Materials vocabulary Flashcards
What are the units of stress?
Nm^-2
pascal, Pa, for short
What is the symbol for stress?
σ
the Greek letter sigma
What is stress?
Force applied divided by the cross sectional area it is applied to (like pressure…)
What is the formula for stress?
stress = force/area
What is elastic deformation?
A temporary change in shape that occurs when a stress is applied, the object returns to its original shape when the stress is removed.
What is NOT lost when an object returns to original shape after elastic deformation?
Energy
What is plastic deformation?
A permanent change of shape that occurs when a stress is applied to an object
What are the units of strength?
Pascals, Pa
Nm^-2
Define the word ‘strong’
A large stress is needed to cause a failure
Define the word ‘strength’
The maximum stress a material can withstand before a failure
Why do some materials have two values for strength?
There are two types of failure: yielding and breaking/fracture
Define the word ‘weak’
Only a small stress is needed to cause a failure
What are the units of toughness?
J/m^2 (usually)
During which kind of deformation are large amounts of energy absorbed?
Plastic
What are the two definitions of toughness?
- The energy required to create a new surface area (J/m^2)
- The energy absorbed per unit volume (J/m^3)
What is a tough material?
- One that needs a large amount of energy to break it
- One that undergoes a lot of plastic deformation before breaking (usually)
- One that resists crack propagation
What is crack propagation?
Any widening, lengthening or increase in numbers of cracks in an object
What makes a material brittle?
(The opposite of tough)
- Does not undergo much plastic deformation
- Breaks soon after reaching the elastic limit
- Cracks pass through material easily (and tend to be sudden and catastrophic)
What are the units of hardness?
Pascals, Pa (Nm^-2)
What makes a material hard?
Being resistive to dents and scratches
What makes a material ductile?
Can be drawn easily into a wire
What makes a material malleable?
Can be easily hammered or pressed into shape
What is the difference between strength and toughness?
Strength is how much stress is needed to cause a failure whereas toughness is how much energy can be absorbed (through plastic deformation) before a failure
