Long-term stress

Plastics testing encompasses the entire range of polymeric material characterizations, from chemical stmcture to material response to environmental effects. Whether the analysis or property testing is for quaUty control of a specific lot of plastic or for the determination of the material s response to long-term stress, a variety of test techniques is available for the researcher. 

Dislocations are known to be responsible for die short-term plastic (nonelastic) properties of substances, which represents departure from die elastic behaviour described by Hooke s law. Their concentration determines, in part, not only dris immediate transport of planes of atoms drrough die solid at moderate temperatures, but also plays a decisive role in die behaviour of metals under long-term stress. In processes which occur slowly over a long period of time such as secondaiy creep, die dislocation distribution cannot be considered geometrically fixed widrin a solid because of die applied suess. 

Fig. 2-22 Viscoelastic creep behavior typical of many TPs under long-term stress to rupture (a) input stress vs. time profile and (b) output strain vs. time profile.

Basics Creep data can be very useful to the designer. In the interest of sound design-procedure, the necessary long-term creep information should be obtained on the perspective specific plastic, under the conditions of product usage (Chapter 5, MECHANICAL PROPERTY, Long-Term Stress Relaxation/Creep). In addition to the creep data, a stress-strain diagram under similar conditions should be obtained. The combined information will provide the basis for calculating the predictability of the plastic performance. 

California black oak and white fir and less often on incense cedar in the San Bernardino Mountains. No direct effects of oxidants have been noted on the mistletoe plant itself under field conditions. The true mistletoe obtains mainly water from its host and would be indirectly affected by debilitation of die host tree. The dwarf mistletoes Arceuthobium spp.) are common on ponderosa, Jeffry, and sugar pines in the San Bernardino National Forest. They depend on their host for both water and carbohydrates. Heavily infected or broomed" branches on ponderosa or Jeffrey pines severely injured by ozone often have more annual needle whorls retained than do uninfected branches on the remainder of the tree. The needles are also greener. It can be hypothesized that the infected branch is a carbohydrate sink where a pooling of carbohydrates occurs higher carbohydrate concentrations may be instrumental in either preventing or helping to repair ozone injury to needles on the broomed branches. In the long term, stresses from mistletoe and ozone are probably additive and hasten tree death. 

The causes of blood deficiency are linked directly with deficiency of these four organs. It is often seen in chronic diseases, improper diet or in conditions of starvation. It can also occur following heavy blood loss. Long-term stress and overwork can directly weaken the Heart, Spleen, Liver and Kidney, causing blood deficiency. 

Internal dryness is caused directly by the consumption of body fluids. Some diseases, medicines, herbs or food products can injure the body fluids and cause dryness. Long-term stress and emotional disturbance can also cause consumption of the body fluids. As the Lung, Stomach and Large Intestine are mainly involved, the syndrome of dryness can be respectively divided into dryness in the Upper-, Middle- and Lower-Jiao. 

Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Creep deformation is time-dependent deformation. 

Table 6.18 Minimum creepage distances for electrical equipment subject to long-term stresses (the values given for IEC 60 664-1 are only a part of the standard s content)... 

FIGURE 10.18. Electroluminescent spectra of a BDOH-PF LEC initial EL and the EL after long-term stress. The initial EL is blue and the long-term stress EL changes to bluegreen. The PL spectrum of a thin film of BDOH-PF is also shown for comparison. (From Ref. 33.)... 

Great stress, to ensure efficacy, is put on those products containing stimulants, such as caffeine or ephedra, to those on a diet, workout often, or under the age of 18. These products can cause long-term stress to the body if used for extensive amounts of time. Ephedra arises in the concern of the FDA because of the mechanism of ephedra in the human body. The adrenaline-like stimulant can cause dangerous effects to the nervous system and heart. Some of these effects include heart attack, seizure, stroke, and even death. There must be caution because the risk can increase with the dose, and with strenuous exercise. It specifies certain groups (such as women who are pregnant or breast feeding) who should never use these products and lists other conditions, such as diseases and the use of certain medications that rule out the use of ephedrine alkaloids. 

Stability—accelerated, long-term (stress studies generally not performed for drug product)... 

Stability—accelerated, long-term (stress studies generally not performed for drug product), statistical analysis Comprehensive summary table for batch profiles Specifications—methods, tests, limits, rationale, validation... 

Since this loss Is much less than that of the Isoprene block-graft fiber, which lost over 20% In 10 hours. It appears that autooxldatlon la a significant factor In long term stress decay. 

Test Methods for Short-Term and Long-Term Stresses... 

The tests for change in dimensions of a polymer under long-term stress, called creep or cold flow (ASTM-D74-56), are no longer recommended by ASTM. ASTM-D-671 describes suggested tests for fatigue or endurance of plastics under repeated flexure. 

But there are also long-term stresses that may damage the brain and the immune system over time. This happens when our brains remain on orange alert, when we feel helpless, perhaps when we have been ill and when worry and grief consume us. When that happens, we release another set of hormones, the best studied of which is called cortisol. We release this hormone from the cortical (outer shell portion) of our adrenal glands and pump it into our bloodstream. 

The long-term stress-strain behavior of polymers is generally more important than shortterm properties where the product is expected to sustain a stress or strain in service. Creep is dearly the most relevant where the product or component is to be subjected to a more or less constant stress. This is the case for a great many uses of rigid plastics and for such products as rubber mountings. Hence, creep data is often an essential design factor for plastics but is only used for other polymer types when particular applications are in mind. 

Subsequent to these original formulations, a number of refinements to these relationships have been proposed. Observations of persistent deformations after micropipette aspiration for extended periods of time formed the basis for the development of a model for long-term stress relaxation [Markle et al., 1983]. The characteristic times for these relaxations were on the order of 1 to 2 h, and they were thought to correlate with permanent rearrangements of the membrane elastic network. 

Nickel base materials have outstanding resistance to thermal, mechanical, and corrosive stresses. They include up to 15 elements with a portion of more than 50 % in weight and are able to resist long-term stresses up to a homologous temperature, which is the ratio between the temperature load and the melting temperature, of approximately 0.85 (Buergel 2006). Nickel base materials are used in gas turbines, jet engines, space vehicles, and many more modem applications with extreme conditions. 

Figure 3.19 Recovery under a fuUy confined condition. The recovered stress for the pure SMP and foam after stress-controlled programming with two pre-stresses is shown. The inset shows the long-term stress relaxation of the foam programmed by a 47 kPa compressive stress. Source [41] Reproduced with permission from Elsevier...

Under long term stress conditions, in eithercyclic (fatigue) or static (creep) loadings, intrinsic morphological features can become areas of weakness, e.g., cracks can form in polyethylene at the boundaries of spherulites and within them, suggesting that long term failure is due to separation between lamellae. 

The long-term stress, including creep factors and safety factors, that is used in designing structural fabrication. Treating plastic materials to minimize their accumulation of static electricity. 

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