Fracture Mechanics Flashcards
Why does fatigue cracking form?
as a result of a cyclic load being applies. This can be as a result of material discontinuity.
How to know if a material is adequate for its function?
Material is adequate if its strength is > applied stress.
What is the fracture mechanics approach? - 3 main variables
Applies stress will influence both the flaw size and fracture toughness. Of which both of these influence each other. This helps establish flaw tolerance limits.
Are ceramics more compressive or tensile in strength?
Ceramics have higher compressive
strength than tensile.
How does crack extension occur?
When the energy available for growth is sufficient to overcome the resistance of the materials – Creation of a new surface requires energy: surface energy + local deformation/rearrangements etc.
What is the energy supplied by and what are the energy losses in the energy approach to fracture mechanics?
Energy supplied by: stored elastic energy + work done by
external Forces
Other energy loss terms: kinetic energy, bulk plastic
deformation, etc
What are the 3 main energy approaches of laws to obey?
Elastic – energy associated with plastic deformation small
Linear – obeys Hooke’s Law ie., Stress ∝ Strain (homogeneous)
Quasi-static – kinetic energy term small
What is the equation for energy release rate?
G= aπσ^2/E
Where G is the energy release rate, E the youngs modulus and a is 1/2 the crack length.
Meaning that at a constant energy release rate, the stress varies with 1/sqrt a the length.
What is the stress intensity approach to fracture mechanics?
K= σ sqrt(πa)
Fracture occurs when Ki (driving force for fracture) = Kic (measure of material resistance - fracture toughness)
so G=Ki^2/E
How do fracture mechanics predict time dependent crack growth?
da/dN= C(∆K )^m
where da/dN is the crack growth per cycle, ∆K is the stress intensity range, C and m are material constraints.
How is the allowable flaw size defined?
By dividing the critical size by
a safety factor. Predicted
service life = time for ‘a’ to
become max. allowable size.
What are fracture mechanics types? -3
- Linear elastic fracture mechanics - no plastic deformation
- Elastic plastic fracture mechanics - considering plastic deformation
- Dynamic fracture mechanics, viscoelastic fracture mechanics, viscoplastic fracture mechanics. - time is the variable
What are the typical fracture behaviours of metals, polymers and ceramics?
METALS- linear elastic(high strength steel), elastic-plastic/fully plastic(low and medium strength steel), fully plastic (austenitic stainless steel), linear elastic(precipitation - hardened aluminium), viscoplastic(metals @ high temp), dynamic-viscoplastic(metals at high strain rates).
POLYMERS - below Tg(linear elastic/viscoelastic) and above Tg(viscoelastic)
CERAMICS- monolithic(linear elastic), ceramic composites(linear elastic) and high temperatures(Viscoplastic)
How does fracture toughness interact with failure stress and the failure mechanism)
When combination of mechanical performance and high temperature usage is needed – CERAMICS.
As the fracture toughness increases the failure stress increases through the LEFM(causing brittle fracture) to the non-linear fracture mechanics(brittle-ductile transition) to the limit load analysis (failure is insensitive to K and only determined by flow properties here)
