Simultaneous strain and temperature measurement of advanced 3-D braided composite materials using an improved EFPI/FBG system

Simultaneous strain and temperature measurement for advanced 3-D braided composite materials using fibre-optic sensor technology is demonstrated, for the first time. These advanced 3-D braided composites can virtually eliminate the most serious problem of delamination for conventional composites. A tandem in-fibre Bragg-grating (FBG)/extrinsic Fabry–Perot interferometric sensor (EFPI) system with improved accuracy has been used to facilitate simultaneous temperature and strain measurement in this work. The non-symmetric distortion of the optical spectrum of the FBG, due to the combination of the FBG and the EFPI, is observed for the first time. Experimental and theoretical studies indicate that this type of distortion can affect the measurement accuracy seriously and it is mainly caused by the modulation of the periodic output of the EFPI. A simple method has been demonstrated to improve the accuracy for detection of the wavelength-shift of the FBG induced by temperature change. A strain accuracy of ±20μ and a temperature accuracy of ±1°C have been achieved, which can meet the requirements for practical applications of 3-D braided composites.     

Three-dimensional textile structural composites under high strain rate compression: Z-transform and discrete frequency-domain analysis

Z-transform theory was applied to several three-dimensional (3D) textile structural composites, including an angle-interlock woven composite, a multilayer multi-axial warp knitted composite and a 4-step braided composite, to characterize their system dynamic behaviour in the frequency domain. More specifically, the analysis focused on the relationship between the compressive load and the system response under static (strain rate 0.001?s^?1) and impulsive (strain rate up to 2700?s^?1) strain along both the in-plane and out-of-plane directions, respectively. The high strain rate compressions were tested using a split Hopkinson pressure bar apparatus, and the input and output (the stress–strain curve) of the test specimen was obtained by recording the signals using a computer for further analysis. Z-transform was then used to analyze the dynamic response and stability of the composites of different preform structures and at various loading conditions. This is the first such attempt to study the compression behaviour of 3D textile structural composites at various strain rates in the frequency domain in order to reveal their mechanical behaviour and features of the materials from a new perspective.     

Acoustic radiation from a fluid-filled, subsurface vascular tube with internal turbulent flow due to a constriction

The vibration of a thin-walled cylindrical, compliant viscoelastic tube with internal turbulent flow due to an axisymmetric constriction is studied theoretically and experimentally. Vibration of the tube is considered with internal fluid coupling only, and with coupling to internal-flowing fluid and external stagnant fluid or external tissue-like viscoelastic material. The theoretical analysis includes the adaptation of a model for turbulence in the internal fluid and its vibratory excitation of and interaction with the tube wall and surrounding viscoelastic medium. Analytical predictions are compared with experimental measurements conducted on a flow model system using laser Doppler vibrometry to measure tube vibration and the vibration of the surrounding viscoelastic medium. Fluid pressure within the tube was measured with miniature hydrophones. Discrepancies between theory and experiment, as well as the coupled nature of the fluid-structure interaction, are described. This study is relevant to and may lead to further insight into the patency and mechanisms of vascular failure, as well as diagnostic techniques utilizing noninvasive acoustic measurements.     

Microstrain in hydroxyapatite carbon nanotube composites

Synchrotron radiation diffraction data were collected from hydroxyapatite–carbon nanotube bioceramic composites to determine the crystallite size and to measure changes in non‐uniform strain. Estimates of crystallite size and strain were determined by line‐profile fitting of discrete peaks and these were compared with a Rietveld whole‐pattern analysis. Overall the two analysis methods produced very similar numbers. In the commercial hydroxyapatite material, one reflection in particular, (0 2 3), has higher crystallite size and lower strain values in comparison with laboratory‐synthesized material. This could indicate preferential crystal growth in the [0 2 3] direction in the commercial material. From the measured strains in the pure material and the composite, there was a degree of bonding between the matrix and strengthening fibres. However, increasing the amount of carbon nanotubes in the composite has increased the strain in the material, which is undesirable for biomedical implant applications.     

Role of graphene waviness on the thermal conductivity of graphene composites

The waviness effect of graphene nanoplates (GNPs) on the thermal conductivity of GNP-based composites is investigated. Two types of wrinkled GNPs (w-GNPs) and flat GNPs (f-GNPs) are used to fabricate GNP/epoxy composites. Thermal conductivity enhancement is observed in both types of composites. However, under the same processing, f-GNPs exhibit a higher thermal conductivity enhancement than w-GNPs. We finally introduce a concept, the waviness factor, to theoretically analyze the thermal conductivity considering the waviness effect of GNPs. The theoretical predictions are found to show good agreement with experimental observations.     

