An airplane in flight experiences a bending force on the wing as aerodynamic lift tries to raise the wing. This force of lift actually causes the skin on the top of the wing to compress and the skin on the bottom of the wing to be under tension. When the airplane is on the ground sitting on its landing gear, the force of gravity tries to bend the wing downward, subjecting the bottom of the wing to compression and the top of the wing to tension. [Figure 3-22] During the testing that occurs prior to FAA certification, an airplane manufacturer intentionally bends the wing up and down to make sure it can take the stress without failing.
When a shear stress is applied to an object, the force tries to cut or slice through, like a knife cutting through butter. A clevis bolt, which is often used to secure a cable to a part of the airframe, has a shear stress acting on it. As shown in Figure 3-23, a fork fitting is secured to the end of the cable, and the fork attaches to an eye on the airframe with the clevis bolt. When the cable is put under tension, the fork tries to slide off the eye by cutting through the clevis bolt. This bolt would be designed to take very high shear loads.
If the stress acting on an object is great enough, it can cause the object to change its shape or to become distorted. One characteristic of matter is that it tends to be elastic, meaning it can be forced out of shape when a force is applied, and then return to its original shape when the force is removed. When an object becomes distorted by an applied force, the object is said to be strained.
On turbine engine test cells, the thrust of the engine is typically measured by what are called strain gages. When the force (thrust) of the engine is pulling out against the strain gages, the amount of distortion is measured and then translated into the appropriate thrust reading.
A deflecting beam style of torque wrench uses the strain on the drive end of the wrench and the resulting distortion of the beam to indicate the amount of torque on a bolt or nut. [Figure 3-24]