Friday 20 November 2009

What is Modal Analysis

Vibration can occur in any physical system. Properties of the system are the frequencies at which vibration occurs and the modal shapes which the vibrating system assumes, these properties can be analytically determined using Modal Analysis.

Even though Vibration modes analysis is a critical element of a design it is often ignored. Structural elements like complex steel floor systems can be predominantly subjected to noticeable vibration. Natural vibration modes in structural works and mechanical support systems will cut down equipment life, Premature or unexpected failure may occur due to vibration modes this may result in hazardous situations. Potential for failure or damage due to rapid stress cycles of vibration can be assessed by detailed fatigue analysis.

Detailed seismic study also needs an understanding of the natural vibration modes of a system, because during seismic activity the large amount of energy acting on a system varies with frequency.

The fundamental vibration mode shapes and corresponding frequencies are resolved by detailed modal analysis. This can be comparatively simple for basic elements of a simple system and extremely complex when qualifying a complicated mechanical device or a complicated structure exposed to periodic loading. These systems necessitate exact determination of natural frequencies and mode shapes by means of techniques such as Finite Element Analysis.
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Monday 2 November 2009

What is Hooke’s Law?

Hooke’s law is named after the seventeenth century physicist Robert Hooke who discovered it in 1660 (18 July 1635 - 3 March 1703).

Deformation (change of shape) of a solid is caused by a force that can either be compressive or tensile when applied in one direction (plane). Compressive forces try to compress the object (make it smaller or more compact) while tensile forces try to tear it apart. We can study these effects by looking at what happens when you compress or expand a spring. Hooke’s Law describes the relationship between the force applied to a spring and its extension.


Hooke’s Law - the relationship between extension of a spring and the force applied to it.

Deviation from Hooke’s Law
We know that if you have a small spring and you pull it apart too much it stops ’working’. It bends out of shape and loses its springiness. When this happens Hooke’s Law no longer applies, the spring’s behaviour deviates from Hooke’s Law. Depending on what type of material we are dealing the manner in which it deviates from Hooke’s Law is different. We give classify materials by this deviation. The following graphs show the relationship between force and extension for different materials and they all deviate from Hooke’s Law. Remember that a straight line show proportionality so as soon as the graph is no longer a straight line, Hooke’s Law no longer applies.

Brittle material :
This graph shows the relationship between force and extension for a brittle, but strong material.
Note that there is very little extension for a large force but then the material suddenly fractures. Brittleness is the property of a material that makes it break easily without bending. Have you ever dropped something made of glass and seen it shatter? Glass does this because of its brittleness.
Plastic material :
Here the graph shows the relationship between force and extension for a plastic material. The material extends under a small force but it does not fracture.

Ductile material :
In this graph the relationship between force and extension is for a material that is ductile. The material shows plastic behavior over a range of forces before the material finally fractures.
Ductility is the ability of a material to be stretched into a new shape without breaking.
Ductility is one of the characteristic properties of metals.

A good example of this is aluminium, many things are made of aluminium. Aluminium is used for making everything from cool drink cans to aeroplane parts and even engine blocks for cars. Think about squashing and bending a cool drink can. Brittleness is the opposite of ductility.

When a material reaches a point where Hooke’s Law is no longer valid, we say it has reached its limit of proportionality. After this point, the material will not return to its original shape after the force has been removed. We say it has reached its elastic limit.

Tuesday 27 October 2009

Properties and characteristics of Wave

 Contents :
1.0 Definition of Wave
2.0 Characteristics of Waves
2.1 Amplitude
2.2 Wavelength
2.3 Period
2.4 Frequency
2.5 Speed
3.0 Types of Waves
3.1 Transverse waves
3.2 Longitudinal wave
4.0 Properties of Waves
4.1 Reflection
4.2 Refraction
4.3 Interference
4.4 Standing Waves
4.5 Diffraction
4.5.1 Huygen's Principle
4.6 Dispersion
1.0 Definition of waves :
Waves are disturbances which propagate (move) through a medium, Light is a special case, it exhibits wave-like properties but does not require a medium through which to Propagate. Waves occur frequently in nature. The most obvious examples are waves in water, on a dam, in the ocean, or in a bucket. Waves can be viewed as transfer energy rather than the movement of a particle. Particles form the medium through which waves propagate but they are not the wave. Waves in water consist of moving peaks and troughs. A peak is a place where the water rises higher than when the water is still and a trough is a place where the water sinks lower than when the water is still. A single peak or trough we call a pulse. A wave consists of a train of pulses.

