Post impact repair of in-situ polymerisable thermoplastic based carbon fibre composite and its assessment under compression after impact loading

This work investigates the impact response, associated damage, the feasibility of repair and strength recovery of infusible thermoplastic based carbon fibre reinforced composites. Impact behaviour of the laminates is studied using drop tower impact tests at three energy levels i.e. 40 J, 30 J and 20 J resulting in delamination as the primary damage mode at lower energy levels with evidence of fibre breakage at 40 J. Repair is performed by thermally re-consolidating impact damaged specimens under vacuum at a temperature above the glass transition temperature of the matrix. X-ray micro computed tomography (CT) scans and ultrasonic C-Scans reveal significant post repair re-consolidation. Compression testing using compression after impact (CAI) fixture resulted in a retained strength of 78 %, 88 %, and 93 % for impacts at 40 J, 30 J, and 20 J respectively. Post-repair compression strength using CAI tests, recovered 85 % of the pristine strength for 40 J impacts and close to 100 % of pristine strength for both 30 J and 20 J impacts. Thermography during CAI testing revealed a difference in the location of damage initiation under compressive loading for impacted and repaired specimens. The overall results highlight the potential for effective on-site repair and strength recovery using a relatively simple thermal re-consolidation procedure</p

Experimental study of hygrothermal ageing effects on failure modes of non crimp basalt fibre-reinforced epoxy composite

Turbine blades form the main component for energy generation in renewable energy generation technologies such as wind and tidal energy. Non‐crimp fabric (NCF) based fibre reinforced composite materials, with E‐glass and carbon fibres have been widely used as the main materials for blades. However, sustainable composites using naturally derived fibres such as basalt, are being developed to reduce environmental impact. Basalt fibres require no chemical additives, solvents or hazardous materials for production and are recyclable. However, lit tle information is available in the literature on the moisture ageing effects on failure modes of NCF based basalt fibre reinforced epoxy composites. Ageing is particularly important for applications in coastal wind and tidal turbine installations, which are exposed to high humidity. The current study analyses the effect of moisture ageing on flexural, interlaminar shear and in‐plane shear properties and associated failure modes of NCF based basalt fibre reinforced epoxy composite at different stress levels. The results showed no significant impact on flexural stiffness of the composite, but in‐plane shear stiffness and strength (flexural, interlaminar shear and in‐ plane shear) of the composite demonstrated a significant reduction following moisture absorption. Similar fail ure modes were observed in both dry and wet conditions

Failure prediction in a non-crimp basalt fibre reinforced epoxy composite

Failure analysis in non-crimp fabric (NCF) based fibre reinforced composite materials is challenging due to the complexity associated with multiaxial fabric architectures and the interaction of different failure modes. In this study an in-situ 3-point bend test under scanning electron microscopy (SEM) has been performed to analyse the failure behaviour of a NCF basalt epoxy composite manufactured by vacuum assisted resin transfer moulding (VaRTM) technique. The failure mechanisms observed during the in-situ 3-point bend test are compared to those from standard (ex-situ) flexure and interlaminar shear strength (ILSS) tests on the same material. Finite element analysis of each test configuration is also conducted to analyse the in-ply stress distributions and to predict the damage initiating stresses. The study reveals that fibre/matrix debonding leading to matrix cracking in the 90◦ sub-ply is the damage initiating failure mode, with final failure occurring due to fibre buckling in neighbouring 0◦ sub-plies. The finite element analysis reveals that matrix cracking in the 90◦ sub-ply is controlled by shear stresses ranging between 35 and 45 MPa</p

Evaluation of the flexural properties and failure evolution of a hybrid composite manufactured by automated dry fibre placement followed by liquid resin infusion

The market for composites is developing rapidly due to the widespread general requirement for lightweight design in various industries. However, the proliferation of fibre-reinforced composites is limited partly due to the lack of ductility associated with these materials. Hybridisation of fibres has emerged as an important strategy to increase the ductility of polymer composite materials. In the present work, water based dispersion was used to develop a dry fibre carbon tape and a hybrid glass/carbon tape which are both compatible with automated deposition techniques. Dry fibre preforms were manufactured using laser-assisted dry fibre placement (DFP) and laminates were subsequently manufactured using vacuum assisted liquid resin infusion. The hybrid composite exhibited both higher flexural strength (8%) and higher strain-to-failure (58%) relative to the carbon based laminate. To understand the failure modes, X-ray micro-computed tomography was used in conjunction with interrupted flexural tests to monitor the evolution of damage. The progressive failure analysis revealed that for hybrid composites, damage propagation was delayed due to the staggered arrangement of glass and carbon fibres. The majority of damage in the hybrid composite was located in the carbon fibres tows

Handling, inspection and repair of aircraft composites: a pilot study on the awareness of maintenance personnel

Composite materials are enjoying an increasing use in aircraft structures. As more fleets transition from metal to more composite aircraft, the practice of maintenance is also adjusting. Handling, inspection and repair of aircraft composites following non-visible or barely visible damage are among the areas of concern, due to associated cost and safety implications. A pilot study was performed to explore the level of awareness and understanding of aviation maintenance practitioners around these issues. In addition, this research project sought to identify factors related to the technical/engineering judgement capacity of the personnel working with composites and to gauge the need for specialised education and training. A questionnaire survey was administered to a group of 40 professionals working for an aircraft maintenance, repair and overhaul (MRO) organisation. Descriptive statistics in conjunction with analysis of variance (ANOVA) were used to analyse the results. The sources of impact and common areas affected on the aircraft have been identified, with situational awareness suggested as the most important mitigator against impact damage. Over 70% of the participants would refer to their engineering manager for instructions on how to handle composite damage. The need for higher standardisation for composites’ maintenance, repair and handling issues emerged as a common theme across different sections of the survey. Almost all respondents agree on the need for specialised knowledge and training for the handling, repair and inspection of composites. </p

