Dong-Teak Chung

Transient wrinkling analysis of steel web rolling

In this study a mechanism of wrinkles were studied in terms of stress wave propagations and interactions between web edges using the explicit time integration finite element code, LS-DYNA. During the hot rolling of steel web for thickness reduction, the roller speed is constantly changing for constant gage control. It was shown- from this study that this sudden change of speed itself could cause wrinkling without in-plane shear stress introduced to the web span from outside. Tensile stress wave is generated by sudden increase of roller speed, and the wave propagates and reflects back and forth while interacting with free edges of the web to generate compressive stress in CMD resulting in wrinkling in MD. As the thickness reduction of the web repeats, front and end edges are no longer on a straight line. The edge of either end of web span may change to the shape of tongue or fishtail. The width of the web is relatively constant except very front and end. Tail-out phase, when the very last part of the web exits the roller, causes sudden change of web width followed by complete release of tensile force. These extreme disturbances cause complete different behaviors of web: severe wrinkling and large vibration. A precise analysis by using the LS-DYNA is carried out and it is concluded that these are sufficient for the permanent wrinkling, fold-over, see ibid. (Kee-Hyun Shin and Wan-Kee Hong, 1998) (Kee-Hyun, 2000).

DEVELOPMENT OF A NEW RAIN EROSION TEST METHOD

The nose of a missile, flying through raining region with a supersonic speed, is subjected to the rain erosion because the nose is made of a brittle ceramic material. Various test methods are used to address such a phenomenon. However, most of the methods are expensive and/or require complicated facilities. The simple yet very effective rain erosion test method is developed. It consists of (1) a low pressure air gun, (2) a sabot assembly for launching single rain drop, (3) a stopper, and (4) a specimen holder block. The sabot assembly similar to the hypodermic syringe carries specific amount of water toward the stopper launched by the low pressure air gun. When the impact occurs against the stopper which stops the sabot, the water and the steel plunger (at the back of the sabot) continues pushing the sabot to generate a high pressure in the chamber filled with resilient silicon rubber. The pressurized silicon rubber then is squeezed through the small opening in front of the sabot, thus, accelerates the water droplet to a much higher velocity. The velocity of the droplet is measured by the make-screen method, where there are two aluminum foils with an insulating layer in between. The droplet velocity up to 800 m/s is successfully attained using a low pressure air compressor. The specimen made of a ceramic material is placed in front of the high speed water droplet and the rain erosion damage on the surface of the specimen is observed.

A Study on the Silicone Rubber of Sabot Assembly for the Velocity Multiplication of Mini Ball

A mini ball launch system was developed for the study of dynamic fracture of ceramic materials. The principle of velocity multiplication system is similar to two stage gun. The plastic sabot assembly houses steel plunger and the void filled with silicone rubber. The sabot is stopped by the stopper block then the steel plunger inside the sabot compress the silicone rubber to high pressure. Then the compressive energy of the silicone rubber is transferred to the ball. More than ten times of initial speed was attained. In this study, most effective silicone rubber for the highest final speed was chosen out of three different varieties by launch tests.

NUMERICAL AND EXPERIMENTAL APPROACH FOR THE OPTIMAL DESIGN OF A DUAL PLATE UNDER BALLISTIC IMPACT

To predict the behavior of a dual plate composed of 5052-aluminum and 1002-cold rolled steel under ballistic impact, numerical and experimental approaches are attempted. For the accurate numerical simulation of the impact phenomena, the appropriate selection of the key parameter values based on numerical or experimental tests are critical. This study is focused on not only the optimization technique using the numerical simulation but also numerical and experimental procedures to obtain the required parameter values in the simulation. The Johnson-Cook model is used to simulate the mechanical behaviors, and the simplified experimental and the numerical approaches are performed to obtain the material properties of the model. The element erosion scheme for the robust simulation of the ballistic impact problem is applied by adjusting the element erosion criteria of each material based on numerical and experimental results. The adequate mesh size and the aspect ratio are chosen based on parametric studies. Plastic energy is suggested as a response representing the strength of the plate for the optimization under dynamic loading. Optimized thickness of the dual plate is obtained to resist the ballistic impact without penetration as well as to minimize the total weight.

A STUDY OF DRY COATING PROCESS

Dry zinc coating processes similar to the shot pinning was studied in the aspects of environmentally benign process to substitute traditional wet type phosphate coating of the work-piece, which is for protection from the deterioration of the surface quality and the shortening of the lifetime of the die during cold extrusion. Experiment and simulation on the collision and coating process of the zinc coated steel balls onto the steel target was performed. Coating patterns on the target and damage of the zinc shell of the ball with respect to the colliding speed were observed during the experiment. Explicit finite element analysis showed the deformation and the fracture of the zinc shell were similar to the experimental results even though the adhesion of the zinc layer onto the work-piece could not be expressed directly. The optimal velocity of the balls was obtained considering effective zinc coating and maximum lifetime of the ball.

COMPUTATIONAL MODELING OF DYNAMIC BRITTLE FAILURE USING THREE-DIMENSIONAL EXPLICIT FEM

Ceramic materials show good mechanical properties. However, the brittleness makes it difficult to predict their dynamic behavior of ceramic materials, and further the discrete nature of brittle failure requires a three-dimensional computation. In this paper, a computational model was developed for analyzing high velocity impact situations of brittle materials, incorporating with node separation scheme to describe the distinctive behavior of a brittle failure. This material behavior model was applied in a three-dimensional explicit code with Lagrangian description, which was developed to compute problems of dynamic brittle failure using four-node tetrahedral elements. To effectively handle the complex contact situations, involving multiple collisions and self-contact of severely deformed and fragmented bodies, careful considerations were paid to the global and local contact searching. In addition, an edge contact algorithm was developed to handle contacts between sharp edges. To validate the developed comp...