Papers by Keyword: Residual Stress

Abstract: Penstock pipelines in hydroelectric power plants are critical components whose structural integrity is paramount for reliable operation. However, they are subjected to severe operational conditions generating complex stresses, favoring low-cycle fatigue crack initiation, particularly at critical weld zones. This is exacerbated by anomalous operational cycles like repetitive emptying and filling. This study presents a comprehensive methodology combining X-ray Diffraction (XRD) with other Non-Destructive Testing (NDT) techniques to assess residual stresses and their impact on pipeline integrity. XRD quantifies net stress under empty pipeline conditions, determining superposition between intrinsic residual stresses (manufacturing, welding, service-induced) and non-hydrostatic loads (self-weight, geomechanical forces). Diffraction pattern analysis yields crucial stress distribution data [1-5], identifying critical concentration zones prone to fatigue [6-10]. Complementary NDT techniques reveal morphological discontinuities influencing material mechanical behavior. Correlating XRD and morphological findings establishes cause-effect relationships between structural state and measured residual stresses. This integrated methodology offers significant predictive maintenance advantages, providing quantitative assessment of current pipeline state and projecting future performance.

Abstract: The structural integrity of Pelton runners, crucial in hydroelectric power generation, is severely compromised by suboptimal repair practices, leading to premature failures and significant operational losses. This study presents a comprehensive analysis of five Latin American case studies (Colombia, Perú and Guatemala), revealing a direct correlation between uncontrolled residual stress and accelerated component degradation. This study demonstrates the effectiveness of integrating XRD with other Non-Destructive Testing (NDT) techniques, enabling the development of predictive models for crack propagation and the optimization of repair protocols. In addition, the results provide evidence that Pelton wheels repair transcends conventional welding, necessitating a profound understanding of residual stress distribution. Precise XRD-driven residual stress determination, pre-and post-intervention, is pivotal for implementing corrective thermal treatments and extending component lifespan. This study demonstrates that integrating XRD into maintenance protocols for Pelton runners constitutes a paradigm shift in structural integrity management. This innovative approach, by enabling precise residual stress analysis, minimizes catastrophic failures and maximizes operational efficiency within hydroelectric power generation. The findings validate the hypothesis that XRD-driven maintenance strategies significantly enhance component longevity and reliability, thereby revolutionizing industry standards.

Abstract: The 8-inch silicon carbide crystals prepared by physical vapor transport (PVT) offer a low-cost pathway for chip production, significantly enhancing the economies of scale. However, point defects， such as vacancies, interstitial atoms, and dislocated atoms produced by the temperature gradient mismatch and the fluctuation of the C/Si ratio during the growth process, seriously affect the residual stresses and the crystalline quality of the crystals. Using stress birefringence optical path difference and X-ray diffraction rocking curve detection methods, we characterized crystals annealed at different temperature. It is well-known that the residual stress of the wafer exhibits an uneven distribution, with the residual stress at the edge of the wafer significantly higher than that at the center. When the post-growth annealing temperature is below 2000°C, the residual stress of the crystal decreases rapidly due to the annihilation and transformation of point defects. However, when the temperature is increased further to 2200°C, a large number of irreparable and large-sized point defect clusters form, which severely degrade the crystalline quality of the crystal, induces lattice distortion, and lead to the generation of residual stress. Overall, the best residual stress relief is achieved at a post-growth annealing temperature of 2000°C.

Abstract: This study aimed to achieve superior sealing surface quality through a cutting process utilizing a non-rotational cutting tool. Previous research has explored the suppression of chatter vibration using indexable non-rotational cutting tools fabricated from damping alloys. The experiments employed a custom indexable tool composed of a damping material (M2052), with cemented carbide as the insert material. Prior research has indicated that non-rotating cutting tools incorporating damping alloys exhibit enhanced suppression of chatter vibrations, compared with traditional non-rotating tools. This study extends the enquiry to assess the effects of the cutting edge shape on the stability of cutting operations using non-rotational cutting tools with damping alloys. To investigate the effect of the cutting-edge shape on the machined surface, the cutting forces were measured using a dynamometer, the machined surface was measured using a white light interferometer, and the residual stresses were measured using an X-ray residual stress analyzer. Consequently, the insert with a large cutting width had a large variation in the cutting force and caused the generation of compressive residual stress, depending on the conditions. However, it is clear that the insert with a small cutting width exhibited a small variation in the cutting force and generated tensile residual stress, resulting in stable cutting.

Abstract: The purpose of this study is to investigate the effectiveness of laser peening (LP) and shot peening (SP) on the fatigue strength and harmless crack size ( of 3D additively manufactured maraging steel. LP and SP was performed under random condition and pre-optimal condition, respectively. Compressive residual stresses of 510MPa and 1650MPa could be introduced on the surface by LP and SP, respectively. Bending fatigue tests were conducted using base metal (BM) specimen, LP specimen and SP specimen. The fatigue strength of the LP and SP specimens were about 57 and 47% higher than that of BM specimen, respectively. Fatigue fracture was initiated from internal by LP and SP. The semicircular cracks less than 0.3mm and 0.1mm in the depth could be rendered harmless by LP and SP, respectively. The estimated based on fracture mechanics were similar to experimental result. The fatigue strength and was affected by the distribution of the compressive residual stress induced by LP and SP. Thus, the LP and SP process can contribute to improving the reliability of 3D additively manufactured maraging steel. Compressive residual stress is the dominant factor in improving fatigue strength and rendering surface defects harmless.

