Abstract
This paper examines the effect of a broad range of crosshead speed (0.05 to 100 mm/min) and a small range of temperature (25 °C and 45 °C) on the failure behaviour of high density polyethylene (HDPE) specimens containing a) standard size blunt notch and b) standard size blunt notch plus small sharp crack – all tested in air. It was observed that the yield stress properties showed linear increase with the natural logarithm of strain rate. The stress intensity factors under blunt notch and sharp crack conditions also increased linearly with natural logarithm of the crosshead speed. The results indicate that in the practical temperature range of 25 °C and 45 °C under normal atmosphere and increasing strain rates, HDPE specimens with both blunt notches and sharp cracks possess superior fracture properties. SEM microstructure studies of fracture surfaces showed craze initiation mechanisms at lower strain rate, whilst at higher strain rates there is evidence of dimple patterns absorbing the strain energy and creating plastic deformation. The stress intensity factor and the yield strength were higher at 25 °C compared to those at 45 °C
Highlights
High density polyethylene (HDPE) finds applications in high modulus/ high load bearing and long exposure to harsh condition applications typically in ambient temperature ranging between 25 to 45 °C
A ductile behaviour at lower strain rates is evident from the longer extension for both blunt notch and sharp crack samples as was observed in polymers by Chanda and Roy[15], and Perkins[16]
(2) Both blunt notch and sharp crack HDPE are ductile fractured at crosshead speeds of 0.05, 0.5, 5, 50 and 100 mm/min. This is attributed to the failure occurring at temperatures 25 c and 45 C which are above the glass transition state of the HDPE past the brittle state of the polymer
Summary
CHS effects at room temperature, 25 °C. The tensile test results recorded as load-extension curves under various crosshead speeds c are shown in Fig. 2 for blunt notched HDPE samples. Fracture surfaces of (a) and (b) exhibit similar feature, that is, both of them have a smooth parabola slow crack growth zone near the notch tip and the size of the parabola zone diminishes as crosshead speed increases from 0.05 to 0.5 mm/min. Fracture surfaces of both blunt and sharp cracked HDPE have parabola type slow crack growth zone near notch tip at 0.05 and 0.5 mm/min, whilst at 5, 50 and 100 mm/min, they have similar uneven fast. Similar to blunt notched specimens, sharp notched specimens are ductile fractured and there is a crazing-shear yielding fracture mechanism transition as crosshead speed increases from 0.05 to 100 mm/min. It has been observed that there is a crazing-shear yielding mechanism transition as crosshead speed increases from 0.05 to 100 mm/min regardless of blunt notch or sharp crack HDPE.
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