Effect of crack depth on the shift of the ductile-brittle transition curve of steels

Nuclear reactor pressure vessel (RPV) steels degrade due to neutron irradiation during normal operation. As a result, the ductile-brittle transition curve of the steel shifts to higher temperature which decreases operation margins in both the temperature and pressure. The loss of these margins however can be offset somewhat by appealing to arguments based on constraint of potential/postulated shallow cracks. In this paper, it is demonstrated that the fracture toughness values for shallow flaws are higher than those determined from standard deep cracked test specimens based on constraint consideration. The J-A₂ three-term solution is used to characterize the crack-tip stress field where J represents the level of loading and A₂ quantifies the level of constraint. Based on the RKR cleavage model, procedures to quantify the temperature shift between specimens with different constraint levels are developed. The experimental data by Sherry et al. [Sherry AH, Lidbury DPG, Beardsmore DW. Validation of constraint based structural integrity assessment methods. Final report, Report No. AEAT/RJCB/RD01329400/R003, AEA Technology, UK, 2001] for the A533B RPV steel are used to demonstrate the procedure and it is shown that the ductile-brittle transition curve shifts to lower temperature from high constraint to low constraint specimens.     

Statistical scatter of fracture toughness in the ductile-brittle transition of a structural steel

The statistical scatter of fracture toughness in the ductile-brittle transition temperature range was experimentally examined on a 500 MPa class low carbon steel. Fracture toughness tests were replicatedly performed at −60 °C, −20 °C and −10 °C. The tests at −60 °C resulted in a single modal Weibull distribution with a shape parameter of 4 for the critical stress intensity factor converted from J-integral, whereas the Weibull distributions of the critical stress intensity factor at −20 °C and −10 °C showed a bilinear pattern with an elbow point, which caused a wider scatter than that at −60 °C. Such scatter transition behavior was discussed with reference to stable crack initiation. A model of the statistical scatter transition has been proposed in this work and the model reasonably explains the experimental results.     

Experimental T33-stress formulation of test specimen thickness effect on fracture toughness in the transition temperature region

This paper describes a study of the test specimen thickness effect on fracture toughness of a material, in the transition temperature region, for CT specimens. In addition we studied the specimen thickness effect on the T₃₃-stress (the out-of-plane non-singular term in the series of elastic crack-tip stress fields), expecting that T₃₃-stress affected the crack-tip triaxiality and thus constraint in the out-of-plane direction. Finally, an experimental expression for the thickness effect on the fracture toughness using T₃₃-stress is proposed for 0.55% carbon steel S55C. In addition to the fact that T₃₃ (which was negative) seemed to show an upper bound for large B/W, these results indicate the possibility of improving the existing methods for correlating fracture toughness obtained by test specimen with the toughness of actual cracks found in the structure, using T₃₃-stress.     

Numerical investigation of temperature increment effect on bubble dynamics in stagnant water and Al2O3 nanofluid column

Formation, growth, and detachment of injected air bubble from a submerged needle in stagnant liquid at the different temperature of fluid, investigated numerically. Experiments have been done for validation of numerical simulation results. The injection flow rate of air was varied between 600 and 1200?mL/h in experimental study. Bubble formation, growth, and detachment information were obtained using high speed camera and visual photography technique. Young–Laplace equation that was derived from the force balance on the bubble, was utilized in numerical simulation. A novel method was used for solution of Young–Laplace equation. The bubble diameter, instantaneous contact angle, volume, and other characteristics of bubble were studied at different temperature of operating condition. Also, the enhancement of temperature in Al₂O₃ nanofluid with 0.01% volume concentration (φ?=?0.01%) was compared with deionized water results. The results reveal that by increasing the Al₂O₃ nanofluid and the deionized water temperature, the diameter, volume, and center of gravity of bubble decrease; however, the instantaneous contact angle increases. Meanwhile, the size of bubble at Al₂O₃ nanofluid is larger than of that in deionized water at same temperature. Also, the bubble aspect ratio is almost senseless to temperature increment in both deionized water and Al₂O₃ nanofluid. Eventually, the variation of operating temperature and adding of nanoparticle to the deionized water have significant influence on behavior of growing bubble.     

