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Experimental notch fracture testing to calculate KII
Hi,
I am doing a v-notch fracture testing (experimental) with opening angles from 0,30,60,90,120 to 150 deg and need to calculate KII. I need a equation that could be used to calculate KII for both 0 deg and also for non-zero opening notch angles. I am using TestResources machine to do this shear testing and I will get results in terms of load vs. d. The material is PMMA. What could be the appropriate displacement/min or load to break the specimen.
I am also using Ansys to predict the same. My KI value is 0 and KII value seems to be between 1-2 MPa.sqm. The specimen is fixed with 6 holes, 3 holes on each side. and I have applied displacement on that area (3 holes-fixed and 3 holes-applied with Y-displacement, x,z-fixed). Now I want to know what is the force in that area.
Pls let me know the answers.
Thanks in advance.
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Notch is different from crack
Hi,
You can use a loading rate of about 1 mm/min or lesser (if your machine will allow it) for quasi-static loading for PMMA.
Your first picture shows a specimen with a notch and a sharpened crack tip. So this will allow you to calculate KI and KII in Ansys. However, in your second picture you show a specimen with a V-notch. In Williams original paper (1952), the asymptotic derivation is for a notch where a crack is a special case with notch angle of 0. In case of a V-notch, it would be more appropriate to use Generalized SIF, since you cannot split the SIF's into individual components. You also need a crack initiation criterion to predict when fracture is going to happen. One such criterion is the Leguillon's criterion which would be useful in your case.
-Arun
Hi,
Thanks Arun for your comments.
As you said, can I use KII=P*F(a/w)/BW^1/2 to calculate experimentally the KII value.This eqn is not valid for crack.
But in ansys, I have used Generalized SIF eqn. as KII=τ*sqrt(2PI*a). Is this correct.
Thanks.
Generalized SIF
You can use an equation or even Ansys to calculate the value of KII for a crack. However, in case of a notch, you don't have a simple formula to calculate the 'stress intensity factors' as in this case SIF is expressed as K=K1+iK2. Again, it depends on what you want to calculate. If you want the crack initiation load, you should use a criterion to predict the onset. I recommend you read this very famous paper (Hutchinson and Suo, 1992 ) to get a good understanding of the problem. Maybe others can also chip in with their thoughts.
-Arun