Plastication Subject

Plastics also have the ability to recover when the applied stress is removed and to a first approximation this can often be considered as a reversal of creep. This was illustrated in Fig. 1.8 and will be studied again in Section 2.7. At present it is proposed to consider the design methods for plastics subjected to steady forces. 

Polyethylene and polystyrene are examples of plastics subject to environmental stress cracking. Crack resistance tests have shown that surfactants, alcohols, organic acids, vegetable and mineral oils, and ethers provide an active environment for stress cracking of polyethylene. Table 6 lists typical sterile devices and plastic materials used to fabricate them, while Tables 7-9 list the potential effects of sterilization processes on polymeric materials. The effect of gamma irradiation on elastomeric closures has been studied by the Parenteral Drug Association [15]. 

Recent studies have confirmed that the sensitivity of neuronal pathways activated by tussive mechanical stimuli of human airways is subject to plasticity. Subjects with upper respiratory tract infections were found to have reduced thresholds for mechanically evoked cough and in addition coughed more in response to mechanical stimuli than did healthy subjects (Lee and Eccles 2004). It has yet to be determined if mechanically evoked chough is similarly sensitized during chronic lung diseases. 

Stress relaxation under constant strain has been reviewed. It relates to applications which are kinematically evaluated and for which applied displacements are constant in time. Examples are bolted joints or plastics subjected to a mismatch-fit into a much stiffer structure or having a different coefficient of thermal expansion. 

The change in dimensirm of a plastic under load over a period of time (excluding the initial instantaneous elastic deformation). Owing to viscoelastic nature, a plastic subjected to a load for a period of time tends to deform more than it would from the same load released immediately after application. The degree of this deformation depends oti the load duration. Creep is the permanent deformation resulting from prolonged application of stress below the elastic limit. Data obtained in creep test are presented as creep vs. time, with stress and temperature cmistant. Slope of the curve is the creep rate, and the end point of the curve is the time for rupture. Creep at room temperature is called cold flow (ASTM D674). 

The desirability of extending the working life of plastics subjected to the combined deteriorating influences of light and oxygen has led to a marked growth in the number of papers published in this area of research. The converse objective, that of producing materials of limited and controllable outdoor lifetime has also continued to develop. In these circumstances a comprehensive review of all relevant publications would require too much space and so arbitrary choice has been exercised on the content of this chapter. 

I) Scrap in the form of offcuts, rejects, sprues, etc., arising in the manufacture of plastics products. Most of this waste material is recycled by blending it with virgin plastic, subject to the careful control of the levels of contamination in the plastic regrind, the proportion of regrind to virgin materials, and the deterioration in the p%sical properties which may be caused by repeated thermal and mechanical processing. 

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. 

Creep krep vt [ME crepen, fr. OE creopan akin to ON krjupa to creep] (before 12c) Due to its viscoelastic nature, a plastic subjected to a load for a period of time tends to deform more than it would from the same load released immediately after application. The... 

The considerations so far have not taken account of the properties of the material under stress, the only requirement being that it is linear elastic. The material will fail by very rapid crack growth when K, reaches a critical value for the material often called the fracture toughness. Typical values for fracture toughness for different plastics subjected to short-term loading are given in Table 2.4. 

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