Papers by Author: Young Woo Choi

Abstract: Wiper blade is one of inexpensive automotive components, but the component is very important part considering car safety for securing automotive visual field. Therefore study for securing longer endurance of wiper blade was proceeding, that is about variables effecting on endurance. In this paper, it was proved that the effect to extend endurance could be acquired by securing proper friction coefficient. Be compared endurance in Weibull distribution that is reliability technique and seized effect about blade pressure distribution, contact pressure, blade rubber surface treatment etc. for securing proper friction coefficient. Relation between these variables was cleared through test. Also, designed and proposed tester for measuring friction coefficient of wiper blade.

Abstract: The effect of axial loading resulting from torque on the fatigue life of bolts has been investigated. It was found that the fatigue limit of bolts increased by an amount proportional to the increase in torque. There was a linear relationship between torque and fatigue limit. Using the Shigley method, the prediction of fatigue limit from calculated stress concentration factors was also examined. A good correlation was found between the experimental and predicted fatigue limit. The Shigley method was appropriate for the prediction of fatigue life of bolts.

Abstract: Single phase bainite structure which is obtained by the conventional austempering treatment reduces the ductility of ductile cast iron. Because of the reduction of ductility it is possible to worsen the fatigue properties. Therefore, semi austempered ductile iron which is treated from
+ϒ is prepared to investigate the static strength and fatigue properties in comparison with fully austempered ductile iron (is treated from ϒ). In spite of semi austempered ductile iron shows the 86% increase of ductility. Also, semi austempered ductile iron shows the higher fatigue limit and lower fatigue crack growth rate as compared with fully austempered ductile iron. By the fractographical analysis, it is revealed that the ferrite obtained by semi austempering process brings about the plastic deformation(ductile striation) of crack tip and gives the prior path of crack propagation. The relatively low crack growth rate in semi austempered specimen is caused by above fractographical reasons

Abstract: Effect of nitriding on fatigue crack initiation and growth rate has been studied on Ni-Cr-Mo steel. Specimens were nitrided for 15hr at 680°C. The fatigue limit of nitrided specimens were superior to those of annealed(680°C, 15 hr) specimens. Based on detailed observations of slip band and micro crack initiation, it is concluded that the excellent fatigue limit of nitrided specimens is attributed to improved slip initiation resistance by nitriding. The characteristic of fatigue crack growth rate of nitrided specimens was investigated by comparing with those of annealed specimens. It was found that by nitriding the crack growth rate was markedly decreased and the threshold stress intensity factor range was improved. It is concluded that the excellent fatigue limit of nitrided specimens is also attributed to improved fatigue crack growth rate and threshold stress intensity factor range by nitriding.

Abstract: Automobile pedal which is loaded by driver’s input is transmitting load to throttle cable, braking device and clutch device and controlling automobile. Measuring working condition and applying equivalent damage are needed for reliability of developing pedal. The measuring working condition is requiring more investigation with various respects because of widely ranged drivers, road condition and environmental condition. Additionally, when equivalent damage is applied, there are not suitable for test condition if equivalent damage is too high level to apply or unused region. In this study, load history is measured with 95percentile customer. Measured load history is converted to stress history about critical area of pedal by FEM. There are drawn up histogram of pedal cycles and load from stress history with rain flow cycle counting method, calculated relative damage of extended stress history with Palmgren-Miner rule. From the results, calculated total relative damage is applied to calculation method of test time and load. Calculation method for test condition is carried out with three methods which are enforcing with total stress by rain flow cycle counting, representative load and blocked load. Accelerated durability test condition of pedal using with relative damage and acceleration factors are proposed.

Abstract: In this study, the fatigue life evaluation of automatic transfer devices under stress concentrations due to the notch effect is performed. To investigate residual life of a notched component, load histories were obtained through strain measurement. A fatigue test was performed on a specimen imitating a real component and results were compared with each notch root radius of the concentration area. Three-dimensional finite element analysis was also performed to evaluate the local stress fields. Miner’s rule was used to predict the fatigue life calculation. As a result, the predicted life of a notched component was in good agreement with a real component and introduced a special method for measuring load using real machine components.

Abstract: The fatigue life of hexagon head and socket head bolts, attached to vehicle a wheel, is assessed and the estimation of the residual life of existing bolts in vehicle wheel is investigated. Fieldmeasured load histories were applied in this test. Tensile tests and fatigue tests were performed to evaluate the effect of tightening torque and to obtain the basic experimental data. Miner’s rule was used to predict the fatigue life of bolts. The results indicate the prediction of fatigue life of the bolts was in good agreement with the real life of vehicle wheel bolts in this test.

Abstract: Impact behaviors of Aluminum Honeycombs Sandwich Panel (AHSP) by drop weight test were investigated in this study. Two types of specimens with l/2" and l/4" cell size were tested by two impactors with the weight of 5.25kgf and 11.9kgf respectively. Transient, contact and elastic-plastic analyses were performed by finite element method. Impact behavior of AHSP about impact sites appeared nearly the same in low impact energy, but it was different in high impact energy. Face was the strongest about impact and short-edge was the weakest. The damaged area of AHSP was enlarged with the increase of impactor weight that is corresponding to impact energy. After 3-point bending test, fracture modes of AHSP were analyzed with AE counts, lower face sheet was fractured in the long-edge direction first, and then separation between face sheet and core happened. In the short-edge direction after core wrinkled, lower face sheet was torn, impact behavior by FE analysis were increased localized damage in high velocity because the faster velocity of the impact was, the smaller the stress of core was. Consequently, impactor weight had an effect on widely damaged area, while the impact velocity gave rise to localized damaged area.

Abstract: The fatigue life of hexagon head and socket head bolts, attached to vehicle a wheel, is assessed and the estimation of the residual life of existing bolts in vehicle wheel is investigated. Field- measured load histories were applied in this test. Tensile tests and fatigue tests were performed to evaluate the effect of tightening torque and to obtain the basic experimental data. A three-dimensional finite element analysis was also performed to evaluate the local stress fields. Miner’s rule was used to predict the fatigue life of bolts. The results indicate the prediction of fatigue life of the bolts was in good agreement with the real life of vehicle wheel bolts in this test.

Abstract: The Combine is necessary equipment in the agricultural industry. The components of the Combine are worked for a long time under inferior environmental condition. Especially reaping knife is worked in high frequency domain, and submit fatigue load while use. So, fatigue test was performed to obtain the S-N curve of real component, and load history is measured through field test. The local stresses due to these loads have been calculated by FEM. These results have been used as the input values for the multi-axial fatigue analysis of real components. For the assessment of multi-axial fatigue damage, the critical plane methods have been employed. The used parameters of critical plane methods are Morrow’s, Smith-Waston-Topper’s, and Brown-Miller’s, those were modified for the high cycle fatigue region within elastic behavior. In addition, design improvement is performed about shape of reaping knife to increase the endurance limit, and durability test for improved knife is performed. It is found that the fatigue life of improved reaping knife is increased, and durability test result show that life of reaping knife is increased enough.