What is the Tensile test?
Tensile testing also known as Tension Test is a destructive test method and most common type of mechanical test. This test provides information about the tensile strength, yield strength, and ductility of the metallic material. It measures the force required to break a metallic, composite or plastic specimen and the extent to which the specimen stretches or elongates to that breaking point.
Tensile testing is performed on a variety of materials including metals, plastics, elastomers, paper, composites, rubbers, fabrics, adhesives, films, etc.
Tensile testing Procedure
Tensile tests are used to determine how materials will behave under tension load. In a simple tensile test, a sample is typically pulled to its breaking point to determine the ultimate tensile strength of the material. The amount of force (F) applied to the sample and the elongation (∆L) of the sample are measured throughout the test. Material properties are often expressed in terms of stress (force per unit area, σ) and strain (percent change in length, ε). To obtain stress, the force measurements are divided by the sample’s cross sectional area (σ = F/A). Strain measurements are obtained by dividing the change in length by the initial length of the sample (ε = ∆L/L). These values are then presented on an XY plot called a stress-strain curve. Testing and measuring procedures vary based on the material being tested and its intended application.
Tensile Testing Standards & Specification
ASTM E8/E8M – Tensile Testing of Metallic Materials
ISO 6892 – Tensile Testing of Metallic Materials
ASTM D412 – Tensile Testing of Elastomers
ISO 37 – Tensile Testing of Elastomers
ASTM D638 – Tensile Testing of Plastics
ISO 527-2 – Tensile Testing of Plastics
ASTM A370 – Standard Test Methods and Definitions for Mechanical Testing of Steel Products
Importance of Tensile Test
Tensile testing is critical for selecting appropriate materials during research and development. Tensile testing can also be used to verify that materials adhere to minimum strength and elongation requirements.
From suspension bridge cables to safety harnesses, lives may depend on the quality of your materials and products so conducting accurate and reliable tensile tests is an absolute necessity.
Consequences of not adhering to high standards can be severe both in monetary and human costs. Usage of improper materials may result in destruction of property and significant loss of life. The costs of disasters brought about by utilizing substandard materials usually far exceed the costs of carrying out regular tensile testing.
Which are the types of Tensile tests?
Tensile test can be classified in following types based on the purposes as listed below:
- Under continually increasing (smooth) load—classic quasi-static tensile test
- Under constant resting (static) load—static tensile test
- Under alternating load for determination of the cyclic stress strain curve—LCF (low cycle fatigue)
- At room temperature (10 to 35 °C): Standard Tensile Test
- At elevated temperatures (too far over 1000 °C): Elevated Temperature Tensile Test or Hot Tensile Test
- At low temperatures or cryogenic temperature Tensile Test (down to -269 °C)
- At very low test speeds—Creep tests
- At elevated test speeds—High-speed tensile test.
Which Properties do we get from a Tensile test?
Tensile testing gives information about various mechanical properties for a given material. Basic tensile test is performed to know about Yield strength, tensile strength & elongation. Most important of them are listed here:
- Maximum Load
- Deflection of Maximum Load
- Work at Maximum Load
- Load at Break
- Deflection at Break
- Work at Break
- Chord Slope
- Stress (tensile strength, Yield strength)
- Strain (%elongation/ reduction)
- Young’s Modulus: This method of testing is used to determine a sample’s behavior under an axial stretching load. Common tensile test results include elastic limit, tensile strength, yield point, yield strength, elongation, and Young’s Modulus. Young’s Modulus is reported commonly as N/mm2 (lbs/in2), MPA (psi).
Stress strain curves highlights the material’s deformation in response to a tensile, compressive, or torsional load applications. Depending on the material being tested (Either brittle or ductile for example), a stress strain curve can indicate key properties of the material including its elastic region, plastic region, yield point, and ultimate tensile strength.
The stress-strain curve provides design engineers, structural engineers with a long list of important parameters needed for application design. A stress-strain graph tells us many mechanical properties such as strength, toughness, elasticity, yield point, strain energy, resilience, and elongation during loading the tensile specimen. These properties are utmost valuable for many engineering applications having a successful & reliable design.
A typical stress stain curve with various attached points are shown in below figure.
Brittle & Ductile fracture in Tensile Test
Two kinds of fracture can be observed in metals. A brittle fracture, or cohesion fracture, takes place without substantial plastic flow and occurs when the normal stress component exceeds the critical normal stress. Shear fracture will occur when the acting shear stress components exceed the critical shear stress value. Large plastic deformation occurs prior to fracture in shear fracture as a result of sliding atomic planes in the lattice structure. Failure in a material seldom occurs without some shear deformation. The brittle fracture exhibits little deformation and therefore has a flat fracture surface; whereas, the shear fracture exhibits gross deformation and the fracture surface has the familiar jagged pyramid form with shear lips.
The relationship of the resistance to these two types of failure does not remain constant for a given material but is dependent upon the temperature and the speed at which the load is applied. The sliding resistance increases with a decrease in temperature and increase in the velocity of deformation; hence, a design subject to low temperatures and impact loading is of particular concern.