Elastic-plastic fracture mechanics assessment for steam generator tubes with through-wall cracks

The present work provides an elastic‐plastic fracture mechanics (EPFM) assessment scheme for a steam generator tube with a through‐wall crack under internal pressure. Noting that the geometry and material are rather uniform for steam generator tubes, and furthermore the only loading to be considered is internal pressure, an engineering EPFM analysis method is proposed to assess through‐wall cracks in steam generator tubes. Important outcomes of the present work are closed‐form approximations for J and crack opening displacement (COD). Sufficient confidence in the proposed J and COD estimates is gained from good agreements with the finite element results over a wide range of the crack length and pressure magnitude. Another important element of the EPFM assessment is to determine relevant J‐resistance curve for steam generator tubes. To improve the accuracy of predicting tube failure, the present paper also proposes a new method to determine fracture toughness using an actual tubular specimen instead of using a standard specimen, from which J‐resistance curves of steam generator tubes are obtained. Using the proposed J and toughness estimates, maximum pressures of steam generator tubes with through‐wall crack are estimated based on EPFM analysis, which is compared with experimental results and predicted ones based on limit load approach.     

DEVELOPMENT OF A BIOSENSOR WITH IMMOBILIZED L-AMINO ACID OXIDASE FOR DETERMINATION OF L-AMINO ACIDS

The objectives of this research were to investigate conditions for immobilization of L-amino acid oxidase (LAAO) on a preactivated nylon membrane, and to develop an enzyme sensor which can detect ammonia. LAAO was immobilized onto a preactivated nylon membrane. Optimal immobilization conditions were 0.3% glutaraldehyde, 1 mg bovine serum albumin, and 1.6 units of enzyme with 2 h coupling time. The enzymic membrane and an ammonium selective nonactin membrane were attached to an ammonia electrode to fabricate an L-amino acid sensor. L-amino acids were enzymatically degraded by immobilized LAAO, and the ammonia generated was measured by a potentiometric electrode at pH 8.5 and 45C. The sensor showed high relative activities for L-amino acids, and the responses of the sensor for phenylalanine and isoleucine were linear to 10 mM with a detection limit of 0.05 mM. The L-amino acid sensor was applied for monitoring increases in amino acids levels, expressed as L-isoleucine equivalents, during yeast autolysis. Determination of L-amino acids was complete in 3 min. Moreover, the activity of the enzymic membrane was stable for at least 260 assays, and did not noticeably decline for 2 months.     

Influence of Ramberg–Osgood fitting on the determination of plastic displacement rates in creep crack growth testing

This paper investigates the effect of the Ramberg–Osgood (R‐O) fitting procedures on plastic displacement rate estimates in creep crack growth testing, via detailed two‐dimensional and three‐dimensional finite‐element analyses of the standard compact tension specimen. Four different R‐O fitting procedures are considered: (i) fitting the entire true stress–strain data up to the ultimate tensile strength, (ii) fitting the true stress–strain data from 0.1% strain to 0.8 of the true ultimate strain, (iii) fitting the true stress–strain data only up to 5% strain and (iv) fitting the engineering stress–strain data. It is found that the first two fitting procedures can produce significant errors in plastic displacement rate estimates. The last two procedures, on the other hand, provide reasonably accurate plastic displacement rates and thus should be recommended in creep crack growth testing. Several advantages of fitting the engineering stress–strain data over fitting the true stress–strain data only up to 5% strain are discussed.     

Technical note. Agricultural,hydrologic and oceanographic studies in Bangladesh with NOAA AVHRR data

Abstract

This paper presents some results of studies made with NOAA Advanced Very High Resolution Radiometer (AVHRR) data in the fields of agriculture, hydrology and oceanography in Bangladesh. The techniques of processing the raw image data are also discussed. The data for four different dates in 1984 and 1985 have been used. The 1984 data were obtained from NOAA/NESDIS of U.S. A. and the 1985 data were received in real time at the SPARRSO Meteorological Satellite Ground Station. Data from the AVHRR channels 1 and 2 were used to calculate the vegetation index (normalized) which was found to be a useful tool for monitoring vegetation conditions from time to time. These data have also application in monitoring and studying flood conditions in and outside the country. The thermal channels of the AVHRR (channels 4 and 5) have been used for calculating the surface temperatures of the Bay of Bengal. These channels revealed some noteworthy surface features in the Bay.     

