Abstract
In this paper a direct method for obtaining the isostatics or stress trajectories is described. These lines indicate the directions of the principal stresses. The procedure used by the author can be applied almost without regard to the geometrical form of the object under stress. It also possesses the advantage in that the test is nondestructive and that in most cases it will yield values of the stress magnitudes within 10 or 15 per cent of the correct values. The technique consists in covering the surface of the body with a thin coat of lacquer which becomes brittle upon hardening. If the object being tested is painted under zero load and then stressed after the coating has hardened, the layer of lacquer will crack along lines perpendicular to the maximum tensile strain. The companion set of trajectories at right angles can usually be formed through relaxation. As an aid in photoelasticity this procedure has great possibilities as it eliminates the necessity for determining the isoclinic lines which are difficult to obtain.
