EXTERNALLY IMPOSED THERMO-ELASTIC STRESS-STATES TO CONTROL OR DELAY FRACTURE DURING UNSUPPORTED CUTTING OF ALUMINA

Open Access
Author:
Akarapu, Ravindra
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 24, 2006
Committee Members:
  • Albert Eliot Segall, Committee Chair
  • John Richard Hellmann Jr., Committee Member
  • Barbara Shaw, Committee Member
  • David John Green, Committee Member
Keywords:
  • Modeling of laser cutting
  • Laser Cutting of ceramics
  • Active Stressing
  • Unsupported Cutting
  • Mitigate Damage
Abstract:
ABSTRACT Ceramics are extremely hard materials with high melting points. Their extreme hardness and brittle nature makes it almost impossible to use traditional mechanical methods to machine them. Laser cutting, being a thermal technique with zero contact force involved, is efficient in cutting ceramics into desired shape. However, even the use of lasers does not necessarily preclude damage and fracture, especially for manufacturing situations where the work pieces can only be partially supported during the cutting process. While “nail-bed” supports can readily eliminate this problem, they are not always practical for efficient or high-speed manufacturing of complex shapes. Either delaying or controlling fracture is an economical solution for such situations. 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 thermo-elastic stresses due to an off-focused laser might be used to control (or at least, delay) such fractures. As such this research investigated the possibility of reducing the probability of premature fracture from bending and twisting stresses, by using a tailored laser surface heating scenario around a progressing cut to “actively” induce compressive thermal-stresses. A defocused laser was used in addition to the cutting laser for actively superimposing compressive thermal stresses. Numerical models capable of simulating unsupported laser cutting were developed and used to predict parameters (power, spot size, and location relative to the cutting laser) for the additional defocused laser. Numerical models computed temperatures, thermo-mechanical stresses and failure probabilities during unsupported cutting. These models were run for different sets of parameters for the defocused laser and those which resulted in a decrease in the failure probability were considered favorable set of parameters which could potentially delay premature fractures. Finally experiments were conducted using favorable parameters for the defocused laser to test the viability of the concept of “active stressing”. Experiments indeed confirmed the possibility of delaying failure by using an additional defocused laser.