Journal of Composite Materials recent issues

Ultra High Resolution Computed Tomography of Damage in Notched Carbon Fiber--Epoxy Composites

Wright, P., Fu, X., Sinclair, I., Spearing, S.M. — 2008-08-28

This article presents the first use of synchrotron radiation computed tomography (SRCT) to achieve sub-micron resolution of damage in aerospace grade carbon fiber—epoxy composites. The structure and interaction of the damage can be visualized in 3-D on a scale not previously observed in practical engineering configurations. The ability to detect and accurately measure features down to individual fiber breaks provides a valuable platform for future research; from the rigorous evaluation of damage models to understanding the fundamental physical mechanisms governing crack growth in composites. In particular the key role of intra-laminar cracks and delaminations in localizing fiber fractures is unambiguously identified for the first time.

Time Dependent Behavior of a Particle Filled Composite PMMA/ATH at Elevated Temperatures

Basaran, C., Nie, S., Hutchins, C. S. — 2008-08-28

Creep behavior of particle filled acrylic composite materials become a major concern when they are used at elevated temperatures. Therefore, for elevated temperature finite element simulations any constitutive modeling requires time— temperature dependent material properties. Unfortunately, this type of data is very difficult to come across in the literature, due to a very long time needed to conduct creep testing. In this study, the creep properties of acrylic casting dispersion PMMA/ ATH were obtained experimentally and the observed characteristics of this material are presented with the experimental data. The underlying deformation mechanisms and the steady-state creep response are also discussed.

Computational Approach of Dielectric Permitivities in BaTiO3--Epoxy Composites

Ramajo, L., Reboredo, M., Santiago, D., Castro, M., Ramajo, D. — 2008-08-28

A numerical approach using a finite element method (FEM) was performed in order to determine the dielectric constant (^') of BaTiO₃—epoxy composites. In order to diminish computational resources and analyse simple models, composite topology was represented by periodic structures based on FCC configurations, but introducing novel packaging protocols, defining the way composites are filled as particle concentration is increased. The dielectric response of these anisotropic and periodic structures was mathematically represented through a quasi-static approximation using the Laplace equation. The amount of inclusions was varied in order to represent diluted and concentrated systems and structures were assessed for the whole feasible range of volume fractions. The numerical results were compared with experimental data concluding that only packaging protocols that consider higher particle—particle interaction are suitable to represent the dielectric behavior of concentrated-composite materials.

The Cyclic Deformation Behavior of Mg--Y2O3 Nanocomposites

Goh, C.S., Gupta, M., Wei, J., Lee, L.C. — 2008-08-28

Constant stress amplitude cyclic tests were conducted on monolithic Mg and Mg reinforced with 0.5, 1, and 2vol% of nano-size Y₂O₃. The cyclic life of the monolithic Mg and its nanocomposites were found to be comparable at the three stress amplitudes tested. Hardening behavior is observed throughout the whole fatigue life of all the specimens tested. Forest dislocations observed in Mg and Mg nanocomposites are responsible for the cyclic hardening. Addition of nanoparticles provides additional strengthening sources that cause the Mg—Y₂O₃ nanocomposites to harden more intensely that monolithic Mg.

Effects of Production Parameters and Conditioning upon Ballistic Characteristics of Para Aramid Light Armors

Akdemir, A., Candan, C., Sahin, O. S. — 2008-08-28

Composite materials are increasingly used in armors design due to their high strength to weight ratios and energy absorption capacities. It is very important to determine the best production parameters for armor production. On the other hand, the armors must show superior durability against harsh working conditions. So it is crucial to know the ballistic characteristics of such composite armors. In this study the effect of production parameters upon the terminal ballistic properties of para-aramid composite armor were examined under different conditionings. The composite armor plates were produced by hot pressing with four different pressing times. Before ballistic test, the armor plates conditioned at +21, +63, and -35°C at 50% of relative humidity and +21°C immersed into water temperature for 24h, in which weapons, ammunition, and equipment designed for use in the battlefield, are expected to fully operate.

