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Because carbon fiber reinforced polymer (CFRP) laminated composites have electric conductivity, damage monitoring was performed using electrical resistance changes. The authors previously proposed an anisotropic electric-potential function method to calculate the electric-potential field of laminated CFRP composites. The method, however, did not deal with the effects of dents. The present study deals with the effects of dents on the anisotropic electric-potential function method. The dent causes an electric conductivity increase in the through-thickness direction. The increase was modeled and applied to a beam-type specimen. The results were compared with the finite-element method results, and the method using equivalent conductivity in the through-thickness direction was confirmed to be effective. [ABSTRACT FROM AUTHOR]

This paper uses classical laminate theory (CLT) and experimental methods to predict the longitudinal specific modulus of braided high modulus polyethylene (HMPE) rope without a matrix. When applying conventional CLT, the modulus, braided angle of strand, and packing factor (PF), i.e., the cross-sectional area ratio of the strand to the rope, are required. Because the void (space between strands) and PF of braided rope without a matrix readily change during the application of load, and given the difficulty measuring PF experimentally, it is difficult to predict the modulus by conventional CLT. This paper proposes the use of the unit of N/tex in place of conventional MPa for CLT. This study demonstrates that changes in PF due to void changes can be neglected when using the N/tex unit. The predicted longitudinal specific modulus of the rope using N/tex unit was found to be in qualitatively agreement with the longitudinal modulus measured experimentally. [ABSTRACT FROM AUTHOR]

The authors have previously proposed an effective method for analyzing the electric current density of carbon fiber-reinforced polymer (CFRP) composites using an anisotropic electric potential function. The method is based on the assumption that CFRP laminates are homogeneous orthotropic materials. Actual CFRP laminates are, however, heterogeneous materials, having resin-rich layers and carbon fiber layers. In the present study, the electric conductance is calculated on the assumption that CFRP laminates are homogeneous using electrical resistance data measured by a four-probe method, and used for electric current analysis. A new specimen design is proposed to allow the electric current distribution of CFRP laminates to be investigated experimentally. The results obtained confirm that our analysis method is accurate for determining the electric current density of actual CFRP laminates. Thus, the assumption that CFRP laminates may be considered homogeneous orthotropic materials for electric current analysis is experimentally verified. [ABSTRACT FROM PUBLISHER]

Carbon-fiber-reinforced polymer (CFRP) composites are used in aerospace and automobile structures. For many CFRP structures, mechanical metallic fasteners are adopted. In internal structures such as a wing box, damage to CFRP structures around the fastener holes is difficult to see. A simple method of finding damage around fastener holes is thus required for CFRP structures. This study applies self-sensing time-domain reflectometry to detect bearing failure around fastener holes of CFRP structures. In a previous paper, the effect of fasteners was minimized when the micro-strip line (MSL) was placed apart from the fasteners. This study numerically and experimentally investigates the peeling of glass-fiber-reinforced polymer (GFRP) insulator. A bearing failure results in embossing at the edge of a fastener hole. The embossing produces a wedge that causes the peeling of GFRP, generating a gap between the GFRP and CFRP. The gap strongly affects the characteristic impedance of the MSL. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) composites have low inter-lamina strength. One of the monitoring technologies is a self-sensing method that uses the electrical resistance change of a CFRP structure for detecting damage. The electric current distribution is vital information for the self-sensing method when optimizing the arrangement of probes to measure electric potential changes. We have developed a new orthotropic electric potential function analysis approach using affine transformation for unidirectional CFRP. In this study, the orthotropic electric potential function analysis method is improved to calculate the electric current of a thin cross-ply CFRP. Two types of stacking sequences for the beam-type cross-ply laminates were calculated to confirm the effectiveness of the improved method. The electrical voltage changes caused by multiple delamination cracks of a cross-ply laminate are new outcomes of this study. The analytical results were compared with computed results using the finite difference method. Consequently, the new equivalent electric conductance method proved to be effective for calculations of the electric current density of a cross-ply CFRP laminate. Furthermore, the new method for calculating the electric potential difference changes caused by multiple delamination cracks using orthotropic distributed doublet analysis, with the equivalent electric conductance, has also proved to be effective. [ABSTRACT FROM AUTHOR]

When an alternating current is applied to a conductive slab, the induced current impedes the electric current and causes a skin effect. This effect is found in all conductive materials. For laminated carbon fiber reinforced plastics (CFRP), however, the depth of the skin effect has not previously been evaluated. In the present study, therefore, we analytically solve Maxwell's equations to derive the skin effect depth of unidirectional CFRP. Using this result, the skin effect is analyzed for laminated CFRP, and its depth is then derived. The effect is then compared with a newly defined skin effect of anisotropic conductance. For highly toughened CFRP that have resin rich layers, the skin effect of anisotropic conduc-tance is found to be more important than that of the alternating current. [ABSTRACT FROM AUTHOR]

