. The strength of materials; a text-book for engineers and architects. plate of ductile material. The lines across the bar indicatelines initially parallel and the deformation is such as tocause tensile and compressive stresses across the rectanglesshown in dotted lines. Alongside, in diagram {b), is showna sketch of an actual shear failure of a special phosphorbronze taken from a paper by Mr. E. G. Izod.* The most * Proc. I. M. E., 1906. BEHAVIOUR OF MATERIALS UNDER TEST 65 accurate method of testing for shear is by torsion u^^on thintubes. TIMBER Timber is not an isotropic material, its
. The strength of materials; a text-book for engineers and architects. plate of ductile material. The lines across the bar indicatelines initially parallel and the deformation is such as tocause tensile and compressive stresses across the rectanglesshown in dotted lines. Alongside, in diagram {b), is showna sketch of an actual shear failure of a special phosphorbronze taken from a paper by Mr. E. G. Izod.* The most * Proc. I. M. E., 1906. BEHAVIOUR OF MATERIALS UNDER TEST 65 accurate method of testing for shear is by torsion u^^on thintubes. TIMBER Timber is not an isotropic material, its strengthproperties are not the same in all directions, and there isconsiderable variation in the results of tests for the samekind of timber. This is because the strength depends uponthe age of the timber, its dryness, the portion of the treefrom which it has been cut, and even upon the kind of soilupon which it has been grown. The strength of timber is greatest when the weight of ,:cr2^52S3^ -VV- -^M- -rs^ 55 V ?//ym0i^ ^ (CV.) iyv/. ^ Fig. 31.—Shear moisture is about 5 per cent, of the total and decreases toabout half this when the timber is green or very wet. The moisture is usually determined by taking shavings byboring and weighing them before and after drying in anoven at a temperature of about 212° E. In scientific tests of timber, such as Bauschingers tests,*a standard moisture of about 15 per cent, is usually values of strengths of various kinds are tabulatedon p. 82. Tensile Strength.—The tensile strength of timber isvery much greater when the pull is parallel to the grainthan when it is across it. Considerable trouble is experi-* See Unwins Testing of Materials (Longmans). 66 THE STRENGTH OF MATERIALS encecl in gripping the specimen satisfactorily in a tensiontest on account of the tendency to shear or crush the ends. The stress-strain diagram for straight-grained timber fortension parallel to the grain is practically straight up tofrac
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