In this work, a uniformly aligned polyethersulfone (PES) fiber web was designed and fabricated by the melt-spinning PES yarns. Such fiber web was adopted as an interleave to simultaneously improve both mode I and mode II fracture toughness of the carbon fiber/epoxy composite, addressing the issue of transforming the thermoplastic component into a well-controlled uniform and ordered phase structure without being influenced by the liquid forming process. The dissolution behaviors of the PES filaments in the epoxy resin were studied by the optical microscopy with a hot stage. The results indicated that the PES filaments were not dissolvable at the temperature of the resin infusion process, but could be dissolved during the curing process of the epoxy resin in a well controllable way and period. Both the mode I and mode II interlaminar fracture toughness of the composites were investigated as a function of areal densities of the PES fiber webs (7.3, 14.7, 21.2 and 28.3 gsm) carefully. A maximum enhancement up to 120% and 68.8% on mode I and II fracture toughness, respectively was obtained with the introduction of the 28.3 gsm-PES fiber web compared to the laminates without interleaves. Analysis of the fracture surfaces of the laminates elucidated that the distinctive improvement of the interlaminar fracture toughness could be attributed to the characteristic interlaminar structures induced from the phase separation of PES in epoxy resin. Moreover, the interleaved laminates displayed an increase of 18.2% and 43.8% for the interlaminar shear strength (ILLS) and compression-after-impact (CAI) properties, respectably. The tensile and flexural properties of the composite were thereafter explored which indicated slightly enhancement on the strengths.

Simultaneously improving mode I and mode II fracture toughness of the carbon fiber/epoxy composite laminates via interleaved with uniformly aligned PES fiber webs

Laminar carbon fibre composites with weak ply interfaces were toughened with ultra-thin (less than 20 μm) un-bonded non-woven Short Aramid Fibre (SAF) veils in the form of home-made pre-pregs. The effects of SAF veil on impact behaviour and compressive strength after impact of Carbon Fibre Reinforced Polymer laminates with multiple ultra-thin SAF interfacial layers were investigated. Optical observation and X-ray micro-computed tomography were employed for intact inspection of external and internal damage after high energy penetration impact. 50.8% decrease in back-face deflection and inhibited in-plane delamination growth were seen when compared with unmodified laminates. Compression after impact tests were performed after low energy impact to assess damage tolerance. SAF veils increased the residual compressive strength by 38.6% and changed the damage mode from delamination dominated fracture to shear failure.

Improving impact resistance and residual compressive strength of carbon fibre composites using un-bonded non-woven short aramid fibre veil

Unidirectional carbon fiber reinforced polymer (UD-CFRP) composites designed for ultimate tensile strength and stiffness along the fiber direction are vulnerable in the transverse direction. In this study, UD-CFRPs were reinforced in the transverse direction using sparsely distributed hierarchical Aramid pulp (AP) micro/nano-fibers between 2 and 8 g per square meter (gsm) per ply interface. Those AP micro/nano-fibers, a few hundred microns in length, effectively formed an ultra-thin interleaving layer around 20 μm in thickness between carbon fiber plies. Flexural properties along and perpendicular to continuous carbon fiber direction were measured under three-point-bending (3-P-B) to determine the toughening effects of AP. Flexural loads in both longitudinal and transverse directions have been enhanced by 30% and over 100% respectively from those randomly distributed AP micro/nano-fibers with diameters varied from a few hundred nanometers to 10 μm. Even slight increase in the elastic modulus of AP-toughened CFRP along the fiber direction was observed. Total fracture energy absorption, initial cracking and final fracture loads were measured and compared, indicating AP micro/nano-fibers have contributed to the high strength, high toughness and high modulus characteristics of AP-interleaved UD-CFRPs. Details on toughening mechanisms were revealed from SEM and cross-section optical microscopy examinations. Comparisons of unreinforced UD-CFRP and AP-toughened UD-CFRP showed the hierarchical AP micro/nano-fibers indeed played an important role in composite microstructure design and property optimization. The ultra-thin interleaved AP veil around 20 μm in thickness can be conveniently incorporated into pre-preg fabrication so that the extra interleaving step can be eliminated, i.e. the toughened pre-pregs can be used as normal pre-pregs in the final composite forming.

Transverse and longitudinal flexural properties of unidirectional carbon fiber composites interleaved with hierarchical Aramid pulp micro/nano-fibers

This study presents a simple technique for positioning helical carbon nanotubes (CNTs) into chemically-etched vertical micro-channels on an aluminum (Al) substrate for strong adhesive bonding with carbon fiber composites. Helical CNTs with diameter up to 100 nm and length up to 10 μm were dispersed evenly in an acetone-diluted resin pre-coating (RPC) solution consisting of 90 wt% acetone + 10 wt% resin (without hardener), then applied onto the chemically etched Al substrate with vertical micro-channels. CNTs were dragged into those vertical micro-cavities through the capillary action during the evaporation process of acetone. The shear bond strength was more than doubled by the quasi-Z-directional CNTs, confirming the critical role of CNT bridging across the interface between the epoxy adhesive and Al substrate. Scanned electron microscope (SEM) and focused ion beam (FIB) were used to reveal micro-channels on the Al substrate. RPC on the surface of carbon fiber composites is also beneficial.

Yuan Hu

Papers

Low-velocity impact damage research on CFRPs with Kevlar-fiber toughening