Reverse calculation shows for SEN(B) specimens, BS 7448-1 uses rp=0.4 and ASTM E1290-08 uses rp=0.2. I have not found any reference citing these values. I've been told that BS 7448-1 opted rp=0.4 through experiments, but have yet find any reference.
There is a strong basis for the selection of appropriate plastic rotation factor (rp). It basically comes from slip line field theory, the location of the centre of rotation, specimen geometry and of course the crack depth (a/W).In the early use of clip gage technique for CTOD determination, a calibration (both experimental and theoretical) curve was normally used to relate the COD and the knife edge opening Vg. Since explaining the slip line field theory and the manner in which rp is worked out is difficult within this discussion forum, I would suggest you to read the book "Fundamentals of Fracture Mechanics" by John F Knott (Butterworths), especially the sections 2.12 and 6.3. A few references given in these sections will be of great help in understanding the basis of rp.
BYW, the ASTM E 1920 mentions rp =0.44 for SENB and Arc Bend specimens; 0.47 (for a.W in the range of 0.45-0.50) for CT specimens. Where is rp =0.2 in E 1920?Which geometry?
Thank you for your reply Dr. Sivaprasad.
As for BS7448-1, rp=0.4, and it is the same as BS5762:1979. However in DD19:1972, which is a draft document, rp=0.45. Well's paper 'Status of COD in Fracture Mechanics', he showed that rp converges approximately at 0.45. ASTM E1290 '93 and '99 uses rp=0.44. I could not find any reference citing this change.
ASTM E1290 '08 converts J into CTOD. However in the plastic part, it is found that rp=0.2 is used for the knife edge height correction in order to obtain the 'approximate' CMOD. I've just found a reference citing the change.
P.S. I corrected the question to SEN(B) and ASTM E1290-08
I am still not clear where did you get rp=0.2 in E 1290-08? In fact there is no plastic rotation correction at all (!!) in 1290-08 nor any suggestion to relate CTOD and J.
There is however a mention about the CTOD-J relation in E 1820-08 (which is now the combined standard for K, J and CTOD) but here as well the relation is through the blunting line slope (mSy) and not through rp.
In ASTM E1290-08, if you break down Ap, you will find that CMOD is evaluated using rp.
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