Abstract: Increasing usage of the high-strength steels in structural design requires deeper understanding of the residual manufacturing stresses effect on the product service fatigue life. The bending forming process is being examined in this work. High cycle fatigue testing of the specimens before and after the bend shaping is performed by means of the vibrational fatigue method. The manufacturing residual and the fatigue tests stress fields are estimated by means of finite element analysis. The similarity principle is used to compare the fatigue curves constructed for the specimens with different geometries based on their local stress field concentration. A comparison with reference work is provided to support the similarity premise. The implementation of the mean stress correction for the residual stress is evaluated. The goal of this work is to demonstrate a methodological integration of the finite element analysis throughout manufacturing and fatigue testing for accurizing design life estimations. It may also serve as an end-to-end review and provide an outline for similar projects.

Abstract: There are number of different methods and procedures in vibration analysis, where the natural frequencies of the specimen or the system are one of the key parameters. It is known that these frequencies can change under load, for example in response to pre-stressing, but the effect of residual stresses is less known. By developing a suitable method, natural frequencies can be used to predetermine residual stress, therefore this method can be used for example predicting whether it will cause deformation during machining of a part, whether it requires increased attention or how to set the parameters well for vibratory stress relief. The results can be significant cost and time savings, as well as the improvements of the quality. Natural frequency is the frequency of free vibration of an undamped linear vibration system, or in other words at which a system left alone will vibrate after excited by an external force [1]. Metal castings or welded structures may have several natural frequencies which appear as frequency bands or ranges on the measurement images. Based on these, to determine the natural frequency of a component or system, we need to excite a frequency as close as possible to the natural frequency for the resonance to occur. When the resonance is reached, the amplitude of the system is at its maximum, and the natural frequencies of the workpiece can be measured. Traditionally, sensors, usually accelerometers are used to measure the natural frequency. The continuous development of information technology has made it possible to replace these sensors with an acoustic diagnostic system. During this research, we have developed an acoustic diagnostic system and procedure, which can generate the acoustic measurement images. We have evaluated the measurement images in many ways, and many different types of components and materials (mostly iron alloys) were analyzed. In addition, the changes of natural frequencies show a similar pattern in the case of parts before treating with vibratory stress relief as for load tests.

Abstract: The purpose of this study is to evaluate the accuracy of a simulation model for the Rotary Friction Welding (RFW) process of AA6061 aluminum alloy. RFW, widely used in the aerospace and automotive industries, is known for producing strong welds with minimal heat-affected zones (HAZ). Using ABAQUS software, a numerical model was developed to simulate key aspects of the process, such as heat generation, material flow, and axial shortening. The simulation results were compared with experimental data and previous studies to validate the model’s accuracy. The comparison demonstrated the model’s ability to closely replicate reality welding conditions, making it a reliable tool for optimizing welding parameters and improving the RFW process for AA6061.

Abstract: Due to the recent increase in energy consumption, global environmental issues have attracted attentions in order to continue the sustainable development of society [1]. Currently, scrap-and-build becomes a mainstream in the field of constructions, which would use and /or dispose a large amount of materials. This situation results in massive energy consumption with a significant environmental impact [2]. Therefore, it is important to consider sustainable systems in the construction industry. In many countries, a lot of structures had been constructed during the period of rapid economic growth in the middle of the last century, and their constructions are reaching the end of their design lives. In particular, due to the significant increase in the number of automobiles, numerous cracks above the estimated numbers has been observed in the bridges of motorways. Therefore, the improvement of fatigue life is strongly required to keep a good condition of constructions for a long time. In addition, the nuclear power plant is one of important infrastructures in the modern society, and the proper maintenance of nuclear reactor has been required to ensure their safety after the accident at the Fukushima Daiichi Nuclear Power Plant caused by the Great East Japan Earthquake. Stress corrosion cracking is considered as the most important damage event in the maintenance of nuclear reactor. Stress corrosion cracking occurs, when three factors are combined: tensile residual stress due to welding, material sensitization in the heat-affected zone, and the corrosive environment. In other words, eliminating one of these factors might prevent from generating stress corrosion cracks, and it is expected to extend the fatigue life of nuclear reactors [3]. Therefore, shot peening has been conventionally used as a method to reduce fatigue cracks, because the compressive residual stress can be obtained by the surface’s deformation generated by the high-speed impact of shot grain. However, quantitative control of residual stress is not easy in the shot peening process. Because the shot peening treatment involves the combinations of many factors in the collision between shot grains and the specimen’s characteristics, and the mechanism of micro-structural transformation is also important point to consider the process’s effect. In addition, shot grains possibly remains on the specimen surface, and the contamination of shot grains might deteriorate the functionality of metal surface [4].

Abstract: The utilization of hydrogen in the construction of a decarbonized society is expected to expand the application of austenitic stainless steels with high resistance to hydrogen embrittlement as structural materials. However, the residual stress generated during machining causes material deformation, leading to increased costs and decreased productivity. Therefore, cutting methods that can control residual stress are necessary, prompting numerous studies on residual stress. We proposed conditions to reduce deformation and clarify the relationship between the depth of cut and material deformation, as well as the relationship between residual stress and material thickness after machining. In this study, stainless steel (AISI 304) was face milled, and the relationship between the cutting temperature and material deformation after machining was evaluated, as in a previous study. In addition, electrolytic polishing was performed to measure the residual stress in the depth direction, and its relationship with material deformation was evaluated. The experimental results showed no correlation between the cutting temperature and deformation. However, the measurement of the residual stress in the depth direction suggests that the removal of the surface layer by electropolishing may affect material deformation and residual stress.