Mechanical stress, degradation mechanisms

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

Moreover, the strong mechanical stress degrades the individual fibers and makes them less flexible. Because of the higher washing temperatures, fabrics shrink and become more wrinkled. 

The first step in development of the dosage form is preformulation to determine the major degradation pathway and to understand the mechanism of degradation. Oftentimes, the degradation pathway is dependent on the type of stress that is applied (e.g., thermal, light, moisture), and hence, the stress degradation studies may not be predictive of the real-time stability studies. To date, all the commercial mAb formulations are either lyophilized or liquid formulations that are intended to be stored at 2 to 8°C (refrigerated) condition (refer to Table 26.1). 

The mechanical stability of polymers was related to the polymer s conformation in some of the earlier drag-reduction studies. Above a critical stress, degradation was faster the more contracted and entangled the polymer s conformation (5-7). In petroleum applications the mechanical instability of synthetic relative to carbohydrate polymers was well-recognized. The relative stability problems (possibly related to DUEVs (8)) encountered in the use of high molecular weight hydrolyzed poly(acrylamide) (HPAM) led to the development of an inverse-emulsion polymerization technique (9). (Current research directions using this technique are discussed in Chapter 9.)... 

In summary, the important effect of fatigue in NbTi Cu composites is mechanical and electrical damage to the copper matrix. Aside from the small static stress degradation of /c, no additional Ic degradation is caused by fatigue. Copper matrix degradation effects impose a peak-to-peak strain limit in pulsed magnets of 0.3 to 0.4%. 

New methods for evaluation environmental-stress degradation and weathering of plastics have been proposed [25] based on mechanical behavior, acoustic spectroscopy, ultra-acceleration of degradation by plasma irradiation, etc. 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Mechanical and Chemical Properties. Colorants, especially pigments, can affect the tensile, compressive, elongation, stress, and impact properties of a polymer (5). The colorants can act as an interstitial medium and cause microcracks to form in the polymer colorant matrix. This then leads to degradation of the physical properties of the system. Certain chemicals can attack colorants and there can be a loss of physical properties as well as a loss of the chromatic attributes of the colorant. Colorants should always be evaluated in the resin in which they will be used to check for loss of properties that ate needed for the particular appHcations. 

It is not known to what extent each of the previous mechanisms contributes to turbine blade degradation during service. It is also probable that each alloy will respond differently to a particular temperature/stress combination. Figure 21-12 shows the typical variation in stress/rupture life determined at 1350°F (375 °C) with service time for forged Inconel X-750 blades. 

The deterioration of a bond line can occur due to (1) the failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength) (2) the failure of the interface between resin and wood surface (replacement of secondary forces between resin and reactive wood surface sites by water or other non-resin chemicals) (3) the breaking of bonds due to mechanical forces and stresses (the influence of water will cause swelling and therefore movement of the structural components of the wood-based panels). 

The laminate is now degraded to the point where the outer layers carry stress only in the x-direction and the inner layers can carry stress only in the y-direction. In both cases, the stress is parallel to the fibers. Thus, the laminate is completely decoupled, both thermally and mechanically. The only nonzero reduced stiffnesses are... 

The mechanisms by which polymers undergo degradation in the human body are not yet completely understood. Examples of breakdown of these materials are illustrated by the embrittlement and excessive wear of polyester sockets exposed to the mechanical, biochemical and thermal stresses of the physiological milieu, as well as by the fatigue fractures, excessive wear and additional cross-linking (embrittlement) that have been observed in polyethylene sockets. 

Bundy, K. J. and Desai, V. H., Studies of Stress-Corrosion Cracking Behaviour of Surgical Implant Materials using a Fracture Mechanics Approach , in Corrosion and Degradation of Implant Materials, second symposium , (Eds) A. C. Fraker and C. D. Griffin, 73-90, ASTM Publication STP 859, Philadelphia (1985)... 

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