Materials Flashcards
(30 cards)
Describe Hooke’s law. Give its equation and its units.
Hooke’s law states that extension is directly proportional to the force applied.
Force (N) = spring constant (Nm^-1) * extension (m).
What’s the difference between a tensile and a compressive force?
Tensile forces stretch the spring, so have positive extension. Compressive forces squash the spring, so have negative extension.
Describe the full force against extension graph.
Force and extension are directly proportional until the material hits the limit of proportinality, in at which case F and X lose this relationship. This limit only lasts briefly however, as the material reaches its elastic limit, resulting in the material being permanently deformed, and the graph begins to taper off.
Describe elastic deformation
When under tension, the atoms of the material are pulled apart. The atoms can move slightly relative to their equilbrium position, so when the force is removed, the atoms return to their equilibrium position and the material returns to its original state.
Describe plastic deformation
Some atoms move relative to the position of other atoms, so when the force is removed, the atoms don’t return to their original positions.
Explain how to investigate extension of a spring.
Equipment set up
1. Set up equipment as shown in the image
2. add masses one at a time
3. measure extension
4. Plot a graph of force against extension
Define stress and give its units.
Stress = tension (or Force) / Cross-sectional area
It’s the same as pressure, so is also measured in Pascals (A.K.A. Nm^-2)
Define strain and give its units
Strain = extension / original length
Strain has no units, it’s a number or a percentage
Describe the effects of stress on a material
Stress pulls apart the atoms from one another, eventually pulling them apart completely, causing the material to break. This is known as the fracture stress. The maximum stress a material can take is known as the ultimate tensile strength
Explain how work done is shown on a force against extension graph, as well as where it is stored.
It’s the area under the graph. The work done is stored as elastic strain energy in the material until the elastic limit.
Explain how to derive the formula to calculate the energy stored in an elastic material as well as its limit
Work done is the force times the displacement, however the force isn’t constant so we take the average of the force at any given point between 0 and the force, i.e. 1/2F. So work done = 1/2F * x. However, Hooke’s law is being obeyed, so F=kx, and therefore work done = 1/2kx^2. This only applies until the elastic limit of the material, as some work is done to seperate the atoms.
Explain what the Youngs modulus is, give its formula and units.
Up to the limit of proportionality, stress and strain are directly proportional, so stress / strain is a constant, The Youngs Modulus (E). Its units are Nm^-2 as strain has no units.
Describe how to find the youngs modulus of a wire.
- Get a long and thin wire, as it will extend more for the same force, which reduces uncertainty.
- Find the CSA of the wire using a micrometer. Take 3 measurements and calculate an average
- Measure the distance from the clamped end to the marker. This is the original length.
- Increase the weight in steps and measure the extension
- plot a graph of stress against strain
How can the practical to find the youngs modulus also find the fracture stress?
Repeat the investigation, but break the wire intentionally and record the force needed to do so. Then repeat the investigation, but increase the force in smaller increments as you approach the force that broke the wire last time. This will allow you to more accurately find the fracture stress.
Describe how to find the youngs modulus from a stress-strain graph
Calculate the gradient
Describe what is meant by the term brittle, and give an exampls of brittle materials
A brittle material will not deform plastically. When too much force is applied, it will simply snap. Ceramics are brittle materials
Describe what is meant by the term ductile, and give an example of a ductile material
Ductile materials can be drawn into wires (i.e. deforned) without losing their strength. Copper is a ductile material.
Describe what is meant by the term strong and give an example of a strong material
Strong materials resist being deformed by a force without breaking. This includes both pulling and squeezing forces. Steel beams are a strong material
Describe what is meant by the term hard and give an example of a hard material
Hard materials are resistant to cutting, indentation and abrasion. Diamonds are a very hard material
Describe what is meant by the term stiff and give an example of a stiff material
Stiff materials have a high resistance to bending and stretching. It’s measured by the youngs modulus - bigger E, stiffer material (Although I feel transwomen might disagree). The outerlayer of safety equipment needs to be stiff.
Describe what is meant by the term tough and give an example of a tough material
Toughness is measure of how much energy a material can absorb before it breaks. Polymers, in particular types of polythene are very tough.
What is special about the stress strain graphs of ductile materials.
They have what’s known as a yield stress, where a large amount of plastic deformation occurs without an increase in load. This causes a kink in the graph.
Describe the structural properties of metals
- Usually form a crystalline lattice (regular repeating pattern)
- A sea of delocalised electrons, making metals good conductors
- strong electrostatic attraction between ions make metals stiff and tough but also ductile, as the ions can move when a force is applied to them
- When a high amount of stress is applied to a metal, they undergo plastic deformation
- 2 metals can be combined to make an alloy, which makes the metal harder and less ductile.
Describe the structural properties of ceramics
- Made by melting other materials and allowing them to cool
- The atoms are crystalline or polycrystalline (many grains / regions of crystalline structure)
- The quicker a ceramic is cooled, the more likely it is amorphus (the atoms have a random arrangement)
- They have no slip planes like metals do, so they very rarely deform plastically before they fracture.
- Ionically or covalently bonded in a giant rigid structure. The strong bonds make ceraamics stiff, but the rigid structure means they are very brittle.
