![Tensile testing on a coir composite](https://upload.wikimedia.org/wikipedia/commons/2/22/Tensile_testing_on_a_coir_composite.jpg)
Tensile testing, also known as tension testing, is a fundamental
materials science and
engineering
Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
test in which a sample is subjected to a controlled
tension
Tension may refer to:
Science
* Psychological stress
* Tension (physics), a force related to the stretching of an object (the opposite of compression)
* Tension (geology), a stress which stretches rocks in two opposite directions
* Voltage or el ...
until failure. Properties that are directly measured via a tensile test are
ultimate tensile strength
Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
,
breaking strength
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
, maximum
elongation and reduction in area.
From these measurements the following properties can also be determined:
Young's modulus
Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
,
Poisson's ratio
In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Pois ...
,
yield strength
In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...
, and
strain-hardening characteristics. ''Uniaxial tensile testing'' is the most commonly used for obtaining the mechanical characteristics of
isotropic
Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
materials. Some materials use
biaxial tensile testing. The main difference between these testing machines being how load is applied on the materials.
Purposes of tensile testing
Tensile testing might have a variety of purposes, such as:
*Select a material or item for an application
*Predict how a material will perform in use: normal and extreme
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
s.
* Determine if, or verify that, the requirements of a
specification
A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard.
There are different types of technical or engineering specificati ...
,
regulation
Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. For ...
, or
contract
A contract is a legally enforceable agreement between two or more parties that creates, defines, and governs mutual rights and obligations between them. A contract typically involves the transfer of goods, services, money, or a promise to tran ...
are met
* Decide if a
new product development
In business and engineering, new product development (NPD) covers the complete process of bringing a new product (business), product to market, renewing an existing product or introducing a product in a new market. A central aspect of NPD is prod ...
program is on track
* Demonstrate
proof of concept
Proof of concept (POC or PoC), also known as proof of principle, is a realization of a certain method or idea in order to demonstrate its feasibility, or a demonstration in principle with the aim of verifying that some concept or theory has prac ...
* Demonstrate the utility of a proposed
patent
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
* Provide
standard Standard may refer to:
Symbols
* Colours, standards and guidons, kinds of military signs
* Standard (emblem), a type of a large symbol or emblem used for identification
Norms, conventions or requirements
* Standard (metrology), an object th ...
data
In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...
for other scientific, engineering, and
quality assurance
Quality assurance (QA) is the term used in both manufacturing and service industries to describe the systematic efforts taken to ensure that the product(s) delivered to customer(s) meet with the contractual and other agreed upon performance, design ...
functions
* Provide a basis for
Technical communication
Technical communication is used to convey scientific, engineering, or other technical information. Individuals in a variety of contexts and with varied professional credentials engage in technical communication. Some individuals are designated as ...
* Provide a technical means of comparison of several options
* Provide
evidence
Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true. What role evidence plays and how it is conceived varies from field to field.
In epistemology, evidenc ...
in legal proceedings
Tensile specimen
![Tensile specimen-round and flat](https://upload.wikimedia.org/wikipedia/commons/7/71/Tensile_specimen-round_and_flat.jpg)
The preparation of test specimens depends on the purposes of testing and on the governing
test method
A test method is a method for a test in science or engineering, such as a physical test, chemical test, or statistical test. It is a definitive procedure that produces a test result. In order to ensure accurate and relevant test results, a test met ...
or
specification
A specification often refers to a set of documented requirements to be satisfied by a material, design, product, or service. A specification is often a type of technical standard.
There are different types of technical or engineering specificati ...
. A tensile specimen usually has a standardized sample cross-section. It has two shoulders and a gauge (section) in between. The shoulders and grip section are generally larger than the gauge section by 33% so they can be easily gripped. The gauge section's smaller diameter also allows the deformation and failure to 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 is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which i ...
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 specimens may 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 is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, which utilizes ...
from
bar stock
Bar stock, also (colloquially) known as blank, slug or billet, is a common form of raw purified metal, used by industry to manufacture metal parts and products. Bar stock is available in a variety of extrusion shapes and lengths. The most commo ...
, the test specimen can be made from the same piece as the bar stock.
For soft and porous materials, like electrospun nonwovens made of nanofibers, the specimen is usually a sample strip supported by a paper frame to favour its mounting on the machine and to avoid membrane damaging.
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
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
E8.
Equipment
![Inspekt desk 50kN IMGP8563](https://upload.wikimedia.org/wikipedia/commons/0/0c/Inspekt_desk_50kN_IMGP8563.jpg)
Tensile testing is most often carried out at a material testing laboratory. The ASTM D638 is among the most common tensile testing protocols. The ASTM D638 measures plastics tensile properties including ultimate tensile strength, yield strength, elongation and Poisson's ratio.
