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Fracture toughness of wood in mode II

Hi everybody,

I have done some experiments on the fracture toughness in mode II of wood specimens using attached geometry;so using

formula KIIc= 5.11P(3.1415*a)^0.5 /(2BW) I was able to calculate the frcature toughness of wood, but I am quite suprised

why this equation does not iclude depth of the specimens and moreover, I think that I have obtained higher values for the

fracture toughnes values. Is there any other formulation for obtaining the fracture toughness in mode II for this specimen?

P.s. dimension of my specimens is 100*100*63mm

Thank you,



Kejie Zhao's picture

Hi Parisa,

I am not very clear how the coefficients in the formula are derived. However from dimensional analysis, the only length scale appeared in the formula is the crack length and the material size is irrevelant. I think this is based on small scale yielding condition, meaning the material size is much bigger than the crack size, also the yielding regime around the crack tip is much smaller than the crack size. In your experiment, you probably need to check the validity of this condition.

BTW, there is a nice note from Prof.Suo, it would be helpful


Hi Kejie,

Thank you for your reply. I am wondering if I make a finite element model of my specimen because I have load which has been applied on the specimen; is it possible to obtain a  KIIc or not?The only ambiguity in my mind at the moment is crack length how can I handle this?

Thank you,


Dear Parisa,

We have recently developed an efficient method to measure Mode II fracture toughness in materials with preferred interfaces (like wood, bonded polymers, unidirectional composites). Our method is more efficient than previous methods for a variety of reasons:

1) We get a distinct failure load (no non-linear load-displacement curve) which is easy to obtain. This is different from other methods like ENF. 

2) We have no mode mixity. This is a pure Mode II test. 

3) More importantly, there is no friction between the crack lips in our test. Therefore, we get a more reasonable value of KIIC than previous tests. 

Our test is based on the popular Iosipescu shear test fixture. Take a look at this comment for more details.  You can look at our conference paper to ECF for further details. This test might be very well suited for your case. 


Hi Arun,

Thank you for your paper posted long time ago about mode II fracture toughness test with the subject:


I am wondering that may I use it for wood because the A and B factors in paper are very small. I would like to use for an example

 A= 75mm and B=150mm do you think that the prsented formulations will be still valid and  may I use of these values?

Thank you,


Hi Parisa,

Yes, this test will be valid for the values of A and B which you have given. As long as the moments of the loads along the center line are zero, our formulations will be valid. If you are testing wood, you should use an effective elastic modulus value (also shown in our paper), since wood is orthotropic. The values of A and B we have presented are for an Iosipescu fixture. So you can use values according to your needs and the formulas will still be valid.



What is the reffernce for the above mentioned formula used here for the value of mode II fracture toughness in terms of the geometry and crtical load?

Does any one know how to derive a formula KI and KII in terms of the specimen geometry and critical load?

I want to find a formula for composite samples using Iosipescu test and compact tension test (CTS). I have searched a lot for a sample of deriving formula for KI and KII but it was frustrating so far.

I wanted to add a picture of my samples under test but I was not able to upload the picture.



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