Unit 5 - Fatigue Flashcards
What are the characteristics of fatigue failures?
Occur without obvious warning, resulting in a brittle appearing fracture.
No gross deformation at fracture.
Macroscopic fracture surface usually normal to principal tensile stress.
How is a fatigue failure recognised and what are its stages?
Recognised by smooth region (crack propagation) and rough region (ductile failure).
Three stages: 1) Initiation, 2) Fatigue Crack Growth (two distinct stages), 3) Overload/Failure.
What factors are necessary to cause fatigue failure?
(1) Maximum tensile stress of high value.
(2) Large variation/fluctuation in applied stress.
(3) Large number of cycles of the applied stress.
What are the predominant control modes for a fatigue test?
Load controlled (Loading profile constant, Compliance varies).
Strain (or extension) controlled (Compliance constant, Loading profile varies).
What types of fatigue tests are there?
SN – Counting cycles to failure at given stress/strain.
Fatigue crack growth – Variability in FCG rate across materials/environments.
Notched fatigue performance - Effect of Kt on fatigue life.
Threshold tests – Lowest stress intensity for crack growth.
What is the S-N curve and its significance?
Shows stress (S) vs. number of cycles (N) to fatigue failure.
Log scale for N, various stress values plotted.
Low Cycle Fatigue (LCF < 10^6 cycles), High Cycle Fatigue (HCF > 10^6 cycles), Very High Cycle Fatigue (VHCF > 10^9 cycles).
How is an S-N curve determined?
Test first specimen at high stress for short-cycle failure.
Decrease stress for each specimen until run out (no failure) in specified cycles.
Fatigue limit is the highest stress with run out.
What are the structural features of fatigue?
Crack initiation – Early development, removable by thermal anneal.
Slip-band crack growth – Stage I, deepening crack on high shear stress planes.
Crack growth on high tensile stress planes – Stage II, normal to maximum tensile stress.
Ultimate ductile failure – Occurs when crack length exceeds load-bearing capacity.
What are the key conclusions about fatigue loading cycles?
Defined through waveforms (sine, triangular, square, trapezoidal).
LCF characterized by gross plastic deformation, HCF by elastic behavior.
No definitive endurance stress; fatigue limit varies.
Mean stress effects characterized through the Goodman diagram.
Cyclic Stress Strain Curve (CSSC) represents material response to cyclic loading, not monotonic stress/strain.
