Plastic long-term behavior

Stress-relaxation measurements, where stress decay is measured as a function of time at a constant strain, have also been used extensively to predict the long-term behavior of styrene-based plastics (9,12). These tests have also been adapted to measurements in aggressive environments (13). Stress-relaxation measurements are further used to obtain modulus data over a wide temperature range (14). 

This review concerns the long-term behavior of plastics when exposed to conditions that include continuous stresses, environment, excessive heat, abrasion, and continuous contact with liquids. This subject has been reviewed in Chapter 2 (LONG-TERM LOAD BEHAVIOR) but since it is a very important subject the review is continuing. Tests such as those outlined by ASTM D 2990 that describe in detail the specimen preparations and testing procedure are intended to produce consistency in observations and records by various manufacturers, so that they can be correlated to provide meaningful information to product designers. 

The long-term behavior of plastics must, in other words, be investigated using static methods. The best-known method is the time-to-mpture or creep test, whereby a workpiece is subjected to stress CTq at time t = 0 and the stress parameter is maintained at a constant level for the entire duration of the test Time-dependent deformation is then measured. Fig. 26. If the elongation is constant, the stress parameter drops off in time with plastics. This is known as relaxation. This knowledge has applications related, for instance, to screw cmmections (plastic... 

Plastics are affected by stress, impact, temperature, and environmental conditions and these are all time dependent. Long-term behavior is the subject of another book in this series and is not discussed further here. 

An accepted criterion for describing high-temperature performance of a plastic is the deflection temperature under load (DTUL). This is based on a short-term test that identifies the temperature at which a polymer distorts beyond acceptable limits. It has the limitation of not being able to predict long-term behavior but is a convenient method by which to compare materials. 

Plastic viscoelastic nature reacts to a constant creep load over a long period of time by an ever-increasing strain. With the stress being constant, while the strain is increasing, result is a decreasing modulus. This apparent modulus and the data for it are collected from test observations for the purpose of predicting long-term behavior of plastics subjected to a constant stress at selected temperatures. 

Prian L, Barkatt A (1999) Degradation mechanism of fiber-reinforced plastics and its implications to prediction of long-term behavior. J Mater Sci 34(16) 3977-3989 Regester RF (1969) Behavior of fiber reinforced plastic materials in chemical service. Corrosion 25(4) 157-167... 

The short-term behavior of plastics under thermal loading is determined by their softening behavior and physical aging processes, whereas long-term behavior is mainly dominated by chemical degradation. 

The environmental stress cracking is the most common failure reason of polymer parts during their use. There are already many tests to verify the stress crack resistance. Most methods use combinations of an external strain and aggressive liquids to achieve a quick test result. The extrapolation to longer time periods is only successfiil with the help of the expert knowledge of the raw material producers. The development of a new testing method enables a simulation of the material long-term behavior on the basis of a short time test of plastics under the influence of a medium. 

Creep tests are ideally suited for the measurement of long-term polymer properties in aggressive environments. Both the time to failure and the ultimate elongation in such creep tests tend to be reduced. Another test to determine plastic behavior in a corrosive atmosphere is a prestressed creep test in which the specimens are prestressed at different loads, which are lower than the creep load, before the final creep test (11). 

Viscoelastic and rate theory To aid the designer the viscoelastic and rate theories can be used to predict long-term mechanical behavior from short-term creep and relaxation data. Plastic properties are generally affected by relatively small temperature changes or changes in the rate of loading application. 

Chemical The chemical resistance of plastics is well known as reviewed early in this chapter (OTHER BEHAVIOR, Chemical Resistance). The data serves as an excellent initial guide. Most material makers have developed long-term data for commonly used chemicals. Great care must be exercised in this selection because environmental conditions are very impertinent to include in the selection. Note when two materials do not attack a plastic when used separately may be troublesome when used in combination or diluted with water. 

Risks associated with plastic packages may be increased by human behavior and lifestyle. It is common to get into the habit of eating only some kind of foods, but this may increase the risk of continuous exposure to a specific migrant from the packaging used for these favored foodstuffs. Frequent consumption of fast foods or microwave meals packaged in plastic films may also exert negative effects in the long term. 

During the expression of fear-related behaviors, the LA engages the central nucleus of the amygdala (CEA), which, as the principal output nucleus, projects to areas of the hypothalamus and brain stem that mediate the autonomic, endocrine, and behavioral responses associated with fear and anxiety (Schafe et al. 2001). The molecular and cellular mechanisms that underlie synaptic plasticity in amygdala-dependent learned fear is an area of very active investigation (Shumyatsky et al. 2002). Long-term potentiation (LTP) in the LA appears to be a critical mechanism for storing memories of the CS-US associa-... 

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