mae 381 exam 3 ch 8

What is a simple fracture?

The separation of a body into two or more pieces because of imposed stress that is

• static (constant)

• at temperatures well below melting point

How do ductile and brittle materials react differently?

• ductile fracture has significant plastic deformation

• brittle fracture has little to no plastic deformation

• catastrophic failure (no warning)

What makes a material moderately ductile?

• “cup and cone” fracture

• starts by necking

• void nucleation (multiple holes form)

• voids grow and join together

• crack forms from the hole

• fractures

What does cup and cone fracture look like compared to brittle fracture?

What are the characteristics of brittle fracture?

• Rapid crack propagation

• transgranular: fracture passes through the grains (does not follow grain boundaries)

• or intergranular: fracture propagates along grain boundaries (follows)

What is cleavage?

The repetitive breaking of atomic bonds along crystallographic planes

During brittle fracture, when does intergranular fracturing occur?

When grain boundaries are weakened or brittle

Provide an example where intergranular fracture occurs?

Hydrogen embrittlement: when hydrogen diffuses into the grain boundaries, causing it to crack and fracture at stresses below the yield stress

Describe the embrittlement process?

1. hydrogen atoms absorbed by carbon steel alloys either as atomic or molecular form

2. hydrogen diffuses to the metal grain boundaries and form bubbles

3. the bubbles exert pressure on the grains, leading to decrease in ductility + strength

How are high strength steels vulnerable to hydrogen embrittlement?

If they have undergone a surface finishing process such as acid pickling or electroplating

What does ductile-to-brittle transition temperature (DBTT) mean

• some materials are ductile at high temps, but brittle at low temps

Provide examples of DBTT materials/situations

• titanic: hull material had high DBTT, cold water, so the metal was brittle upon impact.

• Important in making ski lifts, refridgeration systems.

• Design strategy: stay above the DBTT!

What are the two components of griffith’s criterion?

• Strain energy: stored with atomic bonds, decreasing as crack propagates. the strain becomes zero when it breaks

• Surface energy: increases with crack advancement, new surface area is created (because new faces exposed)

What is the formula for griffith’s criterion?

U_total = U_surfaceEnergy - U_strainEnergy

What are the two opposing energy components in Griffith's fracture criterion, and how do they change with crack length (a)?

1. Strain energy (U_strain): Decreases as the crack propagates because the newly opened crack faces unload and relax.

2. Surface energy (U_surface): Increases linearly with crack length because new surface area is created by breaking atomic bonds.

In Griffith's total energy curve (U_total), what happens to crack growth before vs. after the peak (a_c)?

• a < a_c (Before peak): Crack will not grow (requires external energy input).

• a > a_c (After peak): Crack grows spontaneously and catastrophically because strain energy release is more energy than it takes to break bonds

In griffith’s total energy curve, what happens when the crack propagates?

• a = a_c

• for brittle materials

• a = half the length of internal crack, or length of edge crack

Why is ductile fracture more complex?

• has both crack propagation and plastic deformation

What direction is stress relative to the crack in ductile materials?

• perpendicular to the elliptical crack

When does a ductile material yield/plastically deform?

• When sigma_max (tip of crack) > sigma_y

• p_t: the crack radius. This becomes blunted/rounded, needing a lot of stress to keep the crack growing. hand in hand with cup and cone deformation

What is the function of an impact test?

• Measures fracture toughness,

• measures the energy needed to fracture a material

When an impact tester is used, how do you know when a material is brittle or ductile?

• Brittle: the material does not absorb much energy, so high return swing

• Ductile: material absorbs more energy, so low return swing (high fracture toughness)

Why are some metals brittle?

• fewer slip systems (BCC, FCC, HCP)

• alloys: they are hardened by solid solution strengthening

• small grain size

• too much cold work

• cold (below DBTT)

What is fatigue?

• Failure that occurs due to fluctuating stress

• every time load is applied and removed, there is a bit of damage left each time that grows

• can occur at stress level significantly lower than the yield/tensile strength

What three steps is fatigue failure characterized by ?

1. Crack initiation - small crack forms at stress concentration

2. Crack propagation - crack grows small amount with each loading cycle

3. Final failure - remaining material cannot support loading

How does a rotating bending fatigue test work?

• Counts the number of cycles until failure for range of loads

• plot stress amplitude vs. cycles to failure (N)

• “S-N curve”

What are the two types of behavior during a rotating bending fatigue test?

1. Materials with a fatigue limit (ferrous alloys)
   fatigue failure will not happen if stress is under the fatigue limit

2. Materials w/o a fatigue limit (copper, aluminum)
   fatigue will eventually occur at any stress value, maybe at high cycle count

What design strategy can help avoid fatigue limits?

• avoid stress concentrations (notches, sharp corners). so you can fillet

What is creep?

• Load deformation over time due to constant stress

• important in metals when temperature is greater than 0.4 times the melting temp

What are the 3 stages of creep?

• Primary: strain rate decreases with time because of strain hardening

• Secondary: Strain rate is constant because strain hardening balanced by recovery

• Tertiary: Accelerated creep rate until failure because of damage to material
  - because of formation of cracks, grain boundary separation

What are the main creep mechanisms?

• Diffusional flow (Higher T, lower sigma)

• Dislocation based flow (Lower T, higher sigma)

What is needed for creep resistance?

• higher melting point

• higher modulus/stiffness

• large grain size → single crystal is best, balanced by cost, weight, etc

Provide examples of materials with fatigue limits

Ferrous alloys

Provide examples of materials without fatigue limits

Copper, aluminum

What are the characteristics of the diffusional flow creep mechanism?

• higher Temp

• lower stress

What are the characteristics of the disocational flow creep mechanism?

Lower temperature, higher stress

Provide examples of the diffusional flow creep mechanisms

• grain boundary diffusion

• stress induced vacancy diffusion

• grain boundary sliding

Provide examples of the dislocation flow creep mechanisms

• dislocation glide

• dislocation climb

When is ductile to brittle transition temperature (DBTT) important

• important for low strength steels (BCC structure) becaues T_crss is temperature dependent

Provide examples of DBTT materials

low carbon steels (high ferrite content)