Size effect in plastically deformed passivated thin films

The flow theory of mechanism-based strain gradient plasticity theory (MSG) developed by Qiu et al. (2003) is extended for incompressible material. The MSG flow theory is used to predict the increase of plastic work hardening for plane strain tension of surface-passivated Cu thin film. The theoretical predictions agree well with experiments for suitably chosen material parameters. Contributed by HWANG Keh-Chih     

Laboratory-scale measurement of scattering from modelled Arctic ice ridges

An experiment is performed to measure acoustic scattering from scale modelled ice ridges in both specular (forward) and non-specular (backward) directions, for comparison with predictions from theoretical models. The experiment uses a 100 kHz transmitter emitting sinusoidal bursts. An array of miniature transducers is used to measure the scattered field as a function of scattering angle. Experimental results are obtained for scattering from different types of rods simulating ice ridges, and also the reflection coefficient from an acrylic block. The results show good agreement with the Twersky model predictions. This experiment establishes an effective technique, using scale models in the laboratory, to compare theoretical predictions and field experimental data.     

Damage and fracture: Classical continuum theories

This paper reports the research results on the continuum theory of damage which goes back to the works of Kachanov, Gurson, and Rabotnov. In these models, internal variables that generally have different mathematical structure are explicitly introduced to constitutive relations. The internal variables describe a non-oriented (using scalar damage parameters) or oriented (using different-order tensors) damage distribution in the material. Then, a fracture criterion is introduced based on mechanical or thermodynamic considerations. Models of this type are still most frequently used in the structural analysis of strength of some materials (e.g., composites). Since damage nucleation and growth are closely related to strain localization, consideration is given to formulations and methods for analyzing the stability of inelastic deformation processes. Much attention is given to the effect of the finite element mesh on simulation results, to solution algorithms for such problems, and to the possibilities of using non-local constitutive models. The studies that use gradient models are also included, because damage formation is associated with sharp spatial variations of kinematic and/or dynamic characteristics which must be described by non-classical constitutive relations (gradient, non-local, micromorphic continuum).     

Visualizing the strain field in semiflexible polymer networks: strain fluctuations and nonlinear rheology of F-actin gels

We image semiflexible polymer networks under shear at the micrometer scale. By tracking embedded probe particles, we determine the local strain field, and directly measure its uniformity, or degree of affineness, on scales of 2-100 microm. The degree of nonaffine strain depends upon the polymer length and cross-link density, consistent with theoretical predictions. We also find a direct correspondence between the uniformity of the microscale strain and the nonlinear elasticity of the networks in the bulk.     

Strain mapping analysis of textile composites

The focus of the work is meso-scale analysis (scale level of the fabric unit cell) of textile composite deformation and failure. The surface strain measurement is used for: (1) experimental investigation, which includes study of strain distribution at various stages of deformation, plasticity detection, damage initiation; (2) numerical validation of the correspondent finite element (FE) models. Two examples are considered: carbon-epoxy triaxial-braided and glass polypropylene-woven composite. The surface strain measurement (by digital image correlation technique) accompanies the tensile tests, aiming at: (1) elastic anisotropic constants characterisation, (2) study of non-linear material behaviour (for the thermoplastic composite), (3) control of homogeneity of the macro-strain distribution, and (4) analysis of damage initiation in brittle composites. Validation of meso-FE models by strain measurements encounters difficulties arising from (1) resolution of the strain measurements, (2) irregularities of the initial structure such as random layer nesting, ply interaction, and deviation of yarns from their theoretical position, which affects the measured strain fields. The paper discusses these difficulties and demonstrates a qualitative agreement with the FE analysis of idealised composite configurations.     

EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE LINEAR AND NON-LINEAR DYNAMIC BEHAVIOUR OF A GLARE 3 HYBRID COMPOSITE PANEL

A theoretical model based on Hamilton's principle and spectral analysis is used to study the non-linear free vibration of hybrid composite plates made of Glare 3, a new aircraft structural material. It consists of alternating layers of metal- and fibre-reinforced composites. In previous work, the theoretical model has been used to calculate the first non-linear mode of fully clamped rectangular composite fibre-reinforced plastic (CFRP) laminated plates. This study concerns determination of the linear dynamic properties of the Glare 3 hybrid composite rectangular panel (G3HCRP) such as natural frequencies and mode shapes. The theoretical model is used to calculate the fundamental non-linear mode shape and associated flexural behaviour of the fully clamped G3HCRP. A series of experimental investigations have been conducted using a G3HCRP in order to determine linear dynamic properties. The response due to random excitation was investigated and the experimental measurements are analyzed and discussed. Comparisons are made with finite element predictions and response estimates given by the ESDU method, the latter being a “design guide” approach used by industry. Concerning the non-linear analysis, the results are given for various plate aspect ratios and vibration amplitudes, showing a higher increase of the induced bending stress near the clamps at large deflections. Comparisons between the dynamic behaviour of an isotropic plate and G3HCRP at large vibration amplitudes are presented and good results are obtained.     

Uncertainty quantification via random domain decomposition and probabilistic collocation on sparse grids

Quantitative predictions of the behavior of many deterministic systems are uncertain due to ubiquitous heterogeneity and insufficient characterization by data. We present a computational approach to quantify predictive uncertainty in complex phenomena, which is modeled by (partial) differential equations with uncertain parameters exhibiting multi-scale variability. The approach is motivated by flow in random composites whose internal architecture (spatial arrangement of constitutive materials) and spatial variability of properties of each material are both uncertain. The proposed two-scale framework combines a random domain decomposition (RDD) and a probabilistic collocation method (PCM) on sparse grids to quantify these two sources of uncertainty, respectively. The use of sparse grid points significantly reduces the overall computational cost, especially for random processes with small correlation lengths. A series of one-, two-, and three-dimensional computational examples demonstrate that the combined RDD–PCM approach yields efficient, robust and non-intrusive approximations for the statistics of diffusion in random composites.     

A microstructure model for finite-element simulation of 3D rectangular braided composite under ballistic penetration

The non-delamination feature of 3D braided composites under transverse impact leads to their potential application in the field of ballistic impact protection. One of the effective ways to investigate the ballistic impact damage of the 3D braided composite is to simulate the penetration process by numerical method, such as finite element method. However the numerical simulations of ballistic impact damage are seldom conducted based on the microstructure level. This paper presents a microstructure model for simulating ballistic impact damage of 4-step 3D braided rectangular composite penetrated by a rigid steel projectile. The microstructure model is based on the same yarn spatial configuration with that of the braided composite and also on the assumptions of the braided yarns appear straight inside the braided preform, bending and then change to other directions only at the surface. The ballistic perforation of the braided composite specimen by a cylindrical-conically steel projectile has been simulated with finite element method. The comparisons between FEA and experimental results show the validity of the microstructure model, especially for the penetration resistance and impact damage of the composite. Compared with the other continuum models of the braided composite, the microstructure model can simulate impact damage more precisely. The velocity history and acceleration history of projectile, and impact damage development of the composite in FEM simulation indicate the different damage and energy absorption mechanisms of the braided composite compared with those of laminated composite.     

Stress analysis of fibrous composites using moiré interferometry

A sensitive moiré fringe interferometer has been developed for measuring in-plane strains in engineering materials and structures. The specific advantage of this system is that it does not have to be used in an environment isolated from external vibration. As a result, it is possible to record deformation contours with a sensitivity and spatial resolution sufficient for the accurate measurement of strains even in areas of high strain gradient. The technique has been used to investigate the behaviour of a carbon fibre reinforced plastic material under fatigue loading. The changes in strain distribution have been measured as a function of the number of cycles undergone, and have been compared with theoretical predictions of the strain distribution around a hole in an orthotropic plate.     

Braided Covariance of the Braided Differential Bialgebras Under Quantized Braided Groups

The braided differential bialgebras on braided matrix algebras (with bothmultiplicative and additive coproducts) and on quantum hyperplanes (withadditive coproduct) are proven to be covariant under the braided coactions ofthe quantized braided groups, which contain the usual quantum group-covarianceas a special case. This means that the above braided differential bialgebras havemore and richer symmetries. It is also shown that the braided matrix algebraitself and the related braided differential algebra constitute two braided rings withthe two above-mentioned coproducts.     

