2.4 TENSILE TESTING (ASTM D638)

2.4 TENSILE TESTING (ASTM D638):

2.4.1 INTRODUCTION:

A tensile test is also known as a tension test is probably one of the most fundamental mechanical tests that can be performed on a material. Tensile tests are simple, relatively inexpensive and determine how the material will react to the forces being applied in tension. Tensile test in board sense is the measurement of the ability of the material to withstand forces that tend to pull it apart and to what extent the material stretches before breaking. As the material is, being pulled you will find its strength along with how much it will elongate. Materials, which exhibit the largest strengths during tensile testing, have the lowest impact values. Tensile modulus is an indication of the relative stiffness of the material, and can be determined from the stress strain diagram. Many plastic are very sensitive to the rate of straining and environmental conditions. Therefore the data obtained from this method cannot be considered valid for applications involving load time scales or environmental widely different from this method. The tensile property data helps in preferential selection of a particular type of plastic from a large group of plastic material and such data are of limited use in actual design of the part. This is because the test does not take in to account the time dependent behavior of plastic material. As the pull of the material is continued until it breaks, a complete tensile profile is obtained. A curve will result showing how it reacts to the forces being applied. The point of failure is of much interest and is typically called its ultimate strength or “UTS” on the chart. Uniaxial tension test provides important information about the mechanical properties of the material[2,12].

2.4.2 SCOPE OF THE EXPERIMENT:

This test method covers determination of tensile properties of un reinforced and reinforced plastics in the form of standard dumbbell shaped specimen when tested under defined condition of pretreatment, temperature, humidity, and testing machine speed. This method can be used for testing materials of any thickness above 1.0mm to14mm. This test method includes the option of determining the Poisson’s ratio at room temperature. Test data obtained by this method are relevant and appropriate for use in engineering design.

2.4.3 SIGNIFICANCE AND USE:

This test method is designed to produce tensile property data for the control and specification of plastic materials. These data are also useful for qualitative characterization, research and development. It is advisable to refer to the material specification before using this test method. Tensile properties may vary with the specimen preparation in exactly the same way. Many plastics are very sensitive to the rate of straining and environmental conditions. Therefore the data obtained from this method cannot be considered valid for application involving load time scales or environmental widely different from this method. When uniaxial tensile force is applied to a solid, the solid stretches in the direction of the applied force but it also contracts in both directions lateral to the applied direction of force. If the solid is homogeneous and isotropic, the material remains elastic under the action of the applied force; the lateral strain bears a constant relationship to the axial strain. This constant is called poisons ratio, is defined as the negative ratio of the reverse to axial strain under uniaxial stress.

2.4.4 APPARATUS:

The tensile testing machine of a constant rate of cross head movement is used. It has a fixed or assembly stationary member carrying a second grip. Self-aligning grips employed for holding the test specimen between the fixed member and the movable member prevents alignment problems.

Fig.2.1 Universal Testing Machine

Driving mechanism: a controlled velocity drive mechanism is used. Some of the commercially available machines use a closed loop servo controlled drive mechanism to provide a high degree of accuracy.

Load indicator: a load indicating mechanism capable of indicating total tensile load with an accuracy of + 1% of the indicated value or better is used.

Extension indicator: An extension indicator commonly known as extensometer is used to determine the distance between the designated points located within the gauge length of the test specimen as it is stretched.

Micrometer: The advent of the new microprocessor technology has virtually eliminated the time consuming manual calculation. Stress, elongation, modulus, energy, statically calculation are performed automatically and presented on a visual display or hard copy print out at the end of test can be obtained.

2.4.5 TEST SPECIMEN:

The Type 1 specimen shall be used where sufficient material having a thickness of 7mm (.28in) or less is available. The Type 2 specimen may be used when a material does not break in a narrow section with the preferred Type 1 specimen. The Type 3 specimen must be used for all materials with a thickness of greater than 7mm (.28in) but more than 14mm (.55in). The Type 4 specimen are used when direct comparison is required between materials in different rigidity cases (that is, non-rigid and semi rigid). The Type 4 specimen shall be used for testing non - rigid plastics with a thickness of 4mm (.16in) or less. The Type 5 specimens may be used where only limited material having a thickness of 4mm (.16in) or less is available for evaluation, or where a large number of specimens are to be exposed in a limited space. The test specimen for reinforced composites, including highly orthotropic laminates, shall conform to the dimensions of the Type 1 specimen[2].

2.4.6 SPECIMEN PREPARATION:

The test specimen can be prepared by machining operation, or die cutting, from the materials in sheet, slab or similar form. Specimens can also be prepared by molding the material to be tested.

2.4.7 SPEED OF TESTING:

Speed of testing shall be relative ratios of the grips or test fixtures during the test. The rate of motion of the driven grip or fixture when the testing machine is running to idle may be used. Choose the speed of testing depending upon the type of material used.

2.4.8 CONDITIONING:

Condition the specimen at 23+/- 2^o C and 50+/-5% relative humidity for not less than 40 hours prior to the test. Conduct the test at 23+/-2 ^o C and 50+/-5% relative humidity unless other wise specified by the contract or the relevant ASTM material specification.

2.4.9 PROCEDURE:

The speed of testing is the relative rate of motion of the grips or test fixture during the test fixture during the test. The test specimen is positioned vertically in the grips or test fixture during the test. The grips are tightened evenly and firmly to prevent any slippage. The speed of testing is set at the proper rate and the machine is started. As the specimen elongates, the resistance of the specimen increases and it is detected by the load cell. This load value is recorded by the instrument. The elongation of the specimen is continued until a rupture of the specimen is observed. Load value at break is also recorded. The tensile strength at yield and the tensile strength at break are calculated.

Tensile Strength = Force (lb) / Cross Section Area ( sq.in)

Tensile Strength at Yield = Maximum Load Recorded (lb) / Cross Section Area ( sq.in )

Tensile Strength at break = Load Recorded at break ( lb ) / Cross Section Area ( sq.in )

1. Mark off the units of stress in lb/in² on the X-axis of the chart. This is done by dividing the force by cross section area of the specimen.

2. Mark off the units of strain in. /in. on the Y-axis. These values are obtained by dividing the chart value by the magnification selected.

3. Carefully draw a tangent KL to the initial straight-line portion of the stress-strain curve.

4. Select any two convenient points on the tangent.

5. Draw a straight line PQ and LM connecting points P and L with Y-axis of the chart.

6. Then the tensile values are calculated as follows.

Tensile Modulus = Difference in corresponding stress / Difference in corresponding strain

7. Elongation at yield,

Strain = Change in length (elongation) / Original length.