Eventos Anais de eventos
COBEM 2019
25th International Congress of Mechanical Engineering
Effects of Loading Span on Cleavage Fracture Toughness of Precracked Charpy-Type Bend Specimens
Submission Author:
Claudio Ruggieri , SP , Brazil
Co-Authors:
Vitor Scarabeli Barbosa, Claudio Ruggieri
Presenter: Vitor Scarabeli Barbosa
doi://10.26678/ABCM.COBEM2019.COB2019-0448
Abstract
Current defect assessment procedures of large engineering structures employ macroscopic measurements of cleavage fracture toughness (such as the J-integral at cleavage instability, Jc , or the critical Crack Tip Opening Displacement, CTOD) derived from laboratory testing of conventional fracture specimens with deep, through cracks (a/W>=0.5). In particular, standard three-point bend specimens are routinely employed in toughness testing of ferritic steels in the ductile-to-brittle transition (DBT) region. These toughness measures must satisfy parametric limits on the crack-tip deformation relative to crack length, specimen thickness and remaining crack ligament such that high constraint conditions, similar to those of small-scale yielding (SSY), are maintained over microstructurally significant size scales at the crack-tip region. However, much previous research shows the potentially strong effects of specimen geometry and loading mode on -values measured over the DBT region for ferritic materials. A case of considerable interest is the utilization of small fracture specimens to facilitate experimental measurements of fracture toughness data in commercial nuclear RPV surveillance programs. In particular, three-point bend testing of precracked Charpy (PCVN) specimens becomes necessary when severe limitations exist on material availability such as, for example, in nuclear irradiation embrittlement studies. However, the measuring capacity of small test specimens (as defined by the maximum applied values of J given by Jmax=b*Sy/M where M represents a nondimensional deformation limit, b denotes the uncracked ligament length and Sy defines the yield stress) for fracture toughness prior to constraint loss may be insufficient for moderate strength pressure vessel and structural steels. Once constraint loss occurs, the measured Jc-values increase markedly as the global plastic deformation interacting with the local crack front fields relaxes the levels of stress triaxiality. Motivated by these observations, this work addresses an experimental investigation on the cleavage fracture behavior of typical structural steels using standard and non-standard PCVN configuration. The primary purpose of this study is to investigate the effects of loading span on cleavage fracture toughness measurements using non-standard bend specimens. Fracture toughness testing conducted on various PCVN geometries extracted from an A572 Grade 50 structural steel plate and an A515 Grade 65 pressure vessel steel plate provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc. The experimental results show a potential effect of loading span on Jc-values which can help to mitigate the effects of constraint loss often observed in smaller fracture specimens. An exploratory application to determine the reference temperature, T0, derived from the Master Curve methodology (which defines the dependence of fracture toughness with temperature for the tested material) also provides additional support for using non-standard bend specimens in routine fracture applications.
Keywords
fracture toughness test, J integral, specimen geometry effects, subsize specimen, reference temperature, cleavage fracture
