Alexander E. Bogdanovich

Electrical Conductivity Study of Carbon Nanotube Yarns, 3-D Hybrid Braids and their Composites

Long continuous yarns consisting solely of carbon nanotubes may be the future of specialty composites requiring unique multi-functional properties. Many of such yarns were incorporated in a hybrid composite here, to demonstrate for the first time, their effect on increasing the electrical conductivity of an otherwise insulating composite. Six-ply nanotube yarns produced by University of Texas at Dallas were used as a raw material in this study. Thirty-six ends of such yarn were utilized in a 3-D braiding process along with nine axial bundles of glass fibers. The experimental study of the electrical conductivity of the produced nanotube yarns, 3-D braids and composites made thereof is described; the results for different tested materials are mutually compared and discussed. Some non-trivial effects attributed to the complex multi-level hierarchy and nano-scale building blocks of the studied materials are revealed. Special attention is paid to a proper interpretation of the obtained experimental results, because the tested materials represent complex discrete networks of numerous electrically conductive elements.

Material-smart analysis of textile-reinforced structures

Abstract A methodology of the material-smart structural analysis proposed in this paper links micromechanical modelling of a complex fibrous reinforcement to a three-dimensional analysis of a structural part. The geometry of the reinforcing textile varies throughout the entire volume of a part, thus its stiffness characteristics are dependent on the spatial coordinates. The methodology addresses this by determining elastic properties of the textile composite in a local region (a ‘meso-volume’) through a processing science model. The ‘global’ structural analysis incorporates a variational formulation of the three-dimensional elasticity problem for an inhomogeneous anisotropic body, a division of the body into meso-volumes having individual elastic properties, further partition of each meso-volume into sub-elements, and application of special deficient spline functions for a local approximation of displacements with respect to all three coordinates. All the necessary external and internal boundary conditions can be satisfied with any prescribed accuracy in the proposed numerical procedure. Several numerical examples for woven and braided composite plates are considered to illustrate the applicability of the methodology and the calculational procedure.

Three-dimensional analysis of anisotropic spatially reinforced structures

Abstract The material-adaptive three-dimensional analysis of inhomogeneous structures based on the mesovolume concept and application of deficient spline functions for displacement approximations is proposed. The general methodology is demonstrated on the example of a brick-type mosaic parallelepiped arbitrarily composed of anisotropic mesovolumes. A partition of each mesovolume into sub-elements, application of deficient spline functions for a local approximation of displacements and, finally, the use of the variational principle allows one to obtain displacements, strains and stresses at any point within the structural part. All of the necessary external and internal boundary conditions (including the conditions of continuity of transverse stresses at interfaces between adjacent mesovolumes) can be satisfied with requisite accuracy by increasing the density of the sub-element mesh. The application of the methodology to textile composite materials is described. Several numerical examples for woven and braided rectangular composite plates and stiffened panels under transverse bending are considered. Some typical effects of stress concentrations due to the material inhomogeneities are demonstrated.

Stochastic theory of composite materials with random waviness of the reinforcements

Abstract A general stochastic theory of the elastic properties of composite materials with continuous randomly curved spatial reinforcement is developed. The theory of random functions is utilized to evaluate the probabilistic characteristics of the local waviness of the reinforcement. A probabilistic extension of the orientation averaging model is developed to evaluate the elastic response of composites with multidirectional reinforcement having stochastic waviness. One fundamental advantage of the developed theory, compared to existing analytical approaches, is that an exact description of the reinforcement waviness is not required for predicting elastic properties. The only essential characteristics used as input data are the mean reinforcement paths and standard deviation of the local tangent, which is a random value characterizing the reinforcement path deflection from the “perfect” one. It is shown that existing approaches for evaluating elastic response of the composite with imperfect continuous fiber reinforcement can be obtained from the developed theory as particular cases. The theory is illustrated with examples of a unidirectional composite and a helically wound composite with randomly curved reinforcements. Numerical examples show that even small local waviness of the reinforcement paths may significantly affect the elastic response of composites considered.

Numerical Analysis of Impact Deformation and Failure in Composite Plates

The analytical problem of transverse impact by a rigid body on a rectan gular composite plate is considered. The calculation of displacements, deformations and stresses in conjunction with the characteristics of impact contact interaction is provided. The analysis of transverse stresses arising under short-time impulse in graphite/epoxy and organic glass/polymeric adhesive laminated plates is performed. The results illustrating dependencies of a contact force on time are given for various levels of mass and velocity of the impactor at the prescribed impact energy. The analysis of impact damage zones in a three-ply and a five-ply graphite/epoxy laminate (particularly, having soft polymeric in terleaves) is presented for the case of 2-2.5 J impact energy and different mass and veloc ity combinations. The changing of the failure mode increasing the impactor velocity (decreasing the mass) is followed up.

