High mechanical stresses

Bridgman, The Effect of High Mechanical Stress On Certain Solid Explosives , JChem-Phys 15, 311-13 (1947) CA 41, 49211... 

Materials used in body implants must meet several essential requirements such as tissue compatibility, enzymatic and hydrolytic stability. They must also be chemically resistant and have good mechanical properties. They must not be toxic, or the surrounding tissue will die. They must be resistant to the body fluids which usually have a high percentage of chloride ions. They must be biologically active if an interfacial bond is to be achieved. In some cases, they must be able to withstand continued high mechanical stresses for many years. 

It has to be underlined that, in comparison to LS containing nucleic acid molecules inside the particle, the production of CLS may be performed obviously without considering the stability problems of nucleic acid molecules. In this view, some preparation procedures are not considered, such as the microemulsion technique [55] that represents a favorable method when working with substances unstable because of the high mechanical stress produced by high-pressure homogenization. 

P.W. Bridgman, "The Effect of High Mechanical Stress on Certain Solid Explosives", JChemPhys 15, 311—13(1947)... 

The structure of a typical composite power transducer for ultrasonic cleaning is illustrated in Fig. 6.32. The PZT toroids are kept in compression (stress 25 MPa) by the bolt to reduce the risk of their fracturing under the high drive fields. The PZT will be of the hard variety ( acceptor -doped see Section 6.3.2) so as to minimize the risk of depolarization under the high mechanical stresses experienced. The associated high Qm value and low electrical tan 3 ensure that the losses are kept within acceptable limits. The structure has the added advantage that any heat developed in the ceramic can be dissipated through the massive metal end pieces. 

The rapid failure as seen in water and acid solution in regions I and II are due to chemical ESC, as discussed before. A plausible explanation of chemical ESC is chain scission caused by the combined action of the high mechanical stress at the (notch) crack tip and chemical attack of the chains due to local hydrolysis. 

Deageing effects can also be brought about by high mechanical stresses. In the extreme cases of yielding and necking, the deageing is complete and the material behaves as if it were actually heated to above Tg. 

In such conditions the brittle ceramic fibres, such as Nicalon SiC and A1203 fibres, remain undamaged during the CVI process. However, conventional techniques for the fabrication of ceramic-matrix composites such as hot pressing take place at extremely high temperatures (2000°C) and under high mechanical stresses (30 MPa), which usually severely damage the fibres. 

For very low viscosity products, introduce sufficient energy for dispersive or distributive mixing by adjusting the feed sequence, such that only a small portion of the solvent or dilution oil is added in the upstream part of the twin-screw extruder and therefore high mechanical stress transfer can be achieved. The remainder of the diluent is introduced in the latter sections of the machine. 

If particulate matter has to be dissolved in a liquid or if a chemical reaction catalyzed by a solid is involved, the particles must be suspended from the vessel bottom, so that the total surface can participate in the process. In continuous processes a stochastically homogeneous distribution of the solid in the bulk of the liquid is required, so that the solid particles can be transported with the liquid from stage to stage (for example in a cascade crystallization process). In this intensive suspension process, the solid is, as a rule, subjected to high mechanical stress, which can result in its attrition. 

If an axially positioned stirrer is operated in a vessel without inserts, the liquid is set in rotation and a vortex is produced. In the case of rapidly rotating stirrers and low viscosity liquids, the vortex can reach the stiner head with the result that the stirrer entrains the gas in the liquid (see section 1.4.5.2). This is generally undesirable because it results in an extraordinarily high mechanical stress on the stirrer shaff, bearings and seal, due to the absence of the liquid bearing . This often leads to the destruction of the stirrer. Even when the vortex formation causes no gas entrainment, rotation of the liquid is always undesirable if a two-phase system with different densities is concerned, since the centrifugal force counteracts the stirring process. 

Mechanical degradation describes the breakdown of molecules in the high flow rate region close to a well as a result of high mechanical stresses on the macromolecules. This short-term effect is important only in the reservoir near the wellbore (and also in some of the polymer handling equipment, in chokes, and so on). 

Every production process that applies high mechanical stress to the sensor could result in a change of zero-offset Hence the calibration of the sensor should be done as late as possible to have a chance to compensate those influences. The sensor shown in Fig. 7.4.7 is calibrated when completely mounted by using one of the connector terminals for communication with the ASIC. Doing this enables one to compensate for production influences and to prevent additional conductor tracks and their potential influence on electromagnetic resistance of the sensor. 

Femoral ball heads of hip endoprostheses made from bioinert ceramics such as alumina or zirconia have to sustain high mechanical stresses, resorp-tion/corrosion by aggressive body fluid and abrasive wear over the lifetime of the implant in the human body of 15-20years. Some important properties of ceramic femoral ball heads are listed in Table 2.3 (Willmann, 1995). Mechanical properties of alumina and zirconia are discussed in Chapter 4.1. 

Fiber flexibility is important for clinical applications. The most likely place for fibers to be bent severely is where they pass through the skin. The wound at the insertion site is at risk of infection and it has to be accessible for inspection and care. This usually means the fibers (and whatever sheath covers them) must be able to move more freely outside the tissue than inside because the catheter may be moved during patient care consequently that entry point is a place for high mechanical stress. An accidental sharp bend may cause a permanent change in plastic fiber transmission but may cause catastrophic failure of glass fibers. 

Films formed from high-viscosity nitrocellulose have good flexibility combined with a high crack resistance. They are therefore employed where high mechanical stress is to be expected (e.g., in leather coatings, putty, adhesives). Only lacquers with low solids contents can be obtained from high-viscosity nitrocellulose. 

In the second stage, these anodes increase in number and area, forming notches with high mechanical stress and hence high dissolution, which is marked by a further denabling of the potential. 

Mineral, organic and metallic fibers, and the surfaced materials made from them such as fleece mats, textiles, and weaves, not only make possible economical manufacturing of materials with specifically targeted physical property improvements based on standard plastics and technical molding compounds, but also help manage high mechanical stress loads, which are often direction-dependent and show local variations, with anisotropic composite structures. 

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