mini-CT Specimens Research Articles

Fracture characterization of structural steel S690Q by using mini-CT specimens

Mini-CT specimens are an interesting alternative when characterising the fracture behaviour of structural materials and there are issues with regard to, for example, the amount of available material, the irradiation level (in nuclear materials) or material inhomogeneities. Furthermore, in ferritic-pearlitic steels, the characterisation of the fracture behaviour within the ductile-to-brittle transition zone (DBTZ) is of particular interest, given that the material may behave very different, in terms of fracture toughness, when operating at different but relatively close temperatures within this zone. In many occasions, the definition of the DBTZ behaviour is performed through the Master Curve (MC) methodology and, thus, by testing standardised fracture specimens (e.g., CT, SENB) and determining the material Reference Temperature (T0). The use of mini-CT specimens to define T0 has been validated in a wide range of steels used in nuclear industry, but its application to structural steels has been scarce. Thus, this work gathers the fracture characterisation results (T0) obtained in structural steel S690Q, comparing them to those obtained by using conventional standardized SENB specimens. It is shown that, for this particular structural steel, the use of miniaturized specimens provides a T0 value (-89.3°C) which is comparable to the value obtained from conventional larger specimens (-110°C).

Size correction scheme to determine fracture toughness with mini-CT geometry in the transition regime

The mini-CT geometry is used to determine relevant fracture toughness data along with the master curve description within the transition regime. However, due to significant loss of constraint experienced at loadings close to J-levels corresponding to the reference temperature T0, the current size correction scheme from ASTM E1921 standard requires a better assessment in order to avoid censoring a large number of mini-CT specimens. Indeed, the current tight ASTM E1921 requirements on the KJc limit value set to ensure that the computed transition temperature T0 is reliable often implies that many samples have to be censored. A large number of censored specimens leads to a significant loss of time, money and material, and of unnecessary activated material in the case of irradiated specimens. Besides, because of the specimen size reduction, more mini-CT samples and lower test temperatures as compared to large sized samples are required to determine a reliable T0 value. In this study, an improved specimen size correction associated to the mini-CT geometry is proposed based on a local approach of brittle fracture combined with FEM simulations. The results show that the proposed size correction reduces the T0 bias compared to the 1T-CT geometry. It also contributes to relaxing the KJc limit for the mini-CT geometry as compared to the ASTM size corrected values.

Effect of the pre-crack non-uniformity on the initiation of brittle fracture in mini-CT specimen

Non-uniform pre-crack fronts often develop during fatigue pre-cracking of mini-CT specimen, resulting in invalid measurement when following the tight requirements of the ASTM standards regarding fatigue pre-crack curvature. Invalid measurements lead to a waste of test resources which is a major issue for irradiated materials. Previous studies have demonstrated a weak effect of excessive pre-crack curvature on the experimentally measured fracture toughness in the transition regime. In this paper, experimental investigations as well as finite element simulations were performed to provide additional confirmation of this conclusion. In order to demonstrate that meaningful results can still be generated in the brittle regime from non-uniformly pre-cracked mini-CT specimens, a local approach of brittle fracture was adopted to quantify the effect of pre-crack front non-uniformity on the initiation of brittle fracture. The final goal is to contribute to consolidate relevant requirements on the pre-crack front curvature.

Effect of pre-crack non-uniformity for mini-CT geometry in ductile tearing regime

The mini-CT specimen attracts the attention from all over the world for application in fracture toughness measurement. However, one of the shortcomings of this geometry is related to the required tight accuracy of the specimen dimensions, in particular the fatigue pre-crack curvature which often violates the requirements of the ASTM standards. Given the limited thickness of mini-CT geometry, non-uniform pre-crack tends to develop during fatigue pre-cracking, resulting in a large proportion of mini-CT specimens being considered invalid.Previous investigations have demonstrated that mini-CT specimens with excessive crack front curvature can still provide meaningful fracture toughness results. In this paper, the effect of pre-crack front non-uniformity on ductile fracture is studied: first, the difference of macro parameters such as the applied load, J-integral and crack tip stress–strain field are investigated to illustrate the varying fracture behavior related to non-uniform pre-crack. Next, two micro-mechanics-based approaches, the Rice-Tracey void growth model and Thomason void coalescence model, are integrated to compare the ductile fracture initiation conditions associated with uniform and non-uniform fatigue pre-crack. Finally, the experimental verification of the ductile fracture simulations is performed for mini-CT specimens with uniform and 30° tilted initial cracks. The results indicate that the pre-crack non-uniformity plays a major role in the redistribution of local J-integral and stress–strain state, further affects the position of crack initiation and the early stages of crack propagation. Nevertheless, the pre-crack non-uniformity has limited effect on the global properties that are usually expected from fracture toughness tests, such as applied load, J-R curve and critical fracture toughness. The requirements in the ASTM E1820 regarding pre-crack front curvature need thus to be relaxed.

