Question

Q8. Using a machine element , describe fatigue in engineering materials . Why is fatigue loading more dangerous than normal loading. As a design engineer, how can you prevent failure by fatigue.

Answer 1

Fatigue loading is the changes observed in a material under the influence of stress generated during cyclic loading. It is represented by plotting stress cycle curve ( S-N curve) where S represents the stress & N represents the number of cycles that caused failure.

There is proggressive and localized structural damage when fatigue loading occurs, and if the local stresses are high enough leads to initiation of crack, the growth of the crack and finally gets fractured.

In material science fatigue is the weakining of the materials by continious loading until it experince a crack. In a corrossive enviroment if a material eperineces fatigue loading, it leads to corrossion fatigue due to combined action of alternating stresses and corrossive environment. This process ruptures and passes the passive protective film, after which corrossion is acclerated. If the loads are above the certain threshold microscopic carcks forms at stress concentrators such as surface, persistent slip bands, grain interference. Eventualaly when the crack reaches the critical size tje crack generates suddenly and structure fracture.

Fatigue is responsible for majority of engineering failures. The most common outcomes are circuit board , engines and many industrial applications which happens by thermal fatigue.

Thermal fatigue is caused by thermal stresses produced by cyclic changes in tempreatures.

Fatigue loading is more dangerous because it can result in sudden catastrophic failures after years and years of maintaince and time to time servicing.

We can prevent material from fatigue failure by keeping these in minds:-

(a). we need to cencentrate on the part or region of the design where stresses are more likely to be concentrated we analyse it and comeup with a better design overcoming the problem.

(b). Dimensions of the part, increase in size leads to lower fatigue limit.

(c). Improve the surface finish to the most extent you can bacuse polished mirror like surafces have a greater strength to withstand stresses.

(d). Avoid structural irregularities by revising desingns, square edges will lead to higjher fatigue strength.

(e). Endurance limit of the material