So waves have peaks and troughs. The following diagram shows the peaks and troughs on a wave.


If we look very carefully we notice that the height of the peaks above the level of the still water is the same as the depth of the troughs below the level of the still water. The size of the peaks and troughs is the same.

2.0 Characteristics of Waves:
2.1 Amplitude :

The characteristic height of a peak and depth of a trough is called the amplitude of the wave. The vertical distance between the bottom of the trough and the top of the peak is twice the amplitude. We use symbols agreed upon by convention to label the characteristic quantities of the waves. Normally the letter A is used for the amplitude of a wave. The units of amplitude are metres (m).


2.2 Wavelength :
Look a little closer at the peaks and the troughs. The distance between two adjacent (next to each other) peaks is the same no matter which two adjacent peaks you choose. So there is a fixed distance between the peaks.

Looking closer you'll notice that the distance between two adjacent troughs is the same no matter which two troughs you look at. But, more importantly, it is the same as the distance between the peaks. This distance which is a characteristic of the wave is called the wavelength.
Waves have a characteristic wavelength. The units are metres (m).


The wavelength is the distance between any two adjacent points which are in phase. Two points in phase are separate by an integer (0,1,2,3,...) number of complete wave cycles. They don't have to be peaks or trough but they must be separated by a complete number of waves.

2.3 Period :
The time between two adjacent peaks is same and also the time between two adjacent troughs always the same, no matter which two adjacent troughs you pick. The time you have been measuring is the time for one wavelength to pass by. We call this time the period and it is a characteristic of the wave.

Waves have a characteristic time interval which we call the period of the wave and denote with the symbol T. It is the time it takes for any two adjacent points which are in phase to pass a fixed point. The units are seconds (s).

2.4 Frequency :
There is another way of characterising the time interval of a wave. We timed how long it takes for one wavelength to pass a fixed point to get the period. We could also turn this around and say how many waves go by in 1 second.

We can easily determine this number, which we call the frequency and denote f. To determine the frequency, how many waves go per second, we work out what fraction of a waves goes by in 1 second by dividing 1 second by the time it takes T. The unit of frequency is the Hz.
Waves have a characteristic frequency. F=1/T

2.5 Speed :
Now if you are watching a wave go by you will notice that they move at a constant velocity. The speed is the distance you travel divided by the time you take to travel that distance. This is excellent because we know that the waves travel a distance equal to wavelength in a time T. This means that we can determine the speed.

There are a number of relationships involving the various characteristic quantities of waves.
A simple example of how this would be useful is how to determine the velocity when you have the frequency and the wavelength. We can take the above equation and substitute the relationship between frequency and period to produce an equation for speed of the form

3.0 Types of Waves :
We agreed that a wave was a moving set of peaks and troughs and we used water as an example. Moving peaks and troughs, with all the characteristics we described, in any medium constitute a wave. It is possible to have waves where the peaks and troughs are perpendicular to the direction of motion, like in the case of water waves. These waves are called transverse waves.

There is another type of wave called a longitudinal wave and it has the peaks and troughs in the same direction as the wave is moving. The question is how do we construct such a wave?

An example of a longitudinal wave is a pressure wave moving through a gas. The peaks in this wave are places where the pressure reaches a peak and the troughs are places where the pressure is a minimum.

4.0 Properties of Waves :
We have discussed some of the simple characteristics of waves that we need to know. Now we can progress onto some more interesting and, perhaps, less intuitive properties of waves.

