Papers by Keyword: Compressive Residual Stress

Abstract: The fatigue strength of maraging steel, which is an ultra-high-strength steel, is relatively low, compared to that of conventional high-strength steel. The fatigue life of a structure is highly dependent on the surface conditions, because fatigue cracks generally start at the surface of the material. In particular, surface cracks considerably degrade the fatigue limit. To expand the application range of maraging steel, it is necessary to improve the fatigue limit, and render the surface cracks harmless. This study aims to investigate the effect of shot peening (SP) on the fatigue strength of maraging steel with surface cracks. The SP application introduced a compressive residual stress from the specimen surface to a depth of 170 μm, and increased the fatigue limit by 77 %. The estimated crack size that can be rendered harmless, based on fracture mechanics, is (0.170 − 0.202) μm in the range As = (1.0 − 0.1). The intersections of the harmless crack sizes were determined at depth. A semicircular surface crack below this value is harmless in terms of fatigue limit. The usefulness of non-destructive inspection (NDI) and non-damaging technology was evaluated in relation to a_hml, a_NDI, a_25,50, and As. Thus, the SP process can improve the reliability of the maraging steel. Compressive residual stress is the dominant factor to improve fatigue strength and render the surface crack harmless.

Abstract: Recently, mechanochemical multifunction cavitation (MC-MFC) was developed to improve the corrosion resistance of the magnesium surface. MFC is a technology that combines water jet peening and ultrasound cavitation. MC-MFC is a technology that adds phosphoric acid to water. It can improve the corrosion resistance by forming a phosphate film on the Mg surface. Conventional anodic oxidation, plating, and chemical vapor deposition can improve corrosion resistance by forming a film on the Mg surface, but it is difficult to improve characteristics such as compressive residual stress on the surface. MFC-treated surfaces have previously imparted various properties such as imparting compressive residual stress necessary to improve the fatigue strength to Al alloys and Cr-Mo steels. In this study, the effect of film formed on MC-MFC processed surface on compressive residual stress was investigated.

Abstract: Compressive residual stress below the surface of material could increase fatigue life as it encounters the tensile loading applied on the material during operation. Shot peening process is a common surface treatment to introduce this stress. This study will investigate on how to introduce the same amount of residual stress by simulation using FEM as introduced in experimental shot peening process. Actual shot peening process was done using a particular sets of parameters and FEM with single shot is used to simplify the simulation procedure. Result shows that using a single shot simulation could also introduce the equivalent amount of residual stress as in the experimental multi-shots shot peening process. This value could be used in further study to study the relaxation of the stress after load is being applied.

Abstract: Aluminium alloys are widely used in engineering fields because of its characteristics. Components used in the fields of automobile, aerospace such as gears, shafts, turbines. In the mechanical field materials can be joined using many processes like riveting, bolted joints or by using permanent joining techniques like welding, brazing or soldering. The welded joints are often used for structural applications due to the reason that they provide nearly the same strength of the parent material. But researches show that even though they provide good strength in tension, they fail much faster when they exposed to fatigue loading. This may be due to various reasons like improper welding sequence, improper preparation or improper welding parameters of the joint materials. The fatigue life may also get reduced due to stress concentration developed during the welding process. Fatigue failures often results in catastrophic failures. Many designs have been reported, have been occurred due to fatigue failures which resulted in heavy loss of human lifespan and goods. Materials with even higher tensile strength are found to fail at less number of cycles after welding.

Abstract: Most of the failure accidents of machine equipment are related to the fatigue crack initiation and growth. Therefore, some research was performed to inspect crack initiation and to repair the cracked materials. Those studies are important for the safe operation of machines and to extend the machine life. We have shown some simple methods for arresting crack growth. Those methods are introduced in the present paper. First, a method of drilling stop holes near the crack tip is shown. In this method, additional holes were drilled in the vicinity of the main stop hole. Second, the method for inserting pins into stop holes is shown. Third, a method of applying a crack arrester for stopping the growth of long cracks is shown.

