Materials Flashcards
What is Hooke’s law?
Extension (∆L) is directly proportional to force applied (F), given that the environmental conditions are kept constant.
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 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. Work done= ½ x F x ΔL
How do you find the Young’s modulus from a stress-strain graph?
The gradient of the line.
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.
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 forces 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 dissipated as heat.
How is the 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 the spring 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
Where would you find the ultimate tensile stress on a stress-strain graph?
The highest point on a graph, it is the maximum stress a material can withstand.
