Delamination properties of z-pinned composites in hot–wet environment

The effect of hot–wet environment (75 °C and 85% relative humidity) on the delamination fracture properties and interlaminar toughening mechanisms of z-pinned carbon fibre–epoxy composite is investigated. The absorption rate of water from the hot–wet environment into the composite is accelerated slightly by z-pins, although the pins did not change the saturation limit of the material. Absorbed water weakens the pin/composite interface and this lowers the ultimate elastic traction load generated by z-pins under mode I interlaminar loading. However, once the pin/composite interface has failed, the traction load and energy required to pull-out the z-pins is not affected by absorbed water. The mode I interlaminar fracture toughness and low-energy impact damage resistance of z-pinned composites is not degraded significantly by exposure to hot–wet environment, and this is because absorbed water does not affect the pull-out traction properties of z-pins.     

Fabrication and characterization of three-dimensional PMR polyimide composites reinforced with woven basalt fabric

A PMR polyimide composite reinforced with three-dimensional (3D) woven basalt fabric is fabricated for medium high temperature applications. The PMR polyimide matrix resin is derived from 4,4′-methylenediamine (MDA), diethyl ester of 3,3′,4,4′-oxydiphthalic (ODPE) and monoethyl ester of Cis-5-norbornene-endo-2,3-dicarboxylic acid (NE). The rheological properties of the PMR polyimide matrix resin are investigated. Based on the curing reaction of the PMR type polyimide and the rheological properties, an optimum two-step fabrication method is proposed. The three dimensional fabric preforms are impregnated with the polyimide resin in a vacuum oven at 70 °C for 1 h followed by removing the solvent and pre-imidization. The composites are then consolidated by an optimized molding procedure. Scanning electron microscopy analysis shows that needle shaped voids are generated in yarns and the void volume fraction is 4.27%. The decomposition temperature and the temperature at 5% weight loss of the composite post-cured at 320 °C for 24 h are 440 °C and 577 °C, respectively. The dielectric constant and the dielectric loss of the composite are measured by circular cavity method at 7–12 GHz. The tensile strength and the modulus in the warp direction of the composite are 436 MPa and 22.7 GPa. The composite shows a layer-by-layer fracture mode in three-point bending test. The flexure strength and modulus in the warp direction of the composite are 673 MPa and 27.1 GPa, respectively.     

Characterization of high-velocity impact damage in CFRP laminates: Part II – prediction by smoothed particle hydrodynamics

High-velocity impact damage in CFRP laminates was studied experimentally and numerically. Part I of this study observed and evaluated near-perforation damage in the laminates and characterized the damage pattern experimentally. Part II predicts the extension of high-velocity impact damage based on smoothed particle hydrodynamics (SPH), which facilitates the analysis of large deformations, contact, and separation of objects. A cross-ply laminate was divided into 0° and 90° layers, and virtual interlayer particles were inserted to express delamination. The damage patterns predicted on the surfaces and cross-sections agreed well with the experiments. The analyzed delamination shape was similar to that resulting from a low-velocity impact, consisting of pairs of fan-shaped delaminations symmetric about the impact point. Finally, the mechanisms of high-velocity impact damage in CFRP laminates are discussed based on the observations and numerical analyses.     

Degradation in fatigue behavior of carbon fiber–vinyl ester based composites due to sea environment

This study focuses on the effect of confined and one sided sea water confinement on the cyclic fatigue behavior of carbon fiber reinforced vinyl ester composites that serve as facings materials for naval sandwich structures. Experimental results for facings yielded failures under much lower number of cycles when fatigued under immersed conditions surrounded by sea water than in air. Water penetrates the matrix resin through diffusion and fiber/matrix interface by capillary action through micro-cracks or inter-layer delaminations. During fatigue loading, its inability to drain during the downward (compressive) cyclic loading and the near incompressibility of water induces an internal pore water pressures which dominates the progressive failure mechanism. Sea water induced fatigue degradation data and resulting microstructure changes are obtained using high resolution X-ray micro-tomography along with the implications for marine composites.     

The tensile fatigue behaviour of a silica nanoparticle-modified glass fibre reinforced epoxy composite

An anhydride-cured thermosetting epoxy polymer was modified by incorporating 10 wt.% of well-dispersed silica nanoparticles. The stress-controlled tensile fatigue behaviour at a stress ratio of R = 0.1 was investigated for bulk specimens of the neat and the nanoparticle-modified epoxy. The addition of the silica nanoparticles increased the fatigue life by about three to four times. The neat and the nanoparticle-modified epoxy resins were used to fabricate glass fibre reinforced plastic (GFRP) composite laminates by resin infusion under flexible tooling (RIFT) technique. Tensile fatigue tests were performed on these composites, during which the matrix cracking and stiffness degradation was monitored. The fatigue life of the GFRP composite was increased by about three to four times due to the silica nanoparticles. Suppressed matrix cracking and reduced crack propagation rate in the nanoparticle-modified matrix were observed to contribute towards the enhanced fatigue life of the GFRP composite employing silica nanoparticle-modified epoxy matrix.     

Tribological performance of carbon nanotube–graphene oxide hybrid/epoxy composites

An investigation is conducted on the effect of the hybrid of multi-wall carbon nanotubes (MWCNTs) and graphene oxide (GO) nanosheets on the tribological performance of epoxy composites at low GO weight fractions of 0.05–0.5 phr. The MWCNT amount is kept constant at 0.5 phr, which is typical for CNT/epoxy composites with enhanced mechanical properties. Friction and wear tests against smooth steel show that the introduction of 0.5 phr MWCNTs into the epoxy matrix increases the friction coefficient and decreases the specific wear rate. When testing the tribological performance of MWCNT/GO hybrids, it is shown that at a high GO amount of 0.5 phr, the friction coefficient is decreased below that of the neat matrix whereas the wear rate is increased above that of the neat matrix. At an optimal hybrid formulation, i.e., 0.5 phr MWCNTs and 0.1 phr GO, a further increase in the friction coefficient and a further reduction in the specific wear rate are observed. The specific wear rate is reduced by about 40% down to a factor of 11 relative to the neat epoxy when the GO content is 0.1 phr.     

Prediction of initiation of transverse cracks in cross-ply CFRP laminates under fatigue loading by fatigue properties of unidirectional CFRP in 90° direction

A fatigue life to the initiation of transverse cracks in cross-ply carbon fiber-reinforced plastic (CFRP) laminates has been predicted using properties of the fatigue strength of unidirectional CFRP in the 90° direction. In the experiments, unidirectional [90]₁₂ laminates were used to obtain a plot of maximum stress versus number of cycles to breaking, and two types of cross-ply laminates of [0/90₄]_S and [0/90₆]_S were used to evaluate the initiation and multiplication of transverse cracks under fatigue loading. Transverse cracks were studied by optical microscopy and soft X-ray photography. Analytical and experimental results showed good agreement, and the fatigue life for transverse crack initiation in cross-ply laminates was predicted successfully from the fatigue strength properties of the unidirectional CFRP in the 90° direction. The prediction results showed a conservative fatigue life than the experimental results.