4.1 Reflection :
When waves strike a barrier they are reflected. This means that waves bounce off things. Sound waves bounce off walls, light waves bounce off mirrors, radar waves bounce off planes and it can explain how bats can fly at night and avoid things as small as telephone wires. The property of reflection is a very important and useful one.

When waves are reflected, the process of reflection has certain properties. If a wave hits an obstacle at a right angle to the surface then the wave is reflected directly backwards. If the wave strikes the obstacle at some other angle then it is not reflected directly backwards. The angle that the wave arrives at is the same as the angle that the reflected wave leaves at. The angle that waves arrives at or is incident at equals the angle the waves leaves at or is reflected at.
4.2 Refraction :
Sometimes waves move from one medium to another. The medium is the substance that is carrying the waves. In our first example this was the water. When the medium properties change it can affect the wave.

Let us start with the simple case of a water wave moving from one depth to another. The speed of the wave depends on the depth. If the wave moves directly from the one medium to the other then we should look closely at the boundary. When a peak arrives at the boundary and moves across it must remain a peak on the other side of the boundary. This means that the peaks pass by at the same time intervals on either side of the boundary. The period and frequency remain the same! But we said the speed of the wave changes, which means that the distance it travels in one time interval is different i.e. the wavelength has changed. Going from one medium to another the period or frequency does not change only the wavelength can change.

Now if we consider a water wave moving at an angle of incidence not 90 degrees towards a change in medium then we immediately know that not the whole wave front will arrive at once. So if a part of the wave arrives and slows down while the rest is still moving faster before it arrives the angle of the wave front is going to change. This is known as refraction. When a wave bends or changes its direction when it goes from one medium to the next. If it slows down it turns towards the perpendicular.
If the wave speeds up in the new medium it turns away from the perpendicular to the medium surface.
When you look at a stick that emerges from water it looks like it is bent. This is because the light from below the surface of the water bends when it leaves the water. Your eyes project the light back in a straight line and so the object looks like it is a different place. 4.3 Interference :
If two waves meet interesting things can happen. Waves are basically collective motion of particles. So when two waves meet they both try to impose their collective motion on the particles. This can have quite different results.

If two identical (same wavelength, amplitude and frequency) waves are both trying to form a peak then they are able to achieve the sum of their efforts. The resulting motion will be a peak which has a height which is the sum of the heights of the two waves. If two waves are both trying to form a trough in the same place then a deeper trough is formed, the depth of which is the sum of the depths of the two waves. Now in this case the two waves have been trying to do the same thing and so add together constructively. This is called constructive interference.

If one wave is trying to form a peak and the other is trying to form a trough then they are competing to do different things. In this case they can cancel out. The amplitude of the resulting wave will depend on the amplitudes of the two waves that are interfering. If the depth of the trough is the same as the height of the peak nothing will happen. If the height of the peak is bigger than the depth of the trough a smaller peak will appear and if the trough is deeper then a less deep trough will appear. This is destructive interference.
4.4 Standing Waves :
When two waves move in opposite directions, through each other, interference takes place. If the two waves have the same frequency and wavelength then a specific type of constructive interference can occur: standing waves can form.

Standing waves are disturbances which don't appear to move, they look like they stay in the same place even though the waves that from them are moving.

4.5 Diffraction :
One of the most interesting, and also very useful, properties of waves is diffraction. When a wave strikes a barrier with a hole only part of the wave can move through the hole. If the hole is similar in size to the wavelength of the wave diffractions occurs. The waves that comes through the hole no longer looks like a straight wave front. It bends around the edges of the hole. If the hole is small enough it acts like a point source of circular waves. This bending around the edges of the hole is called diffraction. To illustrate this behavior we start by with Huygen's principle.

4.5.1 Huygen's Principle :
Huygen's principle states that each point on a wave front acts like a point source or circular waves. The waves emitted from each point interfere to form another wave front on which each point forms a point source. A long straight line of points emitting waves of the same frequency leads to a straight wave front moving away.