Material constraint parameters for the assessment of shallow defects in structural components. Part II: Constraint-based assessment of shallow defects

This Part II paper describes a series of constraint-based R6 assessments of shallow-cracked specimens under equibiaxial loading using material constraint parameters obtained from lookup tables presented in the companion Part I paper. Parameters are derived for an A533B-1 steel forging from a knowledge of the yield and flow behaviour of the material and a calibration of the Beremin model parameter m. Resulting assessments, which use both T- and Q-stress to quantify constraint, are found to be conservative with respect to the available experimental data. The results are also used to predict the influence of T-stress on the Master Curve reference temperature. Available data are broadly in agreement with the predicted trend curve. Finally, the results demonstrate that out-of-plane biaxial loading increases constraint to the extent that the inherent conservatism of the elastic T-stress at high L_r is eroded. Out-of-plane constraint effects are only properly accounted for by the hydrostatic Q-stress and for this reason, the use of T to assess biaxial loading situations should be undertaken with a degree of caution.     

Simple and inexpensive technique for measuring the transition temperature of superconducting thick films from 60 to 300 K using liquid nitrogen and subcooled liquid oxygen

This paper describes a simple and low-cost experimental setup intended for AC susceptibility measurements between 60 and 300 K. A mutual inductance bridge for measuring the transition temperature of high-T_c superconductor thick films was constructed. The device is based on using liquid nitrogen and subcooled liquid oxygen; it allows temperatures down to 60 K to be achieved. Experimental details are described and illustrative measurements on high-T_c superconductor thick films are included.     

Experimental study of the effect of loading condition on fracture surface contact features and crack closure behavior in a carbon steel

To better understand the crack closure effect in the fatigue process, influence of fatigue stress amplitude and R ratio on the contact features of fracture surfaces in an annealed carbon steel was studied via two special experimental approaches: (i) the collection of the fracture debris fallen from the crack surfaces, and (ii) the direct observation of the contact zones on the fracture surface through an ink dyeing method. The results of this study show that the change of fatigue CMOD value as a function of a/W ratio depends strongly on the loading condition; the fatigue stress amplitude and R ratio are the major factors that determine the contacting status between the mating fracture surfaces; the severity of the fracture surface contact can also be characterized by the dropping rate of the fracture debris particles collected during the fatigue test.     

The effect of temperature and loading rate on the crack initiation and propagation energy in carbon steel charpy specimens

The Charpy impact tests were carried out at different temperatures and loading rates. The temperature dependences of crack initiation and propagation in carbon steels 45 and St. 3 under impact testing were determined from the obtained force variation plots. The effect of the impact velocity in the range from 1 to 4.4 m/s on the fracture toughness temperature dependence is estimated. __________ Translated from Problemy Prochnosti, No. 5, pp. 120–127, September–October, 2006.     

Deposition temperature effect on the electric and dielectric properties of InSbSe3 thin films

Thin films of InSbSe₃ compound were obtained by thermal evaporation on to clean glass substrates maintained at various deposition temperatures from 423 to 593 K. At deposition temperature T_d?473 K, the films have an amorphous structure, while those prepared at T_d>473 K have a polycrystalline structure identified by X-ray diffraction analysis. The DC electrical conductivity of the films increases as T_d increases, whereas activation energy decreases with increasing T_d, which reflects a change in the degree of disorder. AC conductivity was studied as a function of frequency in the range (10²-10⁵ Hz) and as a function of deposition temperature. The dependence of T_d on the frequency exponent s in the conductivity-frequency relation confirmed that the mechanism of AC conductivity is correlated barrier hopping with a single polaron hopping mechanism. The discrepancy between DC and AC activation energies was studied as a function of deposition temperature. The maximum barrier height W_m and the density of defect states N were also determined. Finally, the dependence of dielectric constant and dielectric loss on T_d were studied. A Debye-like relaxation of dielectric behavior was observed for crystalline films and is found to be a thermally activated process. The position of maximum dielectric loss is shifted towards higher temperature with T_d treatment and there by reduces the relaxation time.     

