Tensile testing

Last updated January 11, 2024

Tensile testing, also known as tension testing,^[1] is a fundamental materials science and engineering test in which a sample is subjected to a controlled tension until failure. Properties that are directly measured via a tensile test are ultimate tensile strength, breaking strength, maximum elongation and reduction in area.^[2] From these measurements the following properties can also be determined: Young's modulus, Poisson's ratio, yield strength, and strain-hardening characteristics.^[3]Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic 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 forces.
Determine if, or verify that, the requirements of a specification, regulation, or contract are met
Decide if a new product development program is on track
Demonstrate proof of concept
Demonstrate the utility of a proposed patent
Provide standard data for other scientific, engineering, and quality assurance functions
Provide a basis for Technical communication
Provide a technical means of comparison of several options
Provide evidence in legal proceedings

Tensile specimen

The preparation of test specimens depends on the purposes of testing and on the governing test method or specification. 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% ^[4] so they can be easily gripped. The gauge section's smaller diameter also allows the deformation and failure to occur in this area.^[2]^[5]

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.^[6]

In large castings and forgings 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.^[7] For workpieces that are machined from bar stock, 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.^[8]^[9]

Various shoulder styles for tensile specimens. Keys A through C are for round specimens, whereas keys D and E are for flat specimens. Key:

A. A Threaded shoulder for use with a thread
B. A round shoulder for use with serrated grips
C. A butt end shoulder for use with a split collar
D. A flat shoulder for used with serrated grips

E. A flat shoulder with a through hole for a pinned grip

Test specimen nomenclature

The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.^[7]

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.

Flat test specimen^[10]
All values in inches	Plate type (1.5 in. wide)	Sheet type (0.5 in. wide)	Sub-size specimen (0.25 in. wide)
Gauge 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	0.188 ≤ T	0.005 ≤ T ≤ 0.75	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	3⁄8

Round test specimen^[10]
All values in inches	Standard specimen at nominal diameter:		Small specimen at nominal diameter:
All values in inches	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.)	3⁄8	0.25	5⁄16	5⁄32	3⁄32
Length of reduced section (min.)	2.5	1.75	1.25	0.75	5⁄8

Equipment

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 . 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. Testing machines are either electromechanical or hydraulic.^[5]

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.^[11]

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.^[6]

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.^[6] If the initial portion of the stress–strain curve is curved and not linear, it indicates the specimen is misaligned in the testing machine.^[12]

The strain measurements are most commonly measured with an extensometer, but strain gauges are also frequently used on small test specimen or when Poisson's ratio is being measured.^[6] 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.^{[ citation needed ]}

Process

Aluminium tensile test samples after breakage

The "cup" side of the "cup–cone" characteristic failure pattern

Some parts showing the "cup" shape and some showing the "cone" shape

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 , ε, using the following equation:^[5]

\varepsilon ={\frac {\Delta L}{L_{0}}}={\frac {L-L_{0}}{L_{0}}}

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:^[5]

\sigma ={\frac {F_{n}}{A}}

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 .^[5]

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.^[13]

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.^[14]

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.^[15] 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 ( $\varepsilon _{1}$ ) and the average strain ( $\varepsilon _{0}$ ):^[16]

${\text{Percent Bending}}={\frac {\varepsilon _{1}-\varepsilon _{0}}{\varepsilon _{0}}}\times 100$

Percent bending should be under 1% on the wider face of loaded samples, and under 2% on the thinner face. Bending can be caused by misalignment on the loading clamp and asymmetric machining of samples.^[16]

Standards

Metals

ASTM E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" (2013)
ISO 6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)
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 D 3039/D 3039M: "Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials"

Flexible materials

ASTM D638 Standard Test Method for Tensile Properties of Plastics
ASTM D828 Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus
ASTM D882 Standard test method for tensile properties of thin plastic sheeting
ISO 37 rubber, vulcanized or thermoplastic—determination of tensile stress–strain properties

Related Research Articles

Ductility is a mechanical property commonly described as a material's amenability to drawing. In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress before failure. Ductility is an important consideration in engineering and manufacturing. It defines a material's suitability for certain manufacturing operations and its capacity to absorb mechanical overload. Some metals that are generally described as ductile include gold and copper, while platinum is the most ductile of all metals in pure form. However, not all metals experience ductile failure as some can be characterized with brittle failure like cast iron. Polymers generally can be viewed as ductile materials as they typically allow for plastic deformation.

Young's modulus is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Young's modulus is defined as the ratio of the stress applied to the object and the resulting axial strain in the linear elastic region of the material.

