Elastic-plastic FE calculations for a fracture mechanics analysis of a weld in the ligament of a CT specimen

Results of elastic-plastic FE calculations on four plane strain models of a CT-100 specimen, (a) with a weld in the ligament, (b) of base material only, (c) of welding simulated material only and (d) of weld material only, are compared and discussed in the light of fracture mechanics aspects. Regarding the cleavage fracture regime, considerably higher crack opening stresses are obtained in the welded specimen than in the base material specimen, as soon as small-scale yielding occurs. Conservative evaluation of the crack opening stresses of the welded joint can be achieved by using the welding simulated material properties. The J-integrals determined by the energy release rate using the method of virtual crack extension are equal for the four material models up to about 200 N mm⁻¹. Above this level, the behaviour of the welded specimen and that of the base material model are still similar, while a higher force is necessary for the welding simulated material model and the weld material model to reach the same J-value as for the two others. The crack opening profile of the welded specimen is asymmetric to the fusion line which lies in the ligament. The half profile of the heat-affected zone side is comparable to that of the welding simulated material model and the half profile of the weld material side to that of the base material model. A higher maximum of the equivalent strain on the heat-affected zone side than on the weld material side may explain why cracks tend to run into the base material region of the weld under conditions of stable crack growth. General yield of the welded specimen occurs on the base material side, whereas on the weld material side only a small plastic zone is visible, comparable to that in welding simulated material at the same load.     

The J-resistance curve leak-before-break test program on material for the darlington nuclear generating station

The Darlington Leak-Before-Break (DLBB) approach has been developed for large diameter (21, 22, 24 inch) SA106B heat transport (HT) piping and SA105 fittings as a design alternative to pipewhip restraints and in recognition of the questionable benefits of providing such restraints. Ontario Hydro's DLBB approach is based on the elastic plastic fracture mechanics (EPFM) method. In this test program, J-resistance curves were determined from actual pipe heats that were used in the construction of the Darlington heat transport systems (Units 1 and 2). Test blocks were prepared using four different welding procedures for nuclear Class I piping.

The test program was designed to take into account the effect of various factors such as test temperature, crack plane orientation, welding effects, etc., which have influence on fracture properties. A total of 91 tests were conducted. An acceptable lower bound J-resistance curve for the piping steels was obtained by machining maximum thickness specimens from the pipes and by testing side grooved compact tension specimens.

Test results showed that all pipes, welds and heat-affected zone materials within the scope of the DLBB program exhibited uppershelf toughness behaviour. All specimens showed high crack initiation toughness J_Ic, rising J-resistance curve and stable and ductile crack extension. Toughness of product forms depended on the direction of crack extension (circumferential versus axial crack orientation). Toughness of DLBB welds and parent materials at 250°C was lower than that at 20°C.     

Hot tensile properties of simulated heat-affected zone microstructures of 9Cr---1Mo weldment

The heat-affected zones (HAZ) of 9Cr---1Mo steel weldments consist of coarse-grain martensite with δ-ferrite, coarse-grain martensite, fine-grain martensite and intercritical structure. These HAZ microstructures have been simulated with respect to microstructure, hardness and grain size by isothermal heat treatment cycles in the temperature range 973–1573 K. Tensile tests at a strain rate of 3 × 10⁻⁴s⁻¹ over the range 298–873 K showed that 0·2% YS and UTS values for the various microstructural conditions were in the descending order of coarse-grain martensite, coarse-grain martensite with δ-ferrite, fine-grain martensite, base metal and intercritical structure. The presence of δ-ferrite in the coarse-grain region of HAZ restricted the grain growth, the increase in strength and the decrease in ductility. This minimises the problem of cracking in the coarse-grain region of HAZ in this steel which is often encountered in low-alloy Cr---Mo ferritic steels. The regions with intercritical structure showed the lowest strength and fracture energy. Fine-grain martensite exhibited a good combination of strength and ductility in the HAZ.     

