Page 426 - Marine Structural Design

402 Part 111 Fatigue and Fracture

22.2.2 Higher Strength Steel
For ship structures, the yield strength for mild steel is 24 (kgf/mm2)(or 235 N/mz). The
higher strength steel is HT32 (yield strength of 32 kgf7mm') and HT36 (Yamamoto et a1 1986).
The allowable stress for hull girder strength is defined for the individual grades of material.
The use of higher strength steel may lead to reduction of plate wall-thickness. However,
corrosion resistance for higher strength steel is equivalent to that for the mild steel. Therefore
corrosion allowance should also be taken as 2.5 to 3.5 mm. Elastic buckling strength is only
determined by geometrical dimensions, and it is not influenced by the yield strength.
Therefore, Elastic buckling strength my decreased due to the wall-thickness deduction for
higher strength steel. To avoid the reduction of buckling strength, it may be necessary to
reduce the spacing of stiffeners. The post-yielding behaviour for higher strength steel is
different from that for mild steel in that the ratio between linear stress limit and yield strength
is higher for higher strength steel. For instance, the proportional limit for steel of yield strength
between 50 and 60 kgf/mm2 is 0.7 to 0.8, while the proportional limit for mild steel is 0.6.
Hence there is less tensile strain (at tensile failure) for higher strength steel and the strength
redundancy in the post-yield region is less. In the heat affected zone (HAZ), Charpy V-notch
energy for high strength steel may be significantly low. It may be necessary to control the heat
energy in welding process and increase the number of passes in single sided welding.
The weldability of a steel is a measure of the ease of producing a crack-free and sound
structural joint. The carbon equivalent ( Ce9) for evaluating the weldability may be calculated
from the ladle analysis in accordance with the following equation:

ceq =c+-+ C,+M,+V +- Ni+C,, % (22.1)
M,
6 5 15
Selection of C,, and its maximum value is a matter to be agreed between the fabricator and the
steel mill because its value represents the tensile strength and weldability. The higher the C,,,
the higher the tensile strength and the worse the weldability.
Welding procedures should be based on a steel's chemistry instead of the published maximum
alloy content, since most mill runs are usually below the maximum alloy limits set by its
specification. When a mill produces a run of steel, chemical content is also recorded in a Mill
Test Report. If there is any variation in chemical content above the maximum allowable limits,
special welding procedures should be developed to ensure a properly welded joint.
For higher strength steel, the fatigue resistance may not increase as much as the increase of the
stress in the stress concentration areas of the weld details. It is therefore necessary to reduce
stress concentration and improve the fatigue resistance for the weld details.
22.2.3 Prevention of Fracture

During the 2nd world war, accidents occurred due to brittle fracture in welded ships. In the
USA, a throughout investigation was carried out on the temperature dependency of brittle
fracture. It is now known that the toughness is higher if the MdC ratio is higher. With the
development of fracture mechanics, it became clear that brittle fracture is due to the reduction
of the fracture toughness Ks in lower temperature (below 0 OC). In order to determine fracture
toughness, it is necessary to conduct accurate measurement using large test specimens. For
practical purpose, the result of Charpy V-notch impact tests has been correlated with the
fracture toughness KIC and used in the specification for steels used in lower temperature. In
ship design Rules, Charpy V-notch impact tests are not required in production for A-grades, B,