System and method for determining direct damage tolerance allowables

1. A method of determining damage tolerance allowables in a specimen, the method comprising:

preparing the specimen by inducing a notch located in an exterior corner thereof;

applying an initial cyclic load to the specimen, until a first crack emanating from the notch is detected, the initial cyclic load having an initial maximum load and an initial minimum load;

applying a subsequent cyclic load to the specimen until extension of the first crack to form a second crack emanating from the first crack is detected, the subsequent cyclic load having a second maximum load and a second minimum load, wherein the second maximum load is the same as the initial maximum load and the second minimum load is greater than the initial minimum load;

determining a flaw threshold stress based upon the initial maximum load and the initial minimum load for which the first crack starts to emanate from the notch; and

determining a crack threshold stress for which the second crack grows from the first crack based upon the second maximum load and the second minimum load.

2. The method according to claim 1 , wherein the specimen has a square cross-sectional portion and the notch is located in a corner of the square cross-sectional portion.

3. The method according to claim 1 , further comprising:

applying primary cyclic load to a specimen until a number of cycles reaches a predetermined number, the primary cyclic load having a primary maximum load and a primary minimum load.

4. The method according to claim 3 , wherein the initial maximum load and the initial minimum load of the cyclic load form an initial load ratio, and wherein the primary maximum load and the primary minimum load of the primary cyclic load form a primary load ratio.

5. The method according to claim 4 , wherein the initial maximum load is higher than the primary maximum load.

6. The method according to claim 4 , wherein the primary load ratio is equal to the initial load ratio.

7. The method according to claim 1 , further comprising: measuring a length of the first crack.

8. The method according to claim 1 , further comprising: measuring a length of the second crack.

9. The method according to claim 1 , further comprising:

determining a threshold flaw oscillatory load by adding the initial maximum load and the initial minimum load together and then dividing by two.

10. The method according to claim 9 , further comprising:

determining a threshold stress by dividing the threshold flaw oscillatory load by a plane area of the notch.

11. The method according to claim 10 , further comprising:

designing an aircraft structure based upon the threshold stress.

12. The method according to claim 1 , further comprising:

determining a threshold crack oscillatory load by adding the second maximum load and the second minimum load together and then dividing by two.

13. The method according to claim 12 , further comprising:

determining a threshold stress by dividing the threshold crack oscillatory load by a plane area of the notch.

14. The method according to claim 13 , further comprising:

designing an aircraft structure based upon the threshold stress.

15. The method according to claim 1 , further comprising:

determining a threshold flaw mean load by subtracting the initial minimum load from the initial maximum load and then dividing by two.

16. The method according to claim 15 , further comprising:

determining a threshold stress by dividing the threshold flaw mean load by a plane area of the notch.

17. The method according to claim 1 , further comprising:

determining a threshold crack mean load by adding the second maximum load and the second minimum load together and then dividing by two.

18. The method according to claim 17 , further comprising:

determining a threshold stress by dividing the threshold crack mean load by a plane area of the notch.