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Numerical simulation and experiments of titanium alloy engine blades based on laser shock processing
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Numerical simulation and experiments of titanium alloy engine blades based on laser shock processing
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Pengyang Li , Shikun Huang , Haifeng Xu , Yuxi Li , Xiaoli Hou , Quandai Wang , Weiping Fu a , Yingwu Fang b
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Xi’an University of Technology, Xi’an 710048, China b Air Force Engineering University, Xi’an 710077, China
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Article history: Received 12 July 2014 Received in revised form 27 September 2014 Accepted 31 October 2014 Available online xxxx Keywords: Laser shock processing Residual stress FEM simulation TC4 engine blade
Effects of Laser Shock Processing (LSP) on mechanical properties of TC4 titanium alloy blades were numerically evaluated. Finite element models were developed to simulate the residual stress distributions and energy profiles on the TC4 titanium alloy blades under different power density inputs during LSP. The models were validated by comparing the simulation results with the experimental results. Simulations show that the kinetic energy of the blade reaches the maximum value within 0–70 ns. The residual stress distribution results exhibit the decreasing compressive residual stress along the radial direction as well as along with the increased depth. The microstructure of the TC4 titanium alloy blades after LSP from transmission electron microscope (TEM) indicates that simulation results were reasonable. © 2014 Published by Elsevier Masson SAS.
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1. Introduction
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Ti–6Al–4V (TC4) blade is one kind of important engine parts whose basic chemical compositions can be found in the literature [16]. In order to improve surface performances of a TC4 alloy blade, LSP is now used to treat the blade surface. Laser shock processing (LSP) is a relatively new surface treatment technique that has been shown to be effective in improving the fatigue properties of a number of metals and alloys. Potential applications are directed to aircraft/aerospace industries and automotive industries due to excellent fatigue resistance, radiation resistance, corrosion resistance, good machinability and welding performance [7,8,19,21,22]. Gomez-Rosas et al. presented a configuration of the LSP concept for metal surface treatments by underwater laser irradiation at 535 nm and 1064 nm on 6061-T6 aluminum samples [9]. Results demonstrated that the LSP is an effective surface treatment technique to improve fatigue properties of 6061-T6 aluminum alloy. Lu et al. studied the effects of multiple LSP impacts on the residual stress and plastic deformation of the hardening layer of LY2 Al alloy [18], and obtained the refined structure at the top surface of LY2 Al alloy subjected to multiple LSP impacts. Zhang et al. compared the fatigue lives of 7050-T7451 Al alloy with different shocked paths by using the stepped loading method during
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Corresponding author. E-mail addresses: [email protected], [email protected] (P. Li).
http://dx.doi.org/10.1016/j.ast.2014.10.017 1270-9638/© 2014 Published by Elsevier Masson SAS.
two-sided LSP [27]. Dai et al. investigated the surface topography, residual stress and micro-hardness of LY2 Al alloy treated by LSP with different initial surface conditions [4]. Lu et al. investigated the effects of the multiple LSP impacts with different pulse energy on mechanical properties and wear behaviors of AISI 8620 steel [17]. Bhamare et al. used a numerical approach based on 3D nonlinear finite element LSP simulations to explore the relation between the processing parameters and the residual stress distribution of Ti–6Al–2Sn–4Zr–2Mo [2]. Hua et al. investigated the LSP effects on hot corrosion resistance of TC11 titanium alloy [11]. Ren et al. examined the effect of laser shock processing on the fatigue behavior of 7050 specimens hole surface [20]. Irizalp et al. analyzed the tensile properties of 6061-T6 alloy in light of residual stresses which take place at material surface after LSP treatment [12]. Recently, great developments in the field of functionally graded materials (FGM) have been obtained [1,5,15,23–25, 28], FGM can be used in aerospace engine blades for its excellent wear resistance, corrosion resistance and heat resistance, and the corresponding fatigue life of the blade would be obviously increased. Tounsi and co-workers [3,10,26] dealt with thermoelastic stresses in FGM which can also be used in increasing the blade life. On the other hand, present researches suggest that compressive residual stresses have very important merits for improving the fatigue life of blade materials, though limited works found on FEM simulation are focused on residual stress field descriptions on TC4 engine blade surfaces. In this paper, a novel FE model is developed to numerically investigate the energy profiles of TC4 engine blade
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Fig. 1. The flowchart of the LSP.
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during the LSP as well as the residual stress fields on the blade surface with different power inputs. The model is validated comparing the simulation results with LSP experiments. This work will also put forward a new direction of the further research on the plastic deformation mechanism of different titanium alloy materials under high strain rate.
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2. FEM model
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Since the typical loading duration on the blade by the shocking pressure of LSP is very short, it is difficult to grasp the LSP transient dynamic response profile of materials and achieve stable residual stress distributions on the blade surface. A FE model is developed with ANSYS/LS_DYNA software, which has great advantage on explicit and implicit algorithm and thus can accurately handle transient shock wave propagation in the material. A powerful post-processing tool LS_PREPOST was adopted to convert analysis results between ANSYS and LS_DYNA. To obtain the stable residual stress field, the explicit dynamic analysis and implicit static analysis are taken by LS_DYNA program. Using explicit dynamic finite element algorithm, the shock wave propagating process in the component material and the dynamic response of the material under the LSP are analyzed. With implicit finite element algorithm, the elastic strain of component’s internal material is released and stable residual stress field is also obtained. The equilibrium equations of the implicit algorithms was acquired based on the literature [13]. Fig. 1 shows the solution flowchart of the algorithms. Meshing generation has a great influence on the FEM simulation results. The principle of the mesh generation in this work is that mesh should be refined at the laser impact zone and locations where stress and strain changes significantly. In the simulation, the LSP shock spot radius is 2.5 mm. Grid refinement is used in the shock region with the unit size is 0.125 mm and the rest region is meshed with the unit size of 1 mm. Solid164 is used in meshing generation and there are 489 049 units in the computational region on the blade, as shown in Fig. 2. In the simulation the blade is fixed as shown in Fig. 3, i.e. the degree of freedom of both sides of the blade and the tenon end face should be constrained. The shock waves generated by the laser shock are propagated inside the blade. Because of plastic strain dissipation and viscous damping, the computational time to achieve a stable residual stress distribution is quite long. Reasonable damping parameter settings can effectively diminish the
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Fig. 2. The mesh generation of blade.
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Fig. 3. The boundary conditions.
