Perspective Chapter: Fatigue of Materials

*Alireza Khalifeh*

## **Abstract**

This chapter deals with the fatigue fracture of the materials under cyclic loadings. Components of structures and machines may be subjected to cyclic loads and the resulting cyclic stress that can lead to microscopic physical damage and fracture of the materials involved. It has been seen at a stress well below the ultimate strength, this microscopic damage can accumulate under action of cyclic loadings until it develops into a crack that leads to final separation of the component. In addition, the material inherently has cracks and other microscopic defects that grow due to cyclic loads and lead to fracture of machine or structure parts. The failures are more often sudden, unpredictable and catastrophic which may occur after a short period of design life. The main objective in writing this chapter is to present scientific findings and relevant engineering practice involving materials fatigue failures.

**Keywords:** failures analysis, fatigue fracture, cyclic loadings, machinery equipment, structures

## **1. Introduction**

It has been found that a metal subjected to cyclic stress will fail at a stress level much lower than that of a single application load. Fractures occurring under cyclic loadings are known as fatigue fractures [1, 2]. Indeed, one of the main reasons for unpredictable and premature material failures in service is the application of cyclic loads and the occurrence of fatigue [3–8].

Two event that caused a lot of human and financial losses due to fatigue were observed during the 1994 Northridge and 1995 Kobe earthquakes. Investigations have shown that cyclic loading of earthquakes alongside presents of high strain rates, notch and poor material properties were responsible for these premature failures in steel structures [9–11]. It should be noted that earthquake loads in the form of low cycle fatigue (LCF) and extremely low cycle fatigue (ELCF) caused the failure of steel structures [12–15].

Machinery equipment's such as compressors, turbines and pumps are more prone to this type of damage. Numerous destructions in these devices have been reported due to incorrect design or manufacturing defects and have caused loss of production and financial resources [16–19]. Failure of a Ti6Al4V alloy compressor impeller used in a petrochemical plant is shown in **Figure 1**. Investigations revealed fatigue has been responsible in the failure of compressor impeller. Stress concentration in the blade root causes the formation of fatigue cracks and final failure of the part [16].

**Figure 1.** *Fracture of a gas compressor turbine blade [16].*

**Figure 2.** *Fatigue failure of U-bolt of an elevator.*

Another practical example of fatigue failure is shown in **Figure 2**. This failure occurred in an AISI4140 steel material as a result of not considering the metallurgical parameters in the construction of U-bolts for a lift. Experience showed that surface modification technique is a suitable strategy for extending the life of U-bolts under cyclic loadings. The technique consisted of heating, quenching, tempering and transforming the initial ferritic/pearlitic microstructure to tempered martensite with a higher surface hardness. The idea was taken from the fact that surface hardening process produced a reduction in grain size, retained austenite level, compressive residual stress, and as a result significantly improves the fatigue limit of the low alloy

#### *Perspective Chapter: Fatigue of Materials DOI: http://dx.doi.org/10.5772/intechopen.107400*

steels [20, 21]. Thermo-chemical surface treatment such as carburizing and nitriding can also improve the fatigue properties of these steels [20, 22–24].

This type of failure is insidious because it led the equipment's to failure and plants to shut downs without any warning. Three main factors are necessary for this type of failure [2]:


Other factors such as stress concentration, overload, temperature, metallurgical structure, surface finishing, and residual stresses accelerate the occurrence of these type of fractures [2, 16, 17, 25–29].

The purpose of this chapter is to present the fatigue failure of materials and the methods of minimizing such damages for safety, durability and reliability of the products. To achieve this, the mechanical aspects of fatigue are explained first. Then, fatigue damage mechanism and fatigue futures are discussed. In the next step, the author is placed a focus on the types of fatigue failure and their characteristics by stating several practical examples. Finally, it has been dealing with factors that affecting material fatigue properties. It should be noted that the material in this chapter is based on our interaction with fatigue damages of components in the industry as well as based on that were taught university.
