Young-Jin Yum

Detection of delamination in graphite/epoxy composite by electric potential method

The amount of delamination in a graphite/epoxy composite laminate was measured using the resistance change of a plate. Initially cracked DCB (double cantilever beam) composite specimens were fabricated and mode I interlaminar fracture test was performed. Resistance change increased as the crack propagated and the change was larger for the thinner specimen. The relation of resistance change and crack growth rate showed nonlinear behavior and could be simplified as a parabolic equation. A proposed empirical equation which included thickness effect of composite plate is utilized to detect and measure inner flaws such as delamination of a fiber reinforced composite laminate.

Frictional behavior of automotive leaf spring

Frictional characteristics of automotive leaf spring was analyzed experimentally and numerically. Compression test was performed to leaf spring used in commercial trucks from which load-displacement and load-strain results were obtained. As expected, hysteresis phenomenon was observed due to the interleaf friction. Load-strain relation was not symmetric as supports of leaf spring were unequal : one is a simple pin and the other is a pin connected by a shackle. Numerical analysis was performed by using commercial finite element analysis program MARC and the load-displacement hysteresis curve was successfully simulated and the static spring rate was in good agreement with experimental data. However, calculated strain values and friction force were different from those of measured values. Finally it was found that the effect of a shackle was not negligible since it affected loaddisplacement behavior significantly.

Simulation of Electrical Resistivity of Carbon Black Filled Rubber under Elongation

It has been known that the carbon black (CB) network is responsible for the electrical and mechanical behaviors of filled rubber. Due to the complexity involved in the filled rubber in relation to the conductive mechanism of the CB network, there has been little work concerned with simulation of the electrical behavior at large strains. Based upon an infinite circuit model, the electrical resistivity of CB filled rubber under elongation is simulated. For CB (N330) filled natural rubber with volume fraction of 27.5%, the simulated electrical resistivity increases with elongation at small stains, corresponding to the breakup of the agglomerates. The reduction in resistivity at larger strains corresponds to the decrease of the junction width, which results in a decrease of the contact resistance. Good agreement is found between the simulations and the experimental data available in the literature. The simulated results confirm the effects of the breakdown of the CB network and the alignment of CB aggregates under strain on the electrical resistivity.

Simulation of carbon black network in filled rubber

Carbon black (CB) is one of the most important fillers for rubber and plastics materials. How to describe the CB network is a fundamental problem for establishing relationships between the CB network and the mechanical properties of filled rubber. In view of the electrical conductivity of CB, an infinite circuit consisting of numerous contact resistors, interconnected with each other, is proposed to simulate the CB network in filled rubber; the resistances were determined by considering the tunneling conduction mechanism and a Gaussian distribution for the CB aggregate junction width. As an example, the electrical resistivity of CB (N330) filled natural rubber during uniaxial deformation was studied. It was found that the logarithm of resistivity was an approximately linear function of the extension ratio, and the resistivity increased with the increase of average number of primary particles per aggregates. Additionally, some published experimental points lie between the curves calculated for five primary particles and for seven primary particles per aggregate at extension ratios below 1.2. The calculations suggested that the average number of primary particles per aggregate for CB type N330 might be between five and seven.

Effect of Multiwalled Carbon Nanotubes on the Mechanical Properties of Carbon Fiber-Reinforced Polyamide-6/Polypropylene Composites for Lightweight Automotive Parts

The development of lightweight automotive parts is an important issue for improving the efficiency of vehicles. Polymer composites have been widely applied to reduce weight and improve mechanical properties by mixing polymers with carbon fibers, glass fibers, and carbon nanotubes. Polypropylene (PP) has been added to carbon fiber-reinforced nylon-6 (CF/PA6) composite to achieve further weight reduction and water resistance. However, the mechanical properties were reduced by the addition of PP. In this research, multiwalled carbon nanotubes (CNTs) were added to compensate for the reduced mechanical properties experienced when adding PP. Tensile testing and bending tests were carried out to evaluate the mechanical properties. A small amount of CNTs improved the mechanical properties of carbon fiber-reinforced PA6/PP composites. For example, the density of CF/PA6 was reduced from 1.214 to 1.131 g/cm3 (6.8%) by adding 30 wt % PP, and the tensile strength of 30 wt % PP composite was improved from 168 to 173 MPa (3.0%) by adding 0.5 wt % CNTs with small increase of density (1.135 g/cm3). The developed composite will be widely used for lightweight automotive parts with improved mechanical properties.

