Stress Control and Yielding Material

Since the first materials testing machines were used for tensile testing of metals, one option of performing the test has been to control the rate at which you apply load to the specimen, or apply stress. This technique has been successfully used on manual hydraulic machines from the beginning of the 20th century, if not before. Manual valves are opened to adjust the flow rate of oil into the actuator; the loading rate is then checked against a large force dial. Once the desired stressing rate has been achieved, the values are left at the same setting so the flow of oil remains constant. This enables the testing machine to run at the same crosshead separation rate for the remainder of the test, or until the test is expedited.

The same test method is still used in major metals tensile testing standards today, e.g. ISO 6892-1 & ASTM E8. When using a modern test system with electronic controllers, the test setup needs to be slightly different. If the control mode for calculating proof stress is set as "tensile stress" (or similar), the machine will try to continually increase the stress at the rate you requested. During yielding, this can result in the crosshead speed increasing exponentially, resulting in far higher strain rates. When testing strain rate sensitive materials, this can lead to large proof stress discrepancies.

To overcome this, testing systems are set up so that the first stage of a test is in stress control. Once the correct rate is achieved, the crosshead speed is then calculated and locked in. This enables the machine to operate in an equivalent manner as their older hydraulic counterparts.

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How to Test Lap-Shear Specimens

Manufacturing processes are moving away from using traditional bolts and rivets to using new, stronger adhesives to hold together materials such as composites and aluminum. With this increase in bonded manufacturing, it is more important than ever to accurately test the adhesive strength of bonds to prevent catastrophic failures.   

There are several tests which are important for adhesives, but the most difficult to perform well is the lap-shear test. In this test, two pieces of material are bonded together and then pulled apart to generate a shear force on the adhesive with no peeling force.

This test can be performed in various ways, but it’s important to understand each one because some may produce results which are drastically lower than expected due to peeling forces.

Adhesively Bonded Rigid Plastic Lap-Shear Joints

The Wrong Way

1.)  No tabs in grips without offsets

a.)  This method is not preferred because the bond line is not in the center of the test axis. It will produce lower max loads than other methods and may result in more rejected materials. Some people may use this method because it requires simple equipment and minimal specimen prep time.

The Right Ways

2.)  Tabs on specimens in grips without offsets

a.)  To perform this test, a user will bond the two pieces together, and then on each end, bond a small additional piece as a tab. With this method, the center of the test axis is aligned with the center of the adhesive, but it requires expertise and extra specimen prep.

b.)  If the tabs are not the right thickness or if the adhesive bonds are too thick or thin, the values produced may still be lower than expected.

3.)  Placing spacers in grips without offsets

a.)  This method requires the user to place pieces of material in the grips with the specimen to keep the bond line in the center of the grip. This can be difficult because the spacer must be identical to the thickness of the specimen and the bond line. If the thickness is not right, then the results produced will be lower than expected.

4.)  Double lap-shear in grips without offsets

a.)  The typical lap-shear specimen is bonded with two additional pieces of material to produce accurate results. This requires extra time, material, and expertise to prepare the specimens properly.

The Best Way

5.)  Untabbed specimens in grips with offsets

a.)  To perform this test, the jaws of the grip are offset to hold the specimen while keeping the adhesive bond line in the test axis. There is no extra specimen prep required for this type of test allowing it to be performed quickly and produce accurate results. Instron offers several grips that can perform this test including the precision mechanical wedge grips.

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Understanding Melt Flow Testing and Its Importance

Melt flow testing is simply a measure of the flow of a polymer when melted. The result of a melt flow test, called the melt mass-flow rate (MFR) or melt volume-flow rate (MVR), is defined as the amount of mass or volume of a polymer that flows through a small die at a specified temperature and pressure.

The melt flow test itself is simple and straightforward. A small amount of a thermoplastic sample (usually in granule or flake form) is heated in a barrel at a specified temperature, melted to a viscous fluid, and is forced out of a capillary die by a piston loaded with dead weights. Once enough sample extrudate has exited the die, it is removed and weighed, or the volume of the sample is measured by the machine.

ASTM D1238 and ISO 1133 are the most common standards for melt flow tests and define the equipment specifications, as well as test methods. Because various temperatures and weights are allowed for melt flow tests (only suggestions for each material type), test parameters always need to be reported with MFR/MVR results.

Note: Imagery is a simple representation and not to scale.

Typically, the MFR/MVR of a polymer is inversely proportional to its molecular weight. Though not frequently used in R&D settings, melt flow testing is very common in quality control and process control laboratories. It is mostly seen in compounding or manufacturing/converting facilities.


What do you gain from melt flow testing?

• A typical index value for verifying in-house material
• Quality check of entrance materials
• Comparison of new materials in a product development setting
• Evaluation tool for new material suppliers
• Quick comparisons of batches of material 
• Estimation of flow properties for simple extrusion processes 
• Predictions of how a polymer will behave in a number of processing techniques

If you are not currently performing melt flow tests, but think you should be, contact us for guidance on selecting the proper equipment for your laboratory. Read more

Helping to Provide the Good Housekeeping Seal of Approval

Ever wonder how the beauty industry can make their claims? Research and testing is behind the development of beauty products to determine the best technologies, formulas, and products to meet goals. When consumers want to hear the validity of the claims, they look to outside sources for evidence.

Good Housekeeping tested a variety of hair products in their Beauty Lab to find the winners of the Anti-Aging Hair Awards. To test how those products impact the strength of hair, technicians used a comb fixture on an Instron system. The Good Housekeeping team measured "friction experienced when [the fixture] moved through a wet or dry hair swatch".  Simulating what happens in real life instilled confidence in consumers to purchase those winning products.

Instron comb fixture for testing hair products

Here is another example of an Instron system performing a tensile test on hair to determine strength before and after a product is used. Read more

Testing a Student-Made Off-Road Vehicle

When Northeastern University students visited our Norwood, MA headquarters, Instron employees had a chance to ride an off-road vehicle. As a platinum sponsor, Instron assisted the student engineers in their quest to finish first.

Instron employees taking a ride in the student-designed car

For the Baja SAE Collegiate Design Series, students from across the globe design, build, and race off-road vehicles in competitions around the United States. Young engineers are challenged to build an off-road vehicle with open wheels and one-passenger seating. For a year, they work together creating the cars and being tested in an industrial market, where they need to promote and finance their projects.

The Northeastern Baja team focuses on endurance and high-performance in the Baja SAE events. To get ahead the competition, this year's car featured handlebar steering and a six-speed manual transmission. Led by Dalton Colen and Matt Nussbaum, the group finished 8th in the Baja SAE competition in Illinois. In each of the past three years, Northeastern has finished in the top ten for the endurance test and overall race at an event.

We wish them luck next year and hope they come back to visit! Read more