Tensile testing
Encyclopedia
Tensile testing, also known as tension testing, is a fundamental materials science
test in which a sample is subjected to uniaxial tension
until failure. The results from the test are commonly used to select a material for an application, for quality control
, and to predict how a material will react under other types of force
s. Properties that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and reduction in area. From these measurements the following properties can also be determined: Young's modulus
, Poisson's ratio
, yield strength, and strain-hardening characteristics.
The shoulders of the test specimen can be manufactured in various ways to mate to various grips in the testing machine (see the image below). Each system has advantages and disadvantages; for example, shoulders designed for serrated grips are easy and cheap to manufacture, but the alignment of the specimen is dependent on the skill of the technician. On the other hand, a pinned grip assures good alignment. Threaded shoulders and grips also assure good alignment, but the technician must know to thread each shoulder into the grip at least one diameter's length, otherwise the threads can strip before the specimen fractures.
In large castings and forging
s it is common to add extra material, which is designed to be removed from the casting so that test specimens can be made from it. These specimen not be exact representation of the whole workpiece because the grain structure may be different throughout. In smaller workpieces or when critical parts of the casting must be tested, a workpiece may be sacrificed to make the test specimens. For workpieces that are machined
from bar stock
, the test specimen can be made from the same piece as the bar stock.
The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.
A standard specimen is prepared in a round or a square section along the gauge length, depending on the standard used. Both ends of the
specimens should have sufficient length and a surface condition such that they are firmly gripped
during testing. The initial gauge length Lo is standardized (in several countries) and varies with the
diameter (Do) or the cross-sectional area (Ao) of the specimen as listed
The following tables gives examples of test specimen dimensions and tolerances per standard ASTM E8.
. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic
powered and electromagnet
ically powered machines.
The machine must have the proper capabilities for the test specimen being tested. There are three main parameters: force capacity, speed, and precision and accuracy. Force capacity refers to the fact that the machine must be able to generate enough force to fracture the specimen. The machine must be able to apply the force quickly or slowly enough to properly mimic the actual application. Finally, the machine must be able to accurately and precisely measure the gage length and forces applied; for instance, a large machine that is designed to measure long elongations may not work with a brittle material that experiences short elongations prior to fracturing.
Alignment of the test specimen in the testing machine is critical, because if the specimen is misaligned, either at an angle or offset to one side, the machine will exert a bending
force on the specimen. This is especially bad for brittle
materials, because it will dramatically skew the results. This situation can be minimized by using spherical seats or U-joints between the grips and the test machine. A misalignment is indicated when running the test if the initial portion of the stress-strain curve is curved and not linear.
The strain measurements are most commonly measured with an extensometer
, but strain gauge
s are also frequently used on small test specimen or when Poisson's ratio
is being measured. Newer test machines have digital time, force, and elongation measurement systems consisting of electronic sensors connected to a data collection device (often a computer) and software to manipulate and output the data. However, analog machines continue to meet and exceed ASTM, NIST, and ASM metal tensile testing accuracy requirements, continuing to be used today.
s. During the application of tension, the elongation
of the gauge section is recorded against the applied force. The data is manipulated so that it is not specific to the geometry of the test sample. The elongation measurement is used to calculate the engineering strain, ε, using the following equation:
where ΔL is the change in gauge length, L0 is the initial gauge length, and L is the final length. The force measurement is used to calculate the engineering stress, σ, using the following equation:
where F is the force and A is the cross-section of the gauge section. The machine does these calculations as the force increases, so that the data points can be graphed into a stress-strain curve
.
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
test in which a sample is subjected to uniaxial tension
Tension
Tension may refer to:* The Void , also known as Tension in some regions* Tension , a factor that affects knitting gauge...
until failure. The results from the test are commonly used to select a material for an application, for quality control
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...
, and to predict how a material will react under other types of force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s. Properties that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and reduction in area. From these measurements the following properties can also be determined: Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
, Poisson's ratio
Poisson's ratio
Poisson's ratio , named after Siméon Poisson, is the ratio, when a sample object is stretched, of the contraction or transverse strain , to the extension or axial strain ....
, yield strength, and strain-hardening characteristics.
Tensile specimen
A tensile specimen is a standardized sample cross-section. It has two shoulders and a gauge section in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area.The shoulders of the test specimen can be manufactured in various ways to mate to various grips in the testing machine (see the image below). Each system has advantages and disadvantages; for example, shoulders designed for serrated grips are easy and cheap to manufacture, but the alignment of the specimen is dependent on the skill of the technician. On the other hand, a pinned grip assures good alignment. Threaded shoulders and grips also assure good alignment, but the technician must know to thread each shoulder into the grip at least one diameter's length, otherwise the threads can strip before the specimen fractures.
In large castings and forging
Forging
Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons...
s it is common to add extra material, which is designed to be removed from the casting so that test specimens can be made from it. These specimen not be exact representation of the whole workpiece because the grain structure may be different throughout. In smaller workpieces or when critical parts of the casting must be tested, a workpiece may be sacrificed to make the test specimens. For workpieces that are machined
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
from bar stock
Bar stock
Bar stock, also colloquially known as billet, is a common form of raw purified metal, used by industry to manufacture metal parts and products....
, the test specimen can be made from the same piece as the bar stock.
The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.