Elastic T stress estimates for circumferential surface‐cracked cylinders*

On the basis of detailed three‐dimensional (3D) elastic finite element (FE) analyses, this paper provides tractable approximations for elastic T stress solutions for circumferential inner‐surface cracks in cylinders. Internal pressure and global bending moment were considered. The FE model and analysis procedure employed in the analysis were verified using existing solutions for both elastic stress intensity factor and T stress. To cover a practical range, three different values of the ratio of the mean radius of cylinder to the thickness, R_m/t, were selected; furthermore, four different values of the ratio of the crack depth to the thickness, a/t, ranging from 0.1 to 0.75 and three different values of θ/π ranging from 0.1 to 0.4 were selected. On the basis of FE analyses results, polynomial approximations were proposed at three different locations: surface point, middle point and deepest point. On the basis of the detailed 3D elastic FE analysis, the solutions presented are believed to be the most accurate, and thus provide valuable information for structural integrity assessment considering a crack‐tip constraint.     

Estimations of creep fracture mechanics parameters for through-thickness cracked cylinders and finite element validation

This paper compares engineering estimation schemes of C* and creep crack opening displacement (COD) for cylinders with circumferential and axial through‐thickness cracks at elevated temperatures with detailed 3D elastic‐creep finite element results. Engineering estimation schemes include the GE/EPRI method; the reference stress (RS) method where the reference stress is defined based on the plastic limit load; and the enhanced reference stress (ERS) method where the reference stress is defined based on the optimised reference load, recently proposed by the authors. Systematic investigations are made not only on the effect of creep‐deformation behaviour on C* and creep COD, but also on effects of the crack location, the cylinder geometry, the crack length and the loading mode. Comparison of the finite element (FE) results with engineering estimations provides that for idealised power law creep, estimated C* and COD rate results from the GE/EPRI method agree best with FE results, suggesting that published plastic influence functions for plastic J and COD for through‐thickness cracked cylinders are reliable. For general creep‐deformation laws where either primary or tertiary creep is important and thus the GE/EPRI method is hard to apply, on the other hand, the ERS method provides more accurate and robust estimations for C* and COD rate than the reference stress method. As these two methods differ only in the definition of the reference stress, the ERS method maintains benefits of the reference stress method in terms of simplicity, but improves accuracy of the estimated J, C* and COD results.     

Se-precursor effect on ZnSe films grown by OMVPE

The growth of ZnSe by organometallic vapor phase epitaxy (OMVPE) was investigated for diallyl selenide (DASe) and methylallyl selenide (MASe), combined with dimethylzinc: triethylamine (DMZn: NEt₃). The allyl selenide compounds are shown to reduce the growth temperature relative to that needed with diethyl selenide (DESe). The surface morphology of the grown ZnSe films varies on growth temperature and VI/II ratios. Secondary ion mass spectrometry (SIMS) measurements show increasing carbon incorporation with the VI/II ratio, and for a value of VI/II =~4 in MASe and VI/II=~1 in DASe. The amount of incorporated carbon abruptly jumps to concentrations of 10²¹ cm³, whereupon the films become polycrystalline. The results are interpreted in terms of dominant intramolecular decomposition pathways compared to homolysis pathway.     

Stress intensity factors for slanted through-wall cracks based on elastic finite element analyses

Based on detailed 3‐dimensional (3‐D) elastic finite element (FE) analyses, the present paper provides stress intensity factors (SIFs) for plates with slanted through‐wall crack (TWC) and cylinders with slanted circumferential TWC. Regarding loading conditions, axial tension was considered for the plates, whereas axial tension, global bending and internal pressure were considered for the cylinders. To cover a practical range, the geometric variables affecting the SIF were systematically varied. Based on FE results, SIFs along the crack front, including the inner and outer surface points, were provided. The present results can be used to evaluate the fatigue crack growth or stress corrosion cracking behaviour of a slanted TWC and furthermore to perform detailed Leak‐Before‐Break analysis considering a more realistic crack shape.     

Stress intensity factors and crack opening displacements for slanted axial through-wall cracks in pressurized pipes

This paper proposes elastic stress intensity factors and crack opening displacements (CODs) for a slanted axial through-wall cracked cylinder under an internal pressure based on detailed three-dimensional (3D) elastic finite element (FE) analyses. The FE model and analysis procedure were validated against existing solutions for both elastic stress intensity factor and COD of an idealized axial through-wall cracked cylinder. To cover a practical range, four different values of the ratio of the mean radius of cylinder to the thickness ( R _m/ t ) were selected. Furthermore, four different values of the normalized crack length and five different values of the ratio of the crack length at the inner surface to the crack length at the outer surface representing the slant angle were selected. Based on the elastic FE results, the stress intensity factors along the crack front and CODs through the thickness at the centre of the crack were provided. These values were also tabulated for three selected points, that is, the inner and outer surfaces and at the mid-thickness. The present results can be used to evaluate the crack growth rate and leak rate of a slanted axial through-wall crack due to stress corrosion cracking and fatigue. Moreover, the present results can be used to perform a detailed leak-before-break analysis considering more realistic crack shape development.