ENVIRONMENTALLY ASSISTED CRACK GROWTH IN GLASS.

Abstract : Glass exhibits the properties of high compressive strength and low density. However, the surface of unprotected glass contains flaws which grow under low stress in moist conditions to critical size for failure. Various methods were used to investigate and increase the reliability of glass under stress. Data were obtained on failure of unprotected glass plates subjected to biaxial tension at about 50-percent relative humidity. Analysis by extreme-value statistics indicated that the failure condition could be represented by a plane surface in a three-dimensional coordinate system composed of extreme-value probability, log stress, and log time. Removal of surface flaws by etching in 5-percent aqueous hydrofluoric acid increased the mean failure strength from approximately 30,000 psi to 145,000 psi, with a value of 300,000 psi biaxial tension being attained in one case. Increases in lifetimes of one, two, and three orders of magnitude were obtained by protecting the glass from atmospheric moisture by preheating and coating with petrolatum, preheating glass and coating with preheated petrolatum, and experimenting at -30F, respectively. Slight improvement in minimum time to failure was obtained by eliminating the weaker specimens by proof testing. (Author)