استانداردهای سازه های فولادی استرالیا
Abstract
1- Introduction
2- Metallurgical effects and mechanisms of brittle fracture
3- Current international regulations on brittle fracture
4- Proposal for new material selection design rules in AS 4100, NZS 3404.1 and AS/NZS 5100.6
5- Comparison of the proposed selection criteria with Australian and New Zealand requirements
6- Conclusion
References
Abstract
This paper develops a new method to select steel grades manufactured to Australian and New Zealand standards. The current materials selection procedure is currently given in the design standards AS 4100, NZS 3404.1 and AS/NZS 5100.6, which is based on test data on the notch toughness characteristics from a previous generation of steel products originally manufactured in Australia or New Zealand. The existing procedure is limited to temperatures down to −40 °C. Moreover, it does not consider the effect of welding, detailing, stress utilisation, seismic loading rates, defects and other important factors. This paper includes a critical review of other international material selection procedures, before preparing a new design method based on fracture mechanics. The method extends the temperature range down to −120 °C, which is much lower than considered in many other international standards. It also includes New Zealand specific requirements for seismic loading rates. In comparison with the new method, it is demonstrated that the current materials selection procedure is much more conservative for plate thickness up to 75 mm for non-seismic design. The paper presents selection tables that can be considered for the development of new brittle fracture provisions for future versions of the Australian and New Zealand steel structures design standards.
Nature of brittle fracture
A brittle fracture is defined as fracture with little, or no plastic deformation of the failed component. It can be initiated by an overload of a cross-section in combination with material properties and/or geometrical allocation of the stresses (i.e. triaxiality of stresses due to the structural detail). The plastic deformation capability is essential for the avoidance of brittle fracture. It can be affected by: hardening in the weld heat affected zone; triaxiality of stress caused by the design of the structural detail; higher strain rates (e.g. during seismic events); or by neutron embrittlement. Another consideration is the possible inhomogeneity of the material, caused by sulphide inclusions leading to reduced mechanical properties in the through-thickness direction of the material. Other considerations include the loading imposed during fabrication and in service, design load, weld shrinkage and alignment at fabrication, erection stresses caused by poor fit-up, stresses by possible displacements of abutments and loadings caused by seismic events. In the design office, usually only stresses due to design loads are verified by calculation. The other stresses (e.g. residual stresses from shrinkage), are covered by the assurance of a plastic deformation capacity. That assurance is of equal importance as the numerical verification by calculation.