| 1. |
- Brussaard, Lijbert, et al.
(författare)
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Biodiversity and ecosystem functioning in soil
- 1997
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Ingår i: Ambio: a Journal of Human Environment. - 0044-7447. ; 26:8, s. 563-570
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Tidskriftsartikel (refereegranskat)abstract
- We review the current knowledge on biodiversity in soils, its role in ecosystem processes, its importance for human purposes, and its resilience against stress and disturbance. The number of existing species is vastly higher than the number described, even in the macroscopically visible taxa, and biogeographical syntheses are largely lacking. A major effort in taxonomy and the training of a new generation of systematists is imperative. This effort has to be focussed on the groups of soil organisms that, to the best of our knowledge, play key roles in ecosystem functioning. To identify such groups, spheres of influence (SOI) of soil biota - such as the root biota, the shredders of organic matter and the soil bioturbators - are recognized that presumably control ecosystem processes, for example, through interactions with plants. Within those SOI, functional groups of soil organisms are recognized. Research questions of the highest urgency are the assignment of species to functional groups and determining the redundancy of species within functional groups. These priorities follow from the need to address the extent of any loss of functioning in soils, associated with intensive agriculture, forest disturbance, pollution of the environment, and global environmental change. The soil biota considered at present to be most at risk are species-poor functional groups among macrofaunal shredders of organic matter, bioturbators of soil, specialized bacteria like nitrifiers and nitrogen fixers, and fungiforming mycorrhizas. An experimental approach in addressing these research priorities is needed, using longterm and large-scale field experiments and modern methods of geostatistics and geographic information systems.
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| 2. |
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| 3. |
- Choularton, T. W., et al.
(författare)
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The Great Dun Fell Cloud Experiment 1993 : An overview
- 1997
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Ingår i: Atmospheric Environment. - 1352-2310. ; 31:16, s. 2393-2405
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Tidskriftsartikel (refereegranskat)abstract
- The 1993 Ground-based Cloud Experiment on Great Dun Fell used a wide range of measurements of trace gases, aerosol particles and cloud droplets at five sites to study their sources and sinks especially those in cloud. These measurements have been interpreted using a variety of models. The conclusions add to our knowledge of air pollution, acidification of the atmosphere and the ground, eutrophication and climate change. The experiment is designed to use the hill cap cloud as a flow-through reactor, and was conducted in varying levels of pollution typical of much of the rural temperate continental northern hemisphere in spring-time.
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| 4. |
- Erlinge, David, et al.
(författare)
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Phenotype changes of the vascular smooth muscle cell regulate P2 receptor expression as measured by quantitative RT-PCR
- 1998
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Ingår i: Biochemical and Biophysical Research Communications. - : Elsevier BV. - 1090-2104 .- 0006-291X. ; 248:3, s. 864-870
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Tidskriftsartikel (refereegranskat)abstract
- Studies using selective agonists have suggested that the contractile effect of extracellular nucleotides, such as ATP and UTP, in blood vessels is mediated mainly by P2X1 receptors with a smaller contribution of P2Y receptors while the mitogenic effect is mediated by P2Y (P2Y1, P2Y2, P2Y4, and P2Y6) receptors with no effect of P2X1 receptors. This indicates a difference in P2 receptor expression between the contractile and the synthetic phenotype of the SMC. To measure the expression of mRNA for these receptors a competitive RT-PCR assay was developed that utilised synthetic RNA-competitors allowing determination of the number of mRNA copies for each receptor in the samples. In the synthetic phenotype the mitogenic P2Y1 and P2Y2 receptor transcripts were upregulated by 342- and 8-fold, respectively, while the contractile P2X1 receptor is totally downregulated and the P2Y4 and P2Y6 receptors were unchanged. This plasticity of the receptor expression may be important in the transition from the contractile to the synthetic SMC phenotype.
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| 5. |
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| 6. |
- Nivens, David E., et al.
