Chapter 6 - Materials Flashcards
What are tensile forces?
Tensile forces are equal and opposite forces acting on a material to stretch it
What are compressive forces?
Compressive forces are two or more forces together that reduce the length of volume of an object
What are restoring forces?
Restoring forces return a system to its equilibrium position
What is tensile deformation?
Tensile deformation is when an object changes shape or properties under tensile forces
What is compressive deformation?
Compressive deformation is when an object changes shape or properties after compressive forces are exerted
What is Hooke’s Law?
Hooke’s Law states that the extension of a spring is directly proportional to the force applied
In what order does the limit of proportionality and elastic limit come?
The limit of proportionality comes before the elastic limit
How does an elastic body behave before the elastic limit?
Within the elastic limit, a spring experiences elastic deformation meaning it returns to original length when the force is removed
How does an elastic body behave after the elastic limit?
Past the elastic limit, the spring undergoes plastic deformation and experiences permanent structural changes
What is the relationship between force and extension?
Force is directly proportional to extension
Force = Spring Constant x Extension
What is the spring constant?
The spring constant is a measure of the stiffness
What are the steps of the Hooke’s Law Investigation?
Attach spring to one end using clamp, boss, and clam-stand secured to bench
Set up metre ruler
Suspend slotted masses form the spring
Record mass and new length of spring
What are the precautions used for the Hooke’s Law Investigation?
Take readings at eye level reducing parallax errors
Use digital balance
Repeat each reading at least once
Obtain at least 6 recordings
What does it mean if springs are in parallel?
When two springs hold up a mass from different points of connection
What does it mean if springs are in series?
When two springs attached to each other hold up a mass
How can the total spring constant be calculated for parallel and series?
Parallel springs:
ktotal = k1 + k2
Series springs:
1/ktotal = 1/k1 + 1/k2
How does energy behave within the elastic limit?
When a material is compressed/extended within the elastic limit, work done on a material can be fully recovered
How does energy behave during plastic deformation?
Plastic deformation moves atoms to new positions: energy is not recoverable
What is the formula for work done on a spring?
Work done = Force x Extension
Area under force-extension graph
What is the formula for elastic potential energy?
E = 1/2 * k * x2
E = 1/2 * F * x
What is the relationship between elastic potential energy and extension?
E is directly proportional to x2
What is tensile stress?
Tensile stress is the force applied per unit cross-sectional area of the wire
What is the formula for tensile stress?
Tensile stress = Force / Cross-sectional Area
What is the formula for tensile strain?
Tensile Strain = Extension / Original Length
What is the ultimate tensile strength?
Ultimate tensile strength is the maximum stress that a material can withstand before it breaks
What is the breaking strength?
The stress value at the point of fracture is the breaking strength
What is Young Modulus?
Young Modulus is the ratio of stress to strain
What is relationship between stress and strain?
Stress is directly proportional to strain
What is the formula for Young Modulus?
Tensile Stress / Tensile Strain
Pa or Nm-2
What is the failure point?
Catastrophic failure total break of all bonds at one place
What is the dislocation in a material?
The boundary between regions of a material that have slipped causing a misalignment of atomic planes
What is the force required at the yield point?
At the yield point, the force required to move a dislocation is less that the force required to stretch atomic bonds
How do the atoms in an elastic material act?
Metal cations increase in spacing as it is put under tension
Atomic separation returns to initial value when force removed
No energy lost in stretching
How do atoms in a plastic material act?
Planes of atoms slide over each other
Dislocations allow plastic behaviour at lower stress
Dislocations allow bonds to be broken one at a time
What are the effects of alloying?
Foreign atoms pin down dislocations making slips less likely
Planes move over shorter distances
What are the properties of glass?
Brittle material
Strong as little extension for force
No plastic deformation
What are the properties of steel?
Strong material
Not ductile
Breaks suddenly
What are the properties of copper?
Strong material
Ductile
Deforms with plastic flow past elastic limit and yield points
What are the properties of plastic?
Plastic flow from the start
Not strong
Small elastic region
What is plastic flow?
Plastic flow occurs when a material under severe stress starts to behave as a fluid
What is strength?
How much force is needed to break something
Not a fair test as some things are thicker
What is breaking stress?
Breaking Stress = Breaking Force / Area
Force applied to the normal of an area
What is stiffness?
How difficult it is to change the shape of the object
What is brittleness?
Stiff, but not strong
What is elasticity?
Ability of a material to regain its original shape after it is distorted
What is plasticity?
Characteristic of a material to not regain its shape after distortion
What is the relationship between the force as it is loaded and unloaded?
More force is required to load the spring rather than unload
What does the area under a stress-strain graph show?
Total area is work done
Area between x axis and unloading line is elastic potential energy
Area between unloading and loading line is energy lost to heat
How do you obtain an accurate diameter of a wire?
Taking multiple readings of the diameter along the wire
How do brittle materials behave?
Brittle materials show elastic behaviour up to its breaking point without plastic deformation
What are polymeric materials?
Polymeric materials consist of long molecular chains
What do polymeric materials behaviour depend on?
Molecular Structure
Temperature
In what order do the key points appear on a stress-strain graph?
Limit of Proportionality
Elastic Limit
Yield Point 1
Yield Point 2
Ultimate Tensile Strength
Breaking Point
How do you tell from stress-strain graph that a material is brittle or ductile?
Brittle: Breaking point = UTS and little to no plastic deformation
Ductile: Significant plastic deformation and exhibit yield points
How do you tell from stress-strain graph that a material is elastic or plastic?
If it is elastic, it has a linear relationship. If it is plastic, it is non-linear
