Section 5 - Materials Flashcards
What is Hooke’s Law?
Extension (ΔL) is directly proportional to force applied (F), given that the environmental conditions are kept constant
k - stiffness constant
What are the base units for k?
Kgs^-1
What is meant by tensile stress?
The force applied per unit cross sectional area Nm^-2
What is tensile strain?
A measure of how the material strecthes: the extension (ΔL) divided by the original length
No units
What is the difference between elastic and plastic deformation?
Elastic deformation - when the force is removed the object will return to its original shape
Plastic deformation - after the load is removed the object will not return to its original shape
What is breaking stress?
The minimum stress needed to break a material
How does stress affect the material at an atomic level?
- The atoms begin to be pulled apart from one another
- The stress becomes so great that atoms separate completely and the material breaks - breaking stress
What is the Ultimate Tensile Strength?
The maximum stress that the material can withstand - just before the breaking stress
How is a brittle material represented on a stress-strain graph? Why?
- Steep gradient, bends slightly then suddenly stops
- The wire undergoes only a very small increase in strain beyond the linear section before fracture
What is meant when a material is described as brittle?
It doesn’t perform plastically but fractures when the stress reaches a certain value
What is the elastic limit?
The force above which the material will be plastically deformed (permanently stretched)
What does the area underneath a force-extension graph represent?
The work done to deform the material
What is Young’s modulus?
Describes the stiffness of a material
How do you find the Young’s modulus from a stress-strain graph?
The gradient of the line
What does a graph of plastic deformation look like on a force extension graph?
The line is a straight line and then bends across - the unloading line doesn’t go through the origin as the material is permanently extended (stretched)
How can a force-extension graph show Hooke’s Law is being obeyed?
When it is a straight line through the origin - force and extension are directly proportional
What is the limit of proportionality and what does it look like on a force-extension graph?
The point after which Hooke’s law is no longer obeyed, it is shown by the line beginning to curve on a force-extension graph
What is the work done to stretch or compress a material stored?
Elastic strain energy
Why are the loading and unloading lines parallel on a force-extension graph for a plastically deformed material?
The stiffness constant (k) hasn’t changed, the force between the atoms are the same when loading and unloading
Why isn’t all work done stored as elastic strain energy when a stretch is plastic?
Work is done to move atoms apart, so energy is not stored as elastic strain energy but is dissipate as heat
How is dissipation of energy in plastic deformation used to design safer vehicles?
Crumple zones - deform plastically in a crash using the car’s kinetic energy so less is transferred to the passengers
Seat belts - stretch to convert the passenger’s kinetic energy into elastic strain energy
Outline the energy changes that occur when a spring fixed at the top is pulled down and released
The work done in pulling down (stretching it) is stored as elastic strain energy, when the spring is released this is converted to kinetic energy which is converted to gravitational potential energy as the spring rises
Do stress-strain graphs show the behaviour of a material or a specific object?
Material
Where would you find the ultimate tensile stress on a stress-strain graph?
The highest point
What would the stress-strain graph for a ductile material look like?
A ductile material can undergo a large amount of plastic deformation before fracturing
What is the yield point on a stress-strain graph?
The stress at which a large amount of plastic deformation takes place with a constant or reduced load
What are force-extension graphs used for?
Specific to the tested object and depend on its dimensions
What are stress-strain graphs used for?
They describe the general behaviour of a material, as stress and strain are independent of the dimensions
Explain what is meant by the spring constant k of a spring and give the units
force/extension
Nm^-1
What is an advantage and a disadvantage of using a thinner material in an experiment?
Advantage: Greater change in displacement for same load, so percentage error is reduced
Disadvantage: The beam may undergo plastic deformation therefore the graph will no longer be linear
What are the 2 conditions for a system to be in equilibrium?
- The resultant force is 0
- The sum of clockwise moments equals the sum of anticlockwise moments
What is the definition of centre of mass?
- The point where an object’s weight is considered to act
- The point where a force has no torque
Why must the centre of mass be below where the object is hung from?
- The point is a pivot
- So there is no resultant moment
How do you find mass per unit length?
density x area
When large tensions are used on a piece of wire, and the frequency is predicted, how would the actual frequencies differ?
- When the tension is larger, the diameter is reduced
- There is a lower mass per unit length so frequency will be lower than expected
What equation should you always use for short mark questions that seem to need the one suvat you don’t have?
W=Fs
