# TENSILE TESTINGAn Introduction

Tensile testing is a fundamental type of mechanical testing performed by engineers and materials scientists in manufacturing and research facilities all over the world. A tensile test (or tension test) applies force to a material specimen in order to measure the material's response to tensile (or pulling) stress. This type of testing provides insight into the mechanical properties of a material and enables product designers to make informed decisions about when, where, and how to use a given material.

## Why Perform a Tensile Test?

Tensile testing and material characterization are crucial for manufacturers and researchers in all industries. In order for a material to be selected for a new product or use, researchers must ensure that it can withstand the mechanical forces that it will encounter in its end-use application. For example, tire rubber must be elastic enough to absorb inconsistencies in road surfaces, while surgical sutures must be strong enough to hold living tissue together. Furthermore, materials and products might be exposed to mechanical forces for short or long periods of time, through cyclical or repeated use, and in a wide variety of different temperature and environmental conditions. Automotive tires are expected to last for a certain number of miles under a variety of weather conditions, while surgical sutures, though only used once, must maintain a consistent tensile strength for long enough for the body to heal.

In addition to its importance to the R&D process, tensile testing is also used by quality assurance departments to ensure that batches of finished product are meeting the required specifications for tensile properties. This is important from both a safety and a business perspective, as defective products can be dangerous to the end user and can also cause significant harm to manufacturers in the form of product delays, lost revenue, and damaged reputations.

HOW TO PERFORM A TENSILE TEST An Introduction to the Basic Principles of Tensile Testing

Tensile tests are performed on universal testing machines, also known as tensile machines or tensile testing machines. These machines consists of a single or dual column frame equipped with a load cell, testing software, and application-specific grips and accessories such as extensometers. Universal testing machines come in a wide variety of force capacities and can be configured with different fixtures to test any product, component, or material.

Tensile Testing Machine
Tensile testing machines can come in single or dual column configurations depending on their force capacity.
Software
Test software is where operators configure test methods and output results.
The load cell is a transducer that measures the force applied to the test specimen. Instron load cells are accurate down to 1/1000 of load cell capacity.
Grips and Fixtures
A wide range of specimen grips and fixtures are available to accommodate test specimens of different materials, shapes, and sizes.
Strain Measurement
Some test methods require measurement of a specimen's elongation under load. Instron's AVE2 can measure changes to specimen length down to ±1 µm or 0.5% of reading.

SETTING UP A TENSILE TEST

In order to perform a tensile test, an operator must perform a variety of tasks to ensure that the test is being conducted in accordance with internal and/or external testing standards. Depending on the lab, these tasks might be partially or entirely automated, though responsibility for the correctness of the setup always resides with the operator.

Test Method Selection

After you have loaded your specimen into the system and attached your extensometer, it is time to start your test. When setting up your test you will have selected the appropriate test method in your testing software and input any necessary parameters regarding test speed, specimen measurements, or end criteria. After you instruct the system to start, the machine will apply tensile force to your specimen as prescribed by the test method and record data as your specimen responds to the stress. After the test is complete the specimen can be removed and the data exported for further study.

Preparing Specimens

Specimen geometries vary widely depending on the material being tested and the test method or standard being used. Governing bodies such as ASTM and ISO have standardized specimen requirements for different materials, which allows their properties to be reliably compared between different batches and manufacturers.

Tensile specimens are commonly machined or die cast in the shape of dogbones, which provide 'shoulders' designed to be held by the grips of the testing machine and a 'gage length' where the tensile properties will be measured. The dimensions of these shoulders, the gage length between them, and the length and width of the entire specimen are all prescribed by the testing standard.

Inserting Specimen into Grips

Depending on the dimensions and texture of the material, different grip types and jaw face surfaces may be required in order to successfully grip the specimens. Grips are available in a wide variety of force capacities and with rubber-coated, smooth, serrated, and other surface types. In order to ensure that force is applied in the correct direction, different alignment devices are available to assist operators when inserting a specimen into the grips.

Strain Measurement Devices

Strain is a measurement of a specimen's deformation under stress and is a basic part of materials characterization required by most testing standards. Strain measurement devices such as extensometers are typically used in order to take this measurement. Contacting devices such as clip-on extensometers are attached to the specimen after it has been placed in the grips.

