An experimental study on the effect of failure trigger mechanisms on the energy absorption capability of CFRP tubes under axial compression

The energy absorption characteristics of graphite/epoxy tubes of circular cross sections, subjected to quasi-static axial compression, were experimentally investigated. Tubes with chamfered-ends, inward-folding or outward-splaying crush-caps, or combined (chamfered-end and crush-cap) failure trigger mechanisms, were investigated to identify the optimal configuration that would result in the lowest initial peak load while providing the highest possible specific energy absorption (SEA). The chamfer failure trigger proved to be the most effective at lowering the initial peak load while yielding a high SEA. The inward-folding crush-caps were more effective than the outward-splaying crush-caps in terms of decreasing the initial peak load and increasing the SEA. These results were significantly affected by the corner radii of the crush-caps: the smaller the radius the higher the initial peak load and the SEA. It was determined that combining a chamfered tube with an inward-folding crush-cap yielded the lowest initial peak load and the highest SEA.     

风电叶片复合材料拉伸损伤破坏声发射行为

通过风电叶片单向和多向复合材料拉伸力学性能实验, 结合声发射技术, 研究复合材料损伤演化特性及纤维预断缺陷对复合材料力学性能的影响。复合材料单向和加卸载拉伸实验时, 采用声发射实时监测整个损伤破坏过程, 获取复合材料试件的拉伸力学性能、 损伤破坏特征及相应的声发射响应特征。结果表明: 由于纤维预断缺陷的存在, 单向复合材料加载到约30％破坏载荷时, 缺陷位置及相邻区域的基体和界面开始出现明显损伤; 加载到约60％破坏载荷时, 含缺陷层和相邻的层出现明显的层间剪切破坏, 导致刚度的急剧缩减, 声发射撞击累积数明显高于无缺陷试件。含纤维预断多向复合材料加载到约60％破坏载荷时, 纤维预断处树脂基体出现明显损伤; 随相对应力水平的提高, 多向复合材料的Felicity比下降较为平缓。     

Acoustic emission of composite materials in tensile tests at cryogenic temperatures

Tensile tests of fibre reinforced plastics are performed at cryogenic temperatures and simultaneously acoustic emission (AE) is observed to examine the microscopic deformation and fracture processes of these materials. AE behaviours at liquid helium temperature are different from those at liquid nitrogen temperature, although the mechanical behaviours are similar. From these results, correlations of the AE sources with the microscopic deformation and fracture processes are discussed.     

Feasibility and limitations of damage identification in composite materials using acoustic emission

One of the current challenges in health monitoring of composite materials is the use of acoustic emission to identify damage modes. Many classification procedures have been reported in the literature but none of them clearly state limitations to their applicability, making it difficult to quantify them in different testing conditions. In the present paper, a method is described to characterize energy attenuation and how it affects AE signals features based solely on AE signals recorded during mechanical tests. Limitations to damage identification based on AE signals features can therefore be defined. The method is demonstrated on AE signals recorded during tensile tests on four different layups of carbon fiber reinforced polymer composites using signals frequency centroids to describe AE sources.     

A study on the failure detection of composite materials using an acoustic emission

Fiber reinforced composite materials have been increasingly used as structural material in airplanes, in space applications, and in robot arms because of their high specific stiffness and strength. Structural design and nondestructive test techniques have evolved as increased emphasis has been placed on the durability and damage tolerance of these materials. There are several methods used to detect damaged regions of composite materials. Acoustic emission is one of these. It is a suitable technique for detection of a wide range of micro-structural failures in composite materials.

In this paper, an AE signal analyzer was designed and fabricated with a resonance circuit to extract the specified frequency of an acoustic emission signal. From the tests that were completed, the disturbance noise levels, such as impact or mechanical vibration, of the fabricated AE signal analyzer had a very small value in comparison to those of the conventional AE signal analyzer. Also, the fabricated AE signal analyzer was proven to have generally the same crack detection capabilities as a conventional AE signal analyzer, under static and dynamic tensile tests of the composite materials.     

