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

Abstract

As to accurately ascertain the value of dynamic rotation factor r*, strain analysis‐based measurement was performed on the bent low‐blow specimens by means of two approaches. (i) Surface strain analysis on the ligament indicates a critical transition zone from the necking to swelling, where the total strain reaches the minimum as undeformed material. (ii) Hardness measurement on the thickness‐reduction specimen also reflects a typical V‐shape distribution of interior plasticity along the ligament as observed from the surface, suggesting the location of true rotation centre relative to the neutral strain axis where the hardness is nearly invariable without any work hardening. As a result, the obtained value of r* maintains nearly 0.45 for (B × 4B) size drop‐weight tear test‐type specimens after their crack initiation and propagation, well consistent with the values calculated from the slip‐line field theory.

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

“…Some final remarks following their replies are in order. 1 We agree with the description of necking and swelling during initial loading. Up to maximum load when plasticity has spread through the ligament, the neutral axis is plausibly near the location of zero plastic strain.…”

“…It has recently been proposed by Fang et al (hereafter Article 1) based on hardness measurements of deformed specimens that the rotation factor r * for Drop‐Weight Tear Test (DWTT) specimens should be near 0.45 . This is significantly different from the r * , which is approximately 0.57, in the recently‐approved ASTM Standard for determination of the crack‐tip opening angle (CTOA) .…”

“…the Conclusion: ‘The centre of rotation in a specimen in which crack propagation has occurred does not necessarily coincide with the location of minimum hardness.’ With a growing crack, the position of minimum hardness will not coincide with the position of the centre of rotation and cannot be used to calculate r* . 4The utility of FEA in revealing details of the stress and strain distribution is well established. Figure of the ‘Comments’ shows clearly that the hardness minimum on the plane investigated by the authors of the original article (Fang et al ) occurs at r* ≈ 0.44 in agreement with their results. However, the hardness distribution is a result of the entire strain history as explained in the ‘Comments’.…”

“…Analysis of the strain components is also complicated by the movement of the centre of rotation during crack growth. Before the crack begins to grow, the analysis in the original paper (Fang et al …”

“…There is an extensive literature on tunnelling, including tunnelling in DWTT specimens; see, for example, Xu et al 13 Analysis of the strain components is also complicated by the movement of the centre of rotation during crack growth. Before the crack begins to grow, the analysis in the original paper (Fang et al 1 ) is correct. However, once the crack starts to grow and the centre of rotation moves through the material, the position of minimum strain will not be coincident with the centre of rotation.…”