Seung Kee Koh

Elastic-plastic stress analysis and fatigue lifetime prediction of cross-bores in autofrettaged pressure vessels

Elastic-plastic stress analysis has been performed to evaluate the fatigue life of an autofrettaged pressure vessel containing cross-bores subjected to pulsating internal pressure of 200 MPa. Finite element analyses were used to calculate the residual and operating stress distributions of the pressure vessel due to the autofrettage process and pulsating internal pressure, respectively. Theoretical stress concentration factors of 3.06, 2.58, and 2.64 were obtained at the cross-bore of the pressure vessel due to internal pressure, 50%, and 100% autofrettage loadings, respectively. Local stresses and local strains determined from the elastic-plastic finite element analysis were employed to calculate the failure location and fatigue life of the pressure vessel with radial cross-bores, incorporating the low-cycle fatigue properties of the pressure vessel steel and fatigue damage parameters. Increase in the amount of overstrain by autofrettage process moved the crack initiation location from the inner radius toward a mid-wall, and extended the crack initiation life, Predicted fatigue life of the fully autofrettaged pressure vessel with cross-bores increased about 50%, compared to the unautofrettaged pressure vessel. At the autofrettage level higher than 50%, the failure location and fatigue life of the pressure vessel were not significantly influenced by the autofrettage level.

Adhesion improvement between Au films and glass by 1 keV Ar+ ion irradiation

Au films with a thickness of around 1600 A were deposited onto glass substrates at room temperature by an ion beam sputtering method, using a 5 cm cold hollow cathode ion gun at a pressure of 4–8×10−6 Torr. Irradiation of the Au/glass samples was carried out at pressure of 7×10−6 Torr. For the sputter deposition, the Ar+ ion energy was 1 keV and the current density at the surface of the substrate was 15 μA cm−2. Effects of 1 keV Ar+ ion doses (Id) ranging from 1×1016 to 2×1017 Ar+cm−2 on the properties such as crystallinity, surface roughness, and adhesion of the films have been investigated. Rutherford backscattering spectrometry showed that the thickness of Au films and sputtering yield were reduced by increase of Id and rms surface roughness of the films increased from 16 to 118 A, as the ion dose was changed from 0 to 2×1017 Ar+ cm−2. Adhesion of Au films on the glass irradiated at Id=2×1017 Ar+ cm−2 was nine times greater than that of untreated Au films, as determined by a scratch test.

An Improved Hybrid Full-Field Stress Analysis of Circularly Perforated Plate by Photoelasticity and Finite Element Analysis

hybrid experimental-numerical method is presented for determining the stresses around a circular hole in a finite-width, tensile loaded plate. Calculated fringes obtained by FEA provided the information about the external boundary of the hybrid element, and those fringes on straight lines were used for hybrid analysis. In order to see the effects of varying stress field, different numbers of terms in a power-series representation of the complex type stress function were tested. Actual and reconstructed fringes were compared. The hybrid results were highly comparable with those predicted by FEA. The result showed that this approach is effective and promising because isochromatic data along the straight lines in photoelasticity can be conveniently measured by use of phase shifting photoelasticity.

Residual stress analysis of an external grooved thick-walled pressure vessel

Residual stress analysis of an autofrettaged thick-walled pressure vessel containing an external groove was described in order to calculate the stress concentration at the external groove. The autofrettage residual stress distributions of the external grooved thick-walled pressure vessel were simulated using an equivalent thermal loading from the analogy of thermal and autofrettage residual stress fields. Thermal stresses due to the simulated thermal loadings for various degrees of autofrettage overstrain level were computed using finite element methods. Very high stress concentration factors due to autofrettage loadings were obtained at the external groove root that contained a sharp root radius. Experimental measurement of residual stresses for a fully autofrettaged smooth thick-walled pressure vessel using an equivalent saw cut method resulted in very close agreement with the theoretical autofrettage residual stress distributions. The stress analysis results implied that the autofrettage residual stress concentration might cause a cracking problem at the external groove root of the thick-walled pressure vessel, indicating that lower autofrettage overstrain and a groove geometry change were desirable for enhanced durability.

Residual Stress Evaluation and Its Effects on the Fatigue Life of an Autofrettaged Tube

The theoretical residual stresses in the autofrettaged tube were calculated, considering Bauschinger effect caused by the reverse yielding while removing autofrettage pressure. Actual residual stresses were measured by using X-ray diffraction analysis, and compared with the theoretical calculations. From the tension-compression tests, however, the tube steel showed a significant Bauschinger effect, depending on the plastic strain prior to unloading. Fatigue crack growth life of the tube was evaluated by integrating the crack growth rate equation. Stress intensity factor of an inside crack at the tube due to pressure and residual stresses was determined from finite element analysis and superposition principle. Compared to the unautofrettaged tube, the extended fatigue life of the autofrettaged thick-walled tube was obtained depending on the autofrettage level and the Bauschinger effect.

