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This study investigates the influence of macroscale skin-core residual stress and cooling rate on the impact response of aerospace grade carbon fibre/polyphenylenesulphide (CF/PPS). Numerical simulations are developed which analyse the thermal shrinkage and residual stress development of unidirectional (UD) lay-up configurations. Macroscale skin-core residual stresses are then incorporated into low-velocity impact simulations based on an orthotropic elastic material model. Interlaminar delamination is modelled using a bilinear cohesive traction–separation law, and intralaminar failure is modelled using the Chang–Chang strength-based failure criterion. The simulation results are compared with the results of drop tower impact tests showing qualitative agreement in terms of maximum impact force and delamination. The results of this work highlight the importance of cooling rate on the interlaminar delamination and intralaminar failure of CF/PPS. [ABSTRACT FROM AUTHOR]

This study investigates the influence of cooling rate on the residual strain of the carbon fibre/polyphenylenesulfide unidirectional laminates. Three different cooling rates (−300℃/min, −100℃/min and −10℃/min) were applied to simulate a wide range of cooling conditions. The crystallisation behaviour which depends on the cooling rate was evaluated using differential scanning calorimetry. A process monitoring test was then conducted using embedded fibre Bragg grating sensors. In-plane transverse strain of carbon fibre/polyphenylenesulfide unidirectional laminates was measured and the results were presented based on the crystallisation behaviour determined by differential scanning calorimetry. Furthermore, residual strain change after subsequent annealing was examined. This study successfully demonstrates the effectiveness of in situ process monitoring using fibre Bragg grating sensors for evaluating material behaviour of thermoplastic composites during rapid heating and cooling under realistic processing conditions. [ABSTRACT FROM AUTHOR]

In this research, optical fiber sensors and shape memory alloys (SMA) were incorporated into sandwich panels for development of a smart honeycomb sandwich structure with damage detection and shape recovery functions. First, small-diameter fiber Bragg grating (FBG) sensors were embedded in the adhesive layer between a CFRP face-sheet and an aluminum honeycomb core. From the change in the reflection spectrum of the FBG sensors, the debonding between the face-sheet and the core and the deformation of the face-sheet due to impact loading could be well detected. Then, the authors developed the SMA honeycomb core and bonded CFRP face-sheets to the core. When an impact load was applied to the panel, the cell walls of the core were buckled and the face-sheet was bent. However, after the panel was heated over the reverse transformation finish temperature of the SMA, the core buckling disappeared and the deflection of the face-sheet was relieved. Hence the bending stiffness of the panel could be recovered. [ABSTRACT FROM AUTHOR]

This paper presents an overview of the damage growth suppression effects in CFRP laminates using embedded SMA foils, and was conducted to technically verify the application to aircraft structures. In previous studies, the authors confirmed the damage growth suppression of CFRP laminates with embedded pre-strained shape memory alloy (SMA) foils through both coupon testing and structural element testing. Through detailed observation of the damage behavior, we determined that these effects occurred due to the suppression of the strain energy release rate based on the suppression of the crack opening displacement caused by the recovery stress of SMA foils. These results were verified in this study using a demonstrator test article, which was a 1/3-scaled model of a commercial airliner's fuselage structure (B737 class). To demonstrate the damage growth suppression, the lower panel, which was dominated by tension load, was selected as the evaluation area. The evaluation area was an integrated panel including both the 'smart area' (CFRP laminate with embedded pre-strained SMA foils), and the 'conventional area' (standard CFRP laminate), for direct comparison of the damage behavior. The demonstration was conducted at 80°C in the smart area, and room temperature (RT) in the conventional area using a quasi-static load-unload test method. The demonstrator test article identified that the damage onset strain in the smart area was improved by 30% over that in the conventional area. [ABSTRACT FROM AUTHOR]