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non-real oscillations caused by explicit algorithm without losing the accuracy of the simulation results. According to the LS_DYNA manual, the global damping parameter value is set as the twice of the first natural frequency. The first natural frequency of the blade is 239.13 Hz from modal analysis. Hence, the global damping parameter is set as 480. For the shock wave peak pressure of laser shock components surface, Fabbro et al. [6] proposed laser shock wave peak and estimation equations of shock parameters and material properties, which are consistent with experimental measurement. Under the constraint mode, the laser shock component surface produce highpressure plasma detonation wave, and the pressure–time history curve is approximate Gaussian shape. The shock wave pressure duration is almost three times the period of the laser pulse. Fabbro et al. [6] also studied semi-theoretical model of the laser shock wave pressure under the constraint condition and obtained the maximum laser pulse shock pressure. Given the LSP experimental conditions, the corresponding equation of shocked pressure is as follows,
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Fig. 4. The changing of energy with time.
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where α denotes the efficiency coefficient, A denotes the absorbing factor, I 0 denotes the laser power density, and Z denotes the equivalent impedance. The strain rate is an important parameter of materials constitutive law for the material model. The strain rates generated by LSP can exceed 106 s−1 inside the target material. It is found that the Johnson–Cook (J–C) constitutive model is a better description for materials under high-voltage/high-speed shock process [14]. The J–C constitutive law is used in current models to study the ultrahigh strain rate dynamic response of materials under LSP. In the J–C model, the effective stress σ y is expressed as,
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Fig. 5. The displacement of unit over time.
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where A, B, C and m denote the material constants, n denotes the index of the exponential work hardening law, and ε ∗ denotes plastic strain rate. In Eq. (1), the normalized temperature T ∗ is defined as follows,
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where T m denotes the material melting temperature, and T 0 denotes the room temperature. According to Eqs. (2) and (3), the stress of the material is a function of strain, strain rate, and temperature. The values of A, B, C , n, and m are determined from an empirical fit of experimental flow-stress data. 3. Simulation and experiment 3.1. Energy analysis during LSP
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In the FE simulation of the LSP, the energy absorbed by the blade is mainly converted into the internal energy, damping energy, kinetic energy and hourglass energy. Fig. 4 shows curves for each energy varying over time in the model. As can be seen from Fig. 4, due to the shock stress, the total energy rapidly reached the maximum value of 0.059 J shortly from the beginning. Because of damping and hourglass declining, the total energy is stabilized gradually afterwards. Hourglass energy always keeps the value of 0.002 J in the whole analysis process. It means that the structure of the model is reasonable that reduced integration unit does not cause hourglass problems. The kinetic energy reaches the maximum value within 0–70 ns and then it reduced to 0 after about
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Fig. 6. The equivalent stress of unit over time.
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10 000 ns because of the damping effect. There is no stress wave propagating in the material at the moment. It means that the stress field calculated is the final residual stress field and that the loading time is set to 10 000 ns is reasonable. Fig. 5 shows the equivalent displacements of several blade units over time and Fig. 6 the equivalent stresses of several blade units over time. It is known from the equivalent stress of the blade central impact area unit 19 544 that the plastic wave begin to load, the elastic wave begin to unload after the elastic wave quick released in the first time around 100 ns, and the maximum peak stress is 780 MPa. Then, the dynamic yield is reached and the plastic deformation is occurred. The stress amplitude of the oscillation is gradually decreasing. Using the material dynamic response analysis and stress wave theory, it can be concluded that the elastic wave travels at a faster speed than the plastic wave does. The elastic wave catches up with the plastic wave and unloads it. The peak pressure is then declining gradually till it is stable after 3500 ns.
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Fig. 7. The contour of effective stress.
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3.2. Residual stress field analysis
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The parameters of the entire LSP simulation of the blade are as follows: the laser power density is 1.8 GW/cm2 , the laser spot radius is 2.5 mm, the laser pulse width is 30 ns and the shock wave peak pressure calculated by the formula is 3 GPa. After the laser shock processing, a stable residual stress field is calculated on the impact surface area of the blade. The stable equivalent Von Mises stress distribution is shown in Fig. 7 with the maximum equivalent stress is 110 GPa. After importing analyzed results of each time step to ANSYS post-processor, the residual stress values can be obtained by defining a path giving the track direction. Thus, the residual stress distribution of laser shock impact on the blade surface along both radial direction and the in depth direction can be obtained, as shown in Fig. 8. It can be seen from Fig. 8(a) that the maximum compressive residual stress on the impact surface is 348 MPa. The compressive residual stress decreases along the radial direction to zero at the boundary of the impact the residual stress is uniform distribution in the impact area of the blade surface. As shown in Fig. 8(b), the residual compressive stress gradually is decrease with the depth increasing. The compressive stress will be reduce to zero when the depth is about 0.7 mm, that is to say, the depth residual compressive stress layer of the blade processed by laser is 0.7 mm. Further, the pressure from the empirical formula shows that with the increasing laser power density, the laser-induced shock wave peak will increase. Practically, the laser shock often adopt different laser power densities for different materials, and thus yield different strengthening effects. Adjusting the laser power density is actually changing the value of the shock wave pressure. By studying the relationship between the residual stress field and the shock wave peak pressure, the numerical analysis of the laser power density impact can be simply performed by changing shock wave peak pressure. In the current finite element simulations, the shock wave peak pressure was chosen as 2 GPa, 3 GPa, 4 GPa and 5 GPa (cor-
responding laser power density I 0 was 2.7 GW/cm2 , 6.1 GW/cm2 , 10.8 GW/cm2 and 17 GW/cm2 ) while other laser shock parameters remained unchanged such as the laser spot radius r = 1.5 mm and the laser pulse width τ = 25 ns. The simulation results are shown in Fig. 9. Fig. 9 suggests that when the peak pressure is too large, the maximum residual stress is not located at the center of the laser spot, where the maximum shock wave pressure locates, but in the vicinity of the center. This phenomenon is called stress empty and is mainly due to the surface waves (reverse plastic wave and rarefaction wave) generated at the circumference of the circular spot. Surface waves spreading along the radial direction from the center of the spot will generate a large pulse and this pulse will eliminate compressive residual stresses near the spot center and therefore the residual compressive stress at laser spot center is small. The magnitude of the stress empty on the surface, which is the impact of the decrease in regional centers stress, is related to the shock wave peak pressure P max . The greater P max is, the easier the large laser impact region formed and the smaller the stress amplitude of stress hole is. Furthermore, the internal residual stress and the layer depth experienced plastic deformation both increase with the increasing of P max value.
