Sangwook Sihn

Modeling and prediction of bulk properties of open-cell carbon foam

Abstract The emerging ultralightweight material, carbon foam, was modeled with three-dimensional microstructures to develop a basic understanding in correlating microstructural configuration with bulk performance of open-cell foam materials. Because of the randomness and complexity of the microstructure of the carbon foam, representative cell ligaments were first characterized in detail at the microstructural level. The salient microstructural characteristics (or properties) were then correlated with the bulk properties through the present model. In order to implement the varying anisotropic nature of material properties in the foam ligaments, we made an attempt to use a finite element method to implement such variation along the ligaments as well as at a nodal point where the ligaments meet. The model was expected to provide a basis for establishing a process–property relationship and optimizing foam properties. The present model yielded a fairly reasonable prediction of the effective bulk properties of the foams. We observed that the effective elastic properties of the foams were dominated by the bending mode associated with shear deformation. The effective Youngs modulus of the foam was strongly influenced by the ligament moduli, but was not influenced by the ligament Poissons ratio. The effective Poissons ratio of the foam was practically independent of the ligament Youngs modulus, but dependent on the ligament Poissons ratio. The effective Youngs modulus of the carbon foam was dependent more on the transverse Youngs modulus and the shear moduli of the foam ligaments, but less significantly on the ligament longitudinal Youngs modulus. A parametric study indicated that the effective Youngs modulus was significantly improved by increasing the solid modulus in the middle of the foam ligaments, but nearly invariant with that at the nodal point where the ligaments meet. Therefore, appropriate processing schemes toward improving the transverse and shear properties of the foam ligaments in the middle section of the ligaments rather than at the nodal points are highly desirable for enhancing the bulk moduli of the carbon foam.

Importance of interfaces in governing thermal transport in composite materials: modeling and experimental perspectives.

Thermal management in polymeric composite materials has become increasingly critical in the air-vehicle industry because of the increasing thermal load in small-scale composite devices extensively used in electronics and aerospace systems. The thermal transport phenomenon in these small-scale heterogeneous systems is essentially controlled by the interface thermal resistance because of the large surface-to-volume ratio. In this review article, several modeling strategies are discussed for different length scales, complemented by our experimental efforts to tailor the thermal transport properties of polymeric composite materials. Progress in the molecular modeling of thermal transport in thermosets is reviewed along with a discussion on the interface thermal resistance between functionalized carbon nanotube and epoxy resin systems. For the thermal transport in fiber-reinforced composites, various micromechanics-based analytical and numerical modeling schemes are reviewed in predicting the transverse thermal conductivity. Numerical schemes used to realize and scale the interface thermal resistance and the finite mean free path of the energy carrier in the mesoscale are discussed in the frame of the lattice Boltzmann-Peierls-Callaway equation. Finally, guided by modeling, complementary experimental efforts are discussed for exfoliated graphite and vertically aligned nanotubes based composites toward improving their effective thermal conductivity by tailoring interface thermal resistance.

Sandwich Construction with Carbon Foam Core Materials

To study suitability of the emerging ultralightweight carbon foams in load-carrying structures, the state-of-the-art carbon foams from various manufacturers were evaluated as core material in a sandwich construction. The carbon foams were firstly characterized by measuring compressive and shear mechanical properties. The carbon foam possessed highly anisotropic properties between in-plane and through-the-thickness directions. The foams aged at 316°C (600°F) for 100 h in air lost 0.6% of their weight and showed little degradation in the properties within the scatter range of test data. The carbon foams tested at an elevated temperature of 316°C (600°F) showed no degradation in the compressive modulus and strength as compared to the properties measured at a room temperature of 21°C (70°F). Sandwich beams with laminated composite facesheets were fabricated with a carbon foam core and tested under static and fatigue four-point bending loads. The beams under the static loadings showed nearly linear elastic behavior until the maximum failure loads, and then postfailed either in yielding or in brittle mode following the postfailure behavior of the carbon foam core. The failure occurred in the core material in a shear mode. Sandwich beams with carbon foam cores survived after a few hundred or even a few hundred thousands of cyclic loads. Both the moduli and strength of the beams remained nearly unchanged after the fatigue loads. Central displacement and strain measurements at the gauge locations matched well with the analytical solution from a sandwich-beam theory. The shear moduli and strength of the carbon foams calculated from the four-point bending tests were in good agreement with those from torsional tests. The sandwich beam test technique is useful for the determination of the foam shear properties, especially for uncoupled shear properties of only one plane. The sandwich beams with the carbon foam cores tiled together in the middle of the beam exhibited nearly identical load-displacement behavior as well as the shear failure mode to those of the beam made with an intact core. From the modulus point of view, the hole-drilling with the high-density foams is a better way of achieving the weight reduction in the sandwich construction than using the low-density foams. If the materials were removed in the noncritically loaded area (e.g., central section of the beams under the four-point bending), the weight reduction can be achieved without sacrificing much strength property of the core materials as well.

