Progressive Damage In Composite Structures Under Quasi-static Punch-shear Loading

Composite material is a new kind of material/structure with many exclusive advantages, such as high strength and stiffness per unit weight, heat insulation and good erosion resistance. Hence, it has been widely used in aviation, construction, wind energy, transportation, navigation and pressure vessels, etc. Composite materials can be classified into different types based on the reinforcements. Unidirectional plies reinforced laminates are the most commonly used. Besides, there are woven fabric reinforced laminates with various fabric architectures. As one of the hybrid composite structures, sandwich plates also play a great role in engineering applications. Sandwich structures usually consist of two stiff face sheets with a light core between. This arrangement can significantly increase the bending stiffness of the plates with additional benefits such as acoustic insulation function. As being used in complex load cases, composites are inevitable to local loads especially in some defense applications. This can cause serious damage or penetration within certain area and lead to a degradation to the structures. Many researchers have investigated the damage mechanisms of composites under local loading. The quasi-static punch-shear test(QS-PST) was adopted by several researchers to characterise the material behaviour and identify types of damage that occurred at different load/displacement levels. To optimise the composite structural design under local loads, more detailed analysis needs to be conducted according to specific composite structures. In this research, experimental and numerical studies were conducted to investigate the damage mechanisms of composite laminates and composite/Nomex sandwich plates under quasi-static punch-shear loading. Particularly, the following studies have been made in this thesis:A comprehensive literature review has been made for the researches on damage mechanisms of composite structures under quasi-static punch-shear loading, and results from analytical, numerical and experimental studies in recent years were also given in this thesis. Problems remained in this field were pointed out, and the objective and significance of this study were also stressed. To investigate the damage mechanisms of composite structures under quasi-static punch-shear loading, a series of experiments were carried out on carbon satin weave reinforced composite laminates and sandwich plates with the same composite material as facesheet and Nomex core. Then a progressive damage model was built and validated by test results. In specific, the following studies have been completed in this thesis:(1) QS-PSTs were carried out on composite laminates and sandwich plates. The testing method was improved from the test scheme by other researchers and allowed a real-time observation on deformation of the distal face of the specimen during the loading. All the load and displacement of the punch bar were recorded in the testing system for the whole process. In this study, a step characteristics tracking approach was adopted to study the progressive damage in composite structures under quasi-static punch-shear loading. This was accomplished by choosing representative points on load-displacement curves and loading specimens to those chosen load levels. After being carefully grinded, the damaged specimens were then observed under optical microscope and also visualized by XCT. The damage initiation and progression information obtained is important and valuable to mechanisms research of composite structures. In addition, mechanical tests of satin weave reinforced composites were conducted to obtain the basic properties of the laminates and face sheets of sandwich plates.(2) Progressive damage study on composite structures under quasi-static punch-shear loading. The damage evolution of composites was inspected and evaluated using optical microscope and XCT technique. Matrix cracks, delaminations and fibre breakages were clearly identified under microscope. This damage evaluation method is straightforward and reliable but it needs to cut the specimens and cannot provide the volume distribution of damage. To give a comprehensive view of the damage inside of the composites, an X-ray computed tomography technique was adopted in this study to visualise the inner failures without damage the specimens. XCT provided a non-destructive way of inspecting and visualising the inner failure of composite laminates and sandwich plates and was used as an important tool for structural characterisation in 3D non-destructive evaluation.(3) Numerical study on damage mechanisms of composite laminates under quasi-static punch-shear loading. A finite element analysis(FEA) was performed using the ABAQUS/Explicit commercial package with a progressive damage model to simulate the response of satin weave composite laminates under punching. The progressive damage model was based on Hashin 3D failure criteria and mechanical properties discount method to simulate the material degradation during the loading process. It was shown that the model reproduced the contact force-displacement behavior of the composites properly. In addition, the damage modes were also correlated with the test observations. It was found that the load pattern of the plate changed from local shearing to overall bending, which corresponded to the inflection stage of the load-displacement curve. The final failure was caused by major delamination and fibre breakages due to shear and tensile stresses developed during the QS-PS tests. The validated progressive damage model was then used to simulate the face sheet of sandwich plates.(4) Numerical study on damage mechanisms of sandwich plates with satin weave composite face sheet and Nomex core during quasi-static punch-shear loading. The validated progressive damage model was used in face sheets of sandwich plates to reproduce the load-displacement curve and to identify the different damage modes for the whole process. The model accounted for the element deletion to simulate the material separation in real loading conditions. The damage modes obtained from finite model were compared with the observations from the test results and it showed a good correlation. It was indicated that the damage model was properly built and can give reasonable results on damage evolution and failure modes during quasi-static punch-shear loading of composite structures. It can capture the inner damage initiation which cannot easily been observed during testing and helps in revealing the damage mechanisms of composite structures under punching.