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Chapter 4: Testing Materials
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- 14.1: DESCRIBING MATERIALS..
- 2What 3 classes of materials are we going to consider the mechanical properties of?- Ceramics
- Metals
- Polymers. - 3What are 3 mechanical properties of ceramics?- Hard
- Brittle
- Stiff - 4What does hard mean?Hard means that they are difficult to scratch.
- 5What does brittle mean?Brittle means that they will shatter into jagged pieces, as cracks propagate easily.
- 6What does stiff mean? What is stiffness a measure of?Stiff means that they are difficult to stretch or bend. (A measure of the difficulty of changing the shape of an object)
- 7What are examples of ceramics?China and other pottery.
- 8What is an example of a more modern 'engineering ceramic'?Alumina
- 9What do pure metals tend to be?They tend to be soft:
- 10What are metals that can be shaped easily called?Malleable.
- 11What are metals that can be drawn out into wires called?Ductile.
- 12What is the difference between malleability and ductility?Malleability is the ability to deform easily upon the application of a compressive force, and Ductility is doing the same with tensile force.
- 13Give an example of a metal alloy.Steel.
- 14What are metal alloys usually?Metal alloys are usually harder than the pure metals used to make them.
- 15Which 2 things does the material class of polymers include? Give 2 examples for each.- Synthetic materials (such as polythene and perspex)
- Natural materials (such as leather and cotton). - 16What do glassy polymers often have?Similar properties to glass.
- 17What are glassy polymers?Brittle.
- 18What are semi-crystalline polymers?Tough.
- 19What does being tough mean they can do?They can undergo considerable deformation and absorb more energy before breaking compared to more brittle polymers- cracks or defects don't propagate as easily
- 20What is tough the opposite of?Brittle.
- 21What does the choice of materials to use in a product depend on?- The mechanical properties
-Cost and appearance of the finished product. - 22What does strength mean?Can withstand a large pressure (load) without fracturing
- 23What is an elastic material? Give 2 examples.A material which returns to its original shape after being distorted.
Examples are: Rubbers, polymers. - 24What is a plastic material? Give an example.A material that retains a new shape after being distorted.
Example is polystyrene. - 25What is a polymeric material? Give an example.A material composed of long chain molecules.
Examples are polymers. - 26What are crystalline materials? Give 2 examples.Materials with internal structures composed of crystals.
Examples are: Graphite, diamond - 27What is the Creep of a material?The property that allows a material to change shape even though the force remains constant.
- 28What is a composite material? Give an example.A material made from two or more materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components.
Example is:
- Concrete: consists of aggregate held with a matrix of cement. - 294.2: STRETCHING WIRES AND SPRINGS.
- 30What did the English experimenter Robert Hooke find in 1678?He found that for small extensions, the force F is proportional to extension x.
- 31What is this relationship known as? What can it be given as?It is known as Hooke's Law, it can be given as:
F=kx - 32What does k represent in this formula?k is a constant of proportionality called the spring constant.
- 33What does every spring or wire have?Its own value for the spring constant.
- 34What does the spring constant give?It gives a measure of how stiff the specimen is.
- 35What does a large spring constant mean?A large spring constant means that the specimen is stiff and therefore difficult to stretch.
- 36Which 3 things does the value of k depend on?- The material of the wire
- Its length
- Its cross-sectional area. - 37What must be remembered about the spring constant?The spring constant is a value for a specimen rather than for a material.
- 38How will the spring constants of a thin wire and a thick wire made from the same metal compare?A thin wire will have a much smaller k than that of a thick wire made from the same metal.
- 39What can Hooke's law also be applied to?It can be applied to compression (squashing the springs).
- 40What are the limitations of Hooke's Law?Extend a spring too far and it will become easier to stretch and will not return to its original length when you let go.
- 41What does a force/extension graph look like? Where is the: - Area of elastic deformation? -Elastic limit/Proportional limit? -Yield point? -Area of Plastic deformation? -Point of ultimate tensile strength? -Fracture point/ Failure point
- 42What does the graph illustrate?How force varies with extension for larger extensions.
- 43What happens over the first part of the graph?The wire (or spring) stretches/ deforms elastically.
- 44What will a wire that is deformed elastically do when the load is removed?It will return to its original length.
- 45What happens when the extension exceeds the elastic limit?The wire deforms plastically.
- 46What will a wire that is deformed plastically do when the load is removed?It will not return to its original length.
- 47What is the shape of the graph for the elastic region?The graph is linear for nearly all of the elastic region, curving very slightly near the elastic limit.
- 48What is the plastic region of the graph?It is non-linear.
- 49What happens at the fracture point?The wire fractures/breaks.
- 50Is it only wires and springs that follow Hooke's law? Why?No, for small extensions and compressions all materials will show elastic behaviour.
- 51What happens the harder you pull on a catapult? What does this mean?The more energy is stored for the extra stretch, so the further the projectile flies when you release it.