(författare)
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Monitoring microbiol adhesion and biofilm formation by attenuated total reflection/Fourier transform infrared spectroscopy
- 1993
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Ingår i: Journal of Microbiological Methods. - 0167-7012. ; 17:3, s. 199-213
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Tidskriftsartikel (refereegranskat)abstract
- A major problem in accurately defining bacterial adhesion mechanisms and processes occurring in biofilms on surfaces is the lack of techniques that nondestructively provide on-line information about the microorganisms, their extracellular polymers, and metabolites. The attenuated total reflectance (ATR) technique of Fourier transform infrared spectroscopy (FT-IR) is ideally suited to monitor molecular interactions at the solution/internal reflection element (IRE) interface, and we report its application to biofilm research. Two methodologies were utilized to obtain the ATF/FT-IR spectra of living Caulobacter crescentus cells attached to germanium crystals. Initially, spectra of attached bacteria in high purity water produced molecular details of the attachment process without spectral interferences from components of the medium. A growth medium, utilized in the second method, allowed direct examination of the infrared absorption bands associated with the actively growing microorganisms on the surface of the IRE in the spectral region of 2000 to 1200 cm-1. Using the amide II band as a marker for biofilm biomass, the detection limit was determined to be approximately 5 × 105 cells·cm-2. These results proved that the ATR-FT/IR methodologies can be utilized to provide chemical information from bacteria and bacterial products located within approximately 1 μm of the surface without spectral interferences due to components of the medium.
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| 7. |
- Olofsson, A M, et al.
(författare)
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Heparin-binding protein targeted to mitochondrial compartments protects endothelial cells from apoptosis
- 1999
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Ingår i: Journal of Clinical Investigation. - 0021-9738. ; 104:7, s. 885-894
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Tidskriftsartikel (refereegranskat)abstract
- Neutrophil-borne heparin-binding protein (HBP) is a multifunctional protein involved in the progression of inflammation. HBP is stored in neutrophil granules and released upon stimulation of the cells in proximity to endothelial cells. HBP affects endothelial cells in multiple ways; however, the molecular and cellular mechanisms underlying the interaction of HBP with these cells are unknown. Affinity isolation and enzymatic degradation demonstrated that HBP released from human neutrophils binds to endothelial cell-surface proteoglycans, such as syndecans and glypican. Flow cytometry indicated that a significant fraction of proteoglycan-bound HBP is taken up by the endothelial cells, and we used radiolabeled HBP to determine the internalization rate of surface-bound HBP. Confocal and electron microscopy revealed that internalized HBP is targeted to perinuclear compartments of endothelial cells, where it colocalizes with mitochondria. Western blotting of isolated mitochondria from HBP-treated endothelial cells showed that HBP is present in 2 forms - 28 and 22 kDa. Internalized HBP markedly reduced growth factor deprivation-induced caspase-3 activation and protected endothelial cells from apoptosis, suggesting that uptake and intracellular routing of exogenous HBP to mitochondria contributes to the sustained viability of endothelial cells in the context of locally activated neutrophils.
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| 8. |
- Tunlid, Anders, et al.
(författare)
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Infrared monitoring of the adhesion of Catenaria anguillulae zoospores to solid surfaces
- 1991
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Ingår i: Experimental Mycology. - : Elsevier BV. - 0147-5975. ; 15:3, s. 206-214
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Tidskriftsartikel (refereegranskat)abstract
- Electron microscopic studies of nematodes infected with the chytridiomycetous fungusCatenaria anguillulae indicated that zoospores of the fungus adhered to the cuticle of nematodes by a layer of extracellular polymers. The chemical composition of the adhesive polymers and their interaction with a solid surface were examined with Fourier transform infrared spectroscopy, using an attenuated total reflectance cell. On-line monitoring of the adhesion of zoospores to a germanium crystal with this technique showed that the adhesive polymers consisted of a protein(s) containing amide I and II bands. The adsorption of these proteins, measured as the increase in the amide II band, had a rapid initial phase of ca. 20 minutes, followed by a slower increase during the course of incubation. Fluorescein isothiocyanate staining of the attached cells at the end of the experiment showed that the adhesion of the zoospores occurred before the formation of the cyst wall.
