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Journal of Materials Science & Technology 2014年第3期30卷 203-212页

作者： G.M.Owolabi H.A.whitworth Department of Mechanical Engineering Howard University

Studies on microstructurally small fatigue cracks have illustrated that heterogeneous microstructural features such as inclusions, pores, grain size distribution as well as precipitate size distribution and volume fraction create stochasticity in their behavior under cyclic loads. Therefore, to enhance safe-life and damage-tolerance approaches, accurate modeling of the influence of these heterogeneous microstructural features on microstructurally small crack formation and growth from stress raisers is necessary. In this work, computational micromechanics was used to predict the high cycle fatigue of microstructurally small crack formation and growth in notched polycrystalline nickel-base superalloys and to quantify the variability in the driving force for formation and growth of microstructurally small crack from notch root in the matrix with non-metallic inclusions. The framework involves computational modeling to obtain three-dimensional perspectives of microstructural features influencing fatigue crack growth in notched nickel-base superalloys, which accounts for the effects of nonlocal notch root plasticity, loading, microstructural variability, and extrinsic defects on local cyclic plasticity at the microstructure-scale level. This approach can be used to explore sensitivity of minimum fatigue lifetime to microstructures. The simulation results obtained from this framework were calibrated to existing experimental results for polycrystalline nickel-base superalloys.

关键词： Microstructuraliy small crack Crystal plasticity Nickel-base superalloy Fatigue life

Shear Band Formation in AISI 4340 Steel Under Dynamic Impact Loads:Modeling and Experiment

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Acta Metallurgica Sinica(English Letters) 2013年第4期26卷 378-384页

作者： Daniel Odoh Gbadebo Owolabi Akindele Odeshi Horace whitworth Department of Mechanical Engineering Howard University Department of Mechanical Engineering University of Saskatchewan57 Campus DriveSaskatoonSKS7N 5A9Canada

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

关键词： Adiabatic shear bands Finite element model Strain hardening Thermal softening Johnson-Cook model AISI 4340 steel

Cool-season annual pastures with clovers to supplement wintering beef cows nursing calves

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Journal of Animal Science and Biotechnology 2012年第4期3卷 221-226页

作者： Stacey A Gunter Whitney A whitworth T Gregory Montgomery Paul A Beck United States Department of Agriculture Agricultural Research Service Southern Plains Range Research Station University of Arkansas at Monticello Division of Agriculture Southeast Research & Extension Center University of Arkansas Division of Agriculture Southwest Research & Extension Center University of Arkansas

In December of 3 years, 87 beef cows with nursing calves （.594 ± 9.8 kg; calving season, September to November） at side were stratified by body condition score, body weight, cow age, and calf gender and divided randomly into 6 groups assigned to 1 of 6 cool-season annual pastures （0.45 ha/cow） that had been interseeded into a dormant common bermudagrass （Cynodon dactylon [L.] Pers.）/bahiagrass （Paspalum notatum Flugge） sod. Pastures contained 1 of the following 3 seeding mixtures （2 pastures/mixture）： 1） wheat （Triticum aestivurn L.） and ryegrass （Lofium multiflorum Lain., WRG）, 2） wheat and ryegrass plus red clover （Trifolium pretense L, WRR）, or 3） wheat and ryegrass plus white （Trifofium repens L.） and crimson clovers （Trifolium incarnatum L., WRW）. All groups had ad libitum access to grass hay （12% crude protein; 58% total digestible nutrients）. The second week in December, cow estrous cycles were synchronized and artificially inseminated. In late December, a bull was placed with each group for 60-d. Data were analyzed with an analysis of variance using a mixed model containing treatment as the fixed effect and year as the random effect. Body weight and condition scores did not differ （P ≥ 0.27） among cows between February and June. Calf birth weights or average daily gain did not differ （P≥ 0.17） among treatments; however, calves grazing pastures with clovers did tend （P= 0.06） to weigh more than calves grazing grass only. Weaning weight per cow exposed to a bull was greater （P= 0.02） for WRR and WRW than WRG. Cows grazing winter-annual pastures containing clovers tended to wean more calf body weight per cow exposed to a bull than cows grazing the grass only pastures.

