Acoustic emission characteristics from steels Part 2: Acoustic measurements from fracture toughness tests

Acoustic emission measurements have been made in conjunction with fracture toughness tests on nine steels of yield strengths from 247–1606 MNm⁻². Tests made under conditions where failure would be analysed in terms of linear elastic fracture mechanics (brittle behaviour) or general yielding fracture mechanics (ductile behaviour) have shown the emission characteristics at brittle and ductile failure to be significantly different. The applicability of relationships of the form (Acoustic Emission) = Const × (Fracture Toughness)ⁿ has been investigated and the effect of test piece size examined. The relative amounts of emission monitored during the tests have permitted an acoustic ‘rating’ of the steels to be made. The validity of the Kaiser effect has been demonstrated but a detrimental influence of possible corrosion/oxidation at the crack faces has been noted. The significance of the results has been assessed in terms of the application of acoustic emission to provide 100% coverage during the periodic inspection of pressure vessels.     

The effect of a long post weld heat treatment on the integrity of a welded joint in a pressure vessel steel

The effect of a long post weld heat treatment on the microstructure and mechanical properties of a welded joint in a 0·2%C-1·4%Mn-0·5%Mo pressure vessel steel was studied. Multipass submerged-arc welds were made at a heat input of 1·2 and 4·3 kJ mm⁻¹. Individual microstructural regions observed in the heat-affected zone of the actual weld were simulated. These regions were brittle in the as-simulated condition. Post weld heat treatment for periods of up to 40 h at 620°C resulted in a significant improvement in the Charpy impact toughness. At the same time, a loss of the heat-affected zone and weld metal hardness and transverse weld strenghth occurred. A fracture toughness (J_Ic) of 134 kJ m⁻² was measured in the heat-affected zone of the 4·3 kJ mm⁻¹ welds after prolonged post weld heat treatment. The improvement in weldment toughness with post weld heat treatment was primarily attributed to softening of the structure.     

Angle-resolved optical characterisation of an electrochromic device

An electrochromic prototype with WO₃ and NiO as electrochromic layers was analysed in an absolute spectrophotometer. The electrochromic glazing was measured in combination with a clear float glass and a low-e glass in order to simulate a ‘real’ window. Similar measurements were performed on a commercial electrochromic product, i.e., a Gentex Night Vision Safety™ (NVS^®) mirror from Gentex Corporation, and the results were compared. The spectral transmittance was measured, in bleached and coloured state, over the solar wavelength range at the angles of incidence, φ=0, 40, 60 and 70°. The direct solar transmittance, T_sol, the visual transmittance, T_vis, and the angular dependence for these parameters were calculated.     

Radiation cooling of buildings at night

The cooling of small buildings at night by radiation loss to the sky has been investigated by monitoring the thermal performance of two huts: one roofed with galvanised steel decking painted white, which acts as a ‘black body’ for wavelengths greater than 3 μm; the other with aluminium decking to which aluminised ‘Tedlar’ sheet had been glued, the ‘Tedlar’ acting as a selective surface absorbing and radiating mainly in the 8–13 μm band.

The hut with the painted roof was cooled marginally better than that with the ‘Tedlar’ covered roof. Useful cooling powers of 22 Wm⁻² were achieved at a roof temperature of 5°C, ambient 10°C, and the gross cooling power probably exceeded 29 Wm⁻². Calculations based on a simple simulation of the sky radiation yield an upper limit of 40 Wm⁻² for the cooling power of the surfaces and suggest that an ideally selective surface operating under the best possible clear-sky conditions has little advantage over a black body radiator unless the temperature of the surfaces is significantly lower than the ambient air temperature.     

Erosive burning of composite solid propellants

A model is proposed for erosive burning of ammonium perchlorate composite wolid propellants. It is assumed that combustion takes place in the turbulent boundary layer and that the turbulent flame consists of distorted ‘flamelets’ but otherwise retains on a molecular scale the characteristics of a laminar flame. A flat-plate heat-transfer correlation with a factor for transpiration is used, neglecting coupling effects of the chemical reactions in the boundary layer. The model equation derived is used to correlate data available in the literature for erosive burning. The equation is found to be applicable for a ‘blowing’ factor, r₈₈¦G 0·02 to 0·03. It is concluded, tentatively, that the controlling chemical reaction is first-order, possibly the gas-phase decomposition of purchlarm acid or of the complex [NH₃---HClO₄].     

