. The strength of materials; a text-book for engineers and architects. and opposite principal stresses, such as theneutral axis of a beam, can be detected by a dark patch orline. If the material is elastic, the colour produced will be ameasure of the difference of the principal stresses at any stress corresponding to a given colour is determinednumerically by experiment by uniformly loading a smallspecimen until the colour produced is the same as at anyparticular point of the model under consideration where thestress is required. In cases where one of the principal stressesis negligi
. The strength of materials; a text-book for engineers and architects. and opposite principal stresses, such as theneutral axis of a beam, can be detected by a dark patch orline. If the material is elastic, the colour produced will be ameasure of the difference of the principal stresses at any stress corresponding to a given colour is determinednumerically by experiment by uniformly loading a smallspecimen until the colour produced is the same as at anyparticular point of the model under consideration where thestress is required. In cases where one of the principal stressesis negligibly small, the stress thus obtained can be taken asequal to the greater principal stress required. 412 THE kSTRENGTH OF MATERIALS In Professor Cokers experiments very successful resultshave been obtained with models of various structional andmachine details cut out of sheet xylonite. Fig. 195 * shows the results of his experiments upon a tie-bareccentrically loaded; the resulting curves of stress agree verywell with the theoretrical straight-line variation, with the. so Fig. 195.—Stresses in Eccentrically Loaded Tie-bar. exception of that at the highest load, in which the materialbegins to yield at the edges and the straight line bends over asshown. In this case the test-piece showed residual stress atthis edge after the load had been removed. In experiments upon models of standard cement briquettesProfessor Coker found t that the maximum stress was about1*75 times the mean stress (cf. p. 78). * Engineering, January 6, Ibid., December 13, 1912. THE TESTING OF MATERIALS 413 Fig. 196 ^ shows the results of the experiments by thesame investigator upon the distribution of stresses in
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