Plasticity mechanics

For linear elastic materials, Hooke s Law is a constitutive relationship between stress and strain. There have been substantial efforts in identifying similar relationships for plastic solids. In uniaxial tests, the portion of the true stress-true strain curve beyond yielding is often described by 

One simple criterion for yielding under multiaxial stresses is known as the Tresca yield criterion. This approach recognizes that the maximum shear stress is one-half the difference between the maximum and minimum principal stresses. In terms of the uniaxial yield stress r (=2t ) this can be written as 

An alternative approach is the von Mises yield criterion, in which the role of all three principal stresses is introduced, i.e.. 

The two criteria agree for uniaxial and equibiaxial loading. The von Mises criterion is essentially an empirical approach, though it does equate the deviatoric 

The idea of plastic yielding is also applied to granular materials, such as soils and powders. In this case, the shear stress required for deformation depends on the packing density, and the particle shape and surfaee characteristics. The shear resistance is commonly described by the Mohr-Coulomb yield criterion, i.e.. 

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NMDAR. An ionotropic receptor for glutamate. It plays a critical role in synaptic plasticity mechanisms and thus is necessary for several types of learning and memory. 

Neavel, R.C., MCoal Plasticity Mechanism Inferences from Liquefaction Studies , Proceedings of the Coal Agglomeration and Conversion Symposium, Morgantown, W.Va., May 1975 (.published April 1976). 

This P -I type of response curve can also be easily shown to apply to a simple rigid-plastic mechanical system, in the manner shown in Figure 16 (see Refs. 15 and 22). Here, the spring in the system is replaced with a pure Coulomb friction element, with resisting force f, which is independent of displacement once the mass starts to move. All other symbols are defined above. 

Lacour M, Ez-Zaher L, Raymond J. (1991). Plasticity mechanisms in vestibular compensation in the cat are improved by an extract of Ginkgo biloba (EGb 761). Pharmacol Biochem Behav. 140(2) 367-79. 

Sheng M, Kim MJ (2002) Postsynaptic signaling and plasticity mechanisms. Science 298 776-780... 

In classical Pavlovian fear conditioning, an initially neutral cue (conditioned stimulus, CS), through temporal pairing with an aversive unconditioned stimulus (US), acquires the abihty to ehcit a fear response in the absence of the US. This acquired abihty represents a type of associative learning and imphes that plasticity mechanisms underlying fear learning may be similar to those... 

Similar surface abrasion processes can be applied on all thermosetting plastics. Mechanical abrasion methods consist of abrasion by fine sandpaper, carborundum or alumina abrasives, metal wools, or steel shot. The following surface treatment procedure is usually recommended for most thermosetting plastics ... 

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

Plastic materials absorb moisture that may be insignificant or damaging. All plastics, to some degree, are influenced by the amount of moisture or water they contain before processing. Moisture may reduce processing and product performances. With minimal amounts in many plastics, mechanical, physical, electrical, aesthetic, and other properties may be affected or may be of no consequence. For the record let it be known that in the past probably 80% of fabricating problems was due to inadequate drying of all types of plastics. Now it could be down to 40%. 

Rosato, D. V., Plastics Mechanical Engineering Design Handbook, Elsevier, 2003. 

PMMA specimens immersed in methanol. The time-dependent craze behavior was interpreted in terms of a plasticization mechanism incorporating the effect of the fluid Due to its porous nature the craze has a very high area to volume ratio so that penetration of the fluid by only a small distance leads to a complete plasticization of the fibrils and a subsequent drop in the load carrying capacity cr of the fibrils the material effectively behaves as one with a lower craze stress aa/a < 1). 

The local plasticization mechanism is described pictorially in the sketch shown in Fig. 31. The advancing craze, nucleated from free surfaces or other occasional imperfections, cuts into the dispersed population PB-2.76K pools (which at this low molecular weight acts like a relatively low viscosity liquid) and drains the contents of these pools onto the surfaces of the craze. Although the solubility v of the PB-2.76K into PS under standard conditions of room temperature and atmospheric pressure is negligibly small (of the order of 4x 10 ), this should increase greatly in the presence of a negative pressure, according to the expression... 

Comparison between the results obtained in air and in detergent shows that aggressive environment affects the crack resistance only below a certain "critical" value, K ic. This is probably to be attributed to a diffusion-controlled plasticization mechanism, which requires times larger than a certain "critical" time (for crack initiation) or crack speeds lower than a certain "critical" speed (for crack propagation) to be activated. This assumption is confirmed by literature data. 

The role of the phase angle ip and of the finite size of the sample on the initiation of plasticity mechanisms. While it is widely recognized that there is such on effect, it is still very poorly understood and will require further close collaboration between the solid mechanics community and the physics and materials science community. 

Fracture surfaces of the cyclically deformed fatigue specimens were observed at low magnification in order to identify the zones of fatigue initiation and final failure, at higher magnifications in order to identify the regions of microscopic crack formation and growth and microscopic plastic mechanisms. 

Substrate composition and phases Surface hardness and roughness at interface Grain size/microstructure Anisotropy in structure and properties Areal variation and batch-to-batch variation Elastic/plastic mechanical properties Fracture mechanics Flaw population and distribution Strain rate effects... 

Plasticizer Boiling % Plasticizer Mechanical properties % Weight loss... 

Abstract Chalk is the constituent material of numerous oil reservoirs in North Sea. The mechanical behaviour of a saturated chalk has been largely studied. However, different aspects of its behaviour are not yet well understood material characteristics depend on the saturating fluids and chalk response is time-dependent. This paper proposes the PASACHALK numerical model an elasto-plastic constitutive law is presented, which reproduces the different plastic mechanisms of the chalk (pore collapse and shear failure) and the influence of pore fluids. The water sensitivity of this soft rock is explained by the existence of suction effects in chalk. Finally, a simulation of a hypothetical reservoir is proposed to show the response of the elasto-plastic model during depletion phase and water injection phase. 

Experiments performed on chalk samples have shown two plastic mechanisms the pore collapse for high mean stresses (contractant behaviour) and the frictional failure for low mean stresses. The pore collapse could be caused by the breakdown of physico-chemical bonds between the grains inducing some grain-to-grain slip (Monjoie et al. 1990). The frictional failure corresponds to a plastic distortion inducing an increase of porosity. 

The two-evidenced plastic mechanisms are modelled by two yield surfaces combined within a cap model the modified Cam-Clay model is used for pore collapse whereas an internal friction model for friction failure (Collin el al. 2002b). Experimental results show that the chalk strength under extension can be overestimated using an internal friction model, a third yield surface is then adopted to limit traction stresses. 

The basic structure of a typical OLED is shown in Fig. 3.1 [35]. It consists of a transparent conducting anode, typically indium tin oxide (ITO) coated on a glass or plastic mechanical support, the organic layers, and a metal cathode. The thickness of OLEDs (excluding the mechanical support) is typically <0.5 j,m. Under forward bias electrons are injected from the low-workfunction cathode into the electron-transport layer (ETL). Similarly, holes are injected from the high-workfunction ITO into the hole-transport layer (HTL). Due to the applied bias, the electrons and holes drift toward each other, and typically recombine in a recombination zone near, or at, the ETL/HTL interface. A fraction of the recombination events forms radiative excited states. The radiative decay of these states provides the electroluminescence (EL) of the device. 

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