# Failure Response of Simultaneously Pre-Stressed and Laser Irradiated Aluminum Alloys
## 3 Results and Discussion
### Failure Time  
Figure 3 represents the failure time (the time from the start of laser irradiation to the failure/fracture of the specimen) for both Al-6061 and Al-7075 as a function of laser power density for various preloaded speeds. For Al-6061 type specimens, with the increase of laser power density, failure time decreases. However, the decreasing trend is irregular for varying preloaded speeds. For the lowest laser power density of 1.1 kW/cm\({}^{2}\), with the increase of loading speed, there is a remarkable reduction in failure time especially for the maximum loading speed of 4 mm/min. However, for this power density (1.1 kW/cm\({}^{2}\)), the specimens took a comparatively long time for complete failure. The failure time depends both on failure temperature, as well as on the temperature rise rate. The failure temperature belongs to preloading speed, while the temperature rise rate depends on laser power density. By the increase of laser power density to 1.6 kW/cm\({}^{2}\), the failure time drops to almost half or even less values under the same preload speeds, except for 1 mm/min. A further increase in laser power density to higher values causes more reduction in the failure time for the fixed loading speeds. By comparing different curves belonging to different loading speeds, it is found that there is a random effect of loading speeds on Al-6061 failure time. This non-uniformity is due to different thermo-mechanical responses to varying experimental conditions. There is a non-uniform distribution of local temperature and local residual stress during the irradiation process under different conditions. This non-uniformity (depending on the temperature rise rate) gives rise to a difference in the variation trend of thermophysical and mechanical properties, like thermal conductivity, the thermal expansion coefficient, yield strength, etc. The sufficiently high temperature rise rate may result in very high local temperature and local residual stress. The former may induce local softening, and the latter may cause  
Figure 2: Failure behavior of Al-7075 specimens for various laser power densities under the fixed preloading speeds. (**a**) 0.5 mm/min; (**b**) 1 mm/min; (**c**) 2 mm/min; (**d**) 4 mm/min. local damage, which may degrade the mechanical properties of specimens and cause the specimen failure, even at a low level of preloaded speed or stress [24]. For Al-7075 specimens, the failure time also decreased by increasing the laser power density. As compare to Al-6061, Al-7075 showed some regularity in the reduction in failure time for both laser power density and preloaded speeds. The failure time for the lowest power density (1.1 kW/cm\({}^{2}\)) is largest for the 1 mm/min case and relatively has smaller values for higher power densities. In addition, when the laser power density increases from 1.1 to 2.7 kW/cm\({}^{2}\), the failure time decreased by 14.7 s, 15.9 s, 14.1 s and 11.2 s, respectively, corresponding to preloaded speeds of 0.5, 1, 2 and 4 mm/min. This indicates that for higher preloaded speeds, there is a smaller impact of laser power density on the failure time except in the case of 1 mm/min preloaded speed. According to Figure 3, the laser power density and average failure time relationship curve can be obtained as shown in Figure 4. An empirical formula is also obtained by fitting the data points, and then the failure time for different laser power densities can be estimated accordingly. Figure 4: Fitting results of the failure time of Al-6061 and Al-7075 type specimens against laser power density. Figure 3: Failure time of Al-6061 and Al-7075 type specimens against laser power density, under different preloaded speeds.