What is ISO 27306- Method of calculating CTOD fracture toughness for steel components about?
ISO 27306 is a safety standard on metallic materials.
ISO 27306 specifies a method for converting the CTOD (crack-tip opening displacement) fracture toughness obtained from laboratory specimens to an equivalent CTOD for structural components, taking constraint loss into account. This method can also apply to fracture assessment using the stress intensity factor or the J-integral concept. ISO 27306 deals with the unstable fracture that occurs from a crack-like defect or fatigue crack in ferritic structural steels.
The CTOD fracture toughness of structural steels is measured in accordance with the established test methods, ISO 121351) or BS 7448-1. The fracture assessment of a cracked component is done using an established method such as FAD (Failure Assessment Diagram) in the organization concerned, and reference is not made to the details thereof in ISO 27306.
ISO 27306 can be used for eliminating the excessive conservatism frequently associated with the conventional fracture mechanics methods and accurately assessing the unstable fracture initiation limit of structural components from the fracture toughness of the structural steel.
ISO 27306 is also used for rationally determining the fracture toughness of materials to meet the design requirements of the performance of structural components.
Note 1: Unstable fractures accompanied by a significant amount of ductile crack extension and ductile fractures are not included in the scope hereof.
Note 2: ISO 27306 does not intend to address size and temperature effects nor influence of data scatter on the results.
Who is ISO 27306- Method of calculating CTOD fracture toughness for steel components for?
ISO 27306 on the method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components is relevant to:
- Testing authorities
- Steel industries
- Metallurgical engineers
Why should you use ISO 27306- Method of calculating CTOD fracture toughness for steel components?
In fracture assessments of steel structures containing cracks, it has been assumed that the fracture resistance of fracture toughness specimens is equal to the fracture resistance of structural components. However, such an assumption often leads to excessively conservative fracture assessments. This is due to a loss of plastic constraint in structural components, which are subjected to tensile loading. By contrast, fracture toughness specimens hold a constrained stress state near the crack-tip due to bending mode. The loss of constraint is significant for high-strength steels with high yield-to-tensile ratios (= yield stress/tensile strength) which have been extensively developed and widely applied to structures in recent years.
ISO 27306 deals with the initiation of unstable fracture due to cleavage of structural steels. ISO 27306 presents a method for converting the CTOD fracture toughness obtained from the standard fracture toughness specimen [three-point bend or compact specimen with 0,45 ≤ a0/W ≤ 0,55 and B (specimen thickness) = t (plate thickness of structural component)], which are characterized by an extremely severe plastic constraint in the vicinity of the crack-tip, to an equivalent critical CTOD for structural components, which are characterized by less constraint. The reverse procedure is also possible with this method. Thus, this method links fracture toughness tests and fracture performance assessments of structural components by taking account of the loss of plastic constraint in structural components.
Overall, ISO 27306 is useful because the equivalent CTOD ratio, β, proposed in ISO 27306 enables a more accurate fracture assessment of structural components with crack, in comparison with conventional fracture mechanics methods.