Reliable viscoelastic coefficients of matrix resin over ranges of time are indispensable for reliable long-term life prediction of carbon fiber reinforced plastics (CFRPs) and their structures. A new methodology is proposed herein for finding the master curve of the reliable long-term relaxation modulus of matrix resin by combining frequency-dependent and temperature-dependent storage moduli measured using a dynamic mechanical analyzer (DMA) and time-dependent and temperature-dependent creep compliance measured using a creep testing machine. First, the new methodology is introduced based on the linear viscoelastic theory. Second, two viscoelasticity testing methods using DMA tests and creep tests are explained. Third, the short-term storage moduli by DMA and the long-term creep compliances by creep testing machine are measured at various temperatures for a heat-resistant epoxy resin. Fourth, the relaxation moduli for a wide range of time and at various temperatures are constructed using the data measured from DMA and creep testing machine. Finally, based on the modified time-temperature superposition principle, a master curve of the relaxation modulus is obtainable for an extremely wide range of time at a reference temperature.

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The authors proposed a splice-type crack arrester for foam core sandwich panels. The splice-type arrester was intended to provide crack suppression functionality to a core-core splice of a foam core sandwich panel via the installation of carbon-fiber-reinforced plastic (CFRP) material between the two tapered cores and the replacement of sharp core edges with CFRP. The crack suppression ability of this type of arrester under mode II-type loading was investigated via a numerical analysis and an experimental evaluation. In the numerical analysis, the decreased energy release rate at the crack tip caused by this type of arrester was estimated quantitatively via a finite element (FE) analysis and crack closure integral. In the experimental evaluation, a load-displacement diagram obtained from a fracture toughness test showed that the critical load increased as the crack tip approached the leading edge of the arrester. The failure behavior indicated that the interfacial crack was completely suppressed near the leading edge of the arrester. A comparison of two different types of loading showed that the splice-type crack arrester had a greater crack suppression effect under mode I-type loading than under mode II-type loading, whereas the failure mechanisms were similar for both types of loading. Through this research, the interfacial crack suppression mechanism by the crack arrester was confirmed in the different loading conditions, i.e. the mode I type and mode II type. It can be said that this fact enhances the applicability of the crack arrester concept.

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A basic combination of dicyandiamide (DICY) as a latent curing agent and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) as a curing accelerator was used for a general-purpose bifunctional epoxy resin, diglycidyl ether bisphenol A (DGEBA). In this study, the composition of DGEBA/DICY/DCMU was fixed, and only the reaction pathway was changed during curing to investigate the difference in the reaction mechanism and the resulting difference in the crosslinked structure and mechanical/thermal properties. In addition, the activation energies and heat of formation of multiple reactions that govern the curing reaction were investigated using quantum chemistry calculations. As a result, glass transition temperature (Tg) decreased by 10 °C as the dwell cure temperature increased. Young’s modulus increased with curing temperature, although there was no major difference in strength, strain to failure and strain energy. These results suggest that the reaction mechanism differs depending on the dwell cure temperature. FT-IR measurement revealed that in case of lower dwell temperature (100 °C), many cyano groups remain before gelation, due to less dissolvement of DICY. Consequently, anionic polymerization of epoxy mainly occurs because of dimethylamine generated by pyrolysis of DCMU. On the other hand, at a higher dwell cure temperature (160 °C), the cyano group peak decreases and the acid amide peak increases. Consequently, not only anionic polymerization but also the chain structure due to the epoxy-DICY reaction increases, indicating changes in the degree of branching of the initially formed microgels. At 160 °C dwell, more oxazolidinone (1760 cm-1) was produced before gelation compared to low temperature dwell, and some epoxy groups were terminated before microgel formation. It is considered that the connection between microgels progresses and a relatively dense cross-link structure is formed. On the other hand, at low temperature dwell, epoxy group termination by oxazolidinone occurs even after microgel formation, which suppresses network formation between microgels.