Using Mini-CT Specimens for the Fracture Characterization of Ferritic Steels within the Ductile to Brittle Transition Range: A Review

The use of mini-CT specimens for the fracture characterization of structural steels is currently a topic of great interest from both scientific and technical points of view, mainly driven by the needs and requirements of the nuclear industry. In fact, the long-term operation of nuclear plants requires accurate characterization of the reactor pressure vessel materials and evaluation of the embrittlement caused by neutron irradiation without applying excessive conservatism. However, the amount of material placed inside the surveillance capsules used to characterize the resulting degradation is generally small. Consequently, in order to increase the reliability of fracture toughness measurements and reduce the volume of material needed for the tests, it is necessary to develop innovative characterization techniques, among which the use of mini-CT specimens stands out. In this context, this paper provides a review of the use of mini-CT specimens for the fracture characterization of ferritic steels, with particular emphasis on those used by the nuclear industry. The main results obtained so far, revealing the potential of this technique, together with the main scientific and technical issues will be thoroughly discussed. Recommendations for several key topics for future research are also provided.

On the use of mini-CT specimens to define the Master Curve of unirradiated reactor pressure vessel steels with relatively high reference temperatures

The safe operation of nuclear plants requires an accurate characterization of the fracture toughness of reactor pressure vessel materials, which can be reduced over time due to irradiation or thermal aging processes. This necessity is a challenge itself since the availability of specimens inside the surveillance capsules of the vessels is generally scarce. Therefore, innovative techniques have to be applied, in order to increase the reliability of fracture toughness measurements and at the same time to reduce the volume of material needed for the tests. In this paper, the Master Curve (MC) approach has been employed, combined with the use of mini-CT specimens made from two different reactor pressure vessel (RPV) steels (ANP-5 and A533B LUS). The MC methodology allows the fracture toughness of the material to be evaluated by using a single parameter: the reference temperature, T0. This parameter has been previously estimated using mini-CT specimens in a number of unirradiated steels, most of them with relatively low T0 values (-120 °C to −60 °C), providing satisfactory results. This paper adds further validation of the use of mini-CT specimens to define the T0 of unirradiated RPV steels with relatively high values of this parameter. Additionally, the analysis of the fracture surfaces confirms the existence of cleavage fracture following the weakest link theory (i.e., one single initiation point), as required by the Master Curve approach.

Master curve evaluation of ANP-5 steel using mini-CT specimens

The nuclear industry demands analyses that make possible the long-term operation of nuclear power plants (i.e., beyond 40 years). In this sense, one of the main challenges to overcome is the restricted amount of material available to extend the surveillance programs. To mitigate this issue, mini-CT specimens have been proposed for the evaluation of the fracture properties of reactor pressure vessel (RPV) materials, and particularly, the corresponding Master Curve. These specimens can be taken from the broken halves of previously tested Charpy specimens. In this work, mini-CT specimens have been employed to evaluate the reference temperature of the RPV ANP-5 steel in non-irradiated conditions. The results were compared with those obtained by means of conventional fracture tests.

FRACture mechanics TEsting of irradiated RPV steels by means of SUb-sized Specimens: FRACTESUS PROJECT

This work presents the overall structure of the FRACTESUS project and the progress carried out so far. The project is part of the EURATOM work programme 2019-2020, topic NFRP-04: Innovation for Generation II and III reactors. The project developments will contribute to the long-term safe operation of nuclear power plants, addressing the goals of the European Union in terms of sustainable and green energy, where the decarbonisation of the energy system is a priority. FRACTESUS intends to demonstrate the reliability of measuring the fracture toughness of reactor pressure vessel steels by means of sub-sized specimens (e.g., 0.16 CT or mini CT specimens). This will allow, among others, to notably increase the number of specimens available in the surveillance programs of the nuclear power plants.

