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The upper limit of strength (the ''theoretical strength'') has been an active subject of research and speculation for the better part of a century. The subject has recently become important, for two reasons. First, given recent advances in ab initio techniques and computing machines, the limits of strength can be calculated with considerable accuracy, making this one of the very few problems in mechanical behavior that can actually be solved. Second, given recent advances in materials engineering, the limits of strength are being approached in some systems, such as hardened or defect-free films, and their relevance is becoming recognized in others. The present paper discusses some interesting results from recent research on the limits of strength, with an intermixture of speculations based on those results. Topics include the inherent nature of {l_brace}100{r_brace} cleavage and ''pencil slip'' in bcc metals, the inherent ductility of fcc metals, the anomalous properties of Al, and the possibility of measuring ideal strength with nanoindentation.

In the automotive industry, the surface morphology of sheet steel is controlled to improve both paint finish and formability. Formability, in turn, is influenced by the friction between the sheet and stamping tools, which the industry commonly measures with the ‘drawbead simulator’ (DBS) test. This investigation evaluated the use of various surface parameters as predictors of the frictional properties of electrogalvanized (EG) steel sheet as measured in the DBS test. The surfaces of a set of commercial EG steel sheets were measured using a profilometer and tested for friction in a DBS machine. The results were compared to various statistical measures of the sheet surfaces. The best of the simple measures proved to be a combination of two parameters: the arithmetic average roughness ( R a ) and the mean wavelength of the roughness, as determined from the Fourier power spectrum. Surfaces with higher R a were more likely to have lower friction in the DBS tests. But low friction can be obtained with lower values of the R a if the roughness is concentrated at the shorter wavelengths (below 500 μm). Surfaces with roughness distributed in the shorter wavelengths have the additional benefit of being easily painted to a high quality finish.

The vegetation data of the Cymbopogon-Themeda Grassland in the Lichtenburg area was reclassified by a numerical classification technique (T winspan ) and refined by applying Braun-Blanquet procedures. The results, which can be integrated with existing phytosociological classifications of the western Transvaal grassland, are summarized in two phytosociological tables where 12 communities or variants, variously grouped into larger, more comprehensive vegetation units, are identified. These plant communities are ecologically interpreted and described.

Deformation-induced martensitic transformation around a fatigue crack results in the phenomenon of crack closure. By approximating the residual stress field due to the volume expansion in the transformed zone as that of an edge dislocation pair, a closed-form solution of crack opening as a function of external loading is derived. The result indicates that the exact determination of crack closure point by compliance measurement is inherently difficult.

A method to perform Mossbauer spectrum curve fitting with a personal computer is presented. A modified Gauss-Newton algorithm for the least squares determination of nonlinear parameters is used in order to write computer codes that perform satisfactorily within the small memory sizes of personal computers. Mossbauer spectrum curve fitting with an IBM AT personal computer takes about 20 minutes of computing time to fit a pure Fe sextet spectrum and about 2 hours to fit a relatively complicated spectrum. The quality of the curve fitting with the personal computer is identical to that fitted with a mainframe computer.

This study investigated the influence of substrate metallization on the microstructure and creep behavior of the pure tin solder joints. Both symmetric (Cu:Cu) and the asymmetric (Cu:Au/Ni) metallizations were tested. The solder joints with asymmetric substrates had lower creep rates than those with symmetric (Cu:Cu) substrates. (Cu-Ni)/sup 6/Sn/sup 5/ intermetallics formed on all substrates. However, a dramatic and anomalous intermetallic growth was observed on the Ni-side of the asymmetric joints; after typical reflow the intermetallic reached almost one third of the solder joint thickness. It appears that the abnormally thick intermetallic inhibits creep in the asymmetric joint and is the primary cause of the lower creep rates.

This paper describes a computer simulation study of the martensitic transformation in a simple model crystal. The transformation is assumed athermal and irreversible. The elastic energy developed during martensite growth is calculated from the theory of linear elasticity for elastically homogeneous media. The results show that the development of complex, multivariant microstructures is promoted by geometric constraints on the transforming crystal, and is strongly affected by an applied stress. Either constraint or applied stress increases the thermal resistance to the transformation, as measured by the spread between the M s and M f temperatures. Typical microstructures are described and illustrated.

