Scanning electron microscope image of carbon fibers coated in TiO2 nanoparticles (left). Interlaminar shear strength versus the average gauge factor for different composites. The highlighted region shows simultaneous improvements in both properties (right).The strength and light weight of carbon fibers make impressive composites when mixed with polymers ideal for use in aerospace and automotive applications, wind turbine blades, and sporting goods. But carbon fiber polymer matrix composites can fail suddenly and catastrophically without warning. Researchers from Oak Ridge National Laboratory and Virginia Polytechnic Institute and State University believe that the simple addition of nanoparticles to the mix could provide an early warning of failure [Rankin et al., Composites Science & Technology 201 (2020) 108491, https://doi.org/10.1016/j.compscitech.2020.108491 ].
Carbon fiber composites, particularly those based on epoxy, tend to suffer from delamination when the bonds between the fiber and matrix fail. Sudden breakage can happen without any external warning signs, limiting the usefulness of these composites for structural applications. Different ways of monitoring the structural integrity of carbon fiber composites are being explored, such as embedding piezoresistive materials in the material, which change resistance in response to strain. Piezoresistive materials can convert mechanical strain into an electrical signal, which can be detected by sensors to monitor the structural health of a composite component.
Christopher C. Bowland and his colleagues selected TiO2 as a sensing material for composite monitoring because of its piezoresistive properties. By embedding TiO2 nanoparticles in the polymer coating, or sizing, of the carbon fibers, the piezoresistive material is evenly distributed throughout the composite. Sizing is typically applied to carbon fibers after carbonization to protect functionality, make fibers easy to handle, and help bonding with the matrix. Cleverly, the researchers piggy-back on this process to build-in strain-sensing capabilities to composites.
Carbon fiber composites embedded with TiO2 nanoparticles show an increase in resistance in relation to increased strain, which returns to zero as soon as the pressure is removed. The addition of TiO2 nanoparticles produces a significant increase in sensitivity to strain. The researchers also compared the mechanical properties of carbon fibers without TiO2 nanoparticles and with different concentrations from 0.1 wt% up to 4 wt%.
Shear strength measurements, which give an indication of the adhesion between fibers and the matrix, reveal an increase from 0.5 wt% to 1.5 wt% nanoparticles, with the best showing an increase of just under 15%. Higher proportions of nanoparticles, however, lead to a decrease in strength, which the researchers attribute to the agglomeration of particles. While in small quantities nanoparticles deflect cracks and lock fibers together, in larger amounts they can propagate cracks and reduce the overall strength of composites.
Simple three-point bending tests, meanwhile, which give a measure of the viscoelastic properties of composites, show improved damping behavior with 1 wt% nanoparticles. Higher damping values are desirable in composites for the aerospace and automotive sectors for improved vibration control, fatigue reduction, and crash-worthiness.