1.shape memory alloys

overview

A shape-memory alloy: is an alloy that "remembers" its original, cold-forged shape: returning the pre-deformed shape by heating. This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. Shape-memory alloys have applications in industries including medical and aerospace
The three main types of shape-memory alloys are thecopper-zinc-aluminuim-nickel, copper-aluminium-nickel, and nickel-titanium alloys

history

The generic name for the family of nickel-titanium alloys is Nitinol. In 1961, Nitinol, which stands for Nickel Titanium Naval Ordnance Laboratory, was discovered to possess the unique property of having shape memory. William J. Buehler, a researcher at the Naval Ordnance Laboratory in White Oak, Maryland, was the one to discover this shape memory alloy. The actual discovery of the shape memory property of Nitinol came about by accident. At a laboratory management meeting, a strip of Nitinol was presented that was bent out of shape many times. One of the people present, Dr. David S. Muzzey, heated it with his pipe lighter, and surprisingly, the strip stretched back to its original form.

properties

The properties of Nitinol are particular to the exact composition of the metal and the way it was processed. The physical properties of Nitinol include a melting point around 1240 ° C to 1310 ° C, and a density of around 6.5 g/cm³. Various other physical properties tested at different temperatures with various compositions of elements include electrical resitivity, thermoelectric power, Hall coefficient, velocity of sound, damping, heat capacity, magnetic susceptibility, and thermal conductivity . Mechanical properties tested include hardness, impact toughness, fatigue strength, and machinability.
The large force generated upon returning to its original shape is a very useful property. Other useful properties of Nitinol are its "excellent damping characteristics at temperatures below the transition temperature range, its corrosion resistance, its nonmagnetic nature, its low density and its high fatigue strength" . Nitinol is also to an extent impact- and heat-resistant . These properties translate into many uses for Nitinol.

applications

Many of the current applications of Nitinol have been in the field of medicine Tweezers to remove foreign objects through small incisions were invented by NASA. anchors with Nitinol hooks to attach tendons to bone were used for Orel Hershiser's shoulder surgery. Orthodontic wires made out of Nitinol reduces the need to retighten and adjust the wire. These wires also accelerate tooth motion as they revert to their original shapes. Nitinol eyeglass frames can be bent totally out of shape and return to their parent shape upon warming. Nitinol needlewire localizers"used to locate and mark breast tumors so that subsequent surgery can be more exact and less invasive" utilize the metal's shape memory property. Another successful medical application is Nitinol's use as a guide for catheters through blood vessels .

references

http://en.wikipedia.org/wiki/Shape-memory_alloy
http://www.stanford.edu/~richlin1/sma/sma.html

images and videos

Sma wire.jpg
Sma wire.jpg



one-way shape memory
one-way shape memory
Two-way intrinsic shape memory
Two-way intrinsic shape memory











2.piezoelectric materials


overview

The piezoelectric effect describes the relation between a mechanical stress and an electrical voltage in solids.
It is reversbile: an applied mechanical stress will generate a voltage and an applied voltage will change the shape of the solid by a small amount (up to a 4% change in volume).
In physics, the piezoelectric effect can be described as the the link between electrostatics and mechanics.

history

The piezoelectric effect was discovered in 1880 by the Jacques and Pierre Curie brothers. They found out that when a mechanical stress was applied on crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane sugar, electrical charges appeared, and this voltage was proportional to the stress.
First applications were piezoelectric ultrasonic transducers and soon swinging quartz for standards of frequency (quartz clocks).
An everyday life application example is your car's airbag sensor. The material detects the intensity of the shock and sends an electricla signal which triggers the airbag.

properties

The piezoelectric effect occurs only in non conductive materials. Piezoelectric materials can be divided in 2 main groups: crystals and cermaics. The most well-known piezoelectric material is quartz (SiO2).

