Chapter 6

Question 1

What is Poisson’s ratio v?

Answer 1

For elastic deformation, the negative ratio of lateral and axial strains (-εx/εz) that result from an applied axial stress.

Question 2

What is anelasticity?

Answer 2

Time-dependent elastic (nonpermanent) deformation.

Question 3

What is design stress?

Answer 3

The product of the calculated stress level (on the basis of estimated maximum load) and a design factor (which has a value greater than unity). Used to protect against unanticipated failure.

Question 4

What is ductility?

Answer 4

A measure of a material’s ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation (%EL) or percent reduction in area (%RA) from a tensile test.

Question 5

What is elastic deformation?

Answer 5

Deformation that is nonpermanent—that is, totally recovered upon release of an applied stress. Happens at small deformations and has a linear stress-strain curve (some polymers have non-linear curves).

Question 6

What is engineering strain?

Answer 6

The change in gauge length of a specimen (in the direction of an applied stress) divided by its original gauge length.

Question 7

What is engineering stress?

Answer 7

The instantaneous load applied to a specimen divided by its cross-sectional area before any deformation.

Question 8

What is hardness?

Answer 8

The measure of a material’s resistance to deformation by surface indentation or by abrasion.

Question 9

What is modulus of elasticity?

Answer 9

The ratio of stress to strain (σ/ε) when deformation is totally elastic; also a measure of the stiffness of a material (materials with larger elastic moduli deform less).

Question 10

What is plastic deformation?

Answer 10

Deformation that is permanent or nonrecoverable after release of the applied load. It is accompanied by permanent atomic displacements.

Question 11

What is the proportional limit?

Answer 11

The point on a stress-strain curve at which the straight-line proportionality between stress and strain ceases.

Question 12

What is resilience?

Answer 12

The capacity of a material to absorb energy when it is elastically deformed. The energy is recovered when the load is released.

Question 13

What is safe stress?

Answer 13

A stress used for design purposes; for ductile metals, it is the yield strength divided by a factor of safety.

Question 14

What is a shear?

Answer 14

A force applied so as to cause or tend to cause two adjacent parts of the same body to slide relative to each other in a direction parallel to their plane of contact.

Question 15

What is tensile strength?

Answer 15

The maximum engineering stress, in tension, that may be sustained without fracture. Often termed Ultimate (Tensile) Strength.

Question 16

What is toughness?

Answer 16

A mechanical characteristic that may be expressed in three contexts: (1) the measure of a material’s resistance to fracture when a crack (or other stress-concentrating defect) is present; (2) the ability of a material to absorb energy and plastically deform before fracturing; and (3) the total area under the material’s tensile engineering stress-strain curve taken to fracture.

Question 17

What is true strain εT?

Answer 17

The natural logarithm of the ratio of instantaneous gauge length to original gauge length of a specimen being deformed by a uniaxial force.

Question 18

What is true stress?

Answer 18

The instantaneous applied load divided by the instantaneous cross-sectional area of a specimen.

Question 19

What is yield strength σy?

Answer 19

The stress required to produce a very slight yet specified amount of plastic strain; a strain offset of ε = 0.002 is commonly used.

Question 20

What is yielding?

Answer 20

The onset of plastic deformation.

Question 21

What does the symbol σ represent?

Answer 21

Tensile stress = force/initial cross-section area

Question 22

What does the symbol τ represent?

Answer 22

Torsion (shear) = moment/(original cross-section*radius)

Question 23

What is the formula for compression σ?

Answer 23

Same as tensile stress -> σ = force/initial cross-section area

Question 24

What is bi-axial tension?

Answer 24

Where tension acts on an object along two axes (ie x and y)

Question 25

What is hydrostatic compression?

Answer 25

Where an object is compressed from all directions due to a fluid

Question 26

In what direction are tensile stresses applied?

Answer 26

Perpendicular to the cross-sectional area

Question 27

In what direction are shear stresses applied?

Answer 27

Parallel to the cross-sectional area

Question 28

What is the formula for tensile strain εz?

Answer 28

εz = change in length/original length (in direction of force)

Question 29

What is the formula for lateral strain εx?

Answer 29

εx = - change in diameter/original diameter (perpendicular to force direction)

Question 30

What is the formula for shear strain γ?

Answer 30

γ = tanθ

Question 31

What is deflection dependent on?

Answer 31

Material, geometric, and loading parameters

Question 32

What does α represent?

Answer 32

Angle of twist

Question 33

What does G represent?

Answer 33

Elastic shear modulus –> G = τ/γ (shear stress/strain)

Question 34

What does E represent?

Answer 34

Modulus of elasticity or Young’s modulus and is a material property

Question 35

What is Hooke’s Law?

Answer 35

σ = E * ε (stress = modulus of elasticity * strain)

Question 36

What are the general values of Possoin’s Ratio?

Answer 36

Metals ~ 0.33
Ceramics ~ 0.25
Polymers ~ 0.40

For most materials falls between 0.15 and 0.50

Question 37

What does Ur represent?

Answer 37

Resilience –> area under stress-strain curve up until yielding

Question 38

What are the types of measurements for hardness?

Answer 38

• Brinell (sphere of steel or tungsten carbide)
• Vickers (diamond pyramid)
• Knoop (diamond pyramid)
• Rockwell/Superficial (diamond cone and steel spheres)

Question 39

What is working stress σw?

Answer 39

The ratio of yield stress (σy) over the factor of safety (N) which ranges between 1.2 and 4