On the Possible Relaxation of the ASTM E1921 and ASTM E1820 Standard Specifications with Respect to the Use of the Mini-CT Specimen

Specimen miniaturization is gaining a lot of interest in the fracture mechanics community thanks to recent methods accounting for specimen size correction. In particular, the miniature compact tension (mini-CT) specimen is gaining large international interest for irradiated surveillance materials used to monitor irradiation damage in reactor pressure vessels. Because of the limited available amount of some of these materials, eight mini-CT specimens can easily be machined from one broken Charpy specimen, avoiding the need of consuming surveillance materials. Specimen reorientation also becomes possible to investigate anisotropic effects. At this stage, ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range(Superseded), the standard for brittle fracture, and ASTM E1820, Standard Test Method for Measurement of Fracture Toughness (Superseded), the standard for ductile fracture, are used for this geometry, although it does not fully satisfy the requirements of the standards. In particular, the specifications on the specimen dimensions relative to the specimen width are tight, given that the tolerances are a fraction of the specimen width. Given that the actual dimensions of the specimen are taken into account in the calculations, the analysis described in this article shows that the tight dimensional accuracy is not necessary. Moreover, given the limited thickness of the specimen, the curvature of the fatigue precrack tends to deviate from a quasi-straight front. As a result, a large number of mini-CT specimens do not fulfill the standard requirements with respect to crack front curvature. Previous tests demonstrated that actual cracks excessive curvature lead to similar results when compared with artificial fatigue quasi-straight cracks. The objective of this article is to experimentally demonstrate that some of the standard requirements can be relaxed without jeopardizing the quality of test data, in terms of master curve and crack resistance curve.

Compatibility of fracture toughness results in the upper shelf region

Fracture toughness has been used for decades nowadays for structure integrity assessment and it is important mechanical characteristic for new materials as well. In the upper shelf region, where stable fracture behavior is observed, concept of J-R curve is widely apply to determine fracture toughness parameter JIc defined by the intersection of the 0.2 mm offset construction line and the J-R curve according to ASTM E1820. Fracture mechanic methods have been well established in the community especially for standard sized specimens but as it was mainly developed for large structures assessment against the brittle failure, standardized specimens are of large size at present. In many cases, such as evaluation of additively manufactured products properties, residual life of in-service components, local properties of components, weld joint, etc., standard specimens are not possible to use due to size requirements. Therefore, small sized specimens are necessary to use for material characterization. Due to constraint effect, the results obtained from standard and small sized specimens may be different. In the current paper, fracture toughness properties are going to be investigated using different sized samples scaled from mini-Charpy or mini-CT specimens to the standard one. Based on experimental results from several structural metals like steels 34CrNiMo6 and 27NiCrMoV 15-6, a procedure leading to the comparable results from small to standard samples will be proposed.

Fracture toughness assessment of in-service aged primary circuit elbows using mini-CT specimens taken from outer skin

Type CF8M cast duplex stainless steels used in the primary circuit elbows of pressurized water reactors are subject to thermal aging embrittlement at their service temperature, at ∼300°C. This phenomenon affects their fracture toughness properties. In order to assess the residual fracture toughness of these elbows, estimations are made through predictive formulae based on chemical composition and aging conditions, which provide safe values. However, in the case of the most sensitive materials, it is important to obtain more accurate estimations. A new method of determination was thus considered, based on the testing of mini-CT specimens taken from the skin of in-service elbows. The feasibility of using mini-CT specimens to evaluate the tearing resistance of cast duplex stainless steels seems at first sight difficult, in particular because of the very coarse metallurgical structure of these steels: will small specimens be representative of larger volumes (mainly regular 1T–CT specimens) and will they not induce too much scatter? In order to answer such questions, an experimental validation program has been undertaken: the results show that the method is relevant and lead to propose guidelines which aim at optimizing the experimental data analysis. Then the method is applied to an in-service elbow: the results obtained are found to be in good agreement with the toughness estimations given by our predictive formulae. This subsequently contributes to the validation of the general methodology used for the justification of French primary circuit elbows.