Abstract: The applied strain of carbon fibre reinforced plastics (CFRPs) is measurable by their electrical resistance changes. For damage monitoring of laminated CFRPs, piezoresistivity strongly affects the measured electrical resistance change through residual strain relief attributable to delamination cracks. Although several studies of CFRP laminates’ piezoresistivity have been published, this study uses single-ply CFRP for specific piezoresistivity measurements in four directions. A review of the theory of in-plane piezoresistivity reveals orthotropic properties of CFRP piezoresistivity. In the present study, piezoresistivity of multiaxial loading is derived, and the unsymmetrical piezoresistivity matrix is calculated using the measured piezoresistivity here. Effects of multiaxial loading in a misaligned unidirectional laminate are also discussed here. The misaligned laminate causes large shrink in the transverse direction during tensile tests; poor electrical contacts at electrodes increases the electric current in the transverse direction; these two effects cause decrease of electrical resistance for the poor electrical contact specimen with large fibre misalignment. [Copyright &y& Elsevier]

Abstract: Carbon fiber reinforced plastics laminate (CFRP) has a high specific strength and stiffness compared to conventional metals, although it is very sensitive to impact. A low impact leads to a delamination and results in the deterioration of the structural reliability. The present study employs a two-stage electric potential change method (EPCM) to identify delamination. In the methods, delamination is estimated using response surfaces, which require numerous experiments. An equivalent electric conductivity method is introduced to reduce this number. Delaminations are successfully estimated using response surfaces based on the results of the finite element method with equivalent electric conductivity. [Copyright &y& Elsevier]

Since a delamination crack is invisible or difficult to detect visually, the delamination causes low reliability of Carbon-Fiber-Reinforced-Polymer (CFRP) for primary structures. To improve the low reliability, smart systems of delamination identifications in-service are desired. Recently, many researchers have employed an Electrical Resistance Change Method (ERCM) to detect the internal damages of CFRP laminates. The ERCM does not require expensive instruments. Author's group has already experimentally investigated the applicability of the ERCM for strain monitoring, delamination crack monitoring and matrix crack monitoring. In the present article, therefore, these results performed in the previous papers are briefly explained. These successful results enable us to monitor a lot of information of the CFRP laminates by means of the electrical resistance changes in many applications, although a lot of electrodes are required to be mounted. To reduce the number of electrodes, a new electrical potential method for the CFRP plate is proposed here, which enables us to reduce the number of electrodes. The present article compared the new method to the method proposed by Anderson. FEM analyses are performed to confirm the applicability of the new method. [ABSTRACT FROM AUTHOR]

Abstract: For laminated composite materials, the stacking sequence design is indispensable to make efficient use of the material properties. Laminated composites are usually fabricated from unidirectional plies of given thickness with limited fiber orientations to a small set of angles (e.g. 0°, +45°, 45°, and 90°). An improved version of fractal branch and bound method has been proposed to optimize multiple stacking sequences. The improved method, however, cannot be applied to complicated structures, since it is difficult to determine which is the most effective surface for designing such structures. In the present study, therefore, a new iteration fractal branch and bound method is proposed, which does not require empirical knowledge of the target structure. Here, this method is applied to a hat-stiffened panel to enable the stacking sequences of the two panel laminates and hat-type stiffener to be optimized simultaneously. [Copyright &y& Elsevier]

This study examines the damage identification for woven graphite/epoxy composite beams by means of an electrical resistance change method. The method has been proposed by the authors and successfully applied to cross-ply and quasi-isotropic laminates; the method has yet to be applied to woven laminates. In many practical structures, woven plies are adopted to prevent peeling of the surface layer. Therefore, a woven graphite/epoxy composite is selected as the target material of the electrical resistance change method to identify the damage. Beam type specimens consisting of woven laminates are the focus of this article. For the purpose of identification, the response surface is adopted as a solving method for the inverse problem. As a result, the method shows an excellent performance for estimating delamination crack locations and sizes. [ABSTRACT FROM AUTHOR]

Stiffened composite panels are often used as structural components in aircraft in order to avoid buckling. It is well known that stacking sequence optimizations are indispensable for laminated composite structures. Stiffened composite panels usually have more than two stacking sequences because they consist of a panel skin laminate and stiffener laminates. This means that the stacking sequences need to be jointly optimized to achieve structural optimization of the stiffened composite panel. The authors have proposed a new stacking sequence optimization method, called the fractal branch and bound method, for optimizing a single laminate. In the present study, the fractal branch and bound method is extended to optimizing multiple stacking sequences. The extended method is applied for obtaining two optimal stacking sequences for the maximization of the buckling load of a hat-stiffened composite panel. The improved method successfully provides two optimal stacking sequences determined in a short period of time. [ABSTRACT FROM AUTHOR]

The fractal branch and bound method was developed by the authors for optimization of stacking sequences to maximize buckling load of composite structures. The method demands an approximation of a design space with a response surface comprising quadratic polynomials for pruning fractal branches of stacking sequences. Approximation of the objective function with quadratic polynomials was confirmed for buckling load maximizations and flutter speed limit maximizations using lamination parameters as predictors. In the present study, flutter speed maximization with a constraint of buckling load is employed as an example of stacking sequence optimization by means of the fractal branch and bound method with a strength constraint. The present paper describes the theoretical background of the fractal branch and bound method. Then approximations are performed using quadratic polynomials with lamination parameters as predictors. After that, effectiveness of this method for supersonic panel flutter of composite laminates was investigated using two cases. Results indicate that the method was applied successfully; a practical optimal stacking sequence was obtained using modified response surfaces. [ABSTRACT FROM AUTHOR]

