Unit 4 Materials Flashcards
State Hooke’s law
Extension (or compression) of a spring is directly proportional to the force exerted on it
List some properties of Hooke’s law.
Only valid when the limit of proportionality is not exceeded
Obeyed by wires and solid objects made of most materials
Can also be applied to the compression of a spring where the change in length is the amount the spring gets shorter.
Define elastic deformation
A material returns to its original shape and size once any forces on it are removed
Define plastic deformation
A material remains permanently deformed so does not return to its original shape and size once any forces on it are removed
What is tensile stress?
The force stretching the object divided by its cross sectional area
What is tensile strain?
The ratio of the object’s extension over its original length.
What is plastic flow?
Where small stress leads to large strain because the cross sectional area of the material decreases rapidly
What is UTS?
Ultimate tensile strain - maximum stress the material experiences, measure of the material’s strength
What is a material’s breaking point?
The stress in the material at which it breaks
List properties of stiff materials
Steep initial gradients
Large Young Modulus
List properties of strong materials
High UTS and breaking point
List properties of brittle materials
They break without much plastic deformation happening before breaking
List properties of ductile materials
They undergo high plastic deformation before breaking
What is the elastic limit of a material?
The point beyond which a spring or material becomes permanently deformed
Draw + explain a force extension graph of a metal wire stretched beyond its elastic limit.
Unloading line does not go through the origin, wire is permanently stretched
Loading and unloading lines parallel because Young’s modulus is constant
Area between loading and unloading lines is the work done to permanently deform the wire
Draw + explain a force extension graph of a rubber band
The unloading line goes through origin - rubber band returns to its original length
Area under loading curve is the work done to stretch rubber band
Area under unloading curve is the work done by rubber band when unloaded
Area between loading and unloading curves is the difference between energy stored in stretched rubber band and energy recovered when unstretched
Difference between energy stored and energy recovered is because some energy is transferred to the internal energy of the molecules
Draw + explain a force extension graph of a polyethylene strip
Unloading line does not go through origin - polyethylene is permanently stretched
Area between loading and unloading curves is the sum of the work done to deform the strip and energy transferred to the internal energy of the molecules
What does the area under the line of a force extension graph for an object represent?
Work done on object/ Total elastic potential energy stored by it
What happens to the elastic potential energy of an elastically deformed object when the force on it is removed?
Transferred to other stores such as kinetic or gravitational potential
What causes an object to be under tension?
Forces acting on it are stretching it
What causes an object to be under compression?
Forces acting on it are compressing it
What is the Young modulus of a material?
Measure of the material’s stiffness in Nm^-2 or Pa
Define the limit of proportionality of a material
Point up to which its stress and strain are proportional
How can the Young modulus of a material be found?
Gradient of the initial straight line section of its stress - strain graph
What does the area under the straight line section of a material’s stress -strain graph represent?
Energy stored in the material per unit volume
What does the first yield point on a material’s stress - strain graph represent?
Stress at which the material suddenly weakens and stretches plastically without any additional force
How are crumple zones making cars safe?
They plastically deform during a crash so less energy is transferred to the passengers.
