In this paper, the mechanical erosion effect during continuous-wave (CW) laser ablation of GFRP (Glass Fiber Reinforced Polymer) was studied. It happened under the condition of subsonic tangential airflow. In order to know the effect of mechanical erosion in the ablation process. An experimental device was designed to collect mechanical erosion products. The total mass loss of GFRP in the process of laser ablation and mass loss caused by mechanical erosion were measured by experiments. Then the percentage of mechanical erosion was calculated. The experiments were made at different airflow velocities and different laser power densities. Spot center temperature was measured during the experiment. The results show that the percentage of mass loss caused by mechanical erosion will increase and tend to be constant with the increase of airflow velocity. The percentage of mass loss caused by mechanical erosion will increase with the increase of laser power density. The percentage of mass loss caused by mechanical erosion will increase and tend to be constant with the increase of laser irradiation time. The mechanical erosion effect of GFRP is related to its laminated structure. The laminated structure can affect the mechanical erosion mechanism of the material surface. It will cause the regular change of the surface temperature of the material.
Non-uniform temperature field in optical transparent glasses induced by high power Gaussian laser irradiation would introduce temperature gradient which will cause thermal lens effects inside materials. Such thermal lens influences on long pulse laser processing are usually omitted in previous studies. However, thermally-induced refractive index change in situ could have a relatively large impact on subsequent incident laser especially under the action of high power laser. In order to obtain a quantitatively transient results of thermal lens effects caused by long pulse laser, we carried out numerical simulations of temperature rise, refractive index change and their influences on light trail in K9 glass heated by a millisecond pulse laser. Different laser energy and glass thickness were applied to reveal the dependence of thermal lens and laser and glass parameters. Results show that the thermal lens effects would increase refractive index in K9 glass and reconverge subsequent laser beam consequently whose amplitude is proportional to incident laser power and glass thickness. The simulation results are compared with the previous experimental results and are consistent in magnitude of refractive index change. But there is a certain gap in refractive index field distribution in the direction of laser radius due to the neglect of influence of refractive index raise to susequent lasers in temperature simulations. This work would provide a reference in the field of millisecond pulse laser processing optical transparent materials.
Time-resolved surface temperature of single crystal silicon was measured by an infrared radiation pyrometer. The silicon sample was irradiated by two pulsed Nd:YAG lasers with pulse duration of 1ms superposed by 7ns pulses, referred to as combined pulse laser (CPL). The change of the damage radius with the millisecond (ms) laser energy density was studied, and then compared with that of single ms laser irradiation. An axisymmetric numerical model was established for calculation of the temperature field distribution while silicon was irradiated by single ms laser and CPL, respectively. Compared with experimental results, the CPL-silicon damage mechanism was discussed.
Laser-induced damage of optical glasses has been investigated for more than fifty years. Due to the residual scratches, inclusions and other forms of defects at surfaces of optical glasses after the processes of grinding and polishing, it is well known that the sample surface can be damaged more easily than bulk. In order to get the relationship between the damage threshold and the location of the laser spot, we carried out damage experiments on K9 glasses with a 7ns pulse laser. Since ns pulse laser-induced damage of optical glasses always accompanies with the generation of the plasma, a optical microscope connected with a CCD camera was used to observe the plasma flash, which can provide a real time detection of damage sites. The laser pulse was first focused into the bulk, then the spot was moved toward the direction of incident laser beam step by step until the beam was completely focused in ambient air. Damage threshold curves were measured for each focus position, and low thresholds and high thresholds were extracted from those curves. Finally, the relationship between damage thresholds and focus position was analyzed.
A model containing an inclusion matrix heated by a millisecond laser is proposed to calculate temperature and thermal stress fields of K9-glass using a finite element method. First, the evolution of temperature and thermal stress fields is analyzed. Results show that both the upper and lower surfaces are damaged. K9-glass is primarily damaged by the combination of radial and axial stresses. Calculated damage morphology is mainly determined by radial stress. Then damage morphology evolution with the increase of the incident laser energy is investigated, which shows that damage area spreads inward from both the front and rear surfaces. Finally, experimental results of long-pulse laser damage of K9-glass are analyzed. The comparison of numerical results with experimental observations shows a good correlation in damage morphology, which indicates that the built inclusion matrix model is applicable to long-pulse laser damage in K9-glass.
There are residual scratches, inclusions and other forms of defects at surfaces of optical materials after the processes of grinding and polishing, which could either enhance the local electric field or increase the absorption rate of the material. As a result, the laser-induced damage threshold at the surface of the material is reduced greatly. In order to study underlying mechanisms and process of short pulsed laser-induced damage to K9 glass, a spatial axisymmetric model where the K9 glass was irradiated by a laser whose wavelength and pulse width are respectively 1064nm and 10ns was established. Taking into account the fact that the surface of the K9 glass is more likely to be damaged, 2μm-thick layers whose absorption coefficients are larger than bulk were set at both the input and output surfaces in the model. In addition, the model assumed that once the calculated tensile/compressive stress was greater than the tensile/compressive strength of K9 glass, the local absorption coefficient increased. The finite element method(FEM) was applied to calculate the temperature and thermal stress fields in the K9 glass. Results show that only the temperature of a small part of interacted region exceeds the melting point, while most of the damage pit is generated by thermal stress. The simulated damage morphology and the size of the damage region are consistent with those reported in literatures, which indicates that the model built in our work is reasonable.
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