This paper describes a method of finite strain measurement and subsequent stress computation of sheet steel specimens undergoing plastic deformation. A notched sheet specimen made of high-strength steel under a tensile load is observed using stereo vision. The displacement is then determined using stereo digital image correlation. A procedure for estimating the finite strain and corresponding stress from the measured in-plane displacement is presented. First, the deformation gradient tensor is determined. The logarithmic strain components are then determined from the right stretch tensor determined from the deformation gradient tensor. Finally, the stresses are evaluated from the strains based on the plasticity theory. The effectiveness of the proposed method is demonstrated by applying it to the evaluation of the stress triaxiality with the progress of the deformation and the identification of material properties after necking using the virtual fields method. Results show that the stresses in the plastically deformed region can be obtained by the proposed method.

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The influence of carbon fiber on the statistical life of resin-impregnated carbon fiber reinforced thermoplastic epoxy (CF/TPEP) strands under cyclic tension loading is evaluated using our developed accelerated testing methodology (ATM) based on the time–temperature superposition principle which holds for matrix resin viscoelasticity. First, the formulation of fatigue strength used for this study is introduced. Second, the parameters in the formulation are inferred based on results of viscoelasticity tests of resin, static, and fatigue tensile tests of CF/TPEP strands. Finally, long-term fatigue life prediction for CF/TPEP strands of two kinds is performed using the inferred parameters. Results show that the fatigue strengths of both CF/TPEP strands decrease markedly with an increasing number of cycles to failure and temperature. The S–N master curve is definable and obtained from measured static and fatigue strengths and viscoelastic properties of matrix resin. The S–N master curve for T700/TPEP strand decreases remarkably with increasing Nf compared to that for T300/TPEP strand. Results clarified that the T700/TPEP strand fatigue strength achieved using high strength carbon fiber was lower than that of T300/TPEP strands in the long failure time region.

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It is important to understand what mechanical properties of the resin and carbon fibers that make up CFRP affect the lifespan of CFRP in order to guarantee its reliability as a structural material. This study examines the prediction of statistical long-term creep life for resin-impregnated carbon fiber strands using thermoplastic epoxy resin as a matrix (CF/TPEP strand) under dry and wet conditions. First, the viscoelastic behaviors of thermoplastic epoxy neat resin are measured under dry and wet conditions. Second, the static tensile strengths of CF/TPEP strands using carbon fibers of two kinds are measured and evaluated statistically at various temperatures under dry and wet conditions. Then the statistical long-term creep tensile strengths for CF/TPEP strands under dry and wet conditions are predicted by substituting the measured viscoelastic properties and static strength data into the formulae of our developed accelerated testing methodology. Third, the tensile creep strengths for CF/TPEP strand are measured under dry and wet conditions, with comparison to prediction results. Findings demonstrate that the statistical long-term creep tensile strengths for CF/TPEP strands under dry and wet conditions can be predicted by substituting the measured viscoelastic properties of matrix resin and static strength data for CF/TPEP strand into the formulae of our developed testing methodology. The creep strength was found to decrease concomitantly with increasing elapsed time, with acceleration of that decrease with increased temperature and water absorption in the similar manner of relaxation modulus of matrix resin. Although there are large differences in static strength at room temperature depending on the type of carbon fiber, there are also differences in the decreasing rate in creep strength due to temperature and water absorption depending on the type of carbon fiber.

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A single yarn is formed by spinning short natural fibers, and a twist yarn consists of several single yarns to achieve high stiffness and strength of natural fiber composites. Generally, the amount of twist obtained by the spinning process, known as twist per inch (TPI), plays an important role in yarn properties because the twist is essential to hold the fibers together. This study investigated the effect of TPI of natural fiber yarns on resin impregnation properties, such as flow velocity and void generation, and the mechanical properties of natural fiber composites molded using VaRTM. Resin flow velocity increased with increasing TPI because capillary pressure due to the dense structure of fibers became the predominant force for tow impregnation. Mechanical properties also depended heavily on the TPI of yarns. TPI dependence on resin flow velocity was valuable in designing the complicated structure of natural fiber composite molded using VaRTM.

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A novel joining technology for thermoset fiber-reinforced plastics (FRP) has been proposed to achieve a reliable joint structure that can be established in a short time using a welding process. Owing to the formation of a thin layer of thermoplastic resin on the surface of thermoset laminates, composite parts can be joined instantaneously using a heating process. However, the failure of the joints of welded thermoset FRP has not been sufficiently elucidated. Thus, this study conducted a single-lap shear test to investigate the failure behavior of welded thermoset FRP considering the fiber orientation of the substrates. In particular, the fiber angle of the surface ply of the laminates was investigated. The results suggest that the failure mode transition from cohesive failure of the thermoplastic resin layer to substrate failure of the surface ply of the thermoset laminates occurred depending on the fiber angle of the surface ply. Moreover, the lap-shear strength varied according to the failure mode transition and fiber orientation. To analyze the failure mechanisms in detail, a multiscale model was developed to predict the failure of welded thermoset FRP joints. The model was validated by comparing the simulation results with the experimentally evaluated failure mode and stress at failure; they were in good agreement. The simulation results further suggest that failure of the thermoset laminates was induced as the resin matrix adjacent to the carbon fibers was subjected to severe stress when the fiber angle of the near-joint laminae was approximately orthogonal to the loading direction. The developed multiscale model is expected to be utilized in the designing of structure and stacking sequence for future applications of welding technology for thermoset FRP.

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