Exploiting carbon nanotube networks for damage assessment of fiber reinforced composites

An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.     

用交流阻抗法研究碳纤维混凝土导电性

随着水泥基材料的发展 ,人们对水泥基材料提出的更新的要求 ,其中水泥基导电复合材料的研究引起了广泛的兴趣。制备水泥基导电复合材料的方法是在水泥基材料中掺入各种导电组分 ,目前较常用的是掺入碳纤维 ,它不仅可以大幅度提高水泥基复合材料的电导率 ,还能够改善水泥基材料的力学性能、增加其韧性。本研究主要通过对碳纤维混凝土的交流阻抗谱进行分析 ,研究其导电性能与内部微结构的关系。     

Electric field effects on CNTs/vinyl ester suspensions and the resulting electrical and thermal composite properties

In this study, electrical conductivity of a vinyl ester based composite containing low content (0.05, 0.1 and 0.3 wt.%) of double and multi-walled carbon nanotubes with and without amine functional groups (DWCNTs, MWCNTs, DWCNT-NH₂ and MWCNT-NH₂) was investigated. The composite with pristine MWCNTs was found to exhibit the highest electrical conductivity. Experiments aimed to induce an aligned conductive network with application of an alternating current (AC) electric field during cure were carried out on the resin suspensions with MWCNTs. Formation of electric anisotropy within the composite was verified. Light microscopy (LM), scanning electron (SEM) and transmission electron microscopy (TEM) were conducted to visualize dispersion state and the extent of alignment of MWCNTs within the polymer cured with and without application of the electric field. To gain a better understanding of electric field induced effects, glass transition temperature (T_g) of the composites was measured via Differential Scanning Calorimetry (DSC). It was determined that at 0.05 wt.% loading rate of MWCNTs, the composites, cured with application of the AC electric field, possessed a higher T_g than the composites cured without application of the AC electric field.     

Electrostatic self-assembled carbon nanotube/nano carbon black composite fillers reinforced cement-based materials with multifunctionality

Electrostatic self-assembled carbon nanotube (CNT)/nano carbon black (NCB) composite fillers are added into cement mortar to fabricate smart cement-based materials. The grape bunch structure of CNT/NCB composite fillers is beneficial for dispersing CNT/NCB in cement mortar matrix and achieving cooperative improvement effect. The mechanical, electrically conductive, and piezoresistive behaviors of the cement mortar are investigated. The CNT/NCB composite fillers can effectively enhance the flexural strength and electrical conductivity of cement mortars, and endow stable and sensitive piezoresistivity to cement mortar at a low filler content. However, they weaken the compressive strength of cement mortar to some extent. The percolation threshold zone of cement mortar with CNT/NCB composite fillers ranges in the amount of 0.39–1.52 vol.%. The optimal content of CNT/NCB composite fillers is 2.40 vol.% for piezoresistivity and the stress and strain sensitivities can reach 2.69% MPa⁻¹ and 704, respectively.     

老化对水泥乳化沥青胶结料动态力学性能的影响

基于分数导数修正Burgers模型,建立了水泥乳化沥青胶结料（简称复合胶结料）的本构方程。结合不同材料配比的复合胶结料的老化试验及频率扫描试验,分析了老化时间及材料配比等因素对复合胶结料黏弹性力学参数的影响。研究表明,复合胶结料的存储模量与损耗模量均随老化时间的增加而增大,在老化时间为0~8 h时,存储模量及损耗模量增加较快,当老化时间超过8 h后,其增长趋势减缓。对于乳化沥青与水泥的质量比（m_A/m_C）为1.2和1.4的复合胶结料,在不同的加载频率下,其相位角均随老化时间的增加而减小。对于m_A/m_C为1.0的复合胶结料,其相位角随着老化时间的增加呈现先增加后减小的变化规律。随着老化时间的增加,复合胶结料趋向于弹性材料的力学性质。分数导数修正Burgers模型可以较好的描述老化后复合胶结料的黏弹性动态力学行为,模型参数弹性模量E₁、黏度η₁和分数导数r可以描述老化情况下材料黏弹特性的变化规律。     

Facile preparation of a polysiloxane-based hybrid composite with highly-oriented boron nitride nanosheets and an unmodified surface

