1. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE MECHNICAL DEPARTMENT
5th semester
DESIGN OF MACHINE ELEMENT
Stress Concentration & fatigue failure
PREPARED BY:
Nakrani ankit.H( )
Rathod himat.R( )
Vadhel Hitesh j.( )

2. Fatigue Failure
It has been recognized that a metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures.
Fatigue failure is characterized by three stages
Crack Initiation
Crack Propagation
Final Fracture

3. Jack hammer component, shows no yielding before fracture.
Crack initiation site
Propagation zone, striation
Fracture zone
MAE dept., SJSU

4. VW crank shaft – fatigue failure due to cyclic bending and torsional stresses
Propagation zone, striations
aCrack initiation site
Fracture area

5. 928 Porsche timing pulley
Crack started at the fillet

6. Fracture Surface Characteristics
Mode of fracture
Typical surface characteristics
Ductile
Cup and Cone Dimples Dull Surface Inclusion at the bottom of the dimple
Brittle Intergranular
Shiny Grain Boundary cracking
Brittle Transgranular
Shiny Cleavage fractures Flat
Fatigue
Beachmarks Striations (SEM) Initiation sites Propagation zone Final fracture zone

7. Fatigue Failure – Type of Fluctuating Stresses
max
min 2
Alternating stress
Mean stress
m =
max
min 2 +
MAE dept., SJSU

8. Fatigue Failure, S-N Curve
Test specimen geometry for R.R. Moore rotating beam machine. The surface is polished in the axial direction. A constant bending load is applied.
Motor
Load
Rotating beam machine – applies fully reverse bending stress
Typical testing apparatus, pure bending

9. Fatigue Failure, S-N Curve
Finite life
Infinite life
S′e
= endurance limit of the specimen Se ′

10. Concept of Stress Concentration
Theoretical stress concentration factor, Kt
Maximum stress at the discontinuity
Nominal stress, max stress with no discontinuity
Kt is used for normal stresses and Kts for shear stresses.

11. Concept of Stress Concentration
Consider an elliptical hole in an infinitely large plate loaded in uniform tension. Using the theory of elasticity the theoretical stress concentration factor is given by the equation below. 2b 2a
Elliptical hole
For a circular hole Kt = 3
The equation can be applied to a longitudinal crack where b << a

12. Stress Concentration Factor
Stress concentration factor is found using experimental methods.
Photoelasticity – a plane polarized light is passed thru a photelastic material (all transparent plastics) resulting in a colorful fringe pattern indicating the intensity of the stress.

13. Stress Concentration Factor
Brittle Coating – a specially prepared lacquers are usually applied by spraying on the actual part. After air drying, the part is subjected to stress. A pattern of small cracks appear on the surface. Data could be used to locate strain gages for precise measurement of the stress. The method is sensitive to temperature and humidity.

14. Stress Concentration Factor
Electrical strain gauges
The method is the most popular and widely accepted for strain measurements and stress analysis. The strain gauge consists of a grid of strain-sensitive metal foil bonded to a plastic backing material. When the gauge is subjected to a mechanical deformation, its electrical resistance changes proportionally. The change in voltage is converted to strain and the stress is calculated from the strain.
Ken Youssefi

15. Stress Concentration Factor

16. Thank you