Dynamic J-integral, help!!!!!!!!!

Dear All members;

I am derived J_integral for dynamic crack propagation, and i have some things that i not yet understood:

1. Why is the integral path must start on one crack sureface and proceeds counter-clockwise to the oposite crack sureface.

2. Why is the value of J_integral independent the radius of integral path at each time increment.

3. What is the difference if we chose the far path and near (close to) path from the crack tip.  

Would you please explain for me, thank you very much,

Best Regards,

Chau. 

Dear All members;

I am derived J_integral for dynamic crack propagation, and i have some things that i not yet understood:

1. Why is the integral path must start on one crack sureface and proceeds counter-clockwise to the oposite crack sureface.

2. Why is the value of J_integral independent the radius of integral path at each time increment.

3. What is the difference if we chose the far path and near (close to) path from the crack tip.  

Would you please explain for me, thank you very much,

Best Regards,

Chau