Chip Formation During Machining of Glasses

Klocke and Hambriicker [283] defined a viscous shearing or flow zone and calculated the temperature field in the contact zone between a grinding grain and a glass surface. For a certain treatment they calculated the temperature at the surface to be around 700 K. The temperature in the material decreases with increase of away from the surface. 

Schinker [443] explained the increase in temperature during high speed machining of optical glass not only by the external but also by the internal friction caused by the shear stress superimposed by the compressive stress field along the shear planes in the plasticized material. The sliding of material layers against themselves causes friction, which produces heat-supported plasticization. 

Other authors (for instance [89,175,284]) observed the formation of curved continuous chips analogous to those created in metal cutting. These chips display often shear marks on the back. 

Giovannola and Finnic [175] discussed the balance between failure strength (Tf and hypothetical yield stress ay of the material. For normal brittle behavior Tf is smaller than ay, which results in the formation of a crack if a stress is applied. In order for flow to occur prior to brittle fracture, Uf should be greater than ay, which is possible at a local scale, especially at the tip of a tool which causes high-temperature promoting flow. This fact substantiates the assumption that a critical load exists at which a transition from plastic flow to brittle fracture will occur [321]. 

Other authors (such as [47,49,378,413]) took the view that the depth of penetration is the most decisive parameter causing the transition between ductile and brittle behaviour. Bifano et al. [47,48] defined a critical border tension depth dc, see (5.1)  

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