Using a strain based failure assessment diagram for creep–brittle materials

Recently, there has been considerable interest in studying creep crack growth in creep–brittle materials. For example, the methodologies for assessing creep ductile materials, using fracture mechanics parameters like C^* and C_t, have been extended to include creep–brittle materials [1]. This paper begins by examining these recent developments and outlines the difficulties in adopting these developments. An alternative approach is then proposed. This new approach leads from recent work proposed by the authors [2], where a strain based failure assessment diagram (SB-FAD) is used. Experimental results from a series of tests on a simulated heat affected zone of a low alloy steel are explored. The application of the methodology for assessing the initiation and growth of a defect in a creep–brittle material is also demonstrated.     

Evaluation of fracture mechanics parameters for bimaterial compact tension specimens

Abstract

The elastic–plastic fracture mechanics parameter J and its analogous creep fracture parameter C* are widely used to measure the fracture resistance of a material. The non-linear component of the J and C* parameters can be evaluated experimentally using the η factor. For weldments, the η factor is dependent on the relative properties of the base (parent) and weld materials, particularly the mismatch in their yield strengths. In this work, the η factor has been evaluated using non-linear finite element analyses in a standard compact tension C(T) specimen for a power law material. A range of mismatches in base/weld material properties have been considered. A through thickness strip of weld material, of height 2h, has been modelled, which was positioned at the mid height of the specimen. The η factor has been evaluated for a range of crack lengths and power law hardening exponents under both plane stress and plane strain conditions and the results compared with literature where available. For a given crack length and weld width, the η solutions of the undermatched and overmatched conditions examined show a maximum variation of 12% from the mean value. A relationship has been proposed with respect to crack length for the C(T) specimen to describe the decrease in the η factor with an increase in mismatch ratio.     

On the probabilistic failure assessment diagram

Reliability analysis of structures containing defects carried out directly by practising engineers is quite complex and costly. Therefore, a simple and convenient reliability analysis method should be investigated and established. In this paper, the principle and building procedure of a reliability analysis method are described and the feasibility of the method are also explicitly clarified. Based on the assumptions that: (1) the load is a deterministic variable; (2) the yield strength and fracture toughness satisfy normal distribution and their variational coefficients are constant; (3) the initial crack length remains unchangeable, a set of probabilistic failure assessment diagrams (PFAD) called elementary reliability assessment method are established under the simplified option 1 curve of the 3rd version of CEGB R6. The investigation shows that the distribution of random safety factor and failure probability of a structure only depend on the coordinates of the expectation of P(L_r,K_r) in the failure assessment diagram of the simplified option 1 curve and is not influenced by the other factors. The investigation further shows that the failure probability will decrease with the increase of the initial crack length if the expectation of P(L_r,K_r) keeps unchanged. A conservative result will be given when using the PFAD as an elementary reliability analysis method to assess the integrity of a structure and only a simple procedure is required like the deterministic assessment method given in CEGB R6. When the secondary stress exists, the conservativeness of the result assessed will increase with the increase of parameter ρ although the PFAD is only established based on considering primary stress.     

Use of the failure assessment diagram to deduce ductile fracture toughness of the RAFM steel EUROFER97

In the current work we use the failure assessment diagram (R6) to deduce ductile fracture toughness for the reduced activation ferritic/martensitic steel EUROFER97 by applying Small Specimen Testing Technology. Fracture parameters have been determined in quasi-static three-point-bend experiments. The fracture toughness results obtained with option 1 curve of R6 are sensibly independent of specimen geometry, constraint state and initial crack length and agree well with the results obtained by the analysis of crack resistance curves. Application of option 2 curve of R6 results into less conservative fracture toughness values.     

A temperature and strain rate dependent strain hardening law

A temperature and strain rate dependent strain hardening law is proposed to describe stress-strain-temperature-strain rate relations. The formulation is purely empirical, based on experimental data of an alloy steel. Agreement between the proposed temperature-strain rate dependent strain hardening law and experimental data is very good over a wide range of temperatures and strain rates.     

Continuum damage mechanics analyses of type IV creep failure in ferritic steel crossweld specimens

A major high temperature failure mechanism for weldments in ferritic steel steam pipework is circumferential creep cracking within the region of the heat affected zone, adjacent to the parent material, that experiences the lowest temperatures during the welding process. This is commonly known as type IV cracking. In recent years a number of experimental studies have investigated the occurrence of type IV failure in laboratory test pieces, however, there have been few attempts at theoretical modelling of type IV failure to assist in the formulation of design and assessment procedures. This report discusses the use of the creep continuum damage mechanics method for the analysis of the deformation and failure of weldments that are known to fail within the type IV region.The creep behaviour of each of the material regions of a weldment is described with a set of physically based constitutive equations, which incorporate a number of state variables. The finite element creep continuum damage mechanics method is used, with the physically based constitutive equations, to analyse the deformation and failure of the welded testpieces. The computations are shown to be in good agreement with the experimental results. The implications of the analyses are discussed with reference to the assessment of weldments that are susceptible to type IV failure.     