AE Evaluation of Relationship between AE Signals and Fracture Mechanisms for the Weldment of Pressure Vessel Steel

The purpose of this study is to find out the AE characteristics and fracture mechanisms through AE signal analysis for the weldment, PWHT specimen and basemetal of the pressure vessel steel. Charpy sized specimens were taken from the multi-passed weld block. Specimens were given to four point bend and AE tests. Lots of AE signals were emitted from the weldment compared with the basemetal and PWHT specimen. Besides, amplitude for the weldment was the largest, followed by PWHT specimen and basemetal and more AE counts for the weldment were emitted in the process of deformation. Lots of microcracks around the notch for the weldment were formed so that more AE signals were produced. In addition, second phase particle such as MgO for the basemetal acts as AE source. However, in case of weldment, debonding mechanisms between matrix and hard oxides which are formed during welding in air attributed to the emission of AE signals and softened particles for the PWHT specimen cause to produce the low level AE signals.

Elasto-Plastic Stress Distribution in Triplate Joint Assembly under Bending Moment

Pre-produced triplate transition joint assemblies are widely used in shipbuilding industry to make welds between aluminum and steel for a number of years now. The straight-shaped transition joint assemblies are bent during shipbuilding. So it is necessary to study the residual stresses created by punch forming, which would have heavy effects on the quality of structural parts. ABAQUS is a suite of powerful engineering simulation programs, based on the finite element method. In this paper, ABAQUS was used as the main tool to simulate the residual stresses in a triplate transition joint after unloading. Punch-pressing was carried to simulate bending moment in ABAQUS. The triplate is consisted of baselayer (steel: Lloyd’s Shipplate Gr. A), interlayer (pure aluminum: Al99.5) and superlayer (Al-Mg alloy: AlMg4.5Mn). Results from the ABAQUS analysis showed that increasing the radius of punch significantly reduced the von Mises residual stresses in steel. Changes of von Mises residual stresses in interlayer (Al99.5) and superlayer (AlMg4.5Mn) were negligible.

Fatigue Life Simulation of a High Pressure Breech System

In order to evaluate the structural integrity of the breech system used for a thick-walled cylinder subjected to pulsating high internal pressure, fatigue life simulation of a breech system was performed. A stress analysis of the breech was performed to locate the critical region vulnerable to crack initiation. Low-cycle fatigue behavior of the breech material was investigated to obtain the fatigue crack initiation properties. Elastic-plastic finite element stress analysis resulted in a stress concentration at the breech ring groove root. Strains at the breech ring and block were experimentally measured using strain gages and resulted in similar values compared to the calculated strains. Local strain approach was employed to estimate the fatigue life of the breech system for crack initiation at the groove root of the breech ring. Fatigue tests using simulation specimens were performed and an averaged fatigue life was obtained, showing a very good agreement with the calculated fatigue life within a factor of two.

Stress Corrosion Cracking Life Estimation of Hold-Down Spring Screw for Nuclear Fuel Assembly

Hold-down spring screw fractures due to primary water stress corrosion cracking were observed in nuclear fuel assemblies. The screw fastens hold-down springs that are required to maintain the nuclear fuel assembly in contact with upper core plate and permit thermal and irradiation-induced length changes. In order to investigate the primary causes of the screw fractures, the finite element stress analysis and fracture mechanics analysis were performed on the hold-down spring assembly. The elastic-plastic finite element analysis showed that the local stresses at the critical regions of head-shank fillet and thread root significantly exceeded the yield strength of the screw material, resulting in local plastic deformation. Preloading on the screw applied for tightening had beneficial effects on the screw strength by reducing the stress level at the critical regions, compared to the screw without preload. Calculated deflections and strains at the hold-down springs using the finite element analysis were in very close agreements with the experimentally measured deflections and strains. Primary water stress corrosion cracking (PWSCC) life of the Inconel 600 screw was predicted by integrating the Scott’s model and resulted in a life of 1.42years, which was fairly close to the field experience. Cracks were expected to originate at the threaded region of the screw and propagated to the opposite side of the spring, which was confirmed by the fractographic analysis of the fractured screws.

Characteristics of Cu thin films on a glass substrate by partially ionized beam deposition at room temperature

Partially ionized beam deposition of Cu thin films onto glass at room temperature was carried out to fabricate laser mirrors with good structural and reflective properties. At a constant film thickness of 600 A, the grain size of as‐grown Cu films increased with acceleration voltage. There was no indication of defects such as cracks and/or large pores in the film surface as shown in scanning electron microscopy images. Rms surface roughnesses of 600 A thick films were measured by atomic force microscopy images. Rms surface roughness increased when acceleration voltage increased from 0 to 2 kV, but decreased at 3 kV. Rms surface roughness of the film grown at 4 kV, however, increased. At the acceleration voltage of 3 kV, reflectance of the films increased with the film thickness until 600 A and decreased at 800 A. The reflectance results showed that the Cu films deposited at 3 kV had higher reflectance than that of others. Our results suggested that it is possible to grow Cu films with good structural and ...