Miniature Compact Tension Specimens Research Articles

On comparison of fracture toughness of irradiated and thermally aged decommissioned nuclear weld metals with miniature specimen test technique

This paper investigates the fracture toughness of thermally aged and irradiated weld metals extracted from head and beltline regions of a decommissioned nuclear reactor pressure vessel. Miniature compact tension specimens are fabricated from the thermally aged reactor pressure vessel head weld as well from the circumferential and axial beltline welds having fluences of 2.90 × 1016n/cm2 and 7.94 × 1017n/cm2, respectively. Master curve approach in accordance with the ASTM E1921 is applied to determine the reference temperature T0 and it is found to be − 113.5 °C, −104.4 °C, and − 89.3 °C for the reactor pressure vessel head, axial beltline, and circumferential beltline welds, respectively. In addition, the multimodal reference temperature Tm to assess the homogeneity of welds as well as key curve analysis for the quality assurance of testing are also provided. Fractographic analysis identified variations in crack initiation sites and microstructural inhomogeneities, particularly in the irradiated beltline welds. Overall, fracture mechanical testing of the materials demonstrated that miniature compact tension specimens are effective for characterizing the fracture toughness of limited surveillance weld materials, revealing differences due to irradiation and weld location.

Effects of crystallographic orientation and lamellar configuration on fatigue crack propagation in single-colony structures of Ti–6Al–4V alloy: Alternating shear crack growth vs. damage accumulation crack propagation

In this study, the fatigue crack propagation mechanisms in lamellar colonies of Ti–6Al–4V alloy were examined using miniature compact-tension specimens with a single-colony structure at the crack tip. Fatigue tests were performed on colonies with different α-phase crystallographic orientations and lamellar configurations with respect to the notch plane and direction, followed by post-fatigue metallographic analysis. Crack propagation occurs by three main mechanisms: 1) alternating shear due to in-plane prismatic slip, 2) damage accumulation via dislocation–dislocation interaction due to out-of-plane slip, and 3) interlamellar decohesion due to damage accumulation via lamellar interphase boundary–dislocation interaction. The slope of da/dN vs. ΔK is higher for alternating shear crack growth than for damage accumulation crack propagation via dislocation–dislocation interaction. This is due to an incubation period before substantive crack extension in the latter case, which is dominated by degree of strain accommodation associated with the activated slip systems. When the lamellar interphase boundaries are nearly perpendicular to the loading axis, the crack growth rates drastically increase by interlamellar decohesion. This is attributed to the reduced incubation period due to slip incompatibility at the interphase boundary where numerous crack nuclei were formed during damage accumulation process.

Applicability of Miniature Compact Tension Specimens for Fracture Toughness Evaluation of Highly Neutron Irradiated Reactor Pressure Vessel Steels

The applicability of miniature compact tension (Mini-C(T)) specimens to fracture toughness evaluation of neutron-irradiated reactor pressure vessel (RPV) steels was investigated. Three types of RPV steels neutron-irradiated to a high-fluence region were prepared and manufactured as Mini-C(T) specimens according to Japan Electric Association Code (JEAC) 4216-2015. Through careful selection of the test temperature by considering previously obtained mechanical properties data, valid fracture toughness, and reference temperature (To) was obtained with a relatively small number of specimens. Comparing the fracture toughness and To values determined using other larger specimens with those determined using the Mini-C(T) specimens, To values of both unirradiated and irradiated Mini-C(T) specimens were found to be the acceptable margin of error. The scatter of 1T-equivalent fracture toughness values of both unirradiated and irradiated materials obtained using Mini-C(T) specimens did not differ significantly from the values obtained using larger specimens. The correlation between the Charpy 41 J transition temperature (T41J) and the To values agreed very well with that of the data in the literature, regardless of specimen size and fracture toughness of the materials before irradiation. Based on these findings, it was concluded that Mini-C(T) specimens can be applied to fracture toughness evaluation of neutron-irradiated materials without significant specimen size dependence.

Fracture Toughness Evaluation of Reactor Pressure Vessel Steels by Master Curve Method Using Miniature Compact Tension Specimens

We conducted fracture toughness testing on five types of commercially manufactured steel with different ductile-to-brittle transition temperatures. This was performed using specimens of different sizes and shapes, including the precracked Charpy-type (PCCv), 0.4T-CT, 1T-CT, and miniature compact tension specimens (0.16T-CT). Our objective was to investigate the applicability of 0.16T-CT specimens to fracture toughness evaluation by the master curve method for reactor pressure vessel (RPV) steels. The reference temperature (To) values determined from the 0.16T-CT specimens were overall in good agreement with those determined from the 1T-CT specimens. The scatter of the 1T-equivalent fracture toughness values obtained from the 0.16T-CT specimens was equivalent to that obtained from the other larger specimens. Furthermore, we examined the loading rate effect on To for the 0.16T-CT specimens within the quasi-static loading range prescribed by ASTM E1921. The higher loading rate gave rise to a slightly higher To, and this dependency was almost the same for the larger specimens. We suggested an optimum test temperature on the basis of the Charpy transition temperature for determining To using the 0.16T-CT specimens.

