F. Avilés

Self-sensing of elastic strain, matrix yielding and plasticity in multiwall carbon nanotube/vinyl ester composites

The piezoresistive response of multiwalled carbon nanotube/vinyl ester composites containing 0.3, 0.5 and 1% w/w carbon nanotubes (CNTs) loaded in tension and compression is investigated. The change in electrical resistance (ΔR) under tension loading was positive and showed a linear relationship with the applied strain up to failure, with slightly increased sensitivity for decreased CNT content. In compression, a nonlinear and non-monotonic piezoresistive behavior was observed, with ΔR initially decreasing in the elastic regime, leveling off at the onset of yielding and increasing after matrix yielding. The piezoresistive response of the composite is more sensitive to the CNT content for compression than for tension, and the calculated gage factors are higher in the compressive plastic regime. The results show that the piezoresistive signal is dependent on the CNT concentration, loading type and material elastoplastic behavior, and that recording ΔR during mechanical loading can allow self-identification of the elastic and plastic regimes of the composite.

Influence of carbon nanotube on the piezoresistive behavior of multiwall carbon nanotube/polymer composites

The role of the physical properties of multiwall carbon nanotubes on the strain-sensing piezoresistive behavior of multiwall carbon nanotube/polymer composites is systematically studied using three types of multiwall carbon nanotubes as fillers of a brittle thermosetting (vinyl ester) and a tough thermoplastic (polypropylene) polymers under quasi-static tensile loading. Two of the three multiwall carbon nanotubes investigated have similar length, aspect ratio, structural ordering, and surface area, while the third group contains longer multiwall carbon nanotubes with higher structural ordering. The results indicate that longer multiwall carbon nanotubes with higher structural ordering yield higher piezoresistive sensitivity, and therefore are better suited as sensors of elastic and plastic strains of polymer composites. The highest gage factor achieved was approximately 24 and corresponded to the plastic zone of multiwall carbon nanotube/polypropylene composites with the longest nanotubes.

Experimental Study of Debonded Sandwich Panels under Compressive Loading

An experimental study of the in-plane compression failure of sandwich panels consisting of glass/epoxy face sheets over a range of PVC foam cores (H45, H100, and H200) and a balsa wood core containing one or two circular or square interfacial debonds is conducted. For the great majority of the specimens, failure occurred by local buckling of the debonded face sheet followed by rapid debond growth towards the panel edges, perpendicular to the applied load. Panels with the largest debond (10 cm diameter or 9 cm length) displayed some post-buckling strength. Examination of the face and core failure surfaces after total separation showed that the tendency for interface (core/resin) failure increases with increasing core density. It was found that the compression strength strongly decreases with decreasing core stiffness and increasing debond size. The compression strength of panels with H45 core decreased with reduced core thickness. Failure loads for panels with symmetric debonds at both face/core interfaces were practically the same as those for panels containing a single debond. Panels with square debonds failed at lower loads than those with circular debonds of the same area. Circular debonds of 50 mm diameter and square debonds of 45 mm length set the threshold for debond failure of the panels considered here.

Modeling of mesoscale dispersion effect on the piezoresistivity of carbon nanotube-polymer nanocomposites via 3D computational multiscale micromechanics methods

In uniaxial tension and compression experiments, carbon nanotube (CNT)-polymer nanocomposites have demonstrated exceptional mechanical and coupled electrostatic properties in the form of piezoresistivity. In order to better understand the correlation of the piezoresistive response with the CNT dispersion at the mesoscale, a 3D computational multiscale micromechanics model based on finite element analysis is constructed to predict the effective macroscale piezoresistive response of CNT/polymer nanocomposites. The key factors that may contribute to the overall piezoresistive response, i.e. the nanoscale electrical tunneling effect, the inherent CNT piezoresistivity and the CNT mesoscale network effect are incorporated in the model based on a 3D multiscale mechanical–electrostatic coupled code. The results not only explain how different nanoscale mechanisms influence the overall macroscale piezoresistive response through the mesoscale CNT network, but also give reason and provide bounds for the wide range of gauge factors found in the literature offering insight regarding how control of the mesoscale CNT networks can be used to tailor nanocomposite piezoresistive response.

