Universal Tensile Testing Machine
What is a Tensile test machine?
Tensile test machines, also known as tension test machines, are universal testing machines specially configured to evaluate the tensile strength of specimens. Our tensile testers will measure characteristics such as ultimate tensile strength, yield strength, elongation, and modulus. Each tensile test machine is configured to your testing needs by our application engineers with the correct controller, grips, and accessories.
Tensile testing, also known as tension testing, 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.From these measurements the following properties can also be determined: Young’s modulus, Poisson’s ratio, yield strength, and strain-hardening characteristics.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
Components of Tensile Testing Machine
Several variations are in use. Common components include:
- Load frame – Usually consisting of two strong supports for the machine. Some small machines have a single support.
- Load cell – A force transducer or other means of measuring the load is required. Periodic calibration is usually required by governing regulations or quality system.
- Cross head – A movable cross head (crosshead) is controlled to move up or down. Usually this is at a constant speed: sometimes called a constant rate of extension (CRE) machine. Some machines can program the crosshead speed or conduct cyclical testing, testing at constant force, testing at constant deformation, etc. Electromechanical, servo-hydraulic, linear drive, and resonance drive are used.
- Means of measuring extension or deformation – Many tests require a measure of the response of the test specimen to the movement of the cross head. Extensometers are sometimes used.
- Output device – A means of providing the test result is needed. Some older machines have dial or digital displays and chart recorders. Many newer machines have a computer interface for analysis and printing.
- Conditioning – Many tests require controlled conditioning (temperature, humidity, pressure, etc.). The machine can be in a controlled room or a special environmental chamber can be placed around the test specimen for the test.
- Test fixtures, specimen holding jaws, and related sample making equipment are called for in many test methods.
How Does A Tensile Testing Machine Work?
Tensile testing systems are comprised of 4 key components: the frame, grips, load cell, and software for data acquisition and control. Most tension testers can be used for both tension and compression mode.
The tensile testing frame can consist of one or two columns and can rest either on a bench top or directly on the floor. Single column machines are for lower force testing. The benefit of a single column machine is that they are lower in cost compared to the dual column machines. Tensile testing over 500 lbs. on a single column machine is not recommended because the frame starts to slightly bend at forces approaching 1,000 lbs. (5kN). This bending of the frame can affect the results of the test, therefore a dual column machine is ideal for testing to 1,000 lbs. and above. Electro-mechanical tensile testers have recently emerged as the more popular choice over older, servo-hydraulic systems. An electro-mechanical machine is quieter, has a smaller footprint, runs on less overall power, and requires less maintenance when compared to a hydraulic testing machine using a Hydraulic Power Unit (HPU). Electro-mechanical machines are also much easier to move around a factory floor or within a lab. There are only 3 manufacturers in the world who can build new electro-mechanical machines over 1,000 kN. They are the I-brand, the TO brand, and Galdabini which was established in 1890 in Italy.
Grips and Fixtures
The grips and fixtures are the most important part of any universal testing system. Machines can be outfitted with different grips in order to perform various testing procedures on different types of materials. There are over 2,000 different types of grips and fixture combinations across the entire ASTM encyclopedia.
Most Tensile Testers can perform bend, crush, peel, puncture, shear, and tension procedures once they are outfitted with the correct fixture configuration. The size of the machine and the grips must be increased for higher force testing.
Bend Fixtures are used for conducting 3 pt. and 4 point flexural tests. There are two pieces to a bend test fixture. First, the top fin which is used to push down onto the center of the sample. The sample rests on the bottom piece of the fixture which has two supports which make up the span distance. The bottom support spans can be outfitted with different sized contact radii according to the exact ASTM procedure and characteristics of the sample material. The top piece and bottom piece of a 4 pt. bend fixture looks almost identical. Other, more exotic bend fixtures can be used on a tensile tester including bending jigs with articulating rollers and serpentine style 4 pt. and 5 pt. fixtures.
Crush or compression tests are performed with platens. The platens are a pair of flat metal plates which are brought down to squish the sample. Platens are pretty straight forward and can be either circular or square. A compression platen with a spherically seated universal joint can be used to perfectly align the sample. Only one spherically seated platen is required in this setup. Some compression platens, such as those for testing foam, have holes drilled in them to allow for the air to properly escape.
