Relationship of Ideal Work of Fracture to Practical Work of Fracture: Background and Experimental Results
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In recent years, research at the materials group in Texas A&M University has focused on the development of methods for measuring the surface free energy (SFE) of asphalt binders and aggregates. This thermodynamic material property has been successfully used to determine the resistance of asphalt binders to cohesive failure and the resistance of asphalt-aggregate systems to adhesive failure in both dry and wet conditions. In addition, fracture mechanics-based models that incorporate surface free energy have been developed to predict the resistance of asphalt mixtures to fatigue cracking and moisture damage. Given the strong correlation of surface free energy with observed damage in the field, this property has been proposed as a screening tool for selecting materials that can be used to produce durable asphalt mixtures. It is recognized that the magnitude of the ideal cohesive or adhesive bond energy calculated based on surface free energy measurements can be much smaller in magnitude compared to the practical fracture energy measured using mechanical tests, such as the pull -off or peel-off tests. However, evidence in the literature indicates that, despite the large difference in magnitude, there exists a relationship between these two quantities that justifies the use of ideal bond energy to characterize resistance of composite systems to cohesive or adhesive failures. This paper presents a critical review of several studies in the areas of adhesives and polymers that demonstrate and explain the relationship between the ideal work of fracture or bond energy calculated based on surface free energy, and the practical work of fracture obtained from mechanical tests. These studies are very useful in understanding some mechanisms that govern the fracture behavior of asphalt binders and asphalt mixtures, and to develop an experimental setup for conducting pull-off tests on asphalt binders that allow recording the force and displacement throughout the test. The paper presents experimental data regarding cohesive and adhesive failures in asphalt-metal systems obtained from the pull-off test demonstrating the relationship between ideal and practical work of fracture.