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Model brittle matrix composites (SiC fibers and borosilicate glass) demonstrated that good matrix cracking stress is achievable only when a good matrix interfacial bond exists. Improvement in this matrix cracking stress can be seen with increasing fiber volume and lowered radial residual compressive stress. The ability to measure the fiber-matrix normal interfacial strength in a composite was demonstrated using a model system in a cruciform sample geometry. An analytical model of ceramic composites based on observed damage progression was developed based on simultaneous stress application and microscopic observations. The damage state of the ceramic composite was successfully incorporated into the model to generate changes in the mechanical behavior. The measurement of coating modulus in BMCs was demonstrated using a micro indentor technique.

The primary intent of this research was the development of composites having an acceptable combination of toughness and oxidation resistance. The research was closely coordinated with the DARPA/ONR URI program at UCSB concerned with high temperature, high performance composites. The study comprised three tasks. The first task was to study phase equilibria, solidification pathways and toughness properties of intermetallics, such as oxidation resistant Ti-Al-Ta and Nb-Al-Si alloys, and ceramics. These studies were then combined with interface interaction studies between the matrix microstructure and ductilizing/toughening particulates an fibers. The second task was a basic study of interface structure and chemistry as it relates to the mechanical properties of the interface. The third task involved studies on aluminum and copper matrix composites. These studies were concerned with developing an understanding of creep resistance. (JES)

The primary intent of this research was the development of composites having an acceptable combination of toughness and oxidation resistance. The research was closely coordinated with the DARPA/ONR URI program as UCSB concerned with high temperature, high performance composites. The study comprised three tasks. The first task was to study phase equilibria, solidification pathways and toughness properties of intermetallics, such as oxidation resistance Ti-Al-Ta and Nb-Al-Si alloys, and ceramics. These studies were then combined with interface interaction studies between the matrix microstructure and ductilizing/toughening particulates and fibers. The second task was a basic study of interface structure and chemistry as it relates to the mechanical properties of the interface. The third task involved studies on aluminum and copper matrix composites. These studies were concerned with developing an understanding of creep resistance. (JES)

The objective of this program was to derive theoretical models to predict micromechanical fracture events in BMC. Incorporate the micromechanical models into composite laminate theory. Provide experimental confirmation and characterization approaches. Delineate interface interphase effects on BMC response. This is a bibliography of the publications generated by this program.

Understanding the complexity of the fracture mechanics of brittle matrix composite: was a subject of several analytical studies in this work. The studies included finding the effect of imperfect fiber-matrix interfaces, both frictional and nonhomogeneous, on the fracture mechanics of composites; readdressing and questioning the current criteria for crack deflection at fiber-matrix interfaces used for developing optimum composite interfaces comparing axisymmetric and planar approximations of three dimensional fracture mechanic problems, since stiffness reduction and fracture toughness of composites are evaluated using such assumptions. (MM)

In understanding the complexity of the fracture mechanics of brittle matrix composites, knowing reliable values of the mechanical properties of the fiber-matrix interface are important. Since all experiments designed to evaluate the mechanical properties of the fiber-matrix interface rely on curve fitting experimental measurements to analytical results, accurate physical modeling of these tests is imperative. In this final report, two common tests, namely the push-in test and the slice compression test, were analytically simulated. In the push-in test, the effect of extrinsic factors such as indentor shape and size, and push-through radius hole, and intrinsic factors such as transverse isotropy of fibers on load-displacement curves was studied. All factors except the push through hole-radius were found to affect the load-displacement curves. In the slice compression test, comparison was made for the values of maximum protrusion, residual protrusion and debond lengths for a simple shear-lag analysis and a finite element analysis model. Large differences ranging from 15% to 70% were found by using the two different models. Hence, the importance of accurate modeling of the test cannot be ignored. The study also gives tools to an experimentalist for designing reliable experiments for evaluating reliable values of the mechanical properties of the fiber-matrix interface.

This report investigates two of the most common modes of localized failures, namely, periodic fiber-bridged matrix cracks and transverse matrix cracks. A modification of Daniels' bundle theory is combined with Weibull's weakest link theory to model the statistical distribution of the periodic matrix cracking strength for an individual layer. Results of the model predictions are compared with experimental data from the open literature. Extensions to the model are made to account for possible imperfections within the layer (i.e., nonuniform fiber lengths, irregular crack spacing, and degraded in-situ fiber properties), and the results of these studies are presented. A generalized shear-lag analysis is derived which is capable of modeling the development of transverse matrix cracks in material systems having a general multilayer configuration and under states of full in-plane load. A method for computing the effective elastic properties for the damaged layer at the global level is detailed based upon the solution for the effects of the damage at the local level. This methodology is general in nature and is therefore also applicable to (0(sub m)/90(sub n))(sub s) systems. The characteristic stress-strain response for more general cases is shown to be qualitatively correct (experimental data is not available for a quantitative evaluation), and the damage evolution is recorded in terms of the matrix crack density as a function of the applied strain. Probabilistic effects are introduced to account for the statistical nature of the material strengths, thus allowing cumulative distribution curves for the probability of failure to be generated for each of the example laminates. Additionally, Oh and Finney's classic work on fracture location in brittle materials is extended and combined with the shear-lag analysis. The result is an analytical form for predicting the probability density function for the location of the next transverse crack occurrence within a crack bounded region. The results of this study verified qualitatively the validity of assuming a uniform crack spacing (as was done in the shear-lag model).