## Posted tagged ‘nature’

### POISSON’S RATIO

August 23, 2011

When an element is stretched in one direction, it tends to get thinner in the other two directions. Hence, the change in longitudinal and lateral strains are opposite in nature (generally). Poisson’s ratio ν, named after Simeon Poisson, is a measure of this tendency. It is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied load. Since most common materials become thinner in cross section when stretched, Poisson’s ratio for them is positive.

For a perfectly incompressible material, the Poisson’s ratio would be exactly 0.5. Most practical engineering materials have ν between 0.0 and 0.5. Cork is close to 0.0, most steels are around 0.3, and rubber is almost 0.5. A Poisson’s ratio greater than 0.5 cannot be maintained for large amounts of strain because at a certain strain the material would reach zero volume, and any further strain would give the material negative volume.

Some materials, mostly polymer foams, have a negative Poisson’s ratio; if these auxetic materials are stretched in one direction, they become thicker in perpendicular directions.Foams with negative Poisson’s ratios were produced from conventional low density open-cell polymer foams by causing the ribs of each cell to permanently protrude inward, resulting in a re-entrant structure.

An example of the practical application of a particular value of Poisson’s ratio is the cork of a wine bottle. The cork must be easily inserted and removed, yet it also must withstand the pressure from within the bottle. Rubber, with a Poisson’s ratio of 0.5, could not be used for this purpose because it would expand when compressed into the neck of the bottle and would jam. Cork, by contrast, with a Poisson’s ratio of nearly zero, is ideal in this application.

It is anticipated that re-entrant foams may be used in such applications as sponges, robust shock absorbing material, air filters and fasteners. Negative Poisson’s ratio effects can result from non-affine deformation, from certain chiral microstructures, on an atomic scale, or from structural hierarchy. Negative Poisson’s ratio materials can exhibit slow decay of stress according to Saint-Venant’s principle. Later writers have called such materials anti-rubber, auxetic (auxetics), or dilatational. These materials are an example of extreme materials.

### POISSON'S RATIO

August 23, 2011

When an element is stretched in one direction, it tends to get thinner in the other two directions. Hence, the change in longitudinal and lateral strains are opposite in nature (generally). Poisson’s ratio ν, named after Simeon Poisson, is a measure of this tendency. It is defined as the ratio of the contraction strain normal to the applied load divided by the extension strain in the direction of the applied load. Since most common materials become thinner in cross section when stretched, Poisson’s ratio for them is positive.

For a perfectly incompressible material, the Poisson’s ratio would be exactly 0.5. Most practical engineering materials have ν between 0.0 and 0.5. Cork is close to 0.0, most steels are around 0.3, and rubber is almost 0.5. A Poisson’s ratio greater than 0.5 cannot be maintained for large amounts of strain because at a certain strain the material would reach zero volume, and any further strain would give the material negative volume.

Some materials, mostly polymer foams, have a negative Poisson’s ratio; if these auxetic materials are stretched in one direction, they become thicker in perpendicular directions.Foams with negative Poisson’s ratios were produced from conventional low density open-cell polymer foams by causing the ribs of each cell to permanently protrude inward, resulting in a re-entrant structure.

An example of the practical application of a particular value of Poisson’s ratio is the cork of a wine bottle. The cork must be easily inserted and removed, yet it also must withstand the pressure from within the bottle. Rubber, with a Poisson’s ratio of 0.5, could not be used for this purpose because it would expand when compressed into the neck of the bottle and would jam. Cork, by contrast, with a Poisson’s ratio of nearly zero, is ideal in this application.

It is anticipated that re-entrant foams may be used in such applications as sponges, robust shock absorbing material, air filters and fasteners. Negative Poisson’s ratio effects can result from non-affine deformation, from certain chiral microstructures, on an atomic scale, or from structural hierarchy. Negative Poisson’s ratio materials can exhibit slow decay of stress according to Saint-Venant’s principle. Later writers have called such materials anti-rubber, auxetic (auxetics), or dilatational. These materials are an example of extreme materials.

### TERMS IN MEASUREMENT

August 23, 2011

MEASUREMENTS:

A Measurement is the outcome of an opinion formed by observers about some physical quantity.

