Dealing with the Fracture Ductile-to-Brittle Transition Zone of Ferritic Steels Containing Notches: On the Applicability of the Master Curve

ABSTRACT: Characterizing the fracture resistance of ferritic steels operating within their Ductile-to-Brittle Transition Zone (DBTZ) has been successfully addressed through the development of the well-known Master Curve (MC). This tool assumes that fracture, in the presence of crack-like defects, is controlled by weakest-link statistics and follows a three-parameter Weibull distribution. When dealing with notch-type defects, there is no standardized solution to predict the fracture resistance within the DBTZ, but the authors have published some works demonstrating that the MC can also be applied in different ways to characterize ferritic steels containing notches. One of these ways is the direct application of the MC methodology, providing a specific reference temperature (T0N) for each material and notch radius. This work reviews this initial attempt to apply the MC in notched conditions, assessing the validity of the main MC hypotheses (initially valid for cracked conditions) when analyzing notch-type defects and providing experimental validation on steels S275JR, S355J2, S460M and S690Q

Some thoughts about the application of the Master Curve methodology to ferritic steels containing notches

ABSTRACT: The Master Curve (MC) is an engineering tool that allows the fracture toughness of ferritic steels operating within their ductile-to-brittle transition zone to be estimated. It is based on statistical considerations, related to the distribution of cleavage-promoting particles around the crack tip, and assumes that: a) fracture is controlled by weakest link statistics; b) it follows a three parameter Weibull distribution. The authors have previously provided two different approaches for applying the MC in notched conditions. The first one consists of determining the reference temperature (T0) in cracked conditions and applying a subsequent notch correction to estimate the fracture toughness at a given temperature; the second one proposes obtaining directly a notch reference temperature (T0N) for a given notch radius by testing notched specimens. This second approach assumes that both the Weibull parameters (Kmin=20 MPam1/2 and b=4) and the censoring criteria used in cracked conditions are applicable in notched conditions. This paper provides some thoughts about these assumptions with the aim of analysing the applicability of the MC in ferritic steels containing notches, and includes specific validation on steels S460M and S690Q

Fracture Load Estimations for U-Notched and V-Notched 3D Printed PLA and Graphene-Reinforced PLA plates using the ASED Criterion

This paper addresses the estimation of critical loads in FFF (Fused Filament Fabrication) printed polymers and composites containing notches. Particularly, the analysis is focused on the fracture load estimations of 39 PLA (polylactic acid) and 39 graphene reinforced PLA (PLA-Gr) printed plates containing two different types of notches (U- and V-notches) and combining different plate thicknesses and defect length to plate width (a/W) ratios. The addition of graphene (1 wt.%) increases both the yield stress and the ultimate tensile strength, also reducing the strain at rupture and, thus, generating a material whose behavior is closer to linear elasticity. Among the different assessment tools that may be used to estimate critical loads, this work applies the well-known Averaged Strain Energy Density (ASED) criterion, which compares the averaged strain energy over a certain control volume at the notch tip with the corresponding critical value, the latter being a material property. This approach has a linear-elastic nature, so its application to non-fully linear materials may require the use of specific corrections or calibrations. For the two materials analyzed here, PLA and PLA-Gr, it has been observed that the ordinary linear-elastic ASED criterion provides good estimations for the PLA-Gr material, whereas the pristine PLA, with more evident non-linear behavior, the obtainment of accurate results requires a previous specific calibration of the ASED material parameter

Environmental fatigue analysis of nuclear structural components: Assessment procedures, loads, and a case study

ABSTRACT: Nowadays, environmental fatigue assessment is mandatory in many countries, in the design and operational stages of nuclear structural components. The analysis of environmental fatigue can be a complex engineering process that is generally performed following national or international procedures. Such procedures are not always based on the same assumptions, and novel analysts may find a confusing variety of documents. Moreover, once a specific procedure has been chosen for the analysis, it is possible to complete the fatigue assessment by using design transients (and loads) or, alternatively, real loads provided by monitoring systems. In this context, this paper provides a comprehensive review of the different environmental fatigue assessment procedures and a brief description of the different types of load inputs (design vs. real data). The work is completed with a case study, in which the (fatigue) cumulative usage factor is estimated in a particular nuclear component by using one of the abovementioned assessment procedures (NUREG/CR-6909) and two options for the load inputsFunding: This research was funded by EURATOM, grant number 662320

Multi-wall carbon nanotubes do not necessarily improve the fracture behaviour of the epoxy matrix

ABSTRACT: Carbon nanotubes (CNTs) have been widely studied in the literature for their potential benefits as reinforcement in epoxy matrices. However, the enhancement that these nano-materials can provide to the corresponding epoxy matrix is largely dependent on the manufacturing process and other factors such as the wt.% or the nanotube type, showing different effects on the resulting mechanical properties. In this study, five contents of multi-wall carbon nanotubes (MWCNTs) were introduced in an epoxy resin to analyse their effects on both tensile and fracture properties, with the fracture behaviour being characterised in cracked and in notched conditions (with various radii). The experimental results showed a drastic deterioration of the tensile strength for MWCNTs contents higher than 0.1 wt.%, and no benefit was found in the fracture resistance (only conducted for 0.1 wt.% due to the negative effect observed in the tensile results). Therefore, it seems that the CNTs do not always improve the fracture behaviour of the epoxy matrix.The authors of this work would like to express their gratitude to the Spanish Ministry of Science and Innovation for the financial support of the project PGC2018-095400-B-I00 “Comportamiento en fractura de materiales compuestos nano-reforzados con defectos tipo entalla”, on the results of which this paper is based

