Final Exam: Fatigue, Creep
Greater
*Creep* will occur to a _______________(Lesser/Greater) *Extent* at *Higher Temperatures* because it is *Easier* for atoms to *Move Around* (Vacancies, Diffusion, Thermal Vibration, etc.).
Crack(s)
*Fatigue Failure* occurs because, over *Repeated Stress Cycles*, ____________ within the material will *Grow* and, eventually, may create a *Concentrated Load* that is *Greater than Yield* even with an *Applied Load* which is *Less Than Yield*.
Lowers
*Grain Growth* _______________(Raises/Lowers) the *Energy* in the material.
Surface
*Grinding*, *Polishing*, and *Surface Coating* can all *Increase* the *Fatigue Life* by reducing the number of *Cracks* along the material's __________________, which is usually where most *Fatal Cracks* form.
T(rue)
*True/False*: *Creep* can happen at *Stresses Below Yield*.
T(rue)
*True/False*: *Fatigue Failure* will offer *No Signs* of *Failure* beforehand.
fatigue process
1) crack initiation 2) slip-band crack growth (deepening of initial crack on high shear stress STAGE 1) 2) crack growth on planes of high tensile stress (growth of well defined crack in direction normal to max tensile stress STAGE 2) 4) ultimate ductile failure (crack reaches sufficient length so remaining cross section can't support load)
3 causes of fatigue failure
1) max tensile stress of high val 2) large fluctuation in applied stress 3) large number of cycles of applied stress
Fatigue (Failure)
A type of *Failure* which occurs with *Repeated Cycles* of *Stress* that may be *Lower* than *Yield*.
A cup-and-cone fracture is formed when a ductile metal is loaded in tension until failure
Cup and cone fracture
High
For *Most Materials*, *Creep* is only an issue at __________(Low/High) *Temperatures*.
Increases
If the *Applied Stress* _______________(Increases/Decreases), the *Fatigue Life* will *Decrease*.
Nf=
Ni + Nfcg
Secondary
The *Creep Rate* is determined by the _______________ *Stage* of a *Creep Test*.
Fatigue Strength
The *Maximum Stress* without *Failure* at a *Given Number* of *Cycles*.
Final (Stage)
The *Stage* of *Creep Failure* marked by *Crack Formation* and *Propagation* up to *Fracture*.
Primary (Stage)
The *Stage* of *Creep Failure* where the *Rate* of *Strain Slows Down*, and the material experiences *Strain Hardening*.
Secondary (Stage)
The *Stage* of *Creep Failure* where the *Strain Rate* is *Constant* and the system has achieved a *Balance* between *Strain Hardening* and *Recovery*.
Fatigue Limit
The *Stress* below which an *Infinite* number of *Cycles* can be applied without *Fatigue Failure*. This is a result of the Stress vs Cycle curve *Flattening Out*. (AKA Endurance Limit).
Creep
The *Time Dependent Permanent Deformation* of a material that occurs under *Constant Stress*.
Fatigue Life
The number of *Cycles* a material will endure at a *Given Stress* before *Fatigue Failure*.
Grain Boundary Slide
The process by which *Grain Boundaries*, under *Constant Stress* and *High Temperatures*, become *Weak* and start to *Slide* against each other. This leads to *Plastic Deformation*.
Grain Growth
The process by which *Larger Grains* will *Absorb Smaller Grains*.
Shot Peening
The process of *Shooting Particles* at the *Surface* of a material which causes *Plastic Deformation* and induces a *Compressive Stress * on any *Surface Cracks*. (AKA Sand Blasting).
Increase
To avoid *Creep* you should ________________(Increase/Decrease) the *Grain Size* to avoid *Grain Boundary Slip*, but you would normally want to do the opposite to strengthen the material.
Brittle and Ductile
Two types of Fracture modes
Fatigue and Creep
Types of process that produce fractures
Introducing
________________(Introducing/Removing) *Point Defects* and *Impurities* will *Strengthen* the *Material* by increasing the *Elastic Strain Energy* around the imperfection.
