Fracture Mechanics - Crack Propagation and Stress Intensity Factors

Main Question or Discussion Point

I am trying to determine the critical crack length of a part using finite element analysis. The software I am using (ABAQUS) has the ability to perform fracture analysis. I model the part, create a crack, and ABAQUS will output the stress intensity factors (modes I, II, and III) and contour integrals in the area of the crack tip.

Theoretically, in order to determine the critical crack length, I would extend the length of my crack to see how it affects the stress intensity factor. Once the stress intensity factor exceeds the fracture toughness of the material, I'd consider that my critical crack length.

However, I never observe the stress intensity factor exceed the fracture toughness. Initially, the stress intensity factor is increasing as I grow the crack. As I continue to grow the crack, the stress intensity factor reaches a peak and then starts to decrease without ever having exceeded the fracture toughness of the material.

Does this indicate that the crack is arresting and there won't be failure due to the crack?
 

Answers and Replies

Astronuc
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I am trying to determine the critical crack length of a part using finite element analysis. The software I am using (ABAQUS) has the ability to perform fracture analysis. I model the part, create a crack, and ABAQUS will output the stress intensity factors (modes I, II, and III) and contour integrals in the area of the crack tip.

Theoretically, in order to determine the critical crack length, I would extend the length of my crack to see how it affects the stress intensity factor. Once the stress intensity factor exceeds the fracture toughness of the material, I'd consider that my critical crack length.

However, I never observe the stress intensity factor exceed the fracture toughness. Initially, the stress intensity factor is increasing as I grow the crack. As I continue to grow the crack, the stress intensity factor reaches a peak and then starts to decrease without ever having exceeded the fracture toughness of the material.

Does this indicate that the crack is arresting and there won't be failure due to the crack?
Or the constitutive model is wrong, or the model for stress intensity factor is incorrect, or ABAQUS code is wrong.

One needs to compare the calculation against a real experimental measurement to see if the local plastic strain is correct, and if crack propagation rates are correct.
 
Yea, I realize a real world test would be the best option at this point. Unfortunately, the people with the money make the calls.
 

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