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A phenyl–ethynyl terminated polyimide resin based on KAPTON-type backbone structures, 'TriA-X', polyimide is promising as a matrix resin of heat-resistant carbon fiber reinforced plastic composites, but its mechanical properties remain unclear. This study examined off-axis stress–strain behavior of unidirectional carbon fiber (CF)/TriA-X polyimide composites under tensile load at room temperature (RT), 100°C, 150°C, and 200°C to elucidate the matrix resin plastic deformation behavior. Comparing the rupture stress and rupture strain of off-axis specimens at RT and 100°C, they decreased concomitantly with increasing test temperature. Comparison of the rupture stress at 100°C, 150°C and 200°C revealed no significant difference in the rupture stress. A one-parameter plasticity model proposed by Chen and Sun with temperature-dependence of the plasticity model parameters elucidated the nonlinear stress–strain behavior. Model results agreed well with the nonlinear stress–strain behaviors of the CF/TriA-X composites at RT, 100°C, 150°C, and 200°C. [ABSTRACT FROM AUTHOR]

By experimentation, this study investigated ply thickness effects on the filled hole compression (FHC) strength of thin-ply CFRP laminates. The FHC strength of quasi-isotropic (QI) laminates increased by 26–45% with decreasing ply thickness from 0.2 to 0.05 mm. Comparing the case of 0 Nm and 7.0 Nm tightening torque, the FHC strength was more than 28–49% higher for the QI laminates with a tightening torque of 7.0 Nm. The FHC strength of thin-ply QI-CFRP (0.05 mm ply thickness) at a fastener tightening torque of 7 Nm was 675 MPa, which was greater than the un-notched compressive (UNC) strength (590 MPa). Visual observation and X-ray CT results of fractured specimens after FHC testing suggested that bearing damage of the CFRP laminates was prevented under adequate fastener tightening torque. It was inferred that the increase in the FHC strength in thin-ply CFRP was mainly contributed by the increase in the UNC and OHC strength. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper presents the mechanical and gas barrier properties of titanium alloy foil-inserted carbon fiber/epoxy composites (CFRPs) undertaken to improve gas barrier properties for cryogenic propellant tank applications. A newly developed ß-titanium alloy (GumMetal, GM) was applied. A sheet of titanium foil (a-Ti or GM, 0.05 or 0.1mm) was inserted between cross-plied composite laminates ([0°2/90°2]s) without adhesive. Epoxy resin in the prepreg contributes to bonding between the Ti-foil and CFRP. The Ti-alloy foil insert did not strongly affect the tensile or compressive strength. Transverse cracks in the 90° layer never penetrated into the Ti-alloy foil layer before the final failure under tensile loading. Nonlinear stress-strain behaviors attributable to the pseudo-elastic behaviors of both unidirectional CFRP and ß-Ti alloy (GM) were estimated using high-order stiffness and classical lamina theory. The numerical results agreed with the experimentally obtained results. Helium gas leakage under tensile stress was not observed before the final failure of the composites (1.4% of tensile strain). The excellent mechanical and gas barrier properties were successfully demonstrated. [ABSTRACT FROM AUTHOR]

This paper presents fatigue lifetime data of impact-damaged carbon fiber/toughened epoxy composites under compressive loading to elucidate the lifetime prediction methodology based on statistical approaches. Drop-weight impact damage was induced to a composite specimen with impact energy of 6.7J/mm in accordance with ASTM D7136. Postimpact fatigue tests were conducted using a test fixture defined in ASTM D7137 under compres-sion-compression loading = 10) at room temperature. The maximum compressive stress (Smin) was 200, 210, 220, 230, and 240 MPa, and the total number of specimens was 31. The compression-after-impact strength and fatigue lifetime show considerable scattering. This result is apparently derived from the variation in impact damage size. A simple statis-tical model based on the weakest link theory was proposed for predicting the lifetime of impact-damaged composite laminates. Results show that the experimentally obtained data were confirmed consistently using this model. The ratio between the endurance limit at 106 and 107 cycles and the initial static strength was estimated as 0.74 and 0.68 as B-basis allowable values. [ABSTRACT FROM AUTHOR]

This paper presents experimentally obtained results of mode-I and mode-II interlaminar fracture toughness (GICandGIIC) of unidirectional and 5 harness satin (5HS) woven fabric carbon fiber/epoxy composites (CFRPs). The mode-I delamination resistance of 5HS specimens, which was evaluated using a double cantilever beam (DCB) method, depends on the weave pattern and the ply stacking sequence at the delamination growth plane. Higher toughness was observed for crack propagation between surfaces with more transverse bundles (L–T and T–T plane) than those with more longitudinal bundles (L–L plane) because of transverse tow delamination pinning the crack and causing it to arrest. The intrinsic mode-I fracture toughness values of the 0°/0°, 0°/90° and 90°/90° fiber combinations were estimated from theGICvalues obtained from three kinds of 5HS specimens which have different mid-plane stacking patterns (L–L, L–T and T–T). TheGICof 0°/0° combination corresponded to that of unidirectional specimen. TheGICof 0°/90° combination was almost identical to that of 90°/90° combination. Results suggest that the interlaminar fracture toughness of woven fabric composites can be estimated from theGICof 0°/0° and 0°/90° (or 90°/90°) combinations. [ABSTRACT FROM PUBLISHER]