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The aim of this paper is to investigate the influence of the surface modification ratio on the tensile strength of the hybrid-interface-controlled HAp/PLA composite materials. Here, pectin and chitosan were selected as the surface modification polymers for the HAp particles. In addition, o-nitrobenzyl alcohol was selected as the photodissociable protecting groups to avoid the chemical reaction between the surface modification polymers. First, the critical amount of the surface modification polymers for the whole surface modification of the HAp particles was evaluated. Then, the tensile tests were carried out for the hybrid-interface-controlled HAp/PLA composites with the various surface modification ratio. As a result, the FT-IR measurement revealed that the positively and negatively charged region of the HAp surface can be fully modified by pectin of 11 [wt%] and chitosan of 0.6 [wt%], respectively. Based on this result, the surface modification ratio of the hybrid-interface-controlled HAp particles could be evaluated under the assumption that the 50 [%] of the HAp surface is positively charged whereas the another 50 [%] is negatively charged. As a result of the tensile tests of the HAp/PLA composites, the tensile strength drastically decreased by the hydrolysis in the case that the surface modification ratio was 0 [%]. On the other hand, the decrease in the tensile strength was quite small in the case that the surface modification ratio was 75 [%]. It is notable that the hydrolytic degradation in the tensile strength was suppressed in the early stage and then accelerated after the 2-week hydrolysis, in the case that the surface modification ratio was 50 [%]. This result suggests that the simultaneous optimization of the hydrolytic and fracture properties of the hybrid-interface-controlled HAp/PLA composites would be enabled by the control of the surface modification ratio.

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The fundamental mechanical properties of an extendible mast for space use are investigated. The mast has a cylindrical configuration and consists of circumferentially separable three flexible shell sections. Three shell sections are rolled up along the longitudinal direction when the mast is stowed and transported to space. During the deployment in space, three sections are unrolled and connected to neighboring sections with tape fasteners to form the cylindrical mast. In this study, tensile and torsional tests are conducted in order to evaluate the effect of mechanical properties of the tape fasteners on mechanical behaviors, especially torsional behavior, of the mast. Finite element analyses including the effect of the tape fasteners are also conducted. Experimental results show that the addition of tape fasteners to the open-section mast improves its torsional stiffness. The experimentally observed tensile and torsional behaviors of the mast including the effect of tape fasteners are well simulated by the finite element model.

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Recently, novel CF/benzoxazine prepregs for aircraft structures have been developed by ENEOS Corp. in Japan. Thermoplastic particles are dispersed on the surface of these prepregs. This study examines the applicability of our proposed formulations for the statistical time-dependent and temperature-dependent fatigue strength under open hole compression (OHC) for quasi-isotropic CFRP laminates using CF/benzoxazine prepreg. Results demonstrated that the statistical fatigue OHC strength of quasi-isotropic CFRP laminates with and without toughened interlayer can be formulated using the statistically determined static and fatigue OHC strengths. Furthermore, the characteristics for fatigue OHC strength of quasi-isotropic CFRP laminates with toughened interlayer were clarified compared with those of CFRP laminates without a toughened interlayer.

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To widen the application of 3D printed continuous carbon fiber-reinforced thermoplastics (CFRTPs), understating of both their open hole mechanical properties and fracture mechanisms is of a pressing need. In this study, tensile tests were performed on 3D printed CFRTP plates with open holes and a quasi-3D extended finite element method (XFEM) program was used to compare the experimental results with the numerical analysis. The specimens were formed using a commercially available 3D printer followed by drilling of the hole. The laminate structure of the printed specimens was [0/90]2s. An X-ray computed tomography (CT) observation of the fabricated specimens showed no delamination was introduced by the drilling process, even though in-plane and interlayer voids due to filament undulation were observed. Experimentally obtained Young’s modulus and OHT strength were 35.6 ± 1.2 GPa and 185.0 ± 2.7 MPa, respectively. The XFEM accurately predicts both Young’s modulus and OHT strength and their values are 36.1 GPa and 191.0 MPa, respectively. The printed specimens failed to leave brittle fracture due to fiber break at the hole edge, which can be also simulated by the XFEM used in this study.

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Authors performed to repair carbon fiber reinforced thermoplastics (CFRTP) with delamination caused by impact, assuming the condition that heat and pressure could be applied to the specimen from only one side of actual CFRTP structure. For damage repair, a heated spherical indenter was pressed against the damaged specimen from one side to close and bond delamination in the specimen. For shape repair, the specimen regained its original flat shape since specimen deformed into the shape of the indenter. In addition, the validity of the repair method was evaluated through comparison of the flexural modulus in the four-point bending test. As a result, delamination was reduced in damage repair and the flexural modulus of specimen after repair was restored by about 14.3% compared with that after impact, despite the small delamination that occurred in the specimen due to shape repair.

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