A facile technique was developed to fabricate polysiloxane-based hybrid composite films containing boron nitride (BN) nanosheets using a nanopulse-width electric field. BN nanosheets assumed anisotrophic alignment under the electric field, without requiring surface coating with metallic nano particles despite the wide band gap. BN was dispersed by sonication in a pre-polymer polysiloxane mixture. The homogeneous suspension was cast on a glass spacer and subjected to either a DC electric field or a nanopulse-width electric field before the mixture was cured through polymerization. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that BN nanosheets in the polysiloxane matrix were aligned with high anisotropy to the electric field direction, which was perpendicular to the film plane. The transmittance of the film samples, measured by UV–visible spectrometry, indicated that the composite, prepared using a nanopulse-width electric field manifested a significantly improved transmittance, compared with composites prepared without using the electric field.     

Transient piezothermoelastic problem for a laminated composite strip composed of angle-ply and piezoelectric laminae

This paper is concerned with the theoretical treatment of transient piezothermoelastic problem which is developed for a laminated composite strip composed of angle-ply laminae and a piezoelectric material of crystal class mm2, subject to nonuniform heat supply in the width direction. We obtain the exact solution for the two-dimensional temperature change in a transient state and the transient piezothermoelastic response of a simple supported composite strip under the state of generalized plane deformation. As an example, numerical calculations are carried out for a angle-ply laminated composite strip made of an alumina fiber reinforced aluminum composite, associated with a piezoelectric layer of a cadmium selenide solid. Some numerical results for temperature change, displacement, stress and electric potential distributions in a transient state are shown in figures. Furthermore, the influence of the thickness of the piezoelectric layer on the thermal stress or electric field is investigated.     

Experimental validation of modal strain energies based damage identification method for a composite sandwich beam

A vibration-based damage identification method, based on changes in modal strain energies before and after occurrence of damage, is presented for a composite sandwich beam. Experiments were performed to obtain the natural frequencies and mode shapes for validation of the presented method. The observed changes in modal strain energies were used for the prediction of existence and location of damage in a composite sandwich beam. Subsequently, these changes were also used to predict damage extents in the two stages. In the first stage, the proposed method is used to approximate the damage extents in the face and core. These were updated to obtain more accurate values of damage extent at the second stage by using the model updating theory. Experimental and numerical results were presented to demonstrate and verify the effectiveness of this method for several single and multiple damage cases. These damage cases also include interactive damage modes. Results indicate that the proposed method is capable of identifying the location and extent of damage in the faces and core of a composite sandwich beam.     

Self-sensing of damage in carbon fiber polymer-matrix composite by measurement of the electrical resistance or potential away from the damaged region

Self-sensing of damage by measurement of the DC electrical resistance or potential away from the damaged region was demonstrated in quasi-isotropic continuous carbon fiber epoxy-matrix composite laminates under impact at energy up to 5 J. The through-thickness potential was substantial up to 240– 480 mm (at 0.25–99 mA correspondingly) in the longitudinal direction from the position of through-thickness current application, due to current spreading in the longitudinal direction. A model for the current spreading is provided. The fractional change in resistance resulting from damage decreased with increasing distance from the point of impact (diameter of indentation up to 3.5 mm and depth of indentation up to 0.16 mm), such that it was non-zero even at a distance of 150 mm from the point of impact. Both the through-thickness resistance and the oblique resistance were effective indicators. The ability for the resistance measured away from the damaged region to indicate damage in the damaged region is due to the much lower electrical resistivity in the longitudinal than through-thickness or oblique directions in the composite.     

铁电复合物PZT/P[VDF(77)-TrFE(23)]退极化场系数的电场与温度效应

用自建的适合厚片材料测量的介电测试系统研究了不同组分含量的ＰＺＴ／Ｐ（ＶＤＦ（７７）－ＴｒＦＥ（２３））０－３型铁电复合物厚片介电系数的电场和温度依赖性,在低电场和室温附近的温度范围,由Ｂｒｕｇｇｅｍａｎ方程预测的介电系数和实验值较为接近。Ｙａｍａｄａ模型通过退极化场系数Ｌｚ的变化,可拟合复合物介电系数的电场和温度依赖关系。结果显示Ｌｚ随电场的升高而降低,随温度的改变（升温、降温过程）而与复合物介     