A standard specimen is prepared in a round or a square section along the gauge length, depending on the standard used. Both ends of the
specimens should have sufficient length and a surface condition such that they are firmly gripped
during testing. The initial gauge length Lo is standardized (in several countries) and varies with the
diameter (Do) or the cross-sectional area (Ao) of the specimen as listed
| Type specimen | United States(ASTM) | Britain | Germany |
|---|---|---|---|
| Sheet ( Lo / √Ao) | 4.5 | 5.65 | 11.3 |
| Rod ( Lo / Do) | 4.0 | 5.00 | 10.0 |
The following tables gives examples of test specimen dimensions and tolerances per standard ASTM E8.
| All values in inches | Plate type (1.5 in. wide) | Sheet type (0.5 in. wide) | Sub-size specimen (0.25 in. wide) |
|---|---|---|---|
| Gage length | 8.00±0.01 | 2.00±0.005 | 1.000±0.003 |
| Width | 1.5 +0.125 -0.25 | 0.500±0.010 | 0.250±0.005 |
| Thickness | 0.25 < T < | 0.005 ≤ T ≤ 0.25 | 0.005 ≤ T ≤ 0.25 |
| Fillet radius (min.) | 1 | 0.25 | 0.25 |
| Overall length (min.) | 18 | 8 | 4 |
| Length of reduced section (min.) | 9 | 2.25 | 1.25 |
| Length of grip section (min.) | 3 | 2 | 1.25 |
| Width of grip section (approx.) | 2 | 0.75 |
| All values in inches | |Standard specimen at nominal diameter: | |Small specimen at nominal diameter: | |||
|---|---|---|---|---|---|
| 0.500 | 0.350 | 0.25 | 0.160 | 0.113 | |
| Gauge length | 2.00±0.005 | 1.400±0.005 | 1.000±0.005 | 0.640±0.005 | 0.450±0.005 |
| Diameter tolerance | ±0.010 | ±0.007 | ±0.005 | ±0.003 | ±0.002 |
| Fillet radius (min.) | 0.25 | ||||
| Length of reduced section (min.) | 2.5 | 1.75 | 1.25 | 0.75 |
Equipment
The most common testing machine used in tensile testing is the universal testing machineUniversal Testing Machine
A universal testing machine, also known as a universal tester, materials testing machine or materials test frame, is used to test the tensile stress and compressive strength of materials...
. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic
Hydraulic machinery
Hydraulic machines are machinery and tools that use liquid fluid power to do simple work. Heavy equipment is a common example.In this type of machine, hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders and which becomes pressurised according to...
powered and electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
ically powered machines.
The machine must have the proper capabilities for the test specimen being tested. There are three main parameters: force capacity, speed, and precision and accuracy. Force capacity refers to the fact that the machine must be able to generate enough force to fracture the specimen. The machine must be able to apply the force quickly or slowly enough to properly mimic the actual application. Finally, the machine must be able to accurately and precisely measure the gage length and forces applied; for instance, a large machine that is designed to measure long elongations may not work with a brittle material that experiences short elongations prior to fracturing.
Alignment of the test specimen in the testing machine is critical, because if the specimen is misaligned, either at an angle or offset to one side, the machine will exert a bending
Bending
In engineering mechanics, bending characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element. The structural element is assumed to be such that at least one of its dimensions is a small fraction, typically...
force on the specimen. This is especially bad for brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
materials, because it will dramatically skew the results. This situation can be minimized by using spherical seats or U-joints between the grips and the test machine. A misalignment is indicated when running the test if the initial portion of the stress-strain curve is curved and not linear.
The strain measurements are most commonly measured with an extensometer
Extensometer
An extensometer is a device that is used to measure small/big changes in the length of an object. It is useful for stress-strain measurements and tensile tests. Its name comes from "extension-meter". It was invented by Dr. Charles Huston who described it in an article in the Journal of the...
, but strain gauge
Strain gauge
A strain gauge is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable...
s are also frequently used on small test specimen or when Poisson's ratio
Poisson's ratio
Poisson's ratio , named after Siméon Poisson, is the ratio, when a sample object is stretched, of the contraction or transverse strain , to the extension or axial strain ....
is being measured. Newer test machines have digital time, force, and elongation measurement systems consisting of electronic sensors connected to a data collection device (often a computer) and software to manipulate and output the data. However, analog machines continue to meet and exceed ASTM, NIST, and ASM metal tensile testing accuracy requirements, continuing to be used today.
Process
The test process involves placing the test specimen in the testing machine and applying tension to it until it fractureFracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
s. During the application of tension, the elongation
Elongation
In astronomy, a planet's elongation is the angle between the Sun and the planet, as viewed from Earth. Elongations occur when an inner planet’s position, in its orbital path, is at tangent to the view from Earth. Because these inner planets are inside the Earth’s orbits their positions as viewed...
of the gauge section is recorded against the applied force. The data is manipulated so that it is not specific to the geometry of the test sample. The elongation measurement is used to calculate the engineering strain, ε, using the following equation:
where ΔL is the change in gauge length, L0 is the initial gauge length, and L is the final length. The force measurement is used to calculate the engineering stress, σ, using the following equation:
where F is the force and A is the cross-section of the gauge section. The machine does these calculations as the force increases, so that the data points can be graphed into a stress-strain curve
Stress-strain curve
During tensile testing of a material sample, the stress–strain curve is a graphical representation of the relationship between stress, derived from measuring the load applied on the sample, and strain, derived from measuring the deformation of the sample, i.e. elongation, compression, or distortion...
.
Standards
Metals- ASTM E8 Standard Test Methods for Tension Testing of Metallic Materials
- ISO 6892 Metallic materials—Tensile testing at ambient temperature
- JISJISJIS is a three letter acronym that can stand for:*Japanese Industrial Standards**JIS encoding**Shift JIS**Horsepower*Jakarta International School*Jeddah International School*Jerudong International School*Jabriya Indian School*Just in Sequence...
Z2241 Method of tensile test for metallic materials