Al-CLST 1. IV.'O I 



THE INDIA RUBBER WORLD 



a piece falls more than 10 per cent below the specified amount, 

 it shall not be counted in calculating the average strength, pro- 

 viding that in no case shall more than 25 per cent of the pieces 

 tested be eliminated on this account. In other words, the object 

 of test is to determine the quality of the article as a whole. 



(7) Adjusting Test Piece in Grips: The test piece shall be 

 adjusted in the grips, care being taken to insure a uniform 

 distribution of stress over its cross-section. 

 TESTING MACHINES. 



(1) The testing machine shall be power driven and shall 

 meet the following requirements : 



(a) It shall indicate and record correctly the applied load, 

 and its accuracy shall be tested from time to time by calibration 

 with dead weights. The tension indicator shall not jump back 

 more than 0.5 per cent when the specimen breaks. .^ machine 



B = Tension in kilograms indicated by machine at breaking 

 point. 



\V = Width of test piece measured to 0.01-mm. 



T = Thickness of test piece measured to 0.01-mm. 



In other words, the tensile in Kg/cm' is obtained by dividing 

 the breaking load expressed in kilograms by the cross section 

 area of the test piece expressed in square centimeters. 



(2) Elongation: The ultimate elongation should be meas- 

 used with a steel tape graduated to millimeters, attached to tram- 

 mel points fixed on a rod running parallel with the test piece 

 and so arranged that they slide on this rod (Fig. 7). The 

 trammel points are then always kept even with the marks on 

 the test piece and after the piece has broken, the scale will re- 

 cord the distance between the marks at the time of breaking.. 

 If the initial disianre b.-tv-,r<ii t!ir mp.rks is 50 mm. and the 



I'lG. 7. Device for Me-\suring Ui.ti.\i.\te Elon-g.^tion. 



for which the load is applied by tension on a spring should not 

 be used. 



(b) The grips shall separate at a uniform rate of 50 cm. per 

 minute, except in the case of friction pulls where the rate of 

 separation should be 25 mm. per minute, or for testing hard rub- 

 ber the rate of separation should be 4 mm. per minute. 



(c» The grips shall tighten automatically and exert a pres- 

 sure proportionate to the applied tension. The grips shall exert 

 a uniform pressure across the width of the test piece regardless 

 of any variation in the thickness of the rubber. If the nature 

 of the rubber is such that the jaws cut it in course of test, name- 

 ly, pure gum inner tube stocks, a jaw such as shown in Fig. 6 is 

 much more satisfactory. 



(d) .\ device shall be provided for instantly starting and stop- 

 ping the motion of the movable jaws; also means to enable the 

 operator to return the movable jaws rapidly to their initial posi- 

 tion, 



(e> Computing results. Tensile, in kilograms per square cen- 

 timeter equals 



B 

 X 100 



Fig. 9. Pipe Connec- 

 tions FOR Steam Hose 

 Test. 



distance between the marks at the breaking point is 150 mm. the 

 per cent elongation will be 150 mm. minus 50 mm., divided by 

 50 mm. multiplied by 100 equals 200 per cent. Or it may be 

 expressed by 



Lf— Lo 



= 100 = Per cent elongation. 



Lo 

 Where Lo equals initial distance between the marks, Lf equals 

 distance between the marks at the breaking point. 



(3) Set: For determining the set, the apparatus shown 

 in Fig. 8 shall be used. The test specimens shall be prepared the 

 same as the pieces used for tensile and ultimate elongation tests. 

 Its operation is as follows: The specimens being in the grips, 

 one of the spools is moved along the shaft until it engages the 

 corresponding pin, and the shaft is revolved until the specimen 

 has been stretched to 60 per cent of the breaking elongation 

 measured between gage marks. The clamp is tightened to hold 

 the specimen in this position for 10 minutes. The specimen is 

 then released and allowed to rest under no elongation for five 

 minutes. It is then stretched to 60 per cent as before and held 

 10 minutes again, then released and allowed to rest another five 

 minutes. Again stretched to 60 per cent as at first, and held a 

 third 10-minute period, after which it is released and after being 

 allowed to rest five minutes, the final measurement of the dis- 

 tance between the two marks is made. The initial distance be- 



