Material-generated stress

Chemical Reaction. Chemical reaction of particulate material generates stresses within the particle that can lead to fracture. In the case of gas-solid reactions, the particle degradation is also desired because it accelerates the reaction by extending the reactive surface. A relevant commercial example is the particle degradation of solid fuels in combustion processes. This latter topic has been studied by Massimilla and coworkers extensively. The reader is referred for further details to a review given by Chirone et al. (1991). 

Here ty is the material-generated stress, G is the shear modulus and /y is the general deformation, which we still need to define. Applying equation (2-14)... 

In a torsion test, a capstan-shaped specimen is twisted in a viscometer, and the generated stress and strain are measured upto the point of material fracture. Torsion produces what is called a pure stress, a condition that maintains sample shape and volume during the test. The material can fail in shear, tension, compression or in a combination mode, and the test does not dictate the mode of failure (Hamann, 1983). The main disadvantages of torsion are (1) specimen shaping and preparation are usually complex and tedious, and (2) the technique is not applicable to soft or sticky... 

The most straightforward type of tactile sensor. used consists of a contact switch which provides binary high-low information. Other types of mechanism such as strain gauges and carbon fibre materials change their electric resistance under stress, whereas piezoelectric materials generate an electric charge when subjected to mechanical strain. 

In the shape-memory transformation described, only the shape of the parent phase is remembered . It is called the one-way shape-memory effect. It is also possible to produce alloys that display two-way shape-memory effects. In these materials, both the shape of the parent phase and the martensitic phase is remembered . This reversible effect is caused by the fact that the nucleation of the martensite is very sensitive to the stress field. Introduction of lattice defects such as precipitates can restrict the number of variants that form and the positions where they nucleate. Such materials generate the martensitic shape on cooling below the temperature Mf. Cycling between higher and lower temperatures causes the alloy to switch alternately between the two shapes. There is considerable research interest in developing and exploiting two-way shape-memory effect alloys at present. 

The variation in coefficients of thermal expansion between polymers gives difficulties where materials are composites or where several different material types are in close contact On cooling, the differential shrinkages may generate stress between the materials, causing them to fail or to come apart from each other. 

The American Society of Testing Material s test, ASTM C-88, for determining soundness of aggregates is based on the principle of generating stresses by artifically depositing sodium or magnesium sulfate in the pore space. In this procedure, the specimen is impregnated with a concentrated solution of the salt at 20°C. The specimen is then dried at 110 C and cooled to 20°C, after which the... 

One active area of research involves the generation of polymers that can form chemical crosslinks once delivered onsite. For applications where creep (slow, unrecoverable deformation of the material under stress) is a concern, a frequency sweep can be used to get an estimation of relative changes (increases in G at a given frequency, for instance) over time with regards to viscous behavior... 

A side effect of the use of a second phase with markedly different linear coefficients of thermal expansion (CTEs) is the residual stresses that are generated during cooling from the processing temperature. Residual stresses in composite materials, and how they impact mechanical properties, have become an increasingly important subject in materials research in recent years." As researchers continue to push materials closer to their property limits, it has become mote important to understand and control failure. Residual stresses are important because, vriien combined with applied stresses, they can lead to premature structural failure. Thus, the ability to limit residual stresses could improve the thermo-structural capabilities of a material. These stresses in multi-phase materials such as ZrB2-SiC, arise from the difference in the CTE of the phases (for ZrB2 CTE = 6.7 x lO" K for SiC CTE = 4.7x 10- K- ). 

Figure 12.17 shows a master stress relaxation curve, presented as ((7 — (T.)/ (T — (7j) vs. log time. The value of a. is determined for t— oo by an approximation to the experimental stress relaxation master curve. This type of plot has been recommended by one of us [52-61]. It brings out the common features of stress relaxation curves for metals, polymers and other materials. This is true not only for experimental but also for computer generated stress relaxation curves [60-62]. For a discussion of these common features see also [62]. The type of plot recommended first in [52] is also being used successfully for instance by Wortmann and coworkers [63-66]. They investigated a variety of materials including wool fibers and also very stiff aramid fibers such as Kevlar. 

The modes of operation of a DMA are varied. Using a multifrequency mode, the viscoelastic properties of the sample are studied as a function of frequency, with the oscillation amplitude held constant. These tests can be run at single or multiple frequencies, in time sweep, temperature ramp, or temperature step/hold experiments. In multistress/strain mode, frequency and temperature are held constant and the viscoelastic properties are studied as the stress or strain is varied. This mode is used to identify the LVR of the material. With creep relaxation, the stress is held constant and deformation is monitored as a function of time. In stress relaxation, the strain is held constant and the stress is monitored versus time. In the controlled force/strain rate mode, the tanperature is held constant, while stress or strain is ramped at a constant rate. This mode is used to generate stress/ strain plots to obtain Young s modulus. In isostrain mode, strain is held constant during a tanpera-ture ramp to assess shrinkage force in films and fibers. 

The Helfrich Hamiltonian, Eq. (1), does not include a surface tension contribution. Free membrane patches can relax and adjust their area such that they are stress-free. In many situations, however, membranes do experience mechanical stress. For example, an osmotic pressure difference between the inside and the outside of a lipid vesicle generates stress in the vesicle membrane. Stress also occurs in supported bilayer systems, or in model membranes patched to a frame. In contrast to other quantities discussed earlier (bending stiffness etc.), and also in contrast to the surface tension of demixed fluid phases, membrane stress is not a material parameter. Rather, it is akin to a (mechanical or thermodynamic) control parameter, which can be imposed through boundary conditions. 

Today s state of the art substrate and anode material for anode-supported solid oxide fiiel cells is a Ni-YSZ cermet. During operation or shut down accidental air break-in or controlled air feed on the anode side can result in re-oxidation of the metallic nickel. The volume expansion caused by Ni oxidation generates stresses within the substrate, the anode and the electrolyte. Those stresses ean exceed the stability of the components, potentially promoting crack growth. This may lead to degradation of the SOFC or complete failure. 

Acoustic emission is the sound waves produced when a material undergoes stress (internal change), as a result of an external force. Acoustic emission is a phenomenon occurring in, for instance, mechanical loading generating sources of elastic waves. 

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