Topic 4.1 Properties of materials

Guiding Questions and Tasks

1. compare and contrast physical and mechanical force. 2. identify a situation where the following forces might be applied: ­ shear ­ tension ­ compression ­ torsion ­ friction 3. Consider the difference in the lengths of the electricity wires ­ as shown ­ in relation to thermal expansion. 4. Outline the following hardness testing methods: ­ scratch hardness ­ static indentation hardness ­ dynamic hardness 5. Describe the stress / strain curve of the Young’s modulus for rubbers 6. Define toughness 7. Outline the effect temperature has on toughness 8. Compare malleability, ductility, and brittleness 9. List different applications for Piezoelectric materials 10. List different applications for SMAs 11. List different applications for Photochromic materials 12. List different applications for Magneto­rheostatic and Electro­rheostatic materials 13. List different applications for thermoelectric materials 14. Explain why Thermal expansion (expansivity) is an important consideration where two ​ dissimilar materials are joined. 15. Give an example of a design context where density is important 16. Give an example of a design context where electrical resistivity is important 17. Give an example of a design context where thermal conductivity is important 18. Give an example of a design context where hardness is important

Topic 4.1 Properties of materials

Materials are often developed by materials engineers to have specific properties. The development of new materials allows designers to create new products, which solve old problems in new ways. For example, the explosion of plastic materials following the second world war enabled products to be made without using valuable metals.

Essential idea:

Materials are selected for manufacturing products based primarily on their properties.

Nature of design:

The rapid pace of scientific discovery and new technologies has had a major impact on material science, giving designers many more materials from which to choose for their products. These new materials have given scope for “smart” new products or enhanced classic designs. Choosing the right material is a complex and difficult task with physical, aesthetic, mechanical and appropriate properties to consider. Environmental, moral and ethical issues surrounding choice of materials for use in any product, service or system also need to be considered.

Concepts and principles:

• Physical properties: mass, weight, volume, density, electrical resistivity, thermal conductivity, thermal expansion and hardness • Mechanical properties: tensile and compressive strength, stiffness, toughness, ductility, elasticity, plasticity, Young’s modulus, stress and strain • Aesthetic characteristics: taste, smell, appearance and texture • Properties of smart materials: piezoelectricity, shape memory, photochromicity, magneto­rheostatic, electro­rheostatic and thermoelectricity

Guidance:

• Design contexts where physical properties, mechanical properties and/or aesthetic characteristics are important • Design contexts where properties of smart materials are exploited • Using stress/strain graphs and material selection charts to identify appropriate materials

Properties of materials

Properties of materials are categorized according to their Physical Properties or Mechanical ​ ​ ​ Properties.

Physical properties

The physical properties of a material are unaltered by the application of force. Physical properties are listed as: mass, weight, volume, density, electrical resistivity, thermal conductivity, thermal expansion and hardness.

Mass Mass (m) of a body, is a measure of the amount of matter that body contains. It is constant. The SI unit for mass is kilogram (kg).

Weight Weight is a force and represents the mass of an object which is acted upon by gravity and is expressed by Newton’s second law: Force (weight) = m x a g​

2 ​ ag is acceleration due to gravity. The surface of the Earth has an approximate value of 9.8m/s , ​ ​ 2 ​ while on the moon its value is only 1.6m/s .​ Weight is therefore a variable quality. ​ Because it is a force, the SI units for weight are Newtons (N)

Note: People often confuse mass and weight. Remember that weight is a force, and is measured in newtons. Mass is measured in kilograms (kg). [http://www.bbc.co.uk/bitesize/ks3/science/energy_electricity_forces/forces/revision/3/] ​ ​

Volume The amount of 3­dimensional space an object occupies.

Density A measure of how much matter is in a certain volume. A gold bar is quite small but has a mass of 1 kilogram (, so it contains more matter than a similar sized piece of wood. Therefore gold is more dense than wood.

The density of water is about 1 kg per liter (1 liter of water has a mass of 1 kg), so anything that floats has a lower density, and anything that sinks is more dense. [https://www.mathsisfun.com/definitions/density.html] ​ ​

Density is important in relation to product weight and size (for example, for portability). Pre­packaged food is sold by weight or volume, and a particular consistency is required.

Electrical Resistivity Electrical conductivity (σ) and electrical resistivity (ρ)are the measure of how easily free ​ ​ ​ ​ electrons move through a material.

