Sung Mo Yang

Fatigue Analysis of Multi-Lap Spot Welding of High Strength Steel by Quasi Static Tensile- Shear Test

The welding quality of spot weldment is an important factor that significantly affects the strength, stiffness, safety, and other performance characteristics of vehicles. Therefore, quality control and fatigue life evaluation of spot weldment are necessary processes. This paper presents a method for determining the fatigue life of multi-lap spot weldment of a high strength steel sheet. In this method, the fatigue life is estimated by using the lethargy coefficient, which is the total defect coefficient according to rupture stress and time obtained by the quasi static tensile-shear test. Also, in this study, we modified the lethargy coefficient by using the welding current. And, we define a specific lapping constant, which is a characteristic constant of 2 or 3 lap weldments. The fatigue life obtained by the fatigue estimate equation, which contains a specific lapping constant was compared and verified with an experimental value. And we analyzed the relation of lap number, welding current and fatigue life. This method can save processing time and cost for predicting the life cycle of a structure.

Principal Stress Separation Using a Combination of Photoelasticity and Thermoelastic Stress Analysis under Biaxial Stress

Today, stress measurement methods by thermography and by photoelasticity are widely used to make stress distribution visible. However, it is difficult to separate principal stresses using only one of these methods because only the difference of principal stresses is measured in photoelasticity, and only the sum of the principal stresses is measured in thermograpy. Therefore, the inverse analysis problem must be solved to separate the principal stress in the thermoelastic method and the shear difference integration method must be used for the photoelastic method. Although there are some reports separation of the principal stresses under uniaxial stress by combining the two methods, little research under the biaxial stress has been reported due to the difficulty of experimentation. In this research, the principal stresses under biaxial stress are separated by a combined method. Moreover, it is verified that the thermoelastic stress measurement method is effective to evaluate the stress concentration factor.

Quantitative Evaluation of Defect inside Tire by Shearography

In recent years, Shearography has significantly improved capabilities in the areas of unbond and separation detection in tires. Although Shearography has many advantages for qualitative evaluation, the technique remains the problem of quantitative analysis of inside defects, because Shearography needs several effective factors including the amount of shearing, shearing direction and induced load, which exist as barrier for the quantitative analysis of inside defects. Since the factors are highly dependent on inspector’s skill and also affect the in-situ workability. The factors were optimized and the size of cracks inside tire has been quantitatively determined. Introduction A significant portion of premature tire failures is attributable to defects existed in the tire. Excessive stress levels developed by stress concentrations induced by flaws usually cause premature failure of tires. Therefore, one avenue to minimize such occurrences is to identify and reject defective tire. Practical methods for nondestructive testing (NDT) of tires are still being sought. Existing methods for a nondestructive testing of tires include visual inspection, air needle technique, X-rays, ultrasonic testing, wave inspection, phase analyzer, holography and so on[1]. Each method has one or more limitations such as radioactivity, scanning, contacting and sensitivity to environmental vibration rendering is to be either impractical or ineffective as an industrial inspection tool. In 1974, Hung [2] developed new laser interferometry technique called Shearography, which is remarkably insensitive to environmental vibration and is able to inspect object with whole-field and non-contact. This technique has introduced to the aerospace industry for the inspection of composites and honeycomb structures and also offered important new capabilities for the non-destructive testing of tires. Although, comparing with early Shearography, film based Shearography provides some improvement, the use of film still has significant disadvantages [3]. It is expensive and difficult to implement as a process control in today’s production environment. In addition to, one drawback common to all the method is that it lacks the ability of directly determining the criticality of flaws. This drawback makes the decision of acceptance or rejection very difficult. Many of the images still require considerable training to interpret. Therefore, real time inspection technique and easy quantitative analysis technique are required for industrial application. Digital Shearography based on electronic devices, CCD camera and PC, and image processing technique solve many problems in film based Shearography. In this study, industrial application technique of digital Shearography and quantitative analysis technique of defects inside tires are described. Separations and actual voids induced by road driving test or abnormal manufacturing processing are inspected and evaluated Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 696-702 doi:10.4028/www.scientific.net/KEM.270-273.696

Creep-Fatigue Life Design with Various Stress and Temperature Conditions on the Basis of Lethargy Coefficient

High temperature and stress are encounted in power plants and vehicle engines. Therefore, determination of the creep-fatigue life of a material is necessary prior to fabricating equipments. In this study, life design was determined on the basis of the lethargy coefficient for different temperatures, stress and rupture times. SP-Creep test data was compared with computed data. The SP-Creep test was performed to obtain the rupture time for X20CrMoV121 steel. The integration life equation was considered for three cases with various load, temperature and load-temperature. First, the lethargy coefficient was calculated by using the obtained rupture stress and the rupture time that were determined by carrying out the SP-Creep test. Next, life was predicted on the basis of the temperature condition. Finally, it was observed that life decreases considerably due to the coupling effect that results when fatigue and creep occur simultaneously.

Theoretical Assessment of Stress Analysis in Short Fiber Composites

An investigation of composite mechanics to investigate stress transfer mechanism accurately, a modification of the conventional shear lag model was attempted by taking fiber end effects into account in discontinuous composite materials. It was found that the major shortcoming of conventional shear lag theory is not being able to provide sufficiently accurate strengthening predictions in elastic regime when the fiber aspect ratio is very small. The reason is due to its neglect of stress transfer across the fiber ends and the stress concentrations that exist in the matrix regions near the fiber ends. To overcome this shortcoming, a more simplified shear lag model introducing the stress concentration factor which is a function of several variables, such as the modulus ratio, the fiber volume fraction, the fiber aspect ratio, is proposed. It is found that the modulus ratio is the most essential parameter among them. Thus, the stress concentration factor is expressed as a function of modulus ratio in the derivation. It is also found that the proposed model gives a good agreement with finite element results and has the capability to correctly predict the variations of the internal quanitities.

