Effects of Manufacturing Induced Features on Damage Evolution in Carbon-Fibre Epoxy Composite Laminates

Abstract

Growth of carbon-epoxy reinforce polymer (CFRP) composite material usage is attributed to its advantages over conventional metals, such as specific strength-to-weight ratios and freedom of design. The i series vehicles that are produced by the BMW Group are a prime example of increasing use of composites in a high-volume sector. High-volume manufacturing necessitates faster production techniques, such as high-pressure resin transfer moulding (HP-RTM). The integration of resin flow channels into HP-RTM moulds assists with production of complex parts by reducing total flow resistance, decreasing filling time and guiding the resin through the mould. This research aimed to identify the effects of local geometric features, such as resin flow channels and resulting lamina undulations, on the mechanical performance of CFRP composite laminates. Two types of resin flow channels were investigated; a single large resin flow channel and multiple small resin flow channels. These are compared alongside reference specimens without resin flow channels. Optical microscopy and micro-CT were used to study the lamina architecture in the region of the resin flow channel and the damage post-failure. The microscopy identified lamina-level out-of-plane undulations and architectural variations in the top-ply stitches. For the large single channels, the out-of-plane lamina undulations were at their maximum close to the resin injection point, with an average maximum out-of-plane lamina undulation of 0.721 mm. By contrast, the smaller multiple channels had only 0.023 mm of out-of-plane lamina undulation, irrespective of their distance to the resin injection point. AE monitoring was combined with statistical quantitative analysis to provide new insight into damage modes, locations, initiation and propagation for composite laminates with integrated resin flow channels. A five-cycle-based flexural loading protocol was established with AE monitoring to provide insight into damage initiation and propagation events. This utilised load and AE event-based analysis techniques, including the Kaiser Effect and Felicity Ratio, as well as identification of critical acoustic and peak load levels, to identify key stages in the failure process. Compared to the reference specimens without flow channels (R), there were 24% and 40% decreases in the critical acoustic loads in the single channel (SC) and multiple channel (MC) specimens, respectively. Identifying the origin and trends of AE events accumulation assisted in the prediction of the sustained load, based on the measured AE parameters. Those predictions were compared against the experimental results. Peak-to-peak load trend behaviour-based predictions of static flexural loads were up to 4% higher than those of average experimental failure loads of the specimens, and all the predictions were within one standard deviation of their corresponding experimental specimens. While fatigue tests are expensive regarding the time, machinery and labour that are required, they are critical to determining service life. This research explored the possibility of identifying the fatigue endurance limit of these composite laminates without conducting a full-cycle fatigue test. The R and MC specimens showed a strong relationship between fatigue life and five-cycle tests, with service life of up to a million cycles when operated below the peak critical loads. The SC specimens contradicted this, with the fatigue mean loads lower by 52 and 2% than those of the peak critical and acoustic critical loads, respectively. The specimens had both low-and high-fatigue lifecycles. This behaviour was influenced by the differing lamina undulation and top-ply stitch locations. An FE model composite laminates with integrated resin flow channels was developed. Lamina-level architectural variations were incorporated into the model. These architectural features were extracted from micrographs. The strains of the models were compared against the analytical solutions and measured DIC strains. The developed models provide an insight into ply-level axial stresses and interlaminar shear stresses. Overall, the results demonstrated that resin flow channels can have a detrimental effect on a CFRP laminates’ strength, particularly when loaded under compression. The statistical quantitative analysis provided new insight into damage modes, locations, initiation and propagation for composite laminates with integrated resin flow channels. A five-cycle-based understanding of fatigue life provided a new understanding and possibly a new way of estimating good fatigue service life. Decisions about flow channel size, location and number(s) need to be considered regarding the specific loading scenarios for each component.