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

研究了半结晶聚合物（SCP）和纤维复合材料（FRP）的力学性能，SCP和FRP的微观结构分别由非晶相和结晶相以及纤维和聚合物基体组成。已知两相之间的相互作用开始于小变形。SCP和FRP具有较强的粘性和较低的抗损伤能力，其变形的主要机制随加载历史而变化。因此，传统的测试方法不能提供这些材料在使用中的机械性能的完整表征。 拟议研究的长期目标是开发一种基于机制的概念，以定义SCP和FRP的力学性能，以便实验室测试结果可用于评估长期使用中的力学性能。未来五年的研究有两个具体目标。首先是开发两种新的测试方法，一种用于SCP，另一种用于FRP，将与粘性变形相关的机械性能与准静态对应物分离，并使用机械性能的变化来量化其抗损伤能力。另一个目的是研究SCP和FRP的微观结构变化，以确定主导变形过程的机制，并确定机制变化的临界条件。主要机制在三个基本的加载模式的拉伸，压缩和剪切将被考虑。通过将从新的测试方法获得的力学性能与从微观结构变化建立的机制相关联，拟议的研究将为开发基于机制的概念以表征SCP和FRP的力学性能提供基础，并评估该概念用于量化这些材料的承载性能的功效。 这项研究将提供培训，两个博士和两个硕士学生，加上一个本科生在每个夏天。目前，一名博士生正在开发一种新的SCP测试方法，并在将粘性应力分量与准静态对应物分离以建立测试期间相应的刚度变化和损伤发展方面取得了重大进展。一名硕士生将被招募使用这种新的测试方法来检查两种SCP，其非晶相处于不同状态（橡胶态或玻璃态），并表征其对负载下的结晶微观结构的影响。第二位博士生将开发一种新的FRP测试方法，并表征基质中的空隙生成及其演变导致分层的临界条件。第二个硕士学生将研究拉伸和压缩的组合加载模式对SCP中两个阶段的力学性能变化的影响，以评估在不同加载模式的组合下量化损伤发展的可行性。

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.