http://www.uniroma2.it/didattica/tbs2/deposito/Manufacture,_characterisation_and_application_of_cellular_metals_and_metal_foams.pdf

Manufacture, characterisation and application of cellular metals and metal foams

Computational modelling of impact damage in brittle materials

The failure of ceramic materials is caused by the extension of small flaws. Therefore, linear-elastic fracture mechanics can be applied to describe the failure behaviour. The main problem in the application of the simple fracture mechanics relation is the existence of a rising crack growth resistance curve, which is caused by crack bridging forces behind the advancing crack tip or by transformations in front of the crack tip. The increasing crack growth resistance leads to problems in the transformation of results from specimens with macrocracks to components with natural cracks.

Fracture Mechanics of Ceramics

The last decade has provided a virtual explosion of data on the molecular biology and function of osteocytes. Far from being the “passive placeholder in bone,” this cell has been found to have numerous functions, such as acting as an orchestrator of bone remodeling through regulation of both osteoclast and osteoblast activity and also functioning as an endocrine cell. The osteocyte is a source of soluble factors not only to target cells on the bone surface but also to target distant organs, such as kidney, muscle, and other tissues. This cell plays a role in both phosphate metabolism and calcium availability and can remodel its perilacunar matrix. Osteocytes compose 90% to 95% of all bone cells in adult bone and are the longest lived bone cell, up to decades within their mineralized environment. As we age, these cells die, leaving behind empty lacunae that frequently micropetrose. In aged bone such as osteonecrotic bone, empty lacunae are associated with reduced remodeling. Inflammatory factors such as tumor necrosis factor and glucocorticoids used to treat inflammatory disease induce osteocyte cell death, but by different mechanisms with potentially different outcomes. Therefore, healthy, viable osteocytes are necessary for proper functionality of bone and other organs. © 2011 American Society for Bone and Mineral Research.

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The Amazing Osteocyte

A theory for describing the evolution of the median/radial crack system in the far field of sharp-indenter contacts is developed. Analysis is based on a model in which the complex elastic/plastic field beneath the indenter is resolved into elastic and residual components. The elastic component, being reversible, assumes a secondary role in the fracture process: although it does enhance downward (median) extension during the loading half-cycle, it suppresses surface (radial) extension to the extent that significant growth continues during unloading. The residual component accordingly provides the primary driving force for the crack configuration in the final stages of evolution, where the crack tends to near-half-penny geometry. On the hypothesis that the origin of the irreversible field lies in the accommodation of an expanding plastic hardness impression by the surrounding elastic matrix, the ensuing fracture mechanics relations for equilibrium crack growth are found to involve the ratio hardness-to-modulus as well as toughness. Observations of crack evolution in soda-lime glass provide a suitable calibration of indentation coefficients in these relations. The calibrated equations are then demonstrated to be capable of predicting the widely variable median and radial growth characteristics observed in other ceramic materials. The theory is shown to have a vital bearing on important practical areas of ceramics evaluation, including toughness and strength.

Elastic/Plastic Indentation Damage in Ceramics: The Median/Radial Crack System

— The growth of ‘small’, half-penny shaped surface fatigue cracks in a precipitation hardened aluminum alloy is compared with the growth of ‘large’cracks in fracture mechanics type specimens. It is found that the small cracks grow much faster than LEFM equivalent large ones, and also experience growth rate perturbations. It is suggested that localized microplasticity in nominally elastic specimens is responsible for the rapid growth of small cracks and that grain size limitations on the microplastic regions cause transient decelerations and sometimes permanent arrest, in crack growth.

The growth of small fatigue cracks in 7075–t6 aluminum

The compressive deformation behavior of open- and closed-cell aluminum foams was assessed under static and dynamic loading conditions. High strain rate experiments were conducted in our laboratory using a split Hopkinson pressure bar system at strain rates ranging from 400 to 2500 s−1. A strain rate effect was demonstrated for Alporas, a closed-cell aluminum. foam. The strain-rate effect was more significant for a higher density (i.e. 15% relative density) Alporas foam and is attributed to the kinetics of gas flow through the cell structure. The experimental results are discussed in reference to recent findings by other investigators on the dynamic behavior of similar open- and closed-cell aluminum foams.

High strain rate compression of closed-cell aluminium foams

Osteocyte lacunae tissue strain in cortical bone

Indentation microfracture in the Palmqvist crack regime: implications for fracture toughness evaluation by the indentation method

https://www2.lbl.gov/ritchie/Library/PDF/1986_Ritchie_MatSciEng_SmallFatigueCracks.pdf

James Lankford

Papers

The growth of small fatigue cracks in 7075–t6 aluminum

High strain rate compression of closed-cell aluminium foams

Osteocyte lacunae tissue strain in cortical bone

Indentation microfracture in the Palmqvist crack regime: implications for fracture toughness evaluation by the indentation method

Small fatigue cracks: A statement of the problem and potential solutions