2.2. Detailed FEM numerical approach
It is obvious that FEM approach is a unified way that covers all fields of strength
analyses. In the long run, only a few matters might be left to the discretion of a
designer who intends to perform a standardized FEA. Until early 1990s, the
advantage of FEM approach was quite limited compared with the simplified
analytical approach. The extent of finite element model and its mesh size were
restricted within the unsatisfactory levels because of the insufficient capacity of the
memory and CPU power in addition to the discouraging computation cost.
Following the successful progress of the modeling technique in the military,
aerospace and automobile research, the simulations of non-linear elasto-plastic
response of complicated hull structure is now practicable. Geometrical stress
concentration and local bending of plates in the multi-axial stress field can be
considered more systematically than ever as well as quasi-uniform membrane stress,
provided that due consideration is taken on the mesh size and pattern of finite
elements. Time-dependent strain hardening and strain rate effects on the material
properties are also counted at every step of the progressive deformation. The finite
element models can be recycled many times and the results can be reviewed
repeatedly from various aspects on demand.
Compared with the implicit finite element code, the explicit finite element code is
considered to be more suitable for the collision and grounding simulations of large
ships at present. The great advantage of the explicit finite element code is that the
simulation model of numerous finite elements can be handled, while the
disadvantage is the reduced accuracy due to explicit formulation itself. The
disadvantage, however, can be resolved practically by the adoption of fine mesh
size and proper time increment.
According to the latest collision simulations carried out at the Regulation
Research panel Nos. 46 and 76 (RR46&76) sponsored by the Shipbuilding Research
Association of Japan (JSRA), the required CPU time for the simulation period of
about 1 s is less than one week when a finite element model assembled by about720,000 elements is employed. The desk-top engineering workstation with enhanced
performance is priced about 25,000 US$, which tends to fall down steadily.
Although considerable man-power is required to assemble a finite element model
with curved hull form and the accuracy of the results is still open for further
improvement, FEM approach has become practical and the most powerful way at
present. More sophisticated coupling of structural response with fluid, brittle
unstable fracture, etc., are pending issues to be challenged to incorporate in the
future.