In this interview, AZoMaterials speaks with Andy Simon, Product Manager for Mechanical Testing and DMA instruments, and Dr. Shoab Ahmed Chowdhury, Applications Scientist, from Waters Materials Sciences and TA Instruments, about dynamic mechanical analysis (DMA), high-force DMA characterization, and how product-specific testing can help evaluate performance under real-world service conditions.
Can you please introduce yourselves and your roles at Waters Materials Sciences and TA Instruments?
Andy Simon: I’m Andy Simon, and I’m the Product Manager here at Waters Materials Sciences for our mechanical testing and DMA instruments.
At Waters, we provide a broad range of material characterization technologies. These include thermal analysis (e.g., DSC and TGA), rheology for measuring liquid flow, thermophysical property measurements, and physical and mechanical testing.
One of our key focuses is on mechanical testing and DMA. We provide conventional or benchtop DMA instruments, such as the DMA 850, as well as mechanical test load frames that allow us to perform a wide variety of tests, including high-force DMA.
Dr. Shoab Ahmed Chowdhury: I’m Shoab Ahmed Chowdhury. My work focuses on mechanical testing systems, and I’ve been working specifically on high-force DMA characterization in the final stages of product development.
What is DMA, and what does it tell us about a material or product?
Andy Simon: DMA measures the elastic and viscous components of a material or product.
All materials have some amount of viscoelastic nature. Really, no material is purely elastic or purely viscous. With purely elastic behavior, stress and strain are perfectly in phase; with a purely viscous material, such as an ideal fluid, there is a 90-degree phase relationship. Real materials fall somewhere between those two behaviors.
A common way to represent the elastic and viscous nature of a material is through storage and loss modulus. Storage modulus represents the energy stored in the material, reflecting its elastic behavior. Loss modulus represents the damping or lost energy.
We can also look at tan delta, the ratio of the loss modulus to the storage modulus, which represents the material's damping characteristics.
What types of DMA experiments can be performed using a high-force DMA system?
Andy Simon: We can perform the same common DMA experiments as on conventional benchtop DMAs.
One is a strain or amplitude sweep, in which we start at a small deformation and progressively increase it to see how the material responds to different levels of strain. For an elastomer, this can be used to measure the Payne effect or identify the linear viscoelastic region.
Another is a temperature ramp. We produce a continuous oscillating mechanical motion at a constant frequency, most commonly 1 Hz, while continuously ramping the temperature. This is a very quick way to measure the temperature dependency of a sample, measure the glass transition temperature, and look for other transitions.
We can also perform frequency sweeps to investigate a material's rate dependence. That is important because you cannot necessarily take data at 1 Hz and assume that the same behavior carries through to higher frequencies.
Time sweeps are another option. Here, we keep the parameters the same and continually run the test. This is similar to traditional fatigue testing, but we are still measuring DMA properties and looking at how they change over time. That can detect much more subtle changes in the material as it degrades or wears.
Why is high-force DMA needed if conventional benchtop DMA is already available?
Dr. Shoab Ahmed Chowdhury: Conventional and high-force DMA instruments are highly complementary.
The DMA 850 is mainly optimized for the most sensitive measurements of the materials themselves, providing very sensitive data for conventional DMA characterization.
High-force DMA becomes valuable when materials are formed into structures such as foams or lattice arrangements, or when multiple different material layers are present within a product. These structures and combinations generally require higher forces, higher deformation, and often product-specific measurements to evaluate performance under service conditions.
Larger samples also require more test space, larger fixtures, and environmental chambers. In other cases, a sample may physically fit within a conventional DMA yet have very high stiffness, requiring a higher force capacity.
For higher-strain applications, such as some elastomer characterization, the required displacement can also be beyond the maximum limit of a conventional DMA.
So there is some crossover, but the instruments are really complementary at different stages of the material and product development workflow.
In a recent project, you used DMA to characterize tennis balls. How did you achieve this?
Dr. Shoab Ahmed Chowdhury: We tested three tennis balls: beginner, tour, and championship. All three had the same geometry, with very similar diameter and height, and we tested them using the TA Instruments Apex 1 mechanical testing instrument.
