Chapter 4 - Testing Materials

Question 1

What does hard mean?

Answer 1

Difficult to dent a materials surface. e.g ceramic

Question 2

What does brittle mean?

Answer 2

When a material breaks by snapping cleanly. It undergoes little or no plastic deformation before fracture.

Question 3

What does stiff mean?

Answer 3

When a material has small extension per unit force (difficult to bend/stretch). Indicated by Young modulus.

Question 4

What does malleable mean?

Answer 4

When it is easy to press or hammer a sheet of material into a required shape.

Question 5

What does ductile mean?

Answer 5

When a material can easily be drawn into a wire. Metals are often ductile because the non-directional metallic bonds allow ions to slide past one another.

Question 6

What does tough mean?

Answer 6

When a material does not break or snap easily, and when a material adsorbs a lot of energy before fracture when deforming plastically. Tough materials are resistant to the propagation of cracks.

Question 7

Give three properties of ceramics.

Answer 7

• > Hard
• > Brittle
• > Stiff

Question 8

Give three examples of a pure metals and a property of theirs.

Answer 8

• > Lead
• > Copper
• > Gold

All soft and malleable

Question 9

Give a use and property of glass polymers.

Answer 9

Used to replace glass lenses and they are brittle.

Question 10

Give a property of semi-crystalline polymers.

Answer 10

Tough

Question 11

What is Hooke’s Law?

Answer 11

For small extensions, the force on a spring is proportional to the extension.

Question 12

What is the equation for Hooke’s Law?

Answer 12

• > F = kx

- > force = spring constant * extension

Question 13

What does the spring constant determine?

Answer 13

The stiffness of the spring. The greater the constant for the specific specimen, the more difficult it is to stretch.

Question 14

What is elastic deformation?

Answer 14

When a material deforms elastically it regains its original shape after deformation.

Question 15

What is compression?

Answer 15

Compressive forces are squashing forces. An object is in compression when two forces act on it in opposite directions to make the object compress along the line of action of the forces.

Question 16

What is the elastic limit?

Answer 16

The elastic limit is the maximum stress at which an object returns to its original shape after the deforming stress is removed.

Question 17

What is plastic deformation?

Answer 17

When a material deforms plastically it undergoes permanent stretching or distortion before breaking.

Question 18

What is a fracture?

Answer 18

An object fractures when it breaks into two or more pieces when placed under stress.

Question 19

What is the equation for energy stored in a spring? Where is the it shown on a graph?

Answer 19

E = 1/2 kx^2

Shown as the area under the graph

Question 20

What is fracture stress?

Answer 20

Fracture stress is the stress at which fracture occurs. The fracture stress of a material in tension is sometimes called the tensile strength.

Question 21

What is yield stress?

Answer 21

The stress at which a specimen begins to yield (where plastic deformation begins).

Question 22

What is the word and symbol equation for stress?

Answer 22

Stress = force / cross-sectional area

“sigma” (N m^-2 or Pa) = F (N) / A (m^2)

Question 23

What is strain?

Answer 23

Strain is the change of length per unit length. Strain is the ratio of two lengths and therefore has no unit. Often represented by the greek letter, epsilon.

Question 24

What is the word and symbol equations for strain?

Answer 24

Strain = extension / original length

“Epsilon” = x / L

Question 25

What is the Young modulus?

Answer 25

The ratio of stress over strain, measured in N m^- 2 or Pa. It gives a measure of the stiffness of the material itself and not just a specimen.

Question 26

What is the equation for Young modulus?

Answer 26

E = stress / strain
= “sigma” / “epsilon”
= (F/A) / (x/L)
= FL / xA