K.S. Han

Failure of carbon/epoxy composite tubes under combined axial and torsional loading 1. Experimental results and prediction of biaxial strength by the use of neural networks

Abstract Biaxial tests have been conducted on cross-ply carbon/epoxy composite tube under combined torsion and axial tension/compression up to failure. Strength properties and distributions were evaluated with reference to the biaxial loading ratio. The scatter of the biaxial strength data was analyzed by using a Weibull distribution function. Artificial neural networks were introduced to predict failure strength by means of the error back-propagation algorithm for learning, providing a different and new approach to the representation of complicated behavior of composite materials. Further prediction is made from experimental data by the use of Tsai–Wu theory and a combined optimized tensor polynomial theory. Comparison shows that the artificial neural network has the smallest root-mean-square error of the three prediction methods.

Effect of volume fraction of carbon fibers on wear behavior of Al/Al2O3/C hybrid metal matrix composites

Wear behavior of Al/Al2O3/C hybrid metal matrix composites fabricated by squeeze casting method was characterized. The effects of volume fraction of carbon fiber on wear behavior of hybrid composites was investigated. Wear behavior of Al/Al2O3/C composites was characterized by the dry spindle wear test under various sliding speeds. The wear resistance of Al/Al2O3/C composites was remarkably improved over Al/Al2O3 composites by adding carbon fibers to Al/Al2O3/C composites. Specifically, at the intermediate sliding speed the wear resistance of Al/Al2O3/C composites containing 8 vol.% carbon fiber was found to be better than that of the rest of the carbon hybrid composites. From fractographic studies, damaged sections in wear surfaces of hybrid composites at intermediate sliding speed were not observed due to the formation of solid lubrication film. The solid lubrication film which was formed as a result of adding carbon fibers improved the wear resistance of carbon hybrid composites because this film reduced the high friction force between MMCs and counter material.

Wear properties of Saffil/Al, Saffil/Al2O3/Al and Saffil/SiC/Al hybrid metal matrix composites

Abstract The purpose of this study is to investigate the wear properties of Saffil/Al, Saffil/Al 2 O 3 /Al and Saffil/SiC/Al hybrid metal matrix composites (MMCs) fabricated by squeeze casting method. Wear tests were done on a pin-on-disk friction and wear tester under both dry and lubricated conditions. The wear properties of the three composites were evaluated in many respects. The effects of Saffil fibers, Al 2 O 3 particles and SiC particles on the wear behavior of the composites were elucidated. Wear mechanisms were analyzed by observing the worn surfaces of the composites. The variation of coefficient of friction (COF) during the wear process was recorded by using a computer. Under dry sliding condition, Saffil/SiC/Al showed the best wear resistance under high temperature and high load, while the wear resistances of Saffil/Al and Saffil/Al 2 O 3 /Al were very similar. Under dry sliding condition, the dominant wear mechanism was abrasive wear under mild load and room temperature, and the dominant wear mechanism changed to adhesive wear as load or temperature increased. Molten wear occurred at high temperature. Compared with the dry sliding condition, all three composites showed excellent wear resistance when lubricated by liquid paraffin. Under lubricated condition, Saffil/Al showed the best wear resistance among them, and its COF value was the smallest. The dominant wear mechanism of the composites under lubricated condition was microploughing, but microcracking also occurred to them to different extents.

Influence of particle size and shape on electrical and mechanical properties of graphite reinforced conductive polymer composites for the bipolar plate of PEM fuel cells

Graphite reinforced conductive polymer composites (CPCs) with high filler loadings were fabricated by compression molding technique. Various sizes and shapes of graphite particles were mixed with phenol resin to impart the electrical conductivity in composites. Fabricated CPCs showed good electrical conductivity (>100 S/cm) and flexural strength (>40 MPa) for the bipolar plate of polymer electrolyte membrane (PEM) fuel cells. The electrical conductivity of CPCs was affected by the formation of conductive networks among graphite particles. CPCs made of sphere-type particles (SG-CPCs) had the same physical density regardless of particle size; and they also showed the same bulk electrical conductivity. This means that there is a close correlation between the electrical conductivity and the densification level, or density, of graphite/phenol compounds. The particle shape was also a principal factor in influencing electrical conductivity. In this study, the electrical conductivity of CPCs made of flake-type graphite particles (FG-CPCs) was higher than that of SG-CPCs due to the difference of the densification characteristic. The flexural strength of SG-CPCs tended to increase with decreasing graphite particle size because the interfacial coherence between graphite particle and phenol resin increased as graphite particle size decreased. This influence of interfacial coherence was also founded in the variation of particle shape. FG-CPCs have higher flexural strength than SG-CPCs because a flake-type particle has larger specific area than a sphere-type particle.

