Material State Changes Research Articles

Incorporation of Material Variability in the Johnson Cook Model

The Johnson Cook (JC) material model is often used for modeling hypervelocity impacts (HVI) because it is capable of capturing high rate deformation and temperature effects. Within the JC model, material damage leading to fracture is aggregated using a path dependent damage parameter. Contributions to the damage parameter are calculated at each cycle as the quotient of incremental effective plastic strain over the effective failure strain. The effective failure strain is a function of the stress state, strain rate, and temperature which changes locally throughout the simulation.Solid bodies often demonstrate variability in resilience resulting from material inclusions and defects. Therefore, in addition to the deterministic, state-based variability in the effective failure strain inherent to the JC model, efforts are often made to capture the effect of general material non-uniformity. Although other approaches may be available, the Weibull probability distribution is often employed within failure analyses to allow simulations to diverge from a completely uniform solution.In this paper, we investigate several methods of augmenting the JC damage model with Weibull variability. The implementation of each method provides a means for measured material uncertainty to enter the calculation. We focus on a standard three-parameter Weibull probability density function (pdf) although the methods proposed can be used with any probability distribution. Characteristics of the pdf are preserved within a solid body at its initial state but differ in the effect the JC effective failure strain has on these characteristics as the material state changes. The Weibull pdf under various loading conditions is discussed, and simulation results incorporating these methods are compared with those employing only the JC damage model or Weibull variability.

SLM加工中材料状态变化对温度场影响的研究

SLM加工中材料状态变化对温度场影响的研究

<em>Octopus tetricus</em> (Mollusca: Cephalopoda) as an ecosystem engineer

Summary: The Sydney octopus (Octopus tetricus) occurs in unusual numbers on a shell bed of its prey remains that have accumulated as an extended midden where additional octopuses excavate dens. Here, O tetricus are ecosystem engineers, organisms that modulate availability of resources to other species and to their own species by causing physical state changes in materials. A community of invertebrate grazers and scavengers has developed on the shell bed. Fishes are attracted to the shell bed in numbers significantly greater than in nearby habitats. Large predators, including wobbegong sharks, were attracted to and fed on concentrations of fish, inhibiting the activities of the original engineers, the octopuses. Positive feedbacks included the accumulation of shell debris, increasing shelter availability for additional octopuses and aggregating fish. Negative feedbacks included reductions of nearby prey size and availability, aggression among octopuses, and predator limitation to octopus activity that would otherwise maintain the shell bed.

Effects of Soil Aging on Mechanical and Hydraulic Properties of a Silty Soil

The aging phenomenon, which produces changes in material state over time, is associated with significant modification of mechanical and physical soil properties. This change should be accounted for during geotechnical design. Although soils sometimes improve with aging, the opposite effect is occasionally observed. This paper describes a study performed to investigate the effect of aging on the mechanical behavior and the permeability of a silty soil. Undrained unconsolidated triaxial shear tests and triaxial permeability tests were performed on disturbed and compacted samples. Upon conclusion of these tests, the samples were sealed from air and moisture. The results show an important increase in both the undrained shear strength and the deformation modulus caused by silt rigidification during the aging process. These changes cause an over estimation of laboratory measured shear strength. For instance, the increase in the deformation modulus and undrained cohesion can approach 100 % for an approximate 328 day storage period. Sample permeability was found to decrease with aging. This reduction can be ascribed to several causes including micro-organisms growth, secondary sample consolidation and progressive filling caused by the migration of very fine particles. These phenomena might have negated the expected increase in permeability with aging time reported in the literature.

