Föreläsning 5 - fracture and fatique

Question 1

Name some mechanical properties

Answer 1

• Stiffness (E-module): How much a material is deformed elastically when loaded
• Yield limit (sigma_y): At which stress the material begins to be plasticized.
• Fracture toughness (K_1C): Measure of resistance to propagating a crack.

Question 2

What made the metal of the Titanic so brittle?

Answer 2

The sea temp. was -2 degrees Celsius.

Question 3

What is a big difference between modern steel and the steel used for the Titanic?

Answer 3

Modern steel is more ductile. The Titanic steel was way more brittle under the same impact loading . It broke with little deformation.

Question 4

How does fracture toughness work?

Answer 4

• Fracture toughness K_1C (MPam^(1/2)), 1 for modus, c for critical
• Fracture when K_1C > K_1
• Takes into account load and crack length
• Depends on the energy required to drive the crack
• K_1C = sqrt(E*G_c)

Question 5

What does a brittle breaking surface look like?

Answer 5

flat

Question 6

How do ductile fractures work?

Answer 6

• Metals contain inclusions that provide stress concentrations.
• Pores from around the enclosures, they grow and form the breaking surface.

Question 7

What is fatigue?

Answer 7

Fatigue failures occur due to cyclig loading at stresses below a materials yield strength.

Depends on the amplitude of the stress and the number of cycles.

Loading cycles can be in the millions for an aircraft; fatigue testing must employ millions of fatigue cycles to provide meaningful design data.

Begins with a slag.

Question 8

What are the types of cyclic loading?

Answer 8

a) Low amplitude acoustic vibration
b) High-cycle fatigue: cycling below the yield strength
c) Low cycle fatigue: cycling above the yield stength but below the tensile strength

High cycling fatigue loading is most significant in engineering terms.

Question 9

What is stress amplitude? What is mean stress?

Answer 9

stress amplitude:
sigma_a = delta sigma/ 2 = (sigma_max - sigma_min)/2

Mean stress:
sigma_m = (sigma_max + sigma_min)/2

See slides for nicer equations :)

Question 10

What are S-N Curves?

Answer 10

Stress plotted as a function of number of cycles.

It’s how fatigue characteristics are plotted.

See good pic in slides.

Endurance limit sigma_e: stress amplitude below which fracture does not occur at all or only after a very large number of cycles (>10^7)

Question 11

What are striations?

Answer 11

Striations are marks produced on the fracture surface that show the incremental growth of a fatigue crack. A striation marks the position of the crack tip at the time it was made.

Question 12

Describe the life span Nf (number of cycles to failure)

Answer 12

• Crack growth to break consists of three phases:
• initiation, N_i
• propagation N_p
• Final fracture
• The first two provide the service life:
• N_f = N_i + N_p
• The initialization starts where the stress locally is highest.
• This occurs in case of radii defects in the material, grinding marks etc.
• Initialization often starts at an outer surface
• Therefore, the surface condition becomes important
• The load (stress) controls growth
• N_p will dominate

Question 13

What happens to a material when the temperature rises?

Answer 13

• Atoms begin to vibrate
• Atomic bonds get weaker
• Atoms move more easily (diffusion)
• Phase transformations
• Chemical reactions

There can be linear or non-linear change of properties

Question 14

What is melting temp.? What is glass transition temp.?

Answer 14

Melting temperature:
- Crystalline materials
- Metals, ceramics
- From solid to low-viscous liquid at melting temp. T_m

Glass transition temperature:
- Material amorphous
- Thermo plastics, glass
- Gradual transition from solid to viscous at T_g, the glass transition temperature

Question 15

Why do we have a max operating temp. and min operating temp.?

Answer 15

For example, the max operating temp. is limited by:
- Impaired mechanical properties
- Phase transformations and chemical changes
- Oxidation

For example, the min operating temp. is limited by:
- Brittleness

Determined by practical experience of the use of the material

Question 16

Describe thermal expansion

Answer 16

Thermal elingation = thermal expansion coefficient * change in temp

Thermal expansion gives rise to stresses. e.g different materials, temperature gradients.

There are stresses when different materials are joined. Stresses due to thermal gradients
=> thermal fatigue

Question 17

Describe thermal conductivity

Answer 17

Mechanisms:
- Crystal vibrations (fononer)
- Electrons

Crystal vibrations and electrons transfer energy from high temperature areas to low energy areas.

Question 18

What is creep?

Answer 18

In materials science, creep (sometimes called cold flow) is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses.

It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

• occurs at over approximately 1/2 of the melting temperature.
• Provides plastic (permanent) deformation
• Two types: diffusion creep, power-law crawling

Primary crawl = rapid deformation until dislocation faces obstacles

Steady- state = continous strain rate

Tertiary creep = damage in material

Question 19

How are creep fractures obtained?

Answer 19

When pores have grown to fracture.