Ilsup Chung

Evaluation of Fretting Fatigue Behavior for Inconel Alloy at 320

Inconel alloys are generally used as steam generator tubes in nuclear power plants. These alloys are highnickel chromium alloys that exhibit excellent resistance to aqueous corrosion. In this paper, the effects of elevated temperatures such as an operating temperature of on the fretting fatigue behavior of inconel 600 and 690. We observed that the plain and fretting fatigue limits at were slightly lower than those at room temperature. The frictional forces varied depending on the number of load cycles. After each test, we studied the fretting fatigue mechanisms via SEM observations. These results can be used for structural integrity evaluations at elevated temperatures and for studying fretting damage in steam generator systems.

Fretting Wear Characteristics of Inconel 690 Tubes in Room Temperature

Abstract A fretting wear test rig for cross contacting tube specimens, which employs a piezoelectric actuator, has been developed. Along with the simple loading scheme using dead weights, the rig is very simple to be used also. The accuracy was found acceptable. Inconel 690 tubes were tested in room temperature and ambient condition. Normal load and sliding amplitude range up to 35N and 100μm, respectively. The sizes of wear scar and the wear volumes were measured, and wear coefficients have been calculated based on those. A study on the fretting wear mechanism of the tubes has been attempted via microscopic observation. Rugged wear surfaces are induced by the separation and adhesion of particles and formation and subsequent fracture of surface layers. Lapped specimens were also tested and abrasive wear seems to be playing a dominant role. 1. 서 론 원자력 발전소의 증기 발생기에는 다수의 튜브 다발이 설치되어 있다. 원자로에서의 가열된 냉각수는 증기 발생기로 보내져 튜브의 내부를 통과한 뒤 다시 원자로로 공급된다. 한편, 튜브 다발의 외부에는 1 차 측의 냉각수와는 철저히 격리된 또 다른 루프의 냉각수가 순환하며 관의 벽을 사이에 두고 열을 전달받는다. 이렇게 가열된 2차 측 냉각수는 증기화하여 터빈으로 보내진다. 이 과정에서 유동으로 인한 진동은 튜브 다발을 지지하는 구조체(egg crate)와 튜브 사이에 미세한 상대 운동을 일으키며, 냉각수의 높은 압력과 함께 발생하는 상대 운동은 튜브 재료의 마멸을 초래할 수 있다. 비슷한 구조로 원자로에 설치되는 연료봉 피복관 또한 유사한 문제점에 노출될 수 있어 관련 (1~3)연구자들의 관심을 모으고 있다.두 개의 고체 표면 사이의 미세한 상대 운동으로 인한 접촉 재료의 마멸 현상을 프레팅 마멸이라 하며, 주로 진동에 노출되는 기계적 체결 부위나 전기 접점 등에서 많이 발생된다. 재료의 손실량이 크지 않음에도 불구하고 피로 파괴를 가속시키거나 성능을 저하시키는 요인이 되기도 한다.

A Study on the Large Deflection of Flat Spring Subjected to Follower Load by a Rotating Pin

The mechanical spring is one of widely used machine elements. Among various kinds, flat-type spring loaded by a rotating pin was studied. A flat spring was simplified to a cantilever beam, and numerical analysis was attempted. Since the loading pin rotates about a separate axis from the fixed spring or vice versa, the location, direction, and magnitude of the contact force including normal contact and friction loads vary accordingly. Meanwhile, the spring is deformed substantially as the relative motion progresses. Therefore, this problem needs to be formulated taking the follower loading characteristics and geometrical non-linearity into account. Derived nonlinear differential equation was solved to yield the spring deflection, contact force and the torque to rotate the pin, and the result was compared with a finite element solution. Also, the influences of principal design parameters were studied. The proposed methodology is expected to be useful for the design of pin-loaded flat spring and the prevention of mechanical failures in the form of yielding or fatigue failure of spring or severe wear of the components.

Creep-Induced Tension Loosening of CRT Tension Mask

Tension mask is a part of CRT type devices, which is installed right behind glass-made front panel. Numerous slits on the thin metal sheet enable the electron beams emitted from posterior gun to be focused, resulting in enhanced definition. Flattened and enlarged displays necessitate the imposition of pretension on the masks, in order to improve the robustness of display quality against vibration or impact. High temperature assembly process subsequent to pretensioning, however, degenerates creep resistance of mask material, and common mask may become susceptible to undesirable elongation due to creep. Once tensile stress becomes high enough to induce creep deformation, pretension is substantially loosened. In this study, tension mask assembly is modeled as a combined structure of beams and wire array, and a numerical simulation is attempted for pretensioning followed by high temperature process. Based on a model study, creep occurrence is found to be probable and its adverse influence is quantified. As fur maintaining high tensile force, simply increasing pretension does not seem to be helpful. Instead, the structure of frame needs to be modified somehow, or material for mask needs to be selected properly.