Papers by Keyword: Residual Stress Field

Abstract: With the development of high temperature protective material, thermal barrier coatings are used in the field of aircraft and industrial steam turbine. They have characteristics of complex structure and long-term work in high temperature environment, etc. Because of multilayer material physical properties mismatch, oxidation, interface roughness, creep stress and so on, the coatings system is easy to produce cracks, expand and link near the interface of coating during thermal cycling. By establishing I, II, III, IV four numerical models of crack fracture mechanisms, assuming that the thickness of oxidation layer is constant, considering the influence of interface roughness and material properties mismatch in the process of thermal cycling, it gets the results of residual stress field near the crack and the influence of the interface. Compared to the presence of defects, four kinds of crack failure mechanisms are analyzed. Energy release rate is calculated by the virtual crack closure method, which can conclude the cracks of I, IV tip stress concentration phenomena are obvious, crack II is easy to expand, due to the temperature load, crack III does not appear stress concentration phenomenon at the tip.

Abstract: Abstract.The connection mode of reducer with straight tube on both sides are the welding connection. There are two weld at the both side of reducer and there has a great influence on residual stress and deformation in the process of welding . Based on the particularity of reducer welding, the paper is focus on the residual stress and deformation in the process of welding, using large-scale finite element analysis software ANSYS .The DN500X450 reducer model is established.The welding temperature field and residual stress field is analysis and calculation and analysis the influence on temperature and stress distribution of reducer. The results show that the maximum of the temperature and the residual stress is located in the big side and reduce the welding seam, and the obvious deformation also find in the big side and reduce joint . The reducing pipe’s distribution of temperature field and residual stress field are obtained,providing the basis to establish properly and optimize of welding process.

Abstract: The aim of this article is to analyze the residual stresses field in a TC4 titanium alloy blade by laser shock processing (LSP).LSP is a new surface processing technology, it uses the laser shock wave to act on the surface of the target and form residual compressive stresses field. The ABAQUS software is applied to simulate the LSP of TC4 titanium alloy blade, and the distributions of the residual stresses field are analysed.After single LSP,the maximum value of residual stress on the surface is 309 MPa.The residual stresses on the surface increase first and then decrease.The residual stresses at the depth continue decreasing with the increase of the depth.After multiple LSP,the maximum value of residual stress on the surface is increased and plastically affected depth is increased.

Abstract: Different experimental methods are being used in laboratories for automatic measurement of fatigue crack growth rates and threshold values. Such data belong to mechanical properties essential for an assessment of residual life of components and structures containing cracks of length more than several millimetres. However, if the material contains small crack-like defects like inclusions or pores, knowledge about local resistance against fatigue growth of physically short cracks becomes very important. Damage mechanisms are even more complicated in case of a presence of subsurface residual stress field, e.g. due to technological effects. Experimental difficulties connected with investigation of short fatigue crack growth (FCG) can be reduced in case of use of automatic indirect methods. The aim of the work described in this paper was to explore possibilities and limits of the use of DCPD (direct current potential drop) method for physically short crack measurement and to use the optimised method for an evaluation of local damage process of initiation and early growth of short fatigue crack in residual stress field induced by shot peening.

Abstract: Laser shock processing (LSP) is an innovative surface treatment technique. According to the theory of residual stress field formation by laser shock wave, laser overlapping shock processing of LY12CZ aluminium alloy was analyzed. The diagonal shock process is simulated by FEM using LS-DYNA codes, and the residual stress field in different angle of fall and pressure are predicted. The results indicate that the value of residual tension stress can be increased when diagonal shock, and the value of residual compressive stress will be decreased. The simulated results can provide the basis for experimental studying the diagonal laser shock processing and laser facular overlap.

Abstract: Typical specimens of AZ31B Magnesium alloy were processed by single point and continuous laser shock peening (LSP). The selected laser energy was 25 J, spot diameter was 8 mm, peening spacing was 8 mm and peening times were 2. The obtained value of residual compressive stresses were -144.3 MPa and -230 MPa for single and continuous LSP respectively, and the magnitude of residual stress was in direct proportion to the depth of deformation in definite micro-deformation range. The average surface micro-hardness in the laser spot zone was 92.42 HV, which increased by 26% as compared to 73.2 HV of substrate, the depth of hardened layer was about 0.3 mm, and the maximum micro-hardness was about 109.86 HV beneath surface of 0.05~0.075 mm. Large amount of crystal chunks appeared at the crystal grain boundaries and inside the grains, and the average grain size decreased from the untreated 7 μm to the peened 4 μm. The results show that the nucleation of fatigue crack can be retarded and the mechanical properties of AZ31B magnesium alloy sheet can be improved greatly with LSP process.

Abstract: A 3D finite element model is established to simulate the ultrasonic shot peening process by using a finite element software ABAQUS. The residual stress distribution of the AISI 304 stainless steel induced by ultrasonic shot peening (USP) is predicted by finite element analysis. Ultrasonic shot peening (USP) process can cause a compressive residual stress layer on the surface of the material. During the simulation, many factors, e.g., ultrasonic shot peening duration, initial residual stress, hourglass, etc., are taken into consideration for the purpose of optimizing the process. The simulation results show that ultrasonic shot peening can produce a compressive residual stress layer on the surface of the material even if there is initial residual tensile stress (250MPa) and the longer peening duration. The residual stress of simulation were compared with the experiment data which were obtained under the same ultrasonic shot peening parameters and have a good agreement with the measurement values by X-ray diffraction method. In conclusion, ultrasonic shot peening is an effective method for protecting weldments against stress corrosion cracking by introducing the compressive residual stress layer into the surface of stainless steel.

Abstract: In order to prevent stress corrosion cracking (SCC) of austenitic stainless steel weldment, a novel technique -laser peening- has been introduced which is able to convert surface tensile stresses in the weldment to surface compressive stresses. In this paper, water-immersed welded specimens of type 304 stainless steel are peened by Q-switched Nd: glass laser with a wavelength of 1.06μm. A residual compressive layer is produced on the specimen surface by scanning laser pulse to cover the heat affected zone (HAZ) along the weld line. The residual compressive stress value based on X-ray measure is significant enough to prevent the initiation of stress corrosion cracking that is also validated by the finite element analysis. The SCC accelerated tests for comparing the potential against SCC of the specimens, which are unpeened and peened by laser pulse with different laser spot coverage, are carried on. The results show that stress corrosion cracks on the surface of unpeened specimen occur after immersion for 25 hours in boiling 42% MgCl2 solutions at 143°C. In contrast, the specimen with 45% laser spot coverage cracks after immersion for 300 hours, while, cracks are observed on the surface of laser-peened specimen with 80% laser spot coverage after 985 hours of testing. The results show that laser peening is an effective technique for improving the residual stress distribution and the SCC resistance of 304 stainless steel weldment.

Abstract: Exact closed-form stress intensity factor (SIF) solutions have been developed for a mode- I through-thickness cracks in an infinite plate. Centre-crack problems have been analysed comprehensively in the literature, but the focus has been on the effect of simple loading about the crack centre. In the current work, the formula of Sih-Paris-Erdogan has been extended to consider the SIF difference on the left and right crack tips, under the local influence of general asymmetric and symmetric stress field. Exact SIF magnification factors convenient for computations have been derived that simultaneously circumvent the problem of crack-tip stress singularity. The solutions so obtained are applied to generate the residual SIFs that would act on a crack growing under the influence of the residual stress fields associated with welded plates and cold-worked holes using the measured residual stress profiles.