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Title: Influence Of Thermal Aging On Pe Pipe Performance
Author: m. Dicks, P. Blackett
Source: American Gas Association 1985
Year Published: 1985
Abstract: Thermal aging is 10 be understood as the reduction in toughness of a material as a result of the effect of oxygen and/or air af elevated temperatures. An example of such effects is seen in stress rupture tests at higher temperatures. The effects of aging are embrittlcment and a reduction in physical properties. Such changes in properties can be seen in the initial stage of material damage by measurement of the elongation at break. Early research work has shown that elongation at break of less than 200 percent is a clear indication of the onset of damage caused by aging. These early tests were conducted on PE 34 type materials using molded specimens kept in aerated water baths at various temperatures. Results of these tests showed that at temperatures below 104 F the 50 year design life could be reached without encountering any deterioration due to aging. The prediction of long-term polyethylene pipe performance using the Arrhenius and/or rate process methods of extrapolation is based upon the assumption that PE materials do not change chemically with time. This means that the molecular weight will not be drastically reduced because of broken carbon bonds (c-c). However, such degradation mechanisms have been observed with first generation HDPE pipe materials at high temperatures in the presence of oxygen. As a result, a great deal of work has been done to predict the onset of such degradation. This work resulted in a wide acceptance of the 200 percent elongation at break criteria. Figure 1 shows a typical stress strain curve for polyethylene exhibiting the classic behavior with an elongation at yield of approximately 10 percent with elongation at break of 600-800 percent. Fracture of a specimen at the 200 percent mark or lower is indicative of aging damage causing the onset of brittle behavior. It was found that the time until the beginning of aging in the range from 73F to 176F follows the Arrhenius equation:

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