Elucidating the mechanism of defect formation in the 7050-T7451 aluminum alloy by laser shock peening
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The laser energy directly affects the strengthening of materials during laser shock peening (LSP). In this paper, the effect of propagation and attenuation shock wave on LSP was studied. Combined with the strengthening effect and defect degree, the best impact parameters and the effects of different impact pressures were obtained. The simulation and experimental results demonstrated when the unloading wave chased the shock wave and caught up at 100-200µm in the depth direction, the possibility of strengthening defects increased and the occurrence position deepened with the increase of laser energy. There was no strengthening defect on the surface of alloy when the impact pressure 4σHEL > (P) > 3.5σHEL. Strengthening defects appeared in the form of cracks 184.61µm from the surface of the alloy. When P > 4σHEL, the strengthening defects appeared on both the surface and subsurface, where the surface defects in the form of holes. The subsurface defects in the form of cracks that appeared 200µm away from the surface resulted from the unloading wave meeting the shock wave. Based on the research and the Hugoniot elastic limit, the optimal impact pressure range to demonstrate the best strengthening effects and to mitigate the formation of strengthening defects is 2σHEL < P < 3.5σHEL.Widely used structural steel SM45C was waterjet peened in air to investigate the optimum peening conditions for the induced compressive residual stress which is beneficial to improve the fatigue strength of the material. After peening the residual stresses generated by the waterjet peening on the surface were measured using X-ray diffraction technique and the results were analyzed to characterize the peening conditions for the maximum compressive stress. Also, the surface roughnesses of peened specimen were measured and the results showed the superior surface quality with minimal changes in surface topography before and after the waterjet peening process.
Laser peening
Fatigue limit
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The mechanism of laser shock peening and application of the coating and limiting layer are studied. Thelaser shock peening experiment of LY2 aerial aluminum alloy is carried out and its capability is tested.The influence on the fatigue life and hardness of aerial aluminum alloy after laser shock peening are analyzed.
Laser peening
Limiting
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The large thin-walled components in aerospace industry, such as wing panels of aircraft, are usually shaped with peen forming process by blasting numerous shots on the component to introduce compressive residual stresses in the surface layer. To obtain an objective shape, peening parameters are designed to introduce reasonable peening stresses, which are determined with physical trial and error traditionally. In this paper, an automatic numerical approach based on ABAQUS software is proposed to determine the peening stresses inversely from the objective shape. The approach consists of representation of peening stress field, monitoring of simulating shape, and performance of automatic calculation. With the approach, peening stresses in strip peened plates are inversely determined from measured bending curvatures. The variations of the peening stress field with peening parameters are obtained. The obtained peening stresses are compared with the results of regularly impacting simulations. The comparison shows an acceptable agreement and indicates the potential of the approach to determine the needed peening stresses for forming the aerospace components.
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It is a well known but puzzling result that zones within star formation regions sometimes show molecular hydrogen emission at very high (∼100 km s−1) velocities. These kinds of observations are somewhat difficult to explain because non-magnetized, J-type shock waves of velocities above ∼20 km s−1 mostly dissociate the molecules present in the preshock medium, and therefore produce almost no H2 emission. We quantify this result by presenting models of steady shock waves moving into a molecular environment, which show that the H2 molecules are indeed dissociated in the immediate postshock region for higher shock velocities. We argue that the total destruction of molecules by high-velocity shocks is a direct result of the assumption of an instantaneous ‘turning on’ of the flow that is generally done in computing shock models. We present models in which a shock wave gradually accelerates over a period of ∼1000 yr as would be expected, for example, from the ‘turning on’ of an outflow from a young star. We find that such shock waves are indeed able to accelerate significant masses of molecular material to velocities of ∼100 km s−1, and are a plausible explanation for widely observed high-velocity H2 emission.
Outflow
Hydrogen molecule
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We analyze a range of VSP datasets for evidence of fracture related attenuation anisotropy, focusing on three attributes: P-wave attenuation anisotropy, differential shear-wave attenuation and frequency dependent shear-wave splitting. We find examples of all three phenomena and are able to reproduce the behaviour with well constrained, unified, theoretical models. Our results suggest a correlation between attenuation anisotropy and fracture properties. It is apparent that in all cases the reservoir displays much higher attenuation than the overburden. Measuring relative attenuation appears to be more robust than measuring absolute attenuation.
Overburden
Shear waves
Anelastic attenuation factor
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A simple artificial neural network is considered for real-time estimation of excess atmospheric attenuation on a satellite communication link with known attenuation at two frequencies. All atmospheric contributors to attenuation are considered except for gases. The network has a two-layer feed-forward structure with 32 neurons in the hidden layer. Its performance is evaluated by computer simulation using 447 hours of measured attenuation data at 20, 40, and 50 GHz. Estimated attenuation tracks well the measured attenuation at 50 GHz. Estimation error standard deviation is 0.36 dB. RMS error is a function of attenuation: it increases slowly with attenuation, but the ratio of error to attenuation decreases with increasing attenuation. This approach accurately estimates excess attenuation without requiring assumptions, but required training data. (4 pages)
Correction for attenuation
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The objective of this program was to determine the peening stress profiles of rod peened aluminum structural alloys versus shot peened material to define the effective depth of the compressed surface layer.
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Cavitation Shotless Peening (CSP) is a new method of surface modification. Cavitation impacts induced by collapses of cavitation bubbles produce compressive residual stress and work hardening on the material surface. In the case of CSP, shots were not required and that is why we call it Cavitation Shotless Peening. At CSP, cavitation was induced by high-speed submerged water jet with cavitation, i.e., a cavitating jet, whose intensity and occurring region of cavitation impacts could be controlled by parameters such as upstream pressure and nozzle size. The authors have already revealed that lifetime of forging die treated by CSP was extended by about 50% compared with the non-peened forging die. In this paper, in order to make clear the mechanism of increase of lifetime of forging die, alloy tool steel (JIS SKD61) was tested both in non-peened and peened conditions. Compressive residual stress was measured by an X-ray diffraction method. Comparison between the non-peened specimen and cavitation shotless peened specimen revealed that improved mechanical properties leading to longer life-time of the forging die were favourable in CSP.
Work hardening
Alloy steel
Laser peening
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Shock wave simulations have been carried out to 800 planes for BCC iron using a Morse potential. Analysis showed that the Hugoniot conservation relations are obeyed even for non-constant shock profiles if appropriate averages over the complete shock region are used. Various definitions of temperature for the shocked region were examined. The anomalies reported by Tsai and MacDonald (1973) were not observed.
Constant (computer programming)
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We have observed the structure and velocity of laser-driven shock waves in aluminum foils. We have measured shock velocities as high as 13 km/s and shock luminosity rise-times less than 50 ps, and we have inferred pressures of 200 GPa and shock-front thicknesses 0.7 \ensuremath{\mu}m. These results suggest that such techniques may be used for measuring equation-of-state parameters and studying the detailed structure of shock fronts.
Shock front
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