Mechanical properties and microstructural characterization of semi-crystalline polymers and fiber composites

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The proposed study is concerned about mechanical performance of semi-crystalline polymers (SCP) and fiber composites (FRP) of which the microstructure consists of two phases, i.e. amorphous and crystalline phases in SCP and fiber and polymer matrix in FRP. Interaction between the two phases is known to start at small deformation. With a strong viscous behavior and low resistance to damage development, the dominant mechanisms for deformation of SCP and FRP vary with the loading history. As a result, conventional test methods cannot provide full characterization of mechanical performance of these materials in service.    The long-term goal of the proposed study is to develop a mechanism-based concept to define mechanical properties for SCP and FRP so that the lab test results can be used to evaluate mechanical performance in the long-term service. Study in the next five years has two specific objectives. The first is to develop two new test methods, one for SCP and the other for FRP, to separate mechanical properties associated with the viscous deformation from those with the quasi-static counterpart, and to use changes of the mechanical properties to quantify their resistance to damage. The other objective is to examine microstructural changes of SCP and FRP to identify mechanisms that dominate the deformation process, and to determine the critical conditions for the mechanism changes. Dominant mechanisms in three fundamental loading modes of tension, compression and shear will be considered. By correlating the mechanical properties obtained from the new test methods with mechanisms established from the microstructural changes, the proposed study will provide a foundation for developing a mechanism-based concept to characterize mechanical properties for SCP and FRP, and to evaluate efficacy of this concept for quantifying the load-carrying performance of these materials.    The study will provide training to two PhD and two MSc students, plus one undergraduate student in each summer. Currently, one PhD student is developing a new test method for SCP, and has made significant progress in separating the viscous stress component from the quasi-static counterpart to establish the corresponding stiffness change and damage development during the test. An MSc student will be recruited to use this new test method to examine two SCPs with their amorphous phases in different states (rubbery or glassy), and to characterize their influence on the crystalline microstructure under loading. The second PhD student will develop a new test method for FRP and to characterize the void generation in the matrix and critical conditions for its evolution to cause delamination. The second MSc student will study the influence of a combined loading mode of tension and compression on the mechanical property changes for each of the two phases in SCP, to evaluate the feasibility of quantifying the damage development when subjected to a combination of different loading modes.