A Micromechanical Damage Model for Carbon Fiber Composites at Reduced Temperatures

Peterson, E. C., Patil, R. R., Kallmeyer, A. R., Kellogg, K. G. — 2008-08-28

Fiber-reinforced composites are seeing increased use in civil infrastructure applications where they may be exposed to moisture, sub-ambient temperatures (below —40°C in northern regions), and reduced-temperature (freeze— thaw) thermal cycling. These environments may induce internal damage in the composite due to residual stresses caused by mismatches in coefficients of thermal expansion between the constituents. In this study, a micromechanical damage model is presented for the prediction of matrix crack development in a unidirectional carbon/epoxy composite resulting from exposure to sub-ambient temperatures. Two thermal loadings are considered: cool-down from the cure temperature of the composite (121°C) to 25°C (T= -96°C) and to -50°C (T=-171°C). A finite element model is utilized to determine the internal stresses due to differences in the CTE of the constituents, and a shear-lag model is developed to predict subsequent crack spacing in the matrix. The influence of fiber spacing (or fiber volume fraction) is addressed. Model predictions indicate that, at 25°C, the internal stresses are not large enough to cause matrix cracking in this composite. However, at -50°C, the longitudinal tensile stress in the matrix exceeds the strength of the epoxy at most typical fiber volume fractions, which will result in matrix cracking. The shear-lag model was used to predict the subsequent crack spacing, and demonstrated a strong dependence on service temperature and fiber spacing. The model predictions provide good qualitative agreement with experimental observations, and indicate that the development of damage in a composite should be considered in the design of composite structures for reduced temperature environments.

Relationship Between Electrical and Mechanical Loss Tangents of SLG Reinforced Phenolic Composites: Pilot Study

Ku, H., Cardona, F., Ball, J., Jacobson, W., Trada, M. — 2008-08-28

The mechanical properties of ceramic microspheres (SLG) reinforced phenolic resin composites have been measured and evaluated in earlier studies. This basic but critical and important data has created interest in the relevant industry in Australia. This study is therefore carried out to measure and evaluate the dielectric and mechanical properties of the composites with a view to benefit the relevant industry. The relationship between the two properties will also be studied. The original contributions of this article are that samples post-cured in conventional ovens have higher electrical loss as well as mechanical loss than their counterparts post-cured in microwaves. The storage modulus of all samples post-cured conventionally is higher than its counterpart. This is in line with the fact that it is a softer material with lower glass transition temperature. They also have higher mechanical loss tangent as well as loss modulus. For all percentages by weight of SLG, the glass transition temperature for the microwave cured sample was higher and the composite was stiffer; the opposite was true for the conventionally cured sample.

Happy 40th Anniversary

Tsai, S.W. — 2008-08-18

Distribution of Micro Stresses and Interfacial Tractions in Unidirectional Composites

Jin, K.-K., Yuanchen Huang, , Lee, Y.-H., Sung Kyu Ha, — 2008-08-18

A micromechanical approach is developed to determine the micro stress within a unidirectional composite under various mechanical and thermal loading conditions. Based on linear stress—strain relations, the concept of a stress amplification factor is introduced, and the correlations between macro stress and micro stress are explicitly expressed in mathematical equations. Three unit cell models, square, hexagonal, and diamond fiber arrays, are analyzed and compared using three-dimensional finite element methods. Subsequently, effective material properties, the distribution of micro stress in the fiber/matrix, as well as traction distribution at the fiber—matrix interface, and the effect of different interfacial stiffness, are obtained.

