What is fatigue deformation?
Fatigue is a plastic deformation of material caused by fluctuating or alternating stress or strain resulting in
localized, progressive microstructural changes which leads to nucleation and propagation of fatigue crack and
ultimately leads to failure/fracture of the material.
** During fatigue: Damage accumulates in every cycle over a large section of fatigue life and once the crack
nucleates and grows to its critical size the propagation happens rapidly.
Practical examples of Fatigue deformation
1.
Bi-cycle frames in motion experience cyclic loading
2.
Aircraft (landing gear, Fuselage)
3.
Bridge, Fly-over etc.
4.
Trains, Wheels, axels, railway tracks
*Fatigue can be both mechanical as well as thermal.
* Any component loading under cyclic stress will fail at stress level much lower than the monotonic tensile
strength of the material, even sometime it may drop to below the YS depending upon the application.
Characteristics of Fatigue in a material
1. It happens under repeated/ cyclic stress or strain and eventually leads to fracture.
2. Fatigue failure occurs without significant plastic deformation.
3. Happens at a stress value much lower than the tensile stress.
Factors affecting fatigue failure
1. A maximum tensile stress of reasonably high values as tension mode is a crack opening mode.
2. Significant variation in the applied stress range.
3. No. of cycles endured by the material. Higher the value lower will be the remaining life.
Factors accelerating fatigue failure
Stress concentration; Environment; Residual stress; Corrosion; Overloading; Microstructure
Fatigue failure occurs by-
1.
Nucleation and growth of a fresh crack
2.
Propagation of a pre-existing crack
Types of Fatigue testing
1.
Constant load: Stress controlled mode (use hour-glass shaped unnotched specimen for high cycle fatigue
(HCF) life determination).
2.
Constant displacement: Strain controlled mode (Use dog-bone shaped unnotched specimen for low cycle
fatigue (LCF) life determination).
3.
Constant stress intensity: K controlled mode (Use compact tensile (CT) specimen with pre notch to
determine the crack growth rate in cyclic loading.
Microstructural features of a fatigue fractured sample
1.
Rachets or beach marks can be observed in low magnification under optical microscope whereas striation
marks can be seen under high resolution microscope.
2.
Fatigue crack always starts from the surface, which is clearly visible on the fracture surface.
3.
The width of beach marks are a function of the loading frequency; lower the frequency higher will be the
width.
4.
The ductile fracture beyond the beach marks signifies fracture due to overloading.