An investigation of fracture and fatigue in a metal/polymer composite
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INTRODUCTION
THERE has been increasing pressure on the automotive industry to produce “environmentally friendly” nongasoline engines that utilize alternative fuels which reduce NOx emissions.[1,2] This has stimulated extensive research efforts that have led to the development of engines that operate on compressed/liquefied natural gas.[3] Unlike their gasoline or diesel engine counterparts, conventional surface/boundary lubricants cannot be applied to engines that operate using compressed/liquefied natural gas. There is, therefore, a need to develop alternative concepts for the lubrication of liquefied natural gas engines. One approach that has been explored in recent years involves the use of polymer/metal composites designed to provide an internal supply of polymeric lubricant during the extended service of an engine.[4] Such composites provide internal sources of polymeric materials that flow to the surfaces to provide boundary/surface lubrication. They also offer the added advantage of “self-healing,” since the flow of polymeric material can “heal” damaged areas on the surface during extended service. In this way, a new generation of polymer/metal composites can be engineered with in-built wear resistance.[5] However, there are some concerns about the fatigue and fracture resistance of metal/polymer composites.[6] Preliminary studies have shown that such composites may be susceptible to fatigue damage under the severe thermomechanical environments that are encountered in automotive engines.[4] There is, therefore, a need to develop models for the prediction of fracture and fatigue damage in polymer/ metal composites. However, the development of such models is hindered by the lack of physical insights into the micromechanisms of fatigue and fracture in polymer/metal composites. E. KUNG, Undergraduate Research Assistant, C. MERCER and S. ALLAMEH, Staff Scientists, and W.O. SOBOYEJO, Professor, are with the Princeton Materials Institute and the Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544. O. POPOOLA, Staff Scientist, is with Ford Scientific Research Labs., Dearborn, MI 48121. Manuscript submitted November 8, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
The current article presents the results of a combined experimental and analytical effort to develop physically based models for the prediction of resistance-curve behavior and fatigue-crack growth in polymer/metal composites. For comparison, the fatigue and fracture behavior of a nonpolymer “matrix” material is also examined in detail. Following a description of the microstructures, the micromechanisms of fatigue and fracture are elucidated at room temperature. The observed mechanism of crack-tip shielding (ligament bridging) is then modeled using mechanics concepts. Finally, the implications of the results are discussed for the design of durable polymer/metal coatings.
II. MATERIALS The materials that were used in this study were supplied by the Ford Scientific Research Laboratories (Dearborn, MI). They were
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