Mechanical behavior of a continuous fiber reinforced aluminum matrix composite subjected to transverse and thermal loading

The transverse properties of an aluminum alloy metal matrix composite reinforced by continuous alumina fibers were investigated. The composite is subjected to both mechanical and cyclic thermal loading. The results of an experimental program indicate that the shakedown concept of structural mechanics provides a means of describing the material behavior. When the loading conditions are within the shakedown region, the material finally responds in an elastic manner after initial plastic response, and for loading conditions outside the shakedown region, the material exhibits a rapid incremental plastic strain accumulation. The failure strain varies by an order of magnitude according to the operating conditions. Hence, for high mechanical and low thermal loading, the failure strains is small; for low mechanical and high thermal loading, the failure strain is large

Reduction of thermal stresses in continuous fiber reinforced metal matrix composites with interface layers

The potential of using an interface layer to reduce thermal stresses in the matrix of composites with a mismatch in coefficients of thermal expansion of fiber and matrix was investigated. It was found that compliant layers, with properties of readily available materials, do not have the potential to reduce thermal stresses significantly. However, interface layers with high coefficient of thermal expansion can compensate for the mismatch and reduce thermal stresses in the matrix significantly

Long term integrity for space station power systems

A study was made of the High Temperature Design Codes ASME N47, British R5, and the French RCC-MR Rules. It is concluded that all these codes provide a good basis of design for space application. The new British R5 is the most complete since it deals with the problem of defects. The ASME N47 was subjected longer to practical application and scrutiny. A draft code is introduced, and a proposed draft for high temperature design in which attempts were made to identify gaps and improvements is suggested. The design is limited by creep characteristics. In these circumstances, life is strongly affected by the selected value of the factor of safety. The factor of safety of primary loads adopted in the codes is 1.5. Maybe a lower value of 1.25 is permissible for use in space. Long term creep rupture data for HAYNES 188 is deficient and it is suggested that extrapolation methods be investigated

Relationship of ovarian function to histochemical reactions

Histochemistry is accepted as a method for investigating body tissues and organ function, and although much research has been made into many human tissues, the ovary alone would appear to have been neglected; there do exist a few reports which deal with the histochemical findings in ovarian tumours, and also with the vasculature, but the present writer has not found any reports of any organised or formal study of the histochemistry of the human ovary. Such a study was undertaken by the writer, and it is this work and the results obtained which form the Thesis. In the first instance the mature human ovary was studied, sections being treated by a variety of staining techniques, the main ones being the Schiff reaction and various qualitative modifications of it, and the Sudan reaction. As a result of this a lipo-mucoprotein substance was discovered in the ovary, this substance being particularly associated with the follicular system, being found in the granulosa layer of the developing follicle, to a greater extent in the corpus luteum, and to a slight degree in association with the corpus albicans. Closer examination revealed that this substance was visualised in two forms - first, as granules within cells, and second, as globules apparently not contained within cells. (Figs. 1 and 2). It has been noted that there is greater evidence of this Schiff positive material in the corpus luteum associated with the pregnancy state than at any other time. Apparently not the result of break-down products, it seemed that this substance was intimately associated with activity of the ovary. Study of foetal/baby ovaries and of postmenopausal ovaries revealed that activity in these organs was not an infrequent occurrence, and particularly with regard to the foetal/baby ovaries, this activity was especially associated with a maternal pregnancy toxaemia. As in the mature ovaries, Schiff positive material was found related to the follicular bodies within these foetal/baby and post-menopausal ovaries. It seemed that the lipo-mucoprotein found in these ovaries might possibly be related to gonadotrophins, probably of pituitary origin, or perhaps with the ovarian hormones themselves; but whether the Schiff positive material revealed the presence of the actual hormones or merely a carrier substance, cannot be stated. In an attempt to resolve the question of the true nature and significance of this Schiff positive material, certain animal experiments were carried out. Animals - rabbits, rats, mice were injected with urine from both pregnant and non-pregnant women, and also with various hormonal preparations. As a result it was shown that urine from pregnant women and gonadotrophic hormones produced activity in the experimental animals, and further, that Schiff positive material, similar to that found in the human ovary, was associated with this activity, and in similar sites. The true nature of the material, however, remains unresolved. Discussion of the results of the investigation leads to the following conclusions: 1) The human ovary, when active, exhibits the presence of some substance which is demonstrable by histochemical procedures, in particular the Schiff reaction and its various modifications. 2) The Schiff positive substance is complex and would appear to be a mixture of lipoid and mucoprotein, the protein moiety probably containing tryptophan. 3) This Schiff positive substance is especially associated with the follicular system of the ovary. 4) It seems unlikely that this substance is either an artefact or due to simple breakdown products, but is in fact representative of some positive active phase in the ovarian cycle. 5) It seems more than likely that what in the past have been referred to as ovarian interstitial cells are in fact a terminal stage of the Schiff positive substance which is now reported. 6) Animal experiments show that the ovaries of rabbits, rats, and mice respond to stimulation in a way similar to that of the human ovary, and exhibit Schiff positive material under similar conditions. 7) Further investigation will be required in order to establish identification of the Schiff positive material

