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