3.2. Stretching span and equivalent failure strain
Needless to say, the stretching of the shells in collision or that of bottom
transverses in grounding contributes deeply to the increased crashworthiness. The
problem is, therefore, that the estimation of the crashworthiness is too sensitive to
both the defined failure strain (given criterion) and the estimated cumulative strain.
The failure, i.e., the rupture can initiate at the weakest point of the linkage of the
structures quite independent of their mean stretching capability. Then it comes down
to struggling to quantify the localized plastic strain in way of highly stressedfree from the notches in uniaxial tension stress field. Therefore, the difference
between 30% and 5–15% is considered to include each correction factor for the
geometrical stress concentration, multi-axial stress field, plate bending stress,
additional stress as an associated member of a stiffener, additional stress as a
compound member such as the bending of the double side structure or the HGHB,
possible notch effect incidental to the deterioration in service, etc. It is evident that
the equivalent failure strain depends on, at least the structural arrangement,
stiffening system and collision position along the unsupported (stretching) span.
connection details as similar to the estimation of the fatigue strength.
Fixed equivalent failure strain efe scattering from 5% to 15% has been adopted in
the simplified analytical approach in general. efe is defined to be mean uniform plastic
strain along the assumed (rather long) stretching span. Moreover, actual efe is usually
calculated for the membrane component of a plate member only. A lot of material
tests have shown that efe of ordinary mild steel is about 30% (saturated when the
gauge length is 200mm or above), which is defined for the machined flat specimen