Experimental approach to mechanical property variability through a high-density polyethylene gas pipe wall

摘要

An experimental investigation was designed to establish the distribution of mechanical properties throughout a high-density polyethylene (HDPE) gas pipe wall. The proposed approach used a continuous and uniform filament that was automatically machined from the pipe on a precision lathe at a very low cutting speed and an optimal depth of cut to minimize heating and structural disturbances. Typical engineering stress-strain curves, in every layer, were obtained on a testing machine especially designed for polymers, and they were statistically analyzed. The stress-strain behavior of HDPE pipe material could basically be divided into three distinctive zones, the second of which remained important. The average stress level illustrating cold drawing for a given layer was almost constant throughout the pipe wall. The measured stresses and moduli correlated very well with the pipe thickness, and they increased from the outer layers toward the inner layers. This was explained by the crystallinity evolution because the pipe production process was based on a convective water-cooling system with a temperature gradient, which generated residual stresses. Computed statistical stress-strain correlations at yielding, the onset of cold drawing, and fracture points revealed acceptable linear relations for an error level of p ≤ 0.05. On the other hand, an increasing linear correlation characterized the relationship of the yield stress and elastic modulus. This result was confirmed by literature for standard specimens, prepared by compression molding, that did not represent an actual pipe structure with respect to an extrusion thermomechanical history. Such an approach to mechanical property variability within an HDPE pipe wall highlighted the complexity of the hierarchical structure behavior in terms of stress-strain and long-term brittle failure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 272-281, 2005.