- 52What is the force F required to stretch a string by x if the force is proportional to extension?The force F is kx.
- 53However, why is the energy stored in a stretched spring not simply force x displacement, Fx?Because the force grows steadily larger as the spring is stretched.
- 54As the force starts at 0 and rises linearly to finish at F, what is the average force?F/2
- 55As a result of this, what is the energy stored in an elastically stretched spring given by?E = 1/2(Fx)
- 56Because F=kx, what can this also be written as?E = 1/2(kx^2)
- 57What happens to energy when a body deforms elastically?The energy stored = The energy transferred stretching the spring.
- 58Because of the previous equations, what can the energy transferred stretching a material be found from?It can be found from the area under the line of a force/extension graph.
- 594.3: STRESS, STRAIN, & THE YOUNG MODULUS.
- 60To make a fair comparison between materials, what concept do we use? What is this?We use the concept of stress, which is force per unit area.
- 61What is the fracture stress?The stress at which a material breaks.
- 62What is the yield stress of materials?The stress at which a material begins to deform plastically and become permanently deformed.
- 63What are the units of stress the same as?The units of pressure (pascal, Pa)
- 64How much does 1kg weigh on the earth's surface?About 9.8N
- 65How will the stretches of a long wire and a shorter wire of the same cross-sectional area compare if the same load is applied to both?The long wire will stretch more than the shorter wire.
- 66How can we make a fair comparison?We can calculate the strain.
- 67What is strain?Strain is the fractional increase in length.
- 68How does strain differ from extension?Unlike extension, strain does not depend on the original length of the specimen.
- 69Does strain have a unit? Why?Strain has no unit because it is a ratio of 2 lengths.
- 70What is strain often given as? What must you do before using strain in calculations?Strain is often given as a %, but you must convert % strain back into the ratio of x/L before using them in calculations.
- 71For a given force F, how does doubling the length L affect the extension x?For a given force F, doubling the length L doubles the extension x.
- 72What does the stress-strain graph of a brittle material, such as glass, show?It shows very little plastic deformation; the graph is linear for all its length.
- 73What is mild steel?A tough material.
- 74What does this mean?It means that it undergoes considerable plastic deformation before fracture.
- 75What can this explain?It can explain why tough materials have rounded edges on fracturing rather than the sharp, jagged edges of brittle materials.
- 76What may happen to a rod of steel under tension?A rod of steel may 'neck' under tension, meaning that part of it becomes narrower than the rest.
- 77When would necking happen on a graph?In the plastic region, after the point of ultimate tensile strength.
- 78What does the Young Modulus give a measure of?It gives a measure of the constant stiffness of a material rather than the stiffness of a particular specimen.
- 79What are the 2 units for Young's ModulusNm^-2 or Pa
- 80Why are the units for Young's Modulus the same as for stress?Because strain is a ratio and is dimensionless.
- 81Why is Young's Modulus often a very large number?Because F and L are large values, divided by the product of the much smaller x and A:
E = (FL)/(xA) - 82On a graph, what would a large strain for a small stress tell you about the material? How would the gradient look?That is it flexible and easy to stretch.
Less steep gradient. - 83On a graph, what would a small strain for a large stress tell you about the material? How would the gradient look?That it is stiff and hard to stretch.
Steeper gradient. - 84What would the Young modulus be for a stiffer material?Larger.
- 85In which region of a stress-strain graph can Young's modulus be defined?The linear elastic region.
- 86What would a stress-strain graph look like?Same as a force-extension graph.
- 87What is the ultimate tensile stress?The highest point on a stress-strain curve.
- 884.4: CHOOSING MATERIALS.
- 89Knowledge of what 3 properties of materials in important in construction? Why?- Fracture Stress
- Yield Stress
- Young's Modulus
Allows people to predict the behaviour of materials under stress. - 90When is a material in tension?When a force is acting in a direction to stretch the material.
- 91What is the force described as in this case?A tensile force.
- 92On the other hand, what does a compressive force do?A compressive force tends to compress or squash the material.
- 93What can some materials be?Strong in compression but weak in tension.
- 94In general, for yield stress, what is the order of the 3 classes of materials in descending order?Metals --> Ceramics --> Polymers.
- 95When choosing a material, give 2 examples of other physical properties that are important.- Density
- Toughness. - 96What is density?Density is mass per unit volume
Density = (mass/Volume) - 97Give 2 examples of non-physical properties that are important.Cost and availability.
- 98What do material selection charts allow?They allow quick comparisons to be made between different classes of materials.
- 99What do scientists use on these charts? Why?Scientists use logarithmic scales on these charts to display the large range of values.
- 100Do logarithmic scales have a 0 on them? What do they do?Logarithmic scales don't have a 0 on them. They get smaller at the lower end but never reach 0.
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- AS - Level Physics (AS) Fichas sobre Chapter 4: Testing Materials, creado por Matt Clancy el 13/04/2016.
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- Matt Clancy
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