Fracture I Flashcards
What is the toughness of ductile fracture?
High
What is the toughness of brittle fracture?
Low
What is the extent of plastic deformation for ductile fracture?
Extensive
What is the extent of plastic deformation for brittle fracture?
Little to none
What is the appearance of the fracture surface for ductile fracture?
Dull and fibrous
What is the appearance of the brittle fracture?
Shiny and crystalline
What is the morphology of the ductile fracture?
- Dimples (microscopic)
- Cup and cone
- Evidence of shear
(failure along paths of
maximum shear)
What is the morphology of the brittle fracture?
- Flat surface
- Evidence of cleavage
of grains - Chevron and fanshaped patterns
What is the difference between highly ductile fracture, moderately ductile fracture, and brittle fracture?
Highly Ductile Fracture: Specimen necks down to a point
Moderately Ductile Fracture: Some necking
Brittle Fracture: No plastic deformation
What is fracture toughness?
- A material property
- A measure of a material’s resistance to brittle fracture when a
crack is present
What is the stress concentration factor (Kt) governed by?
The crack tip radius of curvature.
What happens when a crack propagates:
- There is a release of elastic strain energy
- Energy is consumed to extend the crack by creating new fracture
surfaces
What did Griffith propose when a crack will propagate?
- A crack will propagate when the release of
elastic strain energy exceeds the energy required to form new
crack surfaces
What happens when a crack of length a forms on the surface of an
elastic solid
- The material within the two triangular volumes is
unloaded and elastic energy is released - Crack surface area per unit thickness created is 2a
What is the maximum length for an internal crack?
For an internal crack, maximum length is 2a
What are the characteristics of fracture toughness?
- A material property
- A measure of a material’s resistance to brittle fracture when a crack
is present - Not equivalent to the stress concentration factor Kt
What are the three modes of loading associated with fracture toughness?
Opening mode (Mode I), Sliding mode (Mode II), Tearing mode (Mode III)
What does Mode I of loading describe?
Mode I describes plane strain, in which the crack opens and there is no
strain perpendicular to the front and back faces
Describe in 5 steps how the brittle fracture process occurs (Written Response).
- Necking begins when the ultimate tensile strength is exceeded
- Cavities previously present in material are enlarged
- Cavities coalescence to form crack
- Crack propagates
- When the applied stress can no longer be sustained, fracture occurs
What methods can be used to detect cracks?
Visual inspection, non-destructive testing
techniques (NDT), die penetrant, ultrasonic methods
What does Y depend on in the plane strain fracture toughness formula?
Y is dependent on the geometry of the specimen, crack, and the
manner of loading
Draw a diagram of what a highly ductile fracture looks like.
Check Fracture annotated slides
Draw a diagram of what a moderately ductile fracture looks like.
Check Fracture annotated slides
Draw a diagram of what brittle fracture looks like.
Check Fracture annotated slides
What type of dimples are shown during ductile fracture in a uniaxial tensile test?
Spherical dimples
What type of dimples are shown during ductile fracture under shear loading?
parabolic dimples
What do mathematicians use the stress concentration factor for?
To
study the stress fields at the crack tip
What is the reality of the radius of curvature for crack tips
In reality, crack tips have some finite radius of curvature
What does the radius of curvature approach with sharp cracks? What does it imply?
0 because it implies that the stress at the crack tip approaches infinity!
Why is drilling holes at the end of detectable cracks effective?
It reduces the maximum stress at the crack tip
What is the relationship between yield strength, ductility, and plane strain fracture toughness?
Increase in yield strength = decrease in plane strain fracture toughness = decrease in ductility
What is a key concept in fracture mechanics about crack tips and stress intensity factors?
Even with two very different
geometries and crack dimensions, the same stress field ahead of the
crack tip is observed if the same stress intensity factor K exists.
What is possible when a crack is detected before failure?
- If detected before failure we can remove the component from service
According to Griffith’s theory of brittle fracture, what types of energy are associated with crack propagation?
- Elastic energy released due to the unloading of a volume of material
- Strain energy absorbed to create new crack surface area
What is the stress concentration factor?
Measure of stress
amplification at the crack tip
What types of defects do all manufactured materials contain?
- Porosities/Cavities
- Inclusions (contamination arising from processing)
- Surface damage accumulated during manufacturing or during
service (nicks and cracks) - Dislocations
Write down the equation for the elastic energy released per unit thickness
when a crack extends a distance a
Check Fracture annotated slides (slide 20)
Write down the equation for the surface energy required per unit
thickness to extend a crack from 0 to a
Check Fracture annotated slides (slide 20)
What concept can be used to detect cracks in pressure vessels?
“leak before break”
What are the macroscopic features of brittle fracture?
Chevron markings and fan-shaped ridges
What are the microscopic features of brittle fracture?
Transgranular fracture and Intergranular fracture
What is one way to improve the effectiveness of drilling holes at the end of detectable cracks to reduce the maximum stress at the crack tip?
The effect is greatly improved if a compressive
residual stress around the hole can be induced.
