The usefulness of dynamic mechanical analysis over other forms of mechanical testing is the well-defined periodic loading regime that is used. A periodic force or displacement, usually sinusoidal, is applied to the sample and the resultant displacement or force is measured. This measurement includes the amplitude of the signals and also the phase difference between them.

Note that only instruments producing linear displacement will be considered here. Rotational instruments are required to produce torsional shear and these are mainly sold as rheometers and normally used for the analysis of liquids. Solid samples can be tested on such instruments with suitable bar clamps, but it may be difficult to obtain measurements through the glass transition, due to stiffness range considerations.

The essential measurement from DMA (Dynamic Mechanical Analysis) is complex stiffness. The stiffness is always returned as a value in N/m (newtons per metre). It is the range of stiffness that can be covered in a single experiment that determines a DMA’s usefulness. This range should be a minimum of four decades. Most commercial DMAs will cope with stiff samples very well and this makes the upper force limit unimportant. It is the lower stiffness limit that frequently causes problems. A good instrument will measure a stiffness of at least 500 N/m. Most instruments measure this quantity with high precision, as their force and displacement calibrations are traceable to secondary standards. The conversion of this stiffness to an accurate modulus is frequently a major source of errors, usually due to an inappropriate choice of sample geometry. The measured stiffness is as much a function of sample geometry as modulus. Therefore, the correct choice of sample size and geometry is of paramount importance.

Figure 1 shows the response to a perfectly elastic material. The phase lag, δ, between the force and displacement is zero. This is not the case for viscoelastic materials where a delay occurs in the displacement response to an applied force. Figure 2 shows the response for a perfectly viscous material, such as liquid that flows freely. The phase lag, δ, is now 90◦; i.e. the displacement amplitude is exactly one quarter cycle behind the applied force amplitude. In DMA (Dynamic Mechanical Analysis) instruments, the force and displacement are resolved into in- and out of-phase components (see Figure 3), thus defining the storage or real modulus and the loss or imaginary modulus respectively. The proportion of deformation that is in-phase represents energy elastically stored and recoverable (hence storage or real modulus), whilst the proportion of 90◦ out-of-phase deformation is due to viscous flow, or other dissipative energy processes (hence imaginary or loss modulus). The damping factor, tan δ, is defined as the ratio of loss to storage modulus and by definition the ratio of energy lost to energy stored. 

Figure 1. Force and displacement signals for elastic material (δ = 0◦).

Figure 2. Force and displacement signals for viscous material (δ = 90◦).

Figure 3. Resolution of measured signals into in- and out-of-phase components.

Molecular structure characterization is the main reason that DMA (Dynamic Mechanical Analysis) was developed. The ability to explore the molecular structure via a simple thermal scanning test, requiring a relatively small amount of sample (0.5–2 g), gave the polymer chemists a powerful characterization tool. It can be argued that solid state NMR, dielectric studies and electron spin resonance give more detailed information, which they invariably do, but generally the apparatus is more expensive, harder to use, experiments take longer and specimens frequently require special preparation. This is why DMA has emerged as the dominant tool for the structural evaluation of polymers.

Now there are four main areas of DMA (Dynamic Mechanical Analysis) use, which are molecular structure characterization, general material analysis, food and biomedical testing and the derivation of engineering data. The first category is explained above. Clearly, this explains the use of DMA in polymer synthesis labs where it is important to check what has been made. It is hard to estimate how much DMA use this accounts for, but I estimate this to be below 40% of all instruments, split approximately equally between industry and academia.

The vast expansion of the use of DMA has occurred in the field of polymeric material analysis. First, DMA (Dynamic Mechanical Analysis) is one of the best techniques for assessing the amorphous content of a material. It is important to know how much amorphous material is present in a number of situations. Since DMA is sensitive to molecular structure it is frequently used to check one sample against another that is meant to be the same. Also, processing can have a large effect on final properties. For thermoplastics DMA is sensitive to the level of crystallinity, physical age state and polymerization. For thermosets, the state of cure can be readily determined and as DMA is a mechanical test useful information can be obtained on interfacial properties for composite materials. To a certain extent this category overlaps with the first, except that fewer measurements are made and a complete molecular profile of the material under test is not obtained. This general usage probably accounts for 40–50% of all DMA.

An important and growing sector of DMA application is within the food and bioscience sector. Many samples are measured for the reasons given above, namely the determination of Tg, checking similarity of samples. Generally, the techniques are similar to those required for the analysis of polymers. However, the temperature range is usually less (−50–200◦C) and the water content frequently plays a pivotal part in the sample’s properties. It is for this reason that controlled humidity testing is being incorporated with TMA and DMA apparatus. I would not like to put a figure on how much these sectors contribute to overall DMA usage, but it is over 10%.

A more specialized area of use of DMA is the derivation of engineering data. DMAs are capable of generation of modulus and damping factor (tan δ) data over a wide range of frequency and temperature. In such applications great care has to be taken that meaningful data are generated. Instruments such as tensile testing machines have undergone considerable development to ensure that accurate moduli are generated. For example, large sample test pieces with favourable geometry for the mode of deformation are used; clip-on extensometers measure the strain very accurately and this avoids any strain that may occur within the clamping mechanism. This is hard to achieve in DMA as one of the main design considerations is to keep the sample small to facilitate changing the temperature rapidly and lightness for high-frequency measurements. Therefore, much thought has to be given to sample geometry and the sample size for accurate modulus determinations. Generally, this tends to be a more specialized area of DMA (Dynamic Mechanical Analysis) use. It is mainly the preserve of producers of acoustic damping material (including the military), critical component designers and software designers who need accurate values for software packages such as those used for mould filling and finite element analysis. Other mechanical testing equipment is normally used to source static modulus or viscosity data, but DMA is usually the only source when high frequency or damping factor data are required. In my opinion such use accounts for less than 10% of all measurements.