A model of litter decomposition in a South African savanna is described. Decomposing material is divided into a number of particle classes, distinguishing material of different ages, origins and size. Each particle class is further divided into a number of chemical constituents. Processes modelled include ageing, physical disintegration, consumption by termites and decay. Sources of fresh litter include tree and grass leaves. Results from a standard run of the model and a number of runs with changed parameter values to test the model's sensitivity to these are presented and discussed. From exercise of the model it appears that the most important processes relate to soil-feeding termites, with decay and physical breakdown being the background against which termites operate.

Grass communities identified through association‐analysis were used for determining the effects of several methods for eradicating and controlling macchia. Follow‐up burns at three frequencies applied over a ten‐year period to control macchia were found to have masked most differences in species composition of the grass sward which may have arisen as a result of eradication treatments of burning, spraying and cutting. However, Ely onurus argenteus Nees and Harpechloa falx (L.) Kuntze were absent only from stumped plots. Themeda triandra Forsk. and Apochaetae hispida (L.f.) Phipps survived in a vegetative state in unburned, control plots for about 20 years in spite of competition from a dense, bushy growth of macchia (Cliffortia paucistaminea H. Weim., C. linearifolia Eckl. & Zeyh. and Erica brownleeae Bolus). Themeda triandra was best adapted to withstand frequent burning while Heteropogon contortus (L.) Beauv. ex Roem. & Schult. was suppressed by this treatment. Certain other grasses may have su...

In this paper we report the behavior of the plastic deformation of an idealized crystal made by stacking parallel slip planes. Each slip plane is assumed to contain active sources of dislocations leading to a constant density of non-interacting dislocations in the plane which glide through randomly distributed localized point obstacles, representing small precipitates. The dislocation is assumed to have a constant line tension and the dislocation-obstacle interaction is taken to have a simple step form. Using results of computer simulation of thermally activated glide through random arrays of point obstacles we modeled deformation as a function of temperature and applied stress, determining the strain rate and the morphological characteristics of slip.

The linear elastic theory is used to determine the preferred habit of the α2 Fe16N2 precipitate in α iron, assuming that the precipitate is a coherent, tetragonal phase having the form of a thin circular disc, and neglecting both surface tension effects and the difference in elastic constants between the precipitate and the iron matrix. With these approximations the precipitate habit depends on the tetragonality of the transformation strain connecting the matrix and precipitate lattices, the aspect ratio of the disc, and the elastic constants. The preferred habit of Fe16N2 in α iron is found to vary with the precipitate aspect ratio (K). The preferred habit changes continuously from {100} for K

It has been postulated that ethanol-induced changes in carbohydrate hemeostasis are mediated in part by alterations in pancreatic endocrine function. In this study ethanol (40, 120 and 240 mg/dl) produced dose-related suppression of immunoreactive insulin (IRI) release from isolated rat islets perifused with glucose and theophylline. Acetaldehyde inhibited IRI release at a concentration (39 mg/dl) much higher than can be achieved in vivo. A high concentration (29 mg/dl) of acetate did not influence IRI release. These results suggest that the ability of ethanol to inhibit IRI release resides principally in the parent compound and not its metabolites.

This paper discusses the different types of creep behavior reported for the steady-state deformation of the superplastic Zn-22% Al eutectoid alloy at low stress levels. All existing theories of creep behavior predict that at sufficiently low stresses the steady-state creep will be diffusion-controlled and will follow a power-law with a strain rate exponent of m = 1.0. The question is whether this diffusional creep mechanism sets the lower bound on superplastic deformation in typical samples of Zn-22% Al, or whether another type of creep behavior intervenes.

The functional roles of materials science and materials engineering in modern alloy design are discussed. It is suggested that following the end of World War II evolutionary trends in both methodology and language have tended to divide, rather than unite, these two branches of knowledge. Over a period of several decades the net effect has been, on the one hand, to isolate materials scientists from materials engineers, and thereby prevent them from making greater contributions to technology and, on the other hand, to deny to materials engineers the potential benefits of the vast understanding achieved by the materials scientist. Several new programs in the materials education field are described which have as their aim a partial redress of this undersirable situation. The current state of development of alloy design, as seen from an academic viewpoint, is described with the aid of four illustrative examples taken from current student theses research. These examples are: (1) a b.c.c. iron alloy having unusual toughness at temperatures of liquid nitrogen and below; (2) a series of medium alloy steels whose strength and toughness are equivalent to those of the high alloy maraging steels; (3) an ultrahigh-strength Fe - Cr - C steel of great toughness which was developed from fundamental considerations of microstructure and substructure; and (4) b.c.c. iron alloys which utilize intermetallic compounds, rather than alloy carbides, for elevated temperature dispersion hardening.