CLASSIFICATION OF MEASUREMENTS:

• Standards –  ( Reproduce the value of given quantity )
• Fixed Gauges – (Check Dimensions)
• Measuring Instruments – (Determine the measured value)

NEEDS FOR MEASUREMENT:

1. To Determine the true dimensions of a part.

2. To increase our knowledge and understanding of the world.

3. Needed for ensuring public health and human safety.

4. To convert physical parameters into meaningful numbers.

5. To test if the elements that constitute the system function as per the design.

6. For evaluating the performance of a system.

7. For studying some basic laws of nature.

8. To ensure interchangeability with a view to promoting mass production.

9. To evaluate the response of the system to particular point.

10. To check the limitations of theory in actual situations.

11. To establish the validity of design and for finding new data and new designs.

METHODS OF MEASUREMENT:

1. Direct Comparison

2. Indirect Comparison

3. Comparative Method

4. Coincidence Method

5. Fundamental Method

6. Contact Method

7. Transposition Method

8. Complementary Method

9. Deflection Method

Direct Method:

Measurements are directly obtained.

Ex:Vernier Caliper,Scales.

Indirect Method:

Obtained by measuring other quantities.

Ex:Diameter measurement by using three wires.

Comparative Method:

It’s compared with other known value.

Ex:Comparators.

Coincidence Method:

Measurements coincide with certain lines and signals.

Fundamental Method:

Measuring a quantity directly in related with the definition of that quantity.

Contact Method:

Sensor/Measuring tip touch the surface area.

Ex:Vernier Caliper.

Transposition Method:

Quantity to be measured is first balanced by a known value and then balanced by an other new known value.

Ex:Determination of mass by balancing methods.

Complementary Method:

The value of quantity to be measured is combined with known value of the same quantity.

Ex:Volume determination by liquid displacement.

Deflection Method:

The value to be measured is directly indicated by a deflection of pointer.

Ex:Pressure Measurement.

TERMS OF MEASUREMENT:

Precision:

The ability of the instrument to reproduce it’s readings or observation again and again for constant input signal.

Accuracy:

Closeness/conformity to the true value of the quantity under measurement.

Error:

The difference between true value and measured value is known as measurement error.

Error = Vt – Vm

Reliability:

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.

### TERMS IN MEASUREMENT

August 23, 2011

MEASUREMENTS:

A Measurement is the outcome of an opinion formed by observers about some physical quantity.

CLASSIFICATION OF MEASUREMENTS:

• Standards –  ( Reproduce the value of given quantity )
• Fixed Gauges – (Check Dimensions)
• Measuring Instruments – (Determine the measured value)

NEEDS FOR MEASUREMENT:

1. To Determine the true dimensions of a part.

2. To increase our knowledge and understanding of the world.

3. Needed for ensuring public health and human safety.

4. To convert physical parameters into meaningful numbers.

5. To test if the elements that constitute the system function as per the design.

6. For evaluating the performance of a system.

7. For studying some basic laws of nature.

8. To ensure interchangeability with a view to promoting mass production.

9. To evaluate the response of the system to particular point.

10. To check the limitations of theory in actual situations.

11. To establish the validity of design and for finding new data and new designs.

METHODS OF MEASUREMENT:

1. Direct Comparison

2. Indirect Comparison

3. Comparative Method

4. Coincidence Method

5. Fundamental Method

6. Contact Method

7. Transposition Method

8. Complementary Method

9. Deflection Method

Direct Method:

Measurements are directly obtained.

Ex:Vernier Caliper,Scales.

Indirect Method:

Obtained by measuring other quantities.

Ex:Diameter measurement by using three wires.

Comparative Method:

It’s compared with other known value.

Ex:Comparators.

Coincidence Method:

Measurements coincide with certain lines and signals.

Fundamental Method:

Measuring a quantity directly in related with the definition of that quantity.

Contact Method:

Sensor/Measuring tip touch the surface area.

Ex:Vernier Caliper.

Transposition Method:

Quantity to be measured is first balanced by a known value and then balanced by an other new known value.

Ex:Determination of mass by balancing methods.

Complementary Method:

The value of quantity to be measured is combined with known value of the same quantity.

Ex:Volume determination by liquid displacement.

Deflection Method:

The value to be measured is directly indicated by a deflection of pointer.

Ex:Pressure Measurement.

TERMS OF MEASUREMENT:

Precision:

The ability of the instrument to reproduce it’s readings or observation again and again for constant input signal.

Accuracy:

Closeness/conformity to the true value of the quantity under measurement.

Error:

The difference between true value and measured value is known as measurement error.

Error = Vt – Vm

Reliability:

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.