# Core strength of inner and outer race

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The ultimate strength of the inner and outer ring core and the corresponding hardness depth are used for a check of case core interface. The actual shear stress at hardness depth is compared with the shear yield stress tyield and the shear fatigue strength ta which are calculated according to [Harris](bibliography.md#harris_rumbarger_butterfield_dg03_2009) from the input value of ultimate strength:

$\tau_{yield} = 0.425\bullet R_m ; \tau_{a} = 0.6\bullet\tau_{yield}$

If the material is though hardened, the software can be set to calculate the hardness depth automatically. Then no warnings will occur, just a larger hardness depth is calculated on load increase.