Habitat-Mediated Variation in the Importance of Ecosystem Engineers for Secondary Cavity Nesters in a Nest Web

Through physical state changes in biotic or abiotic materials, ecosystem engineers modulate resource availability to other organisms and are major drivers of evolutionary and ecological dynamics. Understanding whether and how ecosystem engineers are interchangeable for resource users in different habitats is a largely neglected topic in ecosystem engineering research that can improve our understanding of the structure of communities. We addressed this issue in a cavity-nest web (1999–2011). In aspen groves, the presence of mountain bluebird (Sialia currucoides) and tree swallow (Tachycineta bicolour) nests was positively related to the density of cavities supplied by northern flickers (Colaptes auratus), which provided the most abundant cavities (1.61 cavities/ha). Flickers in aspen groves provided numerous nesting cavities to bluebirds (66%) and swallows (46%), despite previous research showing that flicker cavities are avoided by swallows. In continuous mixed forests, however, the presence of nesting swallows was mainly related to cavity density of red-naped sapsuckers (Sphyrapicus nuchalis), which provided the most abundant cavities (0.52 cavities/ha), and to cavity density of hairy woodpeckers (Picoides villosus), which provided few (0.14 cavities/ha) but high-quality cavities. Overall, sapsuckers and hairy woodpeckers provided 86% of nesting cavities to swallows in continuous forests. In contrast, the presence of nesting bluebirds in continuous forests was associated with the density of cavities supplied by all the ecosystem engineers. These results suggest that (i) habitat type may mediate the associations between ecosystem engineers and resource users, and (ii) different ecosystem engineers may be interchangeable for resource users depending on the quantity and quality of resources that each engineer supplies in each habitat type. We, therefore, urge the incorporation of the variation in the quantity and quality of resources provided by ecosystem engineers across habitats into models that assess community dynamics to improve our understanding of the importance of ecosystem engineers in shaping ecological communities.

Impact of In-Cell Water Management on the Endurance of Polymer Electrolyte Membrane Fuel Cells

Water management is critical for achieving optimal performance and endurance of polymer-electrolyte membrane fuel cells. The authors conducted endurance experiments of polymer-electrolyte membrane fuel cells using two water management schemes: a solid plate (SP) approach and a water-transport plate (WTP) approach. In addition to constant-current hold tests at 1 A/cm2 to investigate the voltage decay, diagnostic tests, i.e., polarization curves, hydrogen crossover, cathode cyclic voltammetry, fluoride emission, and membrane tensile stress-strain, are also conducted to investigate the cell performance and membrane material state changes. At cell temperature 85°C, anode RH 96% and cathode RH 75%, and no backpressure, constant current test results indicate significant differences of the degradation behavior between SP cells and WTP cells, suggesting different degradation rate controlling mechanisms. The representative WTP cell remains operational over 1500 hours of testing without failure. The representative SP cell fails catastrophically after 872 hours of testing. The WTP cell shows a much lower performance loss rate in terms of voltage decay and electrochemically-active surface area (ECSA) reduction at the cathode catalyst layers and less membrane mechanical decay than that of the SP cell. The improved performance and endurance is attributed to the better water management capability of the WTP cell.

Nano-mechanics Foundations and Experimental Methodologies for Multiphysics Prognosis of Functional Behaviour in Heterogeneous Functional Materials (HeteroFoaM)

The present paper will discuss the nano-mechanics foundations and experimental methodologies that provide a foundation for the prognosis of the functional behavior of nano-structured heterogeneous materials for energy systems. The principle objective of this work at the fundamental level is the creation of conceptual foundations for prognosis of heterogeneous functional materials. Our approach is to focus on the mechanics associated with the nano-structural configurations and interfaces (function and geometry) of the active phases. The paper will discuss conceptual foundations, experimental methods, and computational and analysis methods that associate global functional response of interest to energy conversion and storage with the multiphysics of nano-structured heterogeneous materials. New experimental room-temperature, non-invasive interrogation methods for establishing internal integrity and for following material state changes in solid oxide fuel cells as a function of operation history will also be introduced and discussed as an example of the application of these concepts.

