Seung Han Yang

Surface Quality Studies with Respect to Grinding Burn of New Typical Nickel-Based Superalloy

Excessive grinding temperature leads to thermal damage that greatly deteriorates the surface quality and has an adverse effect on in-service strength and fatigue properties. Grinding burn is the common type of thermal damage that has been becoming the main conscious issue in grinding difficult-to-cut materials with regard to high efficiency and good integrity as well as high accuracy. Directionally solidified nickel-based superalloy DZ4 is a kind of very important structural material developed nowadays domestically, but the very poor machinability hinders its broad utilization in aircraft. This paper unveiled its grinding burn mechanism by experimentally analyses of grinding process characteristics such as grinding forces and grinding temperature, and also by metallurgical examinations for the surface integrity. With the onset of grinding burn, this paper gives the conclusions as follows. The critical grinding temperature is 950°C. The grinding force ratio Fn/Ft changes remarkably and can be used as the characteristic parameter for the real time grinding burn monitoring. There is no evident change in burn color from slight burn to severe burn, which is much different from that of carbon steel grinding. The residual tensile stress of burned surface layer is very high, and is almost 22 times as much as that of the normal case. The mechanical strength and heat resistance of the DZ4 alloy deteriorates obviously with the decrease of strengthening particles x92 phase. The large and irregularly distributed x92 phases in surface layer severely affect the coincidence of performance of DZ4. The surface hardness of DZ4 alloy weakens by 25% and the depth of affected layer exceeds 0.5mm. This paper will be beneficial to the catastrophic failure inspections and to the development of advanced grinding technologies related to avoidance of grinding burn.

Prediction of Formability of Aluminum Alloy 5454 Sheet

Key Words: Formability(성형성), Aluminum Alloy(알루미늄합금), Plastic Instability(소성불안정), FormingLimit Diagram(성형한계도)초록: 자동차산업에서대기오염을줄이고연비를향상시키기위해경량화가중요한과제로여겨지고있다. 이를위해알루미늄소재의적용이증가하고있다. 판재를차체에적용하기위해서는주로프레스가공공정을거치게된다. 이때, 재료, 제품설계및프레스공정의부적절한가공변수의사용으로인하여파단, 주름, 및스프링백등에의한다양한형태의가공불량이발생한다. 따라서이들변수들의적절한조화뿐만아니라엄격한공정관리가요구된다. 이에본연구에서는자동차판재에주로사용되는Al5454 재료에대한이론적으로유도한소성불안정조건을구하고, MATLAB을이용하여성형한계도를도출하였다. 또한, 장출인장실험을통해얻어진실험값과이론적으로도출한성형한계도와의비교를수행하였다.Abstract: In the automobile industry, reducing the weight is the most important objective for reducing airpollution and improving the fuel efficiency. For this reason, the application of aluminum sheets is increasing.When the sheets are applied to the automobile, using inappropriate variables for the material, product design,and press processing can generate tearing, wrinkling, and spring-back problems, which are the main types offailure in the manufacturing process. Therefore, it is necessary to reduce these failures by harmonizing themany variables and strictly managing the processes. In this research, we study the theoretical plasticityinstability of Al5454 and obtain the forming limit diagram (FLD) using MATLAB. Moreover, we comparethe theoretical FLD with an experimental FLD obtained from a stretching test.

A Study on Particular Abnormal Gait Using Accelerometer and Gyro Sensor

Recently, technologies to help the elderly or disabled people who have difficulty in walking are being developed. In order to develop these technologies, it is necessary to construct a system that gathers the gait data of people and analysis of these data is also important. In this research, we constructed the development of sensor system which consists of pressure sensor, three-axis accelerometer and two-axis gyro sensor. We used k-means clustering algorithm to classify the data for characterization, and then calculated the symmetry index with histogram which was produced from each cluster. We collected gait data from sensors attached on two subjects. The experiment was conducted for two kinds of gait status. One is walking with normal gait; the other is walking with abnormal gait (abnormal gait means that the subject walks by dragging the right leg intentionally). With the result from the analysis of acceleration component, we were able to confirm that the analysis technique of this data could be used to determine gait symmetry. In addition, by adding gyro components in the analysis, we could find that the symmetry index was appropriate to express symmetry better.