Experimental Study of the Principal Stresses Separation Using a Combination of Photoelasticity and Thermoelastic Stress Analysis

In this paper, we report the experimental results of the principal stresses separation using a combination stress analysis of photoelasticity and thermoelasticity. Experimental investigation was carried out on specimens made of epoxy resin with a center hole under constant tensile loads. In case of photoelasticity test, only the difference of the stress is obtained, and in case of thermoelastic stress analysis, only the sum of the stress is measured. However, it is difficult to separate principal stress by only either of them. Then, Mohrs stresses circle was employed as the most easy and convenient method to separate of principal stresses. Moreover, by using different shape of center hole at the specimens, the change of the stress concentration factor at the vicinity of center hole of the test specimen by data of photoelasticity and thermoelastic stress analysis was obtained. The effectiveness of the principal stresses separation from the photoelastic and thermoelastic stresses measured values was verified. We also confirmed the capacity of application of the thermoelastic stress analysis to evaluate stress concentration factor.

Evaluation of Mechanical Property for Pb-free Solder/Ni Plate Joints with Artificial Aging Time

Thus far, solders used in electronics remain lead-based. Pb-free solutions in electronic components and systems are receiving increased attention in the semiconductor and electronics industries. Pb-free materials currently in used are Sn-37Pb, Sn-4Ag and Sn-4Ag-0.5Cu/Ni plate joints. In this study, solder alloys were used at high temperatures for artificial aging processing that was performed at for 0hr, 100hr, 200hr, 400hr, 600hr and 1000hr. The SP test was conducted at and . As a result, the maximum shear strength of all the specimens decreased with the increase in artificial aging time and temperature of the SP test. In addition, Pb-free solders showed higher total fracture energy compared with Sn-37Pb at high temperatures. The mechanical properties of Sn-4Ag-0.5Cu solder/Ni plate joints remained in excellent conditions in electronic parking systems at high temperatures.

Tensile Strength Characteristic of Al6061-T6 and Al7003-T6 Aluminum by the Change in the Friction Welding Process Variable

Recently, the various welding methods are used in the coupling method of product for the streamlining of complex products, simplification of manufacturing process, and decreasing process time a variety of industries. In this welding area, the friction welding technique has many advantages such as the mass production of rapid and highly reliable product and eases of automation, reduction of process time etc. so the domestic/external many studies are conducting. In this study, the Spindle speed, Up-set pressure as the necessary process variables for friction welding were change variously by using the round bar specimen of Al6061-T6 and Al7003-T6 Aluminum material, so the friction welding performance of the specimen by each change was compared and analyzed. In order to comprehend the friction welding performance, the frictional pressure was fixed with 20MPa, the spindle velocity was changed with 1,000, 1,500, 2,200rpm, and the Up-set pressure was changed with 35, 50, 65MPa for producing, so the performance by the specimen was compared and analyzed through the final tensile strength test. In addition, the effects of the friction welding on the specimen were examined through the analysis on the welding part hardness of the each friction welded aluminum specimen. In the result of the study, it showed the highest welding characteristic with the tensile strength 250.5MPa by applying 1,000rpm spindle velocity, 50MPa Up-set pressure compared to the lowest tensile strength 171.6MPa. In addition, as the result of hardness test, the hardness of specimen after conducting the friction welding of specimen decreased compared to the hardness of the pure material in the welding boundary, so it is judged that the measure to minimize the effects of the frictional heat by minimizing the friction welding time is needed.

Material Property Evaluation of High Temperature Creep on Pb-free Solder Alloy Joint to Reflow Time by Shear Punch-creep Test

In this study, shear punch-creep (SP-Creep) at Sn-4Ag/Cu pad the joint was tested by using environment-friendly Pb-free solder alloy Sn-4Ag of electronic components. Pb eutectic alloy (Sn-37Pb) joints limited to environmental issues with reflow time (10sec, 30sec, 100sec, 300sec) according to two types of solder alloy joints are compared and evaluated by creep strain rate, rupture time and IMC (Intermetallic Compound) behavior. As the results, reflow time increases with increasing thickness of IMC can be seen at overall 100sec later in case of two solder joints on the IMC thickness of Sn-4Ag solder joints thicker than Sn-37Pb solder joints. In addition, when considering creep evaluation factors, lead-free solder alloy Sn-4Ag has excellent creep resistance more than Pb eutectic alloy. For this reason, the two solder joints, such as in the IMC (Cu6Sn5) was formed. However, the creep resistance of Sn-4Ag solder joints was largely increased in the precipitation strengthening effect of dispersed Ag3Sn with interface more than Sn-37Pb solder joints.

Study on Friction Characteristic of Sintered Friction Component for Synchronizer-Ring of Diesel Vehicle

The speed change performance of transmissions has become a serious issue because of the increase in the inertia moment that has accompanied increases in engine output and transmission size. Therefore, it is necessary to develop better wear resistant friction materials. In this study, an appropriate sintered friction component for the synchronizer ring of a diesel manual transmission was developed, and its bonding characteristics were analyzed. That is, a process for bonding an Fe-based base material and Cu-based sintered friction material was developed. BSE and EDX analyses of this bonding layer were conducted, along with a shear strength test, to determine the bonding characteristics.