Fiber and interface strength distribution studies with the single-fiber composite test

Application of the single-fiber composite (sfc) tension test for fiber and interface strength determination is discussed. Fiber breaking and fiber/matrix debond propagation are modelled by Poisson processes. Fiber fragment length distribution as well as debond length dependency upon the applied stress are derived and their interrelation revealed.

Acoustic emission monitoring of the sfc during a test is utilized to obtain the dependency of mean fragment length upon stress and consequently on the Weibull distribution shape and scale parameters. Excellent agreement with data obtained by notoriously complicated conventional fiber tests is observed.     

Material characterization of laminated composite materials using a three-point-bending technique

A three-point-bending technique is presented for identifying the elastic constants of laminated composite materials. In the proposed technique, three strains in the axial, lateral, and 45° directions on the bottom surface at the mid-span of a symmetric angle-ply beam subjected to three-point-bending testing are measured for elastic constants identification. The narrow beam theory together with the trial elastic constants is used to predict the theoretical strains of the beam. The theoretical and experimental strains of the beams are then used in a stochastic optimization method to identify the elastic constants of the beam. The accuracy and applications of the proposed technique are demonstrated by means of a number of examples on the elastic constants identification of graphite/epoxy (Gr/ep) or glass/epoxy (Gl/ep) laminated composite materials. The effects of specimen aspect ratio and thickness on the accuracy of the proposed method are investigated.     

Progression of failure in fiber-reinforced materials

Decohesion is an important failure mode associated with fiber-reinforced composite materials. Analysis of failure progression at the fiber-matrix interfaces in fiber-reinforced composite materials is considered using a softening decohesion model consistent with thermodynamic concepts. In this model, the initiation of failure is given directly by a failure criterion. Damage is interpreted by the development of a discontinuity of displacement. The formulation describing the potential development of damage is governed by a discrete decohesive constitutive equation. Numerical simulations are performed using the direct boundary element method. Incremental decohesion simulations illustrate the progressive evolution of debonding zones and the propagation of cracks along the interfaces. The effect of decohesion on the macroscopic response of composite materials is also investigated.     

Acoustic structural health monitoring of composite materials : Damage identification and evaluation in cross ply laminates using acoustic emission and ultrasonics

The characterisation of the damage state of composite structures is often performed using the acoustic behaviour of the composite system. This behaviour is expected to change significantly as the damage is accumulating in the composite. It is indisputable that different damage mechanisms are activated within the composite laminate during loading scenario. These “damage entities” are acting in different space and time scales within the service life of the structure and may be interdependent. It has been argued that different damage mechanisms attribute distinct acoustic behaviour to the composite system. Loading of cross-ply laminates in particular leads to the accumulation of distinct damage mechanisms, such as matrix cracking, delamination between successive plies and fibre rupture at the final stage of loading. As highlighted in this work, the acoustic emission activity is directly linked to the structural health state of the laminate. At the same time, significant changes on the wave propagation characteristics are reported and correlated to damage accumulation in the composite laminate. In the case of cross ply laminates, experimental tests and numerical simulations indicate that, typical to the presence of transverse cracking and/or delamination, is the increase of the pulse velocity and the transmission efficiency of a propagated ultrasonic wave, an indication that the intact longitudinal plies act as wave guides, as the transverse ply deteriorates. Further to transverse cracking and delamination, the accumulation of longitudinal fibre breaks becomes dominant causing the catastrophic failure of the composite and is expected to be directly linked to the acoustic behaviour of the composite, as the stiffness loss results to the velocity decrease of the propagated wave. In view of the above, the scope of the current work is to assess the efficiency of acoustic emission and ultrasonic transmission as a combined methodology for the assessment of the introduced damage and furthermore as a structural health monitoring tool.     

