Young Ze Lee

Non-linear fracture mechanics analyses of part circumferential surface cracked pipes

This paper provides engineering estimates of non-linear fracture mechanics parameters for pipes with part circumferential inner surface cracks, subject to internal pressure and global bending. Solutions are given in the form of two different approaches, the GE/EPRI approach and the reference stress approach. For the GE/EPRI approach, the plastic influence functions for fully plastic J solutions are tabulated based on extensive 3-D FE calculations using deformation plasticity, covering a wide range of pipe and crack geometries. The developed GE/EPRI-type fully plastic J estimation equations are then re-formulated using the concept of the reference stress approach for wider applications. The proposed reference stress based estimates are validated against detailed 3-D elastic-plastic and elastic-creep FE results. For a total of 26 cases considered in this paper, agreement between the proposed reference stress based J and C* estimates and the FE results is excellent. An important aspect of the proposed estimates is that they not only are simple and accurate but also can be used to estimate J and C* at an arbitrary point along the crack front.

Nondestructive Evaluation of the Characteristics of Degraded Materials Using Backward Radiated Ultrasound

The frequency dependency of Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in two kinds of degraded specimens by scuffing or corrosion. Then, the frequency dependency is compared with the residual stress distribution or the corrosion-fatigue characteristics for the scuffed or corroded specimens, respectively. The width of the backward radiation profile increases with the increase of the variation in residual stress distribution for the scuffed specimens. In the corroded specimens, the profile width decreases with the increase of the effective aging layer thickness and is inversely proportional to the exponent, m, in the Paris’ law that can predict the crack size increase due to fatigue. The result observed in this study demonstrates high potential of backward radiated ultrasound as a tool for nondestructive evaluation of subsurface gradient of material degradation generated by scuffing or corrosion.

Fretting Wear Characteristics of INCONEL 690 and INCONEL 600 in Water Environment

Fretting is the oscillatory motion with very small amplitudes, which usually occurs between two solid surfaces in contact. Fretting wear is the removal of material from contacting surfaces through fretting action. Fretting wear of steam generator tubes in nuclear power plant becomes a serious problem in recent years. The materials for the tubes usually are INCONEL 690 (I-690) and INCONEL 600 (I-600). In this paper, fretting wear tests for I-690 and I-600 were performed under various applied loads in water at room temperature. Results showed that the fretting wear loss of I-690 and I-600 tubes was largely influenced by stick-slip. The fretting wear mechanisms were the abrasive wear in slip regime and the delamination wear in stick regime. Also, I-690 had somewhat better wear resistance than I-600.

Evaluation of Friction Force, Wear Volume and Scuffing Life of Piston Rings and Cylinder Blocks with Different Surface Roughness for Low Friction Diesel Engine

Wear and scuffing tests were conducted using friction and wear measurement of piston rings and cylinder blocks for the low friction diesel engine. Scuffing, described as sudden catastrophic failure of lubricated sliding surfaces, usually characterized by a sudden rapid increase in friction, temperature and noise, is an important failure mode on sliding surfaces. In this study, the frictional forces, wear amounts and cycles to scuffing in boundary lubricated sliding condition were measured using the reciprocating wear tester. The cylinder blocks with several values of surface roughness were used as reciprocating specimens, and a piece of piston ring was used as fixed pin. As increasing load by several steps in lubricated sliding, the friction signals indicated the state of surface interactions, such as friction forces, changes of lubricating films and scuffing. There were some rapid increases in friction forces just before the scuffing would occur. It was found that there was the optimum value of initial surface roughness to prolong the wear life of sliding surfaces. As decreasing the surface roughness of cylinder blocks, the wear amounts were decreased due to increasing the contact area. There was also the optimum surface roughness to reduce the friction and to prolong the scuffing life.

Influence of Temperature on the Fretting Wear of Advanced Nuclear Fuel Cladding Tube against Supporting Grid

The experimental investigation was performed to find the associated changes in characteristics of fretting wear with various water temperatures. Fretting can be defined as the oscillatory motion with very small amplitudes, which usually occur between two contacting surfaces. The fretting wear, which occurs between cladding tubes of nuclear fuel rod and grids, causes in damages the cladding tubes by flow induced vibration in a nuclear reactor. In this paper, the fretting wear tests were carried out using the zirconium alloy tubes and the grids with increasing the water temperature. The tube materials in water of 20, 50 and 80 were tested with the applied load of 20N and the relative amplitude of 200. The worn surfaces were observed by SEM, EDX and 2D surface profiler. As the water temperature increased, the wear volume was decreased. However, oxide layer was increased on the worn surface. The abrasive wear mechanism was observed at water temperature of 20 and adhesive wear mechanism occurred at water temperature of 50 and 80. As the water temperature increased, surface micro-hardness was decreased. Also, wear depth and wear width were decreased due to increasing stick phenomenon. Stick regime occurred due to the formation of oxide layer on the worn surface with increasing water temperatures.