We started with a compression test. The main objective was to measure stiffness, so we compressed the specimen and then decompressed it to restore its original geometry. This was performed at a slow ramp rate of 0.05 millimeters per second at room temperature.
All three specimens exhibited hysteresis, suggesting energy loss due to heat dissipation. This also indicates viscoelastic characteristics.
We then performed monotonic testing at different speeds to investigate strain-rate dependency. We could see that stiffness changed at different strain rates, with the difference becoming more prominent at higher strain levels.
However, monotonic testing cannot reliably quantify the material's viscoelastic properties, such as energy return through storage modulus, energy loss through loss modulus, or damping through tan delta. For that, we moved to DMA characterization.
What did the temperature-ramp DMA test reveal about the tennis ball material?
Dr. Shoab Ahmed Chowdhury: For the temperature-ramp test, we cut a rectangular section from the tennis ball and tested it under tensile loading.
The test was performed at a ramp rate of 2 °C per minute, with a temperature range from -10 °C to 150 °C.
Looking at the storage modulus onset point, the glass transition temperature was 108.26 °C. From the tan delta peak, the glass transition temperature was approximately 133 °C.
These glass transition temperatures can differ from those of the individual base materials because the tennis ball strip was manufactured from multiple materials.
We also observed a secondary peak in the tan delta curve around 40 °C, which may be an application-specific temperature in real-world conditions. The two transitions may indicate the presence of two different materials in the tennis ball strip.
After that, we moved to a product-specific DMA test using the entire tennis ball. We changed three parameters: mean force, frequency, and temperature.
We performed a mean-force sweep from -5 to 15 N at six frequencies from 0.1 to 20 Hz. Those frequency steps were performed at 10, 25, and 40 °C, with a dynamic deformation amplitude of ±0.5 mm. In total, that gave us 180 different conditions.
Image Credit: Hellen77 /Shutterstock.com
What did the product-specific DMA testing show about the performance of the different tennis balls?
Dr. Shoab Ahmed Chowdhury: When we looked at storage modulus with respect to mean force, stiffness increased with mean load for all three specimens. Stiffness also increased with increasing frequency.
The beginner tennis ball demonstrated a linear response with respect to mean force, whereas the tour and championship balls demonstrated a nonlinear response.
This is one of the reasons we needed the 3D mean-force sweep DMA test. It allowed us to capture the nonlinear stiffness behavior of the final products rather than only looking at the properties of an individual material.
The test also lets us examine the interaction among loading, frequency, and temperature. That is important when we want to understand how the product may behave under different service conditions rather than under one fixed laboratory condition.
How did you apply this DMA methodology to running shoes?
Dr. Shoab Ahmed Chowdhury: The main objective was to characterize shoe performance under different loading and environmental conditions to simulate the in-service performance of a running shoe.
We tested both the heel and forefoot regions by cutting the shoe in half. We used the same 3D DMA approach developed for the tennis-ball study, varying the mean force, frequency, and temperature during testing.
For the heel, the mean-force sweep extended to 755 N, while the forefoot was tested to 435 N. We used nine different frequencies from 0.1 to 30 Hz and six temperatures from 0 to 50 °C. This resulted in 594 conditions for the heel and 432 conditions for the forefoot.
At 0 °C, the storage modulus of the heel increased with frequency. The largest difference was between 1 and 5 Hz, after which the differences gradually reduced. This indicates a stiffer heel response with faster running speed during a cold day.
At 40 °C, storage modulus still increased with frequency, but the differences between the frequency steps were less prominent than at 0 °C. So the heel still becomes stiffer with faster running speed during warmer conditions, but that frequency dependence is not as prominent as it is under cold conditions.
Why did you test the heel and forefoot separately, and what did tan delta reveal about their performance?
Dr. Shoab Ahmed Chowdhury: The heel and forefoot have different functions during running, so we wanted to characterize them separately.
When we looked at tan delta, we were examining the damping characteristics of the shoe. Tan delta is the ratio of loss modulus to storage modulus; the lower the tan delta, the lower the damping capability.
For the heel, damping reduced at higher temperatures. It also generally reduced at higher mean-force steps, although the difference was not as prominent at 0 °C.