The mechanical properties of composites subjected to out-of-plane impact decrease markedly from even barely visible damage. In this study, three damage detection technologies are explained: (1) Technologies for the arrangement of embedded small-diameter optical fibers that have no serious effect on the mechanical properties of composites; (2) Technologies for the egress of the optical fibers using 'the embedded connector for smart structures' that can be trimmed without damaging the optical fibers; and (3) Technologies for a damage detection system that has data acquisition and analysis functions, evaluates the initiation and the position of damage, and visualizes damage information. We conducted an impact test using the composite airframe demonstrator. FBG sensors are embedded in the upper panel of the 1.5 m diameter, 3 m long stiffened cylindrical composite structure for strain measurement, and optical fibers are embedded for optical loss measurement. Damage detection in composite structures using the developed damage detection system was demonstrated. [ABSTRACT FROM AUTHOR]

Abstract: This study examined the effects of multi-walled carbon nanotube (CNT) dispersion on stress–strain behaviors of poly-ether–ether–ketone (PEEK) at room temperature. Tensile test specimens containing 9wt.% and 15wt.% of CNT were fabricated using injection molding. Results of focused ion beam (FIB) observations show that many CNTs in the CNT/PEEK composite are aligned longitudinally. Although the PEEK stress–strain behavior is almost linear up to 1.5% strain, the stress–strain curves of CNT/PEEK composites exhibit considerable nonlinear and hysteretic behaviors from extremely low strain (<0.1%) under both tensile and compressive loading. The experimental results suggest that the viscoelastic deformation effects on nonlinear and hysteresis behaviors are not strong below 1.5% strain. Presumably, the slippage at the CNT–PEEK interface occurs with increasing applied stress because of poor interfacial load-transfer capability. [ABSTRACT FROM AUTHOR]

Abstract: This paper examines the stress distribution around a fiber break in alumina fiber-reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts their tensile strengths using tensile failure simulations. In particular, we discuss the effect of matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic–perfect plastic matrix and an elastic–plastic hardening matrix using finite element analysis. Second, the procedures for simulating fiber damage evolution in the Al2O3/Al composites are presented. The simulation incorporates the analytical solution to the stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites. Coupled with a size-scaling analysis, the simulated results express the size effect on the strengths of the composites seen in experimental results. [Copyright &y& Elsevier]

Abstract: This study examined the nanoscopic damage progression of aligned multi-walled carbon nanotubes (CNT)/epoxy composites under tensile loading using transmission electron microscopy (TEM). Aligned CNT/epoxy composite films (30μm thickness) were processed using a forest-drawn aligned CNT sheet and hot-melt prepreg method. Four film specimens, respectively subjected to tensile stress of 0MPa, 45MPa, 95MPa and 110MPa, were prepared. After tensile loading, each specimen was machined until the thickness became about 100nm using a focused ion beam milling machine (FIB) for TEM observations. Damage of three kinds, i.e. CNT break derived from the disordered CNT structures around metallic catalyst, sword-in-sheath type CNT break, and several patterns of interfacial debonding, was observed clearly. The broken CNTs and interfacial debonding per unit area were counted from TEM photographs. Results show that broken CNTs and interface debonding increased considerably at 95–110MPa, which suggests multiple fracture of CNT under tensile loading. The CNT length at the failure stress (110MPa) was approximately 45μm. Estimated values from the strength of CNTs resemble those from macroscopic stress–strain behavior. [Copyright &y& Elsevier]

Abstract: This paper presents a proposal of a simple and easy method to evaluate the interfacial shear strength (IFSS) of CNT-dispersed polymer composites. An individual multi-wall carbon nanotube (MWNT) was pulled out from a MWNT-dispersed/PEEK composite using a nano-pullout testing system installed in an SEM. The tensile load was measured using the elastic deformation of an AFM cantilever. The pull-out length was controlled by making a through-thickness hole near the specimen edge using a focused ion beam (FIB) system. The IFSS of a MWNT/PEEK composite was measured as 3.5–14MPa, which agrees with the IFSS estimated from the macroscopic stress–strain behavior of the MWNT/PEEK composites. [Copyright &y& Elsevier]