Tensile Testing: A Complete Scientific Guide – TESTEX

The test to determine the material under tension is called tensile testing, also known as tension testing. It is one of the basic methods for testing the mechanical properties of materials and is used to check the performance of the material and whether it meets the relevant standards. This post is concerned with the following.

• What is the tensile test? What can be tested?
• Concepts and terms related to tensile testing
• What is a tensile tester? What materials can be tested?
• Preparation for tensile testing
• Tensile testing procedures
• Tensile testing: the four stages of stress-strain
• Common test standards for tensile testing
• What are the factors affecting tensile strength in tensile testing?

What is the tensile test? What can be tested?

Tensile testing is the application of tensile force to a specimen under certain conditions to determine the resistance of the specimen to an applied load. For example, the strength index and plasticity index of the material, in addition, the plastic deformation of the material can also be derived.

The data obtained from the tensile test can be used to determine the elastic limit, elongation, modulus of elasticity, proportional limit, area reduction, tensile strength, yield point, yield strength and other tensile properties of the material.

Strength indicators

• The strength of a material is the force per unit area of the material in Pa.
• 1 Pa = 1 N/m²
• However, the unit Pa is so small, so that in practical engineering, MPa is often used as a unit of strength, for example the yield strength of steel generally ranges from 100 ~ 2000 MPa.
• 1 MPa = 10⁶ Pa

• Upper yield strength: ReH = FeH / So (So indicates the original cross-sectional area, FeH indicates the axial force corresponding to the upper yield point)
• Lower yield strength: ReL = FeL / So (So indicates the original cross-sectional area, FeL indicates the axial force corresponding to the lower yield point)
• Tensile strength: Rm = Fmax / So (Fmax is the maximum axial force)

The yielding phenomenon is not obvious material, to produce 0.2% residual deformation of the stress value for the yield limit, known as the conditional yield limit or conditional yield strength. External forces greater than this limit will cause the part to fail permanently and cannot be recovered.

• Hard steel (high carbon steel) has high strength, poor plasticity and no obvious yielding phase in the tensile process, so the yield strength cannot be determined directly and the conditional yield strength is used instead of the yield strength.
• Conditional yield strength: Rp0.2, indicating the stress corresponding to a specified plastic elongation of 0.2%.

Plasticity indicator

When a specimen is pulled off, the elastic deformation disappears, but the plastic deformation remains. In engineering, the deformation left behind after the test piece is pulled off is used to indicate the plasticity index of the material. There are two commonly used plasticity indicators.

• Elongation: A = (Lu – Lo) / Lo * 100%
• Section shrinkage: Z = (So – Su) / So * 100%

Tensile testing related concepts and terminology

Stress and strain

Stress: Stress is the force in the area on which it acts, expressed as N/mm², and in metric units as kPa or MPa.

Strain: Strain is the rate of change in the dimensions of a test material, it is the change in dimensions caused by the loading of a stress. As strain is a rate of change, it has no units.

Specimen-related terms

• Cross-sectional area So: The original cross-sectional area of the specimen prior to the test.
• Original scale Lo: the scale of the specimen prior to the application of force.
• Post-break distance Lu: the distance of the specimen after breakage.
• Parallel length Lc: the length of the parallel part of the specimen between the two heads or the two clamped parts.
• Elongation after break A: the ratio of the residual elongation of the specimen after break mark (Lu – Lo) to the original mark Lo, expressed as a percentage.
• Section shrinkage Z: the ratio of the maximum reduction in cross-sectional area (So – Su) to the original cross-sectional area So after fracturing of the specimen, expressed as a percentage.

What is a tensile tester? What materials can be tested?

The tensile tester is also known as universal tensile testing machine. A tensile tester is a mechanical force testing machine used for static loading, stretching, compression, bending, shearing, tearing, peeling and other mechanical properties testing of various materials. The tensile strength machine is an indispensable testing equipment for material development, physical properties testing, teaching and research, quality control, etc. The universal tensile testing machine is very widely used and can be used to test the following kinds of materials.

• Rubber materials: rubber products, hoses, tapes, O-rings, tyres and other rubber materials and products.
• Plastic materials: plastic products, films, tubes, plates, packaging materials, nylon products, waterproof rolls and other plastic materials and products.
• Metal materials: metal products, stainless steel products, bolts, steel wires, alloy products and other metal materials and products.
• Building materials: wood, sheet, glass, concrete, graphite products, etc.

Preparation for tensile testing

The location, direction and number of sampling are three factors that have a significant influence on the results of material properties tests. The location, direction and number of samples to be taken should be in accordance with the product standard ISO 377 or the relevant agreement.