关键词： Annual ryegrass Beef cows Clovers Nitrogen Pasture 1

Partial Protection by Lipoic Acid Against Carboplantin-induced Ototoxicity in Rats

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Biomedical and Environmental Sciences 2005年第3期18卷 198-206页

作者： KAZIM HUSAIN CRAIG whitworth SATU M. SOMANI LEONARD P. RYBAK Departments of Surgery Southern Illinois University School of Medicine Springfield IL 62794 USADepartments of Surgery Southern Illinois University School of Medicine Springfield IL 62794 USADepartments of Pharmacology Southern Illinois University School of Medicine Springfield IL 62794 USADepartments of Surgery Southern Illinois University School of Medicine Springfield IL 62794 USA

Objective To investigate the alterations in auditory brainstem evoked responses (ABRs) and the changes of carboplatin-induced ototoxicity in the cochlear oxidant/antioxidant systems and otoprotection by an antioxidant lipoate. Methods Male wistar rats were divided into four groups and treated as follows: 1) vehicle (saline) control, 2) carboplatin (256 mg/kg, i.p.), 3) lipoate (100 mg/kg, i.p.), 4) lipoate + carboplatin. Post-treatment ABRs were performed after four days and rats were sacrificed with their cochleae harvested and analyzed. Results Carboplatin significantly elevated ABR threshold above the pretreatment thresholds. Lipoate+carboplatin treated rats showed decreased elevation of hearing threshold. Carboplatin significantly depleted cochlear reduced to oxizized glutathione (GSH/GSSG) ratio, whereas lipoate+carboplatin treatment increased GSH/GSSG ratio. Carboplatin significantly decreased cochlear copper zinc-superoxide dismutase (CuZn-SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione reductase (GR) and glutathione-S-transferase (GST) activities and enzyme protein expressions and a significant increase in Mn-SOD activity, protein expression and malondialdehyde (MDA) level. Cochlear antioxidant enzyme activities, enzyme protein expressions and MDA level were partially restored in lipoate+carboplatin treated rats, compared to carboplatin alone. Conclusion Carboplatin-induced ototoxicity is related to impairment of cochlear antioxidant system and otoprotection conferred by lipoate is associated with partial sparing of the cochlear antioxidant defense system.

关键词： Carboplatin Alpha-lipoic acid Ototoxicity Antioxidants Rats

Changes in rainfall partitioning caused by the replacement of native dry forests of Lithraea molleoides by exotic plantations of Pinus elliottii in the dry Chaco mountain forests, central Argentina

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Journal of Arid Land 2020年第5期12卷 717-729页

作者： Samia S CORTÉS Juan I whitworth-HULSE Eduardo L PIOVANO Diego E GURVICH Patricio N MAGLIANO Land Science Research Center(CICTERRA-CONICET)and National University of Córdoba Av.Vélez Sarsfield 1611X5016GCA CórdobaArgentina Multidisciplinary Institute of Plant Biology National University of Córdoba and CONICETCC 495X5000JJC CórdobaArgentina Environmental Study Group Institute of Applied Maths of San LuisNational University of San Luis and CONICETEjército de los Andes 950D5700HHW San LuisArgentina Department of Biology Faculty of ChemistryBiochemistry and PharmacyNational University of San LuisEjército de los Andes 950D5700HHW San LuisArgentina

The replacement of native dry forests by commercial(exotic)tree plantations could generate changes in rainfall partitioning,which further affects the water *** this study,we determined(i)the rainfall partitioning into interception,throughfall and stemflow,(ii)the role of rainfall event size on rainfall partitioning,(iii)the pH of water channelized as throughfall and stemflow,and(iv)the runoff in Lithraea molleoides(a native species)and Pinus elliottii(an exotic species)stands in the dry Chaco mountain forests,central *** average,interception,throughfall and stemflow accounted for 19.3%,79.5%and 1.2%of the gross rainfall in *** stand,and 32.6%,66.7%and 0.7%of the gross rainfall in *** stand,*** of interception,throughfall and stemflow presented positive linear relationships with the increment of rainfall event size for both tree species(P<0.01 in all cases).Percentages of interception,throughfall and stemflow were all related to the increment of rainfall event size,showing different *** increasing rainfall event size,interception exponentially decreased,throughfall asymptotically increased and stemflow linearly *** *** and *** stands presented significant differences in the pH values of water channelized as throughfall(6.3 vs.6.7,respectively;P<0.01)and stemflow(4.5 vs.5.8,respectively;P<0.01).Runoff occupied only 0.3%of the gross rainfall in *** stand and was zero in *** *** results showed that the native species *** presented 13.6%more water reaching the topsoil(i.e.,net rainfall;net rainfall=gross rainfall-interception-runoff)than the exotic species *** study improves our understanding of the effects of native vegetation replacement on the local water balance in the dry forest ecosystems.

