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[[File:Tensile testing on a coir composite.jpg|thumb|Tensile testing on a coir composite. Specimen size is not to standard (Instron).]]
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'''Tensile testing''', also known as '''tension testing''',<ref>{{cite book|last = Czichos|first = Horst|title = Springer Handbook of Materials Measurement Methods|publisher = Springer|location = Berlin|year = 2006|pages = 303–304|url = http://books.google.com/books?id=8lANaR-Pqi4C|isbn = 978-3-540-20785-6}}</ref> is a fundamental [[materials science]] test in which a sample is subjected to a controlled [[tension (physics)|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 (materials science)|elongation]] and reduction in area.<ref name="davis1">{{cite book|last = Davis|first = Joseph R.|title = Tensile testing|publisher = ASM International|year = 2004|edition = 2nd|url = http://books.google.com/books?id=5uRIb3emLY8C|isbn = 978-0-87170-806-9}}</ref> From these measurements the following properties can also be determined: [[Young's modulus]], [[Poisson's ratio]], [[yield strength]], and [[strain-hardening]] characteristics.<ref>{{harvnb|Davis|2004|p=33}}.</ref> '''Uniaxial tensile testing''' is the most commonly used for obtaining the mechanical characteristics of [[isotropy|isotropic]] materials. For [[anisotropy|anisotropic]] materials, such as [[composite material]]s and [[textiles]], [[Plane biaxial tensile test|biaxial tensile testing]] is required.
 
==Tensile specimen==
[[File:Tensile specimen-round and flat.jpg|thumb|Tensile specimens made from an aluminum alloy. The left two specimens have a round cross-section and threaded shoulders. The right two are flat specimen designed to be used with serrated grips.]]
 
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.<ref name="davis1"/><ref name="davis2">{{harvnb|Davis|2004|p=2}}.</ref>
 
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.<ref name="davis9"/>
 
In large [[casting (metalworking)|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 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.<ref name="davis8">{{harvnb|Davis|2004|p=8}}.</ref> For workpieces that are [[machining|machined]] from [[bar stock]], the test specimen can be made from the same piece as the bar stock.
 
{{Double image|left|Tensile specimen shoulders.svg|460|Tensile specimen nomenclature.svg|350|Various shoulder styles for tensile specimens. Keys A through C are for round specimens, whereas keys D and E are for flat specimens. Key:<br/>
A. A Threaded shoulder for use with a threaded grip<br />
B. A round shoulder for use with serrated grips<br />
C. A butt end shoulder for use with a split collar<br />
D. A flat shoulder for used with serrated grips<br />
E. A flat shoulder with a through hole for a pinned grip|Test specimen nomenclature}}
{{clear}}
 
The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.<ref name="davis8"/>
 
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
 
{| class="wikitable"
|-
! 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.
 
{| class="wikitable"
|+ Flat test specimen<ref name="davis52">{{harvnb|Davis|2004|p=52}}.</ref>
|-
! 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 || 0.25 < T < {{frac|3|16}} || 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 || {{frac|3|8}}
|}
 
{| class="wikitable"
|+ Round test specimen<ref name="davis52"/>
! rowspan=2|All values in inches !! colspan=2|Standard specimen at nominal diameter: !! colspan=3|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.) || {{frac|3|8}} || 0.25 || {{frac|5|16}} || {{frac|5|32}} || {{frac|3|32}}
|-
| Length of reduced section (min.) || 2.5 || 1.75 || 1.25 || 0.75 || {{frac|5|8}}
|}
 
==Equipment==
[[File:Inspekt desk 50kN IMGP8563.jpg|thumb|A universal testing machine (Hegewald & Peschke)]]
 
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. There are two types: [[hydraulic machinery|hydraulic]] powered and [[electromagnet]]ically powered machines.<ref name="davis2"/>
 
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 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.<ref name="davis9">{{harvnb|Davis|2004|p=9}}.</ref>
 
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-joint]]s between the grips and the test machine.<ref name="davis9"/> A misalignment is indicated when running the test if the initial portion of the stress-strain curve is curved and not linear.<ref>{{harvnb|Davis|2004|p=11}}.</ref>
 
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.<ref name="davis9"/> 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|date=February 2011}}
 
==Process==
The test process involves placing the test specimen in the testing machine and applying tension to it until it [[fracture]]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 [[Deformation (engineering)|strain]]'', ''ε'', using the following equation:<ref name="davis2"/>
 
:<math>\varepsilon =\frac{\Delta L}{L_0}=\frac{L-L_0}{L_0}</math>
 
where Δ''L'' is the change in gauge length, ''L''<sub>0</sub> 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:<ref name="davis2"/>
 
:<math>\sigma = \frac{F_n}{A}</math>
 
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]]''.<ref name="davis2"/>
 
==Standards==
 
===Metals===
* [[ASTM]] E8/E8M-11: "Standard Test Methods for Tension Testing of Metallic Materials" (2011)
* [[International Organization for Standardization|ISO]] 6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)
* [[International Organization for Standardization|ISO]] 6892-2: "Metallic materials. Tensile testing. Method of test at elevated temperature" (2011)
* [[JIS]] Z2241 Method of tensile test for metallic materials
 
===Flexible Materials===
*[[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
*[[International Organization for Standardization|ISO]] 37 Rubber, vulcanized or thermoplastic—Determination of tensile stress-strain properties
 
==References==
{{Reflist|2}}
 
== External links ==
* [http://www.youtube.com/watch?v=D8U4G5kcpcM Video on the tensile test]
* [http://www.prestogroup.com/plastic-testing-instruments/computer-based-tensile-testing-machine-zeus-ultimo  tensile testing]
 
[[Category:Materials testing]]

Revision as of 21:38, 3 March 2014

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