The Visco-elastic Nature of Polyethylene

VISCO-ELASTICITY

Polyethylene pipe is a visco-elastic construction material . Due to its molecular nature; polyethylene is a complex combination of elastic-like and fluid-like elements. As a result, this material displays properties that are intermediate to crystalline metals and very high viscosity fluids. The visco-elastic nature of polyethylene results in two unique engineering characteristics that are employed in the design of HDPE water piping systems, creep and stress relaxation. Creep is the time dependent viscous flown component of deformation. It refers to the response of polyethylene, over time, to a constant static load. When HDPE is subjected to a constant static load, it deforms immediately to a strain predicted by the stress-strain modulus determined from the tensile stress-strain curve. At high 12 introduction loads, the material continues to deform at an ever decreasing rate, and if the load is high enough, the material may finally yield or rupture. Polyethylene piping materials are designed in accordance with rigid industry standards to assure that, when used in accordance with industry recommended practice, the resultant deformation due to sustained loading, or creep, is too small to be of engineering concern. Stress relaxation is another unique property arising from the visco-elastic nature of polyethylene. When subjected to a constant strain (deformation of a specific degree) that is maintained over time, the load or stress generated by the deformation slowly decreases over time. This stress relaxation response to loading is of considerable importance to the design of polyethylene piping systems. As a visco-elastic material, the response of polyethylene piping systems to loading is time-dependent. The effective modulus of elasticity is significantly reduced by the duration of the loading because of the creep and stress relaxation characteristics of polyethylene. An instantaneous modulus for sudden events such as water hammer can be as high as 150,000 psi at 73°F. For slightly longer duration, but short-term events such as soil settlement and live loadings, the short-term modulus for polyethylene is roughly 110,000 to 120,000 psi at 73° F, and as a long-term property, the modulus is reduced to something on the order of 20,000-30,000 psi. As will be seen in the chapters that follow, this modulus is a key criterion for the long-term design of polyethylene piping systems. This same time-dependent response to loading also gives polyethylene its unique resiliency and resistance to sudden, comparatively short-term loading phenomena. Such is the case with polyethylene’s resistance to water hammer phenomenon, which will be discussed in more detail in subsequent sections of this article.