Investigating the effect of cure schedules and cure initiators on sustainable composites for large offshore structures

This study evaluates the effect of post-cure schedules and cure initiator form on the mechanical properties of Glass fibre reinforced polymer (GFRP) laminates manufactured using an infusible reactive thermoplastic resin. Tensile, flexural, shear and dynamic mechanical analysis tests were conducted. Fractography was also performed. Specimens fabricated using liquid cure initiator and subjected to an elevated temperature post-cure were the control specimens. Ambient cured specimens decreased by no more than 12% in the case of tensile properties (modulus of 90◦ specimens) and by < 14.3% in the case of flexural properties (also modulus in 90◦ specimens). Furthermore, the difference in mechanical properties of 0◦ specimens fabricated using a powder cure initiator was observed to be within ≈ 7% of respective properties of control specimens. In the context of fabricating thick laminates for large-size offshore structures, the results suggest that an extended ambient post-cure cycle in conjunction with an initiator in powder form can be employed instead of an elevated temperature post-cure schedule with initiator in liquid form. This is economically beneficial since it eliminates infrastructure required for elevated temperature curing/post-curing. The risk of porosity induced due to liquid-based initiators is also avoided.</p

Compression characteristics and fractography of in-situ polymerisable thermoplastic and bio-epoxy based non-crimp carbon and glass fiber composites

This experimental work involves characterization and fractography of a bio-based epoxy and an in-situ polymerisable thermoplastic polymer matrix based non-crimp glass and carbon fiber composites under compressive loading. The laminates are characterized under compression loading using a combined loading compression (CLC) fixture. Laminates made using the thermoplastic matrix exhibit higher compressive strength (approx. 20% along fiber direction) compared to the bio-epoxy based laminates. Further, both composites exhibit comparable compressive modulus characteristics. The tested composites are subjected to fractography analysis using Scanning Electron Microscopy (SEM) and Computed tomography (CT). SEM results indicate a difference in fiber-matrix interface characteristics between the thermoplastic matrix and the bioepoxy matrix. Additionally, the CT scans reveal a difference in failure modes due to fiber orientations. A difference between failure mode of the exterior and interior plies of the specimens was also noticed. However, no specific influence of matrix type was observed on the overall macroscopic failure behavior. Highlights • Bio-epoxy and thermoplastic based laminates were characterized in compression. • Post-test fractography was performed using SEM and x-ray CT scans. • Use of thermoplastic matrix exhibits better fiber-matrix adhesion compared to bio-epoxy. • Both laminates performed well in compression under laboratory test conditions.</p

In-situ polymerizable thermoplastic and bio-epoxy based composites for offshore renewable energy applications

This study evaluates various quasi-static mechanical properties of an in-situ poly-merizable thermoplastic and a bio-based thermosetting composite comprising of non-crimp fabric reinforcement for potential use in the next generation of Off-shore Wind and Tidal Power platforms. Mechanical properties are characterized under tensile, flexural, in-plane shear and interlaminar shear loading. Results reveal that the evaluated properties differ based upon matrix type. Fractographic evidence from scanning electron microscopy is used to explain the differences observed and was generally consistent in terms of revealing cohesive failure at the fiber-matrix interface for the thermoplastic composite and contrasting adhesive failure for the thermosetting composite. For glass fiber reinforcement, the thermoplastic composite is superior in terms of flexural 90 properties (+20%) while the thermosetting composite performed better in flexure 0 in terms of both strength (+15%) and modulus (+25%). In terms of interlaminar shear, the thermosetting composite exhibited higher strength (+14%) while Tensile and in-plane shear properties are similar for composites of both resin systems. Overall, neither composite is superior in terms of overall mechanical properties and both matrices show promise as a stepping stone towards the use of more sustainable constituents in offshore structures. Highlights • Quasi-static mechanical performance and failure analysis of relatively sustainable composites are presented. • Failure analysis indicate cohesive failure of the thermoplastic based composite and interfacial failure of the thermosetting based composite. • Proposed composites are benchmarked against the composites manufactured using conventional resins. • Overall, both matrices show promise as a stepping stone towards more sustainable offshore structures.</p

Hydrothermal in-plane-shear strength of carbon fibre/benzoxazine laminates manufactured out-of-autoclave by liquid-resin-infusion

Benzoxazine’s have recently emerged as new candidate resins for elevated temperature structural applications in the aerospace sector offering attractive attributes including infusibility under vacuum, fire-smoke-toxicity performance and room temperature storage/transport. The main objective of this study is to evaluate the hydrothermal in-plane-shear (IPS) strength of carbon-fibre (CF) based laminates manufactured using two benzoxazine (BZ) resin systems (BZ9120 and BZ9130). CF/BZ9130 was evaluated at 160 °C in the wet condition and benchmarked against two commercially available bismaleimide (BMI) resin systems – traditionally considered for wet applications at 160 °C. CF/BZ9120 was evaluated at 120 °C (just below its Tg) in the dry and wet condition and benchmarked against CF/BZ9130. BMI’s remain the benchmark for IPS strength at 160 °C (wet) with 64% retention while BZ9130 only retained 48% of IPS strength at 160 °C (wet) and also exhibited excessive elongation. CF/BZ9130 showed good retention at 120 °C (68% wet) outperforming CF/BZ9120 (48% wet). Positively, both BZ systems performed at least as well as the BMI’s under ambient conditions