‘User friendly’ interactive pre- and post-processing programs allow major increases in productivity in finite element analysis. These programs are designed to reduce the amount of effort required to develop input and summarize output for large finite element programs. The gains in productivity, however, are not without cost. The use of this technology requires additional computer resources and user training. Although the increases in productivity are obvious to those who use this technology, management decisions are based on economic considerations. Therefore, engineering management must understand the benefits and the value of the increases in productivity to effectively evaluate the use of these tools. This paper discusses the concepts which form the basis for improving productivity through the use of interactive pre- and post-processing. Examples, taken from the authors' experience, are presented which illustrate the improvements in productivity which can be obtained using this technology.

The Etosha National Park has been divided into 31 plant communities on the basis of floristic, edaphic and topographic features, employing a Braun – Blanquet type of phytosociological survey. The vegetation and soils of six major groups of plant communities are described briefly, and a vegetation map delineating the extent of 30 plant communities is presented.

The variation in total basal cover from one time of year to another was investigated by means of the bridge and wheel-point methods over a period of seven years. Treatments were: a control plot, one cut annually in winter and one in spring; a fourth plot was burnt and a fifth burnt and cut alternately, in spring. Statistically significant increases and decreases in total basal cover were found between a number of consecutive surveys. Maximum basal cover was usually recorded between April and July and the minimum basal cover between July and October, although exceptions to both ranges were found. Winter and spring basal covers were significantly correlated (P = 0,001 and 0,01 respectively) with the rainfall of the preceding season on the control plot, but no correlation with rainfall was found on the treated plots. The cover of Trachypogon spicatus and Eragrostis curvula increased and decreased, respectively, with or without cutting and burning treatments, over the period of study. Themeda triandra increased in basal cover with treatments, while cover of Setaria perennis decreased. The cover of Heteropogon contortus remained fairly constant, but decreased when no treatments were applied. The various treatments gave similar results, but usually differed from those of the control. It was concluded that the best time for making an annual survey of basal cover by means of a point method is between May and June. At this time the plants may be easily identified and basal cover is at its highest. Keywords: basal cover; burning; cover; cutting; effects; eragrostis curvula; grassland; heteropogon contortus; national botanic garden; pretoria; rainfall; seasonal variation; setaria perennis; south africa; themeda triandra; trachypogon spicatus; treatments; wheel-point method

A continuum model of an interface in a one-component fluid is developed in the special case when the fundamental equation of the fluid may be written in the local form F= F (θ,p,(∇p) 2 , ∇ 2 p) The fluid governed by this equation is shown to behave as a classical fluid when the density is nearly uniform, but has properties associated with interfacial behavior when the local variation of density is large. The material near a two-phase interface in such a fluid is subject to a complex system of multipolar stresses. However, the thermodynamic behavior of the fluid near the interface is governed by the familiar relation pF = μp — P, where P may be regarded as a thermodynamic pressure. Gibbs' thermodynamics of surfaces follows naturally for an interface in this fluid when the Gibbs construction is used. The relation of this model to prior continuum and statistical models of the fluid interface is discussed.

The reaction channel ( π − , NN ) for the absorption of bound negative pions by nuclei is used as a means to study nuclear short range correlations. A three-body partial-wave analysis has been carried out for the final-state scattering which includes a Reid soft-core nucleon-nucleon interaction and an optical potential. This coupled-channel formalism rapidly converges as we eliminate the asymptotic single-nucleon and deuteron interactions. It is found that for 12 C reasonable agreement with experiment cannot be obtained in this model without modification of the high relative-momentum components of bound shell model pair wave functions.