Influence of repair welding on cyclic thermal shock behaviour of a RPV nozzle corner

In piping as well as in RPV-nozzles and in surrounding parts of the cylindrical vessel shells, repeated cold water injections have been observed leading to crack initiation and subcritical crack growth.

In the framework of the German Reactor Safety Research Programme, cyclic thermal shock tests have been carried out at the RPV of the decommissioned HDR (Heissdampfreaktor) near Frankfurt applying extremely conservative temperature transients. The crack propagation under access of oxygenated pressurized water was evaluated by fracture mechanics methods. The validation by non-destructive examination, as well as by destructive testing at a trepan, proved the fracture mechanics results to be conservative.

The depression at the nozzle corner caused by the trepan removal has been repaired by ‘temper bead’ welding without stress relief heat treatment, similar to the ‘half bead’ technique of the ASME Boiler and Pressure Vessel Code.

The subsequent thermal shock stressing (1200 cycles) under similar parameters demonstrated, with respect to the weld repair:

1. (1) no material separation extending into the ferritic RPV-wall;

2. (2) the quality of repair welding is comparable to the quality of the as-delivered condition manufactured in the 1960s in a more conventional manner.

The positive experience derived from this work may be helpful in the future in dealing with any cases of cracking in thick-walled large diameter vessels which could require repair welding when a post-weld stress relief heat treatment is not possible.     

Effect of temperature dependence of electrical resistivity on the cooling performance of a single thermoelectric element

The coefficients of performance (COP) φ₀ and φ for a single thermoelectric (TE) element welded with two metal plates were calculated as functions of temperature difference (ΔT) and thermoelectric figure of merit (ZT) from the conventional thermal rate equations and the new thermal rate ones proposed here, respectively. We made an attempt to take the differences in the Seebeck coefficient , electrical resistivity ρ and thermal conductivity κ of TE materials at the hot and cold sides of a TE element into the thermal rate equations on the assumption that their TE properties change linearly with temperature. However, the difference in κ was neglected even in the new thermal rate equations because its temperature dependence was too small when φ was applied to the high-performance Bi–Te alloys. The normalized temperature dependences at 300 K of and ρ were denoted by A and B, respectively. The term of A in the thermal rate equations was canceled out by the Thomson coefficient, but that of B remained. When B > 0 K⁻¹, φ/φ₀ is enhanced more significantly with an increase of B at larger ΔT and lower ZT, and it reached about 1.20 at ΔT = 80 K for Bi–Te alloys with B ≈ 5 × 10⁻³ K⁻¹. It was thus found that the COP of a cooling module is also affected strongly by B as well as ZT.     

Corrosion fatigue behaviour of A508 class III steel in a simulated PWR water environment at 325°C

Fatigue crack growth data in a simulated PWR environment for A508 Class III steel at 325°C generally exhibited good agreement with crack growth data recorded for A533B steel at 288°C. All data were positioned below the ASME XI high R ‘wet’ line.

It has been shown through sulphur printing and fractography that the morphology and distribution of non-metallic sulphide inclusions play an important part in initiating environmentally assisted crack (EAC) growth which is identifiable by its fan-shaped fracture mode.

In regions of EAC crack growth a significant area of the fatigue fracture surface can still exhibit ductile striated growth. Indeed it has been shown that significant EAC growth occurs when the amount of fan-shaped growth prevalent on the fatigue surface exceeds 20%. Also, fan-shaped growth tended to occur at the centre of the test specimen.     

