Lecture 11 - Toughness and Fatigue

Question 1

What does Toughness mean?

Answer 1

the materials energy absorption and deformation without failure

Question 2

Can failure happen before ultimate stresses?

Answer 2

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!

Question 3

What happens when the cracks reaches the critical length?

Answer 3

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.

Question 4

What are the 3 mechanics we study for dynamics of cracks?

Answer 4

Start, grow, stop

Question 5

What kind of loading can produce cracks?

Answer 5

Static and dynamic load

Question 6

What are the 3 fracture analysis used for toughness?

Answer 6

• Griffith analysis
• stress intensity approach
• strain energy release rate approach

Question 7

What is the (mostly) good approximation for brittle materials i.e. diamond, glasses?

Answer 7

homogenous 
isotropic
linearly elastic
no crack blunting
isothermal conditions

Question 8

What is the postulate of The Griffith analysis?

Answer 8

a crack will propagate if and only if the energy of the system is thereby decreased.

Question 9

What is the length of a bulk crack and a surface crack?

Answer 9

2c and c

Question 10

What is Gc? And what does a high value mean?

Answer 10

Fracture energy, that it is harder to break.

Question 11

When can you use The stress intensity factor approach?

Answer 11

• homogeneous isotropic and a relatively few anisotropic materials (composites)
• states of stress other than the simple uniaxial stress used by Griffith

Question 12

What are the differences between the Griffith analysis and The stress intensity factor approach?

Answer 12

• 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)

Question 13

What can we also call The stress intensity factor approach?

Answer 13

fracture toughness

Question 14

What are the 3 failure modes?

Answer 14

I Opening mode
II In-plane shear mode
III Anti plane-shear mode

Question 15

What are the differences between Plane state of stress and Plane state of strain?

Answer 15

Thin sheet and thick sheet

Question 16

Why is Kc>KIc

Answer 16

Thus the probability that a material will fracture is the highest in a plane state of strain

Question 17

What are the most widely used test specimen for fracture mechanics test on plastics?

Answer 17

the compact tension (CT) specimen [Pacman]

the single-edge-notch bend (SENB specimen) [3-p bending]

Question 18

What is the rule of thumb about KIC?

Answer 18

Rule of thumb: materials having a KIc > approx.10 MPa m1/2 are considered to be tough.

Question 19

What is crashworthiness?

Answer 19

ability to maintain a survivable volume for the occupants and alleviate the loads transmitted to the occupants during potentially survivable accident scenarios

Question 20

What are the main toughening mechanism for crashworthiness?

Answer 20

geometry, structural arrangements, materials and energy

absorption devices used to dissipate the energy, and the interaction of these variables

Question 21

What is the propagation of a crack along an interface between two constituents is controlled by?

Answer 21

The interfacial strain energy release rate, or simply interfacial energy, Gi.

Question 22

What is delamination?

Answer 22

Delamination is the act of crack growth between adjacent laminae

Question 23

What can delamination also be called?

Answer 23

interlaminar fracture

Question 24

What experiments can you do to examine delamination?

Answer 24

• in mode I: double cantilever beam (DCB) test, where GIc is calculated
• in mode II: end-notched flexure (ENF) test, where GIIc is calculated

Question 25

What is PD and ID?

Answer 25

PD (Primary delamination)

ID (Intralayer delamination)

Question 26

Name some of the toughening mechanisms

Answer 26

Interfacial fracture
Crack deflection
Delamination
Fiber fracture (contributes the least)
Frictional sliding during fiber pull-out
matrix contribution to composite toughness