58 
HAYFORD. 
elongation of the piece is proportional to the load and inde- 
pendent of the duration of the application of the load. 
Under each new load the piece takes a new length promptly 
and maintains it unchanged as long as the load does not 
change. The piece returns to its original length promptly 
when the load is removed. The piece is a competent struc- 
ture under these loads. 
As soon as a load is applied which is slightly beyond the 
elastic limit, the behavior of the piece is very different. The 
elongation is no longer proportional to the load. It increases 
with the lapse of time under the load as long as the load is 
maintained. When the load is removed, the piece returns 
only part way to its original length. It has acquired a 
permanent set, a permanent deformation. 
When a load much in excess of the elastic limit is applied, 
the stretching under the load is more rapid and a new fea- 
ture of the failure begins to appear. When the piece was a 
competent structure, the elastic elongation was distributed 
throughout the piece, following a perfectly regular law. 
When the piece is failing under loads slightly beyond the 
elastic limit, the stretching is somewhat irregularly dis- 
tributed through the piece. When the loads are long-con- 
tinued and much beyond the elastic limit, the stretching 
concentrates gradually at the immediate vicinity of the 
weakest point of the piece. The degree of concentration of 
the damage increases until, just before the complete failure 
occurs, practically all of the distortion is taking place in the 
immediate vicinity of the point at which the rupture is to 
take place. 
There are two contrasts which it is important to keep in 
mind between the behavior of this failing test-piece and of 
failing structures generally, on the one hand, and the be- 
havior of an elastic competent structure, on the other hand. 
First, the yielding of an elastic competent structure is 
independent of the length of time the stresses are in action. 
It depends solely upon the elastic constants and the inertia 
of the material. The bridge engineer in designing his com- 
petent structure does not consider the length of time the 
