Investigation of an active stressing technique for delaying fracture during laser cutting of alumina

During a variety of high-speed laser cutting operations, full work-piece support is not always practical or possible. As a result, costly premature fractures and associated damage such as chips, burrs, and cracks (ranging from the micro- all the way to the macro-scale) can result. In most instances, the resulting stresses are primarily mechanical in nature and arise from the bending and/or twisting moments from the still attached scrap. Even if the scrap weight remains relatively constant (as is usually the case), mixed-mode fracture is all but inevitable since the remaining (supporting) section is continuously diminishing as the cut progresses. Given the predominantly mechanical, and therefore predictable nature of the resulting stresses, it is conceivable that intentionally induced compressive stresses due to an off-focus laser might be used to control (or at least, delay) such fractures. In this paper, the possibility of using a tailored laser-heating scenario ahead of a progressing cut to "actively" induce compressive thermal-stresses to control fracture, during cutting of cantilevered plate, was investigated. A numerical simulation of this active stressing approach was achieved by using a customized finite-element formulation that was previously employed to model dual-beam laser machining. During the complex 3-D simulations, probabilistic fracture mechanics was used to quantify the influence of the induced compressive stresses on the time and nature of the fracture. The effect of important parameters such as laser beam diameter, incident power density, positioning of the laser with respect to cut, as well as timing were then studied with respect to the goal of reducing and/or delaying the likelihood of fracture.