Starting Test

After you have loaded your specimen into the system and attached your extensometer, it is time to start your test. When setting up your test you will have selected the appropriate test method in your testing software and input any necessary parameters regarding test speed, specimen measurements, or end criteria. After you instruct the system to start, the machine will apply tensile force to your specimen as prescribed by the test method and record data as your specimen responds to the stress. After the test is complete the specimen can be removed and the data exported for further study.

TENSILE TEST DATA ANALYSIS Understanding the Mechanical Properties of Materials

Measuring a material or product in tension allows manufacturers to obtain a complete profile of its tensile properties. When plotted on a graph, this data results in a stress/strain curve which shows how the material reacted to the forces being applied. While different standards require the measurement of different mechanical properties, the greatest points of interest are usually the point of break or failure, modulus of elasticity, yield strength, and strain.

Ultimate Tensile Strength

One of the most important properties we can determine about a material is its ultimate tensile strength (UTS). This is the maximum stress that a specimen sustains during the test. The UTS may or may not equate to the specimen's strength at break, depending on whether the material is brittle, ductile, or exhibits properties of both. Sometimes a material may be ductile when tested in a lab, but, when placed in service and exposed to extreme cold temperatures, it may transition to brittle behavior.

Hooke's Law

For most materials, the initial portion of the test will exhibit a linear relationship between the applied force or load and the elongation exhibited by the specimen. In this linear region the line obeys the relationship defined as Hooke's Law, where the ratio of stress to strain is a constant. E is the slope of the line in this region where stress (σ) is proportional to strain (ε) and is called the modulus of elasticity or young's modulus:

$$E = {σ\ \overε}$$

Modulus of Elasticity

The modulus of elasticity is a measure of the material's stiffness that only applies in the initial linear region of the curve. Within this linear region the tensile load can be removed from the specimen and the material will return to the exact same condition it had been in prior to the load being applied. At the point when the curve is no longer linear and deviates from the straight-line relationship, Hooke's Law no longer applies, and some permanent deformation occurs in the specimen. This point is called the elastic or proportional limit. From this point on in the tensile test, the material reacts plastically to any further increase in load or stress. It will not return to its original, unstressed condition if the load is removed.

Yield Strength

A material's yield strength is defined as the stress applied to the material at which plastic deformation starts to occur.

Offset Method

For some materials (e.g. metals and plastics), the departure from the linear elastic region cannot be easily identified. Therefore an offset method to determine the yield strength of the material is allowed. This methodology is commonly applied when measuring the yield strength of metals. When testing metals according to ASTM E8/E8M, an offset is specified as a percentage of strain (usually 0.2%). The stress (R) that is determined from the intersection point "r" when the line of the linear elastic region (with slope equal to modulus of elasticity) is drawn from the offset "m" becomes the offset yield strength.

Alternate Moduli

The tensile curves of some materials do not have a very well-defined linear region. In these cases, ASTM Standard E111 provides for alternative methods for determining the modulus of a material, as well as young's modulus. These alternate moduli are the secant modulus and tangent modulus.

Strain

We are also able to find the amount of stretch or elongation that the specimen undergoes during tensile testing. This can be expressed as an absolute measurement in the change in length or as a relative measurement called "strain." Strain itself can be expressed in two different ways, as "engineering strain" and "true strain."

Engineering strain is probably the easiest and the most common expression of strain used. It is the ratio of the change in length to the original length:

$$e = {L-Lₒ \ \over Lₒ} = { \Delta L\over Lₒ}$$

The true strain is similar, but based on the instantaneous length of the specimen as the test progresses, where Li is the instantaneous length and L0 the initial length:

$$ε = In {Lᵢ \ \over Lₒ}$$

View our Tensile Testing and Tensile Testing Machines FAQ for additional information

INSTRON TENSILE TESTING EQUIPMENT Systems, Components, and Parts

Tensile testing machines are available in a variety of different sizes and force capacities ranging from 0.02 N to 2,000 kN. Most low force testing is performed on an electromechanical single-column or dual-column tabletop machine, while higher force applications require floor model frames. Instron's 6800 Series systems are available in capacity ranges up to 300 kN and can perform a wide range of different test types, including tensile, compression, bend, peel, tear, shear, friction, torsion, puncture, and more. Instron's Industrial Series servohydraulic systems are designed for even higher capacity testing of high strength metals, alloys, and advanced composites.