Effects of Fiber Arrangement on Mechanical Behavior of Unidirectional Composites

Yuanchen Huang, , Kyo Kook Jin, , Sung Kyu Ha, — 2008-08-18

Micromechanical approaches are employed to investigate the influence of different fiber arrangement on the mechanical behavior of unidirectional composites (UD) under various loading conditions. A micromechanical model with a random fiber array is generated and used in a finite element analysis together with two frequently used representative volume elements (RVE), or unit cell models of square and hexagonal arrays. The algorithm for generating the random fiber array is verified by comparing the comprehensive performance of a unit cell based on our random array and that of a unit cell based on a real fiber distribution in the UD cross-section. Performance of the random and regular fiber arrays is also evaluated through frequency distributions of stress invariants in matrix and tractions at the fiber—matrix interface due to various loading types. The effects of different loading angles on the overall response of regular arrays to various loading conditions are investigated thoroughly. Finally, the Weibull distribution of the maximum normal interfacial traction in random array is compared with the cumulative probability distribution of transverse strength data acquired from experiment, and good agreement is achieved.

Micro-Mechanics of Failure (MMF) for Continuous Fiber Reinforced Composites

Sung Kyu Ha, , Kyo Kook Jin, , Yuanchen Huang, — 2008-08-18

The micromechanics of failure was developed to predict the failure of continuous fiber reinforced composites. A micromechanical approach using unit cells of square and hexagonal arrays was employed to compute the micro stresses of constituents and at the fiber—matrix interface, which were used to determine the failure initiation of a unidirectional ply. The constituent properties include two tensile and compressive strengths of fiber and matrix, plus normal and shear strengths at the interface. The matrix and interfacial dominated strength properties are determined by matching the micro stresses at the constituent levels with the observed transverse tensile and compressive strengths on the macro ply level. The longitudinal shear failure is then expected to be a result of damage progression after initial failure. Based on the current MMF, in the graphite/epoxy considered in this study both transverse tensile and compressive failure are expected to occur via matrix failure. However, in the glass/epoxy the transverse tensile and compressive failures are respectively caused by matrix failure and interfacial tensile failure. These predictions are compared with predictions from other widely used failure criteria as well as experimental data. Lastly, we predicted the failure of laminates. Instead of using a unidirectional ply-based failure theory, starting with the fiber, matrix, and their interface will lead to a much simpler, more generic theory.

Formulation of Long-term Creep and Fatigue Strengths of Polymer Composites Based on Accelerated Testing Methodology

Miyano, Y., Nakada, M., Cai, H. — 2008-08-18

The applicability of our developed accelerated testing methodology (ATM) based on the time—temperature superposition principle (TTSP) and others for the prediction of long-term fatigue life of polymer matrix composites is performed experimentally and theoretically. A formulation method of master curves of creep and fatigue strengths for polymer matrix composites is introduced based on ATM. The master curves of creep and fatigue strengths for the typical three directions of unidirectional CFRP, that is the longitudinal tensile and compressive loadings and the transverse tensile loading, are formulated using the data measured based on ATM.

Progressive Failure Analysis of Composites

Tay, T.E., Liu, G., Tan, V.B.C., Sun, X.S., Pham, D.C. — 2008-08-18

Recent developments in the aircraft industry towards substantially improving fuel economy and extending flight range have accelerated interest in the use of advanced composites as primary structural materials. The airframes of next-generation airliners will have substantial parts made of light-weight composites. This means that the engineering demands on the performance of fiber-reinforced composites will become greater. There is therefore a need to better understand and predict the multiple complex failure mechanisms in composite structures, and to devise more reliable failure theories and damage progression models. There is a large body of literature on progressive damage analysis in composites, much of which employs damage mechanics and material stiffness degradation methods. This article reviews some of the more recent work in this area and describes the issues pertinent to application in composite structures. The authors' ongoing research efforts in modeling and prediction of progressive damage through the relatively novel element-failure method (EFM), which has been coded into a user-defined UEL code in Abaqus, are discussed. In particular, results for notched composite laminates and pin-loaded (PL) analyses are shown and compared to experimental data. Although EFM is the computational platform on which the damage is advanced in the structure, the results are dependent on the choice of the failure criterion. Various failure criteria are used throughout the cases discussed herein, from the more traditional Tsai—Wu (TW) criterion to the very recently proposed micromechanics-based failure (MMF) criterion. The EFM may also be used with cohesive elements, with the former intended for modeling in-plane damage progression, while the latter for delamination onset and propagation. This hybrid EFM-cohesive element approach is illustrated with an analysis of double-notched composite laminate. The computational models are relatively robust up to and including ultimate load, and enable the mapping of extensive damage patterns in composite structures. They represent a suite of computational tools that extend the capability to model damage and failure propagation beyond initial failure prediction.

