On the use of the crack tip opening angle parameter to explain the ductile crack growth behavior of miniature compact specimens

Experimental crack resistance curves obtained from miniature compact tension, MC(T), specimens were found to be significantly less tough than those obtained on standard one inch 1T-C(T) specimens. In order to investigate the fundamental reasons behind this unexpected result, local approaches to fracture based on the Rice and Tracey void growth model and the crack tip opening angle (CTOA) concept are used. Local crack growth criteria are identified on test results obtained from 1T-C(T) and are used to predict the MC(T) behavior. Results demonstrate that the CTOA parameter is very effective as it allows transferring results from MC(T) to larger specimens, it is easy to implement in a finite element code, it is mesh size insensitive and can be actually measured although its experimental determination is not straightforward. The β parameter derived from the Rice and Tracey void growth model is unable to explain the experimental results. The possible reasons for the poor performance of the β parameter are discussed.     

Validation and application of the Master Curve and reconstitution techniques to a Spanish nuclear vessel

In this work, a methodology based on the Master Curve approach and the reconstitution of specimens is validated and applied to the vessel base metal of the currently in service Spanish boiling water reactor of the Santa María de Garoña nuclear power plant. The extensive experimental program performed consisted in the characterisation of the ductile to brittle transition region with standard and reconstituted specimens using subsized compact tension and Pre-Cracked Charpy V-notch specimens, under non-irradiated and irradiated conditions. Experimental results validated the reconstitution technique down to inserts of 10 mm which allows specimen reorientation and therefore, the comparison of LT and TL material orientations. The 110 specimens tested in this program allowed the Master Curve approach to be validated for the base steel of Santa María de Garoña nuclear power plant. By comparing the results for the compact and Pre-Cracked Charpy V-notch specimens, the existence of a systematic bias between these two geometries has been analysed. Although the neutron irradiation effect on the condition studied is predicted to be very limited (due to the small fluence), the results did allow the irradiation-induced shift of the ductile to brittle transition temperature to be detected. Comparison between the directly measured fracture toughness and the conventional semi-empirical approach proposed by the ASME Code reveals the overconservatism of the latter approach.     

Effect of prior crack growth on cleavage fracture of low toughness precracked Charpy specimens

Cleavage fracture of reactor pressure vessel steels in the upper ductile to brittle transition region generally occurs with prior significant ductile crack growth. For low upper shelf materials and using PreCracked Charpy v-notch (PCCv) specimens that can be obtained from conventional surveillance programs, the effect of prior crack growth could be particularly important. In practice, the shape of the Master Curve and the failure distribution could be affected by ductile crack growth. To quantify the effect in practical applications, the effect of prior ductile on cleavage is evaluated on PCCv specimen.The methodology use finite element calculations to grow a ductile crack and infer the brittle failure probability using the local approach to fracture. It is found that for very low upper shelf toughness materials, ductile crack growth enhances the failure probability, induces a steeper failure distribution and affects the shape of the Master Curve. However, for low toughness materials, the enhanced failure probability due to crack growth is compensated by loss of constraint.     

Assessment of the master curve approach on three reactor pressure vessel steels

This study aims at assessing the applicability of the Master Curve procedure to the measurement of the reference temperature for three well-characterised reactor pressure vessel steels (22NiMoCr37, JSPS, JRQ). The following aspects of the methodology were investigated, using statistical tools such as the Generalised Maximum Likelihood (GML) and Monte Carlo methods: independence of T _o from test temperature and specimen type (configuration/dimensions); formula given in ASTM E1921 for evaluating the standard deviation of the reference temperature, and possibile expressions for estimating the standard deviation of the other Weibull parameters (m and K _min); proposed relationships for estimating the median toughness and standard deviation for T _o(_To) in the case of multi-temperature Master Curve analysis. In reference to the three Reactor Pressure Vessel Steels (RPVS) investigated, the independence of T _o was assessed from the test temperature (within the range prescribed by the following revision of the ASTM standard, T _o±50 °C) and, as far as C(T) specimens are concerned, from the sample dimensions; the well-known 10÷15 °C difference was however found between PCCv and C(T) geometries. Furthermore, using the Monte Carlo method, we assessed the relationship proposed by the ASTM standard for estimating the standard deviation of the reference temperature, which results fairly conservative; an alternative analytical function has been proposed. Clear trends have also been identified for the standard deviation of m (with respect to the number of valid data r) and K _min (with respect to the median toughness). Finally, the proposed relationships for estimating K _Jc,med and _To for the multi-temperature analysis have been validated by comparison with the results of the Monte Carlo method: an excellent agreement was found in terms of _To (better than 0.2 °C).     

