The present study is limited to low modulus materials like solid rocket propellant in which rigid body motions are minimum and the fringe anomaly can be directly related to a defect in the object. The fringe-spacing analysis technique developed is based on experimental and theoretical data generated. The fringe anomaly corresponding to a defect is quantified in terms of the change in fringe-spacing, in comparison to the defect-free region. From the general equation to the holographic interferometric fringe pattern for a representative model [1], an expression for the fringe-spacing in the direction of applied load has been derived as a function of the defect size and the spatial location of the fringe. Theoretical fringe-spacing curves are plotted based on this expression, to generate the necessary data on fringe-spacing variation corresponding to a defect in comparison to a defect- free case, for establishing the theoretical basis of the technique. Calibration experiments are also carried out on solid propellant specimen to verify the theory. A magnification factor for the fringe anomaly corresponding to a defect has also been introduced for defect sizing.

The technique is very useful for identifying fringe anomalies corresponding to inner or very small defects for which visual fringe anomalies are normally absent. Defect sizing is possible by the standardisation procedure developed using theoretical as well as experimental methods. The sensitivity of the technique and accuracy in defect sizing are also presented. The results are repeatable if the holography set up and loading conditions are not altered and rigid body motions are minimised.

REFERENCES

1. V. R. Ravindran, A. V. S. S. S. R. Sarma and V. U. Nayar, Holographic Nondestructive Testing of Low Modulus Materials", Materials Evaluation Vol. 50, pp. 1058-1065, 1992
2. V. R. Ravindran, A. V. S. S. S. R. Sarma and V. U. Nayar, Holographic Nondestructive Testing of Low Modulus Materials Using Bending Loading", British Journal of NDT Vol. 35, pp. 429-432, 1993
3. A. V. S. S. S. R. Sarma, T. G. G. Kutty, V. R. Ravindran, et. al., `` Holographic Nondestructive Testing (HNDT) of Propellant Grains", Materials Evaluation Vol. 42, pp. 1025-1028, 1985