What is Tensile Testing: Methods, Types, and Data Analysis

Tensile testing is one of the types of mechanical testing. It is the simplest and most frequently used testing method to determine material’s properties. It is carried out by determining the load that is needed to deform the material to its failure point. These load values help the engineers and designers to measure mechanical properties of material. It is used in predicting the material quality and to know the behavior of materials in different loading conditions during the application. This article will further elaborate the testing method, equipment and set up to conduct the experiments and its importance and use in practical life.

What is Tensile Testing?

Tensile test is a destructive characterization technique. It is conducted by a tensile testing machine. The machine works by holding a sample by a grip from both the ends of sample and applying load until the failure of sample occurs. It measures the tensile strength of the sample. To know the strength, it is important to already know the original cross-sectional area of sample and the rate of deformation. This rate specifies the change in length of the sample after applying load. The testing method can be hydraulic testing or electrotechnical testing. Their machine contains sample grips to hold the material, a crosshead for moving and applying force and a data procurement system to note the testing parameters.

What is the principle of tensile testing?

Tensile test is used to quantify and qualify the material’s properties. The response to applying load, and to determine its capacity and incapacity to survive the load application with the specific time interval is the criteria of tensile testing. The test is used to identify the tensile features of a sample. The deformation obtained by testing is used to measure the ductility and brittleness in material. It also helps in determining the tensile strength, elastic limit, yield strength, elastic modulus, and toughness.

How many types of tensile tests are there?

Tensile test is mostly carried out in two ways:

1.Hydraulic tensile tests

A hydraulic system is applied in tensile testing machine to apply load. This test method uses single or double action piston which works on Pascal’s law in load application over the sample. The hydraulic system uses an electric servo valve for adjusting the movement of hydraulic liquid. The adjustment also governs the loading rate. This type of tensile test method can be carried out on metal and composites.

2.Electromechanical tensile tests

The electromechanical test method uses motor with varying speed for load application to record the tensile strength. The motor is attached to 4 screws that move the crosshead by using a reduction gearbox. The speed of the motor is governed by a microprocessor which adjusts the speed according to the requirements. This testing method can be applied to polymers, composites, and textile materials.

The Purpose of Tensile Testing

Mechanical properties and the behavior of any material under different loading conditions is determined by the tensile testing. It is an important feature in material and design selection for many applications. Tensile test. This is a basic requirement to know the mechanical features. Designers must have to know the mechanical properties of specimen for design selection.

Test Setup and Execution

Tensile test is conducted by holding a specimen in the testing machine. The original cross section of specimen must be known before testing. The grips of machine pull the sample from both the ends at specified speed until the breakdown of sample occurs. The result of the test is used to determine tensile strength. Other than that modulus of elasticity or Youngs modulus yield strength and tension failure can also be obtained. Compression test can also be done instead of tension failure. For compression testing, experiments have the same setup. The difference is the direction of load application which is reverse and has different grips to hold the sample.

What is ASTM standard for tensile test?

ASTM standard E8 and E8M is designated to uniaxial testing of metals at room temperature. This ASTM standard determines the tensile properties, yield strength, tensile strength. This also determines elongation, strain at failure and cross section reduction in sample. ASTM D638 is designated to determine tensile properties of polymers and plastics. Other ASTM standards for tensile testing for different materials are ASTM-D76, ASTM-D412, ASTM-D828, ASTM-D1876.

Advanced Considerations in Tensile Testing

3D printed materials are required to undergo tensile testing to know the mechanical properties. 3D printed parts have anisotropic properties, and it is essential to conduct extensive testing to determine their tensile features. Tensile properties of 3D printed materials are significant to determine the suitability of material for different applications. It is important to know how strong the material and what elements are contributing to its stiffness to withstand the load during the applications.

Specimen Proportionality

Specimen proportionality is important when the gauge length has direct relationship with initial cross section. The formula is used to express their relationship is:

Where L0 is gauge length. A0 is original cross section and k is the constant. The value of constant is different in every standard. EN specifications has 5.65. whereas ASME codes expressed it as 5.
Welding Procedure Approval

Tensile testing is common in the approval process of welding procedures. It helps in assuring quality and integrity of welds. The test standards in welding procedure for tensile testing are in ISO-15614 and ASME-IX. These standards determine the strength of welds and determine the required standards to be met in welding processes.

Comprehensive Overview of Mechanical Testing

Main purpose of mechanical testing is to assure the safety in final structure. It is important to meet the required standards in a specific environment. So, it is necessary to create a same testing environment to know the material’s behavior in an actual working environment. Mechanical testing is an important feature in the manufacturing processes. Mechanical testing can vary depending on the final structure requirements.

Types of Mechanical Testing

Mechanical testing such as tensile test to know the tensile properties of sample, Charpy V-notch test to know the toughness of sample, hardness test of sample and fatigue test to know the cycling load bearing capacity of sample.