Friction and Wear of POE and PAG Oils in the Carbon Dioxide Environment

The carbon dioxide (R-744) as a natural refrigerant has been an attractive alternative refrigerant to replace HFC (hydrofluorocarbon) refrigerants currently used in air conditioning systems, due to the environmental concerns. Because new compressors with CO2 are going to be operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very important. To develop new compressor, especially scroll type, the friction and wear characteristics of sliding surfaces between a fixed scroll and an orbiting scroll in the scroll compressor were investigated in this paper. The sliding tests of pin-on-disk type were carried out between scroll surfaces under various sliding speeds, normal loads, surface roughness, and refrigerant pressures. Two types of lubricants were used, namely POE (polyol ester) oil and PAG (polyalkylene glycol) oil. The friction forces, wear amounts and surface temperatures were monitored during the tests. Test results showed that the wear amounts increased with increasing normal load and rotating speed, and also decreased with decreasing surface roughness. Also, the results of the sliding tests showed that using PAG oil has an advantage over POE oil in CO2 environment. As the pressure of CO2 refrigerant became higher, the amount of wear and coefficient of friction became larger in CO2/POE mixed environment, but wear and friction remained constant in CO2/PAG mixed environment. The wear coefficients in CO2/PAG and CO2/POE mixed environment were 8.60×10-9 ㎣/N·m and 2.35×10-8 ㎣/N·m, respectively.

The Effect of Various Surface Coatings on Tribological Characteristics of Rotary Compressor Vane in Carbon Dioxide Environment

The tribology of lubricated sliding vanes in the rotary compressor was investigated to replace the HFC refrigerants by the carbon dioxide. The carbon dioxide is a natural refrigerant and very favorable to environment. Because new compressors with the carbon dioxide are going to be operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very important. To develop new compressors, especially rotary type, the friction and wear characteristics of sliding surfaces between a vane and a roller in the rotary compressor were evaluated in this paper. Several hard coatings, such as TiN, CrN, WC/C and two types of nitridings, were applied on vane surfaces in order to improve the tribological characteristics, and their performances were evaluated experimentally. The vane-on-disk type sliding tests were carried out under the various sliding speeds, normal loads, and carbon dioxide pressures with an exclusive high pressure wear tester. From the tests wear volume of vane surfaces applied various coatings were compared. During the tests coefficients of friction and surface temperatures were monitored. Test results showed that WC/C coatings showed good tribological properties. TiN and CrN coated vanes showed good wear resistance properties but produced high friction.

Wear Transitions of Tube-Support Materials for a Nuclear Steam Generator through Sliding Wear Test and Fretting Wear Test

Tubes in nuclear steam generators are held up by supports because the tubes are long and slender. Fluid flows of high-pressure and high-temperature in the tubes cause oscillating motions between tubes and supports. This is called as FIV (flow induced vibration), which causes fretting wear in contact parts of tube-support. The fretting wear of tube-support can threaten the safety of nuclear power plant. Therefore, a research about the fretting wear characteristics of tube-support is required. This work is focused on fretting wear transitions from mild wear to severe wear of tube-support materials by various loads and relative displacements. The transition is defined on the basis of the changes in wear amount. To investigate the transition, the fretting wear tester was contrived to prevent the reduction of relative displacement between tube and support by increasing the load. The tube and support materials were Inconel 690 and 409 SS, respectively. The results show that the transition of tube-support wear is caused by the changes of the dominant wear mechanism depending on the applied load and the relative displacement.

Tribological Performance of Multi-Walled Carbon Nanotubes in Mineral Oils under Boundary Lubricated Sliding

The tribological performance of multi-walled carbon nanotubes (MWNT) in mineral oils is investigated at ambient temperature. The frictional forces, wear amounts and cycles to scuffing of the oils with nanotubes and without those were measured using the ball-on-disk tester. It was found that there were little differences in the frictional forces and wear amounts of two oils. However, the scuffing times of oils with nanotubes were much longer than those of oils without nanotubes in sliding tests. The nanotubes were very effective on maintaining the oil gap and protecting the surfaces in boundary lubricated sliding.

Transition of Friction and Wear by Stick-Slip Phenomenon in Various Environments under Fretting Conditions

The fretting wear arises when contacting surfaces undergo oscillatory tangential displacement of small amplitude. Depending on the degree of stick and slip there are three kinds of the contact motions, such as gross-slip, partial-slip and stick-slip. The fretting damage occurs most severely when the transition from gross-slip to partial-slip happens. In this paper, the transitions of friction and wear under fretting were investigated by ball-on-disk wear tests in various environments, which were dry friction of air and nitrogen, and wet friction of mineral oil and engine oil. The transition from partial-slip to stick-slip firstly occurred in nitrogen environment, and then in air. Later, the transition occurred at higher load in mineral oil, and then lastly in engine oil.