited investigation into the behavior penetration-deflection transition. Work presented here expands this investigation on transition, exposing trends and behavior that emerge as certain dimensionless groups are varied. Principles of linear elastic fracture mechanics and, as in previous work, cohesive-theory are applied to a bi-material system in tension through the use of the commercial finite element analysis package ABAQUS. Dimensionless groups, including strength ratios, toughness ratios, fracture-length scales, and substrate toughness scales are varied systematically to show resulting system behavior in a generalized fashion. In using the cohesive-zone method, aspects of previous stress-based and energy-based formulations are reproduced. It is also shown where these formulations cease to be valid, revealing unique and previously undetected transitional interface fracture behavior. The results presented here will prove valuable in interface design as the described generalized trends can be used as references in the design of new layered and composite systems. / Graduation date: 2013

Dissertation (Ph. D.)--University of Akron, Dept. of Polymer Engineering, 2005. / "December, 2005." Title from electronic dissertation title page (viewed 09/17/2006) Advisor, Arkady I. Leonov; Co-Advisor, Joseph P. Padovan; Committee members, Joseph P. Padovan, Gary R. Hamed, Erol Sancaktar, Rudolph J. Scavuzzo, Jr.; Department Chair, Sadhan C. Jana; Dean of the College, Frank N. Kelley; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 140-146). Available also in a digital version from Dissertation Abstracts.

Thesis (M.A.)--University of Texas at Austin, 1990. / Vita. Includes bibliographical references (leaves 96-101).

Thesis (M.Eng.)--Memorial University of Newfoundland, 2002. / Bibliography: leaves 90-96.

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In this paper we develop a computational procedure to investigate linear fracture of two-dimensional problems in isotropic linearly elastic media. A symmetric Galerkin boundary element method (SGBEM), based on a weakly singular, weak-form traction integral equation, is adopted to model these fractures. In particular we consider multiple interacting cracks in an unbounded domain subject to internal pressure and remote stress. The growth of the cracks is driven by either linearly dependent injection pressures or volumes in each crack. A variety of crack geometries are investigated. / text

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The main aim of this work was to investigate the fatigue crack growth resistance and fracture toughness of selectively reinforced Al alloys. In such bimaterials, the crack growth resistance is affected by the failure mechanism, the direction of crack approach to the interface and by the conflict between the elastic-plastic mismatch and residual stresses. When the crack approaches the interface from the composite side, in the A12124 based bimaterials, the fatigue crack growth rate is reduced below "composite only" values by the compressive residual stress, although the elastic-plastic mismatch was expected to cause the opposite effect. In the A16061 based bimaterials, although some crack deceleration is also observed, fatigue crack growth rates are above the "composite only" values presumably because these bimaterials have lower compressive residual stress and higher plastic mismatch near the interface. After crossing the interface, the crack driving force is affected by closure mechanisms developed on the composite side of the crack wake. Conversely, when the crack grows from the Al alloy side, for both A12124 and A16061 based bimaterials, the crack growth rate is mainly reduced by the elastic-plastic mismatch. After crossing the interface, the crack driving force is well described by the thermal residual stresses, unless a crack tip deflection reduces the Mode I near tip stresses. In a fracture toughness test, when the pre-crack tip is in the composite side of the A12124 based bimaterials, KQ(5%) values are increased above "composite only" values presumably due to the compressive residual stresses and despite the amplification of the crack driving force from the elastic-plastic mismatch. In the A16061 based bimaterials, due to the higher plastic mismatch and lower compressive residual stresses, KQ(5%) values are below "composite only" values. Additionally, for all bimaterials, KQ(5%) values increase if the pre-crack tip is closer to the interface. When the crack propagates, it extends to the interface, bifurcates and arrests. The load then had to be increased to promote the onset of plastic collapse. The crack tip blunting and deflection mechanism increases the toughness attained at the onset of plastic collapse of the bimaterials above "Al alloy only" values. Conversely, if the pre-crack tip is in the Al alloy side, the final failure is deduced to occur when damage accumulated on the composite side links to the pre-crack tip. When the pre-crack tip is at 2.0mm from the interface, for the A12124 based bimaterials, KQ(5%) values are in general lower than the "A12124 only" value due to the tensile residual stresses. For the A16061 based bimaterials, KQ(5%) values are as high as the "A16061 only" value presumably due to the higher plastic mismatch and lower tensile residual stress of such bimaterials. Additionally, for all bimaterials, KQ(5%) values increase if the precrack tip is at 0.5mm from the interface. If the pre-crack tip is at 2.0mm from the interface, Kerit and Scrit values of the bimaterial are higher than the "Al alloy only" value and this is deduced to be due to the increase in the elastic-plastic mismatch shielding and by delayed critical particle damage within the composite side. At 0.5mm from the interface, Keritt and Scrit values are reduced and this is deduced to be because both the near tip tensile residual stress is higher and critical particle damage occurs earlier on the composite side; moreover, the unreinforced Al alloy layer is thinner and the damage on the composite side is deduced to link more easily to the pre-crack tip. For a constant particle size, there is an optimum particle volume fraction in which both Kerit and Scrit values are maximised with respect to a specific pre-crack tip position.