FORMS OF TEST BARS. 305 



from the head being simply made by cutting a nick in the 

 bar ; B has proportions intermediate between A and C. 

 The results of tests made on these three bars have been 

 shown by experiment to be very dissimilar. 



In A and B the gauge marks are lin. distant from the 

 shoulders. The bar A will probably give the normal 

 results to be expected from a bar having no heads and 

 turned parallel, both as regards strength and ductility. 

 In the case of C the maximum strength will be very 

 much higher than that given by A. and the elongation 

 and reduction will be practically nil. The third bar, B, 

 may be expected to give results intermediate between the 

 others ; the maximum strength will probably be greater, 

 but not very markedly so, than that of A ; the total elon- 

 gation will be greater and the reduction in area less. 



*+--- - I*- 1 



FIG. 156. STROMEYER'S TESTS. 



These results, which experiment has shown to be 

 generally true for ductile metals, are due, in the first 

 place, to the influence of the larger unstrained heads, 

 in retarding the natural flow of the metal during the 

 period of drawing out. In the shorter bars, the normal 

 elongation and reduction are interfered with. The greater 

 elongation in the case of the shorter bars is due to the 

 cause which has been already mentioned, namely, the 

 larger proportion which the local elongation forms of 

 the whole. 



Professor Unwin* quotes two experiments illustrating 

 these facts. Two bars were tested, similar in form to 

 those at B and C (Fig. 155), the longer specimen having its 

 larger diameter 2iri. and the diameter of the part tested 

 1-^in. by 2in. long ; the other specimen was nicked as at 

 C, the diameters being respectively lin. and 2in. as in 



* Unwin's " Testing," page 82. 

 V-2 