Thermodynamic evaluation of water splitting by a cation-excessive (Ni, Mn) ferrite

Water-splitting potential by cation-excessive (Ni, Mn) Ferrite, Ni_{0.5(1 + )}Fe_{1.99(1 + )}O₄ was evaluated using the standard Gibbs free energy change (ΔG°) for the cation-excessive ferrite formation in different O₂ partial pressures. The cation-excessive degree ranged from 0.026 to 0.16 in pO₂ values of 7.9 × 10⁻⁷ to 1.0 × 10⁻¹. From the relation between value of (Ni, Mn) ferrite and oxygen partial pressure, equilibrium constant K(th) was determined. Furthermore ΔH°s for O₂ releasing and water-splitting reactions with cation-excessive (Ni, Mn) ferrite were evaluated from the van't Hoff plot and compared with that for magnetite-wüstite system; where ΔH°s were assumed to be the same values for both (Ni, Mn) ferrite and magnetite-wüstite system: +300 kJ for O₂ releasing and −35 −63 kJ for water-splitting. ΔG°s evaluated for water-splitting with cation-excessive (Ni, Mn) ferrite and wüstite were −38 kJ and −35 kJ, respectively, at 298K. It suggests that water splitting with cation-excessive (Ni, Mn) ferrite proceed easily compared with that with wüstite. ΔS°s for water-splitting are −0.93 kJ K⁻¹ for the former and −0.83 kJ K⁻¹ for the latter. H₂ generation rates by reaction with H₂O for (Ni, Mn) ferrite were studied at temperatures of 573 K-1073 K. It reached the maximum at 1000 K while it proceeds preferentially below 830 K from thermodynamics.     

Determination of the kinetic parameters of oat straw using thermogravimetric analysis

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques were used to study the thermochemical behaviour of three varieties of oat straw (Sentinel, Shaw, and Tibor). The thermal degradation of oat straw was studied in an oxidizing atmosphere (15% oxygen and 85% nitrogen) from ambient temperature to a temperature of 700°C using a heating rate of 20°C min⁻¹. Two distinct reaction zones were observed on the TGA and DTA curves. The kinetic parameters (order of reaction, activation energy, and pre-exponential factor) were determined for each zone separately by applying thermo-analytical techniques to the reaction kinetics. Higher thermal degradation rates were observed in the first reaction zone due to rapid release of volatiles as compared to those in the second reaction zone. The activation energies were in the range of 83–102 and 58–75 kJ mol⁻¹ for the first and the second reaction zones, respectively.     

Measurement of crack propagation velocity in nuclear pressure vessel steel (SA516 gr. 70)

An attempt has been made to develop a simple, reliable and cost-effective device for measuring the dynamic crack propagation velocity in a nuclear pressure vessel steel (SA516 gr. 70). The experimental method is described and a simple digital approach is proposed. The experimentally determined dynamic crack velocity has been utilized to obtain elastic dynamic stress intensity factors by INSAMCR (a two-dimensional dynamic finite element code which is a modified version of SAMCR developed by Dr Schwartz at the University of Maryland). A relationship between instantaneous crack tip velocities and dynamic stress intensity factors for pressure vessel steels is estimated using dynamic crack propagation velocities determined by a proposed measuring device. The relationship between the dynamic stress intensity factor and time history and the dynamic arrest toughness for each test are obtained using the generation mode dynamic finite element analysis. A function ƒ(å) = 1·356 − 2·672å + 6·494å² − 4·539å³ + 1·461å⁴ is suggested which may be useful to predict the relationship between the dynamic fracture toughness (K(å)) and the dynamic crack arrest toughness (K_Ia) for SA516 gr. 70 steel (say K(å) = K_Ia ƒ(å) where å is the dynamic crack propagation velocity).     

