Je Hoon Oh

Cure Cycle for Thick Glass/Epoxy Composite Laminates

Duringthe curingprocess of thick glass/epoxy composite laminates, substantial amounts of temperature lagand overshoot at the center of the laminates is usually experienced due to the large thickness and low thermal conductivity of the glass/epoxy composites, which require a long time for full and uniform consolidation. In this work, the temperature profiles of a 20mm thick unidirectional glass/epoxy laminate duringan autoclave vacuum bag process were measured and compared with the numerically calculated results. For the calculation of distributions of the temperature, degree of cure, resin pressure, exothermic heat and required time for full consolidation by three-dimensional finite element analyses, the effects of convective heat transfer coefficient and geometry of mold and bagging assembly on the temperature profiles were taken into consideration. Based on the numerical results, an optimized cure cycle with the coolingand reheatingsteps was developed by minimizingthe objective function to reduce the temperature overshoot in the composite. From the experimental and numerical results, it was found that the measured temperature profiles were in good agreement with the numerical ones, and conventional cure cycles recommended by prepregmanufacturers for thin laminates should be modified to prevent temperature overshoot and to obtain full consolidation.

Optimum bolted joints for hybrid composite materials

Abstract The optimum bolted joints for hybrid composite materials composed of glass-epoxy and carbon-epoxy under tensile loading were investigated. The design parameters considered for the bolted joints were ply angle, stacking sequence, the ratio of glass-epoxy to carbon-epoxy, the outer diameters of washers and the clamping pressure. As bearing failure was desirable for bolted joints, the geometry of the bolted joint specimen was designed to undergo bearing failure only. By inspecting the fracture surfaces of the specimens it was found that delamination on the loaded periphery of the holes and extensive damage on the edge region constrained by a washer occurred. To assess the delamination of the hybrid composite materials, three-dimensional stress analysis of the bolted joint was performed using a commercial finite-element software and compared with the experimental results.

Pulsed light sintering characteristics of inkjet-printed nanosilver films on a polymer substrate

In this work, the microstructures of inkjet-printed nanosilver films sintered by intense pulsed light (IPL) were systematically analyzed and correlated with the electrical properties. Nanosilver films with various dimensions were inkjet-printed and sintered at different light intensities to investigate the effects of the film dimension and light intensity on the sintering characteristics. For comparison purposes, the same inkjet-printed films were also thermally sintered at 210 °C for 1 h. Consecutive light pulses from a xenon lamp induced film swelling and the corresponding hollow microstructures of the inkjet nanosilver films. The resistance of IPL-sintered films was inversely proportional to the light intensity, and the resultant conductivity comparable to the thermally sintered one was achieved within just a few tens of ms, without damaging a polymer substrate. While all the thermally sintered patterns experienced shrinkage during the sintering process, the IPL-sintered ones could keep their initial dimension at a certain light intensity.

Two-step flash light sintering process for crack-free inkjet-printed Ag films

In this paper, a two-step flash light sintering process for inkjet-printed Ag films is investigated with the aim of improving the quality of sintered Ag films. The flash light sintering process is divided into two steps: a preheating step and a main sintering step. The preheating step is used to remove the organic binder without abrupt vaporization. The main sintering step is used to complete the necking connections among the silver nanoparticles and achieve high electrical conductivity. The process minimizes the damage on the polymer substrate and the interface between the sintered Ag film and polymer substrate. The electrical conductivity is calculated by measuring the resistance and cross-sectional area with an LCR meter and 3D optical profiler, respectively. It is found that the resistivity of the optimal flash light-sintered Ag films (36.32 n? m), which is 228.86% of that of bulk silver, is lower than that of thermally sintered ones (40.84 n? m). Additionally, the polyimide film used as the substrate is preserved with the inkjet-printed pattern shape during the flash light sintering process without delamination or defects.

Solvent and substrate effects on inkjet-printed dots and lines of silver nanoparticle colloids

The shape changes of inkjet-printed dots and lines were investigated by varying the primary solvent of nanosilver colloids, surface wettability and substrate temperature. The morphological changes in dots and lines in array patterns due to the interaction between neighboring dots or lines during evaporation was also examined for two different nanosilver colloids. In order to examine the effect of solvent evaporation rate, two different solvents with different boiling points (BP) were employed for nanosilver inks. With a fluorocarbon film coating and subsequent ultraviolet ozone (UV/O3) treatment, various surface wettability conditions were obtained on silicon (Si) wafers. Substrate temperature was varied from room temperature to 80 °C, and droplets from a 50 µm diameter nozzle were printed onto the substrate after optimizing the ejection of individual droplets. The results indicate that the shapes and sizes of dots and lines are sensitive to changes in both surface energy and substrate temperature, and the ink with a higher BP solvent produces larger dots under the same surface condition due to its slower evaporation. Dots and lines with better quality are achieved using the ink with a lower BP solvent. The morphological changes in dot and line arrays are dependent on the evaporation rate of the primary solvent as well as the distance between neighboring features. As a result, selecting a proper solvent for nanosilver ink is very crucial for controlling the shape and morphology of inkjet-printed patterns.