Time- and Temperature-Dependent Failures of a Metal-to-Composites Bonded Joint with PMMA Adhesive Material:

A metal–composite bonded joint with a PMMA adhesive material was fabricated and tested under various loading conditions to help a short-term and long-term design of the bonded-jointed structures. The bonded joint is composed of a cast-iron rod and a GFRP composite pipe as adherends, and ductile PMMA as adhesive material (Plexus AO425). Tubular-lap bonded joints were fabricated conveniently and consistently with a devised mold and a curing condition to yield a reliable test data set. Experiment was performed to measure creep compliances of the PMMA material, andconstant strain rate (CSR) andfatigue strengths of the bonded joint under various loading and temperature conditions. We observed the CSR and fatigue strengths significantly depended on time and temperature, but little on direction of loadings and size of joints. The bonded joints failed under consistent damage mechanism in the combination of the cohesive and interfacial failure modes, which mainly occurred in the adhesive material and at the interface between the cast-iron rodandthe adhesive material, respectively. In order to further substantiate the utility of the time–temperature superposition principle that has been extended to describe destructive behavior of composite materials, we made the first attempt to apply the superposition principle to failure behavior of the bonded joints. The superpostion principle yielded smooth master curves for the creep compliance of the adhesive material as well as the CSR and fatigue strengths of the bonded joint. Time–temperature shift factors of the non-destructive creep compliance of the adhesive material were closely related to destructive CSR strength of the bonded joint. The master curves that were created by shifting fatigue curves with shift factors of the CSR strength yielded an extended range of the long-term life prediction at any temperatures, frequencies and stress ratios. It was shown that the life prediction of the bonded joints by using the master curves made a good agreement with the experimental results.

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

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.

Micromechanical analysis for transverse thermal conductivity of composites

Micromechanical analyses were conducted for the prediction of transverse thermal conductivity of laminated composites. We reproduced and reinvestigated both analytic and numerical models with regular and randomly distributed fibers in matrix material. A parametric study was conducted for wide ranges of fiber volume fractions and fiber-to-matrix thermal conductivity ratios. The numerical solutions using finite element (FE) analysis were compared with various analytic solutions from simple and enhanced rule or mixtures and an effective inclusion method (EIM). It was found that the EIM yields a reasonably agreeable solution with the FE solution using a hexagonal-array of regular fiber distribution for wide ranges of fiber volume fraction and fiber-to-matrix thermal conductivity ratios, which makes the EIM a useful method in predicting various multiphysical transverse properties of composites. Comparison of the results from the regular- and random-fiber models indicates that the transverse thermal conductivity of composites can significantly be affected by the random fiber distributions, especially at high fiber volume fractions. A similar conclusion was made for the foams with random pore distribution. It was shown that the predictions with the random fiber distribution agree well with the experimental data.

Development of a three-dimensional mixed variational model for woven composites. II. Numerical solution and validation

Abstract A mixed three-dimensional variational model, derived in an adjoining paper, is solved numerically for stress analysis with a finite element approach. Since the mixed model calculates the stress field by taking variations of displacement and stresses independently and satisfying equilibrium of stresses pointwise, accurate interlaminar stresses are predicted at the yarn interface. The interface continuity conditions are implemented through a penalty method by adding an additional variational energy of two constraint conditions: the displacements must be continuous along the interface between two stacked subregions, and interfacial normal and shear stresses must be in equilibrium at the interface. After performing the thickness integration, the three-dimensional variational energy equation is evaluated for each yarn (subregion) two-dimensionally with 16 stress-related and 13 displacement-related unknown variables. Rayleigh–Ritz approximation yields a system of linear equations by taking derivatives of the variational energy equation with respect to the independent unknown variables. The present mixed method is applied to analyze a flat laminated composite with a free edge, and the representative volume element of woven fabric composites. The displacement and stress results of the present method are compared and validated with the conventional displacement-based finite element solutions and/or the previous analytic solution.

Development of a three-dimensional mixed variational model for woven composites. I. Mathematical formulation

Abstract A mixed three-dimensional variational model has been derived for stress analysis of a representative volume element of woven fabric composites, based on the Reissner variational principle. In this model, each yarn is modeled as a homogeneous orthotropic (in its own material axes) medium, and the matrix regions that exist around the wavy yarns are also represented as separate regions in the model. In order to accurately predict the characteristic damage (crack initiation and its propagation), the equilibrium of stresses is satisfied pointwise everywhere in the model, and the yarn-interface stress compatibility is enforced in the model. The variational principle yields a set of second-order partial differential equations, which can numerically be solved by either by finite element or finite difference approaches. A solution procedure with representative results is given in an adjoining paper.

Prediction of Delamination Resistance in Laminated Composites with Electrospun Nano -Interlayers using a Cohesive Zone Model

Nano -interlayers were fabricated with polycarbonate by using an electrospinning process to improve delamination resistance in laminated composites. The laminates were laid up with and without the nano -interlayers having the stacking sequence of (30/ -30/90 )s and tested under uniaxial tensile loading. Five nano -interlayers in the form of the electrospun nanofiber mats were placed at every ply interfaces. Thickness difference of the laminates with and without five nano -interlayers is less than 0.001 mm. First -ply -failure stress, delamination stress and ultim ate strength increased 8.4%, 8.1% and 9.8% with the addition of the nano -interlayers as compared with the pristine specimens. The number of microcracks at the delamination stress decreases significantly by 21.6% with the addition of the nano -interlayers. Therefore, the ultra thin electrospun nano -interlayer cannot only suppress the microcracking and delamination damage by reducing the interlaminar stress concentration at the free edges in the laminated composites, but increase the ultimate strength proper ty as well. A nonlinear finite element analysis was performed using interface elements based on a cohesive zone model to predict the delamination resistance of the laminates with the insertion of the electrospun nano -interlayers under the un idirectional tensile displacement loading . The laminated composite with the interface elements simulating the nano -interlayers was modeled with 3 -D eight -node hexahedral solid elements and interface decohesi on elements having a bilinear constitutive relation. Paramete r studies were carried out with various critical energy release rates and interfacial strengths .

Thin Ply Composites

Large tows of carbon fiber can be spread by a simple, non-intrusive process from which plies down to 1/6 of the conventional 5-mil thickness can be obtained. Laminates made from such plies showed remarkable resistance to micro cracking, delamination and splitting under both static and fatigue loading. With thick-thin ply hybrids, superior performance, lower cost and lower minimum gage can all lead to applications not feasible with 5-mil thick plies.