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Accelerated stress testing options for PE-HD resins


Martin Chytil, Deputy Head of Department and Senior Research Scientist at Orlen Unipetrol’s Polymer Institute Brno, discusses options for accelerated stress testing of PE-HD


Mechanical testing of a polymeric material such as PE-HD can be performed on either a short-term or long-term basis, the latter crucial for applications such as blow-moulded containers and pipes. There are two fundamental modes of


failure of PE-HD responsible for reduction of lifetime: Rapid Crack Propagation (RCP) which is a ductile type of failure; and Slow Crack Growth (SCG) which is a brittle or quasi-brittle fracture. SCG is initiated by formation of crazes


— flat deformation zones consisting of a network of cross-tied fibrils at so-called stress concentrators (pre-created notches, heterogeneity in the molecular network, or a foreign particle) — under external stress. The crazes propagate perpendicularly to the external stress, creating fibrils that span the craze but


eventually fail (the craze-crack transition). For brittle failure to occur, stress must


be kept at below 0.5σy (the yield point). Crack propagation is slow but can be substantially accelerated by an aggressive environment — Environmental Stress Cracking (ESC). The traditional method for assessing


lifetime of PE-HD pressure pipe is based on internal hydrostatic pressure loading by water measured at minimum of three temperatures and varying pressures. Experimental data is then extrapolated to 20°C and time of 50 years. PE-HD pipe grades such as PE80 and PE100 are classified in this way, with the number relating to the minimal required stress at the point of the extrapolation (so 80 corresponds to 8 Mpa). It is a costly and slow process.


Modes of PE-HD pipe failure are shown in this chart. The lower image shows the craze fibrillar network created in brittle fracture of PE-HD


Standard tests for characterisation of PE-HD test specimens include the Pennsylvania Edge Notch Test (PENT) and Full Notch Creep Test (FNCT). Both are based on uniaxial tension loading of test specimens of rectangular or squared cross-section having a notch in the middle – either a front notch (PENT) or circumferential full notch (FNCT). Testing is carried out at elevated temperature – usually 80°C – either in air (PENT) or in a solution of a nonionic surfactant (FNCT). Despite an acceleration over traditional pipe tests, FNCT and PENT have become tedious for new PE-HD pipe grades with a high resistance to stress cracking, such as PE100 and PE100 RC, as testing times extend to thousands of hours. Alternative accelerated methods include Strain Hardening (SH) or Cracked Round Bar test (CRB). The SH test is a modified tensile test performed at 80°C and constant strain rate to very high strain, typically 1100%. At high tensile elongation, strain hardening occurs accompanied with the load increase with the strain. The slope of the curve in the SH region is proportional to the Strain Hardening modulus,


, which correlates


well with PE stress crack resistance (SCR). Duration of this test is typically 20 min or less per specimen. The CRB test is a cyclic fatigue test


using cylindrical test specimens with a circumferential notch in the middle that are stressed at a sinusoidal load at frequency 10 Hz (for PE-HD) at 23°C. The number of cycles to failure (Nf) is plotted against level of stress (Dσ). Advantages of CRB include the absence of a corrosive environment (detergent), testing at laboratory temperature, and a relatively short duration even for PE100 RC grades. Accelerated testing of new PE pipe


grade lifetimes, including SH and CRB testing, is very important and is a key capability at Orlen Unipetrol.


www.unipetrol.cz/en


IMAGE: UNIPETROL


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