4.6 Dispersion :
Dispersion is a property of waves where the speed of the wave through a medium depends on the frequency. So if two waves enter the same dispersive medium and have different frequencies they will have different speeds in that medium even if they both entered with the same speed.

Monday 26 October 2009

What the atom is made up of?

The Greek word atom means indivisible. The discovery of the fact that an atom is actually a complex system and can be broken in pieces was the most important step and pivoting point in the development of modern physics.

It was discovered by Rutherford in 1911, that an atom consists of a positively charged nucleus and negative electrons moving around it. At first, people tried to visualize an atom as a microscopic analog of our solar system where planets move around the sun. This naive planetary model assumes that in the world of very small objects the Newton laws of classical mechanics are valid. This, however, is not the case.

The microscopic world is governed by quantum mechanics which does not have such notion as trajectory. Instead, it describes the dynamics of particles in terms of quantum states that are characterized by probability distributions of various observable quantities.
For example, an electron in the atom is not moving along a certain trajectory but rather along all imaginable trajectories with different probabilities. If we were trying to catch this electron, after many such attempts we would discover that the electron can be found anywhere around the nucleus, even very close to and very far from it. However, the probabilities of finding the electron at different distances from the nucleus would be different. What is amazing: the most probable distance corresponds to the classical trajectory!

You can visualize the electron inside an atom as moving around the nucleus chaotically and extremely fast so that for our \mental eyes" it forms a cloud. In some places this cloud is denser while in other places more thin. The density of the cloud corresponds to the probability of finding the electron in a particular place. Space distribution of this density (probability) is what we can calculate using quantum mechanics. Results of such calculation for hydrogen atom are shown in Fig. As was mentioned above, the most probable distance (maximum of the curve) coincides with the Bohr radius.

Quantum mechanical equation for any bound system (like an atom) can have solutions only at a discrete set of energies E1;E2;E3 : : : , etc. There are simply no solutions for the energies E in between these values, such as, for instance, E1 < E < E2. This is why a bound system of microscopic particles cannot have an arbitrary energy and can only be in one of the quantum states. Each of such states has certain energy and certain space configuration, i.e. distribution of the probability.

A bound quantum system can make transitions from one quantum state to another either spontaneously or as a result of interaction with other systems. The energy conservation law is one of the most fundamental and is valid in quantum world as well as in classical world. This means that any transition between the states with energies Ei and Ej is accompanied with either emission or absorption of the energy ¢E = jEi ¡ Ej j. This is how an atom emits light.
Electron is a very light particle. Its mass is negligible as compared to the total mass of the atom. For example, in the lightest of all atoms, hydrogen, the electron constitutes only 0.054% of the atomic mass. In the silicon atoms that are the main component of the rocks around us, all 14 electrons make up only 0.027% of the mass. Thus, when holding a heavy rock in your hand, you actually feel the collective weight of all the nuclei that are inside it.

How Fibre Optics works?

Water can be directed from one place to another by confining it within a pipe. In the same way light can be directed from one place to another by confining it within a single glass Fibre.

The light is kept within the fibre by total internal reflection. The amount of light which can be carried by a single Fibre is very small so it is usual to form a light tube tapping a few thousand Fibres together. On great advantage of such a light tube is flexibility; it can be ties in knots and still function. However since total internal reflection only occurs when light is going from a medium to a less dense medium, it is necessary to coat each fibre with glass of a lower refractive index. Otherwise light would leak from one fibre at their points of contact.

Light tube can be used to bring light from a lamp to an object, thus illuminating the object. A second light tube can then used to carry light from the illuminated object to an observer, thus enabling the object to be seen and photographed. The procedure has been used to photograph the digestive system the reproductive system and many other parts of the human body. In the case of the light tube carrying light from the object to the observer, it is vital that the individual fibres in the tube do not cross each other, otherwise the image will become garbled. Like radio waves, light waves are electromagnetic. However, their shorter wavelength and higher frequency means that a single light beam can carry far more telephone conservations at one time compared with a radio wave.