In engineering, deformation refers to the change in size or shape of an object. Displacements are the absolute change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain is the relative internal change in shape of an infinitesimal cube of material and can be expressed as a non-dimensional change in length or angle of distortion of the cube. Strains are related to the forces acting on the cube, which are known as stress, by a stress-strain curve. The relationship between stress and strain is generally linear and reversible up until the yield point and the deformation is elastic. The linear relationship for a material is known as Young's modulus. Above the yield point, some degree of permanent distortion remains after unloading and is termed plastic deformation. The determination of the stress and strain throughout a solid object is given by the field of strength of materials and for a structure by structural analysis.

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 strain can be determined. These curves reveal many of the properties of a material, such as the Young's modulus, the yield strength and the ultimate tensile strength.

<span class="mw-page-title-main">Compressive strength</span> Capacity of a material or structure to withstand loads tending to reduce size

In mechanics, compressive strength is the capacity of a material or structure to withstand loads tending to reduce size. In other words, compressive strength resists compression, whereas tensile strength resists tension. In the study of strength of materials, tensile strength, compressive strength, and shear strength can be analyzed independently.

Stress–strain analysis is an engineering discipline that uses many methods to determine the stresses and strains in materials and structures subjected to forces. In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.

$<span class="mw-page-title-main">Toughness</span> Material ability to absorb energy and plastically deform without fracturing$

In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing. Toughness is the strength with which the material opposes rupture. One definition of material toughness is the amount of energy per unit volume that a material can absorb before rupturing. This measure of toughness is different from that used for fracture toughness, which describes the capacity of materials to resist fracture. Toughness requires a balance of strength and ductility.

In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and immediately return to their original state once the stress is removed.

In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a crack with thin components having plane stress conditions and thick components having plane strain conditions. Plane strain conditions give the lowest fracture toughness value which is a material property. The critical value of stress intensity factor in mode I loading measured under plane strain conditions is known as the plane strain fracture toughness, denoted $. When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation . Fracture toughness is a quantitative way of expressing a material's resistance to crack propagation and standard values for a given material are generally available.$

<span class="mw-page-title-main">Three-point flexural test</span> Standard procedure for measuring modulus of elasticity in bending

The three-point bending flexural test provides values for the modulus of elasticity in bending $, flexural stress, flexural strain and the flexural stress-strain response of the material. This test is performed on a universal testing machine with a three-point or four-point bend fixture. The main advantage of a three-point flexural test is the ease of the specimen preparation and testing. However, this method has also some disadvantages: the results of the testing method are sensitive to specimen and loading geometry and strain rate.$

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 a tensometer. The "universal" part of the name reflects that it can perform many standard tensile and compression tests on materials, components, and structures.

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 in an article in the Journal of the Franklin Institute in 1879. Huston later gave the rights to Fairbanks & Ewing, a major manufacturer of testing machines and scales.

The split-Hopkinson pressure bar, named after Bertram Hopkinson, sometimes also called a Kolsky bar, is an apparatus for testing the dynamic stress–strain response of materials.

The Larson–Miller relation, also widely known as the Larson–Miller parameter and often abbreviated LMP, is a parametric relation used to extrapolate experimental data on creep and rupture life of engineering materials.

<span class="mw-page-title-main">Compact tension specimen</span>

A compact tension specimen (CT) is a type of standard notched specimen in accordance with ASTM and ISO standards. Compact tension specimens are used extensively in the area of fracture mechanics and corrosion testing, in order to establish fracture toughness and fatigue crack growth data for a material.

Rising Step Load Testing is a testing system that can apply loads in tension or bending to evaluate hydrogen-induced cracking. It was specifically designed to conduct the accelerated ASTM F1624 step-modified, slow strain rate tests on a variety of test coupons or structural components. It can also function to conduct conventional ASTM E8 tensile tests; ASTM F519 200-hr Sustained Load Tests with subsequent programmable step loads to rupture for increased reliability; and ASTM G129 Slow Strain Rate Tensile tests.

Slow strain rate testing (SSRT), also called constant extension rate tensile testing (CERT), is a popular test used by research scientists to study stress corrosion cracking. It involves a slow dynamic strain applied at a constant extension rate in the environment of interest. These test results are compared to those for similar tests in a, known to be inert, environment. A 50-year history of the SSRT has recently been published by its creator. The test has also been standardized and two ASTM symposia devoted to it.

Tear resistance is a measure of how well a material can withstand the effects of tearing. It is a useful engineering measurement for a wide variety of materials by many different test methods.

