Active stressing and the micromanipulation of stress-states for delaying fracture during unsupported laser cutting

During a variety of high-speed cutting operations that can include both laser and traditional saw methods, full workpiece support is not always practical or even possible. As a result, costly premature fractures and associated damage such as chips, burrs, and cracks (micro- to macroscale) 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. Under these conditions, mixed-mode fracture is all but inevitable since the supporting section is continuously diminishing as the cut progresses. Given these conditions, 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, a technique of using a tailored CO₂ laser-heating scenario ahead of a progressing cut to "actively" induce compressive thermoelastic stresses to control fracture of a cantilevered plate was developed with guidance from numerical simulations. Simulations of the activestressing approach were achieved by using a customized finite-element formulation that was previously employed to model dual-beam laser machining. However, in this instance probabilistic fracture-mechanics was used to quantify the influence of the induced compressive-stresses on the time and nature of the fracture. Experiments were also conducted to test the feasibility of the active-stressing approach. The effect of important parameters such as the beam diameter, incident power density, and the positioning of the second beam with respect to the progressing cut was then studied with the goal of reducing and/or delaying the likelihood of fracture.