Material state change relationships to fracture path development for large-strain fatigue of composite materials

The long-term performance of engineering structures is typically discussed in terms of such concepts as structural integrity, durability, damage tolerance, fracture toughness, etc. These familiar concepts are usually addressed by considering balance equations, crack growth relationships, constitutive equations with constant material properties, and constant or cyclically applied load conditions. The loading histories are represented by changing stress (or strain) states only. For many situations, especially for those associated with high-performance engineering structures, the local state of the material may also change during service, so that the properties used in the equations are functions of time and history of applied conditions. For example, the local values of stiffness, strength, and conductivity are altered by material degradation to create "property fields" that replace the global constants, and introduce time and history into the governing equations. The present paper will examine a small set of such problems, which involve the accumulation of distributed damage and the development of an eventual fracture path leading to failure. Specifically, the paper discusses this problem in the context of material state changes measured by impedance variations as a method of following the details of fracture path development. An analysis and interpretations of observations will be presented, and limitations and opportunities associated with this general concept will be discussed.

Spatial and temporal hotspots of termite‐driven decomposition in the Serengeti

Ecosystem engineers are organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Termites (Insecta, Isoptera) are among the most important ecosystem engineers in tropical ecosystems. We used a field experiment in the tall grasslands of Serengeti National Park, Tanzania, to investigate 1) the consumption by termites of grass litter and dung baits along the landscape gradient of catena position, and 2) seasonal variation in litter and dung removal. Our maps of termitaria and patterns of bait removal revealed clear spatial and temporal hotspots of termite activity. In the dry season termites removed more baits at the top‐catena positions than at the bottom positions, but there was no effect of catena position in the wet season. Spatial hotspots of termite activity overlapped with those of both mammalian herbivores and predators. Within the framework of ecosystem engineering, this study suggests that intraspecific aspects of spatial heterogeneity and temporal variability deserve much greater consideration.

Material state changes as a basis for prognosis in aeronautical structures

AbstractThe long-term performance of aeronautical structures is typically discussed in terms of concepts such as structural integrity, durability, damage tolerance, fracture toughness, etc. These familiar concepts are usually addressed by considering balance equations, crack growth relationships, and constitutive equations with constant material properties, and constant or cyclically applied load conditions. Loading histories are represented by changing stress (or strain) states, only. But for many situations, especially associated with high performance aircraft, the local state of the material may also change during service, so that the properties used in those equations are functions of time and history of applied conditions. For example, local values of stiffness, strength, and conductivity are altered by material degradation to create ‘property fields’ that replace the global constants, and introduce time and history into the governing equations. The present paper will examine a small set of such problems and offer a construct for using related solutions to estimate future performance based on history of use and current material state, a concept typically called prognosis.

Material State Changes in Heterogeneous Material Systems: Multiphysics Foundations for Design, Durability and Prognosis - Invited

not Available.

The Implications of Niche Construction and Ecosystem Engineering for Conservation Biology

Abstract Although strategies to conserve biodiversity (e.g., the establishment of reserves and the management of flagship, umbrella, indicator, and keystone species) are valuable, they entail practical and conceptual difficulties. A focus on niche construction and ecosystem engineering, however, could provide new insights and methods for conservation biology. Many organisms modulate the availability of resources to other species by causing state changes in biotic or abiotic materials (ecosystem engineering), in the process frequently changing the selection to which the ecosystem engineers and other organisms are exposed (niche construction). We describe growing evidence that organisms have significant nontrophic impacts on ecosystem structure, function, and biodiversity, and outline established means of identifying key species involved in niche construction. On the basis of this engineering perspective, we propose a number of measures that could be employed to enhance conservation efforts.