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| 9. |
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| 10. |
- Agardh, Carl-David, et al.
(författare)
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Glucose levels and insulin secretion during a 75 g glucose challenge test in normal pregnancy
- 1996
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Ingår i: Journal of Internal Medicine. - : Wiley. - 1365-2796 .- 0954-6820. ; 240:5, s. 303-309
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: The aim of the study was to evaluate glucose levels and insulin secretion early in pregnancy and at a time when gestational diabetes mellitus frequently occurs in order to define reference values for glucose tolerance during pregnancy. The results were also related to maternal factors that might identify subjects at risk of developing gestational diabetes mellitus as well as foetal factors that might be a result of impaired glucose tolerance during pregnancy. DESIGN: A prospective study. SETTING: All Caucasian women attending one antenatal out-patient care unit were offered a 75 g oral glucose tolerance test at the 17th and 32nd week of gestation. SUBJECTS: A total of 586 consecutive pregnant women were included in the study. All 586 women were examined by repeated blood glucose measurements and 298 agreed to perform oral glucose tolerance tests as well. MAIN OUTCOME MEASURES: Venous whole blood glucose values were measured in the fasting state and in samples obtained 15, 30, 45, 60, 75, 90 and 120 min after oral intake of 75 g glucose. Serum insulin and C-peptide were also measured at these times. In all subjects, a random blood glucose sample was taken at the first visit, and thereafter at the 20th, 30th and 36th week of gestation. Information was also obtained from all subjects regarding body mass index, weight gain during pregnancy, smoking habits, family history of diabetes and hypertension, hypertension during pregnancy, past obstetric history, parity, and fetal outcome. RESULTS: The glucose tolerance was significantly impaired at the 32nd week of gestation compared with the 17th week of gestation. The mean +2SD 2 h glucose value during the oral glucose tolerance test at the 32nd week of gestation was 8.0 mmol L-1. Impaired glucose tolerance was characterised by increased insulin resistance, with a significant rise in serum insulin and C-peptide concentrations and in the insulin/glucose index during the oral glucose tolerance test at the 32nd week of gestation. Maternal factors associated with an impaired glucose tolerance were a family history of diabetes mellitus, smoking, a weight gain more than 18 kg during pregnancy, and glucosuria, while a family history of hypertension and hypertension present during pregnancy were not. Foetal factors that might be a result of impaired glucose tolerance during pregnancy, e.g. macrosomia and prematurity as well as complicated deliveries such as vacuum extraction/forceps or Caesarean section, all tended to be associated with higher blood glucose values. The same pattern was seen when the Apgar score was < 7. CONCLUSIONS: The results from this study show that the present cut-off values for diagnosis of gestational diabetes mellitus should be revised. Even if some maternal factors might indicate an increased risk for impaired glucose tolerance during pregnancy, they are probably not enough to detect women with gestational diabetes mellitus. Therefore, a screening programme for gestational diabetes should be considered.
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| 11. |
- Aitken, Candice L., et al.
(författare)
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Comparison of three methods used for fusion of SPECT-CT images of liver matastases
- 1998
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Ingår i: Fusion98, International Conference on Multisource-Mulltisensor Information Fusion. - : CSREA Press. - 1892512009 ; , s. 435-442
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Konferensbidrag (refereegranskat)abstract
- We compare three methods for fusing SPECT-CT images: ImageMatch - an automatic three-dimensional/two-dimensional method developed by Focus Imaging; IBM Visualization Data Explorer - a three-diemensional interactive method developed by Internation Business Machines, Inc.; and qsh - an interactive three-dimensional/two-dimensional method developed at New York University. While many fusion methods have proved successful for registering brain images, most methods have been less successful for thoracic and abdominal images. We use images of liver metastases obtained with a radiolabeled breast tumor-directed antibody to illustrate the strengths and weakness of the methods reviewed. The images used are typical clinical images from eigth patients. We conclude that an optimal image fusion program should combine the strengths of each of the methods reviewed.