Temperature has the greatest effect on resistivity.The reasons for these changes in resistivity can be explained by considering the flow of current through the material. The flow of current is actually the movement of electrons from one atom to another under the influence of an electric field. Electrons are very small negatively charged particles and will be repelled by a negative electric charge and attracted by a positive electric charge. Therefore if an electric potential is applied across a conductor (positive at one end, negative at the other) electrons will "migrate" from atom to atom towards the positive terminal. Only some electrons are free to migrate however. Others within each atom are held so tightly to their particular atom that even an electric field will not dislodge them. The current flowing in the material is therefore due to the movement of "free electrons" and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor (many free electrons) or a good insulator (hardly any free electrons). The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate. In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material, the more collisions are caused and hence the greater the resistance to current flow. [http://www.learnabout­electronics.org/Resistors/resistors_01a.php] ​ ​

Electrical resistivity is particularly important in selecting materials as conductors or insulators.

Thermal conductivity Thermal conductivity (K) is the measure of the efficiency with which thermal energy will travel through a material. The higher the thermal conductivity, greater is the rate at which the heat will flow. Metals have a high thermal conductivity, while polymers and ceramics have a low thermal conductivity and are insulators rather than conductors of heat. When a temperature gradient exists, temperature will flow from high to low.

Thermal conductivity is important for objects that will be heated or must conduct or be insulated against heat gain or loss.

Thermal expansion Thermal expansion (expansivity) is the measure of a material’s increase in dimensions when that material is heated. When a material is heated, the gain in thermal energy causes an increase in the atomic vibrations, which leads to an increase in atomic separation, which in turn leads to an increase in the material’s overall dimensions.

Consider railway lines, which are lengths of steel. As these are heated by the sun, they expand. If it were not for the expansion space [see figure on the right] then the rails would buckle.

Thermal expansion (expansivity) is important where two dissimilar materials are joined. These may then experience large temperature changes while staying joined.

Hardness Hardness is the ability of a material to resist scratching or abrasion. Hardness tests fall into three broad categories: Scratch Hardness Test Static Indentation Hardness Test Dynamic Hardness

Hardness is important where resistance to penetration or scratching is required. Ceramic floor tiles are extremely hard and resistant to scratching. Another design context would be bearings and brake pads.

Mechanical properties

The mechanical properties of a material describe how it will react to the application of force. There are many different types of force including: ● shear ● tension ● compression ● torsion ● friction ● electrical ● gravitational

Tensile and Compressive Strength The compressive modulus of a material gives the ratio of the compressive stress applied to a material compared to the resulting compression, essentially how easy it is to squash the material between thumb and finger or in a vice. The compressive strength gives the stress that is needed to destroy the sample by crushing.

The tensile strength of a material is the stress required to destroy that sample by tension. As with compressive stresses there will be a clear failure point if the material is brittle. Ductile materials however have several significant points:

● The limit of proportionality, beyond which the sample no longer obeys Hooke’s Law. ● The elastic limit, beyond which the sample will be permanently distorted. ● The yield stress beyond which there is a significant increase in the ease of distortion. ● The ultimate tensile stress, which is the largest stress that the sample can withstand, and ● The breaking stress at which the two ends of the sample are separated.

[http://www.spaceflight.esa.int/impress/text/education/Mechanical%20Properties/index.html ​

[http://www.setareh.arch.vt.edu/safas/fdmtl_imgs/youngs_%20modulus­01­01.png] ​ ​

Tensile strength is important in selecting materials for ropes and cables, for example, for an elevator.

Stiffness Stiffness is the rigidity of an object — the extent to which it resists deformation in response to an applied force. The more flexible an object is, the less stiff it is. Young’s modulus is used to measure a material’s stiffness: a high Young’s modulus means a stiff material. The chart on the right compares the stiffness of glass, carbon steel, aluminium and rubber. It can be seen that brittle materials (glass / ceramics) display linear elastic behaviour and fails with little strain, A soft and tough material, such as low carbon steel, on the other hand, exhibits a very small initial slope, but strain hardened and withstands larger strains before failing.

Stiffness is important when maintaining shape is crucial to performance, for example, an aircraft wing.

Toughness Toughness is the ability of a material to resist the propagation of cracks. A material’s stress­strain curve can be used to give an indication of the overall toughness of the material.

Ductility is a measure of how much something deforms plastically before fracture, but just because a material is ductile does not make it tough. The key to toughness is a good combination of strength and ductility. A material with high strength and high ductility will have more toughness than a material with low strength and high ductility. Therefore, one way to measure toughness is by calculating the area under the stress strain curve from a tensile test. [https://www.nde­ed.org/EducationResources/CommunityCollege/Materials/Mechanical/Toughness.htm] ​ ​

The chart compares the stress­strain curves of High Carbon Steel, Medium Carbon Steel and Low Carbon Steel. It is clear that High carbon steel is the most brittle, while Low Carbon Steel is the most ductile. Comparing the area under the stress­strain curve shows that Medium Carbon Steel is the toughest