Residual strain development in an AS4/PPS thermoplastic composite measured using fibre Bragg grating sensors

Abstract This paper demonstrates the use of fibre Bragg grating (FBG) sensors for the measurement of residual strain development during the consolidation of a thermoplastic composite. During the processing of the carbon fibre-reinforced polyphenylene sulphide (AS4/PPS) laminate, FBG sensors respond to changes in material state, for example the glass-rubber transition and solid–liquid transition. The sensors also permit the observation of wavelength shifts and spectral form changes induced by the contraction of the composite during cooling. The experimental data are compared to a generalized plane strain, thermoelastic numerical model with temperature dependent matrix dominated properties. The model provides solutions for two limiting cases: one where the specimen contracts freely, and one where the specimen is restricted by perfect contact with the mould. Strain values calculated in each case are inserted into optomechanical equations, which convert the strain state in the FBG to a corresponding change in wavelength. In this way, the modelled cases are compared to FBG wavelength shifts during consolidation.

Strength and dynamic fatigue of silicon nitride at intermediate temperatures

The flexure strength distributions and dynamic fatigue of GS44, NT551, and NT154 silicon nitrides were determined at intermediate temperatures (≤850°C) in ambient air. GS44 and NT551 were fabricated by gas pressure sintering and had a larger volume fraction of secondary phase than the hot-isostatically pressed NT154. The inert characteristic strength for the GS44 and NT551 decreased with temperature while that for the NT154 was statistically independent of temperature. The slow crack growth exponent, N, for the GS44 and NT551 exhibited a 70–80% decrease between 700 and 850°C while the strength of NT154 was comparatively independent of stressing rate at 850°C. Associated with those changes in mechanical performances, the coefficient of thermal expansion (CTE) for the GS44 and NT551 sharply increased between 700 and 850°C while that for the NT154 did not. Although the observed change in CTE (and the temperature range where it occurred) did not have any obvious correlation with a change in inert strength at these intermediate temperatures, its presence was an indicator of a threshold temperature where dynamic fatigue resistance was compromised if the temperature exceeded it. The change in material state (and the associated temperature region for which it occurs) is linked to the equilibrium state of the secondary phase in the silicon nitride.

POSITIVE AND NEGATIVE EFFECTS OF ORGANISMS AS PHYSICAL ECOSYSTEM ENGINEERS

Physical ecosystem engineers are organisms that directly or indirectly control the availability of resources to other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance, or creation of habitats. Ecological effects of engineers on many other species occur in virtually all ecosystems because the physical state changes directly create nonfood resources such as living space, directly control abiotic resources, and indirectly modulate abiotic forces that, in turn, affect resource use by other organisms. Trophic interactions and resource competition do not constitute engineering. Engineering can have significant or trivial effects on other species, may involve the physical structure of an organism (like a tree) or structures made by an organism (like a beaver dam), and can, but does not invariably, have feedback effects on the engineer. We argue that engineering has both negative and positive effects on species richness and abundances at small scales, but the net effects are probably positive at larger scales encompassing engineered and nonengineered environments in ecological and evolutionary space and time. Models of the population dynamics of engineers suggest that the engineer/habitat equilibrium is often, but not always, locally stable and may show long-term cycles, with potential ramifications for community and ecosystem stability. As yet, data adequate to parameterize such a model do not exist for any engineer species. Because engineers control flows of energy and materials but do not have to participate in these flows, energy, mass, and stoichiometry do not appear to be useful in predicting which engineers will have big effects. Empirical observations suggest some potential generalizations about which species will be important engineers in which ecosystems. We point out some of the obvious, and not so obvious, ways in which engineering and trophic relations interact, and we call for greater research on physical ecosystem engineers, their impacts, and their interface with trophic relations.

Analysis of a hysteresis motor with overexcitation

The performance of a hysteresis motor can be improved greatly if it is overexcited for a short period when running at synchronous speed. The change in the magnetic state of the rotor hysteresis material when the stator voltage is raised and then reduced to the original value, is described in detail. Based on this, a method for the calculation of the motor performance after overexcitation is proposed, and the effect of overexcitation on the motor performance is clarified by using this method. Good agreement is found between the calculated and the measured results.