Fracture-safe Design - Steel Structures.

The design requirements for steel structures in which brittle fracture is a consideration are given in most structural codes.There are several key factors which need to be considered when determining the risk of brittle fracture in a structure. These are:

(1) minimum operating temperature
(2) loading – in particular, rate of loading
(3) metallurgical features such as parent plate, weld metal or HAZ
(4) thickness of material to be used.

Each of these factors influences the likelihood of brittle fracture occurring.

From experimental data and parametric studies examining the maximum tolerable defect sizes in steels under various operating conditions, codes such as BS 5400-3 and BS 5950-1 provide tables giving the maximum permissible thickness of steel at given operating temperatures.Where a design does not fit into these broad categories attempts have been made to use fracture mechanics to provide a criterion for material selection in terms of Charpy test energy absorption as this information is usually provided by the steel makers. When true fracture mechanics toughness values are available, methods such as those described in References 8 and 9 should be employed to assess the acceptability of flaws in fusion welded structures.

As discussed earlier, at normal operating temperatures and slow rates of loading valid KIc values are not usually obtained for structural steels, and in the offshore and nuclear industries critical CTOD and J tests are widely used. Under these conditions, valid KIc values from low temperature tests will be conservative. If the structure is acceptable when assessed using such conservative data then there is often no great need to pursue the problem, particularly as all forms of fracture mechanics testing are expensive compared with routine quality control tests such as Charpy testing.

In general, the fracture toughness of structural steel increases with increasing temperature and decreasing loading rates. The effect of temperature on fracture toughness is well known. The effect of loading rate may be equally important, not only in designing new structures, but also in understanding the behaviour of existing ones which may have been built from material with low toughness at their service temperature.The shift in the ductile–brittle transition temperature for structural steels can be considerable when comparing loading rates used in slow bend tests with those in Charpy tests. Results from experimental work have shown that the transition temperature for a BS EN 10025 S355 J steel can change from around 0°C to -60°C with decreasing loading rate.

Materials standards set limits for the transition temperatures  of various steel grades based on Charpy tests.When selecting an appropriate steel for a given structure it must be remembered that the Charpy values noted in the standards apply to parent plate. Material toughness varies in the weld and heat-affected zones of welded joints, and these should be checked for adequate toughness. Furthermore, since larger defects may be present in the weld area than the parent plate, appropriate procedures should be adopted to ensure that a welded structure will perform as designed. These could involve non-destructive testing including visual examination. In situations where defects are found, fracture mechanics procedures such as those in References 8 and 9 can be used to assess their significance.

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