Stress Concentration factors on the internal corner of a Stepped Shaft are well understood, and have some standardised calculations/equations: I'd recommend this excellent online calculator, which shows all the background equations further down the page. This allows you to calculate the stress concentration factor in various loading conditions.
You will see for yourself how reducing the fillet radius towards zero drastically raises the stress concentration factor, and you should try to remove the requirement for a 'sharp' corner if at all possible.
You get two effects near the area of sudden changes in cross section.
1 - Stress concentration and contour lines of stress distribution.
2 - Stress wave propogation.
Sharp corners are stress concentration traps. Anticipating the path of stress and desgning the part's geometry according to that helps.
Both of these items one and two affect the performance of the member significantly, specially under repetitive or shock loading.
There are computer simulation sofware that can analyze these up to a high level of precision.