Analytical Sensitivity Analysis of Geometric Errors in a Three-Axis Machine Tool

In this paper, an analytical method is used to perform a sensitivity analysis of geometric errors in a three-axis machine tool. First, an error synthesis model is constructed for evaluating the position volumetric error due to the geometric errors, and then an output variable is defined, such as the magnitude of the position volumetric error. Next, the global sensitivity analysis is executed using an analytical method. Finally, the sensitivity indices are calculated using the quantitative values of the geometric errors.

Study of Local Performance Index of 2-DOF Parallel Manipulator

This study investigates a parallel manipulator that can move over two parallel sliders and in which the end-effector of the manipulator can be adjusted arbitrarily. Through the direct and inverse kinematics of the manipulator, position equations are derived. These equations represent the relationship between the positions of the sliders and the position of the end-effector. The Jacobian matrices of the direct and inverse kinematics are obtained by these equations. By using the condition number defined from these matrices, the local performance index of the manipulator is proposed. By using the simulation results of the performance index, we find that the manipulator can smoothen movements in only one quadrant and that the distribution of the maximal performance index is affected by the ratio of the length of links and the orientation of the end-effector.

PZT-Induced Lamb Waves and Pattern Recognition for On-Line Health Monitoring of Jointed Steel Plates

This paper presents a non-destructive evaluation (NDE) technique for detecting damages on a jointed steel plate on the basis of the time of flight and wavelet coefficient, obtained from wavelet transforms of Lamb wave signals. Probabilistic neural networks (PNNs) and support vector machines (SVMs) were applied for pattern classification. In this study, the applicability of the PNNs and SVMs was investigated for the damages in and out of the Lamb wave path. It has been found that the present methods are very efficient in detecting the damages simulated by the loose bolts on the jointed steel plate.

The forming characteristic in the single-point incremental forming of a complex shape

Incremental sheet forming is popularly applied because of its reduced cost, flexibility, and applicability for small batch production. To reduce the manufacturing cost and time consumption, the incremental forming of a complex shape is implemented by combining two forming methods into one process: positive and negative incremental forming. The experiments were done to confirm the applicable ability of the method with the material aluminium Al3004-P. The finite element simulation was done with the ABAQUS explicit finite element code, and its results were compared with the experimental results.

Development of Hole Expansion Test for Sheet Materials Using Pattern-Recognition Technique

Nowadays, one of the most interested area of automobile industry is the production of vehicle which has collision safety and ability to produce less amount of . The achievement of such a dual performance is done by choosing the materials like dual phase steel, ferrite bainite steel, etc. These steels have been used in automotive chassis and body parts, and also used to be formed by hole flanging to meet the goal of strength and design requirement. The formability of sheet material was experimented by hole expansion test and the judgement relies on human eye and his experience. This manual judgement involves many errors and large deviation. This paper develops the automatic crack recognition system which finds a crack based on CCD image to complement the problem of the current method depending on humans sense.

Kinematics Analysis of a 2-DOF Parallel Manipulator

In this paper, a parallel manipulator is comprised of two sliders and four links. Sliders execute a linear reciprocating motion depending on parallel guides and make the connected links rotate. A couple of links connected by sliders do coupling motion. The end-effector called a link tip has orientation angle. Through the kinematics analysis of this manipulator, we found displacement, velocity and acceleration using direct and inverse kinematics. We used equations that derived from this analysis and determined five constraint conditions. These conditions had much to do with rotation states of links, the relative relation of link length and coupling motion state. To verify those, we suggest a new algorithm regarding constraint conditions of a manipulator. With the result which performed the algorithm, we found out that operation range of coupled links was limited by relative relation of link length and that manipulator was not able to carry out a series of link motion , in case of being the link vertical between two parallel guides.

Comparison between Conventional and Performance Demonstration UT Method by Round Robin Test

The results of ultrasonic testing performed during in-service inspection period for main components of nuclear power plant are affected by the performance of each inspector. To enhance the inspection reliability, the more restrictive qualification is required. KEPRI established Korean performance demonstration (KPD) system for the ultrasonic testing personnel, equipment and procedure which applied to the piping examination of nuclear power plant. A round robin test was conducted to evaluate the reliability of examination result by comparing conventional (DAC) UT and performance demonstration (PD) UT method. Participants took detection/length sizing test and depth sizing test using DAC method and PD method. The results showed that the PD method was preciser and more accurate than the conventional DAC method in length and depth sizing regardless of field experience and PD qualification status.