In developing Type V hydrogen tanks for energy storage and propulsion in commercial airliners, the key design criterion is maintaining tightness under cryogenic conditions. A concern is that anomalies in the laminate could cause microcracks which compromise the tightness. This study examined how resin flow, caused by an expanding mould during curing, creates a gradient in the local fibre volume fraction (FVF) across the laminate. The impact of resin system selection, autoclave cycle parameters, mandrel material, and fibre tension on the FVF gradient was investigated. Cylindrical specimens were manufactured in a process replicating automated fibre placement (AFP), with a piezoelectric sensor measuring contact pressure at the mandrel-laminate interface during the autoclave cycle as an indicator of resin flow and FVF spread. Significant correlations were found between resin flow and all parameters except fibre tension. The resin's viscosity profile and gelation characteristics had the most substantial impact on the FVF spread. Resins with high viscosity and short gelation times lowered the FVF spreads the most. Mandrel materials with lower thermal expansion coefficients (CTE) also reduced FVF gradients, though to a lesser extent. Adjusting the autoclave cycle to lower temperatures for longer periods resulted in the least significant reduction in FVF spread; however, this approach holds high potential for further improvement if more data on resin rheology and cure kinetics are available to optimise the autoclave cycle. These findings provide valuable insights for minimising FVF gradients in the design of carbon fibre-reinforced polymer (CFRP) tanks for liquid hydrogen.

Increasing demands for composite structures and sustainability goals have resulted in the growth of thermoplastic polymer matrix composites and automated manufacturing technologies. The in-situ Automated Fiber Placement (AFP) manufacturing of thermoplastic prepreg tapes has the potential to provide a fast and cost-effective manufacturing solution for large composite structures. However, it is still under development because of the complex mechanisms and short processing times involved which leads to several defect formations, especially gaps and overlaps. One of the primary reasons for the formation of these gaps and overlaps is the tape width deformation during placement.

Current literature on tape width deformation shows that the resulting tape width is influenced by several processing parameters such as temperature, pressure and placement speed. However, results from different studies do not agree with each other, indicating that the tape temperature distribution might be at play. Additionally, the conventionally considered tape width deformation mechanism i.e., transverse squeeze flow has been suggested to be incorrect for the AFP process as the experimental deformations do not agree with the results of the transverse squeeze flow model. Therefore, the research objective for this study was to experimentally investigate the width deformation mechanism and the influence of processing parameters for thermoplastic prepreg tapes using in-situ AFP manufacturing and humm3® (from Heraeus) as the heating device.

The specimens were manufactured according to a full-factorial Design of Experiments (DoE) with two settings (high and low) for the following processing parameters: heated length, nip-point temperature and compaction force. The tape width was measured for all the specimens to investigate the influence of the different processing parameters and some post-processing analyses were carried out to understand the tape width deformation mechanism. This included width measurement in the heating phase of the process, surface roughness analysis, tape cross-section profile inspection and fiber-resin content analysis.

From the post-processing analyses and investigations, it was found that the tape width deforms in the heating as well as the consolidation phase of the process. Additionally, the cross-section images show that the conformable roller led the tape edge profile to have a gradual decrease in thickness with a clear slope and the tape edges show a clear indication of spreading of the fiber-resin mixture due to the presence of both fibers and resin. Moreover, the surface roughness data show an indication of the role of temperature distribution because the as-received tape surface roughness was achieved for the higher temperature and longer heated length settings which are assumed to promote better heat distribution in the material.

The influence of the processing parameters on the tape width deformation did not show clear trends for all specimen configurations. However, the exceptions pointed towards the role of temperature distribution in the tape that led to the overshadowing of the effect of other processing parameters. Considering this, it was observed that the change in heated length did not have a significant effect on the tape width except for one configuration i.e., 300 N, 370 °C, wherein a clear increase with no overlap in data was seen. For the effect of temperature and compaction force, it was found that they have an influence on the tape width for temperatures lower than the melting temperature of the polymer resin (Tm). Additionally, the fiber straining effect on the tape width deformation with compaction force was suggested for the higher temperature specimens.