Investigations on advanced joining methods for composite materials and fibre reinforcements
Davidson, James R.
Fibre Reinforced Plastics (FRPs) are heavily utilised in high performance engineering applications due to their exceptional physical characteristics, including excellent specific strength, stiffness, fatigue loading performance, and corrosion resistance. Typical applications are aircraft primary and secondary structures, body panels in low-volume sports cars, and wind/tidal turbine blades. The rate of composite material uptake is expected to increase in future years, with some researchers anticipating increased utilisation in affordable solutions—over more typical high-cost products and services. With additional uptake, concerns have been raised regarding both financial and environmental costs, particularly in relation to waste disposal and recycling. Improved methodologies for the design and manufacture of composite materials are therefore required to prevent material waste and improve current end-of-life solutions. To contribute towards alleviating these concerns, this thesis addresses inefficiencies relating to the connections/joints in composite structures. More specifically, studies into the joining of FRPs and their reinforcement yarns have been conducted, where discrete investigations are presented for each. In this thesis, a detailed literature review is first provided, which gives background to all aspects considered in this work. An overview of manufacturing techniques, associated environmental concerns, and current waste disposal/recycling solutions is provided, followed by several more technical sections relevant to the experimental/modelling work conducted in this dissertation. Thereafter, focus is directed towards the development of a generalised modelling approach for the preliminary design of pin-loaded composite connections. Firstly, investigations were conducted into the performance of notched composite specimens, such that approximate characteristic curves could be generated for the prediction of failure loads in pin-loaded specimens. To form this empirical (CCA) method, notched/regular tensile and notched/regular compressive strength ratios were required for a spectrum of different combinations of stacking sequence and hole diameter. Layups were chosen where the mean of the magnitude of all ply-angles (APA)—relative to the loading direction—and corresponding variance value (VAR) could be correlated to these ratios. Regular tension, notched tension, regular compression, and notched compression specimens were manufactured and tested in accordance with various international test standards, in which a carbon fibre reinforced plastic (CFRP) prepreg system was evaluated. Results indicated that hole diameter had only marginal impact on notched strength ratios, whilst the stacking sequence—considered in terms of derived APA and VAR parameters—critically affected notched ratios. Utilising a least squares optimisation method, approximate curves for notched strength ratio versus APA were computed, and correspondingly scaled and shifted for each of the evaluated hole sizes. After performing a cubic-interpolated surface fit over these curves and adjusting the values, approximations for both tensile and compressive notched strength ratios could be obtained for any given combination of layup stacking sequence and hole diameter—for the specific material. Results in this section successfully identify trends which correlate stacking sequence and hole diameter to notched performance, whilst also introducing a novel empirical CCA method for estimating the characteristic curve. After proposing the CCA method for approximating characteristic curves, its integration within finite element models (FEMs) was considered. For all simulations, Python scripting was utilised to automatically generate and run analyses, based on a number of user (or optimiser) specified parameters. A detailed description of this FEM-CCA model, along with a mesh convergence study for elements around the specimen hole-edge/pin, is presented. A variation of fuzzy adaptive particle swarm algorithm (FA-PSO) was then considered as an optimisation approach for maximising the strength of the pin-loaded joint. Two optimisation tasks were performed on eight and twelve ply layups, in which layup symmetry was enforced and hole diameters were fixed. The resulting configurations outputted from the FA-PSO algorithm, along with several other non-optimised configurations, were then re-analysed/analysed, where the number of increments was increased to 400—improving predicted failure load precision. To verify the accuracy of the FEM-CCA modelling approach and whether the optimisation algorithm effectively obtained a high-performing solution, experimental tests—equivalent to the models—were performed. To conduct pin-bearing tests, a custom fixture was designed using topology optimisation software to minimise pin-displacement whilst allowing digital image correlation (DIC) to be utilised. During mechanical testing of various configurations, strain fields were correspondingly recorded throughout the load cycle. The Tsai-Wu failure index was observed to correlate most closely with model results, and strain fields produced from DIC were shown to be useful for observing damage progression. The aforementioned work presents a novel strategy for the preliminary design of pin-loaded composite panels, which could be readily extended to more complex specimen geometries and types of fixtures. Alongside the development of a preliminary design approach for mechanically fastened composite joints, an investigation into a novel pneumatic splicing method for creating connections between reinforcing fibres used in FRPs was conducted. Based on preliminary observations that altering the number of pulses fired along overlapped tows had significant influence on connection strength, dry-fibre connections were formed, where the number of pulses fired ranged from one to fifteen pulses per one-hundred-millimeter overlap. In addition to obtaining failure loads, linear stiffness values for the different specimen types were calculated using a series of nested loops in MATLAB 2019a software. For both linear stiffness and failure loads, increasing the number of pulses correlated asymptotically to increased performance. An assessment of the performance of spliced carbon fibre yarns as reinforcing materials in composites was then conducted, in which the best performing configuration from the fibre-only analysis was utilised. Unidirectional plates were manufactured via hand-laying and press curing, and tensile tested in accordance with international standards. To ensure consistent fibre volume fractions across the plate volume, two arrangements were proposed, in which the position of the spliced overlaps were staggered. To benchmark performance, analogous continuous-fibre unidirectional and chopped strand mat specimens were produced and tested. For all specimen configurations, infrared thermography was performed during testing, as a means of presenting the location/concentration of (heat) energy released during specimen failure. In addition to mechanical testing, scanning electron microscopy was performed, and density and fibre volume fractions were obtained for all types of specimen. The mechanical performances achieved by spliced specimens compared favourably with non-spliced configurations, and based on the obtained results, it is anticipated that better performance can be obtained after improvements to manufacturing processes. The results presented in this section indicate that yarns reconstituted via pneumatic splicing can carrying significant loads, whilst maintaining excellent tensile stiffness. It is therefore postulated that reconstituted yarns comprising of spliced reclaimed fibres have the potential to be utilised in manufacturing processes normally limited to continuous tows, such as 3D-printing and weaving. Overall, this thesis has proposed two discrete and novel approaches relating to joining composite structures and their reinforcing yarns, respectively. Firstly, an efficient modelling method has been proposed for pin-loaded laminate composites. The proposed approach was then successfully implemented within a meta-heuristic algorithm to optimise laminate stacking sequence for maximising joint strength. Secondly, a novel preliminary investigation has been conducted relating to the implementation of pneumatic splicing with tows of carbon fibre. Work presented here is—to the authors knowledge—the first published example which considers this joining method in the context of fibre reinforced plastics. Results auspiciously demonstrate that spliced discontinuous tows have the potential to be utilised in numerous composite manufacturing/re-manufacturing solutions.