What is the introduction of tensile test experiment?

One material property that is widely used and recognized is the strength of a material. But what does the word "strength" mean? "Strength" can have many meanings, so let us take a closer look at what is meant by the strength of a material. We will look at a very easy experiment that provides lots of information about the strength or the mechanical behavior of a material, called the tensile test.

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What is a Tensile Test?

The basic idea of a tensile test is to place a sample of a material between two fixtures called "grips" which clamp the material. The material has known dimensions, like length and cross-sectional area. We then begin to apply weight to the material gripped at one end while the other end is fixed. We keep increasing the weight (often called the load or force) while at the same time measuring the change in length of the sample.

Tensile Test Procedure

One can do a very simplified test at home.

If you have a way to hang one end of some material from a solid point that does not move, then you can hang weights on the other end.

Measure the change in length while adding weight until the part begins to stretch and finally breaks.

The result of this test is a graph of load (amount of weight) versus displacement (amount it stretched). Since the amount of weight needed to stretch the material depends on the size of the material (and of course the properties of the material), comparison between materials can be very challenging. The ability to make a proper comparison can be very important to someone designing for structural applications where the material must withstand certain forces.

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Cross-Sectional Areas

We need a way of directly being able to compare different materials, making the &#;strength&#; we report independent of the size of the material. We can do that by simply dividing the load applied to the material (the weight or force) by the initial cross-sectional area. We also divide the amount it moves (displacement) by the initial length of the material. This creates what material scientists refer to as engineering stress (load divided by the initial cross-sectional area) and engineering strain (displacement divided by initial length). By looking at the engineering stress-strain response of a material we can compare the strength of different materials, independently of their sizes.

To use the stress-strain response for designing structures, we can divide the load we want by the engineering stress to determine the cross-sectional area needed to be able to hold that load. For example, a 1/8&#; diameter steel wire can hold a small car. Again, it is not always that simple. We need to understand the different meanings of &#;strength&#; or engineering stress.

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Stress Versus Strain

Now it gets more complicated. Let us take a look at what is meant by the different strength values and also look at other important properties we can get from this simple test. The easiest way is to examine a graph of engineering stress versus engineering strain. Shown below is a graph of a tensile test for a common steel threaded rod, providing a good example of a general metal tensile test. The units of engineering stress are ksi, which stands for a thousand pounds per square inch. Note the reference to area in the units. The units on strain are of course unitless, since we are dividing distance by distance.

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Graph Location 1: Elastic Region

Let us discuss some of the important areas of the graph. First, the point on the graph labeled number 1 indicates the end of the elastic region of the curve. Up to this point, the material stretches in an elastic or reversible manner.

All materials are made up of a collection of atoms. Elasticity can be best understood by imaging the atoms are connected by springs. As we pull on the material, the springs between the atoms get longer and the material lengthens. The elastic portion of the curve is a straight line. A straight line indicates that the material will go back to its original shape when the load is removed.

Graph Location 2: 0.2% Offset Yield Strength

The next portion of the curve of interest is point 2. At this point the curve has begun to bend over, or is no longer linear. This point is known as the 0.2% offset yield strength. It indicates the strength of the material just as it starts to permanently change shape. It is determined as the value of the stress at which a line of the same slope as the initial portion (elastic region) of the curve that is offset by a strain of 0.2% or a value of 0.002 strain intersects the curve.

In our example, the 0.2% offset yield strength is a 88 ksi.

This is a very important aspect of strength. It basically tells us the amount of stress we can apply before the material starts to permanently change shape, putting it on a path to eventual failure. Those who design parts that are used under stress must see that the stress or force on the part never exceeds this value.

Graph Location 3: Maximum Withstand-able Stress

As we move up from point 2 the load or "stress" on the material increases until we reach a maximum applied stress, while the material deforms or changes shape uniformly along the entire gauge length. When we reach point 3, we can determine the tensile strength or maximum stress (or load) the material can support. It is not a very useful property, since the material has permanently deformed at this point. After we reach this point, the stress begins to curve drastically downward. This corresponds to localized deformation, which is observed by a noticeable &#;necking&#; or reduction in the diameter and corresponding cross-section of the sample within a very small region. If we release the load in this area, the material will spring back a little but will still suffer a permanent shape change.

Graph Location 4: Failure or Fracture

Finally, as we follow the curve we eventually reach a point where the material breaks or fails. Of interest here is the final degree to which the material changes shape. This is the &#;ductility&#; of the material. It is determined by the intersection of line number 4, having the same slope as the linear portion of the curve, with the strain axis.

Our example shows a strain of 0.15. The 15% change in length is the amount of &#;ductility&#;.

When the sample fractures or breaks the load is released. Therefore, the atoms elastically stretched will return to their non-loaded positions. Other information about the mechanical response of a material can also be gathered from a fracture test.

