The solid inclusion gauge - A technique for measuring stress in composite propellant charges

The behavior of an inclusion in a host material subjected to a stress system depends primarily on the ratio of the tangent moduli. An inclusion of a suitable material used in the form of a gauge will give an identifiable photoelastic fringe pattern. This pattern is related to the biaxial stresses in the diametral plane of the gauge and is independent of the actual modulus and strains in the host material provided that the moduli ratio is more than 300. A programme of work has been carried out to verify the use of such an inclusion gauge in composite propellant materials. The moduli ratio for the gauge and host material was such that the gauge acted as a rigid inclusion and the study also showed that the gauge response is virtually independent of Poisson's ratio. The values of gauge sensitivity obtained experimentally at various biaxial ratios of applied stresses were close to those derived theoretically, and the values of the two principal stresses in the plane of the gauge are obtained by calculation from three readings taken at discrete positions in the gauge.