The main role of vegetation classification in agricultural land utilization is for the planning and management of vegetated areas. For farm planning, rapid, observationally‐based methods are most practical, but for planning at all scales the slower quantitative methods eliminate personal bias and are more readily reproducible. The main role of ordination lies in providing ecological information on plant‐environmental relationships, and inherently it should do so more efficiently than classification. Both ordination and classification provide information of practical value to agriculture.

The Bankenveld Grassland in the Lichtenburg area was reclassified using T winspan classification and subsequently Braun-Blanquet procedures to refine these results. In two phytosociological tables, two Major Communities, six Communities and eight Variants are identified and described. This new classification can now be included in the comprehensive phytosociological and syntaxonomical synthesis of the western Transvaal Grassland.

The purpose of the study was to determine the ecological status of representative herbaceous species with respect to environmental factors and subhabitats of which the spatial and temporal pattern could be elucidated. The basal cover of the herbaceous vegetation was determined by means of the wheel-point method. Frequency data in each of 200 1 m 2 contiguous quadrats were ordinated by means of detrended correspondence analysis (DECORANA). The distribution of the species along the first axis is a function of conditions representing an open habitat with full sunlight and somewhat drier conditions to the left and a denser and/or shrub habitat with more shade and wetter, more favourable conditions to the right. The gradient along the second axis is that of undisturbed conditions with more perennials at the lower end to disturbed conditions with more annuals at the upper end. From the scatter diagrams it can be concluded that the herbaceous layer is functionally homogeneous, that it represents a seral stage and that Eragrostis pallens and Digitaria eriantha, both with the highest basal cover in the study area, are representative of a seral stage rather than of the climax. The lack of pattern in the herbaceous layer can be attributed mainly to the large number of annual and perennial pioneer and disturbance-indicating species. In a similar study Whittaker et al. (in press) concluded that the woody vegetation dominates the pattern.

The A15 layer of a commercial Airco wire containing 2869 Nb filaments was analyzed as a function of heat treatment. Its microstructure is composed of three morphologically concentric shells. The central shell consists of fine equiaxed grains and has a nearly stoichiometric Sn concentration. High resolution electron microscopic analysis suggests that the fine grains are formed through the polygonization of dislocations. The homogeneous composition through the fine-grained layer is a probable consequence of the small grain size, which permits relatively rapid chemical redistributions through grain boundary diffusion. In contrast, the chemical gradient in the large-grained inner and outer sheels is steep. The microstructure is established by the reaction heat treatment, and determines the critical current. The best combination of grain size, composition, and volume of the fine-grained shell is obtained with an intermediate reaction temperature (700 to 730/sup 0/C); this temperature range also yields the best values of J/sub c/. Various two-step heat treatments were studied and compared to isothermal aging. The best microstructure and, hence, the best critical current characteristic was obtained by aging the specimen at 700/sup 0/C for 4 days followed by 730/sup 0/ for 2 days.

Grass communities identified through association-analysis were used for determining the effects of several methods for eradicating and controlling macchia. Follow-up burns at three frequencies applied over a ten-year period to control macchia were found to have masked most differences in species composition of the grass sward which may have arisen as a result of eradication treatments of burning, spraying and cutting. However, Elyonurus argenteus Nees and Harpechloa falx (L.) Kuntze were absent only from stumped plots. Themeda triandra Forsk. and Apochaetae hispida (L.f.) Phipps survived in a vegetative stae in unburned, control plots for about 20 years in spite of competition from a dense, bushy growth of macchia (Cliffortia paucistaminea H. Weim., C. linearifolia Eckl. & Zeyh. and Erica brownleeae Bolus). Themeda triandra was best adapted to withstand frequent burning while Heteropogon contortus (L.) Beauv. ex Roem. & Shult. was suppressed by this treatment. Certain other grasses may have survived entirely below the soil, and without evidence of aerial portions, in moribund veld. Basal cover was used for recommendations on veld burning management, whether for purposes of water production or for grazing.Keywords: amatole mountains; apochaetae hispida; association analysis; basal cover; botanical composition; burning; cliffortia linearifolia; cliffortia paucistaminea; competition; control; cutting; dohne sourveld; elyonurus argenteus; eradication; erica brownleeae; grass sward; grasses; grazing; gxulu kop; harpechloa falx; heteropogon contortus; keiskammahoek; macchia; management; method; production; south africa; species composition; techniques; themeda triandra; treatments; veld; veld burning

