Xianqian Wu

Shock pressure induced by glass-confined laser shock peening: Experiments, modeling and simulation

The shock pressure generated by the glass confined regime in laser shock peening and its attenuation in the target material are investigated. First, the particle velocity of the target back free surface induced by laser generated shock pressure of this regime is measured using a photonic Doppler velocimetry system. The temporal profile of the particle velocity at the back free surface, where the elastic precursor is captured, manifests a powerful diagnostic capability of this newly developed photonic Doppler velocimetry system for tracking the velocity on short time scales in shock-wave experiments. Second, a coupling pressure analytical model, in which the material constitutive models of confined layers and target material are considered, is proposed to predict the plasma pressure profile at the surface of target. Furthermore, using the predicted shock pressure profile as the input condition, the dynamic response of the target under the shock pressure is simulated by LS-DYNA. The simulated back free surface velocity profile agrees well with that measured by the photonic Doppler velocimetry system. Finally, the attenuation behavior of stress waves and particle velocities in the depth of the target is analyzed, and it indicates an exponential decay. The corresponding empirical formulas for the attenuation behavior are given based on the numerical results

Measurement of fast-changing low velocities by photonic Doppler velocimetry

Despite the increasing popularity of photonic Doppler velocimetry (PDV) in shock wave experiments, its capability of capturing low particle velocities while changing rapidly is still questionable. The paper discusses the performance of short time Fourier transform (STFT) and continuous wavelet transform (CWT) in processing fringe signals of fast-changing low velocities measured by PDV. Two typical experiments are carried out to evaluate the performance. In the laser shock peening test, the CWT gives a better interpretation to the free surface velocity history, where the elastic precursor, main plastic wave, and elastic release wave can be clearly identified. The velocities of stress waves, Hugoniot elastic limit, and the amplitude of shock pressure induced by laser can be obtained from the measurement. In the Kolsky-bar based tests, both methods show validity of processing the longitudinal velocity signal of incident bar, whereas CWT improperly interprets the radial velocity of the shocked sample at the beginning period, indicating the sensitiveness of the CWT to the background noise. STFT is relatively robust in extracting waveforms of low signal-to-noise ratio. Data processing method greatly affects the temporal resolution and velocity resolution of a given fringe signal, usually CWT demonstrates a better local temporal resolution and velocity resolution, due to its adaptability to the local frequency, also due to the finer time-frequency product according to the uncertainty principle.

Cyclic deformation and strain burst behavior of Cu–7at.%Al and Cu–16at.%Al single crystals with different orientations

Abstract [ 1 12] Cu–7at.%Al single crystals as well as [ 1 23], [ 1 17] and [023] Cu–16at.%Al single crystals were cyclically deformed under constant plastic strain control at room temperature. The cyclic hardening curve of [ 1 12] Cu–7at.%Al single crystal can be divided into three stages, namely initial non-hardening stage, rapid hardening stage and saturation stage. The cyclic stress–strain (CSS) curve exhibited a plateau region in the range of γpl, from 1.1×10−3 to 4.5×10−3. The saturation stress of the plateau is about 27 MPa. The occurrence of strain burst depends on the applied plastic strain amplitude γpl. No strain burst was detected when γpl, was below 4.4×10−4 or above 1.1×10−3. The cyclic deformation and strain burst behavior of [ 1 23], [ 1 17] and [023] Cu–16at.%Al single crystals are different from that of [ 1 12] Cu–7at.%Al single crystal. The cyclic deformation of all three differently oriented Cu–16%Al single crystals is unstable with the frequent occurrence of irregular strain bursts. The specimens cyclically hardened at a very low rate and saturation was not attained at high strain amplitudes. The experimental results indicate that the crystallographic orientation has almost no effect on the cyclic deformation and strain burst behavior of Cu–16at.%Al single crystals. The evolution of slip bands during cyclic deformation in both Cu–7at.%Al and Cu–16at.%Al single crystals is similar to the development of Luders band. The percolation of slip bands along the gauge length is generally accompanied by strain bursts. The above experimental results were explained in terms of the variation of slip mode with Al content in the alloys and the corresponding strain localization during cycling.

Atomistic study of temperature and strain rate-dependent phase transformation behaviour of NiTi shape memory alloy under uniaxial compression

Molecular dynamics simulation was conducted to investigate the phase transformation behaviour of nickel–titanium (NiTi, 50%-50% at.%) nanopillar under uniaxial compression at loading rates varying from 3.30 × 107 to 3.30 × 109 s−1 and at temperatures varying from 325 to 600 K. The phase transformation of NiTi was observed to be sensitive to loading rates and temperatures. The phase transformation stress of B2 → B19 increased with increasing temperature while it was insensitive to loading rate. The phase transformation stress of B19 → B19′ → BCO increased with increasing strain rate and decreasing temperature. In addition, reverse phase transformation was observed during compression due to the interaction between the phase transformation of B19 → B19′ → BCO and the deformation twinning/dislocation slide-induced plasticity of the BCO phase, leading to different residual crystal structures after loading. Moreover, a diagram for the phase transformation behaviour of NiTi in the simulated ranges of strain rate and temperature was obtained, from which the contrary experimental observations on the phase transformation behaviour of NiTi from the studies of Nemat-Nasser et al. (Mech. Mater. 37 (2005) p.287) and Liao et al. (J. Appl. Phys. 112 (2012) p.033515) at various strain rates could be well explained.

