Elastic and plastic deformation

Question 1

What is elastic deformation?

Answer 1

A result of an extension of the interatomic bonds, retractable after the load is removed (Reversible).

Question 2

When does elastic deformation occur?

Answer 2

It occurs when a material is loaded within its Elastic Limit and stress and strain are proportional

Question 3

What is the modulus of elasticity also known as?

Answer 3

Young’s modulus

Question 4

What is plastic deformation?

Answer 4

A result of interatomic bonds being broken, and atoms moving to different positions relative to each other (irreversible or permanent )
When the material is loaded beyond the elastic limit

Question 5

What is the plastic deformation process in crystalline solids?

Answer 5

the plastic deformation process is also known as SLIP, as planes of atoms tend to slide over each other into new stable position

Question 6

Where does the slip occur in crystalline solids?

Answer 6

Slip occurs on close packed planes and in close packed directions within the crystal – so that the atoms can follow the shortest path to their new positions_ under the most favourable energetic conditions

Question 7

What is a slip system?

Answer 7

The combination of slip plane and slip direction is known as a slip system.

Question 8

How does plastic deformation occur in non-crystalline solids?

Answer 8

In non-crystalline solids, PD occurs by viscous flow mechanism

Question 9

What is a slip plane?

Answer 9

plane on which easiest slippage occurs

• Highest planar densities (and large inter planar spacing)

Question 10

What is a slip directions?

Answer 10

directions of movement

• Highest linear densities

Question 11

When a slip occurs in crystals what is the energy measured to produce the deformation?

And why is it wrong?

Answer 11

When slip occurs in crystals it has been shown that the energy measured to produce the deformation is approximately 1000 times less than that expected by theoretical calculation.

● Reason for discrepancy: theory assumes a perfect crystal structure and all atoms on slip plane move simultaneously.

Question 12

What are dislocations in crystals?

Answer 12

In practice, slip occurs by incremental movement of discrete slip events or half planes known as dislocations, which are also known as line defects or lines of (potential) energy

Question 13

How do deformations occur in metal and alloys?

Answer 13

In simple terms, it is the movement of these dislocations, individually or in combination, that produces slip in crystals, and therefore, deformations in metals and alloys (and some ceramics).

Question 14

What is edge dislocation? (2)

Answer 14

The edge of an extra portion of a plane of atoms, or a half‐plane, terminates within the crystal

It is a linear defect, that centers on the line defined along the end of the extra half‐plane of atoms (the dislocation line)

Question 15

What is screw dislocation? (2)

Answer 15

Formed by a shear stress that is applied to produce the distortion shown in the Figure
The atomic distortion is also linear and along the dislocation line, AB

Question 16

How do fractures occur in polycrystalline aggregates?

And also alternatively

Answer 16

If each grain in the assembly could slip on only one plane in one direction, the change in shape of one grain would not be compatible with the change in shape of its neighbours.

Small holes and cracks would be generated at the boundaries between the grains. These could propagate and lead to fracture.

• Alternatively, plastic yielding by crystal slip is inhibited and all plasticity is lost on yielding on the planes and in the directions shown.

Question 17

When do slips not occur in some metals?

Answer 17

It is commonly observed that in most metals such cracks do not form. The grains remain in perfect contact with one another, an arbitrary change in shape in one grain being matched by compensating changes in shapes of its surrounding grains.

Question 18

How many slip-systems are there available for use in a FCC crystal?

Answer 18

In FCC crystals, there are four different octahedral planes and each contains three different close‐packed directions. Thus there are twelve independent slip‐systems available for use.

Question 19

What interferes with slipping in FCC crystals?

Answer 19

However, there is still interference with slip due to the necessity to maintain grain contact, and consequently the stress‐strain curves of single crystals and polycrystalline materials differ substantially.

Question 20

How many slip systems are there in a BCC crystal?

Slip planes and slip directions.

Answer 20

• Metals with BCC structures (eg. Fe, Mo, V) have four close‐packed cube‐diagonals which are the slip directions.
  • Because of the variety of slip‐planes possible, it is found that each slip‐direction can be contained in twelve different slip‐planes.
  •Thus, there are 48 independent slip systems available and polycrystalline BCC metals are ductile.

Question 21

How do dislocations occur in metals (cu, al)? (2)

Answer 21

• Dislocation motion easiest
• non-directional bonding - - close-packed directions
  for slip

Question 22

How do dislocations occur in covalent ceramics? (2)

Answer 22

(Si, diamond):

• Motion difficult
• directional (angular) bonding

Question 23

How do dislocations occur in covalent ionic ceramics (NaCl)?

Answer 23

• Motion difficult

- need to avoid nearest neighbors of like sign (- and +)

Question 24

What is work hardening/strain hardening?

Answer 24

During yield there is multiple dislocation movement, which eventually leads to interaction and entanglement of these lines. Entanglement restricts their movements. This restriction and therefore resistance to further deformation is known as work hardening or strain hardening

Question 25

How does strengthening occur?

Answer 25

Any process or treatment that will restrict or prevent the movement of dislocations within the crystals or grains will result in strengthening (and hardening).

Question 26

How can strength of metals be increased by?

Answer 26

decreasing grain size, solid solution strengthening, precipitate hardening, cold working

Question 27

What is a fracture?

Answer 27

when something breaks, or experiences bulk separation under load

Question 28

What is ductile fracture?

Answer 28

accompanied by significant plastic deformation

Question 29

What is brittle fracture?

Answer 29

little or no plastic deformation, catastrophic

Question 30

What is fatigue?

Answer 30

form of failure that results from materials and structures undergoing cyclic or fluctuating loads.

A repeated loading can be tensile, compressive, shear or combinations of these, or vibration

Question 31

What type of fracture is fatigue failure ?

Answer 31

Fatigue failure is brittle-like(relatively little plastic deformation) -even in normally ductile materials.

Question 32

What are the three distinct stages of fatigue failure?

Answer 32

Fatigue failure proceeds in three distinct stages: crack initiation in the areas of stress concentration (near stress raisers), incremental crack propagation, final catastrophic failure.

Question 33

What are the two types of crack propagation?

Answer 33

I: slow propagation along crystal planes with high resolved shear stress. Involves just a few grains, has flat fracture surface
II: faster propagation perpendicular to the applied stress. Crack grows by repetitive blunting and sharpening process at crack tip. Rough fracture surface.

Crack eventually reaches critical dimension and propagates very rapidly

Question 34

What is slow propagation?

Answer 34

I: slow propagation along crystal planes with high resolved shear stress. Involves just a few grains, has flat fracture surface

Question 35

What is faster propagation?

Answer 35

II: faster propagation perpendicular to the applied stress. Crack grows by repetitive blunting and sharpening process at crack tip. Rough fracture surface.

Question 36

What are factors affecting fatigue? (5)

Answer 36

• Stress
– mean and residual
– Amplitude and Frequency
• Surface Treatments
• Microstructure and integrity
• Thermal cycling and corrosion
• Design
– Impose compressive surface stresses
– Remove stress concentrators

Question 37

From a basic tensile stress-strain curve for a material, how would you determine the value for Young’s modulus?

Answer 37

by determining the gradient of the linear (elastic) portion of the curve