Question

Fatigue is a time dependent failure. Select a part of an aircraft that will be subjected to this mode and explain how this might be reduced or accommodated with design and maintenance. You must explain in detail the material properties affected and why.

Answer 1

Fatigue is a repeated process in which loading on any machine element is cyclic and gets repeated.

Air craft engine's propeller blade is affected by fatigue and repeatedly loaded during operation.

Stress concentration in a blade is one of the major cause of any component failure which is repeatedly loaded. This can be due to design errors or manufacturing faults. The material has to be free of any internal flaws. Thus, during design of blade one has to select material such that it should not contain any micro-structure flaws that reduces the overall strength of the component. The coating should be applied on blade otherwise corrosion will occur and its ultimately prone for the material loss and weaken the strength of blade. During operation, one has to make sure that no crack should get developed otherwise, due course of time, the crack widens or propagate and will lead to sudden failure of blade and hence, engine failure.

During design one has to ensure of the following:

1. Material selection with no internal micro-structural flaws. The yield stress of the material should be high enough to have strength against the loading during working of propeller.

2. No sharp edges should be there on blade geometry otherwise, it will lead to higher stress concentration

3. Proper selection of coating material for blade in order to avoid corrosion

During maintenance, one has to

1. Inspect each blade carefully and check for any crack or pitting

2. Coat the blade if coating weakens