**6. Factors affecting material fatigue properties**

The fatigue behavior of the material is very sensitive to design and structure. Three very important factors that affected fatigue properties are the stress concentration, the residual stresses and material selection.

#### **6.1 Effects of microstructure and material properties**

The microstructure significantly affects the fatigue properties [52]. It was found that any changes in the microstructure altering the fatigue behavior especially in the case of high cycle fatigues. Decreasing in grain sizes and increasing in density of dislocation also noticeably improved the fatigue lives. In brass alloys an increase in

fatigue lives observed by cold working and increasing in the dislocation density [1, 2]. The analyses carried out after Northridge earthquake on material consumables showed that the fracture toughness levels of some of electrode materials were very poor and this has been a strong reason for the decrease in fatigue properties of metal structures during these events [53].

In metals, reducing the size of inclusions and impurities significantly increases the fatigue properties. It has been well accepted that second-phase particles in the microstructures play a major role in the fracture of steels and failure resistance can be improved through changes in the volume fraction and morphology of these particles [54–57]. These particles are the centers of stress concentration and cause a decrease in the fatigue properties of the material [58]. Heat treatment is an effective factor in affecting the microstructure and improving fatigue properties.

#### **6.2 Effects of surface**

The source of all fatigue failures is the surface of the components. There is much evidence that fatigue properties are highly sensitive to surface conditions. Surface factors that affect the fatigue behavior consists of surface roughness, changes in the surface properties, and residual stress.

**Surface roughness:** Tests performed on metal samples have shown that the smoother the surface of the parts, the longer their fatigue life in the test [59]. This is due to the fact that local superficial scratches are the stress concentration points and the onset of fatigue cracking.

**Surface properties:** Because fatigue failure is highly dependent on surface conditions, any factor that affects the surface strength also affects its fatigue properties [60, 61]. For example, surface heat treatment of carbonation and nitration, which increase the surface hardness, improve the fatigue properties. On the other hand, carbonation operation, which reduces the surface hardness of the part, reduces the fatigue properties.

**Surface residual stress:** Residual stress is a type of stress that remain in a part after manufacturing processes even without supplementary thermal gradient and external loads. In welded parts due to local heating during welding, complex thermal stress produces during welding which led to residual stress and distortion in component [62–64]. Residual stresses are also created by deformation of formwork and fabrication. The residual stresses are combined with the applied stresses and in the tensile state reduce the fatigue life of the part during dynamic loadings [2]. It is important to note that these type of stresses in the compressed state can increase the fatigue life of components and structures. In fact, there are commercial methods such as ball bearing and surface rolling that produce compressive residual stress and are used to improve fatigue properties [65–69].

#### **6.3 Notch effects**

The manufacturing defects is a factor that produced stress concentration point and reducing the fatigue properties of a material [25, 70, 71]. Investigations on fracture of steel structures in Kobe and Northridge earthquakes have clarified the fatigue brittle fractures triggered by the crack-like defects in the weld metal [53]. In addition, device components usually have stress concentration areas such as fillets, grooves, keyways, and holes which called stress raisers. These areas generically termed notches for brevity and usually reduces the resistance of the equipment to fatigue failures.

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

**Figure 16** provides an example of a notch in a machinery equipment, in particular, the attachment of blade to shroud in a CO2 compressor. Despite carful design to minimize the severity of the notch, a fatigue crack led the equipment to premature failures. Another example is given in **Figure 17**. This is a fracture in beam to column steel structure under seismic loadings during Northridge earthquake where the source of fracture is a notch in welded part (lack of fusion) [72]. Stress raisers may also be due to metallurgical defects such as porosity, impurities, and defects due to crushing and surface decarbonization due to working at high temperatures [71, 73].

Stress intensity factor, *Kt* is a parameter that characterizes the degree of severity of a notch or stress concentration point [1]:

$$K\_t = \frac{\sigma}{\mathcal{S}} \tag{16}$$

Where *σ* is local notch stress and *S* is the nominal stress.

**Figure 16.** *Fatigue failure of impeller of a compressor due to presents of a notch.*

**Figure 17.** *Effects of notch on fracture of steel structures under Northridge earthquake [72].*

#### **Figure 18.**

*Effects of notch on S-N behavior of an aluminum alloy (Kt is estimate of fatigue life and Kf is data's obtain by test) [74].*

On a plot of S versus life Nf, the fatigue life decreases in proportion to *Kt* factor as presented in **Figure 18**.

## **7. Conclusion**

Cyclic loads may lead the machines and structural components to premature failure that is called fatigue. Concern about fatigue failure is due to the fact that it occurs at a stress level much lower than the ultimate strength and in a completely unpredictable way. Macroscopically fatigue failure is seen with a brittle appearance and without any gross deformation in the fracture area. Fatigue failure can be occurred in form of high cycle, low cycle, and extremely low cycle fatigue. There are metallurgical and mechanical parameters that affect the occurrence of fatigue failures. Hostile environment causes corrosion fatigue and decreases the operation life of the components. Presents of notch causes stress concentrations points and accelerated the fatigue failures. Residual stress in the tensile form reduces the fatigue life while in the form of compressive stress increases the life of components.

## **Author details**

Alireza Khalifeh Shiraz Petrochemical Complex, Shiraz, Iran

\*Address all correspondence to: ar.khalifeh@spc.co.ir

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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