The four-point flexural test provides values for the modulus of elasticity in bending $, flexural stress, flexural strain and the flexural stress-strain response of the material. This test is very similar to the three-point bending flexural test. The major difference being that with the addition of a fourth bearing the portion of the beam between the two loading points is put under maximum stress, as opposed to only the material right under the central bearing in the case of three-point bending.$

References

↑ Czichos, Horst (2006). Springer Handbook of Materials Measurement Methods. Berlin: Springer. pp. 303–304. ISBN 978-3-540-20785-6.
1 2 Davis, Joseph R. (2004). Tensile testing (2nd ed.). ASM International. ISBN 978-0-87170-806-9.
↑ Davis 2004 , p. 33.
↑ Common Material Tests. The Engineering Archive. (n.d.). https://theengineeringarchive.com/material-science/page-common-material-tests.html
1 2 3 4 5 Davis 2004 , p. 2.
1 2 3 4 Davis 2004 , p. 9.
1 2 Davis 2004 , p. 8.
↑ Maccaferri, Emanuele; Cocchi, Davide; Mazzocchetti, Laura; Benelli, Tiziana; Brugo, Tommaso Maria; Giorgini, Loris; Zucchelli, Andrea (July 2021). "How Nanofibers Carry the Load: Toward a Universal and Reliable Approach for Tensile Testing of Polymeric Nanofibrous Membranes". Macromolecular Materials and Engineering. 306 (7): 2100183. doi: 10.1002/mame.202100183 . hdl: 11585/821061 .
↑ How to correctly prepare nanofibrous mat specimens for tensile testing. youtube.com
1 2 Davis 2004 , p. 52.
↑ Gedney, 2005
↑ Davis 2004 , p. 11.
↑ Maccaferri, Emanuele; et al. (2021). "How Nanofibers Carry the Load: Toward a Universal and Reliable Approach for Tensile Testing of Polymeric Nanofibrous Membranes". Macromolecular Materials and Engineering. 306 (7). doi: 10.1002/mame.202100183 . hdl: 11585/821061 .
↑ Bissonnette, Benoit; Pigeon, Michel; Vaysburd, Alexander M. (1 July 2007). "Tensile Creep of Concrete: Study of Its Sensitivity to Basic Parameters". Materials Journal. 104 (4): 360–368. doi:10.14359/18825. ProQuest 197938866.
↑ "Lever Arm Test Systems". www.mltest.com. Retrieved 21 May 2022.
1 2 Carroll, Daniel F.; Wiederhorn, Sheldon M.; Roberts, D. E. (September 1989). "Technique for Tensile Creep Testing of Ceramics". Journal of the American Ceramic Society. 72 (9): 1610–1614. doi:10.1111/j.1151-2916.1989.tb06291.x.

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[1] Czichos, Horst (2006). Springer Handbook of Materials Measurement Methods. Berlin: Springer. pp. 303–304. ISBN 978-3-540-20785-6.

[davis1-2] 1 2 Davis, Joseph R. (2004). Tensile testing (2nd ed.). ASM International. ISBN 978-0-87170-806-9.

[3] Davis 2004 , p. 33.

[4] Common Material Tests. The Engineering Archive. (n.d.). https://theengineeringarchive.com/material-science/page-common-material-tests.html

[davis2-5] 1 2 3 4 5 Davis 2004 , p. 2.

[davis9-6] 1 2 3 4 Davis 2004 , p. 9.

[davis8-7] 1 2 Davis 2004 , p. 8.

[8] Maccaferri, Emanuele; Cocchi, Davide; Mazzocchetti, Laura; Benelli, Tiziana; Brugo, Tommaso Maria; Giorgini, Loris; Zucchelli, Andrea (July 2021). "How Nanofibers Carry the Load: Toward a Universal and Reliable Approach for Tensile Testing of Polymeric Nanofibrous Membranes". Macromolecular Materials and Engineering. 306 (7): 2100183. doi: 10.1002/mame.202100183 . hdl: 11585/821061 .

[9] How to correctly prepare nanofibrous mat specimens for tensile testing. youtube.com

[davis52-10] 1 2 Davis 2004 , p. 52.

[11] Gedney, 2005

[12] Davis 2004 , p. 11.

[13] Maccaferri, Emanuele; et al. (2021). "How Nanofibers Carry the Load: Toward a Universal and Reliable Approach for Tensile Testing of Polymeric Nanofibrous Membranes". Macromolecular Materials and Engineering. 306 (7). doi: 10.1002/mame.202100183 . hdl: 11585/821061 .

[14] Bissonnette, Benoit; Pigeon, Michel; Vaysburd, Alexander M. (1 July 2007). "Tensile Creep of Concrete: Study of Its Sensitivity to Basic Parameters". Materials Journal. 104 (4): 360–368. doi:10.14359/18825. ProQuest 197938866.

[15] "Lever Arm Test Systems". www.mltest.com. Retrieved 21 May 2022.

[:0-16] 1 2 Carroll, Daniel F.; Wiederhorn, Sheldon M.; Roberts, D. E. (September 1989). "Technique for Tensile Creep Testing of Ceramics". Journal of the American Ceramic Society. 72 (9): 1610–1614. doi:10.1111/j.1151-2916.1989.tb06291.x.

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