The donut cracks happen naturally when they fry. Generally, they are more cracked on the side that is initially down in the oil. If you overworked the dough, they won't crack as much. Each one cracks differently and to a different degree. Check your oil temperature and make sure the oil isn't too cold. If the donut dough is cold and the oil is right around 350°F, they will crack. Another possibility for donuts not cracking is that your baking powder is too old, and is no longer reactive.