4 Characteristics of Real High-Force DMA and Why They Matter

While many instruments can measure DMA type properties (phase, tan delta, etc), many are not performing ‘real’ DMA experiments.

On the surface, Dynamic Mechanical Analysis seems straightforward: apply periodic stress/strain and measure the sample’s mechanical response including the phase relationship, revealing characteristics like stiffness (storage modulus) and damping (loss modulus).

However, not all manufacturers mean the same thing when they say they can do DMA testing. Some instruments run cyclic tests, then calculate the phase angle between force and displacement – essentially just adding some calculations to a fatigue test. While this ‘calculation’ of storage and loss modulus and tan delta may sound like DMA, it fails to unlock the material insights that real DMA is so well known for and can provide erroneous measurements and conclusions due to its simplistic approach.

A true DMA instrument should have built-in DMA-specific test methods. DMA tests don’t typically follow the same process or contain the same elements as traditional mechanical testing. There are typically many more steps, including changes in temperature, temperature conditioning, etc. Building these procedures in traditional ‘blocks’ is not only burdensome and error prone, but some key test elements are likely not possible. Without having the following built-in test methods where only the basic parameters are set by the user, it is virtually impossible to unlock the insights DMA provides.

Temperature ramps are the most common DMA tests consisting of heating or cooling a material at a constant, gradual rate while applying an oscillatory motion. Temperature ramps are relatively fast tests that evaluate how the storage and loss modulus change with temperature, measure transitions in the material (glass, beta, etc.), and can be used to look for the presence of contamination, impact modifiers, and different blends. The temperature ramp below shows a material’s change in moduli and tan delta as the temperature steadily increases.

Strain sweeps apply an incrementally increasing amplitude at a constant frequency. Strain sweeps are used to determine the linear viscoelastic region by plotting the storage modulus against the amplitude. Strain sweeps are also useful to evaluate how a material’s modulus changes with increasing strain, which is particularly important for elastomer materials.

Frequency sweeps apply sinusoidal oscillations at varying frequencies while keeping temperature and strain/stress amplitude constant, revealing how stiffness (Storage Modulus) and damping (Loss Modulus, tan δ) change with loading rate.

Time-temperature superposition (TTS) is a principle that allows for predicting very long-term (months to years) or short term (kHz to GHz or higher) material properties using an experimentally feasible time or frequency range. Since viscoelastic materials’ properties are temperature-dependent, we can simulate very long and very short time spans by changing the temperature of a material.

Not only does DMA testing require different methods than fatigue testing, but it also requires different analysis methods and tools. Complete analysis software is another key differentiator that makes the difference between weak, oversimplified DMA data or complete and robust insights. Without the proper analysis methods and tools, you could spend hours on your analysis and still have unreliable results.

Fourier Transform based analysis: DMA testing is focused on the time and temperature dependance of materials. Therefore, the test results must be very accurate for the frequency (time) that is being tested. Any waveform distortion can introduce error into the analysis. By using Fourier Transform for data analysis, we focus on the specific frequency for that test condition and reject the waveform distortion and noise that will corrupt the results. This method of rejecting signal noise and waveform distortion allows reliable DMA data even at very small force and displacement amplitudes.

Time temperature superposition is a supported test mode as mentioned above, and TTS analysis should also be facilitated by DMA software. TTS is very powerful but can be very difficult without expertise or user friendly analysis tools. TA Trios has built-in master curve generation wizards to quickly analyze TTS data and generate master curves for your materials and tests.

Glass transition occurs when a polymer is heated and transitions from a hard and brittle material to a softer, rubbery material with more viscous properties. Although the glass transition is commonly reported as a single temperature, it actually occurs over a range of temperatures and can be evaluated in three different ways. It can be as simple as selecting the peaks of the loss modulus or tan delta, but it can also require onset analysis tools for the storage modulus. TRIOS software offers straightforward analysis tools to identify and analyze the glass transition and remove errors from subjective operator analysis.

Onset analysis reports the value of major transitions, such as the glass transition or other secondary relaxations or transitions related to specific molecular motions or curing stages. Onset analysis uses the tangent lines from the adjacent data to project the onset point of that transition. Analytical tools are critical for accurate and repeatable onset analysis as manual methods are highly subjective.

True, reliable DMA includes qualified performance to ensure that artifacts do not influence results. Qualified performance includes:

• Compliance correction: measure the stiffness of the system (frame, install fixtures, and load sensor) to ensure that instrument deflection doesn’t skew the modulus measurement. Even small instrument deflections can generate a large error in modulus measurement.
• Phase correction: adjusts for any delays in the system’s sensors and electronics to accurately measure the material’s true response. Modern electronics are very fast and easily thought of as “instant,” but the reality is that the microsecond level conversion time from the sensor signals to the digital processor can introduce significant phase errors into the data. This must be properly corrected to get trustworthy DMA data.
• Factory demonstrated performance: All ElectroForce instruments with the DMA package are tested at the factory to ensure they meet our DMA performance specification.

A true DMA instrument provides corrections for instrument induced errors to ensure that your reading is true sample data, not based on something the instrument generated in error. Without these checks, you do not know if the measured results are real material behavior or simply instrument artifacts.

DMA testing is designed to quickly measure samples across a wide range of amplitudes, frequencies, and temperatures, often within a single experiment! This wide range of experimental conditions generates material responses that can change multiple orders of magnitude. This range of sample responses makes it virtually impossible for traditional PID controllers to execute DMA experiments. They require samples to stay within a fairly narrow range of response (stiffness) for proper control. An adaptive controller is necessary to ensure the instrument is adapting to the constantly changing sample characteristics without constant user adjustment.

An adaptive FFT-based motion controller is designed for the unique challenges associated with DMA experiments. The adaptive ElectroForce controller uses Fourier Transforms to ensure accurate production of stress and strain on the sample at the desired test frequency, even with rapidly changing sample properties.

Without this control method, most DMA tests would require the operator to re-tune the control system at different DMA test conditions or generate complex, cumbersome, and error-prone gain scheduling tables to teach the instrument how to adapt to the changing sample. These steps all require significantly more operator time and expertise, making efficient DMA testing impossible. Many DMA tests would not be possible without this adaptive controller. Therefore, an adaptive FFT-based motor not only enables research-grade DMA analysis but also makes testing more accessible for users of all levels.

Dynamic Mechanical Analysis is a very powerful tool to understand a material, compare multiple materials, and predict the performance of the material in its end use. To get the benefit of DMA, the test instrument must have real DMA capabilities, not just “DMA Calculations”. That real DMA capability must include DMA specific test methods, analysis models, qualified performance, and adaptive controllers for the rapidly changing material properties. A real DMA instrument provides streamlined and reliable measurements by using DMA specific methods and analysis while quantifying and removing sources of error from the instrument. Without that, you aren’t doing DMA testing, you are just running a fatigue test and calculating the phase lag.

Whether you’re investigating molecular structures of a new material or evaluating product prototype properties, real DMA data is the most reliable source to understand your material and make informed decisions. After more than 60 years of exceptional DMA, the TA Instruments ElectroForce line of High-Force DMA instruments are designed to advance your material studies.