Defects and plasticity in ultrastrong supercrystalline nanocomposites.

Supercrystalline nanocomposites are nanoarchitected materials with a growing range of applications but unexplored in their structural behavior. They typically consist of organically functionalized inorganic nanoparticles arranged into periodic structures analogous to crystalline lattices, including superlattice imperfections induced by processing or mechanical loading. Although featuring a variety of promising functional properties, their lack of mechanical robustness and unknown deformation mechanisms hamper their implementation into devices. We show that supercrystalline materials react to indentation with the same deformation patterns encountered in single crystals. Supercrystals accommodate plastic deformation in the form of pile-ups, dislocations, and slip bands. These phenomena occur, at least partially, also after cross-linking of the organic ligands, which leads to a multifold strengthening of the nanocomposites. The classic shear theories of crystalline materials are found to describe well the behavior of supercrystalline nanocomposites, which result to feature an elastoplastic behavior, accompanied by compaction.

Hierarchical microcrack model for materials exemplified at enamel.

OBJECTIVE: This article investigates the mechanical properties of a material with hierarchically arranged microcracks. METHODS: Hierarchically structured biomaterials such as enamel exhibit superior mechanical properties as being stiff and damage tolerant at the same time. The common mechanical explanation for this behavior is based on the hierarchically structured arrangement of hard minerals and soft organics and their cooperative deformation mechanisms. In situ mechanical experiments with mm-sized bovine enamel bending bars an scanning electron microscope reveal that enamel is able to withstand mechanical loading even if it contains microcracks on different lengths scales. To clarify this issue an analytical hierarchical microcrack model of non-interacting cracks is presented. RESULTS AND SIGNIFICANCE: The model predicts a decrease of the elastic modulus and the fracture strength with increasing levels of hierarchy. The fracture strain on the other hand may decrease or increase with the number of hierarchical levels, depending on the microcrack density. This simple hierarchical microcrack model is able to explain already published experiments with focused ion beam prepared µm-sized enamel cantilevers on different hierarchical levels. In addition it is shown that microcracking during loading in hierarchical materials may lead to substantial pseudoplastic behavior.

Tungsten band edge absorber/emitter based on a monolayer of ceramic microspheres.

We report on a band edge absorber/emitter design for high-temperature applications based on an unstructured tungsten substrate and a monolayer of ceramic microspheres. The absorber was fabricated as a monolayer of ZrO(2) microparticles on a tungsten layer with a HfO(2) nanocoating. The band edge of the absorption is based on critically coupled microsphere resonances. It can be tuned from visible to near-infrared range by varying the diameter of the microparticles. The absorption properties were found to be stable up to 1000°C.

The Schwickerath adhesion test: A fracture mechanics analysis.

OBJECTIVES: The Schwickerath three point bending adhesion test is the basis of the International Standard ISO 9693:1999 procedure for assessing porcelain bonding to metals [1]. It has also been used to evaluate the adhesion of porcelain to zirconia. The purpose of this paper is a fracture mechanics analysis of this test, which allows determination of the crack-length load-displacement and toughness dependence of cracks extending along or near the interface. METHODS: Linear elastic mechanics is used to develop expressions for the strain energy and compliance of Schwickerath geometry specimens as a function of crack extension along or near the interface. From the derivative of the compliance as a function of crack growth the strain energy release rate (G, N/m) is determined. RESULTS: The energy release rate for interface crack extension of Schwickerath geometry specimens is determined. It is found that a simple relationship between the minima of the force-displacement response and the strain energy release rate G exists. Further development enables the predicted force-displacement response as a function of crack length to be derived for different values of G. Experimental results of porcelain bonded to zirconia with and without notches of various lengths machined along the interface verify the expressions and analysis developed. SIGNIFICANCE: With the fracture mechanics analysis developed in this paper it is possible to determine the quality of adhesion in Schwickerath specimens by the interface toughness in addition to the nominal interface shear bond strength. As the toughness of brittle materials has much less scatter than its strength, the interface toughness characterization of the adhesion should allow for a better distinction between the adhesion quality of bonding.

Damage modeling of small-scale experiments on dental enamel with hierarchical microstructure.

Dental enamel is a highly anisotropic and heterogeneous material, which exhibits an optimal reliability with respect to the various loads occurring over years. In this work, enamel's microstructure of parallel aligned rods of mineral fibers is modeled and mechanical properties are evaluated in terms of strength and toughness with the help of a multiscale modeling method. The established model is validated by comparing it with the stress-strain curves identified by microcantilever beam experiments extracted from these rods. Moreover, in order to gain further insight in the damage-tolerant behavior of enamel, the size of crystallites below which the structure becomes insensitive to flaws is studied by a microstructural finite element model. The assumption regarding the fiber strength is verified by a numerical study leading to accordance of fiber size and flaw tolerance size, and the debonding strength is estimated by optimizing the failure behavior of the microstructure on the hierarchical level above the individual fibers. Based on these well-grounded properties, the material behavior is predicted well by homogenization of a representative unit cell including damage, taking imperfections (like microcracks in the present case) into account.

Evaluating risk of re-injury among 1214 army airborne soldiers using a stratified survival model.

