62 
Speed of loading 
467. Fundamentally, speed of loading may be conveniently considered to 
have two effects. First, there is a possible effeot of the microscopic 
behaviour of the steel, the resistance to sliding tending to inorease with 
inoreasing rate of strain, As a result high rates of strain are conducive 
to ths brittle type of failure with small energy absorption. Where the 
fracture does remain ductile, experiments with short specimens have shown 
that very high rates of atrain can lead to a large inorease in the yield 
point of mild steel and the energy absorbed up to fracture can also be 
larger than in a static test. Seoondly, there is a possible macroscopic 
effect depending on the dynamics of the msthod by which impact stresses 
are applied. Thus experiments have been carried out on long wires with a 
tup at the lower end subjected to the impact of a weight at different 
velocities. Above a certain oritical velocity (about 100 ft. per seo. 
in these experiments) it was found that the energy absorption to fracture 
suddenly deoreased, This phenomenon depends on the propagation of plastio 
waves up the wire. 
168. or a normal ductile material with a given stress/strain curve 
concave to the strain axis it is possible to analyse the simple one- 
dimensional propagation of plastio strain up the wire and it is in fact 
found that there is a limit to the particle velocity which oan be 
transmitted. In the experiments under consideration, at the lower speeds 
the effect of the velocity given to the end of the wire could be propagated 
up the wire wherees above the critioal speed the velooity of the tup was 
greater than the maximm particle velocity that oould be propagated, 
It was in fact found that, whereas below the critical speed fracture could 
ocour anywhere in the specimen, above the critical speed, fracture wes 
always at the impact end with the rest of the wire relatively unstrained, 
Thus a low energy absorption my be associated with high speeds of loading 
even though the fracture itself is essentially duotile. 
169. This type of localised fracture with low total energy absorption 
may be of soms importance in connection with the behaviour of ship's plating 
when subjected to underwater explosions. Thus a panel of plating will in 
general be given a higher velooity normal to itself than the framework to 
which it is attached and round the edgea the resultant lateral velocity 
of stretching may be greater than can be propagated through the plate. 
The panel can thus fail round the edges with relatively little energy 
absorption in the main body of the plate. This effeot of edge restraint 
is relatively unexplored but is probably of considerable importance in 
actual ship targets. 