MAE: An Integrated Design Tool for Failure and Life Prediction of Composites

Sihn, S., Jin Woo Park, — 2008-08-18

An advanced strength and life prediction tool, MAE, was developed for the analysis and design of composite structures and components. The MAE integrates three theories and methods: micromechanics of failure (MMF), an accelerated testing method (ATM), and an evolution of damage (EOD). The MAE can serve as a useful tool to predict damage initiation, progression and life under various durability loading and environmental conditions. It can handle the inhomogeneous complex geometries and structural components such as open-hole, filled-hole, bonded/bolted joint, stiffened panel, textile, etc. Therefore, it can help select not only the material and laminates but also optimal geometrical configurations. The MAE modules were implemented and integrated with a commercial finite element software, Abaqus, for better reliability and maintainability. Several examples of strength and life predictions of open-hole and double-edge notched specimens demonstrated the capability of the MAE as an advanced tool for the composite durability design.

Homogenization and Pseudoelastic Behavior of Composite Materials Reinforced with Shape Memory Alloy Fibers

Jarali, C. S., Raja, S. — 2008-08-15

Pseudoelastic behavior of cylindrical shape memory alloy (SMA) fiber embedded in a polymer matrix is investigated by using micromechanic approaches. A homogenization scheme based on Eshelby's equivalent inclusion method is adopted to derive the expressions for strains in the fiber and matrix in terms of the average strain in the composite. The constitutive laws for the SMA fiber and matrix are also expressed in terms of the average strain in the composite. The expressions for the SMA composite stiffness and the inelastic strains tensors are derived using dilute distribution theory and rule of mixtures approach. The composite stiffness and inelastic strain tensors are used in the generalized Hooke's law to compute the transformation stresses and associated hysteresis of the SMA composite. A comparison is also made with the strain energy approach. The computational results in terms of the composite stiffness and the stresses are presented within different fiber volume fraction, using the proposed methods. Finally, the modifications in the modeling approaches are highlighted with analytical case studies involving hysteretic stress—strain behaviors.

Phenomenological Modeling and Numerical Simulation of Relaxation in Bolted Composite Joints

Thoppul, S. D., Gibson, R. F., Ibrahim, R. A. — 2008-08-15

The effects of various bolt preloads, viscoelasticity, and external applied static and dynamic loads on bolt load relaxation in a carbon/epoxy composite bolted joint have been studied. Both phenomenological modeling and finite element analysis (FEA) of bolt-connected three-point bending specimens were employed in the studies. Relaxation of 1.25—4.25% over a period of 30 h was observed depending on the initial preload and applied external loads. Both static and dynamic applied loads were considered. It was observed that for any magnitude of external load the bolt load relaxation decreases with increasing initial preload. These findings emphasize the importance of the magnitude of the preload. Comparing the bolt load relaxation in steel and composite joints for the duration of 30 h, it was concluded that only about 1/3 of the relaxation in composite specimens is due to viscoelastic behavior of the polymer matrix in the composite, and the remaining 2/3 of the relaxation is due to other mechanisms such as bolt thread slip, plasticity and/ or external excitation.