Universal Testing Systems up to 300 kN

Single and dual column table model and floor model testing systems with a force capacity range of 0.02 N (2 gf) to 300 kN.

Industrial Universal Testing Systems up to 2000 kN

Instron’s Industrial Series includes frames with single or dual test spaces and range in force capacity from 300 kN to 2000 kN.

Tensile Grips
For Testing Plastics, Metals, Composites, Elastomers, Textiles, and Components

Pneumatic Side Action Tensile Grips
Catalog no. 2712-XXX

Our most popular tensile grip and installed on more than half of all Instron universal testing machines. These grips are easy to use, extremely versatile, and efficient for high volume testing. Up to 10kN force capacity.

Mechanical Wedge Action Grip
Catalog no. 2716-XXX

A classic, simple, and robust tensile grip design; manual wedge action grips are perfect for metals, composites, and plastic. Designed for easy specimen loading, alignment, and positioning. Up to 250kN force capacity.

Catalog no. 2710-XXX

Screw action grips provide a very simple and efficient method for holding test specimens and are most commonly used for biomedical, plastic film, electronic, and adhesive applications. Up to 10kN force capacity.

Catalog no. 2742-XXX, 2743-XXX

The best grips available for most metal and composite tensile testing applications. A hydraulic grip pump is required if installing these on an electromechanical system. Up to 500kN force capacity.

Self-Tightening Eccentric Roller Grips
Catalog no. 2713-XXX

Simple and efficient self-tightening tensile grips that are perfect for elastomers and thin films. Up to 5kN force capacity.

Pneumatic Cord and Yarn Grips
Catalog no. 2810-410, 2714-XXX

Grips designed specifically for tensile testing yarn, thread, rope, cord, tube, sutures, and wire.

Hydraulic Wedge Action Grips
Catalog no. W-52XX, 53XX

Hydraulic wedge action grips for Instron's high-capacity static hydraulic universal testing systems. Up to 2,000kN force capacity.

Hydraulic Side Action Grips
Catalog no. W-54XX

Hydraulic side-acting DuraSync™ high capacity grips deliver enhanced gripping performance, usability, and operator safety over traditional grip designs. Up to 2,000kN force capacity.

Extensometers
Contacting and Non-Contacting Solutions for Strain Measurement

Automatic Non-Contacting Video Extensometer

A video extensometer is a non-contacting extensometer that can measure deformation by tracking the movement of two attached markers on the specimen, using high-resolution digital camera technology.

Automatic Contacting Extensometer
Catalog no. 2665-750

The AutoX750 maximizes efficiency and revenue while maintaining high quality standards and safe operating conditions.

Static Axial Clip-On Extensometer
Catalog no. 2630-XXX

Instron’s static axial clip-on extensometers are a quick and easy solution for measuring strain and are suitable for use on a wide range of materials such as plastics, metals, and composites.

TENSILE TESTING STANDARDS Standards for Testing Plastics, Elastomers, and Metals

Most tensile testing is performed to established standards published by standards organizations such as ASTM and ISO. Testing standards prescribe acceptable test parameters and results for different types of raw materials such as metals, plastics, elastomers, textiles, and composites, as well as for finished products such as medical devices, automotive parts, and consumer electronics. These standards ensure that materials and products entering the supply chain display predictable mechanical properties and are not likely to fail in their expected end use. Since the cost and safety implications of product failure cannot be overstated, companies are encouraged to invest in high-quality, accurate testing equipment that is designed to help them easily determine whether or not their products meet applicable standards.

ASTM D638 / ISO 527-2

ASTM D638 and ISO 527-2 are two of the most common standards used to evaluate the tensile properties of reinforced and non-reinforced plastics. Though these standards measure many different tensile properties, the most common are tensile strength, tensile modulus, elongation, and Poisson's ratio.

ASTM D412 / ISO 37

ASTM D412 and ISO 37 are the most common standards for determining the tensile properties of vulcanized (thermoset) rubber and thermoplastic elastomers. Compounds in this family are used to create a vast array of products from tires to medical gloves to O-rings. Key measurements for elastomer testing include ultimate elongation and tensile set.