Inhibiting
__________________(Inhibiting/Encouraging) *Dislocation Movement* will *Strengthen* the *Material*.
ductile fracture
a mode of fracture that is attended by extensive gross plastic deformation; slow crack propagation
creep
a time dependent, permanent deformation at high temperatures, occurring at constant load or constant stress
unsafe
above the curve
tension-torsion
applies torsional movement on material to determine strength in torsion and tension ( catheter tubing)
3 types of applied stress
axial (tension-compression), flexural (bending), torsional (twisting)
types of fatigue experiments
bend tension-torsion fatigue crack growth compression
finer grain size
better S-N fatigue resistance
coarse grain
better at elevated temps where creep/fatigue can occur
elastic deformation
completely reversible
more grain boundaries
crack arrest and deflection, reduced crack growth rates
cssc
cyclic stress strain curve determined by connecting stable hysteresis loops from constant strain amplitude fatigue tests of specimens cycled at diff strain amplitudes higher than mssc in cyclic hardening
2 types of fracture
ductile or brittle
smooth region of fracture surface
due to rubbing action as crack propagates
how can striations be missed?
due to small spacing which can't be resolved or lack of ductility at crack tip to produce a ripple by plastic deformation
rough region of fracture surface
fails in ductile manner as cross section reduced, specimen can no longer maintain applied load
sigma e
fatigue limit for complete reversed loading (endurance limit- when graph is level)
intergranular fracture
fracture of polycrystalline materials by crack propagation along grain boundaries
transgranular fracture
fracture of polycrystalline materials by crack propagation through the grains
brittle fracture
fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation
R=-1
fully reversed
creep
happens when your at 40% of the materials melting point
R>0.5
high R
N>10^6 cycles
high cycle fatigue (HCF) elastic + local plasticity
fatigue limit
highest stress at which a run out is obtained
cold working=anisotropy=
increased S-N fatigue resistance when loaded in longitudinal
striation spacing
increases as crack advances
what happens as the R ratio becomes more positive?
increasing mean stress, fatigue limit becomes greater
total fatigue life process
initiation microstructure sensitive growth continuum growth overload failure
how does orientation affect cracking?
it directs the direction of cracking
decreasing peak stress
less damaging
N<10^6 cycles
low cycle fatigue (LCF) gross plasticity
beachmarks
macroscopic markings each beachmark band- period of time over which crack growth occurred can be thousands of striation in 1 beachmark
mean stress=
max stress+min stress ----------------------- 2
stress range=
max stress-min stress
stress amplitude=
max stress-min stress ----------------------- 2
stress concentration factor
max stress/applied stress
stress amplitude alternates about?
mean stress
what can influence S-N behaviour
microstructure, chemistry, heat treatment, grain size, anisotropy, porosity
R ratio for strain control=
min strain ----------- max strain
R ratio=
min stress ----------- max stress
how can fatigue resistance be increased further?
minimising defects andd discontinuities
fatigue crack growth
monitor rate and behaviour of crack growth from pre-exisitng defect
mssc
monotonic stress strain curve higher than cssc in cyclic softening
plastic deformation
not reversible
N=
number of cycles to cause complete fracture
stage 2 FCG
perpendicular to cyclic tensile stress crack then grows to critical size Kc - fracture toughness - rapid failure (microns per cycle) pattern of ripples/striations and beachmarks propagation by plastic blunting process a) crack tip sharp b) tensile applied, crack tip slips along 45 deg planes c) crack widens + grows by plastic shearing, tip gets blunter d) load changed to compression- slip direction reversed e) crack faces crushed together, new crack surface forced into plane of crack- buckles- resharpened crack tip f) ready for advance + re-blunt in next stress cycle
ductile fracture
plastic deformation prior to and during propagation of crack
brittle
rapid rate of crack propagation, no gross deformation and little micro-deformation
compression
simulates processes
stage 1 FCG
slip band opens 45 degrees crack propagates along persistent slip bands low propagation rate (nm per cycle) featureless
cyclic stress crack initiation
slip band with extrusions and intrusions slip planes on +/- 45 degrees
monotonic stress crack initiation
slip planes +/- 45 degrees
cyclic strain controlled fatigue
strain amplitude held constant during cycling need extensometer
fatigue altering conditions
stress conc, corrosion, temp, overload, metallurgical structure, residual stress, combined stress
what is plotted on an S-N curve?
stress v cycles to failure (N)
striation
successive position of an advancing crack front each produced by single cycle of stress defines failure produced by fatigue SEM, TEM form due to slip in crack tip plastic zone
run out
tests at low stresses, up to 10 million cycles
stress concentration
the concentration or amplification of an applied stress at the tip of a notch or small crack
safe
under the curve
N>10^9
very high cycle fatigue (VHCF)
R>0.9
vibration or flutter
R=0
zero to maximum