Aluminum wing spars, like other aluminum structures, have no fatigue limit. However, they are designed using finite element analysis to determine the probability of failure over time so that some minimum spar lifetime is pretty much guaranteed.

Likewise, aluminum fuselages of pressurized airplanes are designed to last a certain number of pressurization/depressurization (ground-air-ground) cycles after which they are usually retired from service.

In fact, the FAA initially "life limits" many new aircraft designs until the design has been in service long enough and examples of the design approaching the initial life limit have been thoroughly inspected for fatigue to justify extending the life limit. A good example is the new Eclipse 500 Very Light Jet, designed for a fatigue life of 20,000 airframe hours and has an initial life limit of 10,000 hours (see Eclipse Press Release announcing completion of static testing).

The wing center section of the U.S. Navy E-2C aiframe has a fatigue life limit of 11,450 flight hours.

The USAF F-15 Eagle fleet was grounded for two months after an accident caused by fatigue failure of a critical upper longeron structure, and during inspections "Time compliance technical order inspections have discovered nine other aircraft with longeron fatigue-cracks. Additionally, approximately 40 percent of inspected aircraft have at least one longeron that does not meet blueprint specifications."

Aircraft Structural Design