Elastic properties and tensile testing

Question 1

Name the 9 mechanical properties of materials discussed in class

Answer 1

1. Elastic (Young’s) Modulus
2. Poisson’s Ratio
3. Shear Modulus
4. Yield Strength
5. Ultimate Tensile Strength
6. Strain to Failure (Ductility)
7. Fracture Toughness
8. Hardness
9. Fatigue Life

Question 2

What are the properties of elastic deformation?

Answer 2

Non-permanent and reversible
Elastic deformation is recoverable; a material
returns to its original shape when an applied load is removed

Question 3

What type of relationship can be present between the force (N) and elongation (m)?

Answer 3

The relationship between
force and elongation can be
linear (as shown) or nonlinear.

Question 4

What is Young’s Modulus?

Answer 4

A measure of material stiffness

Question 5

What is resilience?

Answer 5

The recoverable energy absorbed by a material
during elastic deformation

Question 6

How is modulus of resilience (MOR) determined?

Answer 6

Determined by the area under the stress-strain curve up to
yield point

Question 7

What is the relationship between stored energy, yield stress, and elastic modulus?

Answer 7

Stored energy increases with
increased yield stress and/or
decreased elastic modulus

Question 8

What is the uniaxial tensile test?

Answer 8

This is the most common test used to determine the
relationship between load and deformation in a material

Question 9

What type of properties does the uniaxial tensile test measure?

Answer 9

• Elastic Properties (Elastic Modulus)
• Plastic Properties (Yield Stress)
• Failure Properties (Ultimate Tensile Stress)

Question 10

What are common standardized testing procedures for stress and strain?

Answer 10

ASTM (ASTM E-8), CSA, BS, DIN, JIS

Question 11

What do standards specify and require when it comes to mechanical properties?

Answer 11

• Standards specify size and shape of sample
• Standards require calibration of testing machines

Question 12

What is Poisson’s Ratio and what does it characterize?

Answer 12

• characterizes the
  contraction perpendicular
  to the extension caused by
  a tensile stress
• defined as the negative of
  the ratio between lateral
  and axial strains

Question 13

What is the elastic modulus a function of?:

Answer 13

i) Bond stiffness
ii) Density of bonds (i.e. atomic packing)

Question 14

In Poisson’s ratio, which axis is along the positive strain (elongation)?

Answer 14

The z-axis.

Question 15

In Poisson’s ratio, which axes are along the negative strains (lateral contractions)?

Answer 15

X and y-axis

Question 16

What is the relationship between the lateral strains if the applied stress is uniaxial (only in z-direction), and the material is isotropic?

Answer 16

The lateral strain across the x-axis = lateral strain across the y-axis

Question 17

What does poisson’s ratio depend on?

Answer 17

It depends on atomic
arrangements and bonding

Question 18

When do atoms experience negligible atomic interactions?

Answer 18

At large distances since the atoms are too far apart to have an influence on each other. So there is no attraction or repulsion.

Question 19

When do atoms exert forces on each other?

Answer 19

At small separation distances.

Question 20

How is the equilibrium interatomic distance determined graphically?

Answer 20

By the distance at which the interatomic force is 0.

Question 21

Why do the atoms experience repulsive energy when separated by a
distance less than the equilibrium distance ro?

Answer 21

Due to electrostatic repulsion between positively charged nuclei of atoms and electron clouds of atoms.

Question 22

What is the interatomic force between the atoms at the equilibrium
distance r0 and why?

Answer 22

0 N because the attractive and repulsive forces cancel each other out.

Question 23

What does the minimum potential energy correspond to in the potential interatomic energy vs. interatomic distance graph?

Answer 23

equilibrium spacing, r0

Question 24

What is the relationship between Young’s Modulus and the slope of the interatomic force over interatomic distance? When is this relationship valid?

Answer 24

Young’s Modulus is proportional to the slope of the interatomic force over interatomic distance. This relationship is valid only at the equilibrium distance.

Question 25

What type of material would require a greater stress to elastically deform the material?

Answer 25

A material with high stiffness and high Young’s Modulus requires greater stress to elastically deform the material.

Question 26

What is stiffness?

Answer 26

A material’s resistance to elastic deformation.

Question 27

What is the relationship between Young’s Modulus, stiffness, and elastic strain?

Answer 27

This modulus may be thought of as stiffness, or a
material’s resistance to elastic deformation. The greater the modulus, the stiffer
the material, or the smaller the elastic strain that results from the application of
a given stress.

Question 28

What is a brittle metal?

Answer 28

A metal that experiences
very little or no plastic deformation upon fracture (oppose of ductile)

Question 29

Do the expressions of ductility yield the same numerical value?

Answer 29

No, ductility being expressed in percentage elongation or percentage reduction in area doesn’t yield the same numerical value.

Question 30

Order these types of materials (metals, polymers, ceramics) from the highest Young’s Modulus to the lowest Young’s Modulus.

Answer 30

Ceramics, Metals, and Polymers

Question 31

Why do tensile specimens have a dog-bone geometry?

Answer 31

To localize the deformation of the material in a particular region of the sample during tensile testing.