3.4 materials Flashcards
(24 cards)
What is meant by extension and compression of a string?
Tensile forces act away from the centre of the spring in both direction and will stretch it out causing the spring to extend
Forces acting towards the centre of the spring in both directions is called compression
What happens to the spring when tensile/compressive forces are exerted?
The spring undergoes ‘tensile deformation’ (for tensile forces) or ‘compressive deformation’ (for compressive forces)
What is Hooke’s law?
In a material showing Hooke’s law, the force applied is directly proportional to the extension in length, up to the limit of proportionality F = spring constant x change in length
What is meant by tensile stress?
The force per unit area
What is tensile strain?
A measure of how the material stretches: the extension divided by the original length. This value is a ratio, so it has 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 force is removed, the object will not return to its original shape (limit of proportionality has been exceeded)
How is energy stored during elastic deformation?
The work done is transferred and stored as elastic potential energy
Describe the energy changes that occur during platic deformation
The material is stretched and the energy from the work done is used to break the bonds between the molecules. This causes permanent deformation.
What is breaking stress
The amount of stress a material can take without it breaking
What does the graph of a brittle material look like?
Straight line and then a curve that cuts off immediately - The material breaks without any noticeable yeild
What is meant when a material is described as brittle?
It does not extend much when a force is applied (tensile strain stays low). The material tendds to break rather than stretch under a large force
What is the elastic limit
The point after which plastic deformation occurs. It is sometimes also referred to as the limit of proportionality
What does the area underneath a force-extension graph represent?
The energy stored in the material
Give the equation that calculates elastic strain energy in terms of spring constant and extension
E = 1/2 k Δ L^2
What is Young’s modulus?
tensile stress/ tensile strain
How do you find the young’s modulus from a stress-strain graph?
gradient
What would the graph of a plastically deformed wire look like?
loading line would be a steady incline with curve at the top, from there the unloading line decreases in a straight line back towards the origin but not meeting it (Because the material can’t go back to original form)
A stress strain graph shows just a straight line inclining at a positive gradient with the end labelled as the breaking point. Describe the material’s properties
It is brittle, There is no plastic deformation (it is elastic) and returns to the same length when the stress is removed. It obeys hookes law
A stress strain graph shows an s shaped loading line and a similarly shaped unloading line that meats the loading line at the origin. Describe the material’s properties
It is a polymer, it is elastic and returns to the same length when the stress is removed. It does not obey hookes law
Which measurements need to be taked to calculate the young modulus of a metial in the form of a wire?
initial length, extention (initial and final lengths), weight (from mass x g)
What are common instruments needed to practically determine the young modulus of a wire like material
micrometer or vernier calliper, rule, travelling microscope, scales, newtonmeter
equation for stress?
force/ cross sectional area
equation for strain?
extension/original length
equation for young’s modulus?
stress / strain