Strength Optimization of Laminated Composite Plates

Topal, U., Uzman, U. — 2008-08-15

A procedure to select the optimum fiber orientations and determine the maximum load-bearing capacity of the simply supported symmetrically laminated plates combined the bidirectional tension loads and bending moments is described. The fiber orientation is considered a design variable. The optimization problem consists of two stages. The objective of the first stage is to maximize the strength of the laminated plates by determining the fiber orientations optimally while the objective of the second stage is to maximize each of the bidirectional tension loads and bending moments subject to the Tsai—Wu failure criterion for the optimum fiber orientations obtained from the first stage. The modified feasible direction method (MFD) is used as an optimization procedure. Therefore, a FORTRAN program is used for the finite element analysis and optimization routine. Also, the optimum fiber orientations are obtained using the golden section (GS) method to compare the results. Finally, the effect of the different plate aspect ratio, width-to thickness ratio, the uncertainties in the material properties and the uncertainties in the fiber orientations on the optimum results is investigated and the results are compared.

Shear Behavior of 3-D Biaxial Spacer Weft Knitted Composite under High Strain Rates

Baozhong Sun, , Bohong Gu, — 2008-08-15

The punch shear behavior of 3-D biaxial spacer weft knitted E-glass/ vinyl ester composite was investigated at quasi-static (0.01/s) and high strain rates ranging from 1200 to 3200/s in wale and course directions respectively. The quasi-static shear behavior was tested on a MTS 810.23 material tester and was compared with high strain rate shear behavior from a modified split Hopkinson pressure bar (SHPB) technique. The experimental results indicate that the shear stiffness, failure stress, and failure strain are rate sensitive both in wale and course directions. The shear stiffness and failure stress increase with the the increase of strain rate. The failure strains along the wale and course directions decrease with increasing strain rate. Furthermore, the shear failure becomes more severe in the course direction than in the wale direction at high strain rates. The damage mode in the wale direction at various strain rates is punch shear failure, while that of the course direction is filament tow breakage and matrix cracks. The unique features of this article are to conduct the punch shear tests at high strain rates successfully using a modified SHPB apparatus for the novel 3-D knitted composite and find the strain rate sensitivity both in shear behavior and failure mode. The results will benefit the structural design of the 3-D knitted composite owing to the higher in-plane tension stiffness and strength of the composite than any other 3-D textile structural composites.

Transverse Impact Damage and Energy Absorption of Three-Dimensional Orthogonal Hybrid Woven Composite: Experimental and FEM Simulation

Lv, L., Bohong Gu, — 2008-08-15

This article presents transverse impact behavior of 3D orthogonal Twaron^®/glass hybrid woven composite both in experimental and FEM simulation. The transverse impact behaviors of the 3D woven composite along warp and weft direction were tested with a modified Split Hopkinson Pressure Bar (SHPB) apparatus. The load—displacement curves and damage morphology were obtained to analyze the energy absorptions and impact damage mechanisms of the composite under different impact velocities. A unit-cell model based on the microstructure of the 3D woven composite was established to determine the composite deformation and damage when the composite impacted by a hemisphere-ended steel rod. Incorporated with the unit-cell model, a elasto-plastic constitute equation of the 3D woven composite and the Critical Damage Area (CDA) failure theory of composites have been implemented as a User Defined Material law (VUMAT) for ABAQUS/Explicit. The FEM calculated load—displacement curves, impact deformations and damages are compared with those in experiment. The good agreements of the comparisons prove the validity of the unit-cell model and user-defined subroutine VUMAT. This manifests the applicability of the VUMAT to the design of the 3D orthogonal woven composite structures under other impulsive loading conditions.