Finite element modeling of lamb wave propagation in anisotropic hybrid materials

A finite element approach for modeling of acoustic emission sources and signal propagation in hybrid multi-layered plates is presented. Modeling results are validated by Laser vibrometer measurements and comparison to calculated dispersion curves. We investigate hybrid plates as typically found in composite pressure vessels, composed of fiber reinforced polymers with arbitrary stacking sequences and attached metal or polymer materials. Hybrid plate thickness, the ratio between anisotropic and isotropic materials and material properties are varied. Lamb-wave propagation in a geometry representative of a pressure vessel is modeled. It is demonstrated, that acoustic emission sources in multi-layered structures can cause Lamb-waves superimposed by guided waves within the individual layers.     

Identification of failure mechanisms of metallised glass fibre reinforced composites under tensile loading using acoustic emission analysis

In this work, failure mechanisms of metallised glass fibre reinforced epoxy composites under tensile loading were investigated using acoustic emission analysis. Sandblasting with Al₂O₃ was used to pre-treat the composite surface prior to metallisation, and therefore to improve adhesion. The sandblasting time was varied from 2 s to 6 s. A two-step metallisation process consisting of electroless and subsequent electroplating was used for depositing the copper coating on the pre-treated composite surface. The mechanical pre-treatment had no significant negative effect on the mechanical properties of the composite laminate. The acoustic emission (AE) from the metallised composite was recorded during tensile testing in order to investigate the failure mechanisms. AE-Signals were analysed using pattern recognition and frequency analysis techniques. A correlation between the cumulative absolute AE-energy and the mechanical behaviour of uncoated and coated specimens during tensile testing was successfully observed. It was shown that a stronger adhesion between substrate and coating leads to a lower release of mechanical elastic energy, which could be recorded by means of AE analysis. Furthermore, differences in peak frequency, frequency distribution and the use of pattern recognition techniques allowed classifying the signal into three failure mechanisms for the uncoated samples and four failure mechanisms for the coated samples, namely matrix cracking, fibre-matrix interface failure, fibre breakage and substrate-coating interface failure. Waveform and frequency analysis of the classified signals supported the identification of the failure mechanisms. Furthermore, optical investigation and SEM images of the tested samples and fracture surfaces confirmed the identified mechanisms evaluated by acoustic emission analysis.     

Rapid microwave synthesis of indium filled skutterudites: An energy efficient route to high performance thermoelectric materials

Filled skutterudites are promising thermoelectric materials due to reduced thermal conductivity upon inserting a guest atom or ‘rattler’ into the CoSb₃ structure. By using an indium rattler dimensionless Figure of Merit (ZT) values >1 at 650 K have been reported. The conventional synthesis of these compounds typically takes several days (∼3 days) to obtain the final well-sintered material for property measurements. We report here a microwave-assisted synthesis method that reduces the initial calcination time from 2 days to 2 min. This route significantly reduces the time needed to produce materials suitable for property and device testing.     

Characterization of treated date palm tree fiber as composite reinforcement

Recently, great interest was paid to new technologies dealing with environmental aspect. Preservation of natural resources such as natural fibers forced the composite industry to search and examine “eco-friendly” components. Studies to find alternative reinforcements and resin systems that are environmentally friendly while providing the same performance as their synthetic counterparts are in continuous progress. The aim of this study is to investigate effect of different treatment process on the data palm fiber (DPF). Raw DPF underwent different surface modification methods such as alkali treatment with concentrations 0.5%, 1%, 1.5%, 2.5% and 5%, and acid treatment with 0.3, 0.9 and 1.6 N. All treatments were performed at 100 °C for 1 h. The surface morphology, thermal gravimetry analysis (TGA), Fourier transform infrared spectroscopy (FTIR), mechanical properties and chemical analysis, of treated DPF were investigated. Specimen treated with 1% NaOH showed optimum mechanical properties. Hydrochloric acid treatment resulted in deterioration in mechanical properties.     

