WROUGHT IRON AND STEEL BEAMS. 



193 



Therefore 

 E = 



17920 x 216000 



48 x 0-0506 x 55-9 

 = 28,600,000 Ib. per square inch, 

 or, = 12,750 tons per square inch. 

 The load at which the elastic limit occurs is not so easy to 

 determine in the case of a bending test as in simple ten- 

 sion. In a tension test the stress is uniform over the whole 

 section, and, as soon as the stress gets beyond the limit, 

 the increased increment of strain is easily recognised ; but 

 in a beam the stress is variable over the section, being 

 greatest in those fibres farthest from the neutral plane. So 

 that when the limit is reached it is only in those fibres 

 which are subjected to the maximum stress, while the 

 greater part of the section is still under stresses which are 

 accompanied by elastic strains. The limit effect, therefore, 

 only takes place very gradually, beginning at the outer 



FIG. 92. 



fibres and slowly penetrating inwards towards the neutral 

 plane, and there may be a considerable interval in the 

 loading between the point when the outer layers are 

 stretched beyond the elastic limit and the time when the 

 whole section is brought to this state. The gradual nature 

 of the appearance of the limit will be appreciated by an 

 inspection of the diagram on Fig. 92, whicn has been plotted 

 from the figures given above. 



By placing a straight-edge along the elastic portion of 

 this curve it will be seen that it begins to curve away from 

 the straight at a load of about 18 tons, and that point may 

 02 