One test can provide a variety of information about the material. for example, tensile test can give data about ultimate tensile strength, yield strength, modulus of elasticity or young’s Modulus, tensile properties and many more.

One mechanical property can be determined by various tests such as toughness cand be fine out by Charpy v-notch test and Izod test. For hardness, Vickers, Brinell and Rockwell tests can be conducted. The selection of method depends on the manufacturing process and standard requirements of structure.

Significance of Mechanical Testing

Mechanical testing is important for various reasons. It provides information about durability, safety, and performance of structure. If there is any defect, flaw or weakness is present in structure that can impart prior failure, can be determined at initial stage.

Types of Materials in Tensile Testing

A variety of materials can be tested in tensile testing. These are as:

1.Metals.

The easiest and simplest for mechanical testing are for metals. There are many properties that can be found out such as yield strength, modulus of elasticity or young’s Modulus and other tensile properties.

2.Ceramics

Ceramics are brittle materials. Usually the sample is taken in ‘dog bone’ shape and undergo testing. Their testing is difficult and normally are not tested because of their hardness. This implies it is difficult to create their required shape and to hold it in the machine till their failure.

3.Polymers

Mechanical tests are conducted to know the elasticity or brittleness in polymer. Brittle polymer does not yield point. Therefore, 0.5% strain is sued to know the yield strength or polymers.

4.Textile

Textile is any fiber-based material. Textile testing has a variety of ASTM standards depending on material type. Textiles are tested to obtain tear resistance and to determine the capability to incorporate in a composite like fiber-reinforced material.

5.Composites

Composites are tested to determine the conditions like single or strand of filament, unidirectional or multidirectional laminates or bolted or bonded laminates. Anisotropic composites are tested to know the orientation properties.

Tensile test materials

Different types of materials can undergo tensile test. tensile test shows properties like tensile strength, yield strength, modulus of elasticity etc.

Properties of Metals in Tensile Testing

Tensile properties of different metals are as:

Metal Specimens in Tensile Testing

• Sample strip is cut in width of 25mm and required length.
• Samples are marked in such a way that machine grip has distance of 100mm after sample clamping.
• Distance must be 25mm from both ends of sample and must have equal clamping from both ends.
• The standards follow are ASTM-E8 and ASTM-D638 for metals and polymers, respectively.

Stress-Strain Curve Analysis

Stress-strain curve is a plot of material showing change in length when a load is applied. This curve shows a range of properties of material from elastic to plastic deformation. The data includes young’s modulus, yield strength, tensile strength, and fracture toughness of sample.

Detailed Process of Tensile Testing

Setting Up the Tensile Test

Tensile test is conducted in tensile machine known as universal tensile tester. The tester has a range of sizes and load applying capacity. This variety helps in determining different types of measurement for different materials. Samples are prepared with ASTM-E8 or ASTM-D638 standards in a ‘Dog bone’ shape.

Conducting the Test

The test is conducted by holding the specimen in the testing machine. The machine grips the sample from both ends at a predetermined distance and pulls it slowly lengthwise. The pulling continues until the sample breaks down. Machine provides data in the form of curve which ash change in length and applied load. Load is converted to stress. And change in length is converted to strain to draw stress-strain curve.

Data Analysis and Interpretation

Results obtained from tensile test have valuable data about material’s properties. tensile properties can easily be determined by interpreting results from different formulas of young’s modulus, ductility, or elongation. The results have an important phase of sample failure, upon which many properties depend.

Considerations for Effective Tensile Testing

Tensile testing can be challenging. There are few considerations to ensure before conducting the test:

• Testing temperature:

Metals have different behavior in different temperature range. Low temperature reveals the brittle nature of metals. High temperature shows ductility in metals. Tests conducted at room temperature have different results of metal properties.

• Alloying composition:

Alloy specifically with solute element has a great impact on mechanical properties. like if carbon is introduced in iron, it increases the tensile properties of alloy than pure iron. Similarly, ductility, yield strength, fracture toughness all are affected by alloying composition.

• Mechanical and thermal treatment:

Heat treatments or work hardening can alter the microstructural properties of material. These changes can drastically change the tensile properties in metals. It is important to know the material history thoroughly before conducting tensile test.

• Grain sizes

Mechanical properties are dependent on grain sizes. Small grains size in metals show ductile nature while brittle nature is normally seen in large grain size material. Similarly large grain size has low strength. Grain size can be controlled by heat treatments.

Conclusion

Tensile testing is a basic form of mechanical testing. It is performed by engineers and scientists to reveal the behavior of metal under different loading conditions. In tensile test, force is applied by pulling both ends of sample and the change in length is measured. Universal tensile testing machines are used to conduct the test. The force is then converted to stress and change in cross section is considered as strain. The stress-strain graph is plotted to know the modulus of elasticity young’s modulus, yield strength, tensile strength and fracture toughness of metal. ASTM standards used for metal are ASTM-E8/E8M and for polymers ASTM-D638 is used.