Crack growth data from dynamic tests under contact loading?

Bending and contact strength tests at different loading rates were carried out on a commercial alumina. From the comparison of the two strength tests, it was found that the investigated alumina does not exhibit any subcritical crack growth under contact loading, although a strong subcritical crack growth effect is obvious in bending tests. The different behaviour can be explained as the consequence of mode-II failure accompanied with KI<0 in the contact strength tests and of pure mode-I failure (i.e. KI>0) in the bending tests.