Sampling method

• Sampling directly from the raw material.
• Samples are taken from important areas on the product (the weakest and most dangerous parts).
• Direct testing with physical parts, e.g. reinforcement bars, bolts, screws or chains.
• Testing directly on cast specimens or by machining into specimens.

Processing of specimens

• To prevent the mechanical properties from being affected by cold deformation or heat. Usually machined mainly by cutting.
• Parallel sections should be smooth, free from work hardening, and free from defects such as chips, tool marks and burrs.
• The brittle material clamping part and the parallel section part should have a large radius of round transition.
• For non-machined casting specimens, the surface of the sand, slag, burrs, flying edges, etc. must be clear.

Specimen inspection and marking

• The specimen should be checked before the test to ensure that its appearance meets the requirements.
• The original markings of the specimens are generally marked with fine lines and the method used must not affect the premature fracture of the specimen.
• For extra thin or brittle materials, the specimen can be coated with a fast drying colouring paint in parallel sections and then gently scribed with a marking line.

In addition: the original cross-sectional area So of the specimen needs to be measured and calculated before the test.

Tensile test procedure

1 Prepare the specimen: Prepare the specimen according to the standard requirements and keep records.

2 Adjust the tensile testing machine: change the fixture according to the testing standard and adjust the testing conditions of the tensile machine.

3 Clamp the specimen: first clamp the specimen in the upper chuck, then move the lower chuck to a suitable clamping position, and finally clamp the lower end of the specimen.

4 Check and test run: Check that the above steps are completed. Start the pulling machine and preload a small amount (the load corresponding to the stress must not exceed the proportional limit of the material) and then unload to zero in order to check that the pulling machine is working properly.

5 Start the tensioning machine and carry out the tension test.

6 Remove the test piece and the recording paper.

7 Measure the post-break distance with vernier calipers.

8 Measure the minimum diameter at the neck shrinkage with vernier calipers.

Tensile testing: the four stages of stress-strain

• OB: Elastic stage
• BC: Yield stage
• CD: Reinforcement stage
• DE: Necking stage

In the elastic deformation stage of a metallic material, the stress and strain are proportional to each other, in accordance with Hooke’s law, i.e. σ = Eε, with a scale factor E called the modulus of elasticity.

E = σ/ε

The elastic limit is so close to the proportional limit that in practical engineering, the proportional limit is approximated instead of the elastic limit.

Yield strength: When a metallic material exhibits the phenomenon of yielding, the stress point at which plastic deformation occurs without an increase in force is reached during the test; a distinction should be made between upper and lower yield strengths.

• Upper yield strength: the highest stress before the specimen yields and the force first decreases.
• Lower yield strength: the lowest stress during yielding, not counting the initial transient effect.
• The stress value corresponding to the lower yield point is usually referred to as yield strength.

After the yielding stage, the point C of the curve begins to rise gradually again, indicating that to make the strain increase, the stress must be increased and the material regains its ability to resist deformation, a phenomenon called strengthening, and the CD section is called the strengthening stage (process hardening).

The stress value corresponding to the highest point of the curve is called the tensile strength (or strength limit) of the material, and it is another important indicator of the strength of the material.

When the curve reaches point D, the deformation increases significantly in one of the weaker parts of the specimen (where the material is uneven or defective), the effective cross-section decreases sharply, the necking phenomenon occurs and the specimen is quickly pulled off.

A few common stress-strain curves

The (a) curve is the stress-strain curve for mild steel, which has a jagged yielding phase, with upper and lower yielding, uniform plastic deformation followed by necking and then fracture of the specimen.

The (b) curve is the stress-strain curve for medium carbon steel, which has a yielding phase, but with small fluctuations and almost a straight line, with uniform plastic deformation followed by necking and then fracture of the specimen.

The (c) curve is the stress-strain curve of quenched, medium to low temperature tempered steel, which has no visible yielding phase and produces necking after uniform plastic deformation and then the specimen fractures.

The (d) curve is the stress-strain curve of cast iron, quenched material, which not only has no yielding phase but also fractures suddenly after producing a small amount of uniform plastic deformation.

