An investigation of microcrack formation on machined surfaces.

In recent times a number of workers have pointed out the importance of microcrack formation to the machining process and to surface integrity requirements of machined components. Microcracks have been identified within the shear zone and their presence used to explain some aspects of the chip formation process. It is clear that microcracks represent a major feature in the shear zone in machining, particularily when the workpiece is a two-phase material.The presence of microcracks in the machined surface was reported some fifteen years ago and confirmed recently. A preliminary study carried out on a number of materials machined under various cutting conditions has indicated that microcracks are produced in machined surfaces as a result of machining.However, to the present time there has been no systematic study of the formation of microcracks in the machined surface, and their extent, dimensions, and the conditions under which they are produced are completely unknown. In so far as microcracks may affect the fatigue, corrosion and other properties of the machined surface, it is important that a method of quantifying microcrack formation should be found. A feasibility study has been carried out which has led to the use of the transmission electron microscope as a tool to quantify microcrack formation in the machined surface. A technique based on transmission electron microscopy is described in the present thesis along with its application.Two new parameters, the microcrack area ratio "Ma" and the microcrack density "Md" are introduced and defined. The extent of microcrack formation in the machined surface is studied as a result of varying: (i)- cutting speed, (ii)- undeformed chip thickness, (iii)- rake angle, (iv)- workpiece hardness.The results are discussed and conclusions drawn.Finally, a number of suggestions for future work are formulated which mark the start of a whole new area of research in surface microcracking.