Seon-Jin Kim

압력발생기에서 압력을 생성시키는 얇은 두께의 스테인리스강 피스톤 벨로우즈의 피로 특성을 평가하기 위하여, 본 연구에서는 벨로우즈의 실제 거동(각운동 하중하의 인장 및 압축)을 모사한 피로시험기를 자체 설계 및 제작하여 피로시험을 수행하였다. 피로시험기는 모터와 크랭크를 사용하여 모터의 회전운동을 각운동으로 변환되도록 하고, 변위별 반복수를 측정할 수 있도록 각운동 변위-제어방식으로 구성하였다. 피로시험은 5가지의 변위 레벨에서 각각 2개의 샘플에 대하여 수행하였다. 본 시험을 통하여 피스톤 벨로우즈의 피로 수명과 파괴 양상을 평가하였으며, 균열의 발생은 벽면에 부착된 쪽의 첫 번째 골에서 발생하는 것을 알 수 있었다.

It is well recognized that scatter exists in experimental low-cycle fatigue (LCF) data obtained under a constant-strain amplitude loading condition. The objective of this study is to investigate the statistical variability of LCF data of hot-rolled 316L stainless steel at room temperature (RT). Strain-controlled LCF tests were performed at RT. A triangular waveform was used for fatigue testing with a constant strain rate of 4 × 10-3/s and strain amplitude of ± 0.4. RT LCF specimens exhibited remarkable softening behavior followed by initial cyclic hardening. The coefficient of variation (COV) for LCF property data, such as tensile stress, compressive stress, stress range, and elastic modulus in the 1st and half-life cycles, was 0.7%–6.2%. However, the COV of fatigue life was 20%. Additionally, the fatigue life followed a two-parameter Weibull distribution. The estimated shape and scale parameters were 6.12 and 8471 cycles, respectively. It was found that fatigue life had greater scatter than other LCF property data.

스털링 극저온 냉동기의 압력발생기에 적용되는 SUS304 시트로 하이드로포밍 공법으로 제작된 Ω-형상의 커넥팅 벨로우즈의 피로수명을 평가하기 위하여, 변위-제어 방식의 피로시험기를 설계·제작하여 피로실험을 수행하였다. 그 결과, 변위가 증가할수록 파괴까지의 반복수는 감소하였으며, 변위가 1 mm 이하일 때는 1억 사이클 이상이 되어 무한수명이 되는 것을 확인하였다. 또한 응력해석을 통해 시험과 비교한 결과, 실험에서는 균열의 75%가 1번째 산에서 발생하였으나, 해석에서는 1번째 골에서 최대응력이 발생하는 결과가 나타났다. 마지막으로, 벨로우즈의 실제 벽두께는 골에서보다 산에서의 두께가 감소하였음을 알았다.

In this study, a fiber chain was developed to replace a steel chain using high-modulus polyethylene DM20. The pick count, wrapping count, and inner length were selected as the main design variables of the fiber chain and were analyzed to increase the breaking strength. Orthogonal array experiments were conducted, and the results were analyzed with respect to the breaking strength. The analysis revealed that the pick count and wrapping count had meaningful effects at significance levels within 5%. The main effect analysis revealed that a smaller pick count, larger wrapping count, and longer inner length caused the breaking strength to increase. With the wrapping count fixed at 1, a pick count less than -0.65, and an inner length greater than 0.38, the breaking strength was calculated to be greater than 300 kN. These results are expected be important factors in the derivation of an optimal combination of design variables to attain a fiber chain with a targeted strength.

In this study, three types of synthetic fiber materials were evaluated, namely, DM20, SK78, and T147, to replace steel chains in shipbuilding and offshore fields with fiber chains as there is increasing demand for chains with lighter weights and improved usabilities. The strength and quasi-static stiffness were analyzed to select suitable yarns for the fiber chains. The durability of the yarn was evaluated by performing a 3-T (time to rupture) test as a specific tension level. The results of the experiment revealed excellent dynamic stiffness in DM20 and highest values of the windward and leeward stiffness in T147. 3-T linear design characteristic curves for a specific tension level were derived for the three types of fiber materials. The findings of this study can provide insights for improving strength and durability in fiber chain design.

