TESNILE TESTING

Tensile tests are conducted on materials such as steel to deduce their ultimate tensile strength. The standardized tensile test specimen allows conclusions to be drawn about their characteristics and tensile properties.

Tensile testing is performed on a variety of materials such as Metals, Plastics, Elastomers, Paper, Composites, Rubber, Fabrics, Adhesives, Films, etc.

WHAT IS TENSILE TEST?

A tensile test, also known as a tension test, is one of the most fundamental and common types of mechanical testing. Tensile strength test means application of tensile (pulling) force to a material and measures the specimen's response to the stress. It involves the application of uniaxial force to measure the performance of a test specimen, up to the point of it yielding or breaking, whether sharply or gradually.

In simple terms, it is pulling a specimen apart in a straight line and recording the changes that occur. By doing this, tensile tests help to determine the strength a material is and how much it can elongate. Tensile tests are typically conducted with the helps of force testing or universal testing machines and these are fully standardized.

Tensile test procedures are standardized testing methods in which certain parameters are measured. They are destructive methods, because the specimens to be tested are often loaded beyond the yield strength. The testing on materials is performed by tensile testing machines and universal testing machines. The devices display one-dimensional movements in a stress-strain curve and a force-displacement curve. The determined parameters provide information on the properties and the tensile behaviour of the tested materials.

THE COMMON TENSILE TESTING STANDARDS ARE:
Metals
  • ASTM E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" (2013)
  • ISO 6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)
  • ISO 6892-2: "Metallic materials. Tensile testing. Method of test at elevated temperature" (2011)
  • JIS Z2241 Method of tensile test for metallic materials
  • MPIF Test Standard 10: "Method for the Tensile Properties of Powder Metallurgy (PM) Materials" Standard Test Methods for Tension Testing of Metallic Materials" (2015)
Composites
  • ASTM D 3039/D 3039M: "Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials"
Flexible materials
  • ASTM D638 Standard Test Method for Tensile Properties of Plastics
  • ASTM D828 Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus
  • ASTM D882 Standard test method for tensile properties of thin plastic sheeting
  • ISO 37 rubber, vulcanized or thermoplastic—determination of tensile stress–strain properties

The Standards specify the requirements to be applied to the materials of the tensile test specimen. To perceive the failure strain of some materials, the measured length is used and for other materials, the proportionality factor is utilized. The purpose of the testing procedure is to find out, what load the material can bear without severe deformation and the force at which the material is destroyed.

These tests can be utilized to fathom the properties and the deformation behavior of hard foam, soft-elastic foam, rubber and fiber-reinforced composites are determined.

STRESS VS STRAIN GRAPH
STRESS-STRAIN CURVE

Based on the stress-strain curve generated after the tensile test is conducted, the following parameters are determined:

  • Ultimate tensile strength
  • Young’s modulus
  • Yield strength (lower, upper)
  • Yield point
  • Failure strain
  • Uniform strain
  • Necking
Ultimate tensile strength

The tensile stress of specimen increases continuously until no further increase in force can cause an elongation in the material. After which the specimen tears. In the case of a uniform strain being applied on a material, necking (thinning) occurs at a certain point of the artifact. This so-called waist formation occurs when the maximum tensile force is exceeded on the material. This then helps to determine the tensile stress. The necking (thinning) accelerates until the specimen breaks.

Yield strength

The yield strength or yield stress is the property of a material property. This usually is the stress corresponding to the yield point at which the material begins to deform plastically. The yield strength is often used to determine the maximum allowable load on a mechanical component. This is because it represents the upper limit of forces that can be applied without enforcing a permanent deformation. In some materials, such as aluminium, there is a gradual onset of non-linear behavior, thereby making it difficult to determine the precise yield point. 

This is generally differentiated between the upper and the lower yield strength. The upper yield strength describes the point at which the specimen is deformed plastically for the first time and then the material fibre tears. This results in a decrease of stress and permanent elongation of the specimen. This is also termed as an elongation test.

The lower yield strength describes the point after the first deformation wherein the decrease of the tensile stress is at the highest point. The tensile stress then increases continuously again. For specimen with distinct yield strength, the stress is reduced before it breaks. The strain continues to increase as the material begins to yield.

Young’s modulus

Young’s modulus describes the linear-elastic deformation behaviour. If the yield strength has not yet been reached, the deformation will decrease rapidly if no more force is applied. The parameter is identical to the slope of Hooke’s straight line.

Yield point

The yield point is the stress value above which the specimen gets elongated (changes shape) permanently if the stress is exceeded beyond that particular point/value. Even in the case when more force is not applied, yet the specimen does not go back to its original length/shape.

The yield point of a material is a mechanical property commonly measured during material stress test. The yield point of a material is such that when the material transitions from elastic behaviour to plastic behaviour and then the deformation is permanent. Elastic behaviour is described as the situation when the applied load on a material is ceased/terminated, the material then returns to its original shape. On a stress/strain curve, the elastic region is generally depicted as the portion of the curve with a constant slope.

Yield can be measured in various ways depending on the type of material and type of test being performed (tension, Compression etc). From the tests that are conducted to measure yield we get the values for yield strength and yield strain. These values are often used to evaluate whether or not a material is suitable for certain applications. Yield strength is particularly important as it is needed to determine if a material meets a set Factor of Safety.

The calculation of yield is especially important while testing metals. Yield in metals is typically calculated using the offset yield method, where a line is drawn parallel to the modulus and offset by a predetermined amount. The offset is expressed as a percentage and is determined by the ASTM or ISO standard that is being followed.

For metals, yield is generally calculated at 2% offset. In this case, the yield point is defined as being the point of intersection between the offset line and the stress/strain curve. This is only true for metals that exhibit continuous yielding, rather than discontinuous yielding. This phenomenon generally occurs on certain alloys due to localized yielding.

Not all materials exhibit yield. Composites and ceramics both fail at very low strain without exhibiting any yield. Plastics and Elastomers can exhibit different types of curves (as shown below) during a stress strain test, but mostly are part of any one of the possible three categories. Out of which only one exhibits a true, measurable yield point.

A: A brittle material that will break without yielding, example a filled plastic material.

B: A material that exhibits a zero-slope curve, as in thermoplastics.

C: An elastomeric material where the load is increasingly applied slowly until its failure, such as silicone rubber.

Failure strain

Failure strain is the permanent strain of the artefact after the breakage occurs.

Uniform strain

Uniform strain is the nonproportional strain of a tensile specimen when it reachs the highest force or maximum stress.

CONCLUSION
Why is Tensile Testing Performed?

Tensile testing provides details of the mechanical (tensile) properties of a material. These properties can be plotted on a graph as a stress/ strain curve to depict details such as the point at which the material can probably fail as well as providing details of properties such as modulus of elasticity, strain and yield strength.  

Tensile testing has a various uses such as:

  • Selecting materials for an application
  • Predicting how a material will perform under different forces
  • Determining whether the requirements of a specification, contract or standard are met
  • Demonstrating proof of concept for a new product
  • Proving characteristics for a proposed patent
  • Providing standard quality assurance data for scientific and engineering functions
  • Comparing technical data for different material options
  • Testing materials to provide evidence in legal proceedings

Nowadays tensile tests are used in various industries supported by non-contact, optical measuring devices that include a camera. These measuring devices provide precise measuring results and are also suited for tests in which the specimen maybe destroyed.

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