Effect of Fiber Arrangement on Residual Thermal Stress Distributions in a Unidirectional Composite

A three-dimensional finite element analysis is performed to investigate the effects of fiber arrangements on the residual thermal stresses in unidirectional composites of various fiber volume fractions (FVFs). The fiber arrangements include the regular fiber arrays (square and hexagonal arrays) and a random fiber array. Normal, tangential, and shear stresses at the fiber–matrix interface are first obtained using unit cells of the regular square and hexagonal fiber arrays. To simulate better real fiber arrangements, random fiber distribution is modeled and analyzed using a finite element analysis. Statistical distributions of residual thermal stresses are obtained for various FVFs and compared with the results from regular fiber arrays. The effects of constituent material properties of composites on thermal stresses are also taken into consideration. The results indicate that the random arrangement of fibers has a significant influence on residual thermal stresses especially at high FVFs. The mode stresses of the random fibers are well estimated using the square array whereas the mean stresses are better predicted from the hexagonal array. It is shown that predicted coefficients of thermal expansion are not influenced by the microstructure of composites.

Interfacial Strain Distribution of a Unidirectional Composite with Randomly Distributed Fibers under Transverse Loading

The micromechanical approach was used to investigate the interfacial strain distributions of a unidirectional composite under transverse loading in which fibers were usually found to be randomly packed. Representative volume elements (RVEs) for the analysis were composed of both periodic fiber arrays, such as a square array and a hexagonal array, and a random fiber array. The finite element analysis was performed to analyze the normal, tangential, and shear strains at the interface. In order to verify the RVE for describing a random fiber array, the generated RVE was statistically compared with the one constructed from typical images of a transverse cross-section of a unidirectional composite. Since the strain distributions at the interface experienced periodic characteristics along with its circumference, the Fourier series approximation with proper coefficients was utilized to evaluate the strain distributions at the interface for the periodic and random fiber arrays with respect to fiber volume fractions. From the analysis, it was found that the random arrangement of fibers had a significant influence on the strain distribution at the interface, and the strain distribution in the periodic arrays was one of special cases of that in the random array.

Dynamic characteristics measurements of inkjet-printed thin films of nanosilver suspensions on a flexible plastic substrate

The dynamic properties of inkjet-printed thin films on flexible polyimide (PI) substrates were investigated using the vibration analysis adopting wave approach. In order to fabricate the test specimens, the Ag nanoparticle suspension was inkjet printed on the plasma-treated PI substrate and sintered at different temperatures. The beam-shaped Ag-printed PI specimens with 30 mm length and 0.6 mm width were prepared by pico-second laser pulse cutting and were used as the cantilever beam in the vibration test. From the base-excited response of the beam, the frequency-dependent bending stiffness and loss factor were obtained, which were used to calculate Youngs modulus and loss factor of the inkjet-printed thin films. The influence of the sintering temperature and film thickness on the dynamic properties was investigated and the nanoindentation test was also performed to compare results from each test. Youngs modulus increased and loss factor decreased with increasing sintering temperature and the effect of the film thickness was not significant. Youngs modulus from the vibration analysis was in comparable agreement with that from the nanoindentation test. The proposed method enables the direct determination of the dynamic characteristics of thin films without damaging the thin films as well as removing the substrate.

Precise size control of inkjet-printed droplets on a flexible polymer substrate using plasma surface treatment

The effect of plasma surface treatment on the dried diameter of droplets inkjet-printed on the PI substrate was investigated by varying the plasma process parameters. The sequential design of the experiments technique combining a factorial design with a response surface method was introduced to systematically estimate an accurate empirical response model for two independent design variables: radio frequency (RF) power and gas pressure. C4F8 gas was used for the plasma surface treatment of 90 mm × 90 mm polyimide substrates. Ag ink droplets ejected from a 30 µm nozzle were printed on the plasma-treated substrates, and their measured diameter and standard deviation were used as the response variables. The plasma-treated substrate was also characterized by contact angles and x-ray photoelectron spectroscopy. The results indicate that the droplet diameters and their uniformity are sensitive to changes in both RF power and gas pressure, lower droplet diameters on PI substrates correspond to lower surface energies, and the process condition producing higher F content results in more hydrophobic surface. The resolution of the inkjet printing can be precisely controlled by varying the droplet diameter and uniformity through the C4F8 plasma surface treatment.

Composite robot end effector for manipulating large LCD glass panels

Recently, the design and the manufacture of light robot end effectors with high stiffness have become important in order to reduce the deflection due to the self-weight and weight of glass panel, a part of LCD, as the size of glass panels as well as robot end effectors increases. The best way to reduce the deflection and vibration of end effectors without sacrificing the stiffness of end effectors is to employ fiber reinforced composite materials for main structural materials because composite materials have high specific stiffness and high damping. In this work, the end effector for loading and unloading large glass panels were designed and manufactured using carbon fiber epoxy composite honeycomb sandwich structures. Finite element analysis was used along with an optimization routine to design the composite end effector. A box type sandwich structure was employed to reduce the shear effect arising from the low modulus of honeycomb structure. The carbon fiber epoxy prepreg was hand-laid up on the honeycomb structure and cured in an autoclave. A special process was used to reinforce the two sidewalls of the box type sandwich structure. The weight reduction of the composite end effector was more than 50% compared to the weight of a comparable aluminum end effector. From the experiments, it was also found that the static and dynamic characteristics of the composite end effector were much improved compared to those of the aluminum end effector.