Cars, planes, medical devices and consumer products are using more and more polymer and composite material in their designs. With the increasing use of structural components made from polymers and composites, understanding the viscoelastic properties of these materials becomes critical. Elastomers and polymers display significant viscous behavior that researchers and designers need to understand to make sure the material is appropriate for its intended end use.
Today, Finite Element Analysis (FEA) tools are using information about a material’s viscoelastic properties to more accurately predict how designs that contain these polymers or elastomers will perform. For example, the expanded use of Polymer Matrix Composites (commonly referred to as carbon fiber, or Carbon Fiber Reinforced Plastic) requires extensive dynamic mechanical analysis of the polymer component to predict how the stress will be shared between the polymer matrix and the reinforcement fibers.
Dynamic Mechanical Analysis (DMA) is a common way to measure the elastic (spring) and viscous (damping) behavior of materials and components. This measurement is achieved by subjecting the test specimen to a controlled cyclic stress (or strain) and measuring the force and displacement and their phase relationship.
DMA testing can be as simple as a test at a single frequency at room temperature to measure the Dynamic Stiffness (K*), Elastic Stiffness (K’), and/or Loss Stiffness (K”) of a component or material. DMA can also be very complicated with sweeps of excitation frequency, stress, strain, and temperature, including nesting those sweeps together to use an analysis method called Time Temperature Superposition (TTS). TTS can be used to predict the behavior of a material at temperatures and frequencies which are not feasible to test in practice.
DMA testing has one primary challenge, which is knowing if the data you have collected is accurate. That problem is not unique to DMA testing, but data accuracy is especially challenging to achieve with viscoelastic materials. It is difficult to know if the data is accurate because elastomers and polymers are very sensitive to many different factors including manufacturing processing, aging, temperature, composition (different batches of materials), preload stress/strain, and cyclic stress/strain. Given these variables, it is sometimes difficult to know if unexpected data is due to a bad sample, bad test protocol, or the machine actually producing inaccurate results.
Varying temperatures during Dynamic Mechanical Analysis generates another set of challenges. The two biggest challenges are maintaining the control system (tuning) of the test system as the specimen properties change and ensuring the specimen temperature is even and at the expected value.
The challenges of DMA testing have been addressed to varying degrees depending on the test machine type and the testing requirements. Dynamic Mechanical Analysis as an application is expanding beyond a chemical research tool used primarily by materials scientists, to become an increasingly important design tool used by engineers. DMA machines that are being used for Quality Control typically just need to evaluate if the sample properties are within a range. Some machines take the “black box” approach, which makes the machine very easy to operate, but may mask the potential issues that can generate inaccurate DMA data. Product developers and engineers need much more precise and accurate results than provided by this type of machine.
There are many aspects of the test system that can influence testing accuracy. A good DMA test system must be very stiff to prevent unwanted resonances from impacting the data. It also must have a robust control system, be able to accurately measure forces and displacements, and have powerful software to take full advantage of all those characteristics.
It is also important to validate a system’s measurement accuracy. For example, every MTS Acumen Test System is tested to meet the dynamic measurement specification with a dynamic standard prior to leaving the factory. That verification of dynamic accuracy is performed at the end of installation to ensure the machine is producing accurate data once it is installed at the customer site. Since every Acumen DMA system includes the dynamic standard by default, the customer can repeat the dynamic accuracy measurement as desired to verify that the system is still producing accurate data.
Engineers using elastomers and polymers in their designs will also need to know the strength and fatigue properties of those materials. However, machines dedicated solely to DMA are not capable of these types of testing. Because engineering design teams require more than just DMA data from their testing machine, an impact has been an increased need for either multiple systems or a more versatile test system.
There has been a trend toward larger specimen sizes that is being driven by several factors. The first is the increasing use of composite materials in the automotive, aerospace and construction industries. Also, with composites and filled polymers, larger size samples are needed so that the cross-section of the materials has a statistically representative structure. Finally, viscoelastic property measurements can be sensitive to specimen size, so it is ideal to test material samples that closely resemble the end use component.
The primary impact of the larger specimen sizes is the need for larger and more powerful machines. The DMA machines of old were only capable of applying a few pounds (~30 Newtons) of force. Larger specimens may require hundreds, sometimes even thousands of Newtons of force.
Both the need for multiple types of testing and the trend toward testing larger specimens will drive an increase in multi-use machines for viscoelastic testing, including for DMA applications. Yet, not every materials test system can be classified as a DMA system. Some machines that are marketed as suitable for DMA, gloss over the critical accuracy requirements and only report out results. Some manufacturers simply add calculations to a fatigue testing machine and then call it a DMA machine. Usually multi-use machines require a significant number of compromises, but some, like the MTS Acumen Test System, are equipped to provide accurate results for DMA, monotonic and fatigue testing, allowing an unprecedented level of versatility for a test lab.