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.).

Increases

*Dislocation Movement* _______________(Increases/Decreases) with *Increased Temperature* due to a process called *Dislocation Climb*.

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*.

Weakens

*Grain Growth* ______________(Strengthens/Weakens) the *Material*.

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.

Increase

*Vacancies*, *Diffusion*, and *Thermal Vibration* all ________________(Increase/Decrease) with *Increasing Temperature*.

Fatigue (Failure)

A type of *Failure* which occurs with *Repeated Cycles* of *Stress* that may be *Lower* than *Yield*.

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*.

Residual Compressive Stress

The *Compression* of *Cracks* on the surface of a material as the result of *Surface Coating* which will cause the *Cracks* to *Not Grow*, even when under load cycling.

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*.

Dislocation Climb

The process by which a *Vacancy* moves to the *End* of an *Edge Dislocation*, which, in effect, has caused the edge dislocation to *Shift Upwards* which may allow it to propagate easier.

Recovery

The process of *Annealing* at at *High Temperatures* which *Heals Imperfections* in the *Material*.

Carburizing

The process of *Diffusing Carbon* into a material, such as *Iron*, which can significantly *Strengthen* the material.

Case Hardening

The process of *Diffusing* an atom into the *Surface* of a host in order to strengthen only the *Surface*.

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.

Introducing

________________(Introducing/Removing) *Point Defects* and *Impurities* will *Strengthen* the *Material* by increasing the *Elastic Strain Energy* around the imperfection.

Increasing

__________________(Increasing/Decreasing) the *Angle of Misorientation* between *Grains* will *Strengthen* the *Material*.

Inhibiting

__________________(Inhibiting/Encouraging) *Dislocation Movement* will *Strengthen* the *Material*.

Reducing

____________________(Increasing/Reducing) the *Grain Size* will *Strengthen* the *Material* as *Grain Boundaries* are *Barriers* to *Slip*.