Fatigue and Creep of Ma Mate teri rial alss CME 470 Physical & Mechanical Properties of Materials
J. Ernesto Indacochea University of Illinois at Chicago Civil & Materials Engineering Dept.
Fatigue Materials Materials
ultimately ultimately fail fail if exposed exposed to cyclic cyclic stresses stresses.. Degradation of mechanical properties occurs when loading is repetitive. repetitive. Stresses are below the yield. yield . Fatigue is most important in metals. metals .
https://www.youtube.com/watch?v=LhUclxBUV_E
Fatigue
Fatigue -- Characteristics Recall: Stresses are much less than the yield stresses ~ 90% of all metallic failure are caused by fatigue. Polymers and ceramics (except glasses) can fail by fatigue too. Process consists of crack initiation (originates at point of “ s tress concentration ”, such as corner, notch, metallurgical inclusion or flaw) & propagation. Fracture surface is normal to the applied stress.
Fatigue failure surface of a piston rod . Failure began at a forging flake near the center. Fatigue propagated outward slowly; outer circular region is final failure by brittle fracture. Final failure
Crack initiation
“Crankshaft failure”
Fatigue – Fracture Characteristics
Fatigue Fatigue
damage:
Accrues
with ongoing load application until cracks
initiate. Crack(s) Material
propagate(s). fractures.
fatigue properties are referred to as dynamic properties since the cyclic stresses and strains are applied continuously and at high rates.
The
Fatigue – Factors Considered for Testing Application
of fatigue in design
involves: Prediction
of the life (# of cycles) the material can sustain before cracks form.
The
life before one of these cracks propagates to the critical size.
The
fact that a critical crack size the design process involving fatigue also considers the fracture toughness of the material
Fatigue
Fatigue – Types of Fatigue tests The
fatigue properties of materials may be obtained
via: Constant Constant
stress amplitude test . strain amplitude test.
Fatigue - Constant Stress Amplitude Test Sinusoidal
wave with 2 reversals per cycle.
Terminology: Stress range, sr : s
sm ax sm in
r
Stress amplitude, sa: s s
a
r
2
s
max smin
2
Mean stress, sm:
s
s
m
m ax sm in
2
Stress ratio, R: R
s m in s m ax
Fatigue - Constant Stress Amplitude Test material with an applied stress at the fatigue limit has a 50% probability of failure .
A
This
value is used by designers; an empirical relationship for quenched and tempered steels: Fatigue Limit ≈ ½ sUTS Referred to as fatigue strengths for the corresponding life
Fatigue - Constant Stress Amplitude Test Common
modes for fatigue testing:
Push-pull The The
axial test.
https://www.youtube.com/watch?v=LhUclxBUV_E
rotating beam bending mode.
https://www.youtube.com/watch?v=xbpeNfJFtlI
flexural beam bending mode.
https://www.youtube.com/watch?v=DykiHVrVkKg
push-pull test has most conservative results, i.e., the lowest stresses, fatigue limit, and fatigue strengths for a given life N.
The
Thus,
it is safe to use the push-pull test results for any design application.
Fatigue Testing The
Determine the # of stress cycles at a given stress level a material can support before failure ( Fatigue life). Determine the stress level below which failure by fatigue is unlikely to occur ( Endurance or Fatigue limit )
The
goals of fatigue testing: unsafe
safe
Endurance limit
S-N Curve:
Test parameters:
Start test with relatively large max stress amplitude @ 2/3 of static tensile stress, then count # of cycles to failure
Repeat procedure always using a new sample and using progressively lower max. stress amplitudes.
Data plotted as stress amplitude S vs the log. of the # N of cycles to failure
Fatigue strength
unsafe safe
Fatigue Testing – Types of samples
Fatigue
Fatigue – Probability of Failure Curves
The S-N curves represented in the literature are normally average values.
Creep
Turbine rotors in jet engines & steam generators: components are exposed to static stresses at T high Material’s deformation under these circumstances is known as creep. Creep is undesirable and it is observed in all materials. For metals, becomes significant at T’s > 0.4 Tm.
Amorphous
polymers (plastics & rubbers) are especially
sensitive.
Failure due to creep. Stress rupture of a jet engine turbine blade.
Creep
Creep test involves a constant load (most tests) or stress at Tconst Deformation or strain is measured & plotted as function of time. Schematic of a creep curve:
Creep
Primary or transient creep: continuous decreasing creep rate. Material experiences strain hardens & thus increases its creep resistance.
Secondary creep: rate is constant. Often the stage of longest duration. Balance of two processes strain h ardening & recovery. Steady state creep, is the design parameter for engineering long life applications
Tertiary c reep: acceleration of the creep rate. Microstructure & Metallurgical changes occur: grain boundary separation, internal cracks, voids & cavities.
Creep Stress
& Temperature Effects:
inst antaneous strain increases. steady-state creep rate increases. rupture lifetime is diminished
Creep
Creep rupture tests:
Short-life creep tests, Rupture lifetime.
Presentation of creep rupture tests.
Stress (logarithmic scale) versus rupture lifetime (logarithmic scale) for a low carbon-nickel alloy at 3 temperatures.
Creep Data
Extrapolation Methods:
Larson-Miller parameter:
LM T (C log t r ) o
C= const, about 20; T= °K; tr = rupture time in hours
Data plotted as
vs L-M parameter.
Larson-Miller Parameter
DESIGN EXAMPLE
Using the Larson-Miller data for S-590 iron (right Figure), predict the time to rupture for a component that is subjected to a stress of 140 MPa (20,000 psi) at 800ºC (1073 K). SOLUTION From Figure, at 140 MPa (20,000 psi) the value of the Larson-Miller parameter is 24.0 x 103, for T in K and t, in h; therefore, 24.0 x 10 3 = T(20 + log t,) = 1073(20 + log t,) and, solving for the time, 22.37 = 20 + log t,
t = 233 h (9.7 days)