Environmental Fatigue Analysis of nuclear components within the framework of INCEFA-SCALE project

ABSTRACT: INCEFA-SCALE started in October 2020. The objective of this project is to improve the capacity to predict the lifetime of Nuclear Power Plant (NPP) components subjected to environmental assisted fatigue. The project starts off by analysing the existing data and then provides new environmentally assisted fatigue data which allow the laboratory test outcomes to be applied to components with real geometries and loads. So far, the data mining of different finished projects (INCEFA-PLUS, USNRC, EPRI, MHI and AdFaM) has been carried out, and test conditions for filling the knowledge gaps have been established. Moreover, the test matrix for 2022 has been defined. In this first phase, tests are focused on producing reference data, analysing complex waveforms (variable amplitude) and the effect of the surface finish. The next testing phases will focus on particular conditions: multi-axial tests, notches, stress/strain gradient effect and size effect. Furthermore, the microstructural analysis of common materials and a guideline for fatigue striation measurement on the fracture surface have been developed. This article provides an update on the project status and the advances made in data analysis, mechanical understanding and testing conditionsThis project has received funding from the Euratom research and training program 2019-2020 under grant agreement No 945300. The contributions of all partners in the INCEFA-SCALE project are also acknowledge

Analysis of additively manufactured notched PLA plates using failure assessment diagrams

This paper provides a methodology for the estimation of the load-bearing capacity of additively manufactured (AM) PLA plates containing different types of notches (U-notches, V-notches and holes). The methodology is based on the use of Failure Assessment Diagrams (FADs), which are the main fracture-plastic collapse assessment tool provided by structural integrity assessment procedures, such as BS7910 and API 579-1/ASME FFS-1. When analyzing notch-type defects, the FAD methodology requires the application of a notch correction which, in this work, is based on the Theory of Critical Distances (TCD) and the Creager-Paris stress distribution ahead of the crack-tip. The results show that the FAD methodology can be efficaciously applied in this AM polymer, providing safe conservative estimations of critical loads in U-notched and V-notched plates, and accurate slightly unsafe estimations in plates with central hole. The cracking behavior in the different tested plates is a complex procedure generated by a combination of filament failures and debonding processes.This publication is part of the project “Comportamiento en fractura y efecto entalla en compuestos de matriz termopl´astica obtenidos por fabricaci´on aditiva, PID2021-122324NB-I00” funded by MCIN/ AEI /10.13039/501100011033/FEDER “Una manera de hacer Europa”

Analysis of additively manufactured PLA containing notches using Failure Assessment Diagrams

ABSTRACT: This paper provides a methodology for the estimation of the load-bearing capacity of additively manufactured (AM) PLA specimens that may be applied to both cracked and notched conditions. The methodology is based on the use of Failure Assessment Diagrams (FADs), which are, in practice, the main fracture-plastic collapse assessment tool provided by structural integrity assessment procedures. When dealing with notch-type defects, the methodology requires, additionally, the application of a notch correction that it is based on the Theory of Critical Distances (TCD) and the Creager-Paris stress distribution ahead of the cracktip. The results show that the FAD methodology (alone, in cracked conditions, or in combination with the TCD in notched conditions) can be successfully applied in this AM polymer.This publication is part of the project “Comportamiento en fractura de materiales compuestos nano-reforzados con defectos tipo entalla, PGC2018-095400-B-I00” funded by MCIN/ AEI /10.13039/501100011033/ FEDER "Una manera de hacer Europa

Out-of-plane constraint loss in three point bend specimens with notches

ABSTRACT: This paper presents an experimental and numerical study of the effect of specimen thickness on the effective notch toughness for cleavage fracture measured using Single Edge Notch Bend (SENB) specimens containing a U-notch instead of a fatigue pre-crack. These specimens are typically used to measure a material's effective notch toughness and to assess failure of a structure containing a non-sharp defect using the Notch Failure Assessment Diagram (NFAD). Both the experimental data and the Finite Element (FE) failure predictions show a significant influence of specimen thickness on , over and above the microstructural weakest link effect arising from differences in the volume of the plastic zone. is a function of not only the in-plane effect of the notch radius, but also an out-of-plane constraint loss which itself is enhanced by the presence of the notch radius. Significant out-of-plane constraint loss occurred for notched specimens with a ratio of thickness to width of 0.5, a geometry that if pre-cracked would have met the minimum thickness requirement mandated by ASTM E1820. Doubling the thickness to = 1.0 was sufficient to eliminate the out-of-plane constraint loss. The use of experimentally measured values in an NFAD assessment of a structure may therefore be non-conservative if <1.0.This paper is published with the permission of Wood, the University of Cantabria, and the UK Atomic Energy Authority. The authors of this work would like to express their gratitude for financial support from: - The Spanish Ministry of Science and Innovation, under project MAT2014-58443-P: “Análisis del comportamiento en fractura de componentes estructurales con defectos en condiciones debajo confinamiento tensional”, on which the results of this paper are based; - Wood, via a contribution to the R6 Development Programme; - The Research Councils UK (RCUK) Energy Programme [grant number EP/P012450/1]