Problem: 

The measurement of Poisson’s ratio can be a delicate matter. The equipment used to determine Poisson’s ratio is often quite fragile and extremely sensitive. How do you ensure that the equipment is working properly prior to using it for measurement? Without proper verification techniques, one cannot have confidence in the data generated.

Solution: 

The proper equipment verification techniques used prior to performing the test are critical. Checking the entire measurement system will ensure the test is performed properly and the data generated is accurate.

Poisson’s ratio is a mechanical property often used in Finite Element Analysis (FEA) calculations to predict the stresses a product may see in an end use application. It should be calculated from the initial portion of the load (or stress) versus strain curve. Essentially, it is the measurement of the change in volume of a material during deformation. The Poisson’s ratio of most thermoplastic materials falls in the range of 0.3 to 0.5.

Contact extensometers that get mounted directly on the specimen.

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There are several techniques that can be used to determine Poisson’s ratio of polymeric materials. The most reliable approach to determine this ratio is direct measurement using a strain measuring device on a test specimen of uniform cross-section. The use of bonded strain gages is an extremely accurate technique, but can be time intensive. Bonded strain gages are commonly used for composite materials that have high modulus fibers (such as carbon fiber) bound in some type of thermoset resin system. For thermoplastic materials, the use of biaxial extensometers is the most common approach. There are several types of extensometry techniques such as video and laser extensometers that do not touch the specimens and contact extensometers that get mounted directly on the specimen.

Prior to performing any type of test in a laboratory, proper verification of the test equipment is essential. The verification technique should address the entire system and not just one component. For example, Poisson’s ratio is calculated by plotting the load (or stress) versus axial strain and transverse strain. The equipment should be verified to ensure that the load and strain measuring devices are reading accurately.

Recently, we experienced a unique situation. We were getting ready to use some newly-purchased high heat extensometers for Poisson’s ratio measurements. They came with certification documents. When checked on a high precision extensometer calibrator and the displacement verified using gage blocks, the extensometers read accurately. However, when performing a test on a material with a known Poisson’s ratio, we were receiving a value of about 25 percent lower than it should have been. After recalibrating the extensometers and careful review of our process, we came to the realization that the spring pressure in the transverse extensometer was set too high and was digging into the material, causing the low values. We then changed the spring pressure and the knife edges of the extensometer to blunt edges. These changes yielded proper values. If we had not checked the entire system we could have been issuing data that was about 25 percent lower than it should have been. Even though we verified the individual components of the system, it was not enough to ensure the entire system was working properly.

Materials testing is complex. Thinking through which parameters are measured during a test and incorporating the proper system checks will result in accurate data for your company.

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