The irradiation effects are studied, of solid-state laser on four kinds of plates (three of them are made of metal, the other, of composite), in experiments characterized by relatively large laser spot and the presence of surface flow. The thick iron samples, thin aluminum samples and thin carbon fiber/epoxy resin samples are subjected to air or N<sub>2</sub> surface flow, while the box-shaped samples, containing a thin aluminum plate irradiated by laser, are filled with water. It is found that, besides the common role in all four cases cooling the plate by convective heat transfer, the fluid plays other different roles in different case influencing the dynamic response of the plate. The roles of the fluid in each case are described either with analytical boundary conditions or with differential equations, which are then incorporated into computational models. Numerical simulations are carried out, with results compared with the experiment results to explain the irradiation effects.
Experiments are performed to investigate the laser irradiation effects on thin aluminum alloy sheets subjected to tangential airflow. The wind blower generated airflow with a speed of about 100 m/s along the surface. For comparison, experiments in the absence of airflow are also conducted. Moreover, in order to know whether the combustion reaction takes place during the irradiation, we vary the composition of flow from air to nitrogen. The displacements of the sheets center are measured to see whether the tangential flow has a mechanical effect. The maximum temperature of the sheet is lower than 550 ℃ after 2 seconds irradiation with the laser power density of 173W/cm<sup>2</sup>. Accordingly, the structural parameters of aluminum alloy do not have distinct change and so do the features of sheets. The temperature curves in the air flow and nitrogen flow keep the same and both lower than that in no flow case. Moreover, the displacements measured in three cases do not have obvious difference. These experiment results indicate that the combustion reaction can hardly happen and the tangential flow only has a cooling effect. The maximum temperature reaches 600 ℃ when the laser power density rises to 400 W/cm<sup>2</sup>. Such a high temperature makes that the elastic modulus of aluminum alloy drops rapidly, which greatly softens the alloy sheets. The plastic distort of irradiated sheets confirmed this process. When the power density rises to 450W/cm<sup>2</sup> big melt-through phenomenon is observed and there is viscous dripping under gravity in the no-flow case. However, in air flow and nitrogen flow, we can see the removal of macroscopic unmelted pieces of aluminum alloy sheet. The results indicate that the tangential flow mainly has two effects including cooling the target and removing the unmelted metal when the material is fully softened.
The irradiation effects of 976nm continuous-wave laser on carbon fiber reinforced E-51 resin composite is studied experimentally, with a 0.4Ma tangential airflow or 0.4Ma tangential nitrogen gas flow on the target surface. In order to simulate the thermal response of fiber reinforced resin composite materials subjected to combined laser and tangential gas flow loading, a three-dimensional thermal response model of resin composite materials is developed. In the model, the thermal decomposition of resin is described by a multi-step model. The motion of the decomposition gas is assumed to be one-dimensional, for the case that the laser spot is significantly larger than the thickness of the sample. According the above assumption, the flow of the decomposition gas is considered in the three-dimensional model without introducing any mechanical quantities. The influences of the tangential gas flow, the outflow of the thermal decomposition gas and the ablation（including phase change ablation or oxidative ablation）of the surface material on the laser irradiation effects are included in the surface boundary conditions. The three-dimensional thermal response model is calculated numerically by use of the modified smooth particle hydrodynamics (MSPH) method which is coded with FORTRAN. The model is tested by experimentally measuring the temperature profiles during carbon fiber reinforced E-51 resin composite subjected to combined laser and tangential gas flow. The predicted temperature profiles are in good agreement with experimental temperatures obtained using thermocouples.