steel failure in the gripped area in fatigue test

Dear All

Have you run into the situation that the steel plate fails (through the thickness of 8mm) in the gripped area in the tensile fatigue test.

The recommended gripping force (pressure *piston area=1.15 of the test loading) checked and used but I still get this when the number of cycles approximate 1 million for the falied specimens. The test loading is far more than the fatigue limit of my material.

I  don't have any hole or defects in the solid flat gripped area. 

Thanks for comments and suggestion in advance