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| 12. |
- Andersson, David E.
(författare)
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Hedonic prices and center accessibility : conceptual foundations and an empirical hedonic study of the market for condominium housing in Singapore
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One aim of this study is to compare and evaluateprespecified and data-specific functional forms for theestimation of hedonic price functions. One prespecifiedfunction and three types of data-specific Box-Cox functions areused to estimate hedonic price functions. A second aim of thestudy is to test alternative monocentric and polycentrichypotheses according to a system of successive variablerejections constructed for this study, whereby requiredsignificance levels are influenced by the results of aselection of earlier hedonic price functions.The possibility of constructing internally consistentcriteria for variable elimination is one of the merits ofemploying a prespecified functional form. Other advantagesinclude the simplicity and transparency of the derived results,and the robustness of key parameter estimates in the face ofmisspecified functions. Using a prespecified form will on theother hand as a rule not yield the likeliest functional form tohave generated the data and cannot be relied on to generatewell-behaved residuals.Being assured of well-behaved residuals - as is the casewith data-specific Box-Cox functions - is advantageous, eventhough it makes the detection of potential heteroskedasticitymore difficult. Box-Cox models with more than onetransformation parameter are however especially prone to yielddescriptions of unique data sets at the expense of potentialreplicability. Because of the sensitivity of the t-values ofthe estimated independent variables to the estimated Box-Coxt:ransformation parameters - where the log-likelihood partlydepends on the set of independent variables - it becomesdifficult to avoid an arbitrary selection of independentvariables. This is true of all Box-Cox functions, althoughespecially of functions that estimate more than onetransformation parameter.Although regression results provide detailed information onvariable effects, much of that information is not generalizableowing to interspatial and intertemporal instability. Because ofsupply and demand fluctuations and specification problems, itis argued that is preferable to draw qualitative rather thanquantitative conclusions.An analysis of the Singapore condominium market illustratesthe conceptual discussion of various approaches to hypothesistesting, with an emphasis on testing hypotheses concerning thenumber and nature of urban centers. The data strongly supportthe realism of the monocentric model with one dominantemployment center. Alternative polycentric and service-centerhypotheses are unambiguously rejected. The signs, averageimplicit prices and t-values of the independent variables arerelatively robust over the range of estimated functional forms- whether pre-specified or Box-Cox - given the set of variablesto be included in the hedonic price equation.Keywords:Hedonic prices, Box-Cox function, monocentric,polycentric, accessibility, hypothesis testing, rejection ofvariables, data-specific function, prespecified function.
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| 13. |
- Askerlund, Per, et al.
(författare)
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Reconstitution and Characterization of a Calmodulin-Stimulated Ca-Pumping ATPase Purified from Brassica oleracea L
- 1992
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Ingår i: Plant Physiology. - 0032-0889 .- 1532-2548. ; 100:4, s. 1670-1681
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Tidskriftsartikel (refereegranskat)abstract
- Purification and functional reconstitution of a calmodulin-stimulated Ca(2+)-ATPase from cauliflower (Brassica oleracea L.) is described. Activity was purified about 120-fold from a microsomal fraction using calmodulin-affinity chromatography. The purified fraction showed a polypeptide at 115 kD, which formed a phosphorylated intermediate in the presence of Ca(2+), together with a few polypeptides with lower molecular masses that were not phosphorylated. The ATPase was reconstituted into liposomes by 3-([cholamidopropyl]-dimethylammonio-)1-propanesulfonate (CHAPS) dialysis. The proteoliposomes showed ATP-dependent Ca(2+) uptake and ATPase activity, both of which were stimulated about 4-fold by calmodulin. Specific ATPase activity was about 5 mumol min(-1) (mg protein)(-1), and the Ca(2+)/ATP ratio was 0.1 to 0.5 when the ATPase was reconstituted with entrapped oxalate. The purified, reconstituted Ca(2+)-ATPase was inhibited by vanadate and erythrosin B, but not by cyclopiazonic acid and thapsigargin. Activity was supported by ATP (100%) and GTP (50%) and had a pH optimum of about 7.0. The effect of monovalent and divalent cations (including Ca(2+)) on activity is described. Assay of membranes purified by two-phase partitioning indicated that approximately 95% of the activity was associated with intracellular membranes, but only about 5% with plasma membranes. Sucrose gradient centrifugation suggests that the endoplasmic reticulum is the major cellular location of calmodulin-stimulated Ca(2+)-pumping ATPase in Brassica oleracea inflorescences.