High temperature rate coefficient for the reaction of O(P) with H2 obtained by the resonance absorption of O and H atoms

The reaction of O(³P) with H₂ has been studied behind reflected shock waves in the temperature range of 1713–3532K at total pressures of about 1.4–2.0 bar by Atomic Resonance Absorption Spectroscopy using mixtures of N₂O and H₂ highly diluted in Ar. The O atoms were generated by the fast thermal decomposition of N₂O and the reaction with H₂ was followed by monitoring the time dependent O and H atom concentrations in the postshock reaction zone. For the experimental conditions chosen, the measured O and H atom concentrations were primarily sensitive to the well-known N₂O dissociation and to the studied reaction and hence its rate coefficient could be deduced. The measured rate coefficient data are fitted by the least-squares method to obtain the following three parameter expression: K₄=3.72×10⁶(T/K)^2.17exp(−4080K/T)cm³ mol⁻¹8⁻, which is in excellent agreement with the recent ab initio calculations for the rate coefficient of this reaction in the overlapping temperature range. The present result is also compared to the experimental results reported by earlier investigators.     

Analysing the influences of weld size on fatigue life prediction of FCAW cruciform joints by strain energy concept

The effect of weld size on fatigue life of flux cored arc welded (FCAW) cruciform joints containing lack of penetration (LOP) defect has been analysed by using the strain energy density factor (SEDF) concept. Moreover, new fracture mechanics equations have been developed to predict the fatigue life of the cruciform joints. Load carrying cruciform joints were fabricated from ASTM 517 ‘F’ grade steel. Fatigue crack growth experiments were carried out in a vertical pulsar (SCHENCK 200 kN capacity) with a frequency of 30 Hz under a constant amplitude loading (R=0). It was found that the crack growth rates were relatively lower in the larger welds fabricated by the multipass welding technique than the smaller welds fabricated by the single pass welding technique.     

A human-powered hydrofoil racing-boat: Design and development

A human-powered, hydrofoil-supported, racing boat has been designed, built, tested and developed. At relatively high speeds (4ms⁻¹), this craft provides a more energy efficient means of transport than a conventional displacement hull. An athlete should be capable of powering such a hydrofoil boat through its ‘take-off’ speed of approximately 3·6 ms⁻¹, and then in its ‘foil-borne’ mode of operation, be able to achieve record speeds exceeding 6·0 ms⁻¹. The prototype hydrofoil-supported craft, built for this investigation, required about 287 W of effective power to ‘take-off’, i.e. for the weight of boat plus driver to be entirely hydrofoil-borne, so that the hull was lifted completely out of the water. Due to employing an inappropriate propeller (of only 57% efficiency) and at least 16% excess weight for the boat plus chosen human driver, he could power the craft only to 3·5 ms⁻¹, at which speed the hull had risen so that all but the lowest 5 cm depth of it was above the water. With further developments human-powered hydrofoil craft will fully take off.     

The ignition of oxetane in shock waves and oxidation in a jet-stirred reactor: An experimental and kinetic modeling study

The ignition and oxidation of oxetane have been studied in a single-pulse shock tube under reflected shock wave conditions and also in a jet-stirred reactor (JSR). These experiments cover a wide range of conditions: 1–10 atm, 0.5 ≤ φ ≤ 2.0, 800–1780 K. The ignition delays of oxetane measured in a shock tube have been used to propose an overall dependence of ignition delay time on the concentrations of each component in the gas as: τ = 10^−13.5 exp(13389/T₅)[C₃H₆O]^−0.36[O₂]^−0.59[Ar]^0.088 (units: seconds, moles per cubic decimeters, and Kelvin). Concentration profiles of the reactants, intermediates, and products of the oxidation of oxetane were measured in a JSR. A numerical model, consisting of a detailed kinetic reaction mechanism with 423 reactions (most of them reversible) of 63 species describes the ignition of oxetane in reflected shock waves and its oxidation in a jet-stirred reactor. Fairly good agreement between the observations and the model was obtained. The major reaction paths have been identified through detailed kinetic modeling.     

Instrumented drop-weight test results for normalised and tempered 9Cr1Mo steel

From instrumented drop-weight tests, the nil ductility transition temperature (T_NDT), and a conservative estimate of dynamic fracture toughness (K_Id), at T_NDT for normalised and tempered 9Cr---1Mo steel, are determined to be −25°C and 70 MPa√m, respectively. The latter value agrees well with that determined from pre-cracked Charpy tests. The K_Id/σ_Yd (σ_Yd is the dynamic yield stress) ratio at T_NDT is estimated to be 0·076 √m, in agreement with previous estimates. The uncertainties in crack profile measurement and effect of microstructural variation in the heat affected zone on fracture loads are also discussed.     