SCK-CEN Contribution to the IAEA Round Robin Exercise on WWER-440 RPV Weld Material: Irradiation,Annealing, and Re-Embrittlement

Mechanical properties of WWER-440 RPV weld joints have been studied with account of different states of the material: baseline (unirradiated), irradiated up to the average fast neutron fluence of 6.5 10¹⁹ n/cm², irradiated and eventually annealed, re-irradiated with the accumulated fast neutron fluence of 1.3 10²⁰ n/cm², Tensile, impact fracture, and fracture toughness tests were performed for each state of the material with the use of Charpy specimens (standard, reconstituted, and pre-cracked).     

An Overview of Systematic Reviews of the Role of Vitamin D on Inflammation in Patients with Diabetes and the Potentiality of Its Application on Diabetic Patients with COVID-19

Almost two years have passed since the outbreak reported for the first time in Wuhan of coronavirus disease 2019 (COVID-19), due to severe acute respiratory syndrome (SARS)-CoV-2 coronavirus, rapidly evolved into a pandemic. This infectious disease has stressed global health care systems. The mortality rate is higher, particularly in elderly population and in patients with comorbidities such as hypertension, diabetes mellitus, cardiovascular disease, chronic lung disease, chronic renal disease, and malignancy. Among them, subjects with diabetes have a high risk of developing severe form of COVID-19 and show increased mortality. How diabetes contributes to COVID-19 severity remains unclear. It has been hypothesized that it may be correlated with the effects of hyperglycemia on systemic inflammatory responses and immune system dysfunction. Vitamin D (VD) is a modulator of immune-response. Data from literature showed that vitamin D deficiency in COVID-19 patients increases COVID-19 severity, likely because of its negative impact on immune and inflammatory responses. Therefore, the use of vitamin D might play a role in some aspects of the infection, particularly the inflammatory state and the immune system function of patients. Moreover, a piece of evidence highlighted a link among vitamin D deficiency, obesity and diabetes, all factors associated with COVID-19 severity. Given this background, we performed an overview of the systematic reviews to assess the association between vitamin D supplementation and inflammatory markers in patients with diabetes; furthermore, vitamin D’s possible role in COVID-19 patients was assessed as well. Three databases, namely MEDLINE, PubMed Central and the Cochrane Library of Systematic Reviews, were reviewed to retrieve the pertinent data. The aim of this review is to provide insight into the recent advances about the molecular basis of the relationship between vitamin D, immune response, inflammation, diabetes and COVID-19.     

Application of advanced master curve approaches on WWER-440 reactor pressure vessel steels

The master curve (MC) approach used to measure the transition temperature, T₀, was standarised in the ASTM Standard Test Method E 1921 in 1997. The basic MC approach for analysis of fracture test results is intended for macroscopically homogeneous steels with a body centred cubic (ferritic) structure only. In reality, due to the manufacturing process, the steels in question are seldom fully macroscopically homogeneous.     

Polysialic Acid Sustains the Hypoxia-Induced Migration and Undifferentiated State of Human Glioblastoma Cells

Gliomas are the most common primary malignant brain tumors. Glioblastoma, IDH-wildtype (GBM, CNS WHO grade 4) is the most aggressive form of glioma and is characterized by extensive hypoxic areas that strongly correlate with tumor malignancy. Hypoxia promotes several processes, including stemness, migration, invasion, angiogenesis, and radio- and chemoresistance, that have direct impacts on treatment failure. Thus, there is still an increasing need to identify novel targets to limit GBM relapse. Polysialic acid (PSA) is a carbohydrate composed of a linear polymer of α2,8-linked sialic acids, primarily attached to the Neural Cell Adhesion Molecule (NCAM). It is considered an oncodevelopmental antigen that is re-expressed in various tumors. High levels of PSA-NCAM are associated with high-grade and poorly differentiated tumors. Here, we investigated the effect of PSA inhibition in GBM cells under low oxygen concentrations. Our main results highlight the way in which hypoxia stimulates polysialylation in U87-MG cells and in a GBM primary culture. By lowering PSA levels with the sialic acid analog, F-NANA, we also inhibited GBM cell migration and interfered with their differentiation influenced by the hypoxic microenvironment. Our findings suggest that PSA may represent a possible molecular target for the development of alternative pharmacological strategies to manage a devastating tumor like GBM.