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3.3. LSP experiment
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The laser system includes the primary optical maser oscillator with the Q-switched Nd-glass (neodymium glass rod 8 × 200 mm), the primary laser pre-magnifier with the Ndglass (neodymium glass rod 12 × 250 mm) and the secondary laser main magnifier with the Nd-glass (neodymium glass rod 20 × 500 mm). Fig. 10 shows the schematic experimental setup of LSP. For further discussion on the measurement and calculating method of laser shocked wave, the measurement of shocked wave and theoretical calculation based on Fabbro model were performed. Firstly, the peak pressure of shocked wave was measured during LSP using PVDF (polyvinylidene fluoride) film sensor, since PVDF film sensor has many merits including fast response, high sensitivity and wide measurement range. Secondly, the influence area of LSP zone was measured by comparing the shocked spot mark with the diameter of the shocked dimple. Fig. 11 shows the measured results of shocked spot mark and the shocked diameter of dimple. It turns out that the dimple size is smaller than the spot size: The laser spot size is 4 mm by measuring the shocked spot mark in the aluminum foil and the shocked diameter of dimple is only 2.3 mm. The difference between the shocked spot mark size and shocked dimple diameter is very prominent. In order to improve the efficiency of LSP and the surface density of dimples, an effective method by increasing spot overlapping was used for subsequent industrial implementation. As a result, the residual stresses fields along the radial direction on the blade surface and along the depth direction were measured
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Fig. 8. The distribution of residual stress after LSP.
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Fig. 9. The influence of the laser power density on the target residual stress distribution.
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Fig. 10. The schematic process of LSP.
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Fig. 11. Measured sizes of the shocked mark and dimple.
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Fig. 12. The residual stresses induced by FEM simulations and experiments.
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Fig. 13. TC4 surface nanocrystallization by TEM.
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by LSP. Fig. 12 shows the multiple impacts on the residual stress distribution by simulations and experiments. By comparing the simulation and experimental results on residual stresses induced by multiple impacts, a close match between them was found.
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3.4. Microstructure observation
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In order to study the effect of fatigue prosperities in TC4 blade after LSP, a TC4 sample was prepared to observe the microstructure before and after LSP using a transmission electron microscope (TEM) of model H-800. Fig. 13 shows refinement TEM microstructure morphology, electron diffraction patterns and the corresponding dark field image of TC4 titanium alloy surface structure. It can be seen from Fig. 13 that the surface microstructure refinement of TC4 after LSP that the fatigue performance was improved. The main reasons are as follows: (1) Dislocation strengthening. It can be seen from Fig. 13(a) and (b) that high density dislocations and dislocation cells exist on the surface of titanium alloy after LSP. High density dislocations can improve the yield strength of titanium alloy; on the other hand, the existence of many dislocations as well as their movements can effectively prevent the initiation and propagation of fatigue cracks. Thus the fatigue resistance performance of titanium alloy is improved. (2) Sub-grain strengthening. It can be seen from Fig. 13(c) and (d) that there are a lot of sub-grains existing on the surface of titanium alloy after LSP. The Hall–Petch relation exists between yield strength and the size of the metal crystalline and also between yield strength and the size of sub-grain. (3) Surface nanocrystalline strengthening. For the nanostructure of the titanium alloy surface after LSP, it can be seen from Fig. 13(e) and (f) the small size of nanocrystalline and uncertain orientation relations between neighboring sub-grains, the sliding channel length is only of one sub-grain length scale. Consequently the oxygen diffusion path is very long and the strength of the material is effectively improved.
4. Conclusions
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In this paper, the energy profiles during LSP and residual stress distributions with different power densities for a TC4 titanium alloy blade were discussed by comparing the results of simulation and LSP experiments. Some conclusions are drawn as follows. (1) Numerical simulations and experiments for researches on LSP become necessary to optimize the treatment parameters. According to theoretical studies on the laser shock, the blade shock responses are mainly elastic–plastic. The explicit algorithm from LS_DYAN is adopted to solve the dynamic impact problem and the finite element model of laser shock was established. (2) Simulation results show that the kinetic energy of the blade reaches the maximum value within 0–70 ns. Because of damping effect, it decreases to zero till 10 000 ns. It means that the model has no stress wave spreading in the sample at the moment. The stress field calculated by the model is the final residual stress field. It suggests it is reasonable to set loading time to 10 000 ns. The elastic wave travels at a faster speed to catch up with the plastic wave and then unloads it. The peak pressure value is declining gradually till it is stable after 3500 ns. (3) The residual stress field of the titanium alloy engine blades is simulated. The compressive residual stress is decreasing along the radial direction as well as the depth increasing. The maximum value of residual stress is occurred at the center of the laser spot, but in the vicinity of the center. The magnitudes of the stress hole formed on the surface are related to the shock wave peak pressure P max . The internal residual stress and plastic deformation layer depth increase with the increase of P max . (4) In order to study the effect of fatigue properties in TC4 titanium alloy blade after LSP, a TC4 sample was prepared to observe the microstructure by TEM before and after LSP. The fatigue performance is improved because of the dislocation strengthening, sub-grain strengthening and surface nanocrystalline strengthening by LSP.
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Conflict of interest statement
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None declared.
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Acknowledgements
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The authors would like to thank the support from the Natural Science Foundation of Shanxi Province (2014JM7275), National Natural Science Foundation of China (51275407, 51375381, 51105305).
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Appendix A. Supplementary material
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Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.ast.2014.10.017.
