Engineering Version of Local Approach to Fracture and its Application for RPV Lifetime Prediction

Theoretical basis of the simplified (engineering) version of Local Approach (LA) to fracture is summarized, and the possibility of using this version to predict the lifetime of RPV steels is demonstrated. The concept of “effective” volume, , within which stresses and strains are uniformly distributed, but the magnitude of the probability of cleavage initiation are equal to their real values at given value of , underlies the proposed engineering version of LA. A function was selected to describe the temperature dependence of local cleavage stress, and a calibration procedure was proposed for determining the constants in this dependence from the results of tests of surveillance specimens. For weld metal of WWER-1000 reactors the dependences of brittle strength on fluence were obtained, and the maximum permissible fluence values were determined. It is shown that in addition to the radiation hardening, the radiation-induced reduction in the level of brittle strength, , has a significant effect on the radiation service life of reactor pressure vessel steel.

Location of Initiation Sites in Fracture Toughness Testing Specimens: The Effect of Size and Side Grooves

Cleavage fracture initiates usually at single locations in front of the fatigue crack in some position along the crack front. If the crack driving force along the crack front is uniform, one should expect the initiation sites to be randomly located along the crack front. Finite element analyses have, however, shown that the crack driving force varies along the crack front. Thus, the location of the cleavage initiation sites should reflect this variation in crack driving force. Fracture toughness specimens differ both in geometry and size. Also, the specimens may be side grooved or plane sided. All this can be expected to affect the local crack driving force along the crack front. The local crack driving force for cleavage fracture initiation can be divided into two components. The local KJ value describes the local effective stress intensity, whereas Q or Tstress describes the local constraint. To make things even more complicated, the local constraint is also affected by the local effective stress intensity. All of these are also affected by any ductile tearing occurring prior to cleavage initiation. The testing standards contain specific limitations on specimen sizes and their measuring capacity in order to ensure that the crack driving force in different specimens is sufficiently similar to make the results from different specimen types and sizes comparable. Classically, the fracture toughness test specimens have been comparatively large. Recently more and more work has been devoted to diminish the size of the specimens, to save material. One very promising specimen type is the miniature C(T) specimen with a 4 mm thickness and total height of 10 mm. Based on a recent international round-robin, the miniature C(T) specimen appears to provide compatible Master Curve T0 values as large specimens, but further validation regarding the similitude of the cleavage initiation is required, since the Master Curve is based on the assumption that specimen size does not affect this similitude. In this work, the location of cleavage initiation sites along the crack front are examined for different size and type of fracture toughness specimens, focusing on the miniature C(T) specimen. The location distributions are evaluated in terms of load level, specimen type, size and possible side grooving. It is shown that, as long as the standard requirements are fulfilled, the initiation location distributions for the miniature C(T) specimens are similar to larger conventional specimens. Side grooving is shown to have a minor effect on the initiator locations.

Characterisation of A508 Steel Cleavage Initiation Sites and Local Plasticity Using Surface Matching Technique

Cleavage fracture in ferritic pressure vessel steels is preceded by local plastic deformation that initiates micro-cracks at carbides or second phase particles within the highly stressed region ahead of a crack tip. The objective of this study is to locate initiation sites of failed Compact Tension fracture mechanics specimens, tested at temperatures within the lower ductile to brittle transition region. A surface matching approach was developed to identify regions of local plastic deformation on the fracture surfaces of tested specimens, using confocal microscopy to acquire accurately mapped topographic images of both fractured surfaces, and then subtracting each pair of images in a virtual environment. This methodology is conceptually similar to the fracture surface topography analysis (FRASTA) technique but uses a home developed MATLAB software based on image correlation. The residual mismatch (interference) between the two datasets was used to identify the regions of local plasticity on both fracture surfaces of each test specimen studied that is believed to be associated with the cleavage initiation sites. The size and location of the localised plasticity was found to be consistent with scanning electron microscopy observations of cleavage initiation sites located directly ahead of the fatigue pre-crack tips in tested specimens. Finite element modelling was used to identify the magnitude of stress at the identified regions of cleavage initiation for the specimens studied, providing new insights into the mechanism of cleavage initiation in the lower transition region. This study suggests that this is a promising methodology for the identification of the initiation sites of cleavage fracture.

Effect of Strip Defect on Cleavage Fracture Toughness of Steel 16MnR

A research on the effect of strip defect in the notched specimen of low alloy hot rolled steel 16MnR at -196°C is carried out in this paper. 4-point positive and negative bending experiments of notched specimen with different preloads are carried out to introduce strip defects of different dimensions in the front of notch. And then the residual stress and work hardening is eliminated through high temperature tempering. Bending fracture experiment is carried out at -196°C. Through microscopic observation and the measurement and analysis of mechanical parameters, it is discovered that: when the preload ratio P0/Pgy0.861, the Pf /Pgy rapidly decreases as the P0/Pgy increases. That’s because the increase of local high stress-strain region caused by the strip defect at the front notch end makes the distribution of crack-shaped nucleus active region of cleavage fracture increase, causes the cleavage initiation and leads to discrete numeric values of material notch toughness Pf and W.

Investigation of Cleavage Fracture Behavior in Notched Specimens of a Low Alloy Hot Rolled Steel 16MnR

The cleavage fracture behaviors are studied in notched specimens of a low alloy hot rolled steel 16MnR. The results show that two types of cleavage initiation sites are existed in notched specimens, one being related to the inclusions ahead of notch root (IC type) and the other related to inclusions located ahead of string cracks far from the notch root (SIC type). The types of initiation sites are influenced strongly on temperature, changing from IC type at -196°C to SIC type at -130°C. In both IC and SIC initiation mechanisms, the crack nucleation is induced by inclusions and the final fracture is controlled by propagation of a ferrite grain-sized crack into matrix grain. The cleavage fracture of IC initiation type in notched specimens satisfies a dual-criterion model, i.e. a critical plastic strain ep ³epc for initiating a crack nucleus and a critical tensile stress syy³sf for its propagation. While for SIC initiation type, the dual-criterion model is evolved with the expression of ep+eps³epc and syy+syys³sf.

Effects of Notch Tip Geometry on the Cleavage Fracture of RPV Steel

A series of fracture tests and elastic-plastic FEM analysis were carried out for three-point bend specimens with different notch root radius in the cleavage temperature region of a RPV low alloy steel. The cleavage initiation distance (CID) from the notch tip was measured by SEM photographs on the fracture surface of each specimen. The local cleavage fracture stress σf * was defined as the stress σyy at the cleavage initiation site. The σf * increased with decreasing notch root radius. The CID’s were larger in specimens with bigger root radius. This implies that a larger volume of material and possibly bigger microcracks should be involved in the cleavage fracture process of those specimens. The σf * is considered to have a certain relationship with the level of stress-strain concentration in different geometry specimens.