It is a sure bet that most of us have felt the effects of fatigue at one time or another. If I had to explain why I felt fatigued, I would say that the pressures and stresses of my job were bearing down upon me over a sustained period of time. Some days the stress was heavy and some days not so much but the continued cycling of the stresses combines, over time, to make me feel fatigued.
Many of the parts that we put into airplanes are designed with that situation in mind. There are two types of load stresses acting on aircraft structures; static load and dynamic load. Static strength is the ability of a part to resist a very high applied load only one time. Dynamic strength is the ability of that part to resist an applied fluctuating load of lower intensity many times.
Some of you may have flown on an airliner and noticed the wings fluctuating up and down slightly as the plane flew along. The wings have to be able to yield to the many forces that influence them. Each one of these fluctuations creates a dynamic load on the parts in the wing and it is the challenge of engineers to decide what forces the wing will see, how much the wing must yield to the changing force applications, and how many times the collection of parts in the wing will survive the dynamic load before they become “fatigued”.
In order to determine this, we “fatigue test” parts. A fatigue test is basically a long term tension test using lower loads than the part could sustain in a static environment. For fasteners, we put the bolt in the fatigue testing machine and grab on to the head and the thread of the part and pull on it with predetermined loads. We set the machine up to apply a “high load” which is usually somewhere around 37% to 45% of the ultimate load the fastener is expected to bear under static conditions. We then alternate that “high load” to a “low load” which is typically 10% of the “high load”. One application of both the high and low load is called a “cycle” and the machine applies these cycles very fast. Look at this in terms of a stressful job that might have you so fatigued – you have bad day on Monday and a somewhat stressful day on Tuesday followed by another bad day on Wednesday. If this “cycle” continues then there will likely be a failure.
We can easily illustrate fatigue by using something that is probably littering your desk right now. Grab a paperclip. You can bend it by hand. It was designed to be somewhat flexible. It expands when you slide it over the papers and then contracts to grip the papers – like the wing of the airliner we talked about earlier. You can imagine a tool that could apply enough load on the paperclip (ultimate static strength) to snap it in half but you cannot do it with your fingers. Grab one end of the paperclip and bend it out. You have just applied a dynamic load. If you keep bending the wire it will get easier and easier to bend until the wire breaks. You have just fatigued the metal to the failure point using far less load than it would have taken to cut the wire.
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