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References
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[1] M.M. Alipour, M. Shariyat, Analytical stress analysis of annular FGM sandwich plates with non-uniform shear and normal tractions, employing a zigzagelasticity plate theory, Aerosp. Sci. Technol. 32 (2014) 235–259. [2] S. Bhamare, G. Ramakrishnan, S.R. Mannava, Kristina Langer, V.K. Vasudevan, D. Qian, Simulation-based optimization of laser shock peening process for improved bending fatigue life of Ti–6Al–2Sn–4Zr–2Mo alloy, Surf. Coat. Technol. 232 (2013) 464–474. [3] B. Bouderba, M.S.A. Houari, A. Tounsi, Thermomechanical bending response of FGM thick plates resting on Winkler–Pasternak elastic foundations, Steel Compos. Struct. 14 (2013) 85–104. [4] F.Z. Dai, J.Z. Lu, Y.K. Zhang, K.Y. Luo, Q.W. Wang, L. Zhang, X.J. Hua, Effect of initial surface topography on the surface status of LY2 aluminum alloy treated by laser shock processing, Vacuum 86 (2012) 1482–1487. [5] N.D. Duc, V.T.T. Anh, P.H. Cong, Nonlinear axisymmetric response of FGM shallow spherical shells on elastic foundations under uniform external pressure and temperature, Eur. J. Mech. A, Solids 40 (2014) 80–89. [6] R. Fabbro, J. Fournier, P. Ballard, D. Devaux, J. Virmont, Physical study of laserproduced plasma in confined geometry, J. Appl. Phys. 68 (1990) 775–784. [7] Y.W. Fang, Y.H. Li, W.F. He, P.Y. Li, Effects of laser shock processing with different parameters and ways on residual stresses fields of a TC4 alloy blade, Mater. Sci. Eng. A 559 (2013) 683–692. [8] Y.W. Fang, S.H. Zhao, Y. Wang, J. Sun, P.Y. Li, Distribution of residual stress fields in TC6 blade by LSP, Surf. Eng. 29 (2013) 608–615. [9] G. Gomez-Rosas, C. Rubio-Gonzalez, J.L. Ocana, C. Molpeceres, J.A. Porro, M. Morales, F.J. Casillas, Laser shock processing of 6061-T6 Al alloy with 1064 nm and 532 nm wavelengths, Appl. Surf. Sci. 256 (2010) 5828–5831. [10] M.S.A. Houari, A. Tounsi, O.A. Bég, Thermoelastic bending analysis of functionally graded sandwich plates using a new higher order shear and normal deformation theory, Int. J. Mech. Sci. 2013 (2013) 102–111. [11] Y. Hua, Y. Bai, Y. Ye, Q. Xue, H. Liu, R. Chen, K. Chen, Hot corrosion behavior of TC11 titanium alloy treated by laser shock processing, Appl. Surf. Sci. 283 (2013) 775–780.
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[12] S.G. Irizalp, N. Saklakoglu, E. Akman, A. Demir, Pulsed Nd:YAG laser shock processing effects on mechanical properties of 6061-T6 alloy, Opt. Laser Technol. 56 (2014) 273–277. [13] G. Ivetic, Three-dimensional FEM analysis of laser shock peening of aluminium alloy 2024-T351 thin sheets, Surf. Eng. 27 (2011) 445–453. [14] J.R. Johnson, W.H. Cook, Fracture characteristics of three metals subjected to various strains, strain rates, temperatures and pressures, Eng. Fract. Mech. 21 (1985) 31–48. [15] S. Kumar, K.V.V.S.M. Reddy, A. Kumar, G.R. Devi, Development and characterization of polymer–ceramic continuous fiber reinforced functionally graded composites for aerospace application, Aerosp. Sci. Technol. 26 (2013) 185–191. [16] C.G. Li, H.Z. Fu, Q. Yu, National Defense Industry Press, Beijing, 2002. [17] J.Z. Lu, K.Y. Luo, F.Z. Dai, J.W. Zhong, L.Z. Xu, C.J. Yang, L. Zhang, Q.W. Wang, J.S. Zhong, D.K. Yang, Y.K. Zhang, Effects of multiple laser shock processing (LSP) impacts on mechanical properties and wear behaviors of AISI 8620 steel, Mater. Sci. Eng. A 536 (2012) 57–63. [18] J.Z. Lu, K.Y. Luo, Y.K. Zhang, C.Y. Cui, G.F. Sun, J.Z. Zhou, L. Zhang, J. You, K.M. Chen, J.W. Zhong, Grain refinement of LY2 aluminum alloy induced by ultrahigh plastic strain during multiple laser shock processing impacts, Acta Mater. 58 (2010) 3984–3994. [19] J.Z. Lu, J.W. Zhong, K.Y. Luo, L. Zhang, F.Z. Dai, K.M. Chen, Q.W. Wang, J.S. Zhong, Y.K. Zhang, Micro-structural strengthening mechanism of multiple laser shock processing impacts on AISI 8620 steel, Mater. Sci. Eng. A 528 (2011) 6128–6133. [20] X.D. Ren, Q.B. Zhan, H.M. Yang, F.Z. Dai, C.Y. Cui, G.F. Sun, L. Ruan, The effects of residual stress on fatigue behavior and crack propagation from laser shock processing-worked hole, Mater. Des. 44 (2013) 149–154. [21] X.D. Ren, Q.B. Zhan, S.Q. Yuan, J.Z. Zhou, Y. Wang, N.F. Ren, G.F. Sun, L.M. Zheng, F.Z. Dai, H.M. Yang, W.J. Dai, A finite element analysis of thermal relaxation of residual stress in laser shock processing Ni-based alloy GH4169, Mater. Des. 54 (2014) 708–711. [22] C. Rubio-González, C. Felix-Martinez, G. Gomez-Rosas, J.L. Ocaña, M. Morales, J.A. Porro, Effect of laser shock processing on fatigue crack growth of duplex stainless steel, Mater. Sci. Eng. A 528 (2011) 914–919. [23] H. Saidi, M.S.A. Houari, A. Tounsi, E.A.A. Bedia, Thermoelastic bending response with stretching effect of functionally graded sandwich plates using a novel shear deformation theory, Steel Compos. Struct. 15 (2013). [24] G. Sun, G. Li, S. Hou, S. Zhou, W. Li, Q. Li, Crashworthiness design for functionally graded foam-filled thin-walled structures, Mater. Sci. Eng. A 527 (2010) 1911–1919. [25] K. Swaminathan, D.T. Naveenkumar, Higher order refined computational models for the stability analysis of FGM plates and analytical solutions, Eur. J. Mech. A, Solids 47 (2014) 349–361. [26] A. Tounsi, M.S.A. Houari, S. Benyoucef, E.A.A. Bedia, A refined trigonometric shear deformation theory for thermoelastic bending of functionally graded sandwich plates, Aerosp. Sci. Technol. 24 (2013) 209–220. [27] L. Zhang, J.Z. Lu, Y.K. Zhang, K.Y. Luo, J.W. Zhong, C.Y. Cui, D.J. Kong, H.B. Guan, X.M. Qian, Effects of different shocked paths on fatigue property of 7050-T7451 aluminum alloy during two-sided laser shock processing, Mater. Des. 32 (2011) 480–486. [28] M. Zidi, A. Tounsi, M.S.A. Houari, E.A.A. Bedia, O.A. Bég, Bending analysis of FGM plates under hygro-thermo-mechanical loading using a four variable refined plate theory, Aerosp. Sci. Technol. 34 (2014) 24–34.