In the case of long fibre cables it would be necessary to incorporate a device to boost the intensity of the light to make up for losses due to absorption. Nevertheless the system has great potential for the communication industry, including the possibility of transmitting pictures over long distances.

The reason why light bends when going from one medium to another is because of the change of velocity.

Friday 23 October 2009

Pump - Part 1

 Contents : 

1.0 Introduction to Pump
2.0 Classification of Pumps
3.0 Construction and working Principal of Pumps
Introduction to pump
The pump is one of the most important accessories of prime mover; Pumps are used for increasing pressure of a liquid, feeding the water, forcing the lubricating oil into machine parts and other various industrial and domestic applications.

Rising of water from wells is the earliest form of pumping. Modern applications are much broader. Modern pumps function on one of two principles. By far the majority of pump installations are of the velocity head type (Kinetic Energy). In these devices, the pressure rise is achieved by giving the fluid a movement. At the exit of the machine, this movement is translated into a pressure increase.

The other major type of pump is called positive displacement. These devices are designed to increase the pressure of the liquid while essentially trying to compress the volume.

Many varieties of pumps are available. The selection of the pump class and type for a certain application is influenced by head required, layout, fluid characteristics, intended life, energy, cost and materials of construction. Very important parameters for design of pumps are pressure and liquid flow rate.

Classification of Pumps :
There are many ways of classification of pumps based on their type of construction, application, function, Principal of energy addition etc,
1. Kinetic Energy or Velocity head pumps
1.1 Centrifugal pumps
1.1.1 Volute pump
1.1.2 Diffuser pump
1.1.3 Vertex pump
Sub Classification based on type of flow
1. Axial flow pump (single or multistage)
2. Radial flow pump (single or double suction)
3. Mixed flow pump (single or double suction)
4. Peripheral pump (single or multistage)
1.2. Special Effect
1.2.1 Gas lift pump
1.2.2 Jet pump
1.2.3. Hydraulic ram pump
1.2.4. Electromagnetic pump
2. Positive displacement pumps
2.1. Reciprocating pump
2.1.1 Piston, plunger
2.1.1.1. Direct acting pump (simplex or duplex)
2.1.1.2. Power pump (single / double acting,
simplex, duplex, triplex, multiplex)
2.1.2. Diaphragm pump (mechanically or hydraulic driven,
simplex or multiplex)
2.2. Rotary pump
2.2.1. Single rotor pump
2.2.1.1 Vane Pump
2.2.1.2 Piston pump
2.2.1.3 Screw pump
2.2.1.4 Diaphragm or flexible member
2.2.2. Multiple rotor pumps
2.2.2.1 Gear pump
2.2.2.2 Lobe pump
2.2.2.3 Screw pump
2.2.2.4 Circumferential piston pump

3. Construction and working principal of major pumps

Centrifugal Pumps :
A pump which employs centrifugal force for conveying liquids from one place to other is called centrifugal pump. It is similar to a reversed water turbine. Centrifugal pumps are used in more industrial applications than other type of pump because these pumps offer low initial and maintenance costs. Centrifugal pumps are smooth, non-pulsating in operation and has ability to tolerate non-flow conditions.

The most important parts of the centrifugal pump are the rotating impeller and a stationary casing. The centrifugal pumps are sub classified based on design of the casing as Volute type, vertex type and diffuser type.

In centrifugal pumps kinetic energy of the leaving water from the impeller is converted into potential energy which is utilized to increase the delivery head of the pump.
When the impeller rotates, the liquid is discharged by centrifugal force from its centre, by this action vacuum is created at suction eye which is connected with suction pipe and the liquid from reservoir flows in to the impeller.

Volute pump :
In volute pumps area of flow gradually increases from throat towards the delivery pipe. The increase in area of flow decreases the exit velocity and hence pressure increases in the casing.

Diffuser pump :
In diffuser pump , the fluid passes through a ring of fixed vanes or diffuser after the fluid has left the impeller, that diffuse the liquid, this provids a more controlled flow and a more efficient conversion of velocity head into pressure head. Providing fixed diffuser increases the efficiency of the pump up to 90 percent.