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| 14. |
- Bastviken, David T. E., 1971-, et al.
(författare)
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Experimental measurements of zebra mussel (Dreissena polymorpha) impacts on phytoplankton community composition
- 1998
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Ingår i: Freshwater Biology. - : Wiley. - 0046-5070 .- 1365-2427. ; 39:2, s. 375-386
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Tidskriftsartikel (refereegranskat)abstract
- 1. To investigate direct effects of zebra mussel (Dreissena polymorpha) feeding activities on phytoplankton community composition, short-term microcosm experiments were performed in natural water with complex phytoplankton communities. Both gross effects (without resuspension of mussel excretions) and net effects (with resuspension) were studied. 2. Gross clearance rates were not selective; essentially all taxa were removed at similar rates ranging from 24 to 63 mt mussel(-1) h(-1). Net clearance rates were highly selective; different plankton taxa were removed at very different rates, ranging from 12 to 83% of the gross rates, leading to consistent changes in the phytoplankton community composition. Thus, although zebra mussels can cause most phytoplankton to decline, there is considerable variation among taxa in either pre-digestive selection or post-digestive survival. 3. The direct, short-term effects of zebra mussels on phytoplankton community composition are consistent with some of the major changes observed in the Hudson River since establishment of zebra mussels. 4. We show, with simple calculations, how zebra mussel filtration rate, its selective efficiency on various taxa, and phytoplankton growth rates interact to produce changes in the phytoplankton composition.
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| 15. |
- Bergqvist, David, et al.
(författare)
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Secondary aortoenteric fistula : changes from 1973 to 1993
- 1996
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Ingår i: European Journal of Vascular and Endovascular Surgery. - 1078-5884 .- 1532-2165. ; 11:4, s. 425-428
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Tidskriftsartikel (refereegranskat)abstract
- AIM: To investigate a series of patients with secondary aortoenteric fistulas and compare it with a previous series (1985-93 vs. 1973-84). DESIGN: Retrospective study of medical records. SETTING: Sixteen vascular surgical centers in Sweden. PATIENTS: Twenty-seven patients were identified making an overall incidence of 0.5% of all aortoiliac operations. Among aneurysm patients the incidence was significantly lower than in the previous series. One patient record could not be identified. Fourteen primary operations were for aortic aneurysm, 12 for occlusive disease and one was an aortorenal vein bypass. RESULTS: Symptoms of the fistula occurred after a median interval of 90 months which is significantly later than the previous series (32 months; p<0.05). The commonest presentation was bleeding followed by septis. The median diagnostic delay was 10.5 days, which was significantly shorter than in the previous series. Most fistulas involved the duodenum (88%). One patient died before surgery. The postoperative mortality was 28%, significantly lower than in the previous series (58%) (p<0.05). At the end of follow up (median 43 months) significantly more patients were alive than in the previous series (42% vs 18%) (p<0.05). CONCLUSION: Over a 21 year period there seems to have been a decrease in the frequency of secondary aortoenteric fistulas after aneurysm surgery, a longer interval before they occur, a shorter diagnostic delay, and a better survival.
