Hae Moo Lee

STRESS-STRAIN RELATIONSHIP OF ZR55AL10NI5CU30 METALLIC GLASS ANALYZED BY 3D IMAGES ON SPHERICAL INDENTS

Mechanically polished surface of Zr55Al10Ni5Cu30 metallic glass was indented with a rigid ball 0.5 mm in diameter and its corresponding load-depth curve was recorded automatically. Although a stress-strain relationship beneath the indenter can be analyzed from the raw indentation curve, the current analysis developed for crystalline solids can mislead erroneous properties because it does not consider significant material pile-ups in amorphous metallic glasses. Thus, we proposed a novel indent image processing technique for characterizing the contact and flow properties in the metallic glasses; the contact area was measured by differentiating a three-dimensional indent morphology digitized by a surface profiler and a surface-stretching strain was newly defined in order to estimate the flow properties. Finally, the work-hardening index estimated was about 0.05, comparable with the typical value measured from uniaxial compression in the Zr-based metallic glass.

A study on observation and growth behavior of small surface cracks by remote measurement system

A new experimental system using remote measurement system (RMS) and image processing technique was applied for studying the growth behavior of small surface fatigue cracks in 1Cr −1Mo−0.25V steel at room temperature. The system includes a long distance focusing microscope, a CCD camera, a light source, a 3-axis controller a monitor, a personal computer and a data translation card. The measurement error of the system appeared to be 0.8%. It is possible for this system to measure down to 30 μm of surface fatigue crack length. The length of surface fatigue crack could be successfully measured by the RMS during testing as well as after the test. The growth rate of small cracks on smooth specimens was represented in terms of stress intensity factor and J-integral. At equivalent elastic stress intensity factor levels, the growth rate of small surface cracks was not consistent and faster than that of long crack of CT specimen.

Improved Reliability of Small Punch Testing by Correction of Testing System Stiffness and Measurement of Hydrogen Sensitivity

Conventionally embrittlement of components for petrochemical equipment has been determined by small punch (SP) testing, which involves carrying out mechanical testing of specimens sampled from practical structures to estimate the integrity and residual life of the equipment. The application range of SP testing has been expanded to cryogenic environments, and attempts have recently been made to expand it further to high-pressure hydrogen environments. To examine the effect of hydrogen on pipeline materials before SP testing, the amount of hydrogen permeation was measured under several testing conditions. The test specimens were prepared as follow. First, after obtaining a section (10 mm in width, 10 mm in height, and 0.7 mm in thickness) from a steel pipe made of X70, X65, or X42 as the base metal by wire cut electric discharge processing. Subsequently, grinding both surfaces, the test specimen was finished to maximum surface roughness of less than 6 μm, with a final thickness of 0.5 ± 0.005 mm. The aim of this study was to determine whether sensitivity to hydrogen could be measured by SP testing; hence, the test was conducted at a punch speed of 4 × 10−3 mm/min, which is much slower than the 0.2 ∼ 2 mm/min adopted for conventional SP testing, like CWA 15627. The maximum load measured for all three steels under 10 MPa was not influenced by machine type (servo hydrodynamic or electromechanical), holding time after charging hydrogen, or bolt fastening torque. Different means for correcting SP curves depending on the stiffness of the testing machines were suggested, and their effectiveness throughout testing was demonstrated.Copyright

A Study on the Temperature Calibration for a Small Electrical Resistance Furnace

A severe thermal stress occurs during start up/shutdown transients in thick walled components of high temperature power plants. Thus, a precise consideration of this issue is very important. Many researchers have studied low-cycle fatigue at high temperatures and small box-type electrical resistance furnaces have been developed for small-sized fatigue specimens. However, these small-scale electrical resistance furnaces need precise temperature calibrations because temperature control is difficult in a small space. Thus, a method for the temperature calibration of a box-type electrical resistance furnace is investigated and calibration procedures are proposed in this study.Copyright