An embedded strain gage technique for measuring subsurface strains in boron/epoxy composites was successfully developed and tested. Preliminary tests to establish the feasibility of the technique demonstrated that conventional foil gages do not produce local thickening of the specimen when a patch of scrim cloth is removed, do not change the average material properties, and do not reduce the strength of the specimen. The technique was used to determine surface and subsurface strains in three-, five-, and seven-ply boron/epoxy tension coupons of three different layups. In preparing these specimens an area of scrim cloth corresponding to the gage area was removed and the gages were laid down without any adhesive. Two- and three-element rosettes were used. Ribbon leads were soldered to the gages and taken out from the sides of the specimen. The assembled specimen with the embedded gages and leads was then cured in the autoclave, tabbed, and tested to failure.
Results are presented first in the form of stress-strain curves for each ply and then as strain variations through the thickness with applied stress as a parameter. Values of modulus, Poisson's ratio, and strength were obtained from these tests. Deviations of varying degrees were observed in the strain readings through the thickness. The diagonal (45-deg) strain deviations were higher than those of the transverse strains, which in turn were higher than those of the longitudinal strains. It was also observed that deviations tended to be smaller when the gages were oriented in the direction of the fibers than when they were oriented transverse to them.