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The influence of material volume on the transverse tensile strength of AS4/3501-6 graphite/epoxy composites was investigated. Tension tests of 90° laminates with three different widths and five different thicknesses were conducted. A finite element analysis was performed to determine the influence of the grip on the stress distribution in the coupons and explain the tendency for the distribution of failure locations to be skewed toward the grip. Specimens were instrumented with strain gages and extensometers to ensure good alignment and to measure failure strains. Data indicated that matrix dominated strength properties varied with the volume of material that was stressed, with the strength decreasing as volume increased. Transverse strength data were used in a volumetric scaling law based on Weibull statistics to predict the strength of 90° laminates loaded in three-point bending. Comparisons were also made between transverse strength measurements and out-of-plane interlaminar tensile strength measurements from curved beam bending tests. The significance of observed scale effects on the use of tests for material screening, quality assurance, and design allowables is discussed.
composite material, graphite/epoxy, transverse tensile strength, delamination, matrix crack, scale laws, Weibull statistics
U.S. Army Research Laboratory, Vehicle Structures Directorate, NASA Langley Research Center, Hampton, VA
Analytical Services and Materials, Inc., Hampton, VA