A common method for testing the toughness of a material is to measure its resistance to impact. A notch (V or U profile) is machined into the surface. The impact toughness of a metal is determined by measuring the energy absorbed in the fracture of the specimen. This is simply obtained by noting the height at which the pendulum is released and the height to which the pendulum swings after it has struck the specimen. The height of the pendulum times the weight of the pendulum produces the potential energy and the difference in potential energy of the pendulum at the start and the end of the test is equal to the absorbed energy. [https://www.nde­ed.org/EducationResources/CommunityCollege/Materials/Mechanical/ImpactToughness.htm] ​ ​

Toughness is important where abrasion and cutting may take place. Design issues ● High toughness is particularly important for components which may suffer impact (cars, toys, bikes), or for components where a fracture would be catastrophic (pressure vessels, aircraft). ● Toughness varies with temperature; some materials change from being tough to brittle as temperature decreases (e.g. some steels, rubber). A famous example of this problem in steels was the battleships which broke in two in cold seas during the second World War [http://www­materials.eng.cam.ac.uk/mpsite/properties/non­IE/toughness.html] ​ ​

Constance Tipper Constance Tipper was one of the first women to take the Natural Sciences Tripos, in 1915. Her major research contribution was to discover why during the Second World War the Liberty Ships were breaking in two. Working from the Engineering Department in Cambridge, Tipper established that there is a critical temperature below which the fracture in steel changes from ductile to brittle. The Liberty Ships in the North Atlantic were subjected to such low temperatures that they would have been susceptible to brittle failure. The full implications of her work were not realised until the 1950s but after that, the Tipper test became the standard method for determining this form of brittleness in steel. [http://www­g.eng.cam.ac.uk/125/noflash/1925­1950/tipper.html] ​ ​

Ductility Ductility is the ability of a material to undergo plastic deformation by extrusion, or by application of tensile forces. Not to be confused with malleability, which is the ability of a material to be shaped plastically, generally by compressive forces. The amount of cold work that a metal can withstand without failure therefore depends on the metal’s ductility. The image above shows copper alloy being rolled into a strip, while the image on the right shows copper rod being ‘drawn’ through a die: it is the copper’s ductile property that makes this possible. Copper, aluminum, and steel are examples of ​ ductile metals

Malleability Describes a material which can be plastically deformed and shaped when cold, generally by compressive forces. A malleable material can be plastic shaped with hammering or rolling without fracture. Typical malleable materials are mild steel, gold, lead

Malleable materials are ductile, but ductile materials are not always necessarily malleable.

Elasticity Is the measure of a material to stretch under load, then return to its original dimensions when the load is removed.

Plasticity The plasticity of a material is associated with elongation behaviour that exceeds the elastic region. Continued deformation beyond the elastic limit leads to a more complex deformation where the relationship between stress and strain is no longer linear. When a material is taken beyond its elastic limits and the load is removed, the material no longer returns to its original dimensions, but instead displays some permanent plastic deformation.

Brittleness A material that is unable to undergo plastic deformation is described as brittle. Examples of brittle materials include cast iron, concrete, and some glass products.

Aesthetic characteristics Some aesthetic characteristics are only relevant to food, while others can be applied to more than one material group. Although these properties activate people’s senses, responses to them vary from one individual to another, and they are difficult to quantify scientifically, unlike the other properties. Information received from the senses ● sight ­ colour, reflectivity ● taste ­ sour, sweet, salty, bitter ● hearing ­ pith, frequency, acoustics, absorption ● smell ­ odour, fragrance ● touch ­ texture

Aesthetic appeal can be an important part of a decision to purchase goods so designers often manipulate these characteristics in order to appeal to the tastes of the market segment, which can depend on social and cultural backgrounds. Manipulating these factors can help with product differentiation.

Smart materials Smart materials have properties that react to changes in their environment. This means that one of their properties can be changed by an external condition, such as temperature, light, pressure or electricity. This change is reversible and can be repeated many times. There are a wide range of different smart materials. Each offer different properties that can be changed. [http://www.bbc.co.uk/schools/gcsebitesize/design/electronics/materialsrev5.shtml] ​ ​

Piezoelectricity. When a piezoelectric material is deformed, it gives off a small electrical ​ discharge. When an electric current is passed through it, it increases in size (up to a 4% change in volume). These materials are widely used as sensors in different environments. Piezoelectric materials can be used to measure the force of an impact, for example, in the airbag sensor on a car. The material senses the force of an impact on the car and sends an electric charge to activate the airbag.

