Research Highlights of Polymers Division

Many industries are seeking to take advantage of the weight reductions that can be achieved by replacing metal components and structures with polymer composites. Although this substitution is often successful, one complication that can arise is a change in dynamic response of the structure. In some cases this can be advantageous, as has been found for certain drive shaft applications, but in other instances the changes may cause problems. Unlike metals, however, composites have excellent damping properties that can be used to help control any unwanted vibrations produced by external dynamic loading. Moreover, the great flexibility available in composite structures through changing both materials and designs can be used to alter damping and resonance properties in desirable ways. To take advantage of this flexibility, however, requires a model that predicts behavior of the structure from the properties of the constituents and the design, data on properties of the constituent materials, and knowledge of how these properties relate to the material’s composition and morphology.

An important example of this general problem occurs in off-shore oil drilling. The U.S. needs to reduce its dependence on foreign oil and most of the known reserves are in deep water (below 600 m). The light weight of composite structures is critical for industry to build drilling and production risers that can reach these depths. Industry has developed the technology to make risers, but calculations show that the reduced weight shifts the dynamic response of the structure into a range where interactions with currents and waves could be a problem. This dynamic response of the riser is called vortex induced vibration (VIV), and a commercial software program is used to analyze VIV in riser design. Unlike metal risers, however, composite structures have material damping capability that can counteract problems related to VIV. Unfortunately, the currently available software program does not have the capability to include material damping since it is not a factor with metal structures.

NIST and the University of Houston have addressed this issue as part of a joint program on composite structures. The University of Houston has developed an analysis code that can predict vibrational motions in drilling or production risers from a knowledge of design and material properties. NIST has utilized its capabilities to characterize the dynamic mechanical behavior of various resins systems since they are the source of damping in a composite structure. For the VIV analysis, material property data are needed over a relatively modest range of parameters. For example, the frequencies of interest extend from 0.001 Hz to 20 Hz. In order to improve the response of a material or select a better material, however, it is essential that the behavior be understood in terms of molecular structure and morphology. This requires a knowledge of properties over a much wider range of parameters. Fortunately, time-temperature superposition can often be applied to the data so results over a wide range of frequencies can be obtained. This is illustrated in the figure below where the overlapping points are data generated by NIST for the resin system now being considered for use in risers. By superimposing data at various temperatures, a master curve was generated that predicts the behavior over a wide range of frequencies. This curve shows that the damping in the range of interest for risers can be attributed to the tail of the tan ä peak, which is associated with the glass transition temperature, Tg. Based on this, we formulated and tested a second resin system (solid curves in the figure) that contained a small amount of an elastomeric additive. The additive has some compatibility with the resin so in addition to creating a second phase, it also broadened the glass transition and the corresponding tan ä peak for the resin Tg. As a result, the damping in the range of interest is increased. When this resin system was used in the analysis of vibration for a riser, the behavior was significantly improved.