关键词： drylands ecohydrology land use changes spatial heterogeneity water-limited environments

Functional strategies and distribution of climbing plant communities in different vegetation patches in a subtropical dry forest, central Argentina

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Journal of Plant Ecology 2019年第1期12卷 23-33页

作者： María Cecilia Ferrero Sebastián R.Zeballos Juan I.whitworth-Hulse Melisa A.Giorgis Diego E.Gurvich Instituto Multidisciplinario de Biología Vegetal CONICET-Universidad Nacional de CórdobaAv.Vélez Sársfield 1611CP X5016ZAACórdobaArgentina Facultad de Ciencias Exactas Físicas y NaturalesUniversidad Nacional de CórdobaAv.Vélez Sársfield 299CP X5000JJCCórdobaArgentina

Aims In the context of global change,the impacts of forest structure alter-ation on climbing plants in extra-tropical ecosystems are poorly *** also remains little explored,the functional strategies among climbing plant species and its relationship with the local-scale distribution of climbing plant ***,we aimed at three goals:(i)we studied how climbing plant community com-position responds to the modification of the original forest structure in a subtropical dry forest;(ii)we characterized climbing plant spe-cies according to functional traits related to the acquisition and use of resources;and(iii)we examined whether functional strategies at the community level are also responding to vegetation structure change,a much less addressed topic in the ecology of climbing *** The study was carried out in the Chaco forest of Córdoba Mountains,central *** selected 18 patches of‘native forests’,‘shrublands’and‘Ligustrum lucidum forests’.We counted and identified climbing plant individuals in each patch,including woody and herbaceous *** each spe-cies,we measured 10 functional traits:leaf area,leaf area ratio,leaf dry matter content,leaf tensile strength,specific leaf area,stem density,wood saturated water content,inter-node length,petiole length and plant *** compared climbing plant composition,richness and abundance among the three vegeta-tion ***,we assessed trends of functional variation of climbing plant species by performing a principal component ***,we investigated whether simple mean,weighted mean and Functional Richness index of principal component analysis(PCA)axes differed among the climbing plant communi-ties of the three vegetation *** Findings Multivariate analysis revealed that climbing plant composition of native forests significantly differed from that of shrublands and *** ***,*** forests had the lowest climb-ing plant species *** var

关键词： vines functional traits Chaco Serrano climbing plant species richness community weighted means

Occurrence of Dynamic Shear Bands in AISI 4340 Steel under Impact Loads

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World Journal of Mechanics 2013年第2期3卷 139-145页

作者： Gbadebo Owolabi Daniel Odoh Akindele Odeshi Horace whitworth Department of Mechanical Engineering Howard University Washington DC USA Department of Mechanical Engineering University of Saskatchewan Saskatoon Canada

In this study, occurrence of adiabatic shear bands in AISI 4340 steel under high velocity impact loads is investigated using finite element analysis and experimental tests. The cylindrical steel specimen subjected to impact load was divided into different sections separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower at the section where the adiabatic shear bands are expected to initialize. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in the alloy. The experimental results also show that cracks were initiated and propagated along transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by strain-rates. The simulation results obtained were compared with experimental results obtained for the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

关键词： Adiabatic Shear Band Finite Element Model Strain Hardening Thermal Softening Johnson-Cook Model AISI 4340 Steel

Measurement of the Deformation of Aluminum Alloys under High Strain Rates Using High Speed Digital Cameras

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World Journal of Mechanics 2013年第2期3卷 112-121页

作者： Gbadebo Owolabi Daniel Odoh Alex Peterson Akindele Odeshi Horace whitworth Department of Mechanical Engineering Howard University Washington DC USA Department of Mechanical Engineering University of Saskatchewan Saskatoon Canada

Aluminum alloys exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of aluminum alloy is the driving force for the need to adequately understand its deformation and failure mechanisms under various types of dynamic loading conditions. In this study, full field plastic deformation of AA6061-T6 aluminum alloy at high strain-rates under compressive and torsion loads are measured using split Hopkinson compression, torsion Kolsky bars, and a high speed digital image correlation system. The stress-strain curves obtained using the high speed digital cameras are compared with results obtained from the elastic waves in the compression and torsion bars. A post deformation analysis of the specimen also shows strain localization along narrow adiabatic shear bands in the AA6061-T6 alloy.

关键词： Split Hopkinson Pressure Bar Torsion Kolsky Bar Digital Image Correlation System Adiabatic Shear Band Post Deformation Analysis

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