48 
NATURE OF DEFORMATION OF THE TARGET 
426. For theoretical consideration, it is convenient to consider separately 
the behaviour of the target prior to rupture and the conditions governing 
rupture, [Existing theory relates primarily to the yielding of targets, 
whilst the rupture of targets has been mainly investigated experimentally. 
The major part of the present section will, therefore, be concerned with 
yielding without rupture. Since nearly all naval targets have steel hulls, 
attention will be concentrated on targets constructed of steel. 
Yielding of steel targets 
427. It is first necessary to recognise an essential difference between 
the usual problem of designing structures under static loads and the problem 
of designing to resist impulsive loading produced by an underwater explosion, 
The full curve of fig. 22 
illustrates diagrammatically 
the typical shape of the static 
stress-strain ourve for a mild 
steel specimen tested in simple 
tension, The main features are 
ths initial elastic portion AB, 
the upper and lower yield 
points C', C, the relatively flat ° 
portion CD, the strain hardening 
portion DEF and the final 
failure after the ultimate load 
has been reached at F. 
STRESS 
428. In the normal problem of 
design under statio loads the 
maximm loads to which the t 
atructure is to be subjected 
are given and the traditional 
STRAIN 
method is to design so that for Pig. 22 - Static stress-strain curve 
example, the maximum tensile or mild steel 
stress corresponds to a point S ‘ 
in the elastic range. This design criterion mans that it is reasonably 
certain that inaccuracies in the mthod of design will not result in th 
actual stress exceeding the yield point. It is important to note that, 
since load is the given criterion, the elastic range covers more than half 
the total range of load up to failure. Accordingly, in traditional desi 
the extra saving of ateel by designing to a stress beyond the yield has not 
in general, been considered sufficient to warrant the extra risk of rupture 
and the potential inconvenience of permanent distortion, Problems of statio 
loading have thus been primarily a design question of elastic deformations, 
129. On the other hand, for the problem of impulsive loading due to an 
underwater explosion, the oumilative evidence already discussed suggests 
that the basic oriterion is one of energy rather than load, that is, the 
structure has to absorb a given amount of energy rather than to withstand 
a given maximimm load, Now in the statio tension test the energy absorbed 
is proportional to the area under the full ourve of fig.22; the energy 
x For certain types of structure the actual static load at collapse 
may be many times the load at which the elastio limit is first 
exceeded locally in the structure. It seems probable that plasticity 
will becomes increasingly important in static design problems in the 
future. 
