Fatigue of materials

Introduction to fatigue of materials, causes, identification by using S-N curve method.

1. Fatigue of materials

2. What is fatigue? It is a type of damage/failure in materials; Which is caused by fluctuating or cyclic stresses , Under the effect of fluctuating stress a point comes where the material is fractured. This level of stress is called fatigue stress and the failure is called fatigue failure . The fatigue stress is much lower than the yield stress or tensile stress.

3. Fatigue failure is caused by initiation and propagation of cracks and fracture surface is perpendicular to the direction of an applied tensile stress. It is brittle like in nature even in ductile materials (fracture without prior indication). Ductile fracture: the fracture caused after plastic deformation. Brittle Fracture: the fracture caused without any appreciable plastic deformation.

4. Cyclic stresses Three different stress-time modes are possible. The stress can be axial (tension – compression), bending or torsional in nature. =mean stress =range of stress = =amplitude of stress   Type (a) Type (b) Type (c)

5. Type (a) mode The amplitude is symmetrical about a mean zero stress level. Alternating from a maximum tensile stress to a minimum compressive stress of equal magnitude. It is called a reversed stress cycle .

6. R eversed stress cycle Type (a) mode ( reversed stress cycle .) Tensile Stress applied to a sample→ Released →Compressive Stress Applied →Released. (One cycle completes). Example: Bending load applied to a rotating shaft. (Axles of vehicles )

7. Repeated stress cycle Type (b) mode The maxima and minima are asymmetrical relative to the zero stress level . It is termed as repeated stress cycle . Example is cycling load on a cantilever beam, depending upon the residual stresses inside the material.

8. SPECTRUM LOADING Type (c) mode The stress level vary randomly in amplitude and frequency . Suspension wires in a railroad bridge. Wires carry weight of bridge—under static tensile load. Additional load when a train is on bridge .

9. The S – N curve The specimen is subjected to the cyclic stress at a large maximum stress amplitude . This large maximum stress is usually taken two third of the static tensile strength. Finally the number of cycles to the failure is counted . Then other specimens are taken of the same material and dimensions. And are subjected to the same procedure but at progressively decreasing maximum stress amplitudes. Data are plotted as stress S versus the logarithm of the number of cycles to failure for each specimen. The values of S are normally taken as stress amplitude ( or differently or values can also be used.  

10. The S – N curve The higher the magnitude of stress the smaller the no. of cycles the material is capable of enduring.

11. Fatigue limit For some ferrous and titanium alloys , the S – N curve becomes horizontal at higher N values. This is called fatigue limit or endurance limit . T his fatigue limit actually represents the maximum value of cyclic stress below which fatigue will not occur for an infinite no. of cycles. For many steels the fatigue limit ranges between 35% and 60% of the tensile strength.

12. Fatigue strength Many non ferrous alloys (Al, Cu, Mg) do not have a fatigue limit. Their S – N curve continues downward trend at increasing no. of cycles . Thus fatigue is inevitable for these alloys regardless of the amplitude of the stress. The fatigue response of these materials is specified by fatigue strength . It is defined as the magnitude of the stress at which failure will occur for some specified no. of cycles. (e.g., )  

13. Fatigue Life It is the number of cycles to cause failure at specified stress level.

14. Fatigue surface