Plastic short-term behavior

The tensile modulus is an important property that provides the designer with information for a comparative evaluation of plastic material and also provides a basis for predicting the short-term behavior of a loaded product. Care must be used in applying the tensile modulus data to short-term loads to be sure that the conditions of the test are comparable to those in use. The longer-term modulus is treated under the creep test (Chapter 2). 

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. 

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. 

In computing ordinary short-term characteristics of plastics, the standard stress analysis formulas may be used. For predicting creep and stress-rupture behavior, the method will vary according to circumstances. In viscoelastic materials, relaxation data can be used in Eqs. 2-16 to 2-20 to predict creep deformations. In other cases the rate theory may be used. 

This modulus value is often arbitrarily chosen, although several methods have been suggested for arriving at a suitable value. One is to plot a secant modulus based on 1% strain or that is 0.85% of the initial tangent modulus (Chapter 2, SHORT-TERM LOAD BEHAVIOR). However, for many plastics, particularly the crystalline TPs, this method is too restrictive, so in most practical situations the limiting strain is decided in consultation... 

Plastics have the widest variety and range of mechanical properties of all materials (Figs. 1-8 and 7-1 and 7-2). They vary from basically soft to hard, rigid solids. Great many structural factors determine the nature of their mechanical behavior, such as whether a load occurs over the short term or the long... 

Casadesus G, Shukitt-Hale B, Stellwagen H, Shu X, Lee H-G, Smith M and Joseph J. 2004. Modulation of hippocampal plasticity and cognitive behavior by short-term blueberry supplementation in aged rats. Nutr Neurosci 7 309-316. 

The torsional vibration test according to DIN 53445 is used to investigate temperature-dependent viscoelastic material behavior. This is a short-term test. The time-to-mpture test according to DIN 53444 registers the influence of stress duration, stress type, and temperature. The torsional vibration test provides additional information on the damping behavior of the plastics tested (mechanical loss or damping factor d). Fig. 13 [9]. 

The NEN 7343 is a test method to understand the short-term elution behavior of the powdery waste. A column made of plastic is 5 cm in diameter and more than 20 cm in height. Sample is filled in the column so that the height of the sample in the column becomes more than four times the diameter. A nitric acid solution with pH of 4 is poured from the bottom to the top of the column. It continues until the ratio of liquid volume [ml] to solid weight [g] becomes ten. We will sample the solution at the ratio of 0.1-0.2, 0.2-0.5, 0.5-1.0, 1.0-2.0, 2.0-5.0, and 5.0-10.0, totally six times, and analyze each solution. 

The shape memory behavior of an SMP makes it a very desirable material for use in biomedical applications. Thermally activated SMPs can be programmed and stored in a small secondary shape, and on introduction to the body and water plasticization, recover their large original shape (Beilvert et al., 2014). This property of SMPs can be harnessed for minimally invasive surgery and tissue engineering scaffolds (Beilvert et al., 2014). However, ceU compatibility of an SMP biomaterial needs to be extensively understood to determine its feasibility as a short-term or long-term implant and the impact of its SME on cells. 

This section introduces the behavior and response of both unreinforced and reinforced plastics under loads lasting usually only a few seconds or minutes up to a maximum of fifteen minutes. Such short-term tests are used to define the basic or reference designing and engineering properties of conventional materials. Such properties as tensile strength, compressive strength, flexural strength (the modulus of rupture), shear strength, and associated elastic moduli are often shown on the data sheets provided by suppliers of plastic materials and are in computerized data banks. The influence of such factors as time, temperature, additives and reinforcements, and molecular orientation on the basic behavior of these properties is discussed in turn [1, 2, 10-14, 62-68, 245-87]. 

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. 

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