双电层对水润滑特性影响的实验研究

双电层是固液界面处普遍存在的自然现象,双电层现象使得固液界面处的液体一方中存在过剩电荷,液体流动产生流动电流,液体中同时产生方向相反的传导电流,使带电离子反向流动,阻碍液体的流动,使得液体的等效粘度增加,产生电粘度效应．近年来,双电层效应对润滑的影响逐渐受到人们的重视．理论研究表明,薄膜润滑时双电层效应对摩擦系数和润滑膜厚有明显影响．     

Effects of local damage on vibration characteristics of composite pyramidal truss core sandwich structure

The effects of local damage on the natural frequencies and the corresponding vibration modes of composite pyramidal truss core sandwich structures are studied in the present paper. Hot press molding method is used to fabricate intact and damaged pyramidal truss core sandwich structures, and modal testing is carried out to obtain their natural frequencies. A FEM model is also constructed to investigate their vibration characteristics numerically. It is found that the calculated natural frequencies are in relatively good agreement with the measured results. By using the experimentally validated FEM model, a series of numerical analyses are conducted to further explore the effects of damage extent, damage location, damage form on the vibration characteristics of composite pyramidal truss core sandwich structures as well as the influence of boundary conditions. The conclusion derived from this study is expected to be useful for analyzing practical problems related to structural health monitoring of composite lattice sandwich structures.     

碳纤维水泥基材料的机敏特性研究

水泥基材料是一种高电阻率的惰性材料,通过掺入一定体积含量(0.2 vol%~1.2 vol%)的短切碳纤维,可显著提高其电导率。本文作者研究了碳纤维水泥基材料表观电导特性与其内部微观结构的关系,探讨了电阻率的变化与材料所受外部荷载的关联性。结果表明碳纤维水泥基材料具有实时诊断内部损伤的机敏性:当水泥基材料内部裂纹产生或扩展时,表现为材料电阻率上升;而当水泥基材料内部裂纹闭合时,其电阻率下降。碳纤维水泥基材料的电阻变化与所受荷载呈良好的线形关系;而不含碳纤维的普通水泥基材料在整个受荷过程中,其电导特性则无明显变化。碳纤维水泥基材料电导特性的变化反映了水泥基材料内部损伤状况丰富的信息,根据这一特性可以及时预报水泥基材料内部潜在的损伤状况,有效地防止灾难性的破坏。     

Damage assessment in plate-like structures using a two-stage method based on modal strain energy change and Jaya algorithm

The paper presents an effective two-stage approach based on modal strain energy change and Jaya algorithm for damage assessment in plate-like structures. In the first stage, a newly developed damage indicator, named as normalized modal strain energy-based damage index (nMSEBI), is proposed to help locate potential damage elements more effectively. After eliminating most of the healthy elements, the actual damaged sites and its extent in plate structure are determined in the second stage by minimizing an objective function, which is solved using Jaya algorithm. For finding a suitable objective function used in optimization process, two different objective functions are considered to examine their effects on the performance of the utilized optimization algorithm. The efficiency and accuracy of the proposed two-stage damage detection method are investigated by two numerical examples comprising a concrete plate and a four-layer (0°/90°/90°/0°) laminated composite plate with multiple damage locations. All of the obtained results indicate that even under measurement noise, the proposed method can identify the actual damage sites and estimate the extent of damage with high precision. In addition, the numerical results also show that the computational cost of the optimization process using the objective function based on modal flexibility change is much lower than that using the objective function based on mode shapes change.     

Three-dimensional analysis for rectangular 1–3 piezoelectric fiber-reinforced composite laminates with the interdigitated: electrodes under electromechanical loadings

In this paper, an exact analysis for the rectangular composite laminated plate consisting of 1–3 piezoelectric fiber-reinforced composite layer and orthotropic elastic composite layer subjected to the electric field with the interdigitated electrode (IDE) and arbitrary loads is presented without any simplification. The solution of the derived governing differential equations is obtained through the power series expansion and Fourier expansion methods. An illustrative analysis is carried out to investigate the influence of the stiffness anisotropy, free-strain anisotropy and electric field on the shear curvature of the laminated plate with 1–3 active fiber-reinforced composite layer. The numerical results show that, the magnitude of shear curvature gradually increases as the stiffness anisotropy and the free-strain anisotropy increase, and increasing the electrical field also leads to an increase in shear curvature. Results presented here can be used to assess various approximate theories and enhance the understanding of the static and dynamic response behavior of the 1–3 piezoelectric composite structures.     