Proposed extension of the SINTAP/FITNET thin wall option based on a simple method for reference load determination

A “correct” limit or yield load is an essential element of flaw assessment procedures of the R6 or SINTAP/FITNET type. In this paper the authors propose a definition of this quantity which is based on the SINTAP option 3 failure assessment function. This “reference load” can be determined for any component geometry by finite element analysis. The method is applied to two kinds of thin walled structures (notched plates and curved stiffened panels) and the results demonstrate that the approach is a suitable extension of the existing thin wall module.     

Invasion,persistence and control in epidemic models for plant pathogens: the effect of host demography

Many epidemiological models for plant disease include host demography to describe changes in the availability of susceptible tissue for infection. We compare the effects of using two commonly used formulations for host growth, one linear and the other nonlinear, upon the outcomes for invasion, persistence and control of pathogens in a widely used, generic model for botanical epidemics. The criterion for invasion, reflected in the basic reproductive number, R₀, is unaffected by host demography: R₀ is simply a function of epidemiological parameters alone. When, however, host growth is intrinsically nonlinear, unexpected results arise for persistence and the control of disease. The endemic level of infection (I_∞) also depends upon R₀. We show, however, that the sensitivity of I_∞ to changes in R₀ > 1 depends upon which underlying epidemiological parameter is changed. Increasing R₀ by shortening the infectious period results in a monotonic increase in I_∞. If, however, an increase in R₀ is driven by increases in transmission rates or by decreases in the decay of free-living inoculum, I_∞ first increases (R₀ < 2), but then decreases (R₀ > 2). This counterintuitive result means that increasing the intensity of control can result in more endemic infection.     

Experimental investigation of dimension effect on electrical oscillation in planar device based on VO2 thin film

In this paper, we report on an experimental analysis of dimension effect on a room-temperature electrical oscillation in a planar device using vanadium dioxide (VO₂) thin film. We investigate the variation of the oscillation current (I_O) and frequency (f_O) due to the variation of the dimension of the VO₂ devices, i.e., the length and width of the current channel of the device. For five different VO₂ devices with different dimensions, I_O and f_O are observed at room temperature in a simple circuit composed of a dc voltage source and a standard resistor including one of the VO₂ devices. From the experimental investigation, it is concluded that the peak-to-peak amplitude of I_O and f_O decrease with the increase of the length and width of the current channel. This indicates that f_O depends on not only the external source voltage but also the device dimension.     

Influence of a degraded SrTiO3 layer at the YBa2Cu3O7−δSrTiO3 interface on the dielectric behavior at cryogenic temperature

In an YBa₂Cu₃O_7−δ/SrTiO₃/YBa₂Cu₃O_7−δ parallel capacitor fabricated by a chemical mechanical planarization method, the dielectric constant and loss in the presence of electric fields at 2.2 K were more than 26,000 and less than 0.027, respectively. We propose a multilayer model that explains this behavior. The model assumes that the SrTiO₃ film is composed of single-crystal-like SrTiO₃ layers with a dielectric constant ε_r = 30,000 and degraded SrTiO₃ layers with dielectric constants that vary continuously from 25, the dielectric constant of YBa₂Cu₃O_7−δ, to 30,000. Results of a numerical calculation revealed that the thickness of the single-crystal-like SrTiO₃ layer was more than 92% of a 600-nm-thick SrTiO₃ film.     

