Analysis of bimaterial crack problems in the presence of friction
Abstract
Recently, there has been much interest in the mechanism of fiber pullout and its associated failure modes. This is due to the emergence of brittle matrix composites where the fibers are used to increase the material's toughness rather than its strength as in traditional applications. This increased toughness is often attributed to the process of fiber pullout. The process begins when a crack initiates in the matrix. As the crack propagates, it will eventually intersect a fiber. At this point, if the bond between the fiber and matrix is relatively weak, the crack is deflected along the interface effectively blunting the crack tip. Thus, the strength of the interface is critical. If the bond between the constituents is too strong, the crack will propagate through the fiber as if the material were homogeneous. If the bond is too weak, the transverse properties of the composite are severely degraded. The significance of the interfacial properties in brittle matrix composites has led to many new test methods to characterize them. The most prominent of these are the fiber pushout and pullout tests. These tests involve complex failure mechanisms which are difficult to analyze. As a result, most analyses focus on the load or stress which causes failure without regard to the actual failure mechanisms occurring within the specimen. The thrust of this work is to develop a rational approach for analyzing the fiber pullout test and other associated problems. The present study uses a fracture mechanics approach. However, there are several problems to overcome. Contact occurs between the crack surfaces which can lead to significant amounts of friction. This complication causes the traditional methods for calculating strain energy release rates to breakdown. Therefore, an alternate method is developed. The new method is extensively tested by applying it to several related bimaterial crack problems as well as the fiber pullout test itself. In some special cases, the strain energy release rate can also be calculated directly from its definition. In these cases, the two methods are shown to correlate extremely well.
Degree
Ph.D.
Advisors
Sun, Purdue University.
Subject Area
Aerospace materials|Mechanical engineering
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