Tensile testing
Hemant Yadav
Shaping future engineers through teaching, research, and industry collaboration as an Assistant Professor in Mechanical Engineering
To carry out a tensile test, the operator must complete several steps to ensure the test complies with both internal and external standards. While some labs may automate parts or all of these tasks, the operator remains ultimately responsible for the accuracy of the test setup.
Preparing Specimens
Specimen geometries vary significantly depending on the material being tested and the specific test method or standard applied. Organizations like ASTM and ISO have developed standardized specimen requirements for various materials, enabling consistent comparison of properties across different batches and manufacturers.
Tensile specimens are often machined or cast into a dog-bone shape, featuring 'shoulders' to be gripped by the testing machine and a 'gage length' where tensile properties are measured. The dimensions of the shoulders, the gage length, and the overall size of the specimen are all defined by the relevant testing standard.
In figure 1 standard dimensions of a flat specimen is shown where a, b, c, d, e, f, g and h are the notations for the dimension in table 1.
Specimen into Grips
A critical step in tensile testing is securely positioning the specimen within the grips of the testing machine. Proper alignment is essential to ensure that the tensile load is applied evenly along the axis of the specimen, preventing bending or other distortions that could compromise test accuracy.
For dog-bone specimens, the shoulders are designed to fit perfectly into the grips, ensuring that the tensile force is concentrated in the gage length where measurements are taken. Depending on the type of grips used (hydraulic, pneumatic, or mechanical), the operator must ensure that:
Incorrect insertion or improper grip pressure can lead to premature failure at the grips or inaccurate data collection, so careful setup is essential to a successful tensile test.
Strain Measurement
Strain measurement is a key component of tensile testing, as it quantifies the deformation of a material under applied load. Several types of devices are commonly used to measure strain, each with its own advantages depending on the test requirements and material properties.
Extensometers
Extensometers are widely used in tensile testing to measure the elongation of the specimen directly. These devices can be either contact or non-contact:
Strain Gauges
Strain gauges are small sensors bonded directly onto the specimen's surface. These sensors measure the change in electrical resistance as the specimen stretches, converting it into strain data. Strain gauges are highly accurate and ideal for localized strain measurement but require precise application to ensure reliable results.
Digital Image Correlation (DIC)
DIC is a cutting-edge, non-contact method that uses high-resolution cameras to capture a series of images as the specimen deforms. By tracking surface patterns or applied speckles, the DIC system provides a full-field strain map, offering detailed insights into strain distribution across the specimen. This method is particularly valuable for materials with complex strain behaviors or heterogeneous properties.
Laser Interferometry
Laser interferometry offers a highly precise, non-contact method of measuring strain, typically used for high-precision applications. It involves measuring the interference patterns of laser beams as they reflect off the specimen surface. Laser-based systems are less common in routine testing due to their complexity but are valuable for specialized research applications where extreme accuracy is required.
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Choosing the Right Strain Measurement Device
The selection of a strain measurement device depends on factors such as:
For most standard tensile tests, contact extensometers or strain gauges are sufficient. However, for more advanced or sensitive testing, non-contact methods like DIC or laser interferometry offer greater precision and additional data on strain distribution.
Starting the test
Once the specimen is loaded into the system and the extensometer is attached, the test can begin. Prior to this, the testing software will have been configured with the appropriate test method, along with key parameters such as test speed, specimen dimensions, and end criteria. When the test starts, the machine applies tensile force to the specimen in accordance with the specified method, while recording data on the specimen's response to the applied stress. After completion, the specimen can be removed, and the collected data can be exported for further analysis.
Ultimate Tensile Strength (UTS) is one of the most critical properties of a material, representing the maximum stress a specimen can withstand during testing. The UTS may differ from the strength at break, depending on the material’s behavior—whether it’s brittle, ductile, or a combination of both. For instance, a material that behaves ductile under normal lab conditions could become brittle in service when exposed to extremely cold temperatures, causing its mechanical properties to change.
Hooke's Law - In the initial phase of tensile testing, most materials display a linear relationship between the applied load and the specimen's elongation. This linear behavior follows Hooke's Law, where the ratio of stress to strain remains constant. The constant of proportionality, represented by the slope of this linear region, is known as Young's Modulus or the Modulus of Elasticity. It is denoted by E and indicates that stress (σ) is directly proportional to strain (ε) within this region.
E = σ/ε
The Modulus of Elasticity measures a material’s stiffness and applies only within the initial linear portion of the stress-strain curve. In this region, if the tensile load is removed, the specimen will return to its original state without any permanent deformation. However, once the curve begins to deviate from linearity, Hooke's Law no longer holds, and the material reaches the elastic (or proportional) limit. Beyond this point, any further increase in load causes plastic deformation, meaning the material will not return to its original shape when the load is removed.
Yield strength is the level of stress at which a material begins to undergo plastic deformation, meaning it will no longer return to its original shape after the stress is removed.
Strain- During tensile testing, we can measure the extent of stretching or elongation that a specimen undergoes. This can be reported either as an absolute change in length or as a relative measure known as strain. Strain can be categorized in two forms: engineering strain and true strain, each offering different ways to express the material’s deformation.
Engineering strain is the most commonly used and straightforward expression of strain. It is defined as the ratio of the change in length to the specimen’s original length:
Engineering?Strain = ΔL / original length
where ΔL is the change in length
True strain, on the other hand, is based on the instantaneous length of the specimen throughout the test. It accounts for the continuous change in length as the test progresses and is calculated as:
True?Strain = ln (instantaneous length/initial length)
Similarly, Stress is also having 2 types
Engineering stress is the most commonly used expression of stress in tensile testing. It is defined as the applied force (or load) divided by the specimen’s original cross-sectional area:
Engineering?Stress = applied force/original cross-sectional area of the specimen
True stress takes into account the continuous reduction in the cross-sectional area as the specimen elongates. It is calculated by dividing the applied force by the instantaneous cross-sectional area at any given point during the test:
True?Stress = force applied/instantaneous cross-sectional
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