In situ monitoring of through-thickness strain in glass fiber/epoxy composite laminates using carbon nanotube sensors

The intention of the work presented in this paper was to find a way to measure the through-thickness strain (TTS) in composite laminates. The reason for this was because there did not seem to be any effective technique for the measurement of the TTS, particularly for locations away from the free edge. In the present work, Multiwalled carbon nanotubes (MWCNTs) were added into epoxy resin to make the resin electrically conductive. The modified resin was then incorporated with long glass fibers to make glass/epoxy laminates. The laminate was subjected to a uniaxial load, while the electrical resistances across the thickness of the laminate were measured. Two different types of uniaxial loads were applied. One was along the length of the sample and the other across the thickness of the sample. For the case where lengthwise load was applied, classical lamination theory (CLT) was used to calculate the average TTS (ATTS), and to find stacking sequences that can provide the largest ATTS for a certain load. Strain gage was mounted on the edge of the laminate in order to provide another means to measure the ATTS. It was found that the electrical resistance across the laminate thickness is sensitive to the axial load along the length of the sample. The magnitude of the change in electrical resistance across the laminate thickness is proportional to the strain measured by strain gage. However, while the strain measured by strain gage shows negative strain, there was an increase in the through-thickness electrical resistance (TTER). For the case of thicknesswise load, the strain is negative and the TTER shows decrease. For the thicknesswise load, the change in TTER can be related to the ATTS. For the lengthwise load, even though the magnitude of the change in TTER is proportional to the ATTS, the change in TTER may not be completely due to the ATTS.