Abstract: This paper presents results of the feasibility of carbon/epoxy composites (CFRP) as a future helicopter flexbeam material. Torsional behaviors of unidirectional CFRP and glass/epoxy composites (GFRP) with the same resin matrix were investigated. The initial torsional rigidity of CFRP was almost identical to that of GFRP. The torsional rigidities calculated using finite element analyses (FEA) agreed with the experimental results: the torsional rigidities are governed mainly by the material’s shear stiffness. Torsion fatigue tests were also conducted by controlling the angle of twist of the sinusoidal wave under a constant tensile axial load. No catastrophic failure occurred with either GFRP or CFRP, although decreased amplitudes of torque and torsional rigidities were observed according to the number of cycles. Results of X-ray CT inspections and numerical calculation by FEA revealed that degradation of a torsional rigidity is caused mainly by splitting crack propagation along the fiber direction. The torsion fatigue life of CFRP was superior to that of GFRP. Consequently, results confirmed that CFRP exhibits excellent properties as a torsional element of a helicopter flexbeam in terms of torsional rigidity and tension–torsion fatigue behaviors. [Copyright &y& Elsevier]

Abstract: This study examined the effect of fullerene dispersion on the mechanical properties of carbon-fiber reinforced epoxy matrix composites (CFRPs). Mechanical properties such as tension, compression, open-hole compression, comparession after impact (CAI), binding, short beam shear, and interlaminar fracture toughness were evaluated for [0]8, [90]16, [45/0/−45/90]2S laminates. Tension and compression strengths increased 2–12% by dispersing 0.5% of fullerene into the matrix resin. Furthermore, interlaminar fracture toughness of the composite was improved by about 60%. It was revealed that a small amount of fullerene (0.1–1wt.%) increased the failure strain of epoxy resin itself, thereby improving the CFRP strength. [Copyright &y& Elsevier]

Abstract: This paper presents a sensitive strain-monitoring composite as compared with carbon fiber reinforced plastics (CFRP) using electrical resistance method. Carbon fibers in CFRP are useful as sensors under tensile load to detect fiber fracture and strain of the composites using electrical resistance change. However, the electrical resistance change resulting from fiber breaks is not sensitive at low-strain regimes because of the electrical conduction among neighboring fibers. In this study, surface oxidized SiC fibers were applied to prevent electrical conduction resulting from contact of neighboring fibers. Model specimens were fabricated by embedding as-received or oxidized SiC fibers into epoxy resin. Then, electrical resistance changes were measured under monotonic and loading–unloading tensile tests. Consequently, electrical resistance of oxidized SiC fiber specimens increased remarkably with increasing strain in comparison to as-received SiC fiber specimens as well as CFRP. The fibers were isolated electrically from each other by the SiO2 layer on fiber surface. Cumulative fracture probabilities of oxidized fibers embedded in the model specimens were predicted from measured electrical resistance change. The strength distributions were analyzed based on Weibull statistics. [Copyright &y& Elsevier]

Abstract: Compressive strength of unidirectional carbon–fiber composites is analyzed with emphasis on the nonlinear elasticity of carbon fiber. The deformation and stress state in a kink band is solved numerically using an incremental form of the governing equations, and compressive strength is determined for a given kink band angle and initial fiber misalignment. Constitutive equations are based on one-parameter plasticity model and nonlinear elasticity, which is attributed to matrix plasticity and fiber nonlinearity, respectively. Effects of fiber nonlinear properties on the predicted compressive strength are investigated under various conditions of initial fiber misalignments, kink band angles, and fiber properties. Nonlinear fiber properties turn out to reduce the predicted compressive strength of unidirectional composites in the cases of small fiber misalignments. [Copyright &y& Elsevier]

This paper presents experimental results for the processing and mechanical properties of carbon fiber reinforced composites of a newly developed amorphous, asymmetric, and addition type polyimide (Triple-A PI). The imide oligomers were synthesized from the reaction of 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 4,4′-oxydiaminine (ODA), and phenylethynyl phthalic anhydride (PEPA). Because of amorphous structure, the melting point and melt viscosities of the polymer are relatively lower as compared with similar polyimides such as LaRC™ PETI-5. In spite of the lower molecular weight of the imide oligomer (<2500 g/mol), the cured polymer exhibits excellent mechanical properties because of the irregular and asymmetric structure as well as flexible end-capper. Carbon fiber reinforced composites were fabricated by routing prepreg consolidation. The composites exhibit excellent mechanical properties with high glass transition temperature (>320°C). [ABSTRACT FROM AUTHOR]