Common test standards for tensile testing

• ISO 6892-1
• Metallic materials – Tensile testing – Part 1: Method of test at ambient temperature
• ISO 6892-2
• Metallic materials – Tensile testing – Part 2: Method of test at elevated temperature
• ISO 204
• Metallic materials – Uniaxial creep testing method in tension
• ISO 377
• Steel and steel products – Location and preparation of samples and test pieces for mechanical testing
• ISO 783
• Metallic materials – Tensile testing at elevated temperature
• JIS G0601
• Clad steel plates – Mechanical and technological test
• ISO 3108
• Steel wire ropes for general purposes – Determination of actual breaking load
• EN 10319
• Metallic materials – Tensile stress relaxation testing – Part 1: Procedure for testing machines
• ISO 15579
• Metallic materials – Tensile testing at low temperature
• ASTM B557M
• Test pieces and method for tensile test for wrought aluminium and magnesium alloys products
• DIN EN ISO 2566-1
• Steel – Conversion of elongation values – Part 1: Carbon and low alloy steels
• DIN EN ISO 2566-2
• Steel – Conversion of elongation values – Part 2: Austenitic steels
• ASTM E111-04 & ASTM E1875-00
• Standard test method for Young’s Modulus, Tangent Modulus, and Chord Modulus
• Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio by Sonic Resonance

What are the factors affecting tensile strength in tensile testing?

The main factors affecting the tensile test machine tensile test include: sampling area and sampling method, the shape, size and accuracy of the specimen, measuring instruments, test equipment, test environment temperature, fixture selection, clamping method of the specimen, stretching rate, tensile specimen cross-sectional area, measurement error, etc.

1 Sampling sites and methods

Differences in the sampling site can directly affect the tensile test of metal materials after the elongation, yield strength and tensile strength and other performance indicators. The uneven distribution of metal materials due to composition, organization, structure, defects, processing deformation, etc., makes the mechanical properties of the same batch or even different parts of the same product appear different. In addition, when cutting the sample billet, it is necessary to prevent the mechanical properties from being affected by heat, work hardening and deformation.

2 Specimen shape, size, and accuracy

For the same material in the same state, if the cross-sectional shape is different, the measured results will have a greater effect on the upper yield strength and less on the lower yield strength; the tensile strength of a specimen with a large cross-sectional area (large size) is lower than that of a smaller size and the plasticity index is also reduced; the parallelism and dimensional accuracy within the parallel length of the specimen can also easily affect the test results. The parallelism and dimensional accuracy within the parallel length of the specimen can also easily affect the test results. This is because the measured dimensional value of the specimen may not be the minimum position of the actual sample, which will result in a low test result. Therefore the shape and dimensions of the specimen need to be in accordance with the standard.

3 For measuring instruments

The accuracy of the dimensional measuring instruments and gauges must meet the test requirements. Therefore, before conducting the test, all kinds of measuring instruments must be calibrated and the gauges must be kept clean and clear at the same time.

4 Test equipment

The test machine and the extensometer are two types of test equipment commonly used in tensile testing of metal materials, which directly affect the accuracy and authenticity of the test results. The former is used to measure the value of the force; the latter is mainly used for the determination of displacement or extension. Therefore, it is important to ensure that the testing machine and the extensometer are within the validity period of the test and are regularly calibrated.

5 Test environment temperature

Some metallic materials are highly temperature sensitive and even common metallic materials can lead to inconsistent test measurements if the test temperature varies too much. In general, the yield strength of body-centred cubic metals increases sharply as the temperature drops, while the change is less pronounced for face-centred cubic metals. As the temperature rises, the yield strength of the metal generally decreases.

6 Clamping device selection, the impact of specimen clamping

Incorrect selection of fixtures, specimen clamping and loading and unloading of the extensometer can affect the test results. Mismatch between the clamping device and the shape of the test specimen and the shape of the surface pattern of the clamping device are not suitable, which will cause the clamping device and the specimen not to form a sufficient clamping area, static friction is not enough, resulting in the relative sliding of the clamping device and the specimen during the tensile process, thus affecting the tensile results.

7 Clamping method

Unreasonable clamping methods can easily cause the specimen to slip or break in the jaws, resulting in inaccurate test data or low test data.

8 Stretching rate

The rate of stretching directly affects the stress-strain relationship of the metal material. Different materials are sensitive to different degrees of speed, stretching rate on different materials, the impact of the size of different materials, low strength, good plasticity of the material impact to be large.

9 Determination of the cross-sectional area of a tensile specimen

There are two methods for determining the cross-sectional area of a tensile specimen: one is the ISO 6892 tensile test method for metals and the other is the corresponding product standard for the material. Some product standards specify that the cross-sectional area of a tensile test specimen is to be determined by the cross-sectional area of the nominal size.

10 Methods of measurement of specimen dimensions and human error in measurement

Tensile specimens should be measured with an external diameter micrometer or vernier caliper, depending on the diameter. If the measurement method is not accurate, thus leading to artificially large size measurement. Due to subjective factors and different operating techniques, it can also bring errors to the measurement results.