Acoustic Cavitation
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Cavitation induced by an acoustic field. Microscopic gas bubbles which are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size, and then rapidly collapse. High power ultrasonics usually use the inertial cavitation of microscopic vacuum bubbles for treatment of surfaces, liquids and slurries.
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Auxetic Materials
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Materials which, when stretched, become thicker perpendicularly to the applied force, i.e. they have a negative Poisson's ratio. Such materials have interesting mechanical properties such as high energy absorption and fracture resistance. This may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material and sponge mops.
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Brinelling
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Surface fatigue caused by repeated impact or overloading.
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Cavitation
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The formation of vapour bubbles in a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure. Usually divided into two classes of behavior. Inertial (or transient) cavitation occurs when a void or bubble in a liquid rapidly collapses, producing a shock wave. Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to energy input, e.g. acoustic field.
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Centrifugal Force
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An outward force associated with rotation. Centrifugal force is one of several so-called pseudo-forces (or inertial forces), so named because, unlike real forces, they do not originate in interactions with other bodies situated in the environment of the particle upon which they act. Instead, centrifugal force originates in the rotation of the frame of reference within which observations are made.
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Converse Piezoelectric Effect
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Materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied). For example, lead zirconate titanate crystals will exhibit a maximum shape change of about 0.1% of the original dimension.
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Cooling
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The act of reducing temperature.
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Creep
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The tendency of a solid material to slowly move or deform permanently under the influence of stresses. A result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature. The rate of deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load.
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Decomposition (biological)
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The break down of tissue of a formerly living organism into simpler forms of matter.
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Deformation
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A change in the shape or size of an object due to an applied force. This can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described in terms of strain.
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Particulalrly in the case of solid structures.
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Deposition (physical)
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A process in which gas transforms into solid (also known as desublimation). The reverse of deposition is sublimation. One example of deposition is the process by which, in sub-freezing air, water vapour changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground.
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If preceeded by, for example, sublimation of the solid.
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Elastic Recovery
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The tendancy of a material that has been stretched to return to its original shape.
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For example, through the use of an elastic band or sheet.
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Elasticity
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The physical property of a material when it deforms under stress (e.g. external forces), but returns to its original shape when the stress is removed. The amount of deformation is called the strain. The elastic regime is characterized by a linear relationship between stress and strain, denoted linear elasticity.
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Electrostriction
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A property of all electrical non-conductors, or dielectrics, that causes them to change their shape under the application of an electric field. All dielectrics exhibit some electrostriction, but certain engineered ceramics, known as relaxor ferroelectrics, have extraordinarily high electrostrictive constants,e.g. Lead magnesium niobate (PMN), Lead magnesium niobate-lead titanate (PMN-PT), Lead lanthanum zirconate titanate (PLZT).
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Entropic Explosion
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An explosion in which the reactants undergo a large change in volume without releasing a large amount of heat.
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Explosion
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A sudden increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases.
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Ferromagnetism
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The mechanism by which certain materials (such as iron) form permanent magnets and/or exhibit strong interactions with magnets. Responsible for commonly observed magnetism phenomena, e.g. 'fridge magnets. A material is 'ferromagnetic' only if all its magnetic ions add a positive contribution to the net magnetisation. If some of them subtract from the net magnetisation (i.e. are partially anti-aligned), then the material is 'ferrimagnetic'.
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Fluid Hammer
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A pressure surge or wave resulting when a fluid in motion is forced to stop or change direction suddenly (Momentum Change). Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.
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Force
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That which can cause an object with mass to accelerate. Force has both magnitude & direction, i.e. it's a vector quantity. An object with constant mass will accelerate in proportion to the net force acting upon it and in inverse proportion to its mass (or the net force on an object is equal to the rate of change of momentum it experiences). Forces acting on objects may cause them to rotate or deform, or result in a change in pressure.
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Fractal Forms
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A fractal is generally 'a rough or fragmented geometric shape that can be split into parts, each of which is (at least approximately) a reduced-size copy of the whole,' a property called self-similarity. A mathematical fractal is based on an equation that undergoes iteration, a form of feedback based on recursion.
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Gravitation
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A natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass.
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Groove
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A long and narrow indentation built into a material, generally for the purpose of allowing another material or part to move within the groove and be guided by it.
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Harmonic Oscillator
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A system which, when displaced from its equilibrium position, experiences a restoring force proportional to the displacement. Mechanical examples include pendula (with small angles of displacement) and masses connected to springs.
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For part of the cycle.
