This thesis discusses the analysis of surface cracked configurations in order to develop a fracture prediction criterion suitable for ductile materials. A similar criteria has been successfully developed for brittle materials. However, the criteria has not been applied to ductile materials. Finite element analysis results are presented as well as laboratory test data. The validity of the proposed criterion is addressed and future work is proposed.

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Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 268-286. / Ph.D.

The normal stress ratio theory for crack extension in anisotropic materials is studied analytically and experimentally in order to evaluate its validity. The theory is applied within a macroscopic-level analysis of a single center notch of arbitrary orientation in a unidirectional composite material. The bulk of the analytical work of this study applies an elasticity solution for an infinite plate with a center line crack to obtain critical stress and crack growth direction predictions. An elasticity solution for an infinite plate with a center elliptical flaw is also used to obtain qualitative predictions of the location of crack initiation around the border of an actual rounded notch tip. The analytical portion of the study includes the formulation of a new crack growth theory that includes local shear stress. Predictions of the normal stress ratio theory are obtained for the problems of a unidirectional tensile coupon with a horizontal center notch and a unidirectional losipescu shear specimen with a vertical center notch, each with an arbitrary fiber orientation. These predictions are subsequently compared to experimental results. It is shown that the normal stress ratio theory exhibits a strong ability to correctly predict crack extension direction. Predicted critical stresses correlated well with experimental stresses at crack initiation. Use of the elliptical flaw analysis resulted in significant agreement with observed locations of crack extension, while still providing correct crack extension direction predictions. It is suggested that future analytical studies include application of the normal stress ratio theory as a predictor of critical stresses and its application within a rounded notch tip analysis. Also, future experimental efforts should include performing the critical shear tests identified in this study which could not be performed using the Iosipescu specimen. / M.S.

A novel concept, the fracture efficiency, is proposed in this dissertation as a performance index of fracture specimen designs for adhesive bonds. Evaluated based on a simple quantity, the fracture efficiency parameter, the fracture efficiency represents how much crack driving force a certain specimen design can produce along the bondline for a given maximum non-singular stress in the adherends. In this dissertation, a study of membrane peeling specimens shows that various existing test geometries have little difference in the fracture efficiency. Furthermore, the study shows that it is unlikely to develop new n1embrane peeling tests with high fracture efficiency. Therefore, gross inelastic energy dissipation in the film specimen may occur when using the membrane peeling tests, and may result in large errors in the film adhesion measurement when only elastic material response is considered in the analysis. As a result, the use of a backing material for film adhesion measurement is suggested because of the significant increase in the fracture efficiency when a proper backing material is used. Besides the use as one of the guidelines for fracture specimen designs, it is also demonstrated in this dissertation that the fracture efficiency parameter can also be used to help one determine the validity of a measured bond fracture strength. The study of fracture efficiency leads to new analyses of several existing fracture test geometries, and contributes to a better understanding of the standard blister, island blister, peninsula blister, peel, and cracked lap shear tests. It is found that the island blister and peninsula blister tests can produce very high energy release rates at low pressures by using a small island radius or a small peninsula width. However, these so called "high energy release rate" tests also induce localized high stress concentrations near the debond front and therefore, yielding or rupturing may still unavoidable in such specimens. The significant localized inelastic effect on the measurement of the bond fracture strength is also demonstrated in the peninsula blister experiment for an adhesive tape specimen subjected to a relatively low pressure load. Another important conclusion drawn from the analyses of various membrane peeling tests is that the existing membrane blister tests are special cases of the peel test if compared very near the debond front. Besides analyses for membrane peeling specimens, this dissertation also reports a new closed form solution for the cracked lap shear specimen using a geometrically nonlinear beam-column approach. Excellent agreement for energy release rates has been shown between the closed form solution and geometrically nonlinear finite element analyses. The study also discusses how to design a cracked lap shear specimen of constant energy release rate and high fracture efficiency. / Ph. D.

This work describes the development of an improved mode I fracture testing procedure for adhesively-bonded wood, and demonstrates the sensitivity of this approach. The two significant improvements were: 1) the use of the flat double cantilever beam (DCB) geometry, which has been uncommon for wood and 2) the application of an established and powerful data analysis using a corrected compliance method from beam theory. Three studies were conducted using various wood adhesives and DCB specimens were fabricated from yellowpoplar (Liriodendron tulipifera) sapwood. The sensitivity of this methodology showed significant differences in fracture performance as the degree of cure increased for a phenol formaldehyde adhesive, and yielded maximum strain energy release rate (SERR) values of 370 - 560 J/m2. A second study showed performance differences between two polymeric diphenylmethane diisocyanate (pMDI) adhesives and one polyurethane adhesive. Typical maximum SERR values were 160 and 130 J/m2 for the pMDI adhesives and 160 J/m2 for the polyurethane adhesive. A third study investigated the effect of loading rates on a cross-linked polyvinyl acetate adhesive and maximum SERR values of 370 - 560 J/m2 were achieved. Adhesive penetration and cure were determined by image analysis with fluorescence microscopy, and by micro-dielectric analysis, respectively. Since the geometry of the fracture procedure dictates the absence of wood failure, the resulting fractured surfaces were readily analyzable. The surface analysis techniques of laser ionization mass analysis, solid-state nuclear magnetic resonance and field emission scanning electron microscopy were used to investigate the locus of failure for the smooth fractured surfaces. / Master of Science

Several experimental methods are examined for stress concentration factor determination. Tests are performed on a chosen 2-D specimen using strain gages, brittle coating, brittle specimen failure, photoelasticity, and Moire interferometry. Results are compared to each other and to finite element analysis performed on the same geometry. Strain gaging and photoelasticity were chosen as the best methods for stress concentration factor determination. / Master of Science

This study developed and analyzed a modification of the blister test permitting nearly constant strain energy release rate testing of adhesive bonds. The work consisted of three parts; (1) development of the testing technique to evaluate strain energy release rate and to record the time dependent nature of the fracture process, (2) numerical analysis of the constrained blister test to determine the applicability of an approximate solution for several materials, and (3) development of an. analytical technique to evaluate the strain energy release rate for relatively stiff specimens. / Master of Science