As per ASTM E-1820 for J-R curve, slope of blunting line is given by:
J=2sf*deltaa.
But this blunting line equation may not be true for all material.
It is said the stretched zone width is associated with the blunting before start of actual separation.
I want to know method or procedure to calculate blunting line equation from stretched zone width.
with Regards
AK BIND
E 1820 does not deal with stretch zone. Refer to ESIS P2-92 (attached herewith). Read Appendix 4, page A4.1 in this document.
You have two posibilities, but you need to have used the single specimen procedure (for multi-specimens procedure, it is more complicated to verify the relevant slope) :
- the first one : if you use single specimen procedure, is to conside that the initial slope of your J-Da curve is relevant. At least, you shall consolidate that with several test. However, it assumes that you have no biais or uncertainties in the compliance measurement. If your are not able to predict correctly using complaince the final crack extension measure on the cracked surface (I would said with a accuracy of around 0.1 mm), forgot this solution.
The second one : make a measurement of the stretch zone width (SEM observation our using replica). You have to make a stastical analysis du the the variability of the measurement. Then, this value approxmatively corresponds to the crack extension where the experimental point stop to follow the blunting line (those starting from 0 , not 0.2 mm). It can help to confirm the slope of 2 or to recalculated a value.
However, becareful to the value you consider. In general the slope is between 2 on 4 for steel material for example (4 is more appropriate for austenitic stainless steel for example). Initially ASTM E1820 was open to other values than 2. I don't understand why the now removed the possibility to adjust the value.
For your information, if you compare the ASTM E1820 to ISO 12135 standard, the only major difference is this slope, which is 3.75Rm for the ISO standard, close to a slope of 4 if you consider the ASTM equation.
Just a last advice, if you use a specific slope : you can make one or two experiments stopping the test for a cack extension close to J0.2 and check that the crack extension you get, to verifiy the consistency between the real crack extension and the supposed one considering your blungting line.
Dear Stephane,
I will keep your suggestion in mind while carrying out test. calculation of blunting line equation is described in detail in "An investigation of the blunting line" Engineering Fracture Mechanics, Volume 18, Issue 5, 1983, Pages 1025–1036 by S.-W. Yin et al. Yin has suggested to carried out multiple tests between up to load corresponding to the Jic. What id your view on slope of blunting line of ASTM and ISO standard?
with regards
Dear Arvind,
The recommendation of Yin et al is similar to my advice, with the same goal.
Concerning ASTM and ISO slopes, it is not easy to make a choice. It really depends on the material. It would be usefull one day to have somme recommendation in function of the material in these standard. The ISO is most severe, but is relevant for austenic stainless steels, whereas it seems that the value of 2 is more appropriate for low alloy ferritic steels for example (from may experience on these materials).
I saw once a laboratory who used the ASTM slope on a material which led the operator to conclude to a very high toughness as their (automatic) interpretation provided a J-Da curve joining the limit on the maximum allowable J without any real propagation. Unfortunately, the operator didn't checked the surface crack, which presented around 1 mm of ductile tearing. When we asked to re-assess the test with a slope of 4, the results were consistant with the fracture surface. This highlight the necesary to analyse the consistency of your toughness evaluation with the data.
Dear Stephane,
Thank you very much for your reply. I am going to start the tests for determination of slope of blunting line. I will back to you with my results or for any help if needed.
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