Peel tests are performed at fixed angles in order to determine the strength of the adhesive on the tape. 180 degree and 90 degree peel fixtures are commonly used for peel strength testing. Special peel testing fixtures can be fabricated which allow the user to adjust the angle of the peel test on the fly. Along with the angled peel fixtures, there are also other peel grips such as the floating roller peel fixture and the climbing drum peel fixture.
Shear test fixtures are similar to puncture fixtures in that they are almost always specifically designed according to the test standard. For instance, the rail shear fixture and the Iosipescu shear test fixture performs essentially the same function on the sample, however there are some minor but intricate differences. Basic shear tests for adhesives can be performed with standard tensile grips.
There are many different types of tensile tester grips because pull testing is by far the most popular type of material testing procedure. Roller Grips, Scissor Grips, Vise Grips, Wedge Grips, and Wrap Grips can all be used to test the tensile strength of a material. Certain grips work better in different situations and the topic is too numerous to discuss in this article. The Universal Grip Company has one of the most extensive websites for detailing and explaining different types of tensile grips and fixtures.
The load cell is also an important component to consider. Tests should run between roughly 5 and 95% of the max capacity rating on the load cell. Selecting the correct load cell is vital for accurately measuring the right amount of force and for preventing a machine overload. Smaller load cells can be piggy-backed in series onto larger load cells. This helps with rapid changeover between testing procedures. Most modern tensile testers will be able to recognize which load cell is plugged in and automatically make the appropriate adjustments to calibration and configuration.
Software and Data Acquisition (DAQ)
The software and computer system are also a key piece of the overall testing system. Universal Grip supports the flexible and polished Graphwork 6 software from Galdabini. The communication interface is handled via EtherCAT which is a commonly used and modern network communication protocol. EtherCAT is a superior solution compared to RS232 for transmitting machine to machine data. Some tensile testing manufacturers may offer a separate module in order to control the machine. This extra box is not always necessary and it can add to the overall cost of the machine.
What are the Benefits of Tensile Testing?
Tensile testing provides data on the integrity and safety of materials, components and products, helping manufacturers ensure that their finished products are fit-for-purpose and manufactured to the highest quality.
The data produced in a tensile test can be used in many ways including:
- To determine batch quality
- To determine consistency in manufacture
- To aid in the design process
- To reduce material costs and achieve lean manufacturing goals
- To ensure compliance with international and industry standards
How to Understand Results Obtained from Tensile Testing?
Results from tensile testing should be clear and free from extraneous information that may detract from the results of interest. To facilitate custom presentation, calculations and reporting, results should be portable between software applications such as Microsoft Excel®. Mecmesin tensile testing systems offer varying grades of sophistication for collection, presentation, interrogation and export of results.
What are the Applications of Tensile Testing?
Tensile testing is used to guarantee the quality of components, materials and finished products within a wide range industries. Typical applications of tensile testing are highlighted in the following sections on:
- Aerospace Industry
- Automotive Industry
- Beverage Industry
- Construction Industry
- Electrical and Electronics Industry
- Medical Device Industry
- Packaging Industry
- Paper and Board Industry
- Pharmaceuticals Industry
- Plastics, Rubber and Elastomers Industry
- Safety, Health, Fitness and Leisure Industry
- Textiles Industry
Applications of tensile testing in the aerospace industry include:
- Peel tests on airframe composites
- Shear and tensile strength testing of fasteners e.g. bolts, nuts and screws
- Tensile & material strength testing of adhesive bonds, aircraft textiles and carpets, cables, hoses and tubing, gaskets and o-rings, seat belts, welded and crimped joints, wiring looms and harnesses
Applications of tensile testing in the automotive industry include:
- Quality assessment through tensile testing of interior fittings including: airbags, carpets, dashboards, electrical harness (incl. crimped terminals pull-off force), handles, laminated trim, mirrors, seals and seatbelts and handbrake levers.
