Tensile Testing and Tensile Testing Machines
An Introduction


A tensile test, also known as a tension test, is one of the most fundamental and common types of mechanical testing. A tensile test applies tensile (pulling) force to a material and measures the specimen's response to the stress. By doing this, tensile tests determine how strong a material is and how much it can elongate. Tensile tests are typically conducted on electromechanical or universal testing machines, are simple to perform, and are fully standardized.

 

System Compatibility 






TENSILE TESTING MACHINE
Components and Parts






Tensile tests are performed on tensile testing machines, also known as universal testing machines. A tensile testing machine consists of a test frame that is equipped with a load cell, testing software, and application-specific grips and accessories, such as extensometers. The type of material being tested will determine the type of accessories needed, and a single machine can be adapted to test any material within its force range simply by changing the fixturing.  


tensile testing system setup 


Tensile Testing Machine
1) Load Frame
Tensile testing machine load frames can come in single or dual column configurations depending on their force capacity.
2) Software
Test software is where operators can configure test methods and output results.
3) Load Cell
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.
4) Grips and Fixtures
A wide range of specimen grips and fixtures are available to grip test specimens of different materials, shapes, and sizes.
5) 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.

 




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 such as Instron's 6800 Series, while higher force applications require floor model frames such as the electromechanical 5980 Series. The high capacity frames in Instron's Industrial Series are powered by servo-hydraulic motors.


Low Force Applications Universal Testing Machines 


Low Force Electromechanical

Single column and dual column table model testing systems have a test capacity range from 0.02 N (2 gf) to 50 kN. Low force systems are used for tensile, compression, flex, peel testing, and more.
 


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Instron 5980 Universal Testing Systems 


High Force Electromechanical

Offering up to 600 kN force capacity, the 5980 Series is designed with enhancements that deliver unparalleled accuracy and reliability, improved ergonomics, and an enhanced overall experience for today’s operator.


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Instron Static Hydraulic Industrial Series 


Static Hydraulic Industrial Series

The Industrial Series is comprised of high-capacity, hydraulic testing systems for tension and compression applications, ranging from 300 kN (67,500 lbf) to 2,000 kN (450,000 lbf).
 


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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.
 
 
 



 



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.


tensile testing data 

 

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, or Hooke's law. 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." 

Modulus of Elasticity

The modulus of elasticity is a measure of the material's stiffness which 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 Yield Strength by the offset method.

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 will also be 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, engineering strain formula. The true strain is similar, but based on the instantaneous length of the specimen as the test progresses, true strain formula , where Li is the instantaneous length and L0 the initial length.

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