# Failure Response of Simultaneously Pre-Stressed and Laser Irradiated Aluminum Alloys
## 3 Results and Discussion
### Failure Behavior  
Figure 1a-d demonstrates the failure behavior as a function of laser power density under fixed preloading speed for the Al-6061 type specimens. In Figure 1a, under the lowest laser power density of the 1.1 kW/cm\({}^{2}\) condition, the specimen's yield strength drops smoothly up to complete structural failure. This is because that combination of very low loading speed and low laser power density induced the very small temperature rise even less then the melting point, so concerning the specimen's deformation, and the softening process was relatively slow; while for higher laser power densities (Curves 2-4), the fractured process was shortened with the sharp drop in the specimen's yield strengths with the increase of irradiated laser power densities. As the laser power density increased, the specimen's temperature rose, which boosted the thermal activation process by accelerating the structural defects, annihilating the dislocations and enhancing the softening mechanism. Since the Al-6061 alloy is precipitation hardened and its primary strengthening development is through precipitate growth under controlled heating (aging) to a required state, combined (laser and tensile) loading and elevated temperature induced further precipitate growth (over aging) and strength reduction with the increase of laser power density [25]. In Figure 1b, for the lower laser power densities of 1.1 and 1.6 kW/cm\({}^{2}\), the specimens followed the same failure pattern with a slight difference in failure time, and both curves overlapped each other. The similarity in the deformation behavior might be attributed to limited thermal activated (movement of structural defects, etc.) processes up to specific absorbed energy level for the employed preloaded speed (1 mm/min). Since it has been observed that in some situations the plastic deformation can be halted when the specimen is heated through certain temperature ranges [26],the overlapping of the latter part of the curves represents the delayed time in the specimens' failure instead of sudden fracture. However, the specimens fractured more rapidly with the further increase in laser power density to 2.2 and 2.7 kW/cm\({}^{2}\), respectively. In Figure 1c,d under the preloading speeds of 2 mm/min and 4 mm/min respectively, for the lowest laser power density of 1.1 kW/cm\({}^{2}\), the specimens took a comparatively long time to reach the yield strength and complete failure. Moreover, Curve 1 in Figure 1c and Curves 1 and 2 in Figure 1d showed comparatively different behavior. A more delayed decrease of stress over a considerable range of plastic deformation caused by the adiabatic character of the deformation process at a certain temperature range is observed. After reaching a specific temperature, the load sustained by the specimen started to drop, and fracture occurred shortly. The reason behind the initially slow strength degradation is the less sensitivity to modest temperature increases of strengthening precipitates in Al-6061 specimens, until a temperature where precipitates start to lose coherency [10]. When the preload continues to add strain energy to the continually-heated specimen with limited possibilities for dislocation movement, it eventually becomes energetically favorable to ease the load by fracturing. For the higher laser power densities, the specimen's failure occurred in a similar pattern, and relatively fast reduction of the stress can be seen for all of the preloaded speeds. Figure 2a-d exhibits the failure behavior as a function of preloaded speeds under varying laser power density for the Al-7075 type specimens. In Figure 2a-d, under the lowest laser power density of 1.1 kW/cm\({}^{2}\), the specimen's yield strength drops smoothly and at the end slowed up before complete failure. From this, it can be concluded that loading speed could not produce any affect in mobilizing the thermo-mechanical properties with such low laser power density.