Stacking sequence optimization of laminated plates

Optimum fiber orientations of laminated composite plates for the maximum strength are found under multiple inplane loading conditions. Tsai-Wu failure criterion is taken as objective function. Based on the state space method, effective optimal design formulation is developed and solution procedure is described with the emphasis on the method of calculations of the design sensitivities. Numerical results are presented for the several test problems.

Measurements of fiber orientation and elastic-modulus analysis in short-fiber-reinforced composites

This paper presents an image analysis method for measuring fiber orientations in short-fiber-reinforced composites and a mathematical procedure for predicting the elastic modulus of the composites according to the measured fiber-orientation distribution (FOD). The method determines the FOD from the ratio of fiber matrix perimeter length between two orthogonal planar cross-sections of polished composite samples. The analysis algorithm of the method is much simpler than previously reported methods and is efficiently applied to composites with axially symmetric FOD. To verify the theory, FOD measurement and tensile testing were performed on Al2O3/Al composites fabricated by squeeze casting. The elastic modulus values determined by the tests were compared with the theoretical value, and the agreement was satisfactory.

Failure of carbon/epoxy composite tubes under combined axial and torsional loading. 2. Fracture morphology and failure mechanism

Failure mechanisms of cross-ply composite tubes made by the lapped moulding technique were investigated following biaxial testing, as reported in an earlier study (Part 1). Before mechanical testing the undamaged specimens were inspected to characterize their microstructure, and the source of first material damage was also inspected. From phenomenological failure analysis three types of failure mode were exhibited, depending on the biaxial ratio, and the corresponding failure mechanisms are suggested. By means of fractographic observations of the fracture surface, microscopic failure was investigated as a function of biaxial ratio, and it is suggested how the performance of fiber reinforced composite materials tube for engineering applications might be improved. The main factors involved at low biaxial ratio are matrix strength, the bond strength of the seam, and uniform distribution of fiber and matrix, while at a high biaxial ratio the fiber strength is the main factor.

Stacking Sequence Design of Fiber-Metal Laminate for Maximum Strength

FML (Fiber-Metal Laminate) is a new material combining thinmetal laminate with adhesive fiber prepreg. It has nearly all the advantages of metallic and composite materials, including good plasticity, impact resistance, processibility, light weight and excellent fatigue properties. However, in most FML the fiber prepreg is staked in only one direction, although FML can be designed with a varying stacking sequence angle of fiber prepreg. No work has been published on the optimum design of FML. This paper uses genetic algorithms to study the optimal design of FML under various loading conditions. To analyze FML the finite element method is used based on shear deformation theory. The Tsai-Hill failure criterion and theMiser yield criterion are used as the objective functions of the fiber prepreg and the metal laminate, and the ply orientation angles are the design variables. In the genetic algorithm, tournament selection and the uniform crossovermethod are employed. The elitist model is also used for an effective evolution strategy, and the creeping random search method is adopted so as to approach the solutionwith high accuracy. Optimization results are given for various loading conditions and are compared with CFRP (Carbon Fiber Reinforced Plastic). The results show that FML is better than CFRP in most loading conditions. In particular, FML shows good mechanical performance in point and uniform loading conditions and is more stable to unexpected loading.

Real-Time Monitoring of Composite Wind Turbine Blades Using Fiber Bragg Grating Sensors

The prototype of a 2 MW wind turbine (type U88) has been tested at Taebaek, South Korea since January 2009. A real-time monitoring system with fiber Bragg grating (FBG) sensors was designed and applied to monitor wind turbine blades in operation. Differences according to the operating conditions (yawing, pitching and start-up/normal operation) were monitored accurately in real time with sensors. Additionally, using commercial FEM codes, a GFRP-based composite rotor blade was modeled, and then natural frequencies obtained from the FE modal analysis were compared with the FFT results of measured strain data. Finally, this paper provides an overview of the real-time monitoring system setups and some current test results including modal characteristics.

Fabrication of Graphite Nanofibers Reinforced Metal Matrix Composites by Powder Metallurgy and Their Mechanical and Physical Characteristics

Aluminum metal matrix composites reinforced by graphite nanofibers (GNFs) are fabricated by conventional powder metallurgy methods, and their mechanical and physical characteristics are investigated. Mixing conditions are established by microhardness tests and microscopy observations. The GNF-Al composites are consolidated by hot isostatic pressing (HIP); a high density of composites can be achieved. From microhardness and compression tests, the optimal nanofiber content is determined. The physical properties of the GNF-Al composites are measured by thermal and electrical transport tests. The results indicate that the addition of nanofibers improves the thermal conductivity and the electrical resistivity, but the dimensional stability under high temperatures is not improved. This study may provide experimental information in the design and fabrication of functional metal matrix nanocomposites.