Materials

Question 1

What is Hooke’s law?

Answer 1

Extension (ΔL) is directly proportional to force applied (F), F = kΔL, k = stiffness constant

Question 2

What is meant by tensile stress?

Answer 2

Force applied per unit cross-sectional area, Stress = F / A (Nm⁻²)

Question 3

What is tensile strain?

Answer 3

Extension divided by original length, Strain = ΔL / L (dimensionless)

Question 4

What is the difference between elastic and plastic deformation?

Answer 4

Elastic: returns to original shape; Plastic: permanent deformation

Question 5

What is breaking stress?

Answer 5

Minimum stress needed to break a material

Question 6

What is meant when a material is described as brittle?

Answer 6

Doesn’t deform plastically, breaks when stress reaches a certain value

Question 7

What is the elastic limit?

Answer 7

Force above which material is permanently stretched (plastically deformed)

Question 8

What does the area under a force-extension graph represent?

Answer 8

Work done to deform the material, Work = ½ F ΔL

Question 9

What is Young’s modulus?

Answer 9

E = tensile stress / tensile strain = FL / (ΔL A), measures material stiffness

Question 10

How do you find Young’s modulus from a stress-strain graph?

Answer 10

Gradient of the line

Question 11

Which graph represents a wire that has plastically deformed?

Answer 11

Unloading line doesn’t pass through origin (permanent extension)

Question 12

How can a force-extension graph show Hooke’s Law is being obeyed?

Answer 12

Straight line through the origin; F ∝ ΔL

Question 13

What is the limit of proportionality and how does it appear on a graph?

Answer 13

Point after which Hooke’s Law no longer obeyed; line starts to curve

Question 14

How is work done to stretch or compress a material stored?

Answer 14

As elastic strain energy

Question 15

Why are loading and unloading lines parallel on a graph for plastically deformed material?

Answer 15

Stiffness constant (k) unchanged; atomic forces same

Question 16

Why isn’t all work done stored as elastic strain energy in plastic stretch?

Answer 16

Work moves atoms apart, energy dissipated as heat

Question 17

How is dissipation of energy in plastic deformation used to design safer vehicles?

Answer 17

Crumple zones absorb kinetic energy; seat belts convert energy into elastic strain energy

Question 18

Outline energy changes when a spring fixed at the top is pulled down and released

Answer 18

Work done stretching stored as elastic strain energy → kinetic energy → gravitational potential energy

Question 19

Do stress-strain graphs show behavior of a material or specific object?

Answer 19

Material

Question 20

Where is ultimate tensile stress on a stress-strain graph?

Answer 20

Highest point; maximum stress material can withstand

Question 21

What would the stress-strain graph for a ductile material look like?

Answer 21

Shows large plastic deformation before fracturing