FATIGUE MECHANICAL LIFE DESIGN-A REVIEW-1

 

FATIGUE MECHANICAL LIFE DESIGN-A REVIEW-1

 Abstract

Fatigue is due to cyclic loading and unloading of one kind or the other. It is due to the presence of discontinuities in the material. Mostly fatigue failure is progressive and plastic in nature. It is due to the nucleation, growth and propagation of a micro crack at the point of a discontinuity. There are materials having unlimited fatigue life (plain low carbon steels) as well as limited fatigue life (nonferrous as well as ferrous materials). Fatigue is mostly due to tensile stresses and is random as well as sudden without any warning.

90 % of the service failures are due to fatigue. Lot of work on fatigue failures has already been done and is still continued because of very complex nature of fatigue failures which result in loss of life and property. Fatigue failures thus must be avoided by a proper selection of material, surface finish, stress raisers, residual stresses, reliability, surrounding  environment and temperature as per type the cyclic loading and unloading.

However, fatigue can be reduced by proper selection of fatigue resistant material like composites, by drilling a hole at the point of a probable crack, use of laser peeing and high frequency mechanical impact (HFMI) treatment of welds. Stress fatigue and strain fatigue life approaches have been used for plastic and elastic deformations respectively. This short review paper cannot treat the vast subject thoroughly and the reader is advised to consult more references for additional knowledge.

Introduction

Under cyclic loading and unloading, failure is due to fatigue. Fatigue/endurance limit (σe) represents a stress level.  Below which the material does not fail even after infinite number of cycles. Fatigue is reduction in strength due to a progressive and localized structural damage. Fatigue takes place in a moving component only. For example, in automobiles, ships , aircraft wings and nuclear reactors, jet engines, and turbines.

Fatigue was initially recognized in early 1800 in Europe. It was observed that bridge and railroad components were cracking subjected to repeated loading[1-10].  Three basic factors to cause fatigue are: (1) a sufficiently high tensile stress, (2) a large variation in the applied stress, and (3) a sufficiently large number of repetitions in loading and unloading.

The nominal maximum stress which causes fatigue is much less than the ultimate tensile strength of a brittle material. It is less than the yield stress of a ductile material. If the stress present is above a certain threshold value, microscopic cracks will start at the points of stress concentrations. For example, like a scratch, key way, square holes or sharp corners. The crack then travels along weaker points.  Ultimately it results in a fracture. Fatigue is thus a progressive plastic failure. This phenomenon occurs in three phases namely

(i) crack initiation

(ii) crack propagation

(iii) catastrophic overload failure

There are two types of materials experiencing fatigue. One type which has a fixed endurance limit as plain low carbon steels. These steels do not undergo fatigue even for infinite life. It happens when  the actual stress present in the component is slightly less than the fatigue limit. There are also brittle or ductile materials which do not have a fixed fatigue limit. For example, Cast iron, Copper, Aluminum and their alloys. The design for such materials for a fixed number of cycles 5 x 108(500 million cycles). If the component has 750 RPM with one reversal per cycle, it will have a life of about four years. If the RPM increases, life will reduce [1-16].

Thus importance of fatigue is that it directly governs the useful life of a component under cyclic loading. Lots of research has been carried on fatigue. It is because number of well-known catastrophic fatigue failures which took place all over the world. Fatigue failures must be avoided by a proper selection of material. Surface finish, stress raisers, residual stresses, reliability, surrounding  environment and temperature help in the selection of material.

Salient features of fatigue include randomness and sudden failure without any warning. It is mostly due to tensile stress and the presence of a stress raiser. This is strongly affected by the surrounding environment, temperature, surface finish and residual stresses. Fatigue can be reduced by proper selection of fatigue resistant material like

(i) composites

(ii) drilling a hole at the point of a probable crack

(iii) use of laser peeing and use of high frequency mechanical impact (HFMI) treatment of welds.

Out of different fatigue design approaches, stress life and strain life has been used for plastic and elastic deformations respectively [17-24].