BACKGROUND: Many factors interact to influence an injured individual's risk of sustaining a second injury. However, the quantitative assessment of subsequent injury risk has been limited, primarily due to methodologic constraints. The purpose of this study is to present analytical methodology not previously employed in injury epidemiology to identify risk factors for subsequent injury. METHODS: Data were collected from a retrospective cohort of 1214 U.S. Army Airborne soldiers. Lower extremity and low-back musculoskeletal injuries were identified from outpatient medical records. The Prentice, Williams, and Peterson (PWP) model, stratified by injury event, was used to identify risk factors for initial and subsequent injuries. A Cox proportional hazards model to the time of last injury was used to determine the magnitude of the increased risk associated with having a previous injury history. RESULTS: Risk factors for initial injuries were similar to those seen in other epidemiologic studies of military populations. However, this study found that race/ethnicity, physical fitness, medical provider training, and initial injury types (traumatic versus other) were associated with subsequent injury risk. Additionally, the observed risk of injury was seven times greater among previously injured individuals. CONCLUSIONS: In this population, the risk factors for injury differed by event (initial or subsequent injury), and prior injury history was a risk factor for subsequent injury. The associations between demographic characteristics, the nature of the initial injury, and risk of subsequent injury suggest that changes in the evaluation and medical management of injured individuals may decrease the risk of subsequent injury.

[Blaschko-linear dermatitis in adulthood (Grosshans-Marot disease) with antinuclear antibodies]. / Blaschkolineäre Dermatitis im Erwachsenenalter (Morbus Grosshans-Marot) mit antinukleären Antikörpern.

E. Grosshans and L. Marot first described blaschkitis in 1990 as a linear inflammatory dermatitis following lines of Blaschko in an adult patient. We report another case which occurred in a 38-year-old woman who developed an extensive, linear, erythematosquamous dermatitis involving the face, all limbs and the trunk. The patient's serum tested positive for antinuclear antibodies at a dilution of 1:640. The lesions regressed spontaneously within 4 weeks. Blaschkitis is a distinct entity which corresponds neither to a known inflammatory dermatitis in the lines of Blaschko nor to an hamartoma nor to an X-linked disease. Cutaneous antigenic mosaicism, the expression of which might be induced by a viral infection, could trigger this localized inflammatory T-cell response. This hypothesis relates blaschkitis to other cutaneous autoimmune diseases, as does the presence of antinuclear antibodies. We therefore suggest renaming this type of inflammatory dermatitis Grosshans-Marot disease in honour to the dermatologists who first described the entity.

[Seeping gastrointestinal hemorrhage from telangiectasia in CREST syndrome]. / Gastrointestinale Sickerblutung aus Teleangiektasien bei CREST-Syndrom.

A 55-year-old man with CREST syndrome had marked telangiectasias of the palms, the face and the gastrointestinal tract. The latter caused slow bleeding and anemia. The telangiectasias were definitely associated with the CREST syndrome. Iron substitution was sufficient therapy. If a need for further therapy should arise, endoscopic sclerotherapy could be employed. Gastrointestinal bleeding in the CREST syndrome is rarely described, probably because it is unrecognized or interpreted as hemorrhagic gastritis. The possibility of gastrointestinal telangiectasias and concomitant bleeding should be kept in mind in cases of CREST syndrome.

[Argon plasma coagulation in endoscopic therapy of CREST syndrome associated upper gastrointestinal hemorrhage]. / Die Argonplasmakoagulation in der endoskopischen Therapie einer CREST-Syndrom-assoziierten oberen gastrointestinalen Blutung.

We report on the case of a 55-year-old patient suffering from progressive systemic sclerosis (PSS). The patient was sent to our department when clinical symptoms of an acute upper gastrointestinal hemorrhage occurred. Upper endoscopy showed a watermelon stomach and fresh blood in the stomach. The presence of teleangiectasias in the antrum could be proved histologically. Since the teleangiectasias found in the antrum were the only possible source of the hemorrhage three sessions of endoscopic argon plasma coagulation were performed. Macroscopically, a nearly complete disappearance of teleangiectasias could be achieved. After a follow-up of six months, there have been no clinical signs of another hemorrhage episode. This case shows that the existence of gastrointestinal teleangiectasias should be considered when chronic anemia or acute gastrointestinal hemorrhage occur in patients with PSS. Further it is demonstrated that even extended gastrointestinal teleangiectasias can be successfully treated by endoscopically performed argon plasma coagulation.

Performance of the polypropylene fiber tailstring on the Copper 7 intrauterine device.

New and used polypropylene tailstrings from the Copper 7 (Cu-7) intrauterine device were examined by a combination of analytical techniques. Optical microscopy, scanning acoustic and electron microscopy, x-ray diffraction, energy dispersive x-ray analysis, and chemical etching were employed to elucidate both the surface and interior morphology of new Cu-7 tailstrings. Tailstrings removed from women following varying periods of use were investigated with optical microscopy, scanning and transmission electron microscopy. In addition, a subset of the used tailstrings were cultured to identify the types of microorganisms associated with them. Our findings show that unused Cu-7 tailstrings are in various stages of degradation owing to a combination of factors which include the high-draw ratio employed during manufacturing, the method of packaging, and the use of a particulate colourant. Furthermore, it is evident that used Cu-7 tailstrings undergo major deterioration while in situ because of the unfavorable interactions between the highly drawn polypropylene and the physiological environment. These results indicate that the polypropylene tailstrings as manufactured for use with the Cu-7 IUD fail to meet accepted design criteria for biomedical implants.