ASTM E-1820 recommend side grooving after fatigue pre-cracking. But I think side grooving before fatigue pre-cracking is better because the fatigue pre-crack front will be more uniform. Also, crack growth during fatigue pre-crack, will follow side grooved region.
Please give your comment and and share your experiences.
Two questions in response to the question assmuing that side grooves are made before precracking:
1. what if a fatigue crack/cracks starts from the side-groove/grooves on the crack plane perpendicular to the expected crack growth direction from the notch.
2. The expected pre-crack should follow the sidegrooved plane, but what if it does not! and measuring/conttrolling the length of the pre-crack would be hard optically.
What do you think Arvind Bind Sir.
With regards,
Abhishek
Dear Mr. Gyoko
Rather side grooving before pre-cracking is expected to produce straight crack because side grooving before pre-cracking makes stresses more uniform at crack plane. Please see the attached article. It shows that side grooving before pre0cracking has straighter crack front.
Dear Abhishek Tiwari ,
Most probably, the crack will start from the from the side-groove/grooves on the crack plane perpendicular to the expected crack growth direction from the notch. and I think that side grooving before pre-cracking will make crack front more uniform. See above attached file. Also crack is most likely to follow the side grooved when seen on plane perpendicular to the crack growth direction which is desirable.
AKBIND
Side grooving intends to remove the plane stress region around the sides of the crack, which influences the fracture toughness results.
One thing to note is that when fatigue pre-cracking without side-grooves, it will produce a 'thumbnail' shaped crack tip. When side-grooving is machined into the specimen after pre-cracking, it removes the crack tip contour near the sides of the specimen, resulting in a fairly straight and uniform crack tip.
However, if fatigue pre-crack is applied after sidegrooving, you might get an interaction of perpendicular stress fields around the side-groove-notch tip interface. This might potentially lead to a reverse 'thumbnail' shaped crack tip. I do not think this matter too much for research as it would behave like a 'blunted' chevron crack tip, but one will need to check if it confroms to the standards for use in industry.
Regards,
Wee
Dear Mr. Weeliam Khor
Your are. very right that side grooving before pre-cracking may lead reverse 'thumbnail' shaped crack front or crack front. Chen et al. has reported the same. Please see the my reply to Mr. Gyoko Oh
Dear Mr. Arvind Bind
According to the attached article, there were no obvious differences between the side-grooving operations before and after the precracking in the valid tests of 25.4 mm thick specimens.
On the other hands, the fracture toughness and strength can be classified as two types: static type and dynamic type. When the fracture toughness Kc is determined by static fracture tests where specimens have fatigue pre-cracking, their values are about 50-250 MPam1/2, in the middle and high strength steels. However, in dynamic fracture tests under cyclic loading, the stress intensity factors Kth, Ka(0) are very small and decline to about 2.5 MPam1/2 in the fatigue crack initiation (Ka(0)). The fatigue limits are about half values of tensile strengths in smooth specimens, but decrease greatly in notch specimens relating to the stress concentration factors. The values of Kth, Ka(0), and Kc are necessary to be used separately in the dynamic loading and static loading conditions without getting them mixed up. The Kc means an instability criterion to the final stage of fractures, and it is different from the fatigue crack initiation and stable propagation.
Two reference:
Article Fatigue crack initiation and subsequent crack growth in fill...
Article Experimental and computational analyses on fatigue fracture ...
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