Compressive Properties

Compressive Properties:

1. Compressive properties describe the behaviour of a material when it is subjected to a compressive load at a relatively low and uniform rate of loading.

2. Compressive properties include modulus of elasticity; yield stress, deformation beyond yield point, compressive strength, compressive strain and slenderness ratio. Material processing a low order of ductility may not exhibit yield point.

3. Compressive strength is a value that shows how much force is needed to rupture or crush

a material.

4. Compression tests provide a standard method of obtaining data for research and

development, quality control, acceptance or rejection under specifications and special purposes

5. COMPRESSIVE STRENGTH The maximum load sustained by a test specimen in a compressiive test divide by the oriiginal cross section area of the specimen.

Compressive Properties:

COMPRESSIVE DEFORMATION: - The decrease in length produced in the gauge length of

the test specimen by a compressive load. It is expressed in unit of length.

COMPRESSIVE STRAIN: - The ratio of compressive deformation to the gauge length of the test specimen, i.e., the change in length per unit of original length along the longitudinal axis. It is expressed as dimension ratio.

SLENDERNESS RATIO:- The ratio of the length of a column of uniform cross section to its

least radius of gyration known as slenderness ratio.

MODULUS MODULUS OF OF ELASTICITY ELASTICITY:- The ratio of stress to corresponding strain below the proportional limit of a material. It is expressed as force per unit area, based on the average initial cross- sectional area.

COMPRESSIVE YIELD POINT:- The fist point of stress-strain diagram at which an increase in 

strain occurs without an increase in stress.

Unit:- Kg/Cm2

Test Method: ASTM D 695, ISO-R-604, BS-2782 Method 303, DIN-5345

FORMULA E AND CALCULATION:

Compressive strength: Calculate the compressive strength by dividing the max. Compressive load carried by the specimen during the test by the original minimum carried by the specimen during the test by the original minimum cross sectional area of the specimens cross sectional area of the specimens.  Express the result in MPa. 

                                                           Load (kg)

(1) Compressive strength = ----------------------------------------------------

                                            Original cross sectional area (cm2)

                                                     Maximum load recorded (N)

(2) Compressive strength at yield (N/mm²) = ---------------------

                                                         Cross-section area (mm²)

                                                      Load recorded at break (N)

(3) Compressive strength at (N/mm²) = -------------------------------

                                                     Cross-section area (mm²)

                                                      Difference in stress

(4) Compressive Modulus = --------------------------------------------------

                                        Difference in corresponding strain

                                                  Change in length (Deformation)

(5) Deformation at yield, Strain (ε) = ---------------------------------

                                                   Original length (gauge length)

(6) Percent Deformation = ε x 100

If the specimen gives a yield load that is larger than the load at break, calculate “percent Deformation at yield” otherwise; calculate “percent Deformation at break”.

Factors Affecting Compressive Strength:

Rate of Straining A rate of straining in a test specimen drastically change the compressive strength.

Effect of Plasticizer Soften the material, brings down the compreesive strength and

increase Elongation.

Crystallinity With the increase of crystallinity, compressive strength increases.

Temperature and Humidity Recommended Temperature and Humidity is 23℃ and 55-65%. Flexural Strength decreases as Temperature increases. Moisture works as plasticizer, so it causes then increase in impact.

Method of specimen Injection moulded specimens will have higher value than the

Preparation compressiion speciimen. Molecular Orientation has a significant effect

on compressive strength values. A load applied parallel to the direction of molecular orientation may yield higher value than the load applied perpendicular to the orientation.

Molecular Weight and Molecular Weight Distribution With increase in molecular weight, compressive strength also increases. Smaller molecules in polymer work as plasticizer. So with increase of Molecular Weight Distribution, compressive strength increase.