Resistance-foil strain-gage technology as applied to composite materials
A general review of existing strain-gage technologies as applied to orthotropic-composite materials is given. The specific topics addressed are gage-bonding procedures, transverse-sensitivity effects, errors due to gage misalignment, and temperature-compensation methods. The discussion is supplemented by numerical examples where appropriate. It is shown that the orthotropic behavior of composites can result in experimental error which would not be expected based on practical experience with isotropic materials. In certain cases, the transverse sensitivity of strain gages and/or slight gage misalignment can result in strain-measurement errors exceeding 50 percent.
The accelerated characterization of viscoelastic composite materials
Necessary fundamentals relative to composite materials and viscoelasticity are reviewed. The accelerated characterization techniques of time temperature superposition and time temperature stress superposition are described. An experimental procedure for applying the latter to composites is given along with results obtained on a particular T300/934 graphite/epoxy. The accelerated characterization predictions are found in good agreement with actual long term tests. A postcuring phenomenon is discussed that necessitates thermal conditioning of the specimen prior to testing. A closely related phenomenon of physical aging is described as well as the effect of each on the glass transition temperature and strength. Creep rupture results are provided for a variety of geometries and temperatures for T300/934 graphite/epoxy. The results are found to compare reasonably with a modified kinetic rate theory.
Accuracy of birefringent-coating method for coatings of arbitrary thickness
Rational concepts of the behavior of birefringent coatings are developed for both plane-stress problems and general surface-strain problems. For plane stress in elastic bodies, accuracy of stress or strain determinations along free boundaries is almost entirely independent of coating thickness; however, where Poisson's ratios of coating and structure are unequal, the state of stress in the structure might be influenced by the presence of the coating. In experimental investigations of four geometrical discontinuities with coating thicknesses up to 0.9 in. and materials of unequal Poisson's ratios, no influence of coating thickness on boundary strains was observed. For some three-dimensional problems, local shear tractions developed at the interface introduce a dependence upon coating thickness, but the resultant error in strain determinations was approximately 5% for a case studied experimentally. Strain distributions on singly and multiply connected surfaces in two- or three-dimensional bodies can be studied effectively by the method of birefringent-coating measurements.