A technique for the residual life assessment of high temperature components based on creep-rupture testing on welded miniature specimens

Following the present trend in the development of advanced methodologies for residual life assessment of high temperature components operating in power plants, particularly in non destructive methods, a testing technique has been set up at ENEL/CRAM based on creep-rupture tests in argon on welded miniature specimens.

Five experimental systems for creep-rupture tests in an argon atmosphere have been set up which include high accuracy loading systems, vacuum chambers and extensometer devices.

With the aim of establishing and validating the suitability of the experimental methodology, creep-rupture and interrupted creep testing programmes have been performed on miniature specimens (2 mm diameter and 10 mm gauge length).

On the basis of experience gathered by various European research laboratories, a miniature specimen construction procedure has been developed using a laser welding technique for joining threaded heads to sample material. Moreover, a special device for removing material-reduced samples from in-service components is also in development.

Low alloy ferritic steels, such as virgin 2.25Cr1Mo, 0.5Cr 0.5Mo 0.25V, and IN 738 superalloy miniature specimens have been investigated and the results, compared with those from standard specimens, show a regular trend in deformation vs time.

Additional efforts to provide guidelines for material sampling from each plant component will be required in order to reduce uncertainties in residual life prediction.     

Upgrading of syngas derived from biomass gasification: A thermodynamic analysis

Hydrogen yields in the syngas produced from non-catalytic biomass gasification are generally low. The hydrogen fraction, however, can be increased by converting CO, CH₄, higher hydrocarbons, and tar in a secondary reactor downstream. This paper discusses thermodynamic limits of the synthesis gas upgrading process. The method used in this process is minimization of Gibbs free energy function. The analysis is performed for temperature ranges from 400 to 1300 K, pressure of 1–10 atm (0.1–1 MPa), and different carbon to steam ratios. The study concludes that to get optimum H₂ yields, with negligible CH₄ and coke formation, upgrading syngas is best practiced at a temperature range of 900–1100 K. At these temperatures, H₂ could be possibly increased by 43–124% of its generally observed values at the gasifier exit. The analysis revealed that increasing steam resulted in a positive effect. The study also concluded that increasing pressure from 1 to 3 atm can be applied at a temperature >1000 K to further increase H₂ yields.     

A steel slag–derived Boudouard reaction catalyst for improved performance of direct carbon solid oxide fuel cells

Solid oxide fuel cells (SOFCs) can directly utilize solid carbon as fuel by integrating with the reverse Boudouard reaction in the anode chamber. Efficiency of the Boudouard gasification of solid carbon fuel is one of the crucial factors influencing the performance of direct carbon SOFCs (DC‐SOFCs). In this paper, a novel Boudouard reaction catalyst derived from steel slag was first introduced into DC‐SOFCs for improving the electrochemical performance. The catalytic activity of the steel slag was activated using the molten alkali method to decompose the inert mineral phases of the raw material. The steel slag–derived catalyst was loaded on the activated charcoal by a wet ball milling method. This kind of catalyst can match up to the readily available solid carbon fuels in cost. Promoted by this highly active Boudouard reaction catalyst, the initial Boudouard gasification temperature of the carbon fuel decreased by 99°C, and the producing rate of carbon monoxide doubled. Furthermore, the power outputs of the fuel cells increased from 91 to 159 mW cm^?2, and the fuel utilization increased from 17.10% to 46.43% at 825°C. This study demonstrates that the steel slag–derived catalyst is a promising material for the performance improvement of DC‐SOFCs and may make a valuable contribution to their commercial application.     

A green sulfonated carbon-based catalyst derived from coffee residue for esterification

A green sulfonated carbon-based catalyst was successfully synthesized through sulfonation of incompletely carbonized coffee residue (SCAC catalyst). The sulfonation temperature was investigated and the catalytic activity was tested via esterification of caprylic acid. SCAC-200, the highest caprylic acid conversion at 4 h (71.5%) and initial TOF based on SO₃H acid sites, was synthesized under a carbonization temperature of 600 °C for 4 h and sulfonation temperature of 200 °C for 18 h. Sulfonation temperature plays a dominant role in determining the SO₃H site density of SCAC catalysts because side reactions (condensation/oxidation/dehydrogenation) take place at high sulfonation temperature as indicated by oxygen-to-carbon surface ratio. The activities of the SCAC catalysts were also substantially greater than that of Amberlyst-15. A high catalytic activity and catalyst stability for esterification of HCp were achieved in the SCAC catalysts with high surface area and by balancing strong (SO₃H) and weak (carboxylic and phenolic) acid site densities. Additionally, this catalyst could be regenerated to obtain essentially its initial catalytic activity by MeOH washing. Therefore, the sulfonated coffee residue derived catalyst is promising, economic eco-friendly and potentially substituted for homogeneous H₂SO₄ catalyst for esterification in industries in the near future.     

