A unified statistical model for S-N fatigue curves: probabilistic definition

In recent years, experimental tests exploring the gigacycle fatigue properties of materials suggest the introduction of modifications in well known statistical fatigue life models. Usual fatigue life models, characterized by a single failure mechanism and by the presence of the fatigue limit, have been integrated by models that can take into account the occurrence of two failure mechanisms and do not consider the presence of the fatigue limit. The general case, in which more than two failure mechanisms coexist with the fatigue limit, has not been proposed yet. The paper presents a unified statistical model which can take into account any number of failure mechanisms and the possible presence of the fatigue limit. The case of S-N curves with different fatigue life distributions coexisting for the entire stress range covered by fatigue tests is also considered. The adaptability of the statistical model to the S-N curves proposed in the open literature is demonstrated by qualitative numerical example

Redesigning axial-axial (biaxial) cruciform specimens for very high cycle fatigue ultrasonic testing machines

The necessity to increase performances in terms of lifetime and security in mechanical components or structures is the motivation for intense research in fatigue. Applications range from aeronautics to medical devices. With the development of new materials, there is no longer a fatigue limit in the classical sense, where it was accepted that the fatigue limit is the stress level such that there is no fracture up to 1E7 cycles. The recent development of ultrasonic testing machines where frequencies can go as high as 20 kHz or over enabled tests to be extended to ranges larger than 1E9 in just a few days. This area of studies is now known as Very High Cycle Fatigue (VHCF). On the other hand, most of the existing test equipment in the market for both classical and VHCF are uniaxial test machines. However, critical components used in Engineering applications are usually subjected to complex multi-axial loading conditions. In this paper, it is presented the methodology to redesigning existing cruciform test specimens that can be used to create an in-plane biaxial state of stress when used in ‘uniaxial’ VHCF ultrasonic testing machines (in this case, the term ‘uniaxial’ is used not because of the state of stress created at the centre of the specimen, but because of the direction at which the load is applied). The methodology is explained in such a way that it can be expanded to other existing designs, namely cruciform designs, that are not yet used in VHCF. Also, although the approach is presented in simple and logical terms, it may not be that obvious for those who have a more focused approach on fatigue rather than on modal analysis. It is expected that by contributing to bridging the gap between the sciences of modal analysis and fatigue, this research will help and encourage others exploiting new capabilities in VHCF

Nichtlineare Schalentheorie in konvektiver Beschreibung mit anisotroper Plastizität

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Potenziale und Wirkungen künstlich angelegter Baummikrohabitate

Baummikrohabitate (BMH) sind biodiversitätsfördernde Strukturelemente an Einzelbäumen und haben eine wichtige Funktion für den Artenschutz. Zu den BMH zählen bspw. Specht- und Faulhöhlen, Kronentotholz und epiphytischer Bewuchs. Durch technisierte und ertragsorientierte Waldbewirtschaftungsformen liegen Quantität und Qualität von BMH in Wäldern in der Regel deutlich unter dem natürlichen Potenzial. Die natürliche Entstehung und Entwicklung von BMH dauert oft Jahrzehnte und unterliegt zufälligen Einwirkungen. Um dennoch BMH in größerer Anzahl und unabhängig von natürlichen Wirkungsgefügen und Zeiträumen bereitzustellen, können BMH künstlich erzeugt oder ihre Entwicklung kann durch technische Maßnahmen beschleunigt werden. Die häufigsten und bekanntesten künstlichen BMH sind Nistkästen für diverse Vogelarten sowie Fledermausquartiere. Weit weniger gängig sind die künstliche Schaffung von Kronentotholz und Stammhöhlungen oder das Bereitstellen komplexer Habitatimitate für totholznutzende Gliederfüßer (Arthropoden). Seltenere Zielarten künstlicher BMH sind auch andere Säugetiere wie -Bilche (Gliridae), verschiedene Pilzarten und Flechten. Inwiefern künstlich geschaffene BMH ihre natürlichen Vorbilder erfolgreich imitieren können, ist bislang nicht abschließend bekannt, auch wenn die Zielarten der für diesen Beitrag untersuchten Literatur überproportional häufig gefährdete bzw. seltene Arten umfassen. Der teilweise geringe Kenntnisstand zu den künstlichen BMH ist ein beschränkender Faktor bei deren Anlage und Standortwahl. Insbesondere fehlen präzise Anleitungen für deren Schaffung oder Initiierung. Die vorliegende Literaturstudie trägt die vorhandenen wissenschaftlichen Kenntnisse zur Anlage und Wirksamkeit künstlicher und künstlich initiierter BMH im Wald zusammen

