Materials- Failure and Fracture

Question 1

What is simple fracture?

Answer 1

The separation of a body into two or more pieces in response to an imposed stress which is static (i.e. constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material.

Question 2

What is brittle fracture?

Answer 2

Where there is little or no plastic deformation and there is a low energy absorption before fracture

Question 3

What is ductile fracture?

Answer 3

Where there is substantial plastic deformation and high energy absorption before fracture

Question 4

Differences between ductile and brittle fracture

Answer 4

There is extensive plastic deformation in the vicinity of an advancing crack for ductile but not in brittle. Process proceeds slowly as the crack length is extended in ductile whereas in brittle cracks spread rapidly. Ductile has stable crack (resists further extension unless increase in applied stress). Brittle has unstable cracks (propagation continues spontaneously without increase in magnitude of applied stress). Evidence of twisting/tearing at fracture surface for ductile.

Question 5

How do different ductility materials look when fractured?

Answer 5

Highly ductile material necks down to a point (100% reduction in area). Most ductile materials fracture after a moderate amount of necking and the fracture surface has a fibrous appearance. Brittle materials fracture without any plastic deformation (so no necking, just separated into two).

Question 6

Describe how ductile fracture occurs

Answer 6

First necking begins. Then small cavities/microvoids form in the interior of the cross-section in a line perpendicular to the applied force. These coalesce to form an elliptical crack. Fracture ensues by the rapid propagation of a crack around the outer perimeter of the neck by shear deformation at an angle of about 45°.

Question 7

Describe the initial necking stage of ductile fracture

Answer 7

There is uniform elongation in the sample up to the UTS. At and past this there is plastic instability (necking at the surface). Relatively large amount of strain localised in a disproportionately small region of the material.

Question 8

Describe the small cavity formation stage of ductile fracture

Answer 8

Form inside the material typically due to heterogeneity in the sample. They are easily formed at inclusions, intermetallic or second phase particles and grain boundaries. Particles may fracture first, especially if brittle.

Question 9

Describe the coalescence of cavities stage of ductile fracture

Answer 9

As deformation continues the microvoids enlarge, come together and coalesce to form an elliptical crack. Crack grows perpendicular to its major axis by microvoid coalescence.

Question 10

Describe the crack propagation and shear fracture stage of ductile fracture

Answer 10

Fracture ensues by the rapid propagation of a crack around the outer perimeter of the neck by shear deformation at an angle of about 45° (angle at which shear stress is a maximum). Finally material breaks and final surface is often like a cup-and-cone shape with the central region of the surface having an irregular and fibrous appearance.

Question 11

Appearance of ductile fracture surface due to tension

Answer 11

Results in equiaxed voids (spherical voids a few micrometers in diameter) that coalesce normal to the loading axis. Profile view looks like semi-circle indents lined up on either side of fracture.

Question 12

Appearance of ductile fracture surface due to shear

Answer 12

Results in elongated voids that coalesce in planes of maximum shear stress. Voids point back to the crack origin. At failure dépressions point in opposite directions on opposing fracture surfaces.

Question 13

Appearance of ductile fracture surface due to tensing and bending

Answer 13

Results in elongated voids but the direction of the voids will be in the same direction on both fracture surfaces.

Question 14

How does brittle fracture occur?

Answer 14

Crack usually initiates at some point of stress concentration. Crack propagation is spontaneous and rapid due to low material toughness. Propagation occurs perpendicular to direction of applied stress forming an almost flat fracture surface.

Question 15

What is cleavage?

Answer 15

Crack propagation that corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes.

Question 16

Describe the process of cleavage fracture

Answer 16

Also called transgranular cleavage because the fracture cracks pass through the grains. Separation either side of the crack occurs by tensile force. It is a low-energy fracture. In polycrystalline materials, the crack can change direction at each grain boundary.

Question 17

What is intergranular fracture?

Answer 17

Crack propagation along the grain boundaries. Occurs after embrittlement or weakening of grain boundaries.

Question 18

What does the fracture surface of an amorphous material after brittle fracture look like?

Answer 18

A relatively shiny and smooth surface because they don’t contain multiple crystals so the crack doesn’t change direction as often as it moves through the material.

Question 19

Why are the measured fracture strengths for most materials lower than those predicted ted by theoretical calculations beaded on atomic bonding energies?

Answer 19

Due to the presence of microscopic flaws or cracks that always exist under normal conditions at the surface and within the interior of a body of material. These flaws are a detriment to fracture strength because an applied stress may be amplified or concentrated at the tip. The magnitude of this amplification depends on the crack orientation and geometry.

Question 20

When there is a crack in a material, where is the maximum stress?

Answer 20

At the tip of the crack. If it is internal, both tips of the crack

Question 21

When a stress is applied to a material with a crack, how does the stress vary as you go further from the tip of the crack (perpendicular to the stress)?

Answer 21

The stress decreases exponentially. At positions far removed, the stress is just the nominal stress

Question 22

What is the formula for stress intensity factor Ki?

Answer 22

Ki=Yσsqrt(πa)
Y is constant depending on sample geometry (unitless)
σ is applied stress stress (MPa)
a is crack length (m)
Subscript i denotes the type of crack propagation (mode)

Question 23

When do cracks propagate in relation to stress intensity factor?

Answer 23

When this exceeds a critical value Kic (fracture toughness) with units MPa m^1/2

Question 24

What are the 3 modes of crack surface displacement?

Answer 24

Mode I: opening or tensile mode. One part peels away from the other. Like opening a book.
Mode II: sliding mode. Uses shear force. Like opening a drawer.
Mode III: tearing mode. Uses shear force at a face of the material so that one part of the material slides away from the other but the material is still joined at the opposite face.

Question 25

What are typical KIc values for metals, ceramics and polymers?

Answer 25

Metals: between 15 and 60
Ceramics: between nearly 0 and 5
Polymers: close to 1

Question 26

What is the general shape of the impact energy against temperature curve for the Charpy impact test on a material?

Answer 26

Horizontal line from y axis above the origin. Then curves to straight diagonal line up. Then curves to higher horizontal line