Relation of craze to crack length during slow crack growth phenomena in high-density polyethylene

The craze-crack mechanism occurring in high-density polyethylene (HDPE) causing slow crack growth and environmental stress cracking is investigated in detail with respect to the relation of crack length and the related craze zone. This is essential for the understanding of the resulting features of the formed fracture surface and their interpretation in the context of the transition from crack propagation to ductile shear deformation. It turns out that an already formed craze zone does not inevitably result in formation of a propagating crack, but could also undergo ductile failure. For the examination, the full notch creep test (FNCT) was employed with a subsequent advanced fracture surface analysis that was performed using various imaging techniques: light microscopy, laser scanning microscopy, scanning electron microscopy, and X-ray micro computed tomography scan. FNCT specimens were progressively damaged for increasing durations under standard test conditions applying Arkopal, the standard surfactant solution, and biodiesel as test media were used to analyze the stepwise growth of cracks and crazes. From considerations based on well-established fracture mechanics approaches, a theoretical correlation between the length of the actual crack and the length of the preceding craze zone was established that could be evidenced and affirmed by FNCT fracture surface analysis. Moreover, the yield strength of a HDPE material exposed to a certain medium as detected by a classic tensile test was found to be the crucial value of true stress to induce the transition from crack propagation due to the craze-crack mechanism to shear deformation during FNCT measurements.Highlights Progress of crack formation in high-density polyethylene is analyzed by different imaging techniques Determined growth rates depend on distinction between craze zone and crack The ratio of the present crack to the anteceding craze zone is validated theoretically The transition from crack propagation to ductile shear deformation is identified An already formed craze zone may still fail by ductile mechanisms Cross section of a propagating crack in a HDPE specimen (X-ray micro computed tomography) and resulting fracture surfaces. image