Lightweight damage tolerant composite enclosure structures for shielding electromagnetic interference

This thesis documents the main findings of the experimental investigation aimed at a comprehensive design process of a lightweight, damage-tolerant electromagnetic interference (EMI) shielding composite enclosure. The emphasis was put on the experimentally driven optimisation of composite material configurations from the perspective of both EMI shielding and mechanical performance, where the aspects of damage were considered from the very start of the design process. The employed comprehensive methodology enabled consistent and independent verifications upon which the final design features were based. Since electrical conductivity constitutes the main parameter determining shielding effectiveness (SE) of nonmagnetic materials, two-probe experiments were first conducted to aid in the identification of the crucial parameters affecting the measurement of the volume conductivity properties in both in-plane and through the-thickness direction, with the main aim of obtaining possibly accurate electrical characteristics for the subsequent SE analysis. The clamping pressure was found to have the most significant influence on the recorded values, and increasing the test current by three orders of magnitude to Ampere levels brought about noticeable conductivity reductions attributed to the Joule heating effects. In contrast, additional tests with the electrodes on the same surface revealed a significant effect of current damping ascribed to the electrical anisotropy of carbon/epoxy composites. The primary novelty of the work focused on the electromagnetic (EM) shielding performance of carbon fibre/epoxy cross-ply (CP) and quasi-isotropic (QI) laminates before and after sustaining a controlled impact and lightning strike damage for establishing their EM shielding durability. Following a thorough damage characterisation with a range of non-destructive testing techniques, it was demonstrated that even a small local delamination could bring noticeable changes to the shielding profile in the absorption mode, while the reflection loss was altered only when the morphology of the CFRP laminate surface was intruded by either fibre tufting or fibre breakage. Throughout the entire analysis process, the effects of both lay-up and thickness were examined, indicating nearly identical, absorptiondominated performance of the two lay-ups with a clear influence of laminate thickness. The secondary area of work explored the mechanical performance of enclosure structural elements, focusing on the damage tolerance of sandwich beams intended for the load-bearing bottom panels and the mechanical behaviour of laminate angle sections representing corner segments. For the former, the reduction trends of the residual compressive strength with the pre-conditioned top skin were evaluated using the four-point-bending method for the effects of incident impact energy and drilled hole size, thereby allowing for establishing threshold damage parameter values necessary for the failure mode transition in different sandwich designs. Experimental evaluations of the 90o laminate angle sections helped ascertain failure sequence under the continued opening and closing moment loading, leading to the identification of interlaminar tension-driven delamination (opening moment) and circumferential compressive stress-driven fibre fracture (closing moment) as potentially weakest points to address in the design considerations for composite corner sections. After reviewing individual experiments' outcomes, the down-scaled enclosure prototype was constructed in an optimised configuration, and a possible SE testing method was described.