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| 16. |
- Bergqvist, David, et al.
(författare)
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VII aktivering av kaskadsystem.
- 1995
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Ingår i: Svikt av vitala funktioner. Bakgrund, diagnos och tidig behandling av kirurgiska patienter.. - : In: Renck H. Aniva Torekov. ; , s. 239-
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 17. |
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| 18. |
- Calén, H., et al.
(författare)
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Detector setup for a storage ring with an internal target
- 1996
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 379:1, s. 57-75
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Tidskriftsartikel (refereegranskat)abstract
- A detector setup for the cooler storage ring CELSIUS is described. The setup detects particles produced in interactions between the internal beam and a cluster-jet target. Particles emitted in the forward direction are measured by means of arrays of plastic scintillators and proportional counters. Particles, particularly photons, emitted more isotropically are measured by means of two calorimeters containing CsI(Na) crystals. The performance of the setup is given for neutral meson production in proton-proton and proton-deuteron interactions in the energy range 290-1360 MeV.
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| 19. |
- Esamai, Fabian, et al.
(författare)
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Clinical presentation and diagnosis of cerebral malaria in children in the highlands of western Kenya
- 1999
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Ingår i: East African Medical Journal. - 0012-835X. ; 76:2, s. 89-92
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND:The clinical presentation of cerebral malaria in children in the highlands has not been documented.OBJECTIVE:To describe the presentation of cerebral malaria in the age group one to twelve years.DESIGN:Prospective study conducted from May to September 1997, the rainy season during which malaria occurs in epidemics in the highlands of Kenya.SETTING:Paediatric wards of the Moi Teaching and Referral Hospital, Eldoret which is the Teaching Hospital for Moi University and the referral centre for surrounding districts of Western Kenya, with an altitude of over 2000 metres above sea level.PATIENTS:Twenty three consecutive children aged one to twelve years with cerebral malaria as defined by the WHO were studied. All children were treated with the standard quinine regimen for cerebral malaria.RESULTS:Majority of the children were six to ten years of age with 95.7% having a normal weight for age. 91.3%, 89.5% and 72.2% had fever, headache and convulsions respectively. 68.1% had a short duration of illness (less than three days) with only 9.5% presenting with hypoglycaemia. Severe anaemia was not observed but 72% had mild to moderate anaemia. Hyperparasitaemia (parasite counts greater than 100,000 per microlitre) was found in majority of the cases.CONCLUSION:Cerebral malaria presentation in the highlands is similar to that among non-immune populations and is an acute fulminant illness presenting with coma, hyperparasitaemia, fever and convulsions in children with normal nutritional status.
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| 20. |
- Farrell, Edward J., et al.
(författare)
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Graphical 3D medical image registration and quantification
- 1997
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Ingår i: Journal of medical systems. - 0148-5598 .- 1573-689X. ; 21:3, s. 155-172
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Tidskriftsartikel (refereegranskat)abstract
- We present a graphical three-dimensional method that facilitates image registration and fusion, and provides quantitative geometric and volume information. In particular it enhances the use of functional (radiopharmaceutical) imaging {(SPECT}, {PET)} which, though a powerful clinical tool, has the disadvantage of low spatial resolution and ill-defined boundaries. Registration between functional images and structural images {(MRI}, {CT)} can augment the anatomical context of these functional images.
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| 21. |
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| 22. |
- Galvin, J. M., et al.
