Stress analysis and vibration of elastic and viscoelastic systems (Composite materials and structures)

这项研究计划的目标有两个。一是对复合材料结构中的裂纹和断裂发展有一个了解，以便制定一个失效起始的判据；二是开发高质量的热塑性复合材料结构的有效制造程序。*对于第一个目标，在过去的几年里，申请人找到了一种新的方法，可以在现场检测、定位和量化复合材料结构中发生的损伤，这意味着可以在结构运行时进行检测。该方法非常敏感，可以检测到其他技术可能无法检测到的基质裂缝。该方法简单易行。该方法将用于监测复合材料结构在加载初期的裂纹发展情况。因此，它可以遵循复合材料结构中独有的裂纹跳跃模式。这项技术依赖于通过将碳纳米管掺入基质树脂中而获得的复合材料的导电性。通过良好的混合方法掺入适量的纳米管是至关重要的，以确保电导率的一致性和检测的灵敏度。通过监测加载早期的裂纹跳跃模式，可以揭示复合材料中的变异性。*对于第二个目标，虽然热塑性复合材料具有许多吸引人的特性，如无保质期、良好的抗冲击性能、加工周期短、可回收利用等，但由于其高粘度，加工一直困难，最终零件的结构中通常存在空洞。随着自动纤维贴装机(AFP)的出现，热塑性复合材料的加工有了更多的可能性。在这个项目中，将考虑两个方面。首先，将研究由热固性基复合材料或热塑性复合材料制成的基板与AFP新形成的热塑性复合材料层之间的粘接。这种类型的结构在许多飞机结构中很常见，这些结构由钢筋顶部的蒙皮组成，如肋骨和支柱。将研究不同的工艺条件，如温度、压力和接触时间对粘接质量的影响。将开发出最佳条件。对于第二个方面，在AFP中，将检查热塑性复合材料在熔融温度和固结压力下的变形。此外，还将考察材料在卸压时的松弛行为。所获得的理解将用于制定最佳工艺参数。

The objectives of this research program are two-fold. One is to provide an understanding of the crack and fracture development in composite structures, such that a criterion for failure initiation can be formulated, and the other is to develop procedures for the efficient manufacturing of thermoplastic composite structures with good quality. ******For the first objective, over the past few years, the applicant has found a new method that can detect, locate and quantify damages that occur in the composite structures in-situ, meaning that the detection can be done while the structure is in operation. The method is very sensitive such that it can detect matrix cracks that other techniques may not be able to detect. The method is also simple and easy to use. This method will be used to monitor the crack development in the early stages of loading of the composite structure. As such it can follow the crack jumping pattern which is unique in composite structures. The technique depends on the electrical conductivity of the composites by the incorporation of carbon nanotubes within the resin of the matrix. It is essential that an appropriate amount of nanotubes be incorporated by a good mixing method in order to assure uniformity of the electrical conductivity, and sensitivity for the detection. By monitoring the crack jumping pattern in the early stages of loading, the variability in the composite materials can be revealed. ******For the second objective, while thermoplastic composites offer many attractive characteristics such as no shelf life, good impact resistance, short processing cycle, recyclability etc., due to their high viscosity, processing has been difficult and voids usually exist within the structure of the final part. With the advent of automated fiber placement (AFP) machines, there are more possibilities for the processing of thermoplastic composites. In this project, two aspects will be considered. First, the bonding between substrates made of either thermoset matrix composites or thermoplastic composites, and newly formed layers of thermoplastic composites by AFP will be examined. This type of configuration is common in many aircraft structures, which consist of skins on top of reinforcements such as ribs and spars. The effects of different processing conditions such as temperature, pressure and duration of contact on the quality of the bond will be studied. Optimal conditions will be developed. For the second aspect, in AFP, the deformation of the thermoplastic composite material while they are subjected to melting temperature, and consolidation pressure, will be examined. In addition the relaxation behavior of the material when the pressure is removed will also be examined. The understanding obtained will be used to develop optimal process parameters.