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

这项研究计划的目标是双重的。一个是提供一个理解的复合材料结构中的裂纹和断裂的发展，从而可以制定一个标准的故障启动，另一个是开发程序的高质量的热塑性复合材料结构的有效制造。 对于第一个目标，在过去几年中，申请人已经发现了一种新的方法，该方法可以原位检测、定位和量化复合结构中发生的损伤，这意味着可以在结构运行时进行检测。该方法是非常敏感的，使得它可以检测其他技术可能无法检测的基体裂纹。该方法也是简单和易于使用。该方法将用于监测复合材料结构在加载的早期阶段的裂纹发展。因此，它可以遵循裂纹跳跃模式，这是独特的复合材料结构。该技术取决于通过将碳纳米管并入基体的树脂内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必要的，以确保电导率的均匀性和检测的灵敏度。通过监测加载早期的裂纹跳跃模式，可以揭示复合材料的可变性。 对于第二个目的，虽然热塑性复合材料提供许多有吸引力的特性，例如没有保质期、良好的抗冲击性、短的加工周期、可回收性等，由于它们的高粘度，加工困难，并且在最终部件的结构内通常存在空隙。随着自动纤维铺放（AFP）机器的出现，热塑性复合材料的加工有更多的可能性。本项目将考虑两个方面。首先，将检查由热固性基质复合材料或热塑性复合材料制成的基底与通过AFP新形成的热塑性复合材料层之间的粘结。这种类型的结构在许多飞机结构中很常见，飞机结构由加强件（如肋和翼梁）顶部的蒙皮组成。将研究不同的工艺条件如温度、压力和接触时间对粘结质量的影响。将创造最佳条件。对于第二方面，在AFP中，将检查热塑性复合材料在经受熔融温度和固结压力时的变形。此外，当压力被移除时，材料的松弛行为也将被检查。所获得的理解将用于开发最佳工艺参数。

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.