Self-Sensing of Damage Progression in Unidirectional Multiscale Hierarchical Composites Subjected to Cyclic Tensile Loading

The electrical sensitivity of glass fiber/multiwall carbon nanotube/vinyl ester hierarchical composites containing a tailored electrically-percolated network to self-sense accumulation of structural damage when subjected to cyclic tensile loading-unloading is investigated. The hierarchical composites were designed to contain two architectures differentiated by the location of the multiwall carbon nanotubes (MWCNTs), viz. MWCNTs deposited on the fibers and MWCNTs dispersed within the matrix. The changes in electrical resistance of the hierarchical composites are associated to their structural damage and correlated to acoustic emissions. The results show that such tailored hierarchical composites are able to self-sense damage onset and accumulation upon tensile loading-unloading cycles by means of their electrical response, and that the electrical response depends on the MWCNT location.

Three-dimensional Finite Element Buckling Analysis of Debonded Sandwich Panels:

A three-dimensional (3D) finite element analysis has been conducted to examine the local buckling behavior of foam-cored composite sandwich panels containing a face-core debond. The influences of core stiffness and debond size and shape are examined. The core stiffness was found to have a profound influence on the local buckling load, especially for low modulus cores and smaller debonds. Panels with square debonds buckled at lower loads than those with circular debonds of the same dimension, and buckling loads for both circular and square debonds decreased with increased debond size. The results of the analysis were compared to local buckling loads determined experimentally. Reasonable agreement was found between the predicted and experimentally measured local buckling loads for both circular and square debonds.

First-order shear deformation analysis of the sandwich plate twist specimen

An analytical solution of the deflection of the sandwich twist specimen based on first-order shear deformation theory is presented. The solution utilizes a Fourier series approach for the loading and displacements. Parametric analysis of the influences of face and core stiffnesses, core thickness and panel in-plane size on the plate compliance is conducted. The results are compared to finite element analysis. The first-order shear deformation compliance results are in good agreement with finite element analysis for sandwich panels with soft, shear deformable cores, whereas for stiff cores the first-order shear deformation solution underpredicts the panel compliance. Reasons for the discrepancy and limits for the use of the analysis are discussed.

Face Sheet Buckling of Debonded Sandwich Panels Using a Two-Dimensional Elastic Foundation Approach

A two-dimensional elastic foundation model for analysis of the face sheet buckling behavior of sandwich panels with an embedded rectangular debond has been developed. The model is based on an energy method which assumes that the core behaves as an elastic foundation supporting the face sheets everywhere except over the debond. The face sheet buckling load is presented in a versatile form expressed into two buckling coefficients. Parametric studies are conducted to evaluate the influences of the foundation modulus, face sheet modulus, face sheet anisotropy, debond and panel dimensions, and aspect ratios. It is shown that the critical buckling load is very sensitive to the core modulus and flexural stiffness of the face and less sensitive to the debond size and face anisotropy. Model predictions are compared to experimental results obtained for sandwich panels consisting of glass/epoxy face sheets over a H100 PVC foam core with a range of square face/core debond sizes. Reasonable agreement with experimentally measured local buckling loads is observed for relatively small debonds. For large debonds the model tends to overpredict the buckling load.

Selective damage sensing in multiscale hierarchical composites by tailoring the location of carbon nanotubes

The selectivity in composite damage sensing using the electrical resistance approach is investigated by deliberately placing multiwall carbon nanotubes dispersed within the matrix or deposited onto the fiber surface. To this aim, unidirectional glass fiber/carbon nanotube/vinyl ester specimens with fibers oriented along (0°) and transverse (90°) to the loading direction are subjected to quasi-static tension up to failure. The electrical resistance changes in the composite are correlated to the mechanical strain and acoustic emission events. Using this approach, it is shown that the electrical signal is able to discern between fiber and matrix (or fiber/matrix interface) damage. The electrical resistance of composites with multiwall carbon nanotubes located within the matrix is capable of tracking matrix-dominated damage but is poorly sensitive to fiber breakage. In contrast, the composites with multiwall carbon nanotube–modified fibers exhibit outstanding sensitivity to fiber- and fiber/matrix interface damage.

Deposition of Carbon Nanotubes on Fibers

Abstract This chapter covers deposition and growth of carbon nanotubes (CNTs) on micron-size engineering fibers. The focus is on glass, carbon, and aramid fibers and deposition after synthesis, and the chapter touches on in situ growth and other nonengineering fibers, such as natural fibers. The chapter covers the fundamentals of the dipping and electrophoretic deposition methods, as well as a brief review of in situ growth and other methods for the deposition of CNTs. Applications of such multifunctional CNT-coated fibers for modifying the interface of fiber reinforced polymer composites and as noninvasive elements for structural health monitoring are also briefly discussed.