Strucral property improvements to aerospace sandwich composites using z-pins
Creators: name: Nanayakkara, Asintha
Publisher: name: RMIT University
Year: 2024

description: The general aim of this PhD project is to advance the science and technology of zpinned sandwich composites by performing an in-depth investigation into their mechanical properties, strengthening mechanisms and damage modes. The PhD thesis presents a comprehensive and critical review of the published scientific literature into z-pinned sandwich composites. While past studies often report large improvements to the mechanical performance of sandwich composites due to zpinning, the research is incomplete and gaps exist in the characterisation of these materials. The identification of these gaps in the characterisation of z-pinned sandwich composites provides the basis for the original research work performed in this PhD project.<br><br>The PhD thesis presents a study into the through-thickness compression properties, strengthening mechanisms and damage modes of a sandwich composite structure reinforced with orthogonal z-pins. It was found that less than 4% in z-pin volume content was required to increase greatly the compression modulus (up to 300%), strength (700%) and strain energy absorption capacity (500%). While the compression properties were found to be highly sensitive to the z-pin content, the properties were much less dependent on the end constraint (i.e. built-in column or unsupported column) and diameter of the pins. An investigation into the compressive failure mechanisms of the z-pins within the foam core using acoustic emission monitoring, scanning electron microscopy and x-ray computed microtomography revealed for the first time that the fibrous z-pins failed during both elastic and plastic deformation of the core foam via a complex damage process involving splintering, kinking and fragmentation. It is shown that existing models fail to accurately determine the compression properties due to the complex failure mechanism of the zpins, which are not accounted for in the existing models.<br><br>The PhD thesis presents a comprehensive experimental study into the impact damage resistance, post-impact mechanical properties of z-pinned sandwich composites and localised loading behaviour, which has not been previously investigated to any great detail. The research showed that there was no improvement to the impact damage resistance of the z-pinned sandwich composite at low impact energies (when damage was confined to the impacted face skin). The post-impact compressive stiffness and failure load for the z-pinned sandwich composite remained the same (within experimental scatter) as the unpinned material. Z-pinning was found to be only marginally effective at increasing the damage resistance when the impact energy was high enough to cause core crushing. This study showed that under a localised impact load, z-pins were not particularly effective at increasing the damage resistance or post-impact mechanical properties of sandwich composites and this is attributed to the small number of pins available to resist a localised (point) impact load. It was discovered that increasing the loading area improved the indentation stiffness and crush strength, and this was due to the increased number of z-pins resisting indentation. The experimental indentation results were further analysed against predictions using an indentation model for z-pinned sandwich composites.<br><br>As a final novel study, the effect of z-pinning on the mechanical performance of Tshaped bonded sandwich joints was investigated. Experimental testing revealed that the stiffness, ultimate load and absorbed energy capacity of the sandwich composite joint was improved by z-pinning. The failure load and energy absorption were increased by the z-pins suppressing skin-to-core failure by generating bridging traction loads through the foam core. Pin pull-out tests revealed that z-pins generated high mode I bridging traction loads during frictional pull-out from the face skins, and this increased the load capability and stabilised the fracture process of the sandwich joint. The improvements to the mechanical properties of the T-joint are discussed using mechanical models for the bridging laws of z-pins in composite materials. This research revealed for the first time that z-pinning could be used an as alterative to mechanical fastening for the high strength joining of T-section sandwich composite components.<br><br>The PhD thesis concludes with a summary of the major research findings, a discussion of future research directions into z-pinned sandwich composite panels and joints, and the remaining challenges in the certification of z-pinned sandwich composites for use in aircraft structures.