# 9.8: Why bother if they are the same?

- Page ID
- 30054

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K is useful because stress intensity factors, like stresses, are additive. Irwin showed that any loading state could be broken down into 3 different types of loading that he called modes, I, II and III:

To get the total K, we just sum the contribution from each of the three modes.

This is much more difficult to do with energies, where we would have to think about interaction terms.