Vortex pump :
Vortex casing is a casing in which circular chamber is provided between the casing and the impeller. Vortex casing will increase pump efficiency by reducing eddies formation to a considerable extent.



Monday 12 October 2009

Strain Gauge - Part 2

2.2 Electrical resistance strain gauge :
In electrical resistance strain gauge the displacement or strain is measured as a function of resistance change produced by the displacement in the gauging circuit.
When the conductor is stretched, its length will increase and area of cress section will decrease this will result in change in resistance. Change in resistance per unit strain is defined as Gauge Factor.
Gauge factor indicates the sensitivity of the strain gauge.

Types of electrical resistance strain gauges
Electrical resistance strain gauge with metallic sensing element may be broadly classified in to four groups.
       a. Un-bonded wire strain gauge
b. Bonded wire strain gauge
c. Foil strain gauge
d. Weldable strain gauge
2.2.1 Un-bonded wire strain gauge :
The principal of the un-bonded metallic strain gauge is based on the change in electrical resistance of a metallic wire due to the change in the tension of the wire. This type consists of a stationary frame and a movable platform. Fine wire loops are wounded around the insulated pins with pretension. Relative motion between the platform and the frame increases the tension in two loops, while decreasing tension in the other two loops. These four elements are connected approximately to a four arm Wheat stone bridge. These type strain gauges are used for measurement of acceleration, pressure, force etc.
Strain Gauge 2.2.2 Bonded Wire Strain Gauge :
The bonded metallic type of strain gauge consists of a strain sensitive conductor (wire) mounted on a small piece of paper or plastic backing. In us this gauge is cemented to the surface of the structural member to be tested. The wire grid may be & flat type or wrap-around. In the flat type after attaching the lead wires to the ends of the grids, a second piece of paper is cemented over the wire as cover. in the wrap-around type, the wire is wound around a cylindrical core in the form of a close wound helix. This core is then flattened & cemented between layers of paper for the purpose of protection and insulation. Formerly only wrap-around gauges were available, but generally flat grid gauges are preferred as they are superior to wrap-around gauge in terms of hysterisis, creep, elevated temperature, performance, stability & current carrying capacity.


Strain Gauge
2.2.3 Foil Strain Gauges:
The foil type of strain gauges has a foil grid made up of thin strain sensitive foil. The width of the foil is very large as compared to the thickness (microns) so that larger area of the gauge is for cementing.
Strain Gauge 2.2.4 Weldable Strain gauge:
Weldable strain gauges are easy to install in minutes in any environment compared to bonded type strain gauge. The weldable strain gauge consists of a strain sensitive element, the nickel Chromium or platinum Tungsten, housed within a small diameter stainless steel tube. The strain element is insulated from the tube with highly compacted ceramic insulation. This gauge is subsequenty spot welded to structure under test and provides bonding to transfer the strain. The test specimen which is put into tension or compression, the stress is transmitetd through the weld to mounting flange and in to strain tube. These gauges can be used for static or dynamic applications.

Strain Gauge

2.3 Optical strain gauges
The optical strain gauges are used to measure elongation as well as deflection, following are the two type of optical strain gauges,

      a. Marten’s optical gauge

b. Tuckerman Optical Gauge

2.3.1. Marten’s optical gauge:
These optical stain gauges employs variety of mirror systems to obtain optical magnification.
The well known optical system used in a strain gauge on a single mirror system is marten’s optical gauge.
The pivoted knife edge carries a mirror and the other end of this arm is fastened to specimen as the specimen elongates the measuring knife edge will rotate about its point there by tilting the mirror. The Reflection of the illuminated scale in this mirror is viewed through the telescope.
Strain Gauge 2.3.2 Tuckerman Optical Gauge:
In this instrument, the relative rotation between the fixed mirror and the movable mirror is measured with autocollimator. The autocollimator consists of a lamp source to produce parallel beam of rays and a scale to measure the deflection of the reflected ray.
A tungsten carbide rocker (lozenge) acts as a moving knife; one face of this lozenge is polished to act as a mirror.
If the specimen deforms, rotates the lozenge which in turn deflects the incident ray back to the reticule. Actually three images are visible on the reticule one gives the measurement of strain and other two helping alignment of the gauge. The sensitivity of the gauge is 2 micro strains and this gauge is available with a wide range of gauge length of 6mm. it can measure both static and dynamic strains and cyclic strains up to 180 Hz.

