Optical studies of some impact phenomena and surface hardness measurements

Abstract

An introduction to the phenomena of pressure crack figures is given with special reference to static-impact figures produced on glass and diamond. The optical techniques and the apparatus used are described, including the Inter-ferometric technique used for the detection and measurement of the surface distortions for crack figures obtained first on glass, and then on diamond. The spherical impactors used were of steel (for glass), tungsten carbide and diamond (mainly for diamond). A study is made of the fracture strength properties of ten different types of optical glass and the results are compared with other properties of the glasses, including measurements of their surface hardnesses obtained by an abrading method described in the Appendix. Orientated crack figures have been produced on three different faces of diamond, the shapes being hexagonal on the octahedral and the dodecahedral faces, and square on the cubic face. For the octahedral and a cubic face, the development of these figures is studied as the load is gradually increased; and the mechanism of the crack formation and the accompanying cracking effects within the body of the crystal are discussed in terms of easy cleavage and shattering, respectively. It is found that the octahedral face is definitely the least resistant to fracture by this method, the cubic face appearing to be most resistant: also the critical stresses involved are found to be considerably less than a theoretically calculated value which indicates some form of flaw distribution over the surface analogous to the Griffith cracks for glass. It has also been seen that the octahedral face of diamond can be cracked in this way by using Tungsten Carbide or Sapphire balls, although in the latter case the tip of the ball was shattered by the occurrence of multiple slip. Both for glass and diamond, the observations obtained from the interferograms of the surface distortions is considered to offer strong evidence for the existence of micro plastic flow. No signs of micro slip could be detected in the studies on diamond except possibly in connection with the crystallo-graphic shattering which occurred internally for one test on the octahedral stone.