Evaluación de resultados de fatiga con alto y ultra alto número de ciclos mediante un modelo de regresión de Weibull

El modelo de fatiga de regresión de Weibull, propuesto por Castillo-Canteli, representa una alternativa posible y adecuada para la evaluación y predicción de vidas de fatiga ultra altas (VHCF). Este modelo proporciona una definición probabilística del campo S-N para los posibles mecanismos de rotura determinantes, basada en distribuciones de Weibull de mínimos, así como la existencia de un límite de fatiga asintótico y la propiedad de reducir todo el campo SN a una única función de distribución mediante la variable normalizada V=(log N-B)(log Δσ-C). De este modo, el doble mecanismo de rotura, interno y superficial, que caracteriza las vidas altas (HCF) y ultra altas (VHCF) en fatiga puede ser satisfactoriamente tratado como distribuciones independientes en este particular y complejo caso de muestras con resultados concurrentes, conocido como problema de datos confundidos. Una vez estimados los parámetros de ambas funciones de distribución se procede a la combinación y reconversión de ambas como campo S-N conjunto. El modelo permite establecer una estrategia para optimizar la programación de los ensayos. Por último se presentan dos ejemplos de aplicación, uno de un programa externo experimental y otro de resultados simulados, ambos con ultra alto número de ciclos y dos posible mecanismos de rotura.The fatigue Weibull regression model proposed by Castillo-Canteli represents a possible and adequate alternative for the assessment and prediction of very high cycle fatigue (VHCF) lifetimes. This model provides a probabilistic definition of the S-N field for the two determining failure mechanisms based Weibull distributions for minima, as well as the existence of an asymptotic fatigue limit and the capability to reduce the S-N field to a single cumulative distribution function by considering the normalized variable V=(log N-B) (log Δσ). In this way, both dual fracture mechanisms, i.e. the internal and the surface ones, characterizing the HCF and VHCF data can be adequately interpreted and handled as independent distributions in such a particular and complex case of concurrent populations, known as a confounded data problem. Once the model parameters of both normalized cumulative distribution functions are independently estimated for both failure mechanisms and subsequently combined and reconverted to a joint S-N field whereby. The model allows a test strategy to be established for optimizing the the fatigue program planning. Finally, two examples of application are presented, the first related to an external experimental program, and the second to simulated data both for VCCF with twofold failure mechanisms

Biological Flora of the British Isles: Sorbus torminalis

1.This account presents information on all aspects of the biology of Sorbus torminalis (L.) Crantz (Wild Service-tree) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, and conservation.2.Sorbus torminalis is an uncommon, mostly small tree (but can reach 33 m) native to lowland England and Wales, and temperate and Mediterranean regions of mainland Europe. It is the most shade-tolerant member of the genus in the British Isles and as a result it is more closely associated with woodland than any other British species. Like other British Sorbus species, however, it grows best where competition for space and sunlight is limited. Seedlings are shade tolerant but adults are only moderately so. This, combined with its low competitive ability, restricts the best growth to open areas. In shade, saplings and young adults form a sapling bank, showing reproduction and extensive growth only when released. Sorbus torminalis tolerates a wide range of soil reaction (pH 3.5-8.0) but grows best on calcareous clays and thin soils over limestone.3.Sorbus torminalis is a sexual, diploid, non-apomictic species that has hybridised with a number of other Sorbus species to form microspecies. The hermaphrodite flowers are primarily insect pollinated. Seed production is reliable only in warm years, especially at the edge of its range, although even then seed viability is low. The fruits are primarily dispersed by carnivorous mammals. Seeds display embryo dormancy but most will germinate the first spring after falling.4.This tree is very tolerant of short droughts but only moderately tolerant of frost, hence its southerly and lowland distribution. It faces no particular individual threats although the small size of most populations makes it susceptible to habitat loss and fragmentation, particularly through the loss of open coppiced areas. As a consequence it appears to be declining throughout Britain and Europe despite its wide range of historical uses and the high value of its timber. The extent to which these losses will be offset by increases due to climate change is unknown.This article is protected by copyright. All rights reserved

Ein Beitrag zur Kenntnis der Erregungsuebertragung und ihrer Traegerstrukturen bei Fredericella Sultana (Blumenbach) [Bryozoa]

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