Lecture 11 - Toughness and Fatigue Flashcards
What does Toughness mean?
the materials energy absorption and deformation without failure
Can failure happen before ultimate stresses?
Yes, the presence of cracks can modify the local stresses to such an extent that the elastic stress analyses done so carefully by the designers are insufficient!
What happens when the cracks reaches the critical length?
it can propagate catastrophically through the structure, even though the gross stress is much less than would normally cause yield or failure in a tensile specimen.
What are the 3 mechanics we study for dynamics of cracks?
Start, grow, stop
What kind of loading can produce cracks?
Static and dynamic load
What are the 3 fracture analysis used for toughness?
- Griffith analysis
- stress intensity approach
- strain energy release rate approach
What is the (mostly) good approximation for brittle materials i.e. diamond, glasses?
homogenous isotropic linearly elastic no crack blunting isothermal conditions
What is the postulate of The Griffith analysis?
a crack will propagate if and only if the energy of the system is thereby decreased.
What is the length of a bulk crack and a surface crack?
2c and c
What is Gc? And what does a high value mean?
Fracture energy, that it is harder to break.
When can you use The stress intensity factor approach?
- homogeneous isotropic and a relatively few anisotropic materials (composites)
- states of stress other than the simple uniaxial stress used by Griffith
What are the differences between the Griffith analysis and The stress intensity factor approach?
• the crack profile and hence the displacement field near the crack are assumed to be parabolic not elliptic!
• Energy absorption due to plastic deformation at the crack tip is included
(Griffith’s original analysis an elastic behaviour until fracture was assumed)
What can we also call The stress intensity factor approach?
fracture toughness
What are the 3 failure modes?
I Opening mode
II In-plane shear mode
III Anti plane-shear mode
What are the differences between Plane state of stress and Plane state of strain?
Thin sheet and thick sheet
Why is Kc>KIc
Thus the probability that a material will fracture is the highest in a plane state of strain
What are the most widely used test specimen for fracture mechanics test on plastics?
the compact tension (CT) specimen [Pacman]
the single-edge-notch bend (SENB specimen) [3-p bending]
What is the rule of thumb about KIC?
Rule of thumb: materials having a KIc > approx.10 MPa m1/2 are considered to be tough.
What is crashworthiness?
ability to maintain a survivable volume for the occupants and alleviate the loads transmitted to the occupants during potentially survivable accident scenarios
What are the main toughening mechanism for crashworthiness?
geometry, structural arrangements, materials and energy
absorption devices used to dissipate the energy, and the interaction of these variables
What is the propagation of a crack along an interface between two constituents is controlled by?
The interfacial strain energy release rate, or simply interfacial energy, Gi.
What is delamination?
Delamination is the act of crack growth between adjacent laminae
What can delamination also be called?
interlaminar fracture
What experiments can you do to examine delamination?
- in mode I: double cantilever beam (DCB) test, where GIc is calculated
- in mode II: end-notched flexure (ENF) test, where GIIc is calculated
What is PD and ID?
PD (Primary delamination)
ID (Intralayer delamination)
Name some of the toughening mechanisms
Interfacial fracture Crack deflection Delamination Fiber fracture (contributes the least) Frictional sliding during fiber pull-out matrix contribution to composite toughness