Optimization of interface layers in the design of ceramic fiber reinforced metal matrix composites

The potential of using an interface layer to reduce thermal stresses in the matrix of composites with a mismatch in coefficients of thermal expansion (CTE) of fiber and matrix was investigated. It was found that the performance of the layer can be defined by the product of the CTE and the thickness, and that a compensating layer with a sufficiently high CTE can reduce the thermal stresses in the matrix significantly. A practical procedure offering a window of candidate layer materials is proposed

Materials with periodic internal structure: Computation based on homogenization and comparison with experiment

The combination of thermal and mechanical loading expected in practice means that constitutive equations of metal matrix composites must be developed which deal with time-independent and time-dependent irreversible deformation. Also, the internal state of composites is extremely complicated which underlines the need to formulate macroscopic constitutive equations with a limited number of state variables which represent the internal state at the micro level. One available method for calculating the macro properties of composites in terms of the distribution and properties of the constituent materials is the method of homogenization whose formulation is based on the periodicity of the substructure of the composite. A homogenization procedure was developed which lends itself to the use of the finite element procedure. The efficiency of these procedures, to determine the macroscopic properties of a composite system from its constituent properties, was demonstrated utilizing an aluminum plate perforated by directionally oriented slits. The selection of this problem is based on the fact that, extensive experimental results exist, the macroscopic response is highly anisotropic, and that the slits provide very high stress gradients which severely test the effectiveness of the computational procedures. Furthermore, both elastic and plastic properties were investigated so that the application to practical systems with inelastic deformation should be able to proceed without difficulty. The effectiveness of the procedures was rigorously checked against experimental results and with the predictions of approximate calculations. Using the computational results it is illustrated how macroscopic constitutive equations can be expressed in forms of the elastic and limit load behavior

Pathotypic diversity of Hyaloperonospora brassicae collected from Brassica oleracea

Downy mildew caused by Hyaloperonospora brassicae is an economically destructive disease of brassica crops in many growing regions throughout the world. Specialised pathogenicity of downy mildews from different Brassica species and closely related ornamental or wild relatives has been described from host range studies. Pathotypic variation amongst Hyaloperonospora brassicae isolates from Brassica oleracea has also been described; however, a standard set of B. oleracea lines that could enable reproducible classification of H. brassicae pathotypes was poorly developed. For this purpose, we examined the use of eight genetically refined host lines derived from our previous collaborative work on downy mildew resistance as a differential set to characterise pathotypes in the European population of H. brassicae. Interaction phenotypes for each combination of isolate and host line were assessed following drop inoculation of cotyledons and a spectrum of seven phenotypes was observed based on the level of sporulation on cotyledons and visible host responses. Two host lines were resistant or moderately resistant to the entire collection of isolates, and another was universally susceptible. Five lines showed differential responses to the H. brassicae isolates. A minimum of six pathotypes and five major effect resistance genes are proposed to explain all of the observed interaction phenotypes. The B. oleracea lines from this study can be useful for monitoring pathotype frequencies in H. brassicae populations in the same or other vegetable growing regions, and to assess the potential durability of disease control from different combinations of the predicted downy mildew resistance genes

