Jeong Min Cho

Characterization and Constitutive Modeling of Composite Materials under Static and Dynamic Loading

Composite materials were characterized under quasi-static and dynamic loading and a constitutive model was adapted to describe the nonlinear multi-axial behavior of the materials under varying strain rate. The materials investigated were unidirectional glass fiber/vinylester, and carbon fiber/epoxy composites. Multiaxial static and dynamic experiments were conducted using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. Stress-strain curves were obtained for various loading orientations with respect to the fiber direction at three strain rates, quasi-static, intermediate and high strain rate. A nonlinear constitutive model is proposed to describe the rate-dependent behavior under states of stress including tensile and compressive loading. Experimental results were in good agreement with predictions of the proposed constitutive model.

Multiscale Hybrid Nano/Microcomposites–Processing, Characterization, and Analysis

A review is presented of methods for enhancing the matrix-dominated thermomechanical properties of carbon/epoxy composites by incorporating carbon nanoparticles in the matrix. The materials used were DGEBA epoxy as the basic resin, carbon nanoplatelets, and multi-wall carbon nanotubes. With the latter, a block copolymer dispersant was used to optimize dispersion of the nanotubes. Preforms used were unidirectional carbon fibers (AS4) and five-harness satin weave carbon fabric (AGP370-5H, Hexcel Corp.). Matrix-dominated thermomechanical properties measured were glass transition temperature, compressive modulus and strength, interlaminar shear strength, and in-plane shear properties. Several batches of composite materials were processed and evaluated. They included reference carbon/epoxy composites without nanoparticles, unidirectional carbon/epoxy with carbon nanoplatelets, and carbon fabric/epoxy composites with carbon nanotube loadings of 0.5 and 1 wt%, with and without a copolymer dispersant. Special processing methods were developed, employing solvent-based high shear mixing and sonication. Significant increases in matrix dominated properties were measured. Micromechanical models were proposed to explain the measured enhancements.

Mechanical and failure behavior of composite materials under static and dynamic loading

Composite materials were characterized under quasi-static and dynamic loading and failure theories were developed/expanded to describe static and dynamic failure under multi-axial states of stress. The materials investigated were unidirectional glass fiber/vinylester, and carbon fiber/epoxy composites. Multi-axial static and dynamic experiments were conducted using off-axis specimens to produce stress states combining transverse normal and in-plane shear stresses. A Hopkinson bar apparatus was used for multi-axial characterization of the above materials at high strain rates. Quasi-static and dynamic failure envelopes were obtained by the various available failure theories including the recently introduced Northwestern (NU) theory and compared with experimental results. The NU theory was extended to the dynamic loading regime and was shown to be in excellent agreement with experimental results.

Characterization of Polymeric Foams under Multi-Axial Static and Dynamic Loading

An orthotropic polymeric foam with transverse isotropy (Divinycell H250) used in composite sandwich structures was characterized under multi-axial quasi-static and dynamic loading. Quasi-static tests were conducted along principal material axes as well as along off-axis directions under tension, compression, and shear. An optimum specimen aspect ratio of 10 was selected based on finite element analysis. Stress-controlled and strain-controlled experiments were conducted. The former yielded engineering material constants such as Young’s and shear moduli and Poisson’s ratios; the latter yielded mathematical stiffness constants, i. e., Cij. Intermediate strain rate tests were conducted in a servohydraulic machine. High strain rate tests were conducted using a split Hopkinson Pressure Bar system built for the purpose. This SHPB system was made of polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals. The system was analyzed and calibrated to account for the viscoelastic response of its bars. Material properties of the foam were obtained at three strain rates, quasi-static (10-4 s-1), intermediate (1 s-1 ), and high (103 s-1 ) strain rates.

Strain-Rate-Dependent Behavior of Polymeric Foams

In many applications composite sandwich structures with polymeric foam cores are exposed to high energy and high velocity dynamic loadings producing multi-axial dynamic states of stress. The material studied was a closed cell PVC foam, an orthotropic/transversely isotropic material, exhibiting strain-rate-dependent elastic/viscoplastic behavior. The material was characterized at three strain rates, quasi-static (10 s), intermediate (1 s ) and high (10 s ). Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted under strain control using a split Hopkinson Pressure Bar (SHPB) system made of polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals and results in lower noise to signal ratios and longer loading pulses. It was determined by analysis and verified experimentally that the loading pulses applied, propagated along the polycarbonate rods at nearly constant phase velocity with very low attenuation and dispersion. It was found that the initial elastic properties did not vary with strain rate as opposed the first peak stress which increased noticeably with strain rate.

Mechanical characterization of graphite platelet nanocomposites through molecular mechanics and continuum modeling

Mechanical properties of nanocomposites consisting of epoxy substrate reinforced by randomly oriented graphite platelets are studied with Mori-Tanaka method in collaboration with molecular mechanics. Elastic constants of graphite nanoplatelets which are the inclusion phase of the micromechanical model are calculated based on their molecular force field. The calculated elastic constants are well compared with the both experimental data and other theoretical predictions in literatures. The results from Mori-Tanakas method based on the graphite modulus calculation from molecular mechanics are found that nanocomposite moduli have strong dependence on the aspect ratios of reinforcing particles, but no direct size dependence. The predicted nanocomposite moduli compare favorably with modulus measurement of several graphite particles of various aspect ratios and sizes. The experimental data also shows that particle sizes have very weak effect on nanocomposite moduli.© 2006 ASME