High-speed cutting, especially of enamel, proceeds by brittle fracture. Tooth structure, similar to other materials, undergoes brittle and ductile fracture. Brittle fracture is associated with crack production usually by tensile forces. Ductile fracture involves plastic deformation of material usually proceeding by shear forces. Extensive plastic deformation also may produce local work hardening and encourage brittle fracture. Low-speed cutting tends to proceed by plastic deformation before tooth structure fracture.Ref: Sturdevant's art and science of operative dentistry. 4th edition page 340