Alloy 800H is one of the candidate materials for key structural components such as control rod systems, hot gas duct, core barrels, core supports and shut-down cooling systems for the very high temperature reactor (VHTR) system. The aim of this paper is to compare the low-cycle fatigue behaviors of the alloy 800H base metal and weldments at a high temperature of 850℃. Low cycle fatigue tests were carried out for four different total strain ranges of 0.6, 0.9, 1.2 and 1.5% under constant strain rate of 10-3/s. In all test conditions, the fatigue life of base metal and weldments decreased inversely proportional to the total strain range. The cyclic stress response behavior showed cyclic softening phenomenon for both materials. The low cycle fatigue life for base metal and weldments were predicted by the Coffin-Manson-Basquin law. The results showed that transition life of the weldments is shorter than that of the base metal.

On series of studies on the high temperature low cycle fatigue properties of Alloy 617, this work focuses on the effect of high temperature on low cycle fatigue properties of the Alloy 617. The low cycle fatigue tests were conducted under fully reserved axial total strain range control mode at 0.9, 1.2 and 1.5% at high temperature of 900℃ and 950℃ with a constant strain rate of 10-3/s. In all test conditions, the fatigue resistance for the Alloy 617 was decreased with increasing temperature and total strain range. The temperature was considered to have small influence on the degradation on the low cycle fatigue life at these testing conditions. However, the cyclic deformation was observed to be softened at high temperature low cycle fatigue, higher degree of cyclic softening was found as a function of increasing with temperature and total strain range.

Alloy 800H is considered as a candidate material for the major high temperature components of a very high temperature reactor (VHTR). This paper is to investigate the influence of total strain range on low cycle fatigue characteristics of Alloy 800H in air environment at 750℃. Low cycle fatigue tests were carried out by a series of fully reversed strain-controls, four different total strain ranges of 0.6, 0.9, 1.2 and 1.5% under constant strain rate of 10-3/s. In all test conditions, the fatigue resistance for the Alloy 800H was decreased with increasing the total strain range. The strong effect of total strain range on low cycle fatigue characteristics was observed.

Alloy 800H is currently under consideration to be used in a very high temperature reactor system. This paper describes the experimental low-cycle fatigue study on Alloy 800H base metal and weldments in the air with fully-reversed strain control at 700 °C. The recommended modern Alloy 800H weldments were fabricated by gas tungsten arc welding. The observed fatigue life is found to be comparable between base metal and weldments at this temperature, indicating the welding technique can compete with base material. The influence of hold time was further investigated at the maximum tensile strain of 0.6% total strain range condition for 60 s, and it is found to reduce the fatigue life. For most of the tests, the base metal revealed an initial cyclic hardening stress response, otherwise, the weldment was entirely softened with cycles. The addition of hold time is also found to induce an unequal cyclic stress response in both materials. Accordingly, for continuous low-cycle fatigue tests, fatigue cracks were initiated and propagated in a classical transgranular fracture mode. However, the hold time affected the mixed-mode fracture mode by means of a transition from transgranular to intergranular fracture.

The effect of strain range and temperature on the low-cycle fatigue behaviour and microstructure change during cyclic deformation of Alloy 617 for use in very high temperature gas-cooled reactor components were studied at elevated temperature starting from ambient condition. Increasing the strain range and the temperature was noticed to reduce the fatigue resistance of nickel-based Alloy 617 due to facilitating the transformation behavior of the carbides in the grain interior, precipitates along the grain boundary, and oxidation behavior inducing surface connected precipitates cracking. Initial hardening behavior was observed at room temperature condition during cyclic due to the pile-up dislocation of micro-precipitates. The grain size was also taking a role due to the formation of an obstacle in the matrix. In the high temperature regime, the alloy 617 was found to soften for its entire life due to the fast recovery deformation, proved by its higher plasticity compared with lower temperature. The deformation behavior also showing high environmentally assisted damage. Oxidation behavior was found to become the primary crack initiation, resulting in early intergranular surface cracking.