Fracture of Brittle Materials and Bimaterial Interfaces in the Presence of Compressive Stresses
Type of DegreePhD Dissertation
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The present work is aimed at the experimental investigation of the effect of local compressive stress fields on a crack lying along an interface. This is relevant to problems such as interfaces found in adhesively bonded joints or interfaces found in composite materials. In both examples, the interface is between two materials with significantly different elastic properties, and the fracture behavior along the interface is a significant contributor to bulk material performance. Further, complex, multi-axis load cases arise that can result in either shear-tension or shear-compression load states along the interface. An experimental procedure is developed that utilizes displacement fields measured using digital image correlation method to extract fracture parameters by coupling the full-field data with robust contour integration algorithms built into a commercial finite element code. A three-point bend semi-circular beam geometry is first utilized for the study of mixed-mode fracture in homogeneous materials experiencing quasi-static loads and then extended to dynamic conditions using a single-point impact configuration. The use of the geometry is subsequently extended to experiments where the crack is in the aforementioned state of shear-compression loading. A new test method is then introduced to allow a range of combined shear and compression load states on a crack lying along a bimaterial interface. Each experimental technique is critically evaluated. Detailed results for homogeneous and interface crack specimen geometries are presented to improve the understanding of the effect of compressive stress fields in the vicinity of the crack tip. Under the compressive load states, both the homogeneous and the interface behaviors undergo a marked increase in fracture toughness as the compression stress increases. An empirical relationship is derived to describe the response. The fracture toughness is shown to increase in a near-linear fashion as the compressive stress increases. The same observations were made with respect to the dynamic fracture behavior as well. This contribution serves to quantify the relationship between in-plane stress states and fracture toughness under static and dynamic conditions, as well as along bimaterial interfaces. Another significant contribution from the present study is the development of a numerical technique for improving the data reduction and post processing process. One of the challenges associated with the interface fracture experiments and the dynamic fracture experiments is the ability to reliably locate the crack tip location for post-test analyses. An image processing technique for edge detection is adapted for use in the crack tip identification problem. Analytical investigations are used to demonstrate the performance for a variety of experimental fracture problems.