The constraints placed on a retaining ring in service require that it be made of a non-magnetic alloy which has high strength and toughness and is resistant to degradation in either dry or damp hydrogen. The specific objective adopted for this project was to develop a non-magnetic, hydrogen resistant alloy which could achieve strength levels near 200 ksi yield with good residual toughness while permitting processing primarily through thermal treatment. During the first two years of this project the research has concentrated on achieving strength and toughness objectives, using alloys which have at least tentatively met the other necessary criteria. Given the promising strength and toughness properties obtained in research to date, particularly with precipitation hardened alloys based on the Fe--Ni--Ti--Ta system, fatigue and fracture toughness testing in hydrogen has been initiatd to determine suitability of these alloys for use in a hydrogen environment. Further research will be conducted to optimize alloy properties given results of these tests.

The variation in total basal cover from one time of year to another was investigated by means of the bridge and wheel-point methods over a period of seven years. Treatments were: a control plot, one cut annually in winter and one in spring; a fourth plot was burnt and a fifth burnt and cut alternately, in spring. Statistically significant increases and decreases in total basal cover were found between a number of consecutive surveys. Maximum basal cover was usually recorded between April and July and the minimum basal cover between July and October, although exceptions to both ranges were found. Winter and spring basal covers were significantly correlated (P = 0,001 and 0,01 respectively) with the rainfall of the preceding season on the control plot, but no correlation with rainfall was found on the treated plots. The cover of Trachypogon spicatus and Eragrostis curvula increased and decreased, respectively, with or without cutting and burning treatments, over the period of study. Themeda triandra increased in basal cover with treatments, while cover of Setaria perennis decreased. The cover of Heteropogon contortus remained fairly constant, but decreased when no treatments were applied. The various treatments gave similar results, but usually differed from those of the control. It was concluded that the best time for making an annual survey of basal cover by means of a point method is between May and June. At this time the plants may be easily identified and basal cover is at its highest.Keywords: basal cover; burning; cover; cutting; effects; eragrostis curvula; grassland; heteropogon contortus; national botanic garden; pretoria; rainfall; seasonal variation; setaria perennis; south africa; themeda triandra; trachypogon spicatus; treatments; wheel-point method

Modes of boundary motion in crystals containing controlled amounts of Ag as an alloying element in high-purity Zn crystals were examined. A discontinuous motion found in the alloy crystals at room temperature could be attributed to the diffusivity of solute atoms since such behavior is not observed in the alloy crystals at liquid nitrogen temperature and in the pure crystals at room temperature. A situation similar to the dynamic strain aging seems to have been created around the moving boundary. A further study is necessary to determine the diffusion mechanism. The absence of such discontinuous motion in the pure crystal seems to be due to the lack of sufficient impurity atoms. It appears that the jerky motion observed in pure crystals at room temperature is due to a higher impurity content of approx.100 ppM compared with that of

The S-Process (dual temperature, hydrogen sulfide) for the extraction of heavy water from natural water has been evaluated iii single-stage pilot-plant operation at the Dana Plant, Extraction capacity was demonstrated to check theory within the limits of experimental accuracy, and the bubble cap tray efficiency was demonstrated to be an average of at least 45% versus design estimates of 50%. This report summarizes certain process calculations and presents several novel theoretical concepts of the process theory. (auth)

Statistical analysis, primarily analysis of variance, was applied to evaluate several factors involved in the development of suitable fabrication and processing techniques for the production of lead telluride thermoelectric elements for the SNAP 10A energy conversion system. The analysis methods are described as to their application for determining the effects of various processing steps, estabIishing the value of individual operations, and evaluating the significance of test results. The elimination of unnecessary or detrimental processing steps was accomplished and the number of required tests was substantially reduced by application of these statistical methods to the SNAP 10A production development effort. (auth)

The thickness distortion of a backscatter 14.41 keV γ-ray Mossbauer spectrum is calculated for the case of isotropic γ-ray re-emission in the absorber, and a method of extracting the true absorber material resonant scattering cross section from experimental data is suggested and demonstrated. Some other features of backscatter and transmission 57Fe Mossbauer spectra are compared.