(författare)
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Warping CT scans from nontreatment to treatment position
- 1997
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Ingår i: Radiation oncology investigations (Print). - 1065-7541 .- 1520-6823. ; 5:4, s. 206-212
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a cost-effective technique that optimally utilizes all available diagnostic studies for three-dimensional treatment planning. A simulator unit modified to produce cross-sectional images {(simulator-CT} unit) is used to create a reference data set with the patient in the treatment position. Registration software (qsh) brings other diagnostic studies into agreement with this reference data set. Two cases are presented as examples of the use of this technique. Registration of abdominal scans from the same patient demonstrates the warping of a nontreatment position study to the treatment position. The second case is based on paired data sets through the head, in which the diagnostic study was obtained by using a gantry tilt to follow the base of the skull and to avoid sections passing through the teeth. The registration software provides a method for combining diagnostic studies into a single "master" data set. The success of the transformation depends on the operator's ability to identify corresponding anatomic landmarks for different data sets and on the magnitude of the variation in the patient's position from one procedure to the next. Limitations in image quality and the number of cross-sections obtainable from a {simulator-CT} unit can be partially overcome by using the described technique. Thus, the information contained in nontreatment position diagnostic tests can be used accurately for treatment planning at limited cost.
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| 23. |
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| 24. |
- Lacy, Thomas E., et al.
(författare)
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Effects of damage distribution on evolution
- 1997
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Ingår i: Applications of continuum damage mechanics to fatigue and fracture. - : ASTM International. - 0803124732 ; , s. 131-149
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Bokkapitel (refereegranskat)abstract
- Recent micromechanically inspired phenomenological theories using internal state variable (ISV) representations of damage have been used to predict the thermomechanical behavior of microcracked solids. These models do not, in an explicit manner, account for distributions of microcracks in a representative volume element (RVE) and have been used success-fully only to determine the effective moduli of damaged solids. It has been demonstrated that while the distribution and interaction of damage entities within an RVE generally have a minor effect on the effective moduli, it has a significant effect on the evolution of damage and failure at the macroscale. Damage evolution rates, in general, cannot be described adequately by such theories because of their inability to account for interactions between damage entities in an arbitrary distribution. Key issues pertaining to the development of viable damage evolution equations using a continuum damage mechanics approach are addressed. In particular, limitations associated with the use of ISVs that can be expressed either in terms of macroscopically measurable quantities or through a spatial average of the geometric features of individual damage entities are discussed. Numerical simulations of evolving crack systems in two-dimensional perfectly brittle solids indicate that "effective stress" models may have difficulty in characterizing damage evolution in brittle microcracked solids when the damage consists of cracks of variable size or spatial distributions. An argument for implementing ISVs based on higher-order moments of the damage distribution within an RVE is presented.
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| 25. |
- Lacy, Thomas E., et al.
(författare)
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Gradient concepts for evolution of damage
- 1999
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Ingår i: Mechanics of materials. - 0167-6636 .- 1872-7743. ; 31:12, s. 831-860
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Tidskriftsartikel (refereegranskat)abstract
- While low-order measures of damage have sufficed to describe the stiffness of bodies with distributed voids or cracks, such as the void volume fraction or the crack density tensor of Vakulenko, A.A., Kachanov, M., 1971., addressing the growth of distributed defects demands a more comprehensive description of the details of defect configuration and size distribution. Moreover, interaction of defects over multiple length scales necessitates a methodology to sort out the change of internal structure associated with these scales. To extend the internal state variable approach to evolution, we introduce the notion of multiple scales at which first and second nearest-neighbor effects of nonlocal character are significant, similar to homogenization theory. Further, we introduce the concept of a cutoff radius for nonlocal action associated with a representative volume element (RVE), which exhibits statistical homogeneity of the evolution, and flux of damage gradients averaged over multiple subvolumes. In this way, we enable a local description at length scales below the RVE. The mean mesoscale gradient is introduced to reflect systematic differences in size distribution and position of damage entities in the evolution process. When such a RVE cannot be defined, the evolution is inherently statistically inhomogeneous at all scales of reasonable dimension, and the concept of macroscale gradients of internal variables is the only recourse besides micromechanics. Based on a series of finite element calculations involving evolution of 2D cracks in brittle elastica arranged in random periodic arrays, we examine the evolution of the mean mesoscale gradients and note some preliminary implications for the utility of such an approach.
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