Dynamic response of shear thickening fluid under laser induced shock

The dynamic response of the 57 vol./vol. % dense spherical silica particle-polyethylene glycol suspension at high pressure was investigated through short pulsed laser induced shock experiments. The measured back free surface velocities by a photonic Doppler velocimetry showed that the shock and the particle velocities decreased while the shock wave transmitted in the shear thickening fluid (STF), from which an equation of state for the STF was obtained. In addition, the peak stress decreased and the absorbed energy increased rapidly with increasing the thickness for a thin layer of the STF, which should be attributed to the impact-jammed behavior through compression of particle matrix, the deformation or crack of the hard-sphere particles, and the volume compression of the particles and the polyethylene glycol.

Effect of surface reflectivity on photonic Doppler velocimetry measurement

While photonic Doppler velocimetry (PDV) is becoming a common diagnostic for tracking velocity in shock physical experiments, its validity on measuring surfaces with different reflectivity is not studied. This paper investigates the effects of surface reflectivity on PDV measurement for tracking back free surface velocity in laser shock processing. Credible measurement results for coarse polished surfaces with low reflectivity are obtained, whereas fine polished surfaces with relatively high reflectivity lead to heterodyne fringes with high frequency and corresponding unreasonably fast velocities. This phenomenon reported in the paper is somewhat inconsistent with the general view that PDV has remarkable robustness to large changes in surface reflectivity. The reason might be ascribed to multiple reflections of light, which cause the generation of multiple Doppler shifts. The mixing of the reference light and those Doppler-shifted lights brings out high frequency heterodyne fringes resulting in high velocity. Low surface reflectivity is better suited for PDV measurements.

Geometrical scaling law for laser shock processing

Scaling approach to laser shock processing is studied by dimensional analysis and numerical simulation. The essential dimensionless parameters controlling the shock effect are studied, and a geometrical scaling law correlating the input laser parameters and the output strengthening effect parameters is presented. The numerical results show that there is a competition controlling mechanism between thickness of confined overlay and laser duration for the surface residual stress; the plastically affected depth increases linearly with increasing laser duration, increases quadratically with increasing laser power density, and is almost independent with the thickness of confined overlay. Based on the results, a window of the optimal working parameters is presented.

Experimental study on pressure, stress state, and temperature-dependent dynamic behavior of shear thickening fluid subjected to laser induced shock

The dynamic response of the 57 vol./vol. % dense spherical silica particle-polyethylene glycol suspension at high pressure was investigated through short pulsed laser induced shock experiments by measuring the back free surface velocities of aluminum-shear thickening fluid (STF)-aluminum assembled targets. The results showed that the attenuation behavior of shock wave in the STF was dependent on shock pressure, stress state, and test temperature. The measured back free particle velocities of the targets and shock wave velocities in the STF decreased with the decrease in shock pressure while shocked at the same stress state and the same test temperature. In addition, two types of dragging mechanisms in the STF were observed while shocked at different stress states. For a uniaxial strain state, the impact induced jamming behavior in the STF is the dragging mechanism for the attenuation of shock wave, and a critical shock pressure was required for the impact induced thickening behavior. However, while the shock wave transformed from a uniaxial strain state to a dilatation state after transmitted to a certain distance, beside the dragging effect of impact induced jamming behavior, a strong dragging effect, induced by shear induced thickening behavior, was also observed

Atomistic study on shock behaviour of NiTi shape memory alloy

Abstract The shock behaviour of NiTi shape memory alloy is investigated by using molecular dynamics simulation. The nano-pillar samples of the alloy are subjected to the impact of a piston with a velocity of 350 m/s at initial environment temperatures of 325 and 500 K. At 325 K, we observe two different pathways of the formation of BCO phase, the gradient twins, and the detwinning phenomena, strongly depending on the local stress and the deformation state. As the initial temperature increases to 500 K, the plasticity is dominated by the dislocation movements rather than the twinning at 325 K. The phase transformation and plasticity result in stress attenuation when the stress wave propagates through the nano-pillar. Furthermore, it is interesting to note that multiple stress peaks occur due to the formation of local complex atomic structures with various wave speeds, leading to the catch up and overlap of the stress waves.

Deformation behavior of single crystal silicon induced by laser shock peening

Laser shock peening can significantly improve the fatigue life of metals by introducing plastic deformation and compressive residual stresses near the surface. It has been widely applied on metals for surface strengthening. The plastic deformation behavior of brittle materials such as single crystal silicon under LSP is rarely studied. In the present research, the surface integrity and residual compressive stress of P-type single crystal silicon in <100< orientation shocked by LSP at imposed high temperature were measured to investigate the plastic deformation mechanism at high temperature and high compressive stress. The surface morphology of shocked silicon, observed using optical microscopy, showed that the cracks on the shocked silicon surface became less and the fragments were smaller while the temperature or the laser power density increased, which indicates that the plasticity of single crystal silicon is improved at high stress and temperature. However, the excessive laser power density would lead to local damage of the shocked silicon. The residual stress, measured using Raman scattering method, showed that the compressive residual stresses with magnitude of a few hundreds of MPa were introduced in the surface layer of silicon after LSP at imposed high temperature, and it increased with respect to the temperature and the laser power density. The experimental result indicates the material has experienced the plastic deformation and provides a potential processing method to improve the mechanical behavior of brittle material like single crystal silicon.