Fracture Toughness

Question 1

What is fracture toughness, and what test is typically carried out to determine it?

Answer 1

➢ Fracture toughness is an important property in many alloy
systems including titanium alloys.
➢ It is concerned with the behavior of material containing flaws
(which all engineering materials do). The flaws will cause the
stress to be intensified in regions around the flaw. Fracture
toughness measures the ability of a material containing a flaw to
withstand an applied load.
➢ Typically a test is carried out by applying a tensile stress to a
specimen with a flaw of known size and geometry.

Question 2

What 3 cracks can occur from the tensile test?

Answer 2

1. Edge/External crack (length “a”)
2. Internal crack (length “2a”)
3. Elliptical crack (length “2c”, depth “a”)

Question 3

What is the Stress Intensity Factor (K) equation?

Answer 3

K = Y * sigma * sqrt(pie * a)

K = stress intensity factor
Y = geometrical factor, dependent on material and flaw geometry
sigma = applied stress
a = flaw size

Question 4

What is Kc?

Answer 4

➢ Kc is the value of K required for the crack to propagate.
➢ Kc varies with section thickness, until a certain thickness point
where it becomes independent of thickness.

Question 5

What are the 3 modes of fracture?

Answer 5

1. Opening
2. In-plane shear
3. Out-of-plane shear

Question 6

What is Kic?

Answer 6

➢ KIC values are the published data for materials.
➢ For relatively thin samples KC will depend on, and decrease with,
increasing specimen thickness (B). Eventually becomes
independent of B i.e. it is equal to KIC.
➢ Brittle materials have low KIC values (appreciable plastic
deformation is not possible in front of the advancing crack).
➢ Kic values are relatively large for ductile materials. KIC is a
fundamental property of the material.

Question 7

What is the material thickness (B) equation?

Answer 7

B >/= 2.5 * (Kic/Yield stress)

Kic = K = Y * sigma * sqrt(pie * a)
B = material thickness

Question 8

What do large cracks lead to?

Answer 8

Large cracks lead to high stress intensity factors (K). Also, the same value of stress intensity factor can be achieved with a small crack and high stress as with a large crack and low stress.

Question 9

The ability of a material to resist the growth of a crack depends on what issues?

Answer 9

1. Large flaws reduce the permitted stress. Many manufacturing
   processes will introduce flaws, careful control can help to minimize
   these.
2. Increasing the strength of a given material will tend to reduce the
   fracture toughness and is often associated with reduced ductility.
3. Thicker materials have a lower stress intensity factor (K) than thin
   materials.
4. Increasing temperature usually increases the fracture toughness
   of bcc and hcp materials, fcc materials are relatively unaffected by
   temperature (they are already ductile owing to slip planes being
   more readily available to move irrespective of temperature)
5. The ability of a material to deform is crucial. In ductile materials
   the region in front of the crack tip can deform, in doing so it will
   absorb energy from the advancing crack and effectively act to
   “blunt” the crack. In brittle materials such as glass, or some cast
   irons, the crack propagates easily at low energy levels, as the
   material is unable to absorb energy from the growing crack.
   ➢ In Ti alloys specifically, fracture toughness varies with factors such
   as composition, microstructure and texture.
   ➢ Generally as the strength of the alloy increases the fracture
   toughness decreases. Oxygen content, which can be present as an
   interstitial element in Ti alloys adversely affects the fracture
   toughness and must therefore be kept low.