- Quality assessment through tensile testing of exteriors fittings including: bumper mouldings and trims, door and window seals, emblems and number plates, mirrors and mud flaps
Applications of tensile testing in the beverage industry include:
- Peel strength of induction-sealed foils and labels
- Tensile force required to open ‘ring-pulls’ on bevcans
- Testing cork extraction force
Applications of tensile testing in the construction industry include:
- Bond strength testing of adhesives, mastics, sealants and bonds between brick and foam layers
- Tensile and material strength testing of geotextiles and safety support netting
Electrical and Electronics Industry
Applications of tensile testing in the electrical and electronics industry include:
- Connector withdrawal force
- Pull-off forces of crimped, welded or soldered electrical contacts
- Component-to-PCB pull-off force
- PCB material tensile strength
Medical Device Industry
Applications of tensile testing in the medical device industry include:
- Hypodermic needle-to-hub retention force
- Tensile strength and elongation at break of medical tubing, bandages, dressings and tapes
- Joint strength of IV connector fittings
- Suture-to-needle crimp pull out test
- Tensile strength of suture material and knotting
- Joint strength and material elongation of respiratory masks
- Elongation and tensile strength of examination gloves
- Mechanical strength of orthopaedic implant components
Applications of tensile testing in the packaging industry include:
- Adhesive/peel testing of adhesive bonds, container seals and labels
- Force associated with opening snap-caps, pop-caps and other push pull closures
- Elongation of plastic packaging materials
Paper and Board Industry
Applications of tensile testing in the paper and board industry include:
- Openability of card and paper based packaging
- Folding characteristics of boxes and cartons
- Force to separate multi-part documents
- Durability of documents
Application of tensile testing in the pharmaceuticals industry include:
- Pull off force of phial caps
Plastics, Rubber and Elastomers Industry
Applications of tensile testing in the plastics, rubber and elastomers industry include:
- Joint strength of interlocking plastic components
- Assessment of material tensile properties
- Adhesion / peel testing of plastic labels, ID and credit cards
Safety, Health, Fitness and Leisure Industry
Applications of tensile testing in the safety, health, fitness and leisure industry include:
- Tensile testing of safety support netting
- Ergonomic risk evaluations
- Elastic properties of racquet strings
Applications of tensile testing in the textiles industry include:
- ‘Pull-off’ characteristics of buttons, stitched-on decorations, press studs, poppers, zip fasteners, hook-and-loop fasteners
- Strength testing of vulnerable seams
Tensile Testing Challenges
Non-axial loading is one of the most common causes of incorrect tensile measurements. Even very small off-center loading when using a load sensor or force gage can result in measurement errors of up to 0.5%. It is therefore important to ensure that the alignment of the testing string, i.e., the load cell, top test fixture, sample and bottom test fixture are perpendicular to one another.
Using a properly sized force gage or load cell sensor based on the expected load measurement is important to achieving accurate and repeatable results. A general rule is to use a sensor that is between 20% and 80% of the anticipated load measurement. This will avoid or minimize error results at the low end due to mechanical noise and will help prevent overloading conditions at the upper end of the measurement range. Since most sensors are calibrated and have their accuracy specification based on full scale, the closer you are to zero, the more influence the accepted error has on your measurement.
Having an incorrect test fixture is another common cause to inaccurate tensile measurements. A fixture that is too large for the sample or that applies too much gripping force to the sample during tensile movement can cause the sample to fracture outside the specified gage length area. The test fixture should be sized to the sample’s expected load characteristics. Wedge-action test fixtures work well on ductile samples but tend to be less reliable on brittle materials, since they apply load onto the sample as axial loading increases. Brittle materials tend to test more consistently when pneumatically-operated test fixtures are used that regulate the gripping force onto the sample.
Improper sample preparation can result in inconsistent and incorrect characterization. When testing to a specific international standard, the sample should be prepared to the prescribed dimensions. Force measurement applications will typically use the component in its finished state. Material testing, however, uses specially prepared specimens in various forms and shapes. Their cross-sections may be round, rectangular or square and they have a known standard gage length.
Testing at too fast or too slow a velocity is another common reason why tensile measurement may not be optimum. Where available, testing should be done in accordance to a recognized and accepted testing standard by ASTM, ISO, DIN or others. Test speed is clearly specified in these standards and will ensure a proper measurement is taken.
Tensile results may be significantly affected by temperature. Elasticity of a sample can decrease significantly as temperature increases.