Operation of a commercial solar gel pond

The solar gel pond is an innovative concept which overcomes many of the shortcomings of the conventional salt-gradient solar pond. In this paper, the design, construction and start-up details of a commercial scale pond (400 m²), built for a publishing company in Albuquerque, New Mexico will be presented. A pond with trapezoidal cross section was designed so that shadowing could be minimized and also the ratio of surface area to the volume of the storage zone could be maximized. The publishing company required a minimum of 1 GJ/day (1MBTU/day). Generally it has been noted that in ponds with large volume a lesser percentage of retained energy is lost as edge losses. Based on the above consideration a pond size of 400 m² and 5 m deep with a gel thickness of 60 cm and a mass flow rate (for the heat extraction loop) of 1 × 10⁻⁴ to 1 × 10⁻³ kg/m²·sec was determined to be the optimum size for the publishing company's needs. Two to seven percent salt water was used mainly to keep the gel bags floating on the surface. Tedlar bags were used to contain the gel. During the first year of operation, while the pond was still heating up, the pond obtained a temperature of 60°C and the gel showed no signs of degradation.     

Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

A dynamic fracture-toughness-based reference temperature characterization for the weld metals of modified 9Cr–1Mo steel in two different weld positions

The ductile–brittle transition temperatures (DBTT) of modified 9Cr–1Mo steel welds from two different weld positions, namely down hand (1G) and overhead (4G), have been evaluated and compared using the ASTM E 1921-05 based reference temperature (T₀) approach, but under dynamic-loading conditions. The reference temperatures thus obtained, termed as T₀^dy to signify the dynamic condition, have been found to be higher for the 4G position than the 1G position. A scanning electron microscopic study of the fracture surfaces close to the fatigue crack front reveals that while lath boundary fracture is the dominant mechanism for brittle crack initiation in both the welds, the higher T₀^dy value is linked to the higher concentration of probable crack initiation sites in the 4G position. The experimentally obtained Weibull slope in both the welds has been found to be different (7.526 and 7.205 for the 1G and 4G positions, respectively) from the ‘fixed slope of 4’ assumption, used in ASTM E 1921-05. However, in the present instance, the ‘fixed Weibull slope of 4’ concept yields more conservative T₀^dy values compared to those obtained using the experimentally determined Weibull slope. For these welds, the RT_NDT-based ASME K_IR curve proved to be ultra-conservative compared to the realistic dynamic fracture toughness variation described by the Master Curve indexed with T₀^dy.     

A new electrode for a poly(pyrrole)-based rechargeable battery

The use of dodecylbenzenesulfonate-doped poly(pyrrole) films, PPYDBS, as a secondary battery electrode was studied. The redox and morphologic properties of these films are suitable for battery application. Films were synthesized by electrolysis of pyrrole and sodium dodecylbenzenesulfonate aqueous solutions with a current density of 1.0 mA cm⁻² and were switched in LiC1O₄ 1.0 M propylene carbonate solutions (PC) by cyclic voltammetry. In these experiments an apparent diffusion coefficient of 3.7x10⁻⁹ cm²s⁻¹ has been found. Charge/discharge tests at ±50, ±100, ±150 and ±200 μA cm⁻² were done for a PPYDBS/ LiC1O₄, PC/Li battery. The open-circuit voltage was 3.2 V after 30 h, the specific capacity 53 A h kg⁻¹ and the energy density 154 W h kg⁻¹. These values indicate that this polymer can be used as an electrode in a rechargeable battery.     

Ductile fracture properties for assessing leak-before-break issues in ferritic weldments

A Leak-Before-Break (LBB) approach is being used by Ontario Hydro's Darlington nuclear generating station as a design alternative to pipe rupture restraint hardware on the large diameter piping of the primary heat transport system. The J-resistance curves of four different ferritic weldments, fabricated by either the submerged arc weld (SAW) or shielded metal arc weld (SMAW) process, were determined as part of this program.

Results indicated that the as-welded and post-weld heat treated (PWHT) welds were susceptible to varying degrees of static or dynamic strain aging at 200 and 250°C. Dynamic strain aging effects were most significant for as-welded welds, as evidenced by sudden load drops on the load-displacement curves and ductile crack jumping. The effect of loading displacement rate and PWHT on toughness was assessed and related to the weld's tensile properties and susceptibility to dynamic strain aging. Implications of strain aging to LBB assessments are discussed.     

