Chapter 6: Materials Flashcards
What is meant by the extension and compression of a string?
Tensile forces act away from the centre of the spring in both directions 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?
Spring undergoes tensile deformation or compressive deformation
What is Hooke’s Law?
force applied is directly proportional to the extension in length, up to the limit of proportionality
F = KE
What is tensile stress?
Force per unit area
What is tensile strain?
Measure of how a material stretches: extension divided by original length
What is the difference between elastic and plastic deformation?
Elastic: force is removed and the object will return to its original shape
Plastic: after the force is removed, the object will not return to the original shape
How is energy stored during elastic deformation?
Work done is transferred and stored as elastic potential energy
Describe the energy changes that occur during plastic deformation
As material is stretched, energy from work done is used to break bonds between molecules, causes permanent deformation
What is breaking stress?
the amount of stress a material can take without it breaking
What does it mean when a material is brittle?
Does not extend much when a force is applied (tensile strain stays low), tends to break
What is the elastic limit?
A point along a force extension graph where a material stops deforming elastically and started behaving plastically
What does the area under a force-extension graph represent?
energy stored in a material
Equation calculating elastic strain energy in terms on spring constant and extension
1 half x k x (delta x ) squared
What is Young’s modulus?
tensile stress/ tensile strain
How do you find the Young’s modulus from a stress-strain graph?
gradient
What is the limit of proportionality?
The point along the force-extension graph where a material no longer obeys Hooke’s law as Force is no longer proportional to extension
What is a spring constant (K)?
The force required to stretch/compress a string by 1 meter
Describe a force-extension graph for a metal
The force increases proportionally and then starts to curve, and when unloading, we can extrapolate a straight line down, ensuring the gradient of the unloading curve is the same as the loading curve
What is the force-extension graph for plastic
The loading and unloading curves form a leaf sort of shape, and they have different gradients, and the area between the graph is the same as the energy lost by heat
What is a hysteresis loop?
Loop formed by unloading and loading
Describe a force extension graph for polythene
Loading curve is an uneven curve while the unloading curve is a straight line, and the area between the curves is the energy that never returns
Amorphous
Without a clearly defined shape or form
Techniques and procedures used to determine the Young Modulus for a metal
1) Measure diameter of wire in 3 different places using a micrometer
2) Attach a meter ruler to a work bench so that the lower end is facing the G clamp and place a marker on the wire at 0cm on the ruler
3) Measure the length of wire from the blocks of wood to the marker on the wire when it is taut
4) Attach a mass to the wire and record the total mass attached to the end of the wire in kg. Record new position of the marker
5) Add another 100g and mark the new position and repeat at least 7 times
Techniques and procedures used to investigate force-extension characteristics for arrangements which may include springs, rubber bands, polythene strips
1) Hang spring from a clamp and boss and secure it so that it will not fall off
2) Secure the meter rule vertically to the clamp, using the set square to ensure it is straight and place it adjacent to the spring
3) Record the meter rule reading at the bottom of the spring and place the fiducial marker on this point.
4) Hang mass hanger from the bottom of the spring
5) Add another mass. Record the new meter rule reading, number of masses and total extension
6) Repeat until spring has become permanently stretched
7) Repeat the experiment for the rubber band and polythene strip
What is ultimate tensile strength
The maxima stress a material can withhold before it breaks