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Numerical simulation and experiments of titanium alloy engine blades based on laser shock processing
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Pengyang Li , Shikun Huang , Haifeng Xu , Yuxi Li , Xiaoli Hou , Quandai Wang , Weiping Fu a , Yingwu Fang b
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Article history: Received 12 July 2014 Received in revised form 27 September 2014 Accepted 31 October 2014 Available online xxxx Keywords: Laser shock processing Residual stress FEM simulation TC4 engine blade
Effects of Laser Shock Processing (LSP) on mechanical properties of TC4 titanium alloy blades were numerically evaluated. Finite element models were developed to simulate the residual stress distributions and energy profiles on the TC4 titanium alloy blades under different power density inputs during LSP. The models were validated by comparing the simulation results with the experimental results. Simulations show that the kinetic energy of the blade reaches the maximum value within 0–70 ns. The residual stress distribution results exhibit the decreasing compressive residual stress along the radial direction as well as along with the increased depth. The microstructure of the TC4 titanium alloy blades after LSP from transmission electron microscope (TEM) indicates that simulation results were reasonable. © 2014 Published by Elsevier Masson SAS.
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1. Introduction
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Ti–6Al–4V (TC4) blade is one kind of important engine parts whose basic chemical compositions can be found in the literature [16]. In order to improve surface performances of a TC4 alloy blade, LSP is now used to treat the blade surface. Laser shock processing (LSP) is a relatively new surface treatment technique that has been shown to be effective in improving the fatigue properties of a number of metals and alloys. Potential applications are directed to aircraft/aerospace industries and automotive industries due to excellent fatigue resistance, radiation resistance, corrosion resistance, good machinability and welding performance [7,8,19,21,22]. Gomez-Rosas et al. presented a configuration of the LSP concept for metal surface treatments by underwater laser irradiation at 535 nm and 1064 nm on 6061-T6 aluminum samples [9]. Results demonstrated that the LSP is an effective surface treatment technique to improve fatigue properties of 6061-T6 aluminum alloy. Lu et al. studied the effects of multiple LSP impacts on the residual stress and plastic deformation of the hardening layer of LY2 Al alloy [18], and obtained the refined structure at the top surface of LY2 Al alloy subjected to multiple LSP impacts. Zhang et al. compared the fatigue lives of 7050-T7451 Al alloy with different shocked paths by using the stepped loading method during
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Corresponding author. E-mail addresses: [email protected], [email protected] (P. Li).
http://dx.doi.org/10.1016/j.ast.2014.10.017 1270-9638/© 2014 Published by Elsevier Masson SAS.
two-sided LSP [27]. Dai et al. investigated the surface topography, residual stress and micro-hardness of LY2 Al alloy treated by LSP with different initial surface conditions [4]. Lu et al. investigated the effects of the multiple LSP impacts with different pulse energy on mechanical properties and wear behaviors of AISI 8620 steel [17]. Bhamare et al. used a numerical approach based on 3D nonlinear finite element LSP simulations to explore the relation between the processing parameters and the residual stress distribution of Ti–6Al–2Sn–4Zr–2Mo [2]. Hua et al. investigated the LSP effects on hot corrosion resistance of TC11 titanium alloy [11]. Ren et al. examined the effect of laser shock processing on the fatigue behavior of 7050 specimens hole surface [20]. Irizalp et al. analyzed the tensile properties of 6061-T6 alloy in light of residual stresses which take place at material surface after LSP treatment [12]. Recently, great developments in the field of functionally graded materials (FGM) have been obtained [1,5,15,23–25, 28], FGM can be used in aerospace engine blades for its excellent wear resistance, corrosion resistance and heat resistance, and the corresponding fatigue life of the blade would be obviously increased. Tounsi and co-workers [3,10,26] dealt with thermoelastic stresses in FGM which can also be used in increasing the blade life. On the other hand, present researches suggest that compressive residual stresses have very important merits for improving the fatigue life of blade materials, though limited works found on FEM simulation are focused on residual stress field descriptions on TC4 engine blade surfaces. In this paper, a novel FE model is developed to numerically investigate the energy profiles of TC4 engine blade
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Fig. 1. The flowchart of the LSP.
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during the LSP as well as the residual stress fields on the blade surface with different power inputs. The model is validated comparing the simulation results with LSP experiments. This work will also put forward a new direction of the further research on the plastic deformation mechanism of different titanium alloy materials under high strain rate.
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2. FEM model
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Since the typical loading duration on the blade by the shocking pressure of LSP is very short, it is difficult to grasp the LSP transient dynamic response profile of materials and achieve stable residual stress distributions on the blade surface. A FE model is developed with ANSYS/LS_DYNA software, which has great advantage on explicit and implicit algorithm and thus can accurately handle transient shock wave propagation in the material. A powerful post-processing tool LS_PREPOST was adopted to convert analysis results between ANSYS and LS_DYNA. To obtain the stable residual stress field, the explicit dynamic analysis and implicit static analysis are taken by LS_DYNA program. Using explicit dynamic finite element algorithm, the shock wave propagating process in the component material and the dynamic response of the material under the LSP are analyzed. With implicit finite element algorithm, the elastic strain of component’s internal material is released and stable residual stress field is also obtained. The equilibrium equations of the implicit algorithms was acquired based on the literature [13]. Fig. 1 shows the solution flowchart of the algorithms. Meshing generation has a great influence on the FEM simulation results. The principle of the mesh generation in this work is that mesh should be refined at the laser impact zone and locations where stress and strain changes significantly. In the simulation, the LSP shock spot radius is 2.5 mm. Grid refinement is used in the shock region with the unit size is 0.125 mm and the rest region is meshed with the unit size of 1 mm. Solid164 is used in meshing generation and there are 489 049 units in the computational region on the blade, as shown in Fig. 2. In the simulation the blade is fixed as shown in Fig. 3, i.e. the degree of freedom of both sides of the blade and the tenon end face should be constrained. The shock waves generated by the laser shock are propagated inside the blade. Because of plastic strain dissipation and viscous damping, the computational time to achieve a stable residual stress distribution is quite long. Reasonable damping parameter settings can effectively diminish the
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Fig. 2. The mesh generation of blade.
