Crack Tunnelling and Crack Tip Opening Angle in Drop-Weight Tear Test Specimens

Abstract

Interrupted drop-weight tear tests (DWTT) of typical low-strength and highstrength pipe steels were performed at quasi-static and impact loading rates, to quantitatively characterize crack tunnelling and crack-tip opening angle (CTOA). Crack front tunnelling was observed in all interrupted DWTT specimens, but the extent of tunnelling varied with the steel grade and the loading rate. CTOA values measured optically at the surface were higher than those measured at a cross-section close to mid-thickness. This is attributed to the effect of crack tunnelling, i.e. the effect of constraint, on CTOA. CTOAs derived from load vs. load-line displacement, using the simplified single-specimen method, are in good agreement with values measured at the mid-thickness.

Determination of dynamic rotation factor for DWTT specimens by strain analysis and hardness measurement

“…For the reason that the low‐blow energy is not strong enough to break the notched specimen, the recorded force signal smoothly decreases after peak point and eventually ends up with a sudden drop to zero. Similar experimental phenomena have also been reported during the interrupted DWTT …”

“…Similar experimental phenomena have also been reported during the interrupted DWTT. 17 It needs to be noted that, because of the reduction of the impact loading rate (i.e. K˙) that is dominated by the parameters of t f and v 0 , 20 sample 30-25 reflects a significantly decreased load response and remarkably plastic-bending behaviour, differing from sample 30-135.…”

“…Hereafter, 1/4 thickness of the bent specimens was removed from the double original surfaces by a milling machine, as to eliminate the superficial effect on the interior hardness determination. Positional hardness distribution along the midline of residual ligament at the T/4 section from both surfaces of the plate was measured and averaged, concerning the crack tunnelling effect also reported by Xu, illustrated in Fig. .…”

Characterization of stable ductile crack propagation by CTOA: Review of theory and applications

“…To ensure that CTOA is measured in the steady‐state portion of the record, ln (P/P m ) values between −0.5 and −1.2 for the 10 mm deep notched specimen and to ln (P/P m ) values between −0.7 and −1.4 for the 5 mm deep press‐notched specimen are used. Crack‐tip opening angles derived from load vs load‐line displacement (ie, using the S‐SSM) are in good agreement with values measured at midthickness …”

Comments and replies on the paper ‘Determination of dynamic rotation factor for DWTT specimens by strain analysis and hardness measurement’, FFEMS, 2016, 39, 830–838, by J.Fang, J.W.Zhang, and Y.D.Zhou

“…This now seems to be appreciated by the authors. There is significant tunnelling of the crack front, making surface observations of questionable value in determining the overall response. There is an extensive literature on tunnelling, including tunnelling in DWTT specimens; see, for example, Xu et al …”