Dynamic mechanical measurements: Comparison between bending and torsion methods on a graphite‐reinforced and a rubber‐modified epoxy

Dynamic mechanical measurements in the cantilever bending and torsion rectangular modes were compared on two epoxy samples: one unidirectionally reinforced by 42 vol.% graphite fibers, the other modified by 15 wt.% carboxy terminated acrylonitrile butadiene rubber. Two forced oscillation instruments were used: cantilever bending on a Polymer Labs DMTA and torsion with a Rheometrics System Four. In the cantilever mode, the storage modulus on both samples was measured at about two to three times too low because of compliance of the instrument. The two instruments agreed and appeared to measure accurately tan δ on the graphite‐reinforced samples. Agreement was only fair on tan δ for the rubber‐modified sample. Multiple frequency data were used to separate the glass‐transition temperature of the rubber from the epoxy β transition. Activation energies of these transitions are within the range reported. This study demonstrates the value of forced oscillation methods and the importance of selecting sample dimensions to fit instrument limits.