Mechanical performance of Buckminster fullerene-reinforced composite with interface defects using finite element method through homogenization techniques

Several times, preferred properties are not accomplished when the fiber-reinforced composites are tested after they are prepared and the similar observations are also observed in the case of nanoparticle-reinforced composites. While the composites are manufactured under controlled conditions with stringent quality measures, it is complex to circumvent fiber-matrix interfacial debonding. The amount of debonding decides the final mechanical properties of composite material. In this study, an effort is made to quantify the debonding effect of nanoparticles on mechanical properties of nanoparticle-filled composites using finite element models with homogenization approach. Buckminster fullerene and conventional fiber T300 are selected as reinforcement medium. The predictions revealed that the longitudinal Young’s modulus is not affected by debonding.     

Analysis and monitoring of the stressed state in solids by gas sensors

Gas release from solids under mechanical stressing of specimens up to failure was studied. The amount of hydrogen being released vs. strain developed in a specimen under step-by-step loading was measured. The hydrogen evolved in strained metals (iron, nickel, copper, aluminum, etc.) and some epoxy-resin-based composites was detected by chemical sensors in a gas atmosphere at normal atmospheric pressure. The amount of hydrogen was shown to grow with the amount of plastic strain and peak upon failure. A method for strain analysis in solids was worked out. It was applied to analyze the stressed state of the rotor material of a centrifuge during its operation.     

Mechanical properties of flat braided composite with flexible interphase

Textile composites have been used extensively as industrial materials because of the excellent mechanical properties resulting from the continuously oriented fiber bundle. In a study of the mechanical properties, it is important to consider the fiber/matrix interface property as for other composite materials. In a recent study, the fiber/matrix interface is regarded as an interphase that has its own material constants and thickness; consequently, the mechanical properties of a composite can be controlled by specifically designing the interphase. In this study, we applied this concept to braided composites with flexible resin as interphase for the purpose of designing the interphase. In a static tensile test, though there were no improvements in Noncut specimens (normal braided composites), but a Cut specimen (each side of the Noncut specimen was cut) with flexible interphase was improved in fracture load and displacement. The observation of the specimen edge was carried out and it was confirmed that the progress of debonding at the fiber bundle intersection was interrupted by a flexible interphase, and a matrix crack did not occur in the Cut specimen with flexible interphase. In a fiber bundle pull-out test, it was confirmed that debonding progressed not into the fiber/resin interface but into the flexible interphase in the specimen with flexible interphase, and the interfacial property at the fiber bundle intersection was improved.     

Bulk strain energy density in randomly reinforced polymer composites with antifriction dispersed additives

Bulk strain energy density was numerically simulated for epoxy-phenol-based composites randomly reinforced with short polyimide fibers, with antifriction dispersed polytetrafluoroethylene (PTFE) additives. A mathematical model was constructed using the notion of a stress concentration operator (fourth-rank tensor) that relates volume averaged, or external, stresses within a heterogeneous material with their local values within an individual heterogeneity. The simulation was based on a generalized singular approximation of random field theory used to solve a stochastic differential equation of equilibrium of an elastic medium. This approximation yields an explicit expression for stress concentration in a composite material. The explicit expression allows one to analyze the distribution of bulk strain energy density depending on the composition, structure, volume and mass fraction of heterogeneities, and on the type and value of applied load. We studied how the considered energy characteristic depends on the type of external mechanical loading and concentration of isotropic components in the model composites. It is shown that with the increasing concentration of polyimide fibers at a fixed concentration of PTFE inclusions, the bulk strain energy density values of all components decrease and approach each other independently of the type of external loading. The form of these dependences is nonlinear. A change in the mass fraction of dispersed PTFE inclusions in the model composites exerts little effect on local energy values of all components under any of the considered applied external loads.     

基于光纤光栅绝对测量技术的高耐久智能钢拉杆

针对目前钢拉杆构件缺少施工荷载下的内力测试和服役期间的长期监测手段问题,采用玻璃钢封装技术研制开发出一种新型的基于光纤光栅绝对测量传感技术的高耐久性智能钢拉杆。在理论屈服荷载85%的拉伸荷载下,智能钢拉杆感知的应变具有很好的线性度和重复性,由其得到的测试杆力与实验张拉力吻合良好,误差在4%以内。该智能钢拉杆具有抗电磁干扰、传感距离长、成本低、耐腐蚀、绝对测量等优点,既可以方便给出任何阶段的受力状态,也可以作为一个大应变式荷载传感器对其相邻的结构进行荷载监测,适合用于恶劣服役环境下的钢拉杆工程结构。