Construction of Vehicle Door Impact Beam Using Hot Stamping Technology

A vehicle door impact beam made of a thin sheet of steel has been constructed using hot stamping technology with the aim of ensuring occupant safety in the event of a side collision. This technology has been used to increase the strength of the vehicle body parts and to reduce the weight of the door impact beam as well as the number of work processes. Mechanical tests were performed to determine the material properties of the hot-stamped specimen and the results of the tests were used as input data in stamping and structural simulation in order to obtain the optimal design of door impact beam. The strength of the hot-stamped door impact beam increased to a value that was 102% higher than that of conventional pipe-shaped door impact beam. A weight reduction of 34% was also achieved.

Characteristic Frequency of Carbon Black Filled Rubber

Due to the high electrical conductivity of carbon black (CB) particles, the presence of CB improves the conductivity of filled rubbers. The impedance spectra of CB filled rubber were simulated using an infinite resistor‐capacitor (RC) circuit by considering the tunneling conduction mechanism for the CB contact regions. The calculated results had a similar appearance to the experimental results, i.e., the Cole‐Cole plot was a semi‐circle and a peak appeared in a plot of the imaginary component of complex impedance with respect to the frequency from which the characteristic frequency was obtained. For a simple RC circuit in parallel connection, the logarithm of the characteristic frequency should be a linear function of the 1/3 power of the average number of primary particles per aggregate. A slight deviation from the line found in the simulations was attributed to the network contribution. Additionally, low CB loading had little effect on the characteristic frequency, in accordance with the experimental data, while high loading had a marked effect. Furthermore, the calculations showed that the characteristic frequency was affected not only by the distribution of individual CB aggregates, but also by the percentage of agglomerates at high CB loading.

Optimization of the fabrication conditions and effects of multi-walled carbon nanotubes on the tensile properties of various glass fibers/unsaturated polyester resin composites

Abstract Conventional glass fiber/unsaturated polyester resin (UPR) composites were fabricated by four layers to verify the optimum fabrication conditions such as the initial curing temperature, fiber changes and vacuum. Before that another optimum condition for stir mixing multi-walled carbon nanotubes (MWCNTs) into UPR was further investigated based on the initial curing temperature. Exothermic temperature measurements, density measurements, thermo-gravimetric analysis and tensile testing were utilized to identify the effects of these factors on the UPR and composite laminates. Consequently, an initial curing temperature range of 25°C–35°C was recommended to start the polymerization. In addition, the fiber combination and applying vacuum were also represented as the other optimum conditions in composite fabrication. The optimum initial curing temperature was applied for modifying UPR with 0.1 wt.% MWCNTs. Then, all of optimum conditions were used to fabricate glass fibers/modified UPR composites. The higher tensile strength and modulus of fiber combination with the adding of MWCNTs were obtained at 25.29 MPa and 1.39 GPa, respectively. The results indicate that simple MWCNT mixing can be used in industry due to the reduction of the fabrication time and better tensile properties.

Experimental and finite element creep analyses of plastics used in automotive instrument panel

Creep behavior of PPF and PC/ABS was studied which are widely used plastics in automotive instrument panel. Material properties were obtained for various temperatures from tensile tests and three point bending tests were performed for different loadings and temperatures. Time-hardening version of power law creep equation were obtained from the experiment and creep behavior each plastic could be represented in simple form. FE creep analysis results matched well with the experimental results while the difference between numerical and experimental results became larger as load increased.

Creep Behavior of Plastics Used in Automobile Instrument Panels

Tensile and creep tests were performed at various temperatures to investigate the mechanical properties of plastics used in automotive instrument panels. Mechanical properties such as Youngs modulus and Poissons ratios changed markedly with the test temperature. Three-point bending creep tests were performed for three kinds of plastics under four loading conditions. Coefficients in the time-hardening power law creep equation were obtained from the experiment, and the creep behavior was represented by a simple expression. The results of finite element creep analysis showed good agreement with the experimental results, while the difference between the numerical and experimental results increased with the load.