Composites

Composites are a combination of two or more materials that are bonded together to improve their chemical, physical, mechanical or electrical properties.

Fibres Fibres are a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread with a length to thickness ratio of at least 80.

Matrix composition The "Matrix Composition" of composites refers to how composite materials are formed together. A composite is a material that has been combined with two or more other materials to improve the properties of the materials. When one material is embedded into another material, the product is a composite. The separate materials then reinforce each other, resulting in an overall stronger and more useful material. It is where one material acts as a glue 'matrix' holding the other material in place, such as Glass reinforced fibre (fibreglass).

New materials It is possible to enhance the properties of a material by adding another material with the properties that are wanted, an example would be improving the toughness of concrete by adding steel rods. Concrete is hard (high compressive strength) and weak in tension but steel is tough and has high tension but not too hard, therefore by adding steel to concrete, we get a hard and tough composite material that is ideal for building. Something important to consider when making composites is thermal expansivity as two materials with different rates of expansion would break each other.

Smart materials Smart materials have one or more properties that can be dramatically altered, for example, viscosity, volume, and conductivity. The property that can be altered influences the application of smart material.

Smart materials Smart materials include piezoelectric materials, magneto-rheostatic materials, electro-rheostatic materials, and shape memory alloys. Some everyday items are already incorporating smart materials such as coffee pots, cars, the International Space Station, eye glasses, and the number of applications for them is growing steadily. Watch smart material demo Here is an example of a smart material. It is specifically a shape memory alloy. If you watch the first minute or so, you'll get the idea of how it works. Watch further for more detail into where it can be used in real life situations, although it is formatted like an advertisement!

Piezoelectric material 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). They 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 and electric charge to activate the airbag. Look at the following website for more information. ckissues/august97/features/airbag/airbag.html

A BBQ lighter uses something called piezoelectricity to generate a nice spark that lights the grill. "Piezo," in Greek, means "pressure," and you find piezoelectric materials in a number of different places. For example, all of these products depend on the piezoelectric effect: The push-button igniter in a gas BBQ grill or fireplace Push-button cigarette lighters Piezoelectric beepers (common in digital watches and anything electronic that goes "beep") Piezoelectric tweeters in stereo speakers Sound-generating arrays for sonar, fish finders and ultrasound devices Crystal microphones Phonograph needles Quartz crystals used in most digital clocks and timers as the time base

Electro-rheostatic and magneto- rheostatic Electro-rheostatic (ER) and magneto- rheostatic (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.

Shape memory alloys SMAs are metals that exhibit pseudo- elasticity and shape memory effect due to rearrangement of the molecules in the material. Pseudo-elasticity occurs without a change in temperature. The load on the SMA causes molecular rearrangement, which reverses when the load is decreased and the material springs back into its original shape.

Applications for pseudo-elasticity include eye glasses frames, medical tools and antennas for mobile phones. The frames of reading glasses have been made of shape memory alloy as they can undergo large deformations in their high temperature state and then instantly revert back to their original shape when the stress is removed. Another application is using SMAs to construct aircraft engines which reduces aircraft's engine noise greatly. SMAs have also been widely used in dental work, used most commonly in braces, in the case where they are broken or bent, thus temperature can be changed to send them back to their original shape.

One application of shape memory effect is for robotic limbs (hands, arms, 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.