Initial and progressive failure analysis of laminated composite structures under dynamic loading

Abstract The problem of theoretical prediction of the initial failure and ply-by-ply failure processes in laminated composite structures under dynamic loading is under consideration. A history of deformation can be predicted at any point of a structure using the proposed analytical techniques. The phenometological. second-order tensor-polynomial and maximum stress failure criteria are used to calculate the lower bound of an applied dynamic load. This lower bound corresponds to a start of failure in a structural part. A ply-by-ply failure model is then developed. Using the model, some higher bound for a critical dynamic load impulse value, corresponding to the total exhaustion of a load-bearing capacity by all of the layers in a laminated structure can be predicted. The analysis is applied to thin-walled imperfect laminated graphite/epoxy cylindrical shells, loaded with a short-time impulse of axial compression or external pressure. A general approach to the 3D dynamic deformation analysis of a brick-type mosaic plate and its interaction with a rigid impactor is proposed The approach allows one to model both the initial and damage induced inhomogeneities in a composite structure under dynamic impulsive or impact loading cases.

Aligned carbon nanotube sheet piezoresistive strain sensors

Carbon nanotubes (CNTs) have a unique set of properties that may be useful in the production of next generation structural health monitoring composites. This research introduces a novel CNT based material system for strain and damage sensing applications. An aligned sheet of interconnected CNTs was drawn from a chemical vapor deposition grown CNT array and then bonded to the surface of glass fiber/epoxy composite coupons. Various types of mechanical tests were conducted, accompanied by real-time electrical data acquisition, in order to evaluate the electro-mechanical behavior of the developed sensing material. Specimens were loaded in the longitudinal and transverse CNT sheet orientations to investigate the anisotropy of the piezoresistive effect. The CNT sheets exhibited good sensing stability, linearity, sensitivity and repeatability within a practical strain range; which are crucial sensor features for health monitoring. It was also demonstrated that the CNT orientation in the sheet had a dramatic effect on the sensitivity, thus validating the usefulness of this sensing material for directional strain/damage monitoring. Finally, pre-straining of the CNT sheet sensors was conducted to further enhance the linearity of electro-mechanical response and long-term stability of the sensors during cyclic loading.

Quasi-static and fatigue tensile behavior of a 3D rotary braided carbon/epoxy composite

This paper presents an experimental study of the quasi-static and fatigue behaviour of a three-dimensional braided carbon/epoxy composite. The study involves a three-dimensional braided carbon preform and composite samples produced at 3TEX Inc. on their 576-carrier rotary braiding machine. The first part of the paper describes the preform and composite sample fabrication procedures, the fiber volume fraction determination and the porosity evaluation using micro-computed tomography three-dimensional observation. The second part is devoted to experimental study of the quasi-static tensile response of the material, including the acoustic emission monitoring and the microscopic damage detection. The third part is dedicated to the fatigue tensile–tensile behaviour illustrated by the fatigue life curve and the micro-computed tomography images of the damage imparted after different number of cycles. The fourth part presents results of the quasi-static tensile tests of preliminarily cyclically loaded specimens. These results provide significant insight into the influence of the damage imparted during fatigue loading on the subsequent quasi-static behaviour.

High Through-Thickness Thermal Conductivity Composites Based on Three-Dimensional Woven Fiber Architectures

Composites based on laminates of uniaxial or biaxial fiber reinforcements exhibit low through-thickness thermal conductivity, due to low matrix thermal conductivity and the number of interfaces in the thermal path. For applications near heat-generating components, the use of laminate composites in the surrounding structure can be limited by this inability to transport heat through the thickness. Three-dimensional orthogonal weaving provides an efficient, cost-effective method of placing high thermal conductivity yarns in the through-thickness Z direction of a preform to yield structural composites with high through-thickness thermal conductivity and with in-plane strength or stiffness comparable to laminates. Three basic research efforts investigated the efficacy of this approach: 1) weaving trials to determine the ability to three-dimensional orthogonal weave pitch carbon fibers and plied copper wires in the Z direction of the preforms, 2) through-thickness thermal conductivity testing of composites based on three-dimensional preforms, and 3) thermal modeling to describe several of the phenomena observed during thermal conductivity tests. During the testing, composites based on the three-dimensional orthogonal preforms with Z fiber volume fractions of only 5.5% measured a 12-fold increase in through-thickness thermal conductivity over a laminate composite, 8.4 versus 0.7 W/m K.

Applications of a meso-volume-based analysis for textile composite structures

Abstract The application of structural analysis to textile composites is examined in this paper. The particular requirements when modeling these unique materials are introduced and discussed. Techniques for quantifying the material inhomogeneities through three-dimensional geometric modeling are introduced, and methods of translating them into elastic properties are presented. Some basic ideas on the application of spline functions to the stress-field analysis in textile composites are proposed. Analytical techniques based on the idea of a meso-volume are discussed, and the particular continuity equations required to relate the materials to the analysis are presented. An example is shown to demonstrate the application of this method in the particular case of cylindrical bending of a textile composite plate.