Dynamic Mechanical Analysis Modes

A tensile test gives you one modulus value at one loading rate and one temperature. Dynamic mechanical analysis gives you a series of modulus values over a temperature sweep, with the elastic and viscous parts separated. The deformation mode determines the sample geometry and the materials used for testing. The following article describes the types of modes, the associated standards, and testing specifications.

What Is Dynamic Mechanical Analysis?

Dynamic mechanical analysis applies a small oscillating deformation to a specimen and measures the force response while temperature or frequency changes on a controlled schedule. Polymers respond out of phase with the applied strain, and the size of that phase lag says how much of the material’s response is elastic and how much is viscous.
Three values result from DMA. The storage modulus, E’, gives the elastic properties and determines the stiffness. The loss modulus, E”, gives the viscous properties and shows the energy lost per cycle. The tan delta value gives the ratio between the two and shows the damping capacity. All these parameters show an abrupt change around the glass transition temperature, where the E’ decreases by two to three orders of magnitude.
Frequency matters as much as temperature. Polymers stiffen at higher frequency in the same way they stiffen at lower temperature, which is the basis for time-temperature superposition. Running sweeps at several frequencies and shifting them onto one curve extends the usable range far beyond what a single run covers.

Types of Dynamic Mechanical Analysis

Deformation mode is the first decision, and specimen form usually determines it.

1. Three-point bending. The specimen rests on two supports with the drive shaft pushing at the center. ASTM D5023 covers this mode for plastics. It suits rigid bars, gives a good signal on stiff materials, and needs no gripping, avoiding the clamping problems that soft specimens create in other modes.
2. Tension. The specimen is held at both ends and oscillated in tension about a static offset. ASTM D5026 covers plastics in tension. Films, fibers, and thin sheets go here, since they buckle in any compressive mode.
3. Torsion. The rectangular bar is twisted along its longitudinal axis, yielding the shear modulus instead of the tensile modulus. This method covers an extremely wide range of stiffness in one setup, making it the right approach for materials that soften significantly in the entire sweep range.
4. Dual and single cantilever. The specimen is clamped at one or both ends and driven from the center or the free end. Clamping introduces its own compliance, and over-tightened clamps on a soft polymer put the specimen under enough compressive stress to shift the apparent transition.
5. Compression and shear sandwich. Elastomers, gels, and foams that cannot support themselves in bending get tested between platens or bonded between shear plates.

Dynamic Mechanical Test Methods and Standards

ASTM D4065 is worth reading before writing any DMA requirement, because it sets what has to appear in the report: mode, frequency, heating rate, specimen dimensions, strain amplitude, and clamping arrangement. A DMA result without those parameters cannot be reproduced, and two labs quoting different heating rates will report transition temperatures several degrees apart.

Also read: ASTM D5023: Dynamic Mechanical Testing of Plastics

How to Choose the Right Dynamic Mechanical Test

• What form is the specimen in? A molded bar points to three-point bending under D5023. A film or a thin coating on a removable substrate points to tension under D5026. A soft elastomer that will not hold a bending span points to torsion or shear. This eliminates most of the table immediately.
• How far does the stiffness change across the sweep? A substance that undergoes its glass transition sees a huge decrease in modulus. The bending fixture will be inaccurate when the specimen starts to sag, which means that a broad temperature range on a thermoplastic may work well in torsion or tension.
• What are you actually trying to get? A transition temperature needs a temperature sweep at one frequency. A design modulus at a service frequency needs a frequency sweep at fixed temperature. A long-term creep or relaxation prediction needs multiple frequencies and time-temperature superposition, which is a longer program.
• Does a specification name a method? Aerospace composite qualification normally involves D7028, which specifies the heating rate, the mode, and the construction of the onset storage modulus used for reporting the transition. Using a tan delta peak based on a different standard does not fulfill this criterion.

Another parameter that affects the result but is rarely mentioned is strain amplitude. The theory behind DMA assumes that the material is in the linear viscoelastic regime, while filled elastomers enter the nonlinear regime at much smaller strains than 1%. Start with the strain scan at constant temperature and choose the amplitude within the linear plateau.

Dynamic Mechanical Analysis FAQs