**4. Fatigue features**

In macroscopic scale fatigue failure is seen with a brittle appearance and without any gross deformation in the failure. The fatigue failure surface usually consists of a smooth area due to crack growth and surface wear on each other, and a rough area formed when the load is unbearable [36]. Another characteristic of fatigue failure is the beach marks, fine and arch-shaped lines, that starts from the place of crack initiation and progresses to the area of ductile fracture as it is seen in **Figure 7**. They are also known as macroscopic striations.

It should be noted that striations are not always formed on the fracture surfaces. Inert environments, high strength materials for examples steels with hardness above 30 HRC, aqueous environments or high temperature air, creep fatigue condition, Low Δ*K* value for example stainless steel at Δ*K* <30*MPa* ffiffiffiffi *m*p Mode II and III fracture [36]. **Figure 8** shows a fatigue fracture of an AISI 316 L stainless steel in absence of striations.

**Figure 8.** *Surface fatigue failure of a duplex stainless steel due to high vibration in services.*

#### **Figure 9.**

*Microscopic striation in a fatigue fracture surface of a ductile material (a), brittle type of striations (b) [37].*

The microscopic evaluation of fracture surface is generally carried out with a scanning electron microscopy (SEM). The most important characteristic of fatigue fracture that is manifested in failure fracture surface is the presence of striations. Seriations are small groves extended perpendicular to the crack growth direction as seen in **Figure 9**. In general, striations indicate the growth rate of cracks in microscopic scales where each striation correspond to one load cycle. It is important to note that striation in brittle materials are different from ductile materials [36]. In ductile state the striation profile is wavy and smooth while brittle striations are irregular or saw tooth-like as seen in **Figure 9a, b**.
