As I stated before, there are always some cracks, but they are normally really small, just in the order of microns and only detectable by special inspection methods. They do not reduce strength of the spar, because metal has the ability to ´bridge´ such cracks by deforming plastically at the crack tip. Nevertheless such cracks grow with flight numbers, most of them so slow that they never become critical.
One interesting example of aircraft fatigue cracking is the
Lusaka Accident where a 707 lost its stabilizer due to fatigue cracking of a spar cap.
´The total number of flights between the initiation of the fatigue crack and the final failure of the upper chord is estimated as being in the order of 7,200 flights with some 3,500 flights of propagation occurring across the exposed top surface of the rear chord´
This means the crack was existing a few 1000 flight cycles before it became visible (but was not detected), and afterward grew further for thousands of flights before total failure of the spar occured.
You don´t worry about the thousands of holes every wing spar has (rivet holes), so don´t worry about the hundreds of cracks that are much smaller than the holes