Surface complexation models: An evaluation of model parameter estimation using FITEQL and oxide mineral titration data

The ability of surface complexation models (SCMs) to fit sets of titration data as a function of changes in model parameters was evaluated using FITEQL and acid-base titration data of [alpha]-FeOOH, [alpha]-Al2O3, and TiO2. Three SCMs were evaluated: the triple-layer model (TLM), the constant capacitance model (CCM), and the diffuse-layer model (DLM). For all models evaluated, increasing the model input value for the total number of surface sites caused a decrease in the best-fit Log K values of the surface protolysis constants. In the case of the CCM, the best-fit surface protolysis constants were relatively insensitive to changes in the value of the capacitance fitting parameter, C1, particularly for values of C1 greater than 1.2 F/m2. Similarly, the best-fit values of TLM surface electrolyte binding constants were less influenced by changes in the value of C1 when C1 was greater than 1.2 F/m2. For a given C1 value, the best-fit TLM values of the electrolyte binding constants were sensitive to changes in [Delta]pKa up to [Delta]pKa values of 3. For [Delta]pKa values above 3, no changes in the best-fit electrolyte binding constants were observed. Effects of the quality and extent of titration data on the best-fit values for surface constants are discussed for each model. A method is suggested for choosing a unique set of parameter values for each of the models.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29417/1/0000493.pd

Age and synchronicity of planktonic foraminiferal bioevents across the Cenomanian–Turonian boundary interval (Late Cretaceous)

The upper Cenomanian \u2013 lower Turonian is a key-stratigraphic interval, as it encompasses the Late Cretaceous supergreenhouse and a major perturbation of the global carbon cycle (i. e., Oceanic Anoxic Event 2) as evidenced by a global positive carbon isotope excursion and by the nearly world-wide deposition of organic-rich marine facies. A turnover in planktonic foraminiferal assemblages and in other marine organisms is documented across this stratigraphic interval, but reconstruction of the timing and identification of the cause and effect relationships between environmental perturbations and organism response require a highly- resolved stratigraphic framework. The appearance and extinction levels of planktonic foraminiferal species generally allow accurate intra- and supra-basinal correlations. However, bioevents cannot be assumed to be globally synchronous, because the stratigraphic and geographic distribution of species is modulated by ecological preferences exhibited by each taxon and controlled by oceanic circulation, often resulting in earlier or delayed events in certain geographic areas (i. e., diachronous datums). The aim of this study is to test the synchronicity of the planktonic foraminiferal bioevents recognized across the C/T boundary and to provide the most reliable sequence of events for correlation of low to mid-latitude localities. For this purpose, we have compiled a highly-resolved biostratigraphic analysis of the European reference section for the C/T boundary at Eastbourne, Gun Gardens (UK), and core S57 (Tarfaya, Morocco), and correlated the sequence of bioevents identified with those recorded in other coeval sections available in the literature, including the GSSP section for the base of the Turonian Stage at Rock Canyon, Pueblo (Colorado), where we calculated reliable estimates of planktonic foraminiferal events that are well-constrained by radioisotopically and astrochronologically dated bentonite layers. Results indicate that the extinctions of Thalmanninella deeckei, Thalmanninella greenhornensis, Rotalipora cushmani and \u201cGlobigerinelloides\u201d bentonensis in the latest Cenomanian are reliable bioevents for correlation. In addition, our analysis highlights other promising lowest occurrences (LOs) that need to be better constrained by bio- and chemostratigraphy, including the LO of Marginotruncana schneegansi falling close to the C/T boundary. By contrast, the appearance of Helvetoglobotruncana helvetica and of some Dicarinella species, the extinction of anaticinellids and the onset of the \u201cHeterohelix\u201d shift are likely diachronous across low to mid-latitude localities. Finally, our study suggests that different species concepts among authors, different sample size and sampling resolution, as well as species paleoecology are important factors that control the stratigraphic position at which bioevents are identified