Sources of Variability in Small-Scale Tensile Testing of Carbon Fiber Reinforced Resins

Abstract

Scaling down the dimensions of standardized materials testing methods can provide opportunities for comparison between established and emerging materials. However, scaled-down testing methods can be sensitive to a variety of testing variables and may not always produce material properties that are comparable to standardized methods. In this work, we developed a method for tensile testing small fiber-reinforced composite specimens and used it to test unidirectional IM7/8552 laminates. Specimens with dimensions around one-third of the ASTM standard recommended size were loaded in tension using aluminum inserts inside hydraulic collet grips, without the need for specimen end tabs. To investigate specific variables that can lead to deviation in the measured tensile strength and modulus of the laminate, 14 different specimen sets were tested. Overall, the testing method was not sensitive to the specimen dimensions over the range of lengths, widths, and thicknesses that were investigated. The ability to fabricate very small specimens with consistent quality was the main challenge to generating accurate and precise tensile data. Uncertainty in the cross-sectional area measurement for very small specimens can also contribute significantly to the deviation in the measured tensile properties. Using an alignment jig significantly reduced the scatter in the tensile strength measurement. The developed testing method consistently and accurately measures tensile modulus but tends to report strength values that are lower than reference values. We consider the method to be a useful and reliable tool for comparing tensile properties of small composite specimens, but not a direct replacement for testing using ASTM D3039 or other established testing standards.