Correlation Between Physico-Mechanical and Free Volume Properties of Gaur-Gum Filled Polyurethane/Polymethyl Methacrylate Biodegradable Composites

Kumar, H., Ranganathaiah, C., Siddaramaiah, — 2008-08-15

This study reports characterization of polyurethane/polymethyl methacrylate (PU/PMMA, 50/50) Semi-Interpenetrating Polymer Network (SIPN) prepared by in-situ polymerization, and filled with different weight percent (wt%) of natural polymer, Gaur-Gum (GG). These are characterized for density, tensile strength and percentage elongation at break. Positron annihilation lifetime measurements have also been carried out to measure the free volume of these composites. The results show good correlation between free volume content of the composites with the mechanical properties. These natural polymer-filled composites were subjected to biodegradation using a specific microorganism, Aspergillus niger. The influence of A. niger on composites has also been studied. The results show that the weight loss of the specimens exposed to A. niger media increases with increase in gaur gum in the composite. Interestingly the weight loss of GG-filled composites seems to be more than the quantum of GG incorporated which is a clear indication that degradation of PU/PMMA system is responsible for the weight loss.

Strategies to Reduce Bond-line Read-out in Adhesive Joined SMC Automotive Body Panels

Basu, S. K., Kia, H. G. — 2008-08-15

Parameter sensitivity analysis on a 2D finite element model of adhesive-joined SMC hat/plate structure was performed to evaluate the relative impact of design, application, and material parameters on the bond-line read-out (BLRO) defect. Based on the sensitivity analysis and the ease of implementation, a list of strategies in the order of their effectiveness in reducing the BLRO defect is proposed. Choosing a low modulus, low cure temperature, and low shrinkage adhesive, raising the stiffness of the exterior SMC panel, and lowering the stiffness of the interior SMC panel in the regions around the bond-line were found to reduce the BLRO defect.

The Relationship Between Rheological Behavior and Toughening Mechanism of Toughened Poly(Ethylene Terephthalate)

Yuan Zong, , Yongfeng Cheng, , Gance Dai, — 2008-07-22

Mechanical properties, morphology, and rheological behavior of poly(ethylene terephthalate) (PET) with different compatible agents were studied. The results showed that the addition of ethylene-butylacrylate-glycidyl methacrylate copolymer (PTW) significantly improved the impact-strength property of PET over maleic anhydride grafted polyethylene octene elastomer copolymer (POE-g-MA), whereas silane coupling agent KH570 grafted polyethylene octane elastomer (POE-g-KH570) had no obvious contribution to the toughness of PET. Scanning electron microscopy micrographs indicated that the size of POE-g-MA and PTW particles were smaller that that of POE-g-KH570 and dispersed uniformly due to their interaction with PET. The rheological tests revealed the viscoelastic properties of PET/PTW and PET/POE-g-MA were different from that of PET/POE-g-KH570 ascribed to the high elastic of the compatibilizers and the finely dispersed particle. And it explained that the toughening mechanism of PET toughened system was mostly due to shear yielding.

Microstructure and Mechanical Properties of Carboxylated Carbon Nanotubes/Poly(L-lactic acid) Composite

Jiangtao Feng, , Jiehe Sui, , Wei Cai, , Zhiyong Gao, — 2008-07-22

Poly(L-lactic acid) (PLLA)/MWNTs composites were prepared by mixing solubilized PLLA with solutions of MWNTs treated by four kind of acids. Fourier transform infrared (FT-IR) spectra revealed that carboxyl groups were grafted to the surface of MWNTs. The water solubility showed that the MWNTs treated by HNO₃/H₂O₂ and HNO₃/H₂SO₄ could suspend in the air at room temperature for more than 100 days. Thermogravimetric analysis (TGA) showed that MWNTs treated by HNO₃/H₂O₂ and HNO₃/H₂SO₄ obtained relatively high COOH content. Mechanical properties of composites showed that the Young's modulus of the carboxylated MWNTs/PLLA composites increased compared to the pure PLLA. Scanning electron microscopy (SEM) images of fracture morphology confirmed that the dispersion of carboxylated MWNTs was more homogeneous than the pristine MWNTs in polymer matrix. MWNTs treated by HNO₃/H₂O₂ could get more COOH group and less damage.