Investigation of the impact of atmospheric particles on solar radiation at Nsukka, Nigeria

Using a well-calibrated integrating nephelometer of sensitivity 10⁻⁷/m, we have successfully measured the scattering extinction coefficient of atmospheric fine particles, b_sp at Nsukka, Nigeria, at latitude 6.8°N and longitude 7.35°E and an elevation of 488 m above sea level.

The values of b_sp recorded are 10.0 × 10⁻⁴/m (maximum) and 0.17 × 10⁻⁴/m (minimum). We have also investigated the impact of the scattering extinction coefficient on solar radiation. Results of our investigation reveal that, for high b_sp, mean insolation is generally low, and the reverse is the case for low b_sp. It appears that there is no correlation between instantaneous (hourly) extinction coefficient and insolation for the period covered.     

Fertilizer solar ponds as a clean source of energy: Some observations from small scale experiments

Commercially available NH₂CONH₂ is used to establish a salinity gradient solar pond in a small 1 m² outdoor tank. With a salinity difference of 35% between the upper and lower zone, a temperature difference of 23°C was obtained without any instabilities in the gradient zone. The difference in concentration of solution required to sustain a temperature difference of 40°C across the gradient zone is 520 kg/m³. By economically using runoff into the fertilizer cycle of an agricultural system the estimated cost of fertilizer solar pond generated heat is Rs. 1.10/kWh.     

Bio-oils from arid land plants: Flash pyrolysis of Euphorbia characias bagasse

The devolatilization of the bagasse obtained by solvent extraction of dried Euphorbia characias, a bushy plant growing in arid land of the Mediterranean area, was investigated under rapid heating conditions at atmospheric pressure using a bench-scale fluidized bed pyrolyser. Particle heating rates exceeded 10⁴°C s⁻¹. Bagasse was fed continuously at the rate of 6 g h⁻¹ directly into a sand bed fluidized by nitrogen operating in the temperature range of 400°–750°C. The yields of oils, gases and chars are reported. A maximum oil yield of 44% (wt/wt) (moisture free bagasse) was obtained at 500°C. Yields of gases, CO, CO₂, C₁–C₄ hydrocarbons increased with the rise in temperature, reaching a maximum at 750°C. Elemental analyses showed that the composition of oils and chars was dependent on pyrolysis temperature. The nitrogen content is fairly high; an upgrading process could be necessary for its remotion before the use of the bio-oil as combustible. The other characteristics of oils fall in the range of oils derived from other biomass feedstocks. Chars have a high HHV (15.36 MJ kg⁻¹ at 500°C), representing a valuable fuel.     

High temperature solar heated seasonal storage system for low temperature heating of buildings

A preliminary study of a solar-heated low-temperature space-heating system with seasonal storage in the ground has been performed. The system performance has been evaluated using the simulation models TRNSYS and MINSUN together with the ground storage module DST. The study implies an economically feasible design for a total annual heat demand of about 2500 MWh. The main objective was to perform a study on Anneberg, a planned residential area of 90 single-family houses with 1080 MWh total heat demand. The suggested heating system with a solar fraction of 60% includes 3000 m² of solar collectors but electrical heaters to produce peak heating. The floor heating system was designed for 30°C supply temperature. The temperature of the seasonal storage unit, a borehole array in crystalline rock of 60,000 m³, varies between 30 and 45°C over the year. The total annual heating costs, which include all costs (including capital, energy, maintenance etc.) associated with the heating system, were investigated for three different systems: solar heating (1000 SEK MWh⁻¹), small-scale district heating (1100 SEK MWh⁻¹) and individual ground-coupled heat pumps (920 SEK MWh⁻¹). The heat loss from the Anneberg storage system was 42% of the collected solar energy. This heat loss would be reduced in a larger storage system, so a case where the size of the proposed solar heating system was enlarged by a factor of three was also investigated. The total annual cost of the solar heating system was reduced by about 20% to about 800 SEK MWh⁻¹, which is lower than the best conventional alternative.     

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.     

Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electronic ground state ( ²Π) with oxygen atoms were determined at 2.5 Torr total pressure in the temperature range 302–757 K. Excimer laser photolysis (ELP) of chlorine isocyanate (ClNCO) produced NCO radicals detected by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a negative temperature dependence, described by the two-parameter equation: k_NCO+O(T) = (4.3_−2.2^+3.2) × 10⁻⁸ × T^{−1.14_−0.12+0.08} cm³ molecule⁻¹ s⁻¹. Measurements at 298 K and total pressures of 2.5 and 9.9 Torr, respectively, indicated a slight pressure dependence. For the reaction of NCO radicals with hydrogen atoms, the rate constant k_NCO+H = (2.2 ± 1.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹was obtained at 298 K and a total pressure of 2.6 Torr for the first time by a direct measurement. From a single measurement k = (3.8 ± 1.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ was determined at 548 K and 2.4 Torr total pressure. In addition, rate constants for the reactions of NCO radicals with molecular oxygen (O₂), carbon dioxide (CO₂), molecular hydrogen (H₂), and carbon monoxide (CO), which is a dissociation product of CO₂ in a microwave discharge, were measured at two different temperatures. At room temperature these reactions were slow and at the detection limit of the ELP/LIF technique. However, at elevated temperatures at least the rate constants of the reactions NCO + O₂ and NCO + H₂ become significantly larger and, therefore, should be taken into account, when modeling combustion processes under certain conditions.     

Performance of repair welds on aged 2.25Cr–1Mo boiler header welds

In order to clarify the performance of repair welds on power boiler, thick parts such as header and steam piping, an ex-service aged 2.25Cr–1Mo header was repaired using SMAW with postweld heat treatment and the mechanical properties of the repair welded joints were experimentally evaluated.Creep rupture life of the repair welded joint was almost same as that of service-degraded base metal and heat-affected zones. It was proved that the life reduction would not be caused by repair welding. In creep–fatigue tests with strain holding, some type of repair welded joints was fractured at the heat affected zone caused by repair welding. This may be caused by strain concentration at the heat-affected zone under strain holding. Charpy impact toughness of the simulated heat affected zone due to repair welding was much higher than that of service-degraded base metal. It was proved that the toughness would be restored by repair welding.     

Experimental study of effects of prior overload on fracture toughness of A533B steel

This paper presents the results of an extensive study carried out to examine the effects of prior overloading over the entire fracture transition regime for 50-mm thick A533B steel. The main variables examined are temperature, crack orientation with respect to the rolling direction, level of prior overload, the initial crack length, and the statistical variation of prior overload effects. It is found that the effect of prior overload on fracture toughness at lower temperatures is dependent on orientation, so that in the L-T orientation for short and medium cracks (0·2 and 0·5 a/W) there is a benefit throughout the transition regime of 50-mm thick A533B steel. In the T-L orientation no benefit is obtained for temperatures greater than the initiation of tearing temperatures. Above these temperatures the prior overload sequence lowers the fracture toughness. For L-T orientation long cracks (a/W = 0·7) it is found for temperatures lower than −140°C that prior overload apparently increases the toughness. At higher temperatures there is a loss of toughness even though failure is cleavage dominated up to −80°C.

On the lower shelf at −170°C in the L-T orientation the fracture toughness variability after preloading is found (based on a sample of 14 specimens) to exhibit a bimodal distribution. This distribution is similar to that exhibited by non-preloaded material.     

Electrical and electrochemical performance characteristics of large capacity lithium-ion cells

We are currently evaluating large capacity (20–40 Ah) Bluestar (cylindrical) and Yardney (prismatic) lithium-ion cells for their electrical and electrochemical performance characteristics at different temperatures. The cell resistances were nearly constant from room temperature down to −20°C, but increased by over 10 times at −40°C. The specific energies and powers, as well as the energy densities and power densities are high and did not reach a plateau even at the highest discharge rates tested. For example, the prismatic lithium-ion cells gave close to 280 Wh dm⁻³ from a 4 A discharge and 249 Wh dm⁻³ at 20 A, both at room temperature. For the same current range the specific energy values were 102 Wh kg⁻¹ and 91 Wh kg⁻¹. Cycle life and other electrical and electrochemical properties of the cells will be presented.