Chapter 4 - Testing Materials Flashcards
What does hard mean?
Difficult to dent a materials surface. e.g ceramic
What does brittle mean?
When a material breaks by snapping cleanly. It undergoes little or no plastic deformation before fracture.
What does stiff mean?
When a material has small extension per unit force (difficult to bend/stretch). Indicated by Young modulus.
What does malleable mean?
When it is easy to press or hammer a sheet of material into a required shape.
What does ductile mean?
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.
What does tough mean?
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.
Give three properties of ceramics.
- > Hard
- > Brittle
- > Stiff
Give three examples of a pure metals and a property of theirs.
- > Lead
- > Copper
- > Gold
All soft and malleable
Give a use and property of glass polymers.
Used to replace glass lenses and they are brittle.
Give a property of semi-crystalline polymers.
Tough
What is Hooke’s Law?
For small extensions, the force on a spring is proportional to the extension.
What is the equation for Hooke’s Law?
- > F = kx
- > force = spring constant * extension
What does the spring constant determine?
The stiffness of the spring. The greater the constant for the specific specimen, the more difficult it is to stretch.
What is elastic deformation?
When a material deforms elastically it regains its original shape after deformation.
What is compression?
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.
What is the elastic limit?
The elastic limit is the maximum stress at which an object returns to its original shape after the deforming stress is removed.
What is plastic deformation?
When a material deforms plastically it undergoes permanent stretching or distortion before breaking.
What is a fracture?
An object fractures when it breaks into two or more pieces when placed under stress.
What is the equation for energy stored in a spring? Where is the it shown on a graph?
E = 1/2 kx^2
Shown as the area under the graph
What is fracture stress?
Fracture stress is the stress at which fracture occurs. The fracture stress of a material in tension is sometimes called the tensile strength.
What is yield stress?
The stress at which a specimen begins to yield (where plastic deformation begins).
What is the word and symbol equation for stress?
Stress = force / cross-sectional area
“sigma” (N m^-2 or Pa) = F (N) / A (m^2)
What is strain?
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.
What is the word and symbol equations for strain?
Strain = extension / original length
“Epsilon” = x / L
What is the Young modulus?
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.
What is the equation for Young modulus?
E = stress / strain
= “sigma” / “epsilon”
= (F/A) / (x/L)
= FL / xA
