CREEP AND FATIGUE CRACK GROWTH IN PE-HD PIPE GRADES - EFFECT OF TEST TEMPERATURE AND R-RATIO G. Pinter, W. Balika and R.W. Lang Institute of Materials Science and Testing of Plastics University of Leoben, A-8700 Leoben, Austria ABSTRACT The creep crack growth (CCG) and fatigue crack growth (FCG) behavior of two PE-HD pipe grades was studied based on a linear elastic fracture mechanics (LEFM) methodology. The FCG-tests were performed under a sinusoidal load at a frequency of 1 Hz and R-ratios (Fmin/Fmax) of 0.1, 0.3 and 0.5; the test temperatures were 23 (only FCG), 60 and 80 °C. The results showed that FCG rates in PE-HD are caused by a combination of cyclic-induced and creep-induced damage, depending on the mean stress level. While for given values of KImax (FCG tests) and KI (CCG tests), respectively, at low test temperatures the cyclic component of the applied stress dominates crack growth rates with CCG rates (R = 1) being lower than the FCG rates (R < 1), at high test temperatures the creep component becomes increasingly important in affecting crack growth rates so that CCG rates even exceed FCG rates. The point of inversion from fatigue to creep dominated failure on the temperature scale apparently depends on molecular and morphological characteristics of the PE-HD type and occurs at around 80 °C for PE-HD 1 and around 60 °C for PE-HD 2 in this investigation. INTRODUCTION Based on empirical findings of the failure behavior of pressurized plastics pipes in laboratory tests and on experiences with long-term field failures of pipes, linear elastic fracture mechanics (LEFM) concepts are frequently applied to also study the crack growth behavior of pipe grade poly(ethylenes) (PE). In the past the crack propagation behavior of various PE types was investigated using crack growth tests under static loads (so called creep crack growth (CCG) tests) [1-4]. More recently cyclic loading conditions (fatigue crack growth (FCG)) were used by some research groups to characterize the crack propagation resistance of PE [58]. However, while some studies on the failure behavior of notched specimens under static and fatigue loading conditions for a given PE type have been reported [9,10], so far hardly any information is available on a correlation between the kinetics of crack growth behavior under static and cyclic loads using LEFM concepts. Hence, it is the main objective of this paper to investigate the effects of R-ratio (minimum load to maximum load, Fmin/Fmax, in a fatigue cycle) on FCG behavior at various temperatures and to compare the results with the kinetics of crack growth under static loads. To gain some insight into the micromechanisms of crack growth under the various loading conditions, fracture surfaces were studied systematically via scanning electron microscopy (SEM).
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