Materials Flashcards
What is a tensile and compressive force?
Tensile produce extention
Compressive shorten an object
State Hooke’s Law.
The extention of a spring is directly proportional to the force applied as long as the elastic limit is not exceeded.
What is elastic deformation and when does it occur?
When force is taken goes or origional shape, occurs within the linear region.
What is plastic deformation and when does it occur?
Cannot go to origonal shape, occurs after limit of proportionality.
Give another name for limit of proportionality?
Elastic limit.
What does the ‘k’ represent in Hooke’s law?
Force constant is the measure of stiffness of the spring. Larger the FC the harder it is to extend.
Give two examples other than a spring where Hooke’s law can be applied.
Wires under tention, concrete columns under compression.
Investigate Hooke’s Law.
Spring on clamp with boss and clamp stand. Secure to bench G-clamp. Add mass measure extention with ruler.
Where can we find work down on a graph?
Area under force-extention graph.
What happens to work done before elastic limit is reached?
It is fully recovered.
Why is some energy lost when material is past elastic limit?
Enery used to move atoms to new permanent positions.
What are the two formilas for elastic potential difference?
E = 1/2 F x
E = 1/2 k x^2
Explain enery transfers in bungee cords.
Free falls, slack taken up, jumper comes to a stop. Elastic potetial into gravitation potential and kinetic, jumper accelerates up.
What must operators of bungee jumping take into account.
Wieght, height of jump and cord properties.
Describe curves of metal wire.
Draw curve.
Unloading = loading. (till elastic limit)
Obeys Hooke’s Law.
Describe curves of rubber.
Draw curve.
Does not obey Hooke’s Law but will return to origonal length.
Unloading not equal to loading.
Hysteresis loop.
Decribe curve of polythene.
Draw curve.
Does not obey Hooke’s Law.
Does not go back to origonal length.
Unloading does not equal loading.
Give an example where we need plastic deformation/
A steel sheet pressed into a car part must retain this shape.
Why do we have tensile stress and strain?
Easier to compare materials. Doesnt take into account length, diameter or tension.
Formula for stress and unit?
Froce/cross sectional area
Nm^2 (Pa)
Formula for strain and unit?
Extention/length
No unit
Draw stress-strain graph for a metal with labelled points.
Check
Difference between limit of proportionality and elastic limit?
Elastic limit is boundary between elastic and plastic deformation.
Limit of proportionality is up to where is obeys Hooke’s Law.
What are yeild points are where do they apply?
Where materils extends rapidly. Not always present. in ductile materials.
What is the UTS?
Ultimate tensile stress is the max stress a material can go under before it breaks.
What is necking?
After UTS a material can be longer and thinner at its weakest point.
What does a high UTS mean?
Strong material.
Where is stress and strain directly proportional?
Within the limit of proportionality.
What is the formula and unit for young Modulus?
Stress/Strain
Nm^2 (Pa)
How to find young modulus on stress-strain graph?
Gradient.
What does a higher E mean?
Stiffer material.
How to investigate the young modulus of a metal wire?
Micrometer to measure diameter take mean. Force is the masses added. Ruler and marker on wire to find extenstion.
Find stress and strain, then E from gradient of graph.
What makes polymeric materials differ in the loading curves?
Give example.
Made of long molecular chains, behave differently depending on structure and temperature.
eg rubber whos elastic but plolythene shows plastic.
Give examples of brittle materials and their loading curve.
Glass and cast iron.
Breaking points same as UTS.
Use a plane to describe examples of use of materials.
Al alloys for wings as high E.
Rotor bladed with ceramics widthstand high temps, strong. (brittle no plastic deformation).
Tyres made of rubber, shock absorber.
Draw stress-strain graph of glass and cast iron.
Check
Descrine ductile materials.
Can be hammered into a wire. Most metals ductile.
Extensive plastic region, necking occurs.
Describe bittle materials. Give example.
Distort very little but break under enough stress.
Very little elastic enery stored.
Can be stiff.
eg concrete
Why can polymeric materials extend so much?
The long molecular chains will striaghten.
