Role of Densification in Deformation of Glasses Under Point Loading

It is proposed as a working hypothesis that the many so-called microplastic effects produced by pressure of hard, sharp points on glasses do not occur by plastic flow, but by densification. An interferometric technique is illustrated, by means of which the existence of densification can readily be demonstrated and its magnitude estimated. It is concluded that the "hardness" number of glasses is best interpreted as a measure of the critical stress for yield by densification; that the hardness number has no necessary relation to tensile strength; and that the technology of glass cutting is dependent on residual stresses associated with densification.