A study to evaluate the effect of scratch tip radius on stress intensity factor (K) controlled slow crack growth (SCG) was performed to establish whether a plastics pipe industry practice to allow scratches 10% the thickness of the pipe, could still be allowed on large diameter pipes with blunt scratches. A series of finite element analyses were done using a 1/4 two-dimensional (2-D) notched pipe model assuming a 12-in diameter pipe with a standard dimentional ratio (SDR) of 11, a notch ten times smaller than its thickness, notch tip ratios ranging from 16 to 0.0459, and linear elastic material behavior. Results indicate K-controlled SCG would occur if the ratio of notch tip radius to notch depth is less than 0.1667, although this ratio is probably very conservative due to scratch tip blunting from the formation of a craze zone ahead of crack tips in polyethylene (PE) pipes. However, for ratios greater than 0.5, ductile failures could be induced for internal pressures yielding high hoop stresses and at high temperatures. This is due to the fact that stress concentration factors for relatively blunt notches can still induce maximum scratch tip stresses several times higher than the hoop stress of an unscratched pipe. The results of this finite element analysis could be validated experimentally using ASTM D2837-01 following notching procedures given in ISO 13479 with modified cutters to obtain several notch tip radii.

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