Impact Loading of Composite and Sandwich Structures

Sammanfattning: Low weight is one of the most important factors in the design process of high speed naval ships, road vehicles and aircrafts. Lower structural weight enables the possibility of down-sizing the propulsion system and thus decrease manufacturing and operating costs as well as reducing the environmental impact.Two efficient ways of reducing the structural weight of a structure is by using high performance composite materials and by using geometrically efficient structures such as the sandwich concept. In addition to good quasi-static performance different structures have dynamic impact requirements. For a road vehicle this might be crash worthiness, an aircraft has to be able to sustain bird strikes or debris impact and a naval ship needs to be protected against blast or ballistic loading. In this thesis important aspects of dynamic loading of composite and sandwich structures are addressed and presented in the appended papers as follows.In paper A the notch sensitivity of non-crimp fabric glass bre composites is investigated. The notch sensitivity is investigated for several different laminate con gurations at varying tensile loading rate. It is shown that the non-crimp fabrics have very low notch sensitivity, especially for laminate con gurations with a large amount of bres in the load direction. Further, the notch sensitivity is shown to be fairly constant with increasing loading rates (up to 100/s).In paper B a heuristic approach is made in order to create an analytical model to predict the residual strength of composite laminates with multiple randomly distributed holes. The basis for this model is a comprehensive experimental programme. It is found that unidirectional laminates with holes predominantly fail through three failure modes: global net-section failure, local net-section failure and local shear failure. Each failure mode can be described by a physical geometric constant which is used to create the analytical model. The analytical model can predict the residual strength of unidirectional laminates with multiple, randomly distributed holes with good accuracy.In paper C and paper D, novel prismatic high performance all-composite sandwich cores are proposed. In paper C an analytical model is developed that predicts the strength and sti ness properties of the suggested cores. In paper D the prismatic cores are manufactured and tested in shear loading and out-of-plane compression loading. Further, the analytical model is used to create failure mechanism maps to map out the overall behaviour of the different core con gurations. The novel cores show very high speci c strength and sti ness and are potential candidates as cores in high performance naval ship hulls.In paper E the dynamic properties of prismatic composite cores are investigated. The dynamic out-of-plane strength of an unit cell is tested experimentally in a gas gun - Kolsky bar set-up. Especially, different failure mechanisms and their e ect on the structural strength are investigated. It is found that cores with low relative density (slender core members) show very large inertial stabilisation e ects and have a dynamic strength that can be more than seven times higher than the quasi-static strength. Cores with higher relative density show less increase in dynamic strength. The main reason for the dynamic strengthening is due to the strain rate sensitivity of the parent material rather than inertial stabilisation of the core members.