Aircraft undergo complex loading during their operation and lifecycle. For example, take off, landing, turbulence, and flight/ground maneuvers are all instances where significant loading occurs. The cyclic loading and unloading activates a failure mechanism called fatigue, which is most prevalent at the highest stressed regions.
Many aircraft are assembled using riveted construction, whereby small holes (i.e., fastener holes) are drilled in the exterior panels and underlying structure during assembly. These fastener holes are stress concentrations, which is where fatigue cracks often form. These fatigue cracks will continue to grow during subsequent load cycles until they reach a critical size, at which point the loading can no longer be supported and fracture will occur.
The surface created as a result of a part failing (i.e., fracture surface) contains useful information about the material and the events leading up to the failure. For example, the image below shows one half of a test specimen after fracture and fatigue.
The test specimen contained a typical aircraft fastener hole in the center. A fatigue test was performed on the specimen using a fatigue testing machine (i.e., cyclic loading and unloading). During the fatigue test a fatigue crack grew until it reached a critical size, then the specimen quickly broke in half (fracture). Following the fatigue test, a failure analysis can be performed to examine the details of the fracture surface.
The fatigue crack on the specimen is identified as the lighter gray portion (right of center). The largest load cycles create distinctive markings on the surface (i.e., beach marks), which help illustrate the shape of the crack at a single point in time during the fatigue test. By tracing these beach marks back in time (like counting tree rings), it is possible to determine that a fatigue crack grew from the location where the fastener hole met the countersink.
Fatigue analysis methods are used to estimate the time for a crack to grow to a critical size. This information helps plan inspection schedules and ensures the safe operation of the aircraft.
If you have any questions about fatigue analysis or fatigue tests, please contact us.
Representative fracture surface showing fatigue cracking