The wings are the most relevant parts of an airplane. They maintain the flight by the reaction forces against the air. Now, talking about the wing as a structural part of a plane, this is taken to the limit. Civil structures are tested, the aerospace structures are subject to dynamic and static forces, sometimes, until they break.
The most particular test performed on a wing is called “wing-up bending test”, the wing is pulled up until they reach the structural limit. This allows us to observe the flex of the part. Remember the wing is one of the strongest parts of the plane, and in a mid-flight, it carries all the weight and even additional forces. These tests provide important data on how well the wing is designed, and proposes the improvements needed on the structure.
The wings on the planes are bent many times over a period of time. The repetitions are taken into account. Therefore, it is considered a fatigue test, and this shows how many flights to be operated during the life of the aircraft, simulating the landings and take-offs. Thus, the limit cycles of the wing or plane are determined, and in similar way some particular components of the plane.
An example with a Boeing 787, the manufacturer made the tests and one is called ultimate-load wing-up bending test. The wings on the 787 are flexed upward “approximately 25 feet”, a displacement beyond can be subjected on its operation.
So, the wing tests are a key on the airplanes design and is a standard procedure in the design phases. The next time you see the wings, be aware that these amazing parts are taken to the maximum and safest margins. They were tested multiple times in a controlled space by the manufacturer.