Stress-Induced Martensite in Front of Crack Tips in NiTi Shape Memory Alloys: Modeling Versus Experiments

NiTi-based shape memory alloys (SMAs) exhibit an unusual stress distribution at the crack tip as compared to common engineering materials, due to a stress-induced martensitic transformation resulting from highly localized stresses. Understanding the fracture mechanics of NiTi-based SMAs is critical to many of their applications. Here, we develop an analytical model, which predicts the boundaries of the transformation region in the crack tip vicinity of NiTi-based SMAs. The proposed model is based on a recent analytical approach which uses modified linear elastic fracture mechanics concepts to predict the crack tip stress distribution and transformation region in SMAs but, unfortunately, it applies only to the plane stress condition. To overcome this limitation, the proposed model accounts for stress triaxiality, which plays an important role in restricting crack tip plastic deformations in common ductile metals as well as the stress-induced martensite in NiTi SMAs. The effects of triaxial stress at the crack tip are taken into account by including a new parameter, the transformation constraint factor, which is based on the plastic constraint factor of elasto-plastic materials. The predictions of the model are compared with synchrotron x-ray micro-diffraction observations and satisfactory agreement is observed between the two results. Finally, the evolution of crack tip transformation boundaries during fracture tests of miniature compact tension specimens is predicted and the effects of applied load and crack length are discussed.

Use of Broken Charpy V-notch Specimens from a Surveillance Program for Fracture Toughness Determination

Abstract Broken Charpy specimens from the surveillance program of a Belgian nuclear power plant are used to obtain the fracture toughness in the transition regime using the master curve concept. Two approaches are used. The first one is based on the reconstitution of Charpy specimens that are subsequently precracked and tested in three-point bending. The second approach is an innovative one that makes use of miniature Compact Tension specimens, which are machined from the broken pieces of the original Charpy specimens. Both approaches lead to consistent results. The recently developed miniature Compact Tension design has the advantage to use less material and is less costly in terms of machining operation. For the material investigated in this research, the current regulation, based on a semi-empirical approach to obtain the lower bound fracture toughness, is demonstrated to be conservative as compared to the direct fracture toughness determination.

Fracture toughness of provisional resins for fixed prosthodontics

Six resins for treatment restorations were tested to determine fracture resistance. The resins were broadly categorized chemically as: one epimine, two poly(methyl methacrylate)s, one composite, and two poly (R' methacrylate)s. Miniature compact tension specimens were molded and then cured for 5 minutes at atmospheric pressure or at an additional pressure of approximately 0.14 MPa. A commercially available pressure vessel was used for curing. The epimine and two poly(methyl methacrylate) resins demonstrated the greatest fracture toughness whereas poly (R' methacrylate) had the lowest. The composite, representing a new class of provisional material, recorded an intermediate fracture toughness. The use of a pressure vessel during polymerization did not significantly increase the fracture toughness of the six resins. There was minimal effect of pressure on the surface of the specimens but it reduced the internal porosity evident in the atmospheric pressure-cured specimens.

Effects of ‘Streaks’ on the Fracture Toughness of Chromium–Molybdenum Rail Steel

AbstractMicrostructural inhomogenities or ‘streaks’ of lower bainite were found in the webs of experimental ingot and continuously cast Cr–Mo steel rails. The effect of these inhomogenities on the fracture toughness behaviour of rail web steels was investigated by means of miniature compact tension specimens (W = 13.5 mm) and compared to results obtained from similar specimens taken from 1% Cr and carbon rail steels.It was concluded that the bainite ‘streaks’ did not lead to a loss of fracture toughness and that KQ values obtained from miniature specimens were comparable to those determined from standard compact tension specimens (W = 25.4 mm) taken from rail heads of corresponding rail steels. Resume Des non-homogeneites de microstructure ou “traits” de bainite, inferieure ont ete trouve dans les âmes des rails fabriques en aciers differents, Cr–Mo experimentaux, de coulee continue et de coulee en lingot. L'effet de ces non-homogeneites sur le comportement en tenacite a la rupture des aciers a âme de rai...