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Fig. 3. The boundary conditions.
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non-real oscillations caused by explicit algorithm without losing the accuracy of the simulation results. According to the LS_DYNA manual, the global damping parameter value is set as the twice of the first natural frequency. The first natural frequency of the blade is 239.13 Hz from modal analysis. Hence, the global damping parameter is set as 480. For the shock wave peak pressure of laser shock components surface, Fabbro et al. [6] proposed laser shock wave peak and estimation equations of shock parameters and material properties, which are consistent with experimental measurement. Under the constraint mode, the laser shock component surface produce highpressure plasma detonation wave, and the pressure–time history curve is approximate Gaussian shape. The shock wave pressure duration is almost three times the period of the laser pulse. Fabbro et al. [6] also studied semi-theoretical model of the laser shock wave pressure under the constraint condition and obtained the maximum laser pulse shock pressure. Given the LSP experimental conditions, the corresponding equation of shocked pressure is as follows,
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Fig. 4. The changing of energy with time.
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where α denotes the efficiency coefficient, A denotes the absorbing factor, I 0 denotes the laser power density, and Z denotes the equivalent impedance. The strain rate is an important parameter of materials constitutive law for the material model. The strain rates generated by LSP can exceed 106 s−1 inside the target material. It is found that the Johnson–Cook (J–C) constitutive model is a better description for materials under high-voltage/high-speed shock process [14]. The J–C constitutive law is used in current models to study the ultrahigh strain rate dynamic response of materials under LSP. In the J–C model, the effective stress σ y is expressed as,
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Fig. 5. The displacement of unit over time.
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where A, B, C and m denote the material constants, n denotes the index of the exponential work hardening law, and ε ∗ denotes plastic strain rate. In Eq. (1), the normalized temperature T ∗ is defined as follows,
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In the FE simulation of the LSP, the energy absorbed by the blade is mainly converted into the internal energy, damping energy, kinetic energy and hourglass energy. Fig. 4 shows curves for each energy varying over time in the model. As can be seen from Fig. 4, due to the shock stress, the total energy rapidly reached the maximum value of 0.059 J shortly from the beginning. Because of damping and hourglass declining, the total energy is stabilized gradually afterwards. Hourglass energy always keeps the value of 0.002 J in the whole analysis process. It means that the structure of the model is reasonable that reduced integration unit does not cause hourglass problems. The kinetic energy reaches the maximum value within 0–70 ns and then it reduced to 0 after about
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Fig. 6. The equivalent stress of unit over time.
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10 000 ns because of the damping effect. There is no stress wave propagating in the material at the moment. It means that the stress field calculated is the final residual stress field and that the loading time is set to 10 000 ns is reasonable. Fig. 5 shows the equivalent displacements of several blade units over time and Fig. 6 the equivalent stresses of several blade units over time. It is known from the equivalent stress of the blade central impact area unit 19 544 that the plastic wave begin to load, the elastic wave begin to unload after the elastic wave quick released in the first time around 100 ns, and the maximum peak stress is 780 MPa. Then, the dynamic yield is reached and the plastic deformation is occurred. The stress amplitude of the oscillation is gradually decreasing. Using the material dynamic response analysis and stress wave theory, it can be concluded that the elastic wave travels at a faster speed than the plastic wave does. The elastic wave catches up with the plastic wave and unloads it. The peak pressure is then declining gradually till it is stable after 3500 ns.
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Fig. 7. The contour of effective stress.
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3.2. Residual stress field analysis
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The parameters of the entire LSP simulation of the blade are as follows: the laser power density is 1.8 GW/cm2 , the laser spot radius is 2.5 mm, the laser pulse width is 30 ns and the shock wave peak pressure calculated by the formula is 3 GPa. After the laser shock processing, a stable residual stress field is calculated on the impact surface area of the blade. The stable equivalent Von Mises stress distribution is shown in Fig. 7 with the maximum equivalent stress is 110 GPa. After importing analyzed results of each time step to ANSYS post-processor, the residual stress values can be obtained by defining a path giving the track direction. Thus, the residual stress distribution of laser shock impact on the blade surface along both radial direction and the in depth direction can be obtained, as shown in Fig. 8. It can be seen from Fig. 8(a) that the maximum compressive residual stress on the impact surface is 348 MPa. The compressive residual stress decreases along the radial direction to zero at the boundary of the impact the residual stress is uniform distribution in the impact area of the blade surface. As shown in Fig. 8(b), the residual compressive stress gradually is decrease with the depth increasing. The compressive stress will be reduce to zero when the depth is about 0.7 mm, that is to say, the depth residual compressive stress layer of the blade processed by laser is 0.7 mm. Further, the pressure from the empirical formula shows that with the increasing laser power density, the laser-induced shock wave peak will increase. Practically, the laser shock often adopt different laser power densities for different materials, and thus yield different strengthening effects. Adjusting the laser power density is actually changing the value of the shock wave pressure. By studying the relationship between the residual stress field and the shock wave peak pressure, the numerical analysis of the laser power density impact can be simply performed by changing shock wave peak pressure. In the current finite element simulations, the shock wave peak pressure was chosen as 2 GPa, 3 GPa, 4 GPa and 5 GPa (cor-
responding laser power density I 0 was 2.7 GW/cm2 , 6.1 GW/cm2 , 10.8 GW/cm2 and 17 GW/cm2 ) while other laser shock parameters remained unchanged such as the laser spot radius r = 1.5 mm and the laser pulse width τ = 25 ns. The simulation results are shown in Fig. 9. Fig. 9 suggests that when the peak pressure is too large, the maximum residual stress is not located at the center of the laser spot, where the maximum shock wave pressure locates, but in the vicinity of the center. This phenomenon is called stress empty and is mainly due to the surface waves (reverse plastic wave and rarefaction wave) generated at the circumference of the circular spot. Surface waves spreading along the radial direction from the center of the spot will generate a large pulse and this pulse will eliminate compressive residual stresses near the spot center and therefore the residual compressive stress at laser spot center is small. The magnitude of the stress empty on the surface, which is the impact of the decrease in regional centers stress, is related to the shock wave peak pressure P max . The greater P max is, the easier the large laser impact region formed and the smaller the stress amplitude of stress hole is. Furthermore, the internal residual stress and the layer depth experienced plastic deformation both increase with the increasing of P max value.
