Module 3 Materials Flashcards
What is Elasticity?
Elasticity is the ability of a body to return to it’s original size and shape, once the deforming force/stress is removed.
Define Tensile deformation?
Tensile deformation occurs when a string,wire or spring has forces acting at either end. The forces are equal and opposite facing away from each other (180 degrees away) causing an extension in the length of the body.
Describe the 4 points you need to identify in a force extension graph.
Also describe the shape of the graph at each point.
The first point to identify is “Limit of proportionality ”.
This is a singular point in the graph located after a straight line from the origin and before a slight curve to this straight line.
The second is “Elastic Limit”.
This is a singular point, originating after the slight curve to the straight line from the origin and before a deeper curve towards the extension axis.
The third section rather than a point is “Plastic Deformation”. This is right after the point of “Elastic Limit”. It is usually a curvy line with the gradient dipping more towards the extension axis as more weight is applied.
The final point is “Fracture”. This is signified with the curvy line from before stopping at a singular point.
Describe the events taking place at the 4 points you need to identify from a Force Extension graph.
At the Limit of Proportionality: Hooke’s law is obeyed up till this point, Force applied and Extension have a linear relationship. After this point, Hooke’s law is no longer obeyed but for small increments of weights, the material still behaves elastically, till the elastic limit.
At Elastic Limit: After this point the material no longer behaves elastically and starts to deform plastically.
During Plastic Deformation: The material keeps on deforming plastically, if the force is removed the material would not return to it’s original shape and size.
During Fracture: The material snaps
Describe a method to measure the extension against force for a wire.
Also mention the steps for safety.
Tie one end of the wire to clamp and clamp the clamp to the end of a table. At the other end tie the wire to a hooked weight and let it drop from the other end of the table. Draw a marker on the now taught wire between the two ends and place a ruler underneath it. Add weights and measure by how much the marker moves from the original place on the ruler. This is the extension.
You should wear safety glasses and keep a cushion underneath the springs.
Define Hooke’s law.
Give formulae.
Hooke’s law states that before the elastic limit, Force applied and extension of the object are proportional to each other.
This is given by F=kx
What is k in F=kx called?
What are it’s units?
Force constant. In addition, spring constant but this is only for springs
The units are N/m
How is the force constant calculated for springs in series and parallel?
For springs in series; 1/k=(1/k1 + 1/k2 + 1/k3)
For springs in parallel; k=(k1 + k2 + k3)
How is work done on a spring calculated?
E = 1/2 Fx or
E = 1/2 kx^2
(Work done on spring = elastic potential energy stored.
F is halved as force changes as extension changes…so an average of the force is calculated,by halving the final force.)
Also the are under the force- extension graph is equivalent to the work done.
What happens to energy stored in a spring when tension is released after it deformed plastically?
Some of the energy is lost as heat and the majority is used to reverse the compression or extension, however not fully
Define the force extension characteristics of rubber bands.
Rubber bands,while deforming elastically, are harder to extend at the start then get easier and then hard again. When force is unloaded they loose a lot of elastic potential energy as heat energy and after a few cycles get warm to the touch.
What is the area of a graph of stress against strain?
It is the work done or elastic potential energy stored per unit volume in a material
How do you calculate the energy wasted by material while loading and unloading.
Draw a line matching the highest point on the graph to the point directly underneath it on the x axis.
The area between this line and the line connecting to the final length of the material is equal to the heat loss