The most common testing machine used in tensile testing is the ''
universal testing machine
A universal testing machine (UTM), also known as a universal tester, materials testing machine or materials test frame, is used to test the tensile strength and compressive strength of materials. An earlier name for a tensile testing machine is ...
''. 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
Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...
powered and
electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the ...
ically powered machines.
The electromechanical machine uses an electric motor, gear reduction system and one, two or four screws to move the crosshead up or down. A range of crosshead speeds can be achieved by changing the speed of the motor. The speed of the crosshead, and consequently the load rate, can be controlled by a microprocessor in the closed-loop servo controller. A hydraulic testing machine uses either a single- or dual-acting piston to move the crosshead up or down. Manually operated testing systems are also available. Manual configurations require the operator to adjust a needle valve in order to control the load rate. A general comparison shows that the electromechanical machine is capable of a wide range of test speeds and long crosshead displacements, whereas the hydraulic machine is a cost-effective solution for generating high forces.
The machine must have the proper capabilities for the test specimen being tested. There are four main parameters: force capacity, speed,
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 gauge 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
In applied mechanics, bending (also known as flexure) 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 ...
force on the specimen. This is especially bad for
brittle
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...
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.
If the initial portion of the stress–strain curve is curved and not linear, it indicates the specimen is misaligned in the testing machine.
The strain measurements are most commonly measured with an
extensometer
An extensometer is a device that is used to measure 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 Charles Huston who described it ...
, but
strain gauge
A strain gauge (also spelled strain gage) is a device used to measure strain on 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 ...
s are also frequently used on small test specimen or when
Poisson's ratio
In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Pois ...
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 slowly extending it until it fractures. During this process, 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
Strain may refer to:
Science and technology
* Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes
* Strain (chemistry), a chemical stress of a molecule
* Strain (injury), an injury to a mu ...
'', ''ε'', using the following equation:
:
where Δ''L'' is the change in gauge length, ''L''
0 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 tensile force and A is the nominal cross-section of the specimen. The machine does these calculations as the force increases, so that the data points can be graphed into a ''
stress–strain curve
In engineering and materials science, a stress–strain curve for a material gives the relationship between stress and strain. It is obtained by gradually applying load to a test coupon and measuring the deformation, from which the stress and ...
''.
When dealing with porous and soft materials, as electrospun nanofibrous membranes, the application of the above stress formula is problematic. The membrane thickness, indeed, is dependent on the pressure applied during its measurement, leading to variable thicknesses value. As a consequence, the obtained stress-strain curves show high variability. In this case, the normalization of load with respect to the specimen mass instead of the cross-section area (A) is recommended to obtain reliable tensile results.
Tensile testing creep
Tensile testing can be used to test
creep in materials, a slow plastic deformation of the material from constant applied stresses over extended periods of time. Creep is generally aided by diffusion and dislocation movement. While there are many ways to test creep, tensile testing is useful for materials such as concrete and ceramics that behave differently in tension and compression, and thus possess different tensile and compressive creep rates. As such, understanding the tensile creep is important in the design of concrete for structures that experience tension, such as water holding containers, or for general structural integrity.
Tensile testing of creep generally follows the same testing process as standard testing albeit generally at lower stresses to remain in the creep domain rather than plastic deformation. Additionally, specialized tensile creep testing equipment may include incorporated high temperature furnace components to aid diffusion.
The sample is held at constant temperature and tension, and strain on the material is measured using strain gauges or laser gauges. The measured strain can be fitted with equations governing different mechanisms of creep, such as power law creep or diffusion creep (see
creep for more information). Further analysis can be obtained from examining the sample post fracture. Understanding the creep mechanism and rate be able to aid materials selection and design.
It is important to note that sample alignment is important for tensile testing creep. Off centered loading will result in a bending stress being applied to the sample. Bending can be measured by tracking strain on all sides of the sample. The percent bending can then be defined as the difference between strain on one face (
) and the average strain (
):
Percent bending should be under 1% on the wider face of loaded samples, and under 2% on the thinner face. Bending can caused by misalignment on the loading clamp and asymmetric machining of samples.
Standards
Metals
*
ASTM
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" (2013)
*
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)
*
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* 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
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
D 3039/D 3039M: "Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials"
Flexible materials
*
ASTM
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
D638 Standard Test Method for Tensile Properties of Plastics
*
ASTM
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
D828 Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus
*
ASTM
ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
D882 Standard test method for tensile properties of thin plastic sheeting
*
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
37 rubber, vulcanized or thermoplastic—determination of tensile stress–strain properties
References
External links
Video on the tensile testTensile Tester*
ttps://www.micomlab.com/micom-testing/astm-d638/ Learn more about the ASTM D638 Tensile TestRUBBER BELT TENSILE STRENGTH TEST{{Authority control
Materials testing