Strain Gauge
2.4 Pneumatic strain gauge :
The principal of operation of a pneumatic gauge depends upon the relative discharge of air between a fixed orifice and a variable orifice.
Magnification up to 100,000 times and the gauge length as small as 1mm are possible to achieve by these gauges.
These gauges are suitable for both Static and dynamic strain measurements. These are sensitive, robust and reliable.

Strain Gauge

2.5. Acoustic strain gauge :
In an acoustic strain gauge the variation in length of a wire stretched between two gauge points is measured which alters the natural frequency of the wire.
The magnitude of frequency change for a strain gauge can be increased by decreasing the length of the wire or stress in wire.These gauges are highly accurate and long term reliable. Optical strain gauges are used to measure strains in concrete structure, concrete dams, rock, steel structures etc.

Wednesday 7 October 2009

Strain Gauge - Part 1

Contents: 
1.0 Definition of stain gauge
2.0 Classification of strain gauges
2.1 Mechanical strain gauges
2.1.1 Berry Strain gauge
2.1.2 Huggenbeger Extensometer
2.1.3 Johansson Extensometer
2.2 Electrical Strain gauges
2.3 Optical strain gauges

2.4 Pneumatic strain gauges
2.5 Acoustical strain gauges

1. Definition :
A strain gauge is a device used to measure the strain on a free surface of a structure. Strain gages are the preeminent tool in stress analysis. Strain gauges of all types are essentially employed to measure the linear deformation over a given gauge length. The sense the change in length, Magnify and indicate it in some other form. Strain Gauge is invented by Edward E Simmons and Arthur C Ruge in the year 1938.

2. Classification of Strain Gauges
Depending up on the magnification system, the strain gauges are broadly classified as under,
a. Mechanical strain gauges
b. Electrical strain gauges
c. Optical strain gauges
d. Pneumatic strain gauges
e. Acoustical strain gauges

2.1 Mechanical strain gauges
Mechanical strain gauges are also known as Extensometers used to measure static or gradually varying load conditions. These gauges are usually provided with two knife edges which are clamped firmly in contact with the test component by means of a clamping spring at a specific distance of gauge length. When the specimen under testing is strained the knife edges undergoes displacement, this displacement is amplified by a mechanical linkages and the strain is displaced on a calibrated scale.


Types of Mechanical strain gauges
2.1.1 Berry Strain gauge
These strain gauges uses a lever magnification with dial indicator to show magnified motion. It consists of one rigid frame and two conically pointed contact pointers. One pointer is rigidly fixed to the frame while the other is pivoted at a point on the frame. The displacement in the lever is magnified and indicated in the dial indicator.

Berry strain gauge
2.1.2 Huggenbeger Extensometer
This extensometer has a set of compound levers which are relatively small in size and high magnification factor. These gauges are highly accurate. The movable knife edge rotates the lever at lower pivot, the lever in turn rotates the indicator pointer at upper pivot point with the help of a link.
Stain Gauge
2.1.3 Johansson Extensometer
These extensometers uses tension tape or twisted metal strip between two knife edges. Half of the strip is twisted to one direction and remaining half is twisted to other direction and a pointer is fixed at the center of the strip. On application of load, displacement in the movable knife edge takes place with high amplification due to stretching of twisted metal strip.
Strain Gauge






What is Modal Analysis

Vibration can occur in any physical system. Properties of the system are the frequencies at which vibration occurs and the modal shapes whi...