Stress corrosion cracking of nuclear reactor pressure vessel and piping steels

This paper presents an extensive investigation of stress corrosion cracking of nuclear reactor pressure vessel and piping steels exposed to hot water. Experimental fracture mechanics results are compared with data from the literature and other laboratories. Thus a comprehensive overview of the present knowledge concerning stress corrosion crack growth rates is provided. Several sets of data confirm that ‘fast’ stress corrosion cracks with growth rates between 10⁻⁸ and 10⁻⁷ m/s and threshold stress intensities around 20 MN m^−3/2 can occur under certain conditions. However, it appears possible that specific environmental, mechanical and metallurgical conditions which may prevail in reactors can result in significantly lower stress corrosion crack growth rates. The presently known stress corrosion crack growth rate versus stress intensity curves are discussed with emphasis on their usefulness in establishing safety margins against stress corrosion cracking of components in service. Further substantial research efforts would be helpful to provide a data base which permits well founded predictions as to how stress corrosion cracking in pressure vessels and piping can be reliably excluded or tolerated. It is emphasized, however, that the nucleation of stress corrosion cracks (as opposed to their growth) is difficult and may contribute substantially to the stress corrosion free service behavior of the overwhelming majority of pressure vessels and pipes.     

Fracture propagation control in gas pipelines: A survey of relevant studies

This paper reviews the problem of crack propagation in gas pressurised pipelines. After summarising the fracture control philosophy for pipelines, the main terminology and basic concepts are defined.

A review of the brittle fracture problem summarises Battelle and British Gas studies relating fracture appearance and crack behaviour, and the background to the adoption of the DWTT fracture appearance criterion in specifications is detailed. The application of the strain energy theory to full-scale test results is discussed. A balance between strain energy stored in the pipe wall and fracture energy explains why brittle fractures arrest if the stress is below a critical level.

The problem of ductile fracture propagation is reviewed and the results of full-size tests carried out by British Gas to provide toughness levels for fracture arrest are presented. The nature of ductile fracture propagation is discussed with reference to analogies with fracture initiation. Steady state, constant velocity, fractures imply specific shapes of the G (energy available)-velocity characteristic curves. In particular, a falling G with increasing fracture velocity is necessary. Some published theories to account for this effect are summarised and it is concluded that considerations of inertia in the fractured pipe wall provide the required relationship. Laboratory toughness tests for assessing brittle and ductile fracture resistance in relation to pipeline fracture behaviour are discussed. The relevance of fracture mechanics tests, particularly R curve analysis, is highlighted.

The paper concludes with a list of areas of work which warrant further study.     

On the application of probabilistic fracture mechanics to the reliability and inspection of pressure vessels

A probabilistic fracture mechanics model was developed to analyse the failure probability of a typical high power density reactor pressure vessel. The major causes for the nuclear reactor pressure vessel failure include fatigue, corrosion fatigue and brittle failure. All these causes are greatly affected by the stress loading conditions, material properties (aged by neutron damage), and defects embedded in the structure. Both an analytical first-order second-moment approximation and a hybrid methodology were employed in this study. In addition to the static scatter of the pre-existing cracks and material properties, a random walk model based on the operating history was introduced to represent the random occurrence of the abnormal transient stresses. The failure mode is defined as the brittle failure caused by a critical crack, meaning the stress intensity factor around a critical crack exceeding the fracture toughness of the pressure vessel material. Through a sample study on a typical high power density nuclear power plant, it was found that the vessel failure probability is about 4 × 10⁻⁴ at the 40th year of operation and the failure rate is in the order of 5 × 10⁻⁶ per vessel per year, which had reasonable agreement with the value of 10⁻⁴−10⁻⁶ as reported based on real-world statistics. In addition to the failure probability caused by fatigue crack growth, the reliability of a Low Temperature Overpressure incident was also evaluated.     