Abstract: 40CrNi2Si2MoVA high-strength steel is widely used in aircraft industry because of its unique combination of ultrahigh strength with high fracture toughness. However, 40CrNi2Si2MoVA is vulnerable to both corrosion fatigue and stress corrosion cracking failures with catastrophic consequences for aircraft. The high strength steel 40CrNi2Si2MoVA is enhanced and strengthen by usage of laser shock processing (LSP) with a optimal shock processing parameters, and after a series of comparion experiment in different overlap rate,the surface compressive residual stress in the testing specimen is measured and compared, and the experiment results show that the specimen surface has-962MPa compressive residual stress. Finally, the enhanced effect of compressive residual stress in surface of specimen by laser shock is discussed. The results of laser shock processing has a guiding significance to the engineering applications of high strength steel.

Abstract: For a slant pre-crack under mixed-mode conditions, a method is proposed in which mode Ⅰ and mode Ⅱ stress intensity factors ‹K_I›_mes and ‹K_II›_mes can be directly evaluated from the discontinuous displacement along it. The effect on fatigue pre-crack deformation behavior was discussed by comparing fatigue and annealed cracks. In general, using the biggest tangential stress criterion the direction propagating from the pre-cracks can be predicted by ‹K_I›_mes and ‹K_II›_mes calculated above. So testing for bending fatigue crack propagation under mixed-mode conditions was carried out using fatigue and annealed slant pre-cracks with slant angle β=450 defined as the angle between loading and pre-crack direction in a rectangular plate. As a result, for annealed pre-crack, the estimated fracture angle θ_estcorresponded to the measured oneθ_mes ; On the other hand, for fatigue pre-crack,θ_mes is bigger than θ_est because of the compressive residual stress around the pre-crack .

Abstract: The control of residual stress is crucial in ensuring the integrity of engineering components and Laser Shock Peening (LSP) process can be used to good effect to introduce the beneficial compressive residual stress levels required. It is, however, difficult to use normal laser peening control systems to establish the ideal peening conditions that will result in the best component performance. This paper presents results from a study to optimise the laser peening parameters for a typical titanium super alloy used in high performance turbine blade by investigating how the main peening process parameters influence residual stress profiles resulted by numerical simulations. Statistical Design of Experiments (DoE) was used to limit the number of experiments required for optimisation to be possible. Using this technique and numerical depth profiling methods for residual stress analysis, the maximum compressive residual stresses in Ti-6Al-4V were measured for a range of peening conditions. The results of the detailed process characterisation investigations have shown that, by using careful DoE, it is possible to fully optimise the laser shock peening process to obtain greater benefits than would be possible with traditional control processes.

Abstract: At present, welding technology is not only emphasized in the development of manufacturing technology but also application is expanding. In these systems, application of SUS as high-temperature material which is used for special purposes is attempted, and improvement of manufacturing technologies bear watching together with increase of using rate. Specifically, Wings with surface of three-dimensional shape usually applied to Fastener with purpose of lightweight. However, due to development of welding technology, methods of existing assembly tend to be replaced by welding, recently. Specifically, if laser welding techniques is applied, it minimizes heat-affected zone than other welding techniques. However, in the case of these special welding, residual stress is raised, and it fatally affects fatigue life. In order to remove residual stress and delay effect of fatigue life, shot-peening is executed; it executes shot-peening and verifies delayed effects of fatigue life. The intention of this study is to obtain the optimal conditions of shot-peening

Abstract: Carbonitriding followed by shot peening is an important industrial process to improve the mechanical properties of components, especially by producing compressive residual stresses. In addition, a high hardness and strength produced by this process enhances the surface properties and leads also a high resistance to fatigue. In this study, shot peening with different parameters have been employed to treat the carbonitrided specimens. The measurements of residual stress and residual austenite were performed by X-ray diffraction. It is shown, with a simple eigenstrain model, that residual austenite transformation under shot impact contributes to a significant fraction of residual stresses. When the material (750 HV) is peened with 800 HV shot, it represents about 50%, the remaining is due to plasticity. When it is peened with 640HV shot, 100% of residual stresses can be explained by austenite transformation.