Study on the effect of focal position change on the expansion velocity and propagation mechanism of plasma generated by millisecond pulsed laser-induced fused silica

In this work, by controlling the positional relationship between the target and the focal point, the surface damage, shock wave phenomenon and propagation mechanism involved in the plasma generation of fused silica by millisecond pulsed laser irradiation at different focal positions were studied. Laser energy is an important experimental variable. The dynamic process of plasma was detected by optical shadow method, and the influence of surface film damage on plasma propagation and the propagation mechanism at different focal positions were discussed. The study found that the plasma induced by the pulsed laser at the focus position within 0–20 μs exploded, the micro-droplets formed around 20 μs. At the same time, a shock wave is formed by the compressed air, the micro-droplets are compressed under the action of the shock wave recoil pressure, and the micro-droplets channel phenomenon is observed in the micro-droplets. The peak velocities of plasma and combustion wave appear earlier in the pre-focus position than in the post-focus position. This research provides a reference for the field of laser processing using fused silica as the substrate.

The Characteristics of Glued Tensile Shear Strength Constituted of Wood Cut by CO_(2) Laser

The performance of engineered wood products is highly associated with proper bonding and an efficient cutting method.This paper investigates the influence of CO_(2) laser cutting on the wetting properties,the modified che-mical component of the laser-cut surface,and the strength and adhesive penetration near the bondline.Beech-wood is cut by the laser with varying processing parameters,cutting speeds,gas pressures,and focal point positions.The laser-cut samples were divided into two groups,sanded and non-sanded samples.Polyvinyl acetate adhesive(PVAc)was used to bond the groups of laser-cut samples.After assembly with cold pressing,the tensile shear test was carried out.Numerical modelling was carried out to determine the partial elongation and shear strain of the glue line.Based on this,the shear modulus and linear elasticity of the glue line were estimated.Scan-ning electron microscopy was used to assess the adhesive penetration into the porosity structure of the laser-cut samples,and the depth of the heat-affected zone.The laser-cut surface was analysed by Fourier transform infrared spectroscopy.The wetting properties of the laser-cut surface were investigated by using a contact angle goni-ometer.The numerical model of the strain-stress curve confirmed the experimental model.The highest modulus of the linear elasticity of the glue in the numerical calculation belongs to the joint containing laser-cut samples at a gas pressure of 21(bar).The penetration depth of PVAc adhesive into the porosity structure of the laser-cut sam-ples was similar to that of sawn samples.The deepest heat-affected zone in the laser-cut samples was 150µm.A PVAc drop disappeared immediately on the laser-cut surface without sanding,but gradually on the sanded surface.In contrast,the drop on the sawn surface remained with an angle of 32°–48°.The degradation of hemi-cellulose and lignin was proven by the lower intensity of the C=O and C-O Bonds,compared to the sawn surface.

Effects of laser beam welding parameters on mechanical properties and microstructure of AZ31B magnesium alloy

The effects of laser beam welding process parameters such as laser power,welding speed and focal position on mechanical properties and microstructure of AZ31B magnesium alloy were studied.Nine joints were fabricated using different levels of laser power,welding speed and focal position.Tensile properties of the welded joints were evaluated and correlated with the weld zone microstructure and hardness.It is found that the joints fabricated using a laser power of 2 500 W,welding speed of 5.5 m/min and focal position of-1.5 mm yield superior tensile properties compared with the other joints.The formation of very fine grains in weld region,higher fusion zone hardness and uniformly distributed finer precipitates are the main reasons for superior tensile properties of these joints.

Cognitions and questions regarding crustal deformation and location forecasts of strong earthquakes

Using Global Positioning System(GPS) data to analyze the earthquake preparation characteristics of the Kunlun Ms8.1 and the Wenchuan Ms8.0 earthquakes, we review the main research developments of earthquake forecasting and the mechanisms of earthquake preparation using crustal deformation data from recent periods, and discuss the similarities and differences in the scientific approaches adopted by the Chinese and foreign scholars. We then analyze the deformation characteristics of earthquake preparation, with respect to slip and dip-slip faults. Our results show that, in order to understand the relationship between crustal deformation and earthquake preparation, research focus should be expanded from fault-scale to larger scale regions. Furthermore, the dynamic deformation characteristics associated with earthquake preparation must be considered as a multiscale, spatial-temporal process, in order to obtain the necessary criteria for strong earthquake forecasts.