Mechanism of fatigue failure, fatigue design criteria, fatigue testing

1. FATIGUE OF MATERIALS
Dr. RUDRESH M
ASSISTANT PROFESSOR
DEPARTMENT OF AERONAUTICAL ENGINEEIRNG
DAYANANDA SAGAR COLLEGE OF ENGINEERING

2. FATIGUE
The process of progressive localized permanent structural change occurring in a
material subjected to conditions that produce fluctuating stress and strains at some
point or points and that may culminate in cracks or complete fracture after a sufficient
number of fluctuations.
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

3. STRATEGIES IN FATIGUE
DESIGN
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

4. FATIGUE LIFE(DESIGN)
MODEL
STRESS LIFE MODEL:
• Oldest of the 3 models .
• Used for high cycle fatigue.
• It works only when load amplitudes are predictable and consistent over the life of part.
STRAIN LIFE MODEL:
• This model gives a reasonably accurate picture of the crack initiation stage.
• Combination of fatigue loading and high temperature are better handled.
• Most often applied at LCF.
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

5. FATIGUE LIFE(DESIGN)
MODEL
FATIGUE CRACK GROWTH
MODEL:
• This model uses fracture
mechanics
• This method is applied at LCF
• Most useful in predicting the
remaining life of cracked parts
in service
• Used in conjunction with NDT
especially in aircraft industry
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

6. FATIGUE DESIGN METHODS
IN HOUSE TOOL
• Component used where it was designed is called in house tool
• Designer knows the expected load time history to which part will be subjected
NEW MODEL
• Changing an existing product by making it larger or smaller than previous is called
new model
• More data should be available from previous models
• Broken part from previous models provide the most useful data
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

7. FATIGUE DESIGN METHODS
NEW PRODUCT
• Setting up a major project that is quite different from past practice.
• A space craft or an ocean drilling rig or a new type of tree harvester are examples.
We call it a new product.
• This requires greatest effort in fatigue design, predicting future loads.
DESIGN TO CODES
• Designing a highway bridge or steam boiler.
• The expected loads, acceptable method of analysis and permissible stresses are
specified by the customer or by a code authority. We call it design to code.
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

8. FATIGUE DESIGN CRITERIA
INFINITE LIFE DESIGN
• Unlimited safety is the oldest criterion
• For parts subjected to millions of cycles like engine valves springs, this is a still
good design criterion.
• This criterion may not be economical or practical in many design situations.(ex:
aircraft)
SAFE LIFE DESIGN
• The practice for designing for a finite life is known as “safe life design.”
• Here appropriate, regular inspections may not be practical or possible, hence the
allowable service life must be less than the test life or calculated life.
• Ball bearings and roller bearings are good example.
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

9. FATIGUE DESIGN CRITERIA
FAIL SAFE DESIGN
• Fail safe design requires that if one part fails, the system do not fail.
• Fail safe design recognizes that fatigue cracks may occur and structures are arranged
so that cracks will not lead to failure before they are detected and repaired.
• Engines are fail safe only in multi engine planes.
• A landing gear is not fail safe but it is designed for a safe life.
DAMAGE TOLERANT DESIGN
• This is a refinement of fail safe design’
• It assumes that crack will exist, caused either by processing or by fatigue and uses
fracture mechanics analysis and tests.
• The key items are needed for successful damage tolerant design are residual strength,
fatigue crack growth behavior and NDT.
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

10. MECHANISM OF FATIGUE
FAILURE
necking
void
nucleation
fracture
Crack
propagationCoalescence
of cavities
• Resulting
fracture
surfaces
(steel)
50 mm
particles
serve as void
nucleation
sites.
50 mm
From V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.),
Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton,
J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)
100 mm
Fracture surface of tire cord wire loaded in tension. Courtesy of F.
Roehrig, CC Technologies, Dublin, OH. Used with permission.
• Fatigue failure
1. Crack initiation
2. Crack propagation
3. Final failure
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

11. MECHANISM OF FATIGUE
FAILURE
• Fracture surface with
crack initiation at top.
Surface shows
predominantly dull fibrous
texture where rapid failure
occurred after crack
achieved critical size.
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

12. MECHANISM OF FATIGUE
FAILURE
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

13. FATIGUE TEST SPECIMENS
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

14. RUDRESH.M, DEPT. OF MECHANICAL, EWIT

15. FATIGUE TESTING
MACHINES
• Rotating bending machines
• Reciprocating bending type
• Axial direct stress machines
• Vibrating shaker machines
• Repeated torsion machines
• Multi-axial stress machines
• Computer controlled closed loop machines
• Component testing machines for special applications
• Full scale or prototype fatigue testing systems
RUDRESH.M, DEPT. OF MECHANICAL, EWIT

16. THANK YOU