CHAPTER XI 
BEHAVIOUR OF MATERIALS IN THE TESTING 
MACHINE 
161. Nominal and Actual Stresses.—When a specimen of cross 
sectional area a is placed in simple tension or simple compression by a — 
load P, then f, the direct stress produced, is given by the equation f= P/a. 
The effect of the load is not only 59 
to alter. the length of the specimen, aa 
but also its cross sectional area, 45 ae 
and if in the above equation a is n 
the original area of the cross section, 40 44 
then f is the nominal stress pro- 3 Lpoptesie. A 
duced by the load P. If however “*35 7a 
a is the actual area of the cross S50 “s: >* ae 
section when the load P is on the §°° we i 5 1 
specimen, then / is. the actual stress ae Tio wand aes : 
produced. &: / vot om £ \ 
0-5 20|—7, ai 
Pree beerereTT TTT TTT S315 ie 
S03 Bi | 8 a 
. TENSILE | § mio 8. &! — 
3° TEST : STEEL - + 
"30-1 re ; 5 : ; 
3 o | _| Axis y A i 
0 I 2 xe! ee) 1 38 D095. O15 ae . 
Lengths. Scale % . Extension (in inches) in Binches 
Fia. 241. Fia. 242. ° . 
For loads within the elastic limit of the specimen the difference — 
between the original and actual areas of the cross section is so small that 
it may be neglected. Beyond the elastic limit however, in the case of 
ductile materials, the actual area of the cross section of the specimen may 
differ considerably from the original area, and the actual stress may be 
much greater than the nominal stress when the specimen is in tension, 
and much less when in compression. The foregoing remarks are well — 
illustrated by Figs. 241, 242, and 243, which show the results of two — 
tests, one a tensile test on a specimen of mild steel, and the other a a 
compression test on a specimen of wrought-iron. The full curves in — 
Figs. 242 and.243 show the relation between the nominal stress and the 
strain produced, while the dotted curves show the relation between the — 
actual stress and the strain produced. The specimens were originally — 
170 | 