When we compared the heel and forefoot at 1 Hz and 0 °C, and then again at 30 Hz at the same temperature, damping was higher for the heel under both conditions. The difference between heel and forefoot was much larger at 30 Hz.
In fact, the reduction in damping capability for the forefoot was almost an order of magnitude at the faster running speed. This indicates that forefoot damping can be significantly reduced on colder days when running faster, something the user may not notice when running more slowly on the same cold day.
A lower tan delta does not automatically mean poorer performance, as it depends on the shoe's goal. The optimum tan delta depends on whether comfort and cushioning are the higher priority or whether propulsion is the primary design objective.
Why use DMA rather than simply perform a conventional compression test and measure stiffness?
Dr. Shoab Ahmed Chowdhury: Monotonic testing does not provide a complete performance evaluation, especially for the final product itself.
Our monotonic testing revealed hysteresis and indicated viscoelastic behavior, but it could not quantify how much of the behavior was attributable to the elastic contribution and how much to the viscous contribution.
That quantification is possible with DMA because we can differentiate between the elastic and viscous components of the material.
A monotonic test can indicate viscoelastic properties and that the response will change under different loading, frequency, and temperature conditions. DMA allows you to quantify performance and potentially change your design based on that evaluation.
How does the Apex 1 make this type of high-force DMA and mechanical testing easier to perform?
Dr. Shoab Ahmed Chowdhury: The Apex 1 is a highly versatile instrument that can perform monotonic, fatigue, DMA, and other types of testing in a single instrument. It has a maximum deformation range of 100 mm, and appropriate fixtures and environmental chambers can be used to simulate real-world conditions.
With a traditional mechanical testing workflow, users must complete several steps before they even begin specifying test parameters. These can include selecting the control mode, tuning the system according to material stiffness, determining data-acquisition parameters, and adjusting limits to detect failure.
With the Apex 1, many of these steps have been automated.
TuneIQ determines the sample stiffness and the necessary tuning parameters, so the user does not have to manually tune the system when moving between a soft material and a highly stiff material.
Confident data acquisition automatically determines the optimal number of data points based on the test frequency or duration. Break Detect automatically stops the motor at the point where sample failure occurs during monotonic or fatigue testing.
TRIOS also includes intelligent analysis, allowing users to perform data analysis in accordance with ASTM or ISO standards for monotonic testing, such as ASTM D638 for polymer materials.
The workflow comes down to three steps: install the specimen, provide the test parameters and sample geometry information, and start the test and record the results.
About Andy Simon
Andy Simon is Product Manager for Mechanical Testing and DMA instruments at Waters Materials Sciences. His work focuses on mechanical testing and dynamic mechanical analysis technologies used to characterize the behavior of materials and products.
Within this field, Simon works with both conventional benchtop DMA and high-force mechanical testing platforms. His technical work and educational presentations cover the application of DMA to material characterization, including strain sweeps, temperature ramps, frequency sweeps, time sweeps, and time-temperature superposition, as well as the extension of these techniques to product-level mechanical testing.
Simon also works on the application of mechanical characterization to products and structures that cannot be adequately evaluated using conventional benchtop DMA because of their size, stiffness, required deformation, or service conditions.
About Dr. Shoab Ahmed Chowdhury
Dr. Shoab Ahmed Chowdhury is an Applications Scientist specializing in mechanical testing systems at TA Instruments, part of Waters Materials Sciences. His work focuses on mechanical testing and material characterization, including the use of high-force dynamic mechanical analysis to investigate materials and complete products under application-specific conditions.
Dr. Chowdhury completed his PhD at Clemson University, where his research investigated the long-term performance and environmental response of multi-stable composite materials.
His work at TA Instruments includes DMA, monotonic testing, and fatigue testing across materials and finished products. Through product-level studies such as the tennis-ball and running-shoe investigations discussed here, his work examines how variables including force, frequency, deformation, and temperature affect stiffness, damping, and overall product performance under service conditions.
This information has been sourced, reviewed, and adapted from materials provided by Waters Materials Sciences (formerly TA Instruments).
For more information on this source, please visit TA Instruments.
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