A microporous carbon derived from metal-organic frameworks for long-life lithium sulfur batteries

A polyhedral microporous carbon derived from metal-organic frameworks (ZIF-8) could present good property for sulfur loading and trapping. A melting-evaporation route was adopted to synthesize two sulfur/microporous carbon (S/MC) composites, of which sulfur content is controllable, and ether-based or ester-based electrolytes were used to evaluate the synthesized composites for the lithium sulfur batteries. According to electrochemical results, the S/MC composite with 65.2 wt% S in the ether-based electrolyte exhibited optimized performance as compared with the composite with 65.2 wt% S in the ester-based electrolyte, as well as the composite with 58.6 wt% S in the two kinds of electrolytes. For the S/MC composite with 65.2 wt% S in ether-based electrolyte, the initial discharge capacity could reach up to 1505.9 mAh g⁻¹ and the reversible capacity could be 833.3 mAh g⁻¹ after 40 cycles at 0.1 C. Furthermore, while being respectively evaluated at 0.5, 1.0, and 2.0 C, the discharge capacities could still maintain at 544, 493 and 354 mAh g⁻¹ after 300, 500, and 800 cycles, demonstrating appreciable cyclic reversibility and rate capability.     

The costs of failure: A preliminary assessment of major energy accidents, 1907–2007

A combination of technical complexity, tight coupling, speed, and human fallibility contribute to the unexpected failure of large-scale energy technologies. This study offers a preliminary assessment of the social and economic costs of major energy accidents from 1907 to 2007. It documents 279 incidents that have been responsible for $41 billion in property damage and 182,156 deaths. Such disasters highlight an often-ignored negative externality to energy production and use, and emphasize the need for further research.     

Southeastern United States wood pellets as a global energy resource: a cradle-to-gate life cycle assessment derived from empirical data

Given increased policies driving renewable electricity generation and insufficient local production of woody biomass, many countries are reliant upon the importation of wood pellets. Of current wood pellet exports, the vast majority originates from the Southeastern United States (US). In this paper we present results from a cradle-to-gate, attributional process life cycle assessment in which two production scenarios of wood pellets were modelled for the Southeastern US: one utilising roundwood from a silviculture operation and the other utilising sawmill residues. The system boundary includes all steps from harvesting of the wood biomass, through delivery of the finished wood pellets to a US port facility. For each of the impact categories assessed, wood pellets from sawmill residues resulted in higher values, ranging from 5% to 31%. In relation to Global Warming Potential, roundwood pellets resulted in a 13–21% lower value than pellets produced from sawmill residues, depending upon the allocation method.     

A robust electrocatalytic activity and stability of Pd electrocatalyst derived from carbon coating

Palladium (Pd) as an efficient anodic catalyst has been extensively investigated in direct formic acid fuel cells (DFAFCs); while, Pd catalyst is electrochemically unstable in acidic electrolyte resulting in low stability retarding the widespread application of DFAFCs. In this study, a new method is invented to prevent the Pd nanoparticles from rapid dissolution by carbon layer originated from the carbonization of glucose. Ascribing to the presence of carbon layer, Pd electrocatalyst demonstrates much higher stability in comparison with Pd electrocatalyst without carbon layer in the course of stability tests. Robust electrocatalytic activities toward formic acid and methanol/ethanol oxidation are observed for carbon-stabilized Pd electrocatalyst resulted from the higher content of metallic Pd atoms coming from the carbonization process, in which Pd (II) species are further reduced. Moreover, the fuel cell performance of carbon-stabilized Pd electrocatalyst reaches 90 mW cm_Pd⁻² measured with 1 M formic acid; while, power density of bare Pd electrocatalyst is only 74 mW cm_Pd⁻². This work highlights that carbon layer carbonized from glucose improves not only the stability of Pd electrocatalyst, but also the electrocatalytic activity.     

A simple model of the wind turbine induction zone derived from numerical simulations

The induction zone in front of different wind turbine rotors is studied by means of steady‐state Navier‐Stokes simulations combined with an actuator disk approach. It is shown that, for distances beyond 1 rotor radius upstream of the rotors, the induced velocity is self‐similar and independent of the rotor geometry. On the basis of these findings, a simple analytical model of the induction zone of wind turbines is proposed.