Tracking Hygrothermal Strains of Carbon/Epoxy Composite under Varying Temperature and Humidity

Tsai, C.-L., Chih Hsing Wang, , Chang, J.-J., Yeih, W. — 2008-07-22

An algorithm was developed based on diffusion and hygrothermal expansion theories to quantify the real-time hygrothermal deformation of a composite structure. The algorithm can be used to calculate the dynamic hygrothermal strain of composite laminates that is caused by constantly changing environmental humidity and temperature. Experiments were performed on carbon/ epoxy laminates in a conditioning chamber where relative humidity and temperature were set to vary stepwise and the hygrothermal strains of the specimens were monitored. The hygrothermal strains were calculated theoretically from humidity and thermal history throughout each experiment and confirmed experimentally.

Numerical Modeling of Gas Leakage Through Damaged Composite Laminates

Kumazawa, H., Whitcomb, J. — 2008-07-22

A computational fluid dynamics (CFD) model was developed to predict leakage through a damaged laminate. Crack opening displacements (COD) from a 3D finite element model were fit using a polynomial function. This curve fit was then used in defining the openings in the CFD model. Delamination length at crack intersections was estimated based on experimental data for specimens subjected to thermal loads only. Leakage under combined thermal and biaxial mechanical loadings was predicted with the estimated delamination length. Considering the sensitivity of the predictions to many variables, the calculated leakage under combined thermal and biaxial mechanical loadings was estimated with reasonable accuracy.

Surface Morphology Effects on High-Rate Fracture of an Aluminum/Epoxy Interface

Chul Jin Syn, , Chen, W. W. — 2008-07-22

Crack initiation and propagation along interfaces are influenced by interface morphology under quasi-static loading conditions. In this article, the loading rate and surface topography effects on energy dissipation during dynamic fracture of an aluminum/epoxy interface were experimentally investigated. Four-point bending specimens with a precrack on the interface were dynamically loaded at high rates with stress waves. The aluminum side of the interface on the specimen had four surface roughness levels. The study of surface morphology effects at high rates is the original contribution of this research. The results indicate that surface morphology significantly affect the energy dissipation during the dynamic fracture of the interface. At a specific roughness, fracture toughness and energy dissipation increase with increasing loading rates.

Static and Low Velocity Impact Behavior of Composite Sandwich Panels with an Aluminum Foam Core

Reyes, G. — 2008-07-22

The static and low velocity impact response of aluminum foam based sandwich structures manufactured using thermoplastic composite skins has been studied. The three-point bend (3PB) test geometry was used to evaluate the static properties of the sandwich structures. An examination of the quasi-statically tested specimens revealed failure modes such as indentation, core yielding, and face wrinkling. The low velocity impact behavior of the sandwich systems was investigated using an instrumented dropping weight impact tower and modeled using an energy balance approach. Experimental results revealed that these systems exhibited excellent energy absorbing characteristics under dynamic loading conditions. Furthermore, it has been shown that a simple energy balance model based on the dissipation of energy during the impact event can be used to successfully model the low velocity impact response of the composite reinforced sandwich structures. A breakdown of the energy dissipation revealed that thermoplastic composite reinforced aluminum foam sandwich structures absorb much of the impact energy due to the bending and contact effects.

An Investigation of the Effect of SiC Reinforcement Coating on the Wettability of Al/SiC System

Tekmen, C., Saday, F., Cocen, U., Ljungberg, L.Y. — 2008-07-22

In this study, the wetting behavior of oxide and metallic coated SiC substrate with Al—Si—Mg alloy has been investigated. SiC substrate and particles were coated with SiO₂, TiO₂, and metallic Ni by using thermo-chemical treatment, sol-gel and electroless coating techniques, respectively. Also, the effect of doping elements (Ni, Cu and Fe in TiO₂ coating) on the wettability has been investigated. Coatings were characterized by SEM, EDS, XRD, and by means of surface roughness. Contact angle results demonstrate that metallic Ni coating significantly improves the wettability. However, doping elements did not alter the results due to their detrimental effect on surface roughness.