Lightning Strike Ablation Damage Influence Factors Analysis of Carbon Fiber/Epoxy Composite Based on Coupled Electrical-Thermal Simulation

According to the mathematical analysis model constructed on the basis of energy-balance relationship in lightning strike, and accompany with the simplified calculation strategy of composite resin pyrolysis degree dependent electrical conductivity, an effective three dimensional thermal-electrical coupling analysis finite element model of composite laminate suffered from lightning current was established based on ABAQUS, to elucidate the effects of lighting current waveform parameters and thermal/electrical properties of composite laminate on the extent of ablation damage. Simulated predictions agree well with the composite lightning strike directed effect experimental data, illustrating the potential accuracy of the constructed model. The analytical results revealed that extent of composite lightning strike ablation damage can be characterized by action integral validly, there exist remarkable power function relationships between action integral and visual damage area, projected damage area, maximum damage depth and damage volume of ablation damage, and enhancing the electrical conductivity and specific heat of composite, ablation damage will be descended obviously, power function relationships also exist between electrical conductivity, specific heat and ablation damage, however, the impact of thermal conductivity on the extent of ablation damage is not notable. The conclusions obtained provide some guidance for composite anti-lightning strike structure-function integration design.     

Enhanced crack detection sensitivity of carbon fiber composites by carbon nanotubes directly grown on carbon fibers

The ability of highly conductive hybrid carbon–fiber/carbon nanotube loaded epoxy composites to sense matrix cracking damage in situ is demonstrated. Multi-walled carbon-nanotubes (MWCNTs) are grown perpendicular to and on the surface of a woven carbon–fiber fabric using a chemical vapor deposition process. An increase in sensitivity of resistance change under interlaminar fracture is shown through a series of double cantilever beam (DCB) tests on samples prepared with MWCNTs grown on both sides of carbon–fiber fabric lamina placed at the top and bottom surfaces of an 8-layer test panel whereas samples with MWCNTs inside the samples did not show much increase in sensitivity of resistance change compared with the baseline samples without MWCNTs. The results suggest that the addition of surface positioned hierarchical carbon-nanotube lamina on composite structures has the potential for autonomic sensing of internal matrix damage.     

Residual strength of composite laminates containing scarfed and straight-sided holes

This investigation is motivated by the needs to quantify the load-carrying capacity of composite laminates with scarfed holes, a damage cut-out shape employed to achieve flush repairs of composites. Both experimental testing and analytical modelling were carried out to investigate the damage progression behaviour of composite laminates containing either straight-sided or scarfed holes. Hoop strains were recorded by strain gauges located along the scarf surface and the results indicate a much greater extent of damage progression than specimens containing straight-sided holes. Three different strength-prediction models were employed to quantify the residual strength, including an analytical cohesive zone model developed in this work, an analytical inherent-flaw fracture mechanics method and a finite element-based continuum damage model. Comparisons of the experimental results with the model predictions reveal that the continuum damage model, calibrated using data from coupons with straight-sided holes, provides promising correlation with experimental results.     

电场对PZT/复合材料层合板破坏强度影响的实验研究

采用三点弯曲方法测量在不同外加电场作用下PZT复合材料层合板的破坏强度(MOR),分析得到了正电场能增强PZT复合材料层合板的破坏强度,而负电场降低了PZT复合材料层合板的破坏强度。本文作者还建立了一套AE测试系统来实时探测在加载过程中由于损伤引起的声信号,进而分析并证实了不同载荷下的PZT复合材料层合板的破坏模式,并用超声扫描成像图加以验证。      

Detection of impact damage in CFRP laminates by means of electrical potential techniques

Quasi-isotropic samples of carbon fibre reinforced polymer composite 2 mm thick were manufactured and instrumented with an array of electrodes. The electrodes were used to introduce a DC current and to measure the resultant potential distribution on the top and bottom surfaces of the laminate. Instrumented samples were impacted at energies between 2 and 12 J, and the potential fields measured before and after the impact. The impact damage caused significant changes in local values of potential. Indications of the extent and severity of the impact damage could be best obtained by representing the data as contours of equi-potential change. There was excellent correlation between regions of potential change and impact damage as measured by ultrasonic C scan. The results of finite element calculations of potential change distribution on the surface of current carrying quasi-isotropic laminates containing delaminations of a range of sizes and locations were qualitatively similar to those obtained experimentally, but differed in detail. Results are discussed in terms of the differences between real impact damage and that which was simulated, and the likely performance of a damage detection system based on this principle.