The effect of remodelling and contractility of the actin cytoskeleton on the shear resistance of single cells: a computational and experimental investigation

The biomechanisms that govern the response of chondrocytes to mechanical stimuli are poorly understood. In this study, a series of in vitro tests are performed, in which single chondrocytes are subjected to shear deformation by a horizontally moving probe. Dramatically different probe force–indentation curves are obtained for untreated cells and for cells in which the actin cytoskeleton has been disrupted. Untreated cells exhibit a rapid increase in force upon probe contact followed by yielding behaviour. Cells in which the contractile actin cytoskeleton was removed exhibit a linear force–indentation response. In order to investigate the mechanisms underlying this behaviour, a three-dimensional active modelling framework incorporating stress fibre (SF) remodelling and contractility is used to simulate the in vitro tests. Simulations reveal that the characteristic force–indentation curve observed for untreated chondrocytes occurs as a result of two factors: (i) yielding of SFs due to stretching of the cytoplasm near the probe and (ii) dissociation of SFs due to reduced cytoplasm tension at the front of the cell. In contrast, a passive hyperelastic model predicts a linear force–indentation curve similar to that observed for cells in which the actin cytoskeleton has been disrupted. This combined modelling–experimental study offers a novel insight into the role of the active contractility and remodelling of the actin cytoskeleton in the response of chondrocytes to mechanical loading.     

Evolution of the optical transitions in AlxGa1 − xAs/GaAs quantum well structures grown on GaAs buffers with different surface treatments by molecular beam epitaxy

Al_0.3Ga_0.7As/GaAs Quantum Well structures were grown by molecular beam epitaxy (MBE) on a 500 nm thick GaAs buffer layer subjected to the following surface processes: a) in-situ Cl₂ etching at 70 °C and 200 °C, b) air-exposure for 30 min. The characteristics of these samples were compared to those of a continuously grown sample with no processing (control sample). We obtained the quantum wells energy transitions using photoreflectance spectroscopy as a function of the temperature (8-300 K), in the range of 1.2 to 2.1 eV. The sample etched at 200 °C shows a larger intensity of the quantum well peaks in comparison to the others samples. We studied the temperature dependence of the excitonic energies in the quantum wells (QWs) as well as in GaAs using three different models; the first one proposed by Varshni [4], the second one by Viña et al. [5], and the third one by Pässler and Oelgart [6]. The Pässler model presents the best fitting to the experimental data.     

A numerical investigation into the effect of CRS misuse on the injury potential of children in frontal and side impact crashes

This research focuses on an investigation into the head and neck injuries sustained by toddlers due to CRS misuse under frontal and side impact crashes. A fully deformable FE model incorporating a Hybrid III 3-year-old dummy was developed which has been previously validated for frontal impacts under CMVSS 208 and FMVSS 213 testing conditions. Furthermore, this model has also been validated under near-side impact conditions in accordance to crash tests carried out by NHTSA. In addition, numerical models incorporating a Q3/Q3s prototype child crash test dummies were developed. The objective of this research was to study the effect of seatbelt slack and the absence of the top tether strap on the head and neck injuries sustained by toddlers in a vehicle crash. Numerical simulations were conducted under full frontal and near side impact crash testing conditions in accordance with FMVSS 213 for the Hybrid III 3-year-old dummy and Q3/Q3s dummies in the absence and presence of slack in the seatbelt webbing, and in the absence and presence of the top tether strap. In addition, the effect of using a cross-shaped rigid ISOFIX system was also investigated. An analysis of the head and chest accelerations, neck loads and moments was completed to investigate the potential of injury due to CRS misuse. An increase in HIC₁₅ by approximately 30–40% for the frontal impact and 10–20% for the near-side impact respectively was observed for the Q3 child dummy due to both forms of CRS misuse. In the absence of the top tether strap the forward head excursions were observed to be increased by approximately 70% for the Hybrid III 3-year-old dummy and 40% for the Q3 dummy, respectively. Use of the cross-shaped rigid ISOFIX system illustrated a reduction in head and neck injury parameters, for both frontal and side impact conditions, in the absence and presence of CRS misuse. CRS misuse results in a significant increase in injury parameters and potential for contact related head injuries. Use of a rigid ISOFIX system to restrain a CRS provides better CRS and dummy confinement and reduced injury potential than a flexible ISOFIX system.