This study describes a new method of experimental design to obtain a response surface of buckling load of laminated composites. Many evaluations for genetic algorithms for stacking sequence optimizations require high computational cost. That evaluation cost can be reduced by an approximation using a response surface. For a response surface for stacking sequence optimizations, lamination parameters are adopted as variables of the approximation function of the entire design space instead of ply angles for each ply. This study presents, proposes and investigates a new method of experimental design in detail. For most analytical tools, stacking sequences is demand as input data and lamination parameters cannot be applied directly to the tools. Therefore, the present study proposes and applies a new D-optimal set of laminates to the stacking sequence optimizations of the problem of maximization of buckling load of a composite cylinder. The new experimental design is a set of stacking sequences selected from candidate stacks using D-optimality. Consequently, the D-optimal set of laminates is shown to be effective for design of experiments of response surfaces for maximization of the buckling load of composite structures. [Copyright &y& Elsevier]

A new intron method is proposed to optimize stacking sequences and number of plies of composites. The intron method is developed from the recessive-gene-like repair method that is employed for implementation of stacking-sequence constraints without causing decrease of the design reliability of GA. The intron method is attempted for the optimizations of laminated cylinders to maximize the buckling load and minimize the number of plies. As a result, the intron method successfully obtained the real optimal stacking sequences and the number of plies without decrease of design reliabilities. [ABSTRACT FROM AUTHOR]

Delamination of laminated composites is usually invisible or difficult to detect visually. Delamination causes low reliability for primary structures. Automatic systems for in-service delamination identifications are desired to improve low reliability. The present study employs an electric resistance change method for delamination detection. Since the method adopts reinforcement carbon fiber itself as sensors for delamination detection, this method does not reduce static or fatigue strength; also, the method is applicable to existing structures. Authors have found that the electric resistance change method with response surfaces is very effective experimentally and analytically. However, a large error of estimation remains for estimation of delamination location. In the present study, a new data processing procedure is proposed to improve performance of estimations of delamination location. The new method is applied to laminated composite beams. A delamination crack of a laminated composite beam is monitored with the new method using FEM analyses. As a result, the method reveals excellent performance of estimations of delamination location even for new data not used in regression equations. [Copyright &y& Elsevier]

The present study proposes crack swarm inspection (CSI) for estimating crack location and size in carbon composite laminates from the surface voltage distribution. This technique generates a large number of virtual microscopic cracks, and calculates the surface voltage distribution of the composites using anisotropic electric potential functions and doublet strings. Using genetic algorithms, the virtual microscopic cracks formed a swarm to coincide with the measured surface voltage; thereby, the crack sizes and locations are estimated from the position of the crack swarm. The CSI was applied to crack detection in carbon laminated composite plates; it was confirmed that the existence of cracks in each partitioned section was detected with >80% probability, in reference to the crack location and size information. Furthermore, we also confirmed that the estimation accuracy was affected by the electric current density in the thickness direction, and addressed the recommended electrode interval based on the minimum size of the estimated crack. [ABSTRACT FROM AUTHOR]

Self-sensing carbon-fibre-reinforced polymer (CFRP) that use changes in electrical resistance has been applied to monitor delamination cracks in CFRP composite structures. CFRP laminated composites have orthotropic electric conductivity. Despite the existence of insulator resin-rich layers between each ply, the toughened CFRP laminates have electric conductivity in the thickness direction, which plays important role in delamination monitoring with changes in electrical resistance. Further more, it is also quite important to understand the lightning damage behaviour of CFRP laminate and to perform numerical simulations estimating the current flow in CFRP laminate when lightning event. However, the measured electric conductance has not been confirmed to be a material property for toughened CFRP. The objective of the present study is thus to clarify the mechanism of realizing electric conductivity in the through-thickness direction. During the curing process, alternating current is applied, and impedance is measured for toughened CFRP. It is found that conductivity in the through-thickness direction of toughened CFRP laminates is due to chance carbon fibre contact between plies during the curing process, and is not a material property. [ABSTRACT FROM AUTHOR]

Variable stiffness composite (VSC) is a viable design extension of carbon fiber reinforced plastic (CFRP) laminates to obtain unique mechanical properties by changing the fiber orientation in a curvilinear manner. We optimized a VSC for open-hole tension laminates considering the automatic processing by the tow prepreg curve placement and evaluated the strength improvement with experiments. Multi-objective optimization using fracture criterion and mean curvature as objective functions resulted in higher strength gains for solutions with higher mean curvature and lower processability. Besides, by giving the molding conditions as constraints for optimization, the strength was improved while satisfying the molding conditions. The fabrication of optimum VSCs involved using a tabletop automated fiber placement. Strength tests showed a 34.3% improvement in the curvilinear prepreg path. [ABSTRACT FROM AUTHOR]