Metal Fatigue



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The dictionary defines fatigue as weariness from labor or use. No doubt we all have felt tired or fatigued at the end of a tough day, sometimes close to our breaking point. We want to stop, to give up, to relax. Strangely enough, metals exhibit this same sort of behavior, some sooner than others.

All metals are susceptible to some degree to fatigue damage or failure. True fatigue failures occur over relatively long periods of service life at stress levels higher than those normally considered during the design process. If you bend the wire of a paper clip back and forth, no apparent damage is done the first time or two. However, after repeated bending, the ductility of the wire is fatigued or exhausted, and breakage occurs. This is an illustration of very short-life, or low-cycle, fatigue failure.

Metal fatigue has been studied for well over 100 years. In 1869 A. Wohier, chief locomotive engineer of the Royal Lower Silesian Railways, Germany, discovered such important facts as: it is the number of stress cycles rather than the elapsed time of use that is important, and that ferrous materials can withstand an infinite number of stress cycles, providing the stresses are all below certain limiting values.

Modern fatigue studies took off, so to speak, in the aircraft industry. Aircraft manufacturing companies make extensive use of aluminum alloys in aircraft construction. Aluminum is very susceptible to fatigue damage at even low stress levels. Before fatigue was properly accounted for in design, the aluminum used to fabricate the body parts of the aircraft would very often mysteriously crack, especially with age, in wing and tail section areas where vibration and flexing are most exhibited. When engineers learned to keep the stress levels low in these areas and learned to minimize vibration or to design controlled vibration into these aircraft body sections, fatigue failures began to decline dramatically.

What does all this have to do with the car you drive back and forth to work, you might ask? Most, if not all, of the parts of today’s cars that might be susceptible to fatigue damage are made from medium- to high-strength alloy steels. These parts include the crankshaft, the connecting rods, the pistons, engine springs, and any other parts that exhibit back-and-forth or flexure motion or are repeatedly stressed and unstressed. As we stated before, as long as these parts are made of a ferrous material—steel in these cases—and stresses are held below a certain limiting value, fatigue failures should not occur. All well and good!

Now let’s suppose that you don’t take the advice given in Section 3 in regard to frequent oil changes. Let’s further suppose that you allow the oil to remain in the engine for 10,000 miles or more. That oil is going to be filthy, clogged with combustion products, and by then, probably void of any useful additives. In short, the ability of the oil to properly lubricate any of the moving parts of the engine is in serious question. Friction throughout the engine will increase significantly with subsequent additional loading put on the cooling system to remove the friction heat. As friction increases it will require more engine power to rotate the crankshaft, move the connecting rods, slide the pistons, etc.

As the engine strains to keep all these parts moving, it must apply more and more force on the parts as friction increases. As more force is applied to the moving parts to keep them working, the stress levels in those parts will rise. These higher stress levels are repeated with engine speed at up to 5,000 times per minute. Because these higher stress levels can easily surpass the limiting design value for infinite (long) life, fatigue damage will gradually occur and engine life will decline.

This material is included as a final effort to convince you to change oil regularly. It is so important to engine life that it cannot be overstated.

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