Module 3.4 - Materials Flashcards
How does energy transfer when stretching a spring then releasing?
- As you stretch, the pulling force transfer energy to the ELASTIC store
- When you release, the energy transfers from the elastic to the KINETIC.
Define deformation
The change in shape/size of an object (stretching/squashing/thrusting)
What is elastic deformation?
When a material RETURNS to its original length when a load is removed
What is plastic deformation?
When a material DOES NOT return to its original length when a load is removed - permanent
Describe extension
> Caused by TENSILE forces
Equal and opposite tension forces when stretching
Causes INCREASE in length
POSITIVE extension
Describe compression
> Caused by COMPRESSIVE forces
Equal and opposite but facing EACH OTHER
Causes DECREASE in length
NEGATIVE extension
What happens if a object obeys Hooke’s law?
Its extension will be DIRECTLY PROPORTIONAL to the force applied to deform it, up to its elastic limit
What is Hooke’s law in equation form?
F = kx
F - N
k - N/m
x - m
What does the distance the spring moves whilst stretching equal?
The extension of the spring
How do you found the work done of a spring (energy stored) on a F-x graph?
The area UNDER the graph
How do you work out the Elastic potential energy, E of a material if it obeys Hooke’s law?
E = 1/2Fx = 1/2kx^2
E - J
k - N/m
x - m
F - N
Which - spring, rubber band, plastic strip - has elastic deformation?
Spring and rubber band
Which - spring, rubber band, plastic strip - obey’s Hooke’s law
Spring
For which - spring, rubber band, plastic strip - does all its energy come back out when unloaded?
Spring
Which - spring, rubber band, plastic strip - has plastic deformation?
Plastic strip
Which - spring, rubber band, plastic strip - does not obey Hooke’s law?
Rubber band and plastic strip
For which - spring, rubber band, plastic strip - does some energy remain as thermal?
Rubber band and plastic strip
For which - spring, rubber band, plastic strip - has some of its energy used to break bonds so doesn’t come back out?
Plastic Strip
What happens to force and extension for a spring of fixed length and thickness?
Force and extension remain DIRECTLY PROPORTIONAL
What happens to force and extension as the length of the spring is changed?
Force is directly proportional to extension ÷ length - x/L
What happens to force and extension as the thickness of the spring is changed as well as length?
Force ÷ area is now directly proportional to extension ÷ length
How is stress worked out?
Stress = Force ÷ area
Stress - N/m^2
Force - N
Area - m^2
How is strain worked out?
Strain = extension ÷ length of material
Strain - no units
Extension - m
Length - m
How do you work out Young’s modulus?
E = stress ÷ strain
= (Force x Length) ÷ (Area x Extension)
When is Young’s modulus valid?
When the limit of proportionality is not exceeded
How do you work out Young’s modulus from a stress-strain graph?
The GRADIENT of the ELASTIC region of the graph
Define Ultimate tensile stress/strength (UTS)
MAXIMUM stress that a material can withstand
What is the Breaking stress?
The stress at the FRACTURE point
Describe a strong material
> Can withstand a LARGE FORCE
> HIGH UTS
Describe a stiff material
> DIFFICULT to bend
> HIGH Young’s Modulus
Describe a ductile material
> Can be DRAWN out into a WIRE
> Experiences a lot of PLASTIC deformation before snapping
Describe a brittle material
> Snaps easily
> Experiences NO or LITTLE plastic deformation before snapping
Describe a polymeric material
> Made of long chain molecules called POLYMERS
> Experiences PLASTIC deformation and NECKS before snapping
What does it mean when an object necks?
When stress decreases by a greater proportion than strain