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Hooke's Law
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Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit. Materials for which Hooke's law is a useful approximation are known as linear-elastic or 'Hookean' materials.
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Hydraulic Press
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A hydraulic mechanism for applying a large lifting or compressive force. It is the hydraulic equivalent of a mechanical lever.
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Impact Force
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A high force or shock applied over a short time period. Such a force or acceleration can sometimes have a greater effect than a lower force applied over a proportionally longer time period.
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Length Contraction
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The physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer. This contraction is usually only noticeable at a substantial fraction of the speed of light; the contraction is only in the direction parallel to the direction in which the observed body is travelling.
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Lever
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A rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object.
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Magnetism
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One of the phenomena by which materials exert attractive or repulsive forces on other materials. Some well-known materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron, cobalt, and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field.
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Magnetoelastic Effects
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A group of effects including Magnetostriction (or Joule Magnetostriction), Delta-E Effect, Wiedemann Effect, Magnetovolume Effect, and their Inverses: Villari Effect, Delta-E Effect, Matteucci Effect and the Nagaoka-Honda Effect.
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Magnetostriction
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A property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetisation. The variation of material's magnetisation due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value. This effect can cause losses due to frictional heating in susceptible ferromagnetic cores.
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Magnetovolume Effect
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One of a group of Magnetoelastic Effects. Volume change due to magnetisation (most evident near the Curie temperature)
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Mechanical Force
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A mechanical influence that causes a free body to undergo an acceleration.
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Negative Thermal Expansion
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A physicochemical process in which some materials contract upon heating rather than expanding as most materials do. Materials which undergo this unusual process have a range of potential engineering, photonic, electronic, and structural applications. For example, mixing a negative thermal expansion material with a 'normal' material which expands on heating, could result in a zero expansion composite material.
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Osmotic Pressure
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Osmotic pressure is the pressure applied by a solution to prevent the inward flow of water across a semipermeable membrane.
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Pascal's Law
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(or the Principle of transmission of fluid-pressure) Pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains same.
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Phase Change
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The transformation of a thermodynamic system from one phase to another. Most commonly used to describe transitions between solid, liquid and gaseous states of matter, in rare cases including plasma.
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Piezomagnetism
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A phenomenon observed in some antiferromagnetic crystals. It is characterised by a linear coupling between the system's magnetic polarisation and mechanical strain. In a piezomagnetic, one may induce a spontaneous magnetic moment by applying physical stress, or a physical deformation by applying a magnetic field. Piezomagnetism differs from the related property of magnetostriction.
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Plasticity
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The deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal or plastic being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself.
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Poisson's Effect
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When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression.
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By indirect action of an intermediary material expanding in accordance with Poisson's Effect.
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Pressure Increase
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Pressure is an effect which occurs when a force is applied on a surface. Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point.
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Pressurisation
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The application of pressure in a given situation or environment; and more specifically refers to the process by which atmospheric pressure is maintained in an isolated or semi-isolated atmospheric environment (for instance, in an aircraft, or whilst scuba diving).
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Pseudoelasticity
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(or superelasticity) An elastic (impermanent) response to relatively high stress caused by a phase transformation between the austenitic and martensitic phases of a crystal. Exhibited in Shape memory alloys. Superelastic alloys belong to the larger family of shape memory alloys. Unlike shape-memory alloys, no change in temperature is needed for the alloy to recover its initial shape.
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Settling
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The process by which particulates settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force. For gravity settling, this means that the particles will tend to fall to the bottom of the vessel, forming a slurry at the vessel base.
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Surface Acoustic Wave
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(SAW) An acoustic wave traveling along the surface of a material having some elasticity, with an amplitude that typically decays exponentially with the depth of the substrate. This kind of wave is commonly used in devices called SAW devices in electronics circuits.
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Tension
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The pulling force exerted by a string, cable, chain, or similar object on another object. It is the opposite of compression.
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Tension in one system can be used to induce compression in another.
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Thermal Contraction
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The tendency of matter to reduce in volume (contract) in response to a change in temperature or when a substance is cooled.
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Thermal Expansion
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The tendency of matter to change in volume in response to a change in temperature or when a substance is heated.
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Expansion in one system can be used to induce compression in another.
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Torque
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Torque (also called moment or moment of force) is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist. Torque is the rotational analogue of Force.
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Viscoelasticity
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The property of exhibiting both viscous and elastic characteristics when undergoing deformation.
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Wind
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The flow of air or other gases that compose an atmosphere
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Indirectly, for example via a turbine.
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