Tensile Tests Procedures for Composites

If one pulls on a material until it breaks, one can find out lots of information about the various strengths and mechanical behaviors of a material. In this virtual experiment we will examine the tensile behavior of three different composite fiber materials. They have similar uses but very different properties.

Procedure

A material is gripped at both ends by an apparatus, which slowly pulls lengthwise on the piece until it fractures. The pulling force is called a load, which is plotted against the material length change, or displacement. The load is converted to a stress value and the displacement is converted to a strain value.

About the Materials

Testing materials are the composites fiberglass, Kevlar®, and carbon fiber. Composites are combinations of two or more individual materials with the goal of producing a material having unique properties not found in any single material.

All of these composites use epoxy as a matrix, which &#;glues&#; the fabric like arrangement of the fibers of the respective materials.

Epoxies are thermosetting network polymers, which are very hard and strong, but on the brittle side.

All fabrics are of the same &#;weight,&#; which is a measure of fabric size or weight of a square yard. An example of the fiber material made from fiberglass is shown above left. Kevlar is very similar except it has a yellow color. The carbon has a black color. The samples used in this case are flat bars cut out of larger material using a water jet saw. The three samples are shown below left.

Material Properties

Material Properties Fiberglass Kevlar® Carbon Fiber Density P E E Tensile Strength F G E Compressive Strength G P E Stiffness F G F Fatigue Resistance G-E E G Abrasion Resistance F E F Sanding/Machining E P E Conductivity P P E Heat Resistance E F E Moisture Resistance G F G Resin Compatibility E F E Cost E F P

P=Poor, G=Good, F=Fair, E=Excellent

Experiment

Watch MSE Composite Tensile Test Experiments video

Description: The apparatus pulls on each end of the material until it fractures.

Fiberglass 00:00
Kevlar 01:10
Carbon Fiber 03:09

The video is 5 minutes and 5 seconds with no audio.

Executive Producer Ed Laitila
Host Stephen Forsell
Videographer Britta Lundberg

Final Data

Raw Data for Fiberglass

The displacement increases from zero to a little over 5 mm. The load increases almost linearly from 0 to about 12 kN before dropping almost vertically.

Corrected Data For Fiberglass

The engineering strain increases from zero to about 0.10. The engineering stress increases linearly from zero to about 170 MPa, the fracture strength. The modulus is 1.7 GPa.

Corrected Data for Kevlar

The engineering strain increases from zero to about 0.11. The engineering stress increases linearly from zero to about 265 MPa, the fracture strength. The modulus is 2.3 GPa.

Corrected Data for Carbon Fiber

The engineering strain increases from zero to about 0.10. The engineering stress increases linearly from zero to about 580 MPa, the fracture strength. The modulus is 5.7 GPa.

Conclusions

The carbon fiber composite material has a much higher tensile strength and modulus of elasticity than the other materials. Note they all break in a &#;brittle&#; manner, as the curve is linear until it breaks or fractures with no bending of the curve at high loads. Consequently, there is no permanent change in original shape during this test, and hence no ductility.

Additional Virtual Experiment Examples

You have seen the experiments for the composite materials. Compare the composite material stress-strain curves with those for polymer and steel.

Tensile Test Steel

The necking steel sample has a continuous stress strain relationship. The stress increases almost vertically, then drops gradually.

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Tensile Test Polymer

The stretching polymer sample has a discontinuous stress strain relationship. The stress increases almost vertically, then drops and increases unevenly.

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Test procedure to determine mechanical properties of a specimen.

Tensile testing on a coir composite. Specimen size is not to standard (Instron).

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

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Tensile testing might have a variety of purposes, such as:

Tensile specimen

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Tensile specimens made from an aluminum alloy. The left two specimens have a round cross-section and threaded shoulders. The right two are flat specimens designed to be used with serrated grips. An aluminium alloy tensile specimen, after testing. It has broken, and the surface where it broke can be inspected.

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

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

Test specimen nomenclature

For more information, please visit Cell Instruments.

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

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Round test specimen[10] 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.)

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0.25

5

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16

5

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32

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32

Length of reduced section (min.) 2.5 1.75 1.25 0.75

5

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Equipment

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A universal testing machine (Hegewald & Peschke)

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

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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]

ε = Δ L L 0 = L &#; L 0 L 0 {\displaystyle \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]

σ = F n A {\displaystyle \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

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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 ( ε 1 {\displaystyle \varepsilon _{1}} ) and the average strain ( ε 0 {\displaystyle \varepsilon _{0}} ):[16]

Percent Bending = ε 1 &#; ε 0 ε 0 × 100 {\displaystyle {\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

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Metals

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• ASTM E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" ()
• ISO -1: "Metallic materials. Tensile testing. Method of test at ambient temperature" ()
• ISO -2: "Metallic materials. Tensile testing. Method of test at elevated temperature" ()
• JIS Z 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" ()

Composites

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• ASTM D /D M: "Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials"

Flexible materials

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• 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

References

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