Polymer composites are widely used for high temperature thermal protection (TPS) materials for spacecraft heat shields, and nozzle liners for solid propellant rocket motors. Newly developed silicon containing polymer, abbreviated MSP (Mitsui Silicon Containing Polymer), possesses high decomposition temperature (500°C), and high char yield (94% at 1000°C) after pyrolysis. Therefore, the MSP polymer has potential as matrix resin of high performance ablative composites compared with conventional phenolic resins. In this study, thermal performance and ablation characteristics of a carbon composite with the MSP polymer (CF/MSP) were investigated under supersonic plasma air stream. The cold wall heat fluxes ranged from 1.2 to 11.0 MW/m2. Conventional carbon fiber phenolic (CF/PR) composite was tested at the similar conditions for direct comparison. The mass loss of the CF/MSP composite was far less than that of the CF/PR composite, which is due to higher char yield and consequent high density of char layer. On the other hand, higher internal temperature was measured in the CF/MSP composite. [ABSTRACT FROM AUTHOR]

Abstract: Composite laminates manufactured from thin-ply prepregs are expected to have superior damage resistance properties compared to those from standard prepregs. Although the use of thin-ply prepregs leads to increase in manufacturing cost, the damage resistance properties against matrix cracking and delamination increase. This study investigates several strength properties as well as the damage resistance properties of carbon fiber/toughened epoxy composite laminates for the applicability of thin-ply prepregs to aircraft structures. Specifically, compressive strengths of open hole laminates (OHC strength) and laminates after impact loadings (CAI strength) are investigated by the comparison of results between laminates manufactured from the thin-ply prepregs and the standard prepregs. It is shown that laminates using thin-ply prepregs have superior strength compared to those using standard prepregs. It is expected that the design limit of composite aircraft structures increase by using thin-ply prepregs. [Copyright &y& Elsevier]

Taking a biomimetic approach, this study introduces the use of a Voronoi diagram as an indicator of stiffener arrangement in stiffened composite panels. The locations of generator points that determine the shape of the Voronoi diagram are optimized for buckling problems under compression and shear load. An original optimization algorithm that combines the genetic algorithm and the finite element method is developed and implemented. The Voronoi diagram can also be used as an indicator for conventional, periodical stiffener shapes such as grid, lattice, honeycomb, and isogrid. We conducted parametric studies for periodical stiffener shapes, and demonstrated that these shapes can be designed using Voronoi diagrams. The results revealed that the optimized panel has a higher buckling load than that of conventional panels, and this methodology is effective for designing biomimetic stiffened CFRP panel structures. [ABSTRACT FROM AUTHOR]

Abstract: Composite laminates with thin-ply layers are expected to exhibit superior damage resistance to the standard composite laminates. This study investigated the damage characteristics of carbon fiber/toughened epoxy thin-ply laminates subjected to transverse loadings. Quasi-isotropic laminates were prepared using both standard prepregs and thin-ply prepregs in order to examine the effect of ply thickness on the damage accumulation processes. Clear difference on damage accumulation process between standard laminates and thin-ply laminates was identified; fiber fractures were susceptible to formation in thin-ply laminates. Finally, the reason of the difference on damage process was investigated using finite element analyses, and it was clarified that the accumulated delamination position has a significant effect on the fiber fractures during the indentation. [ABSTRACT FROM AUTHOR]

This study examined lightning strike damage behaviors of carbon fiber reinforced plastic (CFRP) laminates under air (0.1 MPa), reduced-pressure air (0.02 MPa), and N 2 (0.1 MPa) environments to elucidate the effects of atmospheric environments on lightning damage. Simulated lightning current in accordance with SAE-ARP 5412B was applied to each specimen. The maximum current peak value was defined as 40 kA. CFRP laminates under reduced-pressure air (0.02 MPa) exhibited slight damage compared with that under atmospheric-pressure air. By contrast, CFRP laminates under N 2 sustained considerably more damage than under air (0.1 MPa). High-speed observations revealed that the atmospheric gas and pressure affect the arc root behavior. Finite element analysis was conducted based on experimentally obtained results to model lightning strike damage. The arc root length captured using a high-speed camera was incorporated into the analytical model. Numerical analysis results showed good correspondence with experimentally obtained results. These experimental and analytical results indicate that arc root behavior strongly affects lightning strike damage. [ABSTRACT FROM AUTHOR]

Microscopic failure mechanisms in adhesively bonded joints of carbon fiber reinforced polymer (CFRP) structures were studied, and in-situ observation during fracture toughness tests under mode I and mode II loading were carried out. Strain in the adhesive layer was quantitatively evaluated by applying the digital image correlation (DIC) method to micrographs. Strain concentration was observed along the CFRP/adhesive interface under mode I loading. On the other hand, strain concentration and microcracks around the polyester carrier was observed under mode II loading. The macroscopic crack growth occurred due to the coalescing of microcracks under mode II loading. The maximum principal strain in adhesive resin under mode II loading at the onset of crack growth was significantly larger than that under mode I loading. The effects of distance from specimen edge on the crack growth behavior were also studied using the X-ray computed tomography. [ABSTRACT FROM AUTHOR]