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3.3. LSP experiment
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The laser system includes the primary optical maser oscillator with the Q-switched Nd-glass (neodymium glass rod 8 × 200 mm), the primary laser pre-magnifier with the Ndglass (neodymium glass rod 12 × 250 mm) and the secondary laser main magnifier with the Nd-glass (neodymium glass rod 20 × 500 mm). Fig. 10 shows the schematic experimental setup of LSP. For further discussion on the measurement and calculating method of laser shocked wave, the measurement of shocked wave and theoretical calculation based on Fabbro model were performed. Firstly, the peak pressure of shocked wave was measured during LSP using PVDF (polyvinylidene fluoride) film sensor, since PVDF film sensor has many merits including fast response, high sensitivity and wide measurement range. Secondly, the influence area of LSP zone was measured by comparing the shocked spot mark with the diameter of the shocked dimple. Fig. 11 shows the measured results of shocked spot mark and the shocked diameter of dimple. It turns out that the dimple size is smaller than the spot size: The laser spot size is 4 mm by measuring the shocked spot mark in the aluminum foil and the shocked diameter of dimple is only 2.3 mm. The difference between the shocked spot mark size and shocked dimple diameter is very prominent. In order to improve the efficiency of LSP and the surface density of dimples, an effective method by increasing spot overlapping was used for subsequent industrial implementation. As a result, the residual stresses fields along the radial direction on the blade surface and along the depth direction were measured
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Fig. 8. The distribution of residual stress after LSP.
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Fig. 12. The residual stresses induced by FEM simulations and experiments.
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by LSP. Fig. 12 shows the multiple impacts on the residual stress distribution by simulations and experiments. By comparing the simulation and experimental results on residual stresses induced by multiple impacts, a close match between them was found.
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3.4. Microstructure observation
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In order to study the effect of fatigue prosperities in TC4 blade after LSP, a TC4 sample was prepared to observe the microstructure before and after LSP using a transmission electron microscope (TEM) of model H-800. Fig. 13 shows refinement TEM microstructure morphology, electron diffraction patterns and the corresponding dark field image of TC4 titanium alloy surface structure. It can be seen from Fig. 13 that the surface microstructure refinement of TC4 after LSP that the fatigue performance was improved. The main reasons are as follows: (1) Dislocation strengthening. It can be seen from Fig. 13(a) and (b) that high density dislocations and dislocation cells exist on the surface of titanium alloy after LSP. High density dislocations can improve the yield strength of titanium alloy; on the other hand, the existence of many dislocations as well as their movements can effectively prevent the initiation and propagation of fatigue cracks. Thus the fatigue resistance performance of titanium alloy is improved. (2) Sub-grain strengthening. It can be seen from Fig. 13(c) and (d) that there are a lot of sub-grains existing on the surface of titanium alloy after LSP. The Hall–Petch relation exists between yield strength and the size of the metal crystalline and also between yield strength and the size of sub-grain. (3) Surface nanocrystalline strengthening. For the nanostructure of the titanium alloy surface after LSP, it can be seen from Fig. 13(e) and (f) the small size of nanocrystalline and uncertain orientation relations between neighboring sub-grains, the sliding channel length is only of one sub-grain length scale. Consequently the oxygen diffusion path is very long and the strength of the material is effectively improved.
4. Conclusions
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In this paper, the energy profiles during LSP and residual stress distributions with different power densities for a TC4 titanium alloy blade were discussed by comparing the results of simulation and LSP experiments. Some conclusions are drawn as follows. (1) Numerical simulations and experiments for researches on LSP become necessary to optimize the treatment parameters. According to theoretical studies on the laser shock, the blade shock responses are mainly elastic–plastic. The explicit algorithm from LS_DYAN is adopted to solve the dynamic impact problem and the finite element model of laser shock was established. (2) Simulation results show that the kinetic energy of the blade reaches the maximum value within 0–70 ns. Because of damping effect, it decreases to zero till 10 000 ns. It means that the model has no stress wave spreading in the sample at the moment. The stress field calculated by the model is the final residual stress field. It suggests it is reasonable to set loading time to 10 000 ns. The elastic wave travels at a faster speed to catch up with the plastic wave and then unloads it. The peak pressure value is declining gradually till it is stable after 3500 ns. (3) The residual stress field of the titanium alloy engine blades is simulated. The compressive residual stress is decreasing along the radial direction as well as the depth increasing. The maximum value of residual stress is occurred at the center of the laser spot, but in the vicinity of the center. The magnitudes of the stress hole formed on the surface are related to the shock wave peak pressure P max . The internal residual stress and plastic deformation layer depth increase with the increase of P max . (4) In order to study the effect of fatigue properties in TC4 titanium alloy blade after LSP, a TC4 sample was prepared to observe the microstructure by TEM before and after LSP. The fatigue performance is improved because of the dislocation strengthening, sub-grain strengthening and surface nanocrystalline strengthening by LSP.
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P. Li et al. / Aerospace Science and Technology ••• (••••) •••–•••
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Conflict of interest statement
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None declared.
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Acknowledgements
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The authors would like to thank the support from the Natural Science Foundation of Shanxi Province (2014JM7275), National Natural Science Foundation of China (51275407, 51375381, 51105305).
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Appendix A. Supplementary material
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Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.ast.2014.10.017.
