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Research Papers

# Active Stressing and the Micromanipulation of Stress-States for Delaying Fracture During Unsupported Laser Cutting

[+] Author and Article Information
R. Akarapu, A. E. Segall

Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA 16802

J. Manuf. Sci. Eng 130(6), 061004 (Oct 09, 2008) (10 pages) doi:10.1115/1.2977824 History: Received July 11, 2007; Revised July 14, 2008; Published October 09, 2008

## Abstract

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 active-stressing 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.

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## Figures

Figure 3

Weibull plot depicting the variability in the strength of alumina (AD-96) at 25°C, 400°C, 800°C, 1000°C, and 1200°C, respectively

Figure 7

(a) Risk of rupture intensities on the surface, (b) surface zone with failure probability greater than 0.01, (c) S11 (Pa) in the surface zone, and (d) principal stress directions in the surface zone

Figure 8

Principal stress S11 ahead of the cut and along the cutting direction for Xcut=22.68mm

Figure 4

Transient temperature distribution during unsupported cutting at time=0.25s

Figure 5

Comparison of measured and computed failure probability curves for various criteria; the length of cut is normalized by the total length of cut (L=26.4mm)

Figure 6

(a) Principal stress S11 (Pa) developed during unsupported cutting (coarse model) and (b) principal stress S11 (Pa) developed during unsupported cutting (submodel)

Figure 1

Experimental setup and geometrical model of the unsupported cutting

Figure 2

Typical premature fracture during unsupported cutting with Xcut denoting the length at which premature fracture occurred

Figure 9

Principal stress S11 ahead of the cut and along the cutting direction for Xcut=22.68mm

Figure 10

Principal stress S11 ahead of the cut and along the cutting direction for Xcut=22.68mm

Figure 11

Probability of failure plots for active-stressing experiments showing a distinct and advantageous shift via active-stressing

Figure 12

Scatter plot between normalized length of cut and angle of fracture path

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