Mechanical Properties:
Fundamental to the understanding of a material’s performance is a knowledge of how the material will respond to any load.
The important mechanical properties are
1. Tensile tests
2. Flexural properties
3. Compressive properties
4. Creep properties
5. Stress relaxation
6. Impact properties
7. Shear strength
8. Abrasion
9. Hardness tests
Tensile Strength:
Stress: The force applied to produce deformation in a unit area of test specimen.
Strain: The change in length per unit of the original length.
Elongation: The increase in the length of test specimen produced by a tensile load.
Yield Point: The first point on the the stress strain curve at which an increase in strain
occurs without the increase in stress.
Yield Strength: The stress at which a material exhibits a specified limiting deviation
from the proportionality of stress to strain
Proportional limit: The greatest stress at which a material is capable of sustaining the applied load without any deviation from proportionality of stress to strain (Hooke’s Law)
Modulus of Elasticity: Modulus of Elasticity The ratio of stress to corresponding strain below the proportional limit of a material.
Ultimate Strength: The maximum unit stress a material will withstand when subjected to
an applied load in compression, tension or shear.
Secant Modulus: The ratio of the total stress to corresponding strain at any specific point on the stress-strain curve.
Tensile Strength
Standard Test Method for Tensile Properties of Plastics (ASTM D 638), IS-8453, JIS-7113, ISO-1184, BS-2782.
By knowing the amount of deformation (strain) introduced by a given load (stress), the designer can predict the response of the application under its working condition.
Tensile strength: The maximum Tensile stress ( nominal) sustained by a test piece piece during during aa tension tension test test or or Ultimate Ultimate strength strength of of aa material material subjected to tensile loading otherwise, it is a measurement of the ability of a material to withstand forces that to pull it apart and to determine to what extent the material stretches before breaking.
Tensile Modulus: The ratio of tensile stress to corresponding strain at the maximum load. It is an indication of the relative stiffness of a material.
Percentage of Elongation at Yield: The percentage elongation produced in the gauge length of the test piece at the yield tensile stress.
Percentage of Elongation at Yield: The elongation at break, or at maximum load, produced in the gauge length of the test piece, expressed as a percentage of the gauge length.
Formula and Calculations
Force (load) (N)
(1) Tensile strength = ---------------------------------------------------------
Cross-section area of the specimen(mm²)
Maximum load recorded (N)
(2) Tensile strength at yield (N/mm²) = ----------------------------------
Cross Cross section section area area (mm )²
Load recorded at break (N)
(3) Tensile strength at break (N/mm²) = ---------------------------------
Cross section area (mm²)
Difference in stress
(4) Tensile Modulus = ---------------------------------------------------
Difference in corresponding strain
Change in length (elongation)
(5) Elongation at yield, Strain (ε) = ----------------------------------------
Original length (gauge length)
(6) P t Percent El ti Elongation = ε x 100
NOTE: If the specimen gives a yield load that is larger than the load at break, calculate
“percent elongation at yield” otherwise; calculate “percent elongation at break”.
Factors Affecting Tensile Results:
Temperature and Humidity: Recommended Temperature and Humidity is 23℃ and 55 –65 %. Tensile Strength decreases as Temperature increases. Moisture works as, so it causes then decrease in Tensile Strength and
increase the Elongation.
Strain rate: As the strain rate increased the tensile strength increased
Method of specimen Preparation: Injection moulded specimens will have higher value than the compression specimen. Molecular Orientation has a significant effect on tensile Strength values. A load-applied parallel to the direction of molecular orientation may yield higher value than the load applied perpendicular to the orientation. The opposite is true for elongation.
Effect of Plasticizer and filler: Soften the material, brings down the Tensile Strength and increase Elongation.
Crystallinit: With the iincrease of Crystallinity, Tensile Strength increases.
Test Speed: Elongation is high when Test Speed is minimum i.e. 0.05 mm/min and
is lower when Test Speed is maximum i.e. 500 mm/min.
Molecular Weight and Molecular
Weight Distribution: With increase in molecular weight Tensile Strength also increases and Molecular Weight Distribution With increase in molecular weight, Tensile Strength also increases. Smaller molecules in polymer work as plasticizer. So with increase of Molecular Weight Distribution, Elongation decrease and Tensile Strength.
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