applications

High voltage and power sources
Sensors
Actuators
Frequency standard
Piezoelectric motors
Reduction of vibrations and noise
Infertility treatment

references

http://en.wikipedia.org/wiki/Piezoelectricity#Mechanism
http://www.piezomaterials.com/

images and videos

piezoelectric effect
piezoelectric effect
external image 220px-SchemaPiezo.gif







3.Magnetorheological fluid

overview

A magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot of this is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications.
MR fluid is different from a ferrofluid which has smaller particles. MR fluid particles are primarily on the micrometre-scale and are too dense for Brownian Motion to keep them suspended (in the lower density carrier fluid). Ferrofluid particles are primarily nanoparticles that are suspended by Brownian Motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications.
properties
A typical MR fluid consists of 20-40 percent by volume of relatively pure, 3-10 micron diameter iron particles, suspended in a carrier liquid such as mineral oil, synthetic oil, water or glycol. A variety of proprietary additives, similar to those found in commercial lubricants to discourage gravitational setting and promote particle suspension, are commonly added to LORD Corporation’s state-of-the-art MR fluids to enhance lubricity, modify viscosity and inhibit wear. For most engineering applications, a simple Bingham plastic model is effective in describing the essential, field-dependent fluid characteristics.
MR fluids made from iron particles exhibit maximum yield strengths of 50-100 kPa for applied magnetic fields of 150-250 kA/m. MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and usage. Further, because the magnetic polarization mechanism is unaffected by temperature, the performance of MR-based devices is relatively insensitive to temperature over a broad temperature range (including the range for automotive use).
MR fluids are usually applied in one of two modes. MR fluid operating in valve mode, with fixed magnetic poles, may be appropriate for hydraulic controls, servo valves, dampers, and shock absorbers. The direct-shear mode with a moving pole, in turn, would be suitable for clutches and brakes, chucking/locking devices, dampers, breakaway devices and structural composites

modes of operation

An MR fluid is used in one of three main modes of operation, these being flow mode, shear mode and squeeze-flow mode. These modes involve, respectively, fluid flowing as a result of pressure gradient between two stationary plates; fluid between two plates moving relative to one another; and fluid between two plates moving in the direction perpendicular to their planes. In all cases the magnetic field is perpendicular to the planes of the plates, so as to restrict fluid in the direction parallel to the plates.
Flow mode
Mr fluid flowmode.jpg
Mr fluid flowmode.jpg

Shear Mode
Mr fluid shearmode.jpg
Mr fluid shearmode.jpg



Squeeze-Flow Mode
Mr fluid squeezeflowmode.jpg
Mr fluid squeezeflowmode.jpg




applications

The applications of these various modes are numerous. Flow mode can be used in dampers and shock absorbers, by using the movement to be controlled to force the fluid through channels, across which a magnetic field is applied. Shear mode is particularly useful in clutches and brakes - in places where rotational motion must be controlled. Squeeze-flow mode, on the other hand, is most suitable for applications controlling small, millimeter-order movements but involving large forces. This particular flow mode has seen the least investigation so far. Overall, between these three modes of operation, MR fluids can be applied successfully to a wide range of applications. However, some limitations exist which are necessary to mention here.

references

http://en.wikipedia.org/wiki/Magnetorheological_fluid
http://www.lord.com/products-and-solutions/magneto-rheological-(mr).xml








4.auxetics materials

overview


Auxetic materials are materials with a negative Poisson’s Ratio, i.e. become fatter when stretched and thinner when squashed .
  • external image auxetic.gif

Examples of auxetic materials include

Certain rocks and minerals]
Living bone tissue (although this is only suspected]
Specific variants of polytetrafluorethylene polymers such as Gore-Tex]
Paper, all types. If a paper is stretched in an in-plane direction it will expand in its thickness direction due to its network structure]


advantages do auxetic materials

  • Apart from the scientific value of having materials with a negative Poisson`s ration, auxetic materials show enhanced mechanical properties such as:
    • increased shear stiffness
    • increased plane strain fracture toughness
    • increased indentation resistance


references

http://staff.um.edu.mt/jgri1/auxetic/auxetic_f2.html
http://en.wikipedia.org/wiki/Auxetics