Fracture toughness and defect assessment of low-temperature carbon steel flanges

A quantitative fitness-for-purpose assessment has been performed for a range of large carbon steel flanges for use at low temperatures (to ASTM A350 LF2 specifications). Toughness has been measured using fracture toughness tests and Charpy impact tests and results have been related qualitatively to microstructure. The presence of Widmannstatten ferrite has been found to be particularly deleterious to toughness. Local stresses have been predicted using both elastic and elastic-plastic finite element methods (FEMs). Point-loading methods have been used to study the behaviour of cracks in these non-uniform stress fields and to establish critical defect sizes. A critical tensile stress criterion has been invoked to characterise the onset of cleavage fracture. Although under worst-case operating conditions tolerable defect sizes (according to PD6493) are below nominal acceptance limits, for lower bound toughness values, δ_c = 0·009 mm and K_Q = 39 MPa m^1/2, reassessment using point-loading methods suggests in many instances that the material may still be fit for purpose. To guarantee fitness for purpose, δ_c values of approximately 0·020 mm may be required, and the study appears to suggest that the impact toughness requirements of ASTM A350 LF2 could be relaxed.     

Ductile fracture theories for pressurised pipes and containers

The probable mechanisms of fracture which may be encountered in various components may generally be classified in two main groups. In the first the fracture is of ‘plane strain’ type which may occur in components (e.g. thick walled pressure vessels and other heavy section components) where prior to and during a possible fracture propagation the material is not expected to undergo large scale plastic deformations. In this case the underlying fracture theory is rather well-understood, and a criterion based on K_IC usually provides a highly reliable tool to deal with the problem.

The second type of fracture failure falls into the general category of ‘plane stress’ or ‘high energy’ fracture. In a great variety of tubings and containers, due to relatively small wall thickness, large defect size, high material toughness, and high temperature, prior to and during a possible rupture process, around the defect region the material would be expected to undergo large scale plastic deformations. In this case the standard theories of fracture based on the concept of plane strain fracture toughness are not applicable. This type of fracture which is generally accompanied by large inelastic deformations is (somewhat loosely) termed the plane stress fracture for which currently there does not seem to be a universally accepted criterion. In some components an additional complicating factor arises where one is dealing essentially with a shell of given curvature rather than a flat plate.

The theories which are currently in use in practice to analyse plane stress type of fracture are those which are based on the concepts of critical crack opening stretch, K_R-characterisation, J-integral, and the recently proposed plastic instability. In this paper the application of the fracture criteria based on these concepts to the fracture of shells will be discussed and the concept of plastic instability will be developed in some detail. Since there is no widely accepted standard criterion to deal with this type of fracture, one of the aims of the paper will be to provide an up-to-date critical appraisal of the current theories.     

A new method for estimating solar radiation from bright sunshine data

Daily values of H/H₀, the ratio of total horizontal radiation to that outside the atmosphere has been correlated with s/S, bright sunshine as a fraction of daylength for 3 yr measurements in Adana and Ankara, Turkey. Using a maximum-likelihood quadratic fit, we show that monthly averages s/S and its standard deviation σ_s/S can be used to estimate the monthly average H/H₀ as

H/H₀ = 0.204 + 0.758s/S − 0.250[s/S² + σ²_s/S.

Comparison of the estimations of the above equation with measurements from different regions of Turkey indicate that less than 5 per cent relative error is possible. A further correlation σ²_s/S with s/S makes it possible to estimate H/H₀ with just the knowledge of s/S.     

Effect of temperature on the morphological and photovoltaic stability of bulk heterojunction polymer:fullerene solar cells

In high performance polymer:fullerene bulk heterojunction solar cells the nanoscale morphology of interpenetrating acceptor:donor materials is optimized through appropriate preparation conditions such as annealing and choice of solvent, but this initial state-of-the-art morphology will not remain stable during long-term operation. We report the effects of prolonged storage at elevated temperatures on both the morphology and the photovoltaic performance for the model systems MDMO-PPV:PCBM and P3HT:PCBM as compared to ‘High T_g PPV’:PCBM based solar cells, where the ‘High T_g PPV’ is characterized by its high glass transition temperature (138 °C). In situ monitoring of the photocurrent–voltage characteristics at elevated temperatures, in combination with a systematic transmission electron microscopy (TEM) study and complementary optical spectroscopy, reveals distinct degradation kinetics and morphological changes that indicate the occurrence of different underlying physico-chemical mechanisms.