T. Pelton

Abstract As part of an engineering analysis and experimental methodology to characterize prepreg tack, a compression‐to‐tension test was optimized to enhance reproducibility and generate intrinsic property data. With the resulting stress‐strain compression and tension data, a theoretical model was developed to describe tack as a bulk viscoelastic property of a prepreg laminate stack. Using the viscoelastic analysis, four intrinsic material parameters to characterize prepreg tack could be defined. These were 1) relaxed modulus, 2) unrelaxed modulus, 3) relaxation time, and 4) initial void content of the prepreg stack. Relaxed and unrelaxed moduli of the prepreg stack were independent of temperature, while the relaxation time was highly dependent on temperature and matrix viscosity. In addition, the relaxation time was found to be influenced by resin/fiber content and prepreg surface characteristics, which also influenced the void content of the prepreg stack. Using these measured parameters, good agreement was observed between theory and experimental data for both the stress‐strain curve of the tack test and the simplified compression tack index (CTI*), defined as the ratio of output energy of the prepreg stack during tensile unloading to input energy during compressive loading.

Article Perceptions of Prepreg Tack for Manufacturability in Relation to Experimental Measures was published on September 1, 1995 in the journal Science and Engineering of Composite Materials (volume 4, issue 3).

A compression to tension apparatus and a methodology capable of measuring prepreg tack have been analyzed in detail in order to establish fundamental material and operating characteristics. Both intrinsic and extrinsic parameters influencing prepreg tack were identified and analyzed using commercially available carbon fiber/epoxy prepregs and mechanical testing equipment. Two different factors, (1) contact (or wetting) area of adjacent prepreg plies and (2) viscoelastic properties of the prepreg, were found to control prepreg tack. At low temperatures, contact area was the main deformation controlling step, while at high temperatures, the viscoelastic property of the prepreg was found to be dominant. Both interlaminar and intralaminar deformations were observed depending on the prepreg systems examined as well as the operating conditions of the test. In addition, hold time, hold pressure, loading rate, resin content, and out-time were also found to affect prepreg tack. Energy of separation, which may be viewed as a descriptor of prepreg tack, was observed to increase with increasing hold time, hold pressure, and loading rate. Energy of separation also showed a maximum value at a specific resin content for a specific prepreg system, while it decreased with increasing prepreg out-time due to prepreg surface characteristic change rather thanmore » bulk physical aging. Conclusively, it was observed that prepreg tack must be viewed as an extrinsic, bulk, but surface-sensitive, viscoelastic property which depends on material as well as operating conditions.« less