. The strength of materials; a text-book for engineers and architects. so on until thespecimen breaks. The point at which the test-piece breaksshould be noted in decimals of one bend, which are marked onthe graduated arc. The machine is cahbrated by fixing the handle end of thelever in the jaws and applying a known force by means ofa spring-balance and comparing the record made on the stripwith the actual moment appHed. If there is any discrepancybetween the two results, the spring is adjusted until suchdiscrepancy disappears. The number of bends which a give^material can endure before fractur
. The strength of materials; a text-book for engineers and architects. so on until thespecimen breaks. The point at which the test-piece breaksshould be noted in decimals of one bend, which are marked onthe graduated arc. The machine is cahbrated by fixing the handle end of thelever in the jaws and applying a known force by means ofa spring-balance and comparing the record made on the stripwith the actual moment appHed. If there is any discrepancybetween the two results, the spring is adjusted until suchdiscrepancy disappears. The number of bends which a give^material can endure before fracture is a measure of theductihty, and experiment shows that this is approximately THE TESTING OF MATERIALS 399 proportional to the percentage elongation multiplied by thepercentage reduction in area in a standard tensile test. The following empirical results have been found fromexperiment to be approximately true— Yield-point stress in tons per sq. in. _ First bending moment in lb. ft. ~ r55 Ultimate tensile stress in tons -per sq. Longest line in lb. ft. ^ r55. Fig. 186.—Arnolds Testing Machine. The energy required to cause rupture is equal to 16 multi-plied by the number of bends, multiplied by the mean rangeof bending moment in lb. ft. Professor Arnolds Reverse Bending Machine.—Inthis machine, which may also be regarded as a repetition ofstress machine, a bar a, Fig. 186, f in. in diameter, is firmlyheld in a clamp b and passes through a slot in a slide c whichis reciprocated by a shaft d running at a standard speed of650 revolutions per minute. The distance between line ofcontact of the slot with the specimen and the point where the 400 THE STRENGTH OF rilATERIALS latter enters the clamp is 3 inches and the slot is adjusted tocause a deflection of | in. in the specimen on each side. Thenumber of bends which the sj^ecimen endures before fractureis taken as a measure of the capacity of the material to resistfailure by shock. Repeated Impact Testing Machine. — The ma
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