Assessment of notched structural steel components using failure assessment diagrams and the theory of critical distances

Abstract

When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. The proposal of this paper consists, on the one hand, in the application of the Theory of Critical Distances for the estimation of the notch fracture toughness and, therefore, for the conversion of the notched situation into an equivalent cracked situation in which the material develops a higher fracture resistance. On the other hand, once the notch fracture toughness has been defined, the assessment is performed using the Failure Assessment Diagram methodology, and assuming that the notch effect on the limit load is negligible. The methodology has been applied to 336 CT notched fracture specimens made of two different structural steels, covering temperatures from the corresponding lower shelf up to the upper shelf, providing satisfactory results and a noticeable reduction in the overconservatism derived from the analyses in which the notch effect is not considered. INTRODUTION: NOTCH EFFECT, THE THEORY OF CRITICAL DISTANCES AND FAILURE ASSESSMENT DIAGRAMSThere are many situations where the defects responsible for structural failure are not sharp. Actually, notched components develop a fracture resistance that is greater than that developed by cracked components (e.g., [1][2][3][4][5][6][7]) and this, generally, is directly related to the load-bearing capacity of the component. Hence, the development of an adequate methodology for the assessment of the notch effect would reduce the conservatism in many practical situations.There are two main failure criteria in notch theory: the global fracture criterion and local fracture criteria [2,3]. The global criterion establishes that failure occurs when the notch stress intensity factor reaches a critical value, K ρ c :This approach is totally analogous to that used in cracks, but its application is very limited because of the lack of analytical solutions for K ρ (as there are for K I ) or/and standardized procedures for the experimental definition of K ρ c .Local criteria are based on the stress field on the notch tip. Among them, the Point Method (PM), the Line Method (LM) and the Finite Fracture Mechanics stand out [8], all of them being different versions of the Theory of Critical Distances (TCD) and, then, using a characteristic material length parameter (the critical distance, L) when performing fracture assessments [8]:K c is the material fracture toughness and σ 0 is a characteristic material strength parameter (the inherent strength) that must be calibrated. Only in those materials with linear-elastic behaviour at both the macro and the micro scales (e.g., ceramics), does σ 0 coincide with the ultimate tensile strength (σ u ) [8].The notch analysis following these methodologies is relatively simple. For example, the PM [9] establishes that fracture occurs when the stress reaches the inherent strength (σ 0 ) at a distance from the defect tip equal to L/2:For its part, the LM [...

Structural integrity analysis of notched ferritic steels operating within their ductile-to-brittle transition zone: An approach from Failure Assessment Diagrams and the Notch Master Curve

“…This approach is analogous to that proposed by the authors in [7,9], with the particularity of using the NMC for the prediction of the apparent fracture toughness all along the DBTZ, instead of applying a particular correction provided by the TCD for every single temperature. Thus, the methodology presented here constitutes a more general solution for the assessment of notched ferritic steels, with general application within the whole DBTZ, whereas the approach gathered in [7,9] is only applicable at the specific temperatures where L has been calibrated and defined…”

“…[1][2][3][4][5][6][7][8][9]), notched components develop a greater fracture resistance than that developed in cracked conditions. This may have direct consequences on the load-bearing capacity of the component, and also on the corresponding structural integrity.…”

“…Thus, the methodology presented here constitutes a more general solution for the assessment of notched ferritic steels, with general application within the whole DBTZ, whereas the approach gathered in [7,9] is only applicable at the specific temperatures where L has been calibrated and defined…”

“…As also explained in [7,9], in order to complete the FAD analysis it is necessary to define the Lr parameter, which depends on the limit load (equation (8)). Plastic collapse occurs through the yielding of the remnant section, so that, in a perfectly plastic material, it can be defined by the material yield stress and the defect dimensions, with no influence of the radius existing on the defect tip.…”

“…Figure 1 [9] shows an example of this kind of assessment, where the crack-like assessment of the defect leads to unsafe conditions (the assessment point is located above the FAL).…”