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References
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[1] M.M. Alipour, M. Shariyat, Analytical stress analysis of annular FGM sandwich plates with non-uniform shear and normal tractions, employing a zigzagelasticity plate theory, Aerosp. Sci. Technol. 32 (2014) 235–259. [2] S. Bhamare, G. Ramakrishnan, S.R. Mannava, Kristina Langer, V.K. Vasudevan, D. Qian, Simulation-based optimization of laser shock peening process for improved bending fatigue life of Ti–6Al–2Sn–4Zr–2Mo alloy, Surf. Coat. Technol. 232 (2013) 464–474. [3] B. Bouderba, M.S.A. Houari, A. Tounsi, Thermomechanical bending response of FGM thick plates resting on Winkler–Pasternak elastic foundations, Steel Compos. Struct. 14 (2013) 85–104. [4] F.Z. Dai, J.Z. Lu, Y.K. Zhang, K.Y. Luo, Q.W. Wang, L. Zhang, X.J. Hua, Effect of initial surface topography on the surface status of LY2 aluminum alloy treated by laser shock processing, Vacuum 86 (2012) 1482–1487. [5] N.D. Duc, V.T.T. Anh, P.H. Cong, Nonlinear axisymmetric response of FGM shallow spherical shells on elastic foundations under uniform external pressure and temperature, Eur. J. Mech. A, Solids 40 (2014) 80–89. [6] R. Fabbro, J. Fournier, P. Ballard, D. Devaux, J. Virmont, Physical study of laserproduced plasma in confined geometry, J. Appl. Phys. 68 (1990) 775–784. [7] Y.W. Fang, Y.H. Li, W.F. He, P.Y. Li, Effects of laser shock processing with different parameters and ways on residual stresses fields of a TC4 alloy blade, Mater. Sci. Eng. A 559 (2013) 683–692. [8] Y.W. Fang, S.H. Zhao, Y. Wang, J. Sun, P.Y. Li, Distribution of residual stress fields in TC6 blade by LSP, Surf. Eng. 29 (2013) 608–615. [9] G. Gomez-Rosas, C. Rubio-Gonzalez, J.L. Ocana, C. Molpeceres, J.A. Porro, M. Morales, F.J. Casillas, Laser shock processing of 6061-T6 Al alloy with 1064 nm and 532 nm wavelengths, Appl. Surf. Sci. 256 (2010) 5828–5831. [10] M.S.A. Houari, A. Tounsi, O.A. Bég, Thermoelastic bending analysis of functionally graded sandwich plates using a new higher order shear and normal deformation theory, Int. J. Mech. Sci. 2013 (2013) 102–111. [11] Y. Hua, Y. Bai, Y. Ye, Q. Xue, H. Liu, R. Chen, K. Chen, Hot corrosion behavior of TC11 titanium alloy treated by laser shock processing, Appl. Surf. Sci. 283 (2013) 775–780.
7
[12] S.G. Irizalp, N. Saklakoglu, E. Akman, A. Demir, Pulsed Nd:YAG laser shock processing effects on mechanical properties of 6061-T6 alloy, Opt. Laser Technol. 56 (2014) 273–277. [13] G. Ivetic, Three-dimensional FEM analysis of laser shock peening of aluminium alloy 2024-T351 thin sheets, Surf. Eng. 27 (2011) 445–453. [14] J.R. Johnson, W.H. Cook, Fracture characteristics of three metals subjected to various strains, strain rates, temperatures and pressures, Eng. Fract. Mech. 21 (1985) 31–48. [15] S. Kumar, K.V.V.S.M. Reddy, A. Kumar, G.R. Devi, Development and characterization of polymer–ceramic continuous fiber reinforced functionally graded composites for aerospace application, Aerosp. Sci. Technol. 26 (2013) 185–191. [16] C.G. Li, H.Z. Fu, Q. Yu, National Defense Industry Press, Beijing, 2002. [17] J.Z. Lu, K.Y. Luo, F.Z. Dai, J.W. Zhong, L.Z. Xu, C.J. Yang, L. Zhang, Q.W. Wang, J.S. Zhong, D.K. Yang, Y.K. Zhang, Effects of multiple laser shock processing (LSP) impacts on mechanical properties and wear behaviors of AISI 8620 steel, Mater. Sci. Eng. A 536 (2012) 57–63. [18] J.Z. Lu, K.Y. Luo, Y.K. Zhang, C.Y. Cui, G.F. Sun, J.Z. Zhou, L. Zhang, J. You, K.M. Chen, J.W. Zhong, Grain refinement of LY2 aluminum alloy induced by ultrahigh plastic strain during multiple laser shock processing impacts, Acta Mater. 58 (2010) 3984–3994. [19] J.Z. Lu, J.W. Zhong, K.Y. Luo, L. Zhang, F.Z. Dai, K.M. Chen, Q.W. Wang, J.S. Zhong, Y.K. Zhang, Micro-structural strengthening mechanism of multiple laser shock processing impacts on AISI 8620 steel, Mater. Sci. Eng. A 528 (2011) 6128–6133. [20] X.D. Ren, Q.B. Zhan, H.M. Yang, F.Z. Dai, C.Y. Cui, G.F. Sun, L. Ruan, The effects of residual stress on fatigue behavior and crack propagation from laser shock processing-worked hole, Mater. Des. 44 (2013) 149–154. [21] X.D. Ren, Q.B. Zhan, S.Q. Yuan, J.Z. Zhou, Y. Wang, N.F. Ren, G.F. Sun, L.M. Zheng, F.Z. Dai, H.M. Yang, W.J. Dai, A finite element analysis of thermal relaxation of residual stress in laser shock processing Ni-based alloy GH4169, Mater. Des. 54 (2014) 708–711. [22] C. Rubio-González, C. Felix-Martinez, G. Gomez-Rosas, J.L. Ocaña, M. Morales, J.A. Porro, Effect of laser shock processing on fatigue crack growth of duplex stainless steel, Mater. Sci. Eng. A 528 (2011) 914–919. [23] H. Saidi, M.S.A. Houari, A. Tounsi, E.A.A. Bedia, Thermoelastic bending response with stretching effect of functionally graded sandwich plates using a novel shear deformation theory, Steel Compos. Struct. 15 (2013). [24] G. Sun, G. Li, S. Hou, S. Zhou, W. Li, Q. Li, Crashworthiness design for functionally graded foam-filled thin-walled structures, Mater. Sci. Eng. A 527 (2010) 1911–1919. [25] K. Swaminathan, D.T. Naveenkumar, Higher order refined computational models for the stability analysis of FGM plates and analytical solutions, Eur. J. Mech. A, Solids 47 (2014) 349–361. [26] A. Tounsi, M.S.A. Houari, S. Benyoucef, E.A.A. Bedia, A refined trigonometric shear deformation theory for thermoelastic bending of functionally graded sandwich plates, Aerosp. Sci. Technol. 24 (2013) 209–220. [27] L. Zhang, J.Z. Lu, Y.K. Zhang, K.Y. Luo, J.W. Zhong, C.Y. Cui, D.J. Kong, H.B. Guan, X.M. Qian, Effects of different shocked paths on fatigue property of 7050-T7451 aluminum alloy during two-sided laser shock processing, Mater. Des. 32 (2011) 480–486. [28] M. Zidi, A. Tounsi, M.S.A. Houari, E.A.A. Bedia, O.A. Bég, Bending analysis of FGM plates under hygro-thermo-mechanical loading using a four variable refined plate theory, Aerosp. Sci. Technol. 34 (2014) 24–34.
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