What Are Brittle Fractures Of Steel Structures?
Structural steel does not always exhibit a ductile behaviour, and under some circumstances a sudden and catastrophic fracture may occur, even though the nominal tensile stresses are low. Brittle fracture is initiated by the existence or formation of a small crack in a region of high local stress.
Once initiated, the crack may propagate in a ductile (or stable) fashion for which the external forces must supply the energy required to tear the steel. More serious are cracks which propagate at high speed in a brittle (or unstable) fashion, for which some of the internal elastic strain energy stored in steel is released and used to fracture the steel.
Such a crack is self-propagating while there is sufficient internal strain energy, and will continue until arrested by ductile elements in its path which have sufficient deformation capacity to absorb the internal energy released.
The resistance of a structure to brittle fracture depends on the magnitude of local stress concentrations, on the ductility of the steel, and on the three-dimensional geometrical constraints. High local stresses facilitate crack initiation, and so stress concentrations due to poor geometry and loading arrangements (including impact loading) are dangerous.
Also of great importance are flaws and defects in the material, which not only increase the local stresses, but also provide potential sites for crack initiation.
The ductility of a structural steel depends on its composition, heat treatment, and thickness, and varies with temperature and strain rate. Figure 1.11 shows the increase with temperature of the capacity of the steel to absorb energy during impact.
At low temperatures the energy absorption is low and initiation and propagation of brittle fractures are comparatively easy, while at high temperatures the energy absorption is high because of ductile yielding, and the propagation of cracks can be arrested.
Between these two extremes is a transitional range in which crack initiation becomes increasingly difficult. The likelihood of brittle fracture is also increased by high strain rates due to dynamic loading, since the consequent increase in the yield stress reduces the possibility of energy absorption by ductile yielding.
The chemical composition of steel has a marked influence on its ductility: brittleness is increased by the presence of excessive amounts of most non-metallic elements, while ductility is increased by the presence of some metallic elements.
Steel with large grain size tends to be more brittle, and this is significantly influenced by heat treatment of the steel, and by its thickness (the grain size tends to be larger in thicker sections). EC3-1-10 [18] provides values of the maximum thickness t1 for different steel grades and minimum service temperatures, as well as advice on using a more advanced fracture mechanics [34] based approach and guidance on safeguarding against lamellar tearing.
Three-dimensional geometrical constraints, such as those occurring in thicker or more massive elements, also encourage brittleness, because of the higher local stresses, and because of the greater release of energy during cracking and the consequent increase in the ease of propagation of the crack.
The risk of brittle fracture can be reduced by selecting steel types which have ductilities appropriate to the service temperatures, and by designing joints with a view to minimising stress concentrations and geometrical constraints.
Fabrication techniques should be such that they will avoid introducing potentially dangerous flaws or defects. Critical details in important structures may be subjected to inspection procedures aimed at detecting significant flaws.
Of course the designer must give proper consideration to the extra cost of special steels, fabrication techniques, and inspection and correction procedures.