Piezoelectric Technology by Kelly Bauer ​ The same technology that powers your car’s airbag sensor has the ability to harness the power of human footfall and convert it into energy. This idea, applied to running paths, dance floors, sidewalks and train platforms, could revolutionize the alternative energy industry. In 1880, brothers Jacques and Pierre Curie, discovered what is now known as the piezoelectric effect. By applying mechanical stress to crystals such as topaz, quartz, and tourmaline, among others, the brothers were able to create electrical charges and found the voltage to be in proportion to the stress. If the name Curie stands out, it’s not surprising. The Curie family holds five Nobel Prizes and Pierre Curie’s wife, Marie Sklodowska­Curie holds the distinction of being the first woman to win a Nobel Prize, the first person, and only woman, to win more than once and the only person to win twice in more than one area of science. The airbag application of their discovery is a simple one to understand. The sensor in your car’s airbag detects the level of shock when you’re involved in an accident and, if past a certain threshold, sends an electrical signal which initiates the airbag to deploy. This is but one of many ways that we are currently using the piezoelectric effect. Another practical application is sonar technology. Used in ultrasonic transducers both in submarines and also in cars today it helps to determine the distance between vehicles and their potential obstacles. While very helpful, these applications only scratch the surface of what piezoelectric technology can do. Humans produce an immense amount of kinetic energy, but the manufacturing of systems on a scale large enough to effectively capture that energy has not been possible, until recent times. A company called Pavegen has recently developed kinetic energy harvesting and power generating systems of mind boggling size and application. They envision their technology in pavements, school corridors and even recently saw it installed on a football pitch in Rio de Janerio. The pitch has been equipped with 200 kinetic tiles which work with accompanying solar panels to power the lights for 10 hours at a time. The football pitch, which previously suffered frequent blackouts, is now literally people­powered.

The material Pavegen uses is covered with a soft surface, not unlike that found on many playgrounds and can turn 1 step into 7 watts of energy. Pavegen has its sights set on a variety of applications, from treadmills that would use your steps to charge your cell phone while you work out, to producing energy at concerts and large scale events, by placing tiles under the dance floors and common areas. In an age of energy conservation and an ever growing focus on green technology and science, the harvesting of human created kinetic energy is an exciting addition to the alternative energy sector. This technology joins the ranks of solar, wind and wave harnessed energy systems. Fostering inspiration for architects and engineers to design the types of cities and buildings that were previously seen only in science fiction. [http://www.biztekmojo.com/00172/piezoelectric­technology] ​ ​

Shape memory alloys (SMA) Shape­memory alloys (SMAs), are metals that change shape when heated to an activation temperature. When cool, they are malleable and can be shaped like a typical metal. However, when heated to activation, they return to their preset shape. At the atomic level, the crystalline structure of an SMA changes with heat from one regular structure to another. However, while all metals will change shape with heat (i.e. melt), SMAs change shape all in solid phase and this change is reversible. The most commonly used SMA is nitinol (nickel titanium) [http://makezine.com/2012/01/31/skill­builder­working­with­shape­memory­alloy/] ​ ​

Applications for pseudo­elasticity include eyeglasses frames, medical tools and antennas for mobile phones. One application of shape memory effect is for robotic limbs (hands, arms and legs). It is difficult to replicate even simple movements of the human body, for example, the gripping force required to handle different objects (eggs, pens, tools). SMAs are strong and compact and can be used to create smooth, lifelike movements. Computer control of timing and size of an electric current running through the SMA can control the movement of an artificial joint. Other design challenges for artificial joints include development of computer software to control artificial muscle systems, being able to create large enough movements and replicating the speed and accuracy of human reflexes.

Photochromism Photochromicity refers to a material that can described as having a reversible change of colour when exposed to light. One of the most popular applications is for colour­changing sunglass lenses, which can darken as the sun brightens. A chemical either on the surface of the lens or embedded within the glass reacts to ultraviolet light, which causes it to change form and therefore its light absorption spectra.

Photochromic lenses have millions of molecules of substances such as silver chloride or silver halide embedded in them. The molecules are transparent to visible light in the absence of UV light, which is normal for artificial lighting. But when exposed to UV rays, as in direct sunlight, the molecules undergo a chemical process that causes them to change shape. [http://science.howstuffworks.com/innovation/science­questions/question412.htm] ​ ​

Electro­rheostatic (ER) and magneto­rheostatic (MR) Electro­rheostatic (ER) and magneto­rheostatic (MR) materials are fluids that can undergo dramatic changes in their viscosity. They can change from a thick fluid to a solid in a fraction of a second when exposed to a magnetic (for MR materials) or electric (for ER materials) field, and the effect is reversed when the field is removed. MR fluids are being developed for use in car shock absorbers, damping washing machine vibration, prosthetic limbs, exercise equipment and surface polishing of machine parts. ER fluids have mainly been developed for use in clutches and valves, as well as engine mounts designed to reduce noise and vibration in vehicles.

Thermoelectricity Thermoelectricity is, at its simplest, electricity produced directly from heat. It involves the joining of two dissimilar conductors that, when heated, produce a direct current. Thermoelectric circuits have been used in remote areas and space probes to power radio transmitters and receivers.