ASTM E8 / ASTM A370 / ISO 6892

ASTM E8, ASTM A370, and ISO 6892 are major standards for the tensile testing metals and metallic materials. Methods of test control are a major consideration in metals testing, and a thorough understanding of crosshead compliance and strain control is necessary in order to produce accurate test results.

The following is a listing of some of the most common international tensile testing standards.

• ASTM A370 | Standard Test Methods and Definitions for Mechanical Testing of Steel Products
• ASTM A416 | Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete
• ASTM A48 | Standard Specification for Gray Iron Castings
• ASTM A746 | Standard Specification for Ductile Iron Gravity Sewer Pipe
• ASTM A996 | Standard Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement
• ASTM C297 | Standard Test Method for Flatwise Tensile Strength of Sandwich Constructions
• ASTM D1037 | Standard Test Methods for Evaluating Properties of Wood-Base Fiber and Particle Panel Materials
• ASTM D1414 | Standard Test Methods for Rubber O-Rings
• ASTM D1708 | Standard Test Method for Tensile Properties of Plastics by Use of Microtensile Specimens
• ASTM D2256 | Standard Test Method for Tensile Properties of Yarns by the Single-Strand Method
• ASTM D3039 | Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials
• ASTM D4018 | Standard Test Methods for Properties of Continuous Filament Carbon and Graphite Fiber Tows
• ASTM D412 | Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension
• ASTM D4632 | Standard Test Method for Grab Breaking Load and Elongation of Geotextiles
• ASTM D5034 | Standard Test Method for Breaking Strength and Elongation of Textile Fabrics (Grab Test)
• ASTM D5035 | Standard Test Method for Breaking Force and Elongation of Textile Fabrics (Strip Method)
• ASTM D5766 | Standard Test Method for Open-Hole Tensile Strength of Polymer Matrix Composite Laminates
• ASTM D5961 | Standard Test Method for Bearing Response of Polymer Matrix Composite Laminates
• ASTM D638 | Standard Test Method for Tensile Properties of Plastics
• ASTM D7269 | Standard Test Methods for Tensile Testing of Aramid Yarns
• ASTM D882 | Standard Test Method for Tensile Properties of Thin Plastic Sheeting
• ASTM A416 | Standard Specification for Low-Relaxation, Seven-Wire Steel Strand for Prestressed Concrete
• ASTM D885 | Standard Test Methods for Tire Cords, Tire Cord Fabrics, and Industrial Filament Yarns
• ASTM F2150 | Tensile Testing Polymer Hydrogels
• ASTM F606 | Tensile Testing of Fasteners, Washers, Direct Tension Indicators, and Rivets
• ASTM F2516 | Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials
• ASTM E8 | Standard Test Methods for Tension Testing of Metallic Materials
• ISO 10319 | Wide-width tensile test of geosynthetics
• ISO 10555 | Tensile testing of sterile and single-use intravascular catheters
• ISO 11193 | Tensile testing of single-use medical gloves
• ISO 13934 | Tensile testing of fabrics (grab method)
• ISO 15630 | Testing steel for the reinforcement and prestressing of concrete
• ISO 1798 | Tensile strength and elongation at break of flexible cellular polymeric materials
• ISO 1926 | Tensile properties of rigid cellular plastics
• ISO 2062 | Breaking force and elongation at break of yarns
• ISO 3183 | Tensile testing of pipe and tube
• ISO 37 | Tensile properties of vulcanized or thermoplastic rubber
• ISO 527-2 | Tensile properties of plastics
• ISO 527-3 | Tensile testing of thin plastic films and sheets
• ISO 527-4 | Tensile properties of isotropic and orthotropic fiber-reinforced plastic composites
• ISO 6892 | Tensile testing of metals and metallic materials
• ANSI/AWS B4.0 | Standard methods for mechanical testing of welds
• BS EN 319 | Tensile strength of particleboard and fiberboard
• BS EN 2561 | Tensile testing of carbon fiber reinforced plastics
• BS EN 2597 | Tensile properties of unidirectional carbon fiber reinforced plastics
• IS 1608 | Tensile testing metallic materials at ambient temperature
• TAPPI 220, 456, & 494 | Tensile testing of paper

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