Failure of Materials 2 K Fracture mechanics Flashcards
For the Griffith approach:
- When does fast fracture occur?
- What two material properties affect this?
- What is the equation for fracture mechanics
- What is Stress intensity factor, K and its equation
- K describes how big the stress field is in the region of a crack tip, locally, and the stress distribution.
- K is equal to applied stress and root(pi a)
- Can describe the magnitude of the local crack tip stress fields.
Note:
There is another thing, The stress concentration factor, Kt, shows the amount of stress around the hole being so many times higher than what applied stress is.
What is the critical stress intensity factor, Kc, and its equation?
- Critical stress intensity factor, Kc, is the point where fractor stress is reached and fracture will occur.
- Equation below (second equation with Y, adds a dimensionless parameter which doesn’t assume the material is a semi-infinite plate of uniform thickness of 1).
-Y is a dimensionless quantity that takes into account size/geometry as well as manner of load application. - This is because K is dependent on applied stress and crack length. So it will vary depending on those values.
What is the caveat with using critical stress intisity factor, Kc?
- K does take into account the shape of the geometry, applied loading situations, etc.
- K1c (worst-case scenario) is a constant above a certain sample thickness. K1c is the critical stress intensity factor required to initiate and propagate a crack in a material without catastrophic failure.
- But as the sample gets thicker the K value decreases to a certain point when it goes to a constant value, due to constraint.
- Constraint: gives an additional reaction stress near the crack tip, due to the thickness.
- Thin situation: plane stress state.
- Thick situation: plane strain fracture toughness and is found to be a material property. i.e. linear elastic assumptions hold.
What is the equation that relates fracture toughness, K and Griffiths approach?
What is the uses of fracture mechanics?
How does temperature relate to failure, for both brittle and ductile materials?
Brittle failure:
* Temperature doesn’t affect failure.
* Temperature independent, as shown in equation
Ductile failure:
* Ductile failure is dependent on temperature, with yield stress, as its described in the Hall-Petch equation, though only for BCC and HCP crystal structures.
What does a Ductile to Brittle transition (DBT) diagram tell us about the BCC and FCC materials?
- Fcc case: yield always occurs before brittle fracture => ductile failure
- Bcc case: at low temperatures the yield stress > fracture stress, so brittle fracture will occur before yield => brittle failure.
What is the effect of increasing yield stress on the DBT diagram?
- The yield stress increases and shifts up, due to some process like work hardening.
- The fracture stress point doesn’t change
- Therefore, we have made the material more brittle For BCC as there is a greater brittle range.
How can we increase yield and fracture stress to cause DBT enchancement?
- Grain size refinement
- Increase yield stress by grain size refinements. Also, surface fracture stress increases, so overall DBT drops.
- Improving strength and toughness. This is because of the nature of brittle cleaving and dislocation pile-up on grain boundaries.
What is toughness?
- The ability of a material to absorb energy to fracture.
- Ductile failure absorbs more energy and can be considered tougher than brittle failure.
What is the Charpy impact test, and why do we do it?
- We use it to find out how much energy is absorbed by a material in impact.
How is toughness affected by temperature and crystal structure?
- Notice the shapes of the curve.
- Fcc same energy all temperatures
- Bcc and HCP temp dependent, higher temp more energy absorption.
