Determination of third-order elastic constants from stress-shifted resonance frequencies, observed by diffraction of light. Examples: aluminium alums of K, NH4, Cs and CH3NH3

Abstract

Resonance frequencies of thick plane-parallel plates are easily detected by diffraction of light with an accuracy better than 1 p.p.m. The method, which has been earlier employed for the determination of thermoelastic properties, proved to be sufficiently sensitive also for the measurement of stress-induced shifts of resonance frequencies, and is therefore suited for the determination of third-order elastic constants. The technique is easier to handle and much less expensive than other methods in use today. The complete sets of third-order elastic constants for KAl(SO4)2.12H2O, NH4Al(SO4)2.12H2O, CsAl(SO4)2.12H2O and CH3NH3Al(SO4)2.12H2O have been measured. All constants possess negative values. These alums exhibit a behaviour similar to that of the structurally related caesium halides, with the exception of ammonium alum which shows anomalously small constants.