A New Test for Characterization of Interlaminar Tensile Strength of Tape-Laminate Composites

Abstract

Interlaminar tensile strength (ZT) is an important property of polymer matrix composites (PMCs). Accurate characterization of ZT is critical for material screening, failure analysis, and design of laminates. However, current methods for determining ZT suffer from several drawbacks that prevent accurate and unambiguous measurement of this property. With this in mind, the aim of this study is to develop a new test specimen and method for accurate measurements of ZT. This new test is developed such that the measured ZT values can be directly compared to intralaminar tensile strength (YT) obtained using the same specimen geometry and batch of material. Consequently, the ZT test geometry is based on a three-point bend flexure configuration, which has been previously used to measure YT. The material orientation in the proposed specimen configuration begins with the configuration used for measuring YT, where the specimen is subjected to tensile flexure stresses along the transverse (in-plane) 2-direction. This geometry is subsequently modified by extracting the central portion of the specimen, rotating the resulting central core by 90° about the 1-direction, and reattaching it to the other two parts of the specimen using a paste adhesive. In the resulting configuration, the central portion of the specimen is subjected to tension along the 3-direction, which allows for characterization of ZT at the onset of beam failure during flexure loading. The proposed test is evaluated using two microstructurally distinct carbon/epoxy systems, namely Hexcel’s IM7/8552 and Toray’s T800/3900-2C. To obtain a sense of variability in ZT for each material, at least 30 specimens are tested per configuration. Additional testing is performed to determine whether the bonded core and the adhesive used affect the measurements of ZT. Overall, the experimental data demonstrates that the new configuration provides a reliable measure of ZT that is independent of the specimen manufacturing method. More importantly, a direct comparison between ZT and YT demonstrates that the two properties appear to be different, with ZT being the lower of the two. Although no direct links between lower values of ZT and the specimen microstructure are discovered, this effect is likely influenced by the local microstructure of each composite system, and specifically, by the presence and location of resin-rich regions relative to the tensile stresses.