Preparation and characterization of nanostructured MWCNT-TiO2 composite materials for photocatalytic water treatment applications

Nanoscale composite materials containing multi-walled carbon nanotubes (MWCNT) and titania were prepared by using a modified sol-gel method. The composites were comprehensively characterized by thermogravimetric analysis, nitrogen adsorption-desorption isotherm, powder X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis absorption spectroscopy. The analysis revealed the presence of titania crystallites of about 7.5 nm aggregated together with MWCNT in particles of 15-20 nm of diameter. The photoactivity of the prepared materials, under UV or visible irradiation, was tested using the conversion of phenol from model aqueous solutions as probe reaction. A synergy effect on the photocatalytic activities observed for the composite catalysts was discussed in terms of a strong interphase interaction between carbon and TiO₂ phases by comparing the different roles of MWCNT in the composite materials.     

Debonding failure modes of flexural FRP-strengthened RC beams

In this paper, different types of debonding failure modes are described. Then, experimental results of four-point bending tests on FRP strengthened RC beams are presented and debonding failure mechanisms of strengthened beams are investigated using analytical and finite element solutions. Reasonable results could be obtained for modelling of debonding failure load of tested beams.

Existing international codes and guidelines from organizations such as ACI, fib, ISIS, JSCE, SIA, TR55, etc. are presented and compared with the results from the experiments and calculations. A discrepancy of up to 250% was seen between different codes and guidelines for predicting the debonding load. Furthermore, a new recommendation for debonding control is given.     

Fabrication and characterization of machinable Si3N4/h-BN functionally graded materials

A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si₃N₄ ceramics. Silicon nitride/hexagonal boron nitride (h-BN) functionally graded materials (FGMs) were fabricated by hot pressing at 1750°C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si₃N₄/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. Such design can improve the machinability of composite, and at the same time can make the mechanical properties of Si₃N₄ ceramic not to be sacrificed too much. The texture of h-BN and β-Si₃N₄ was observed during hot pressing sintering.     

玻璃纤维复合材料静载荷声发射试验研究

通过对玻璃纤维复合材料试件静力载荷条件下的声发射试验,研究了该种材料在静力试验条件下的损伤破坏过程的声发射特性,分析了该材料损伤各阶段的声发射特性和对应的载荷比,系统而完整地得到了该材料损伤类型特征及声传播特性。试验证明:对玻璃纤维复合材料进行静力载荷条件下的声发射试验研究,可有效并清晰地揭示该材料在静力试验条件下损伤破坏过程中的声发射特性及材料损伤类型特征,为该材料的寿命健康监测、缺陷判定提供评价依据。     

An experimental study of failure initiation and propagation in 2D woven composites under compression

Experimental research was conducted to investigate the compressive failure of orthogonal 2D woven composites. Damage initiation and the effect of weave architecture were studied using a Four Point Bending (FPB) test setup. Scanning Electron Microscope (SEM) and Digital Imaging (DI) observations show the individual failure of the load-aligned tows, which behave as structural elements at the internal reinforcement level. Weave architecture and internal geometry are seen to affect location and morphology of the damage initiation. Damage propagation and the effect of stacking were studied using a reduced Compact Compression (rCC) test setup. Optical micrographs show that stacking configuration can have a significant effect on the damage morphology. Additionally, results also suggest the effect of stacking is a function of the weave architecture and internal geometry.     

Characterization of the dynamic failure behaviour of a glass-fiber/vinyl-ester at different temperatures by means of instrumented Charpy impact testing

Instrumented Charpy impact testing is used to investigate the strength and failure properties of a glass-fiber/vinyl-ester composite. The test technique, originally developed for testing of steel specimens, is presented in its basic aspects; reported are the conventional procedures for determining load, displacement and energy absorption that a specimen experiences, over the entire phase of loading and subsequent failure of the specimen. Techniques are described for generating data of sufficient accuracy when applying the test to composites. In particular, the necessity of utilizing measurement chains of sufficiently high frequency response and striker tups of sufficiently high sensitivity is emphasized. Tests are performed with glass-fiber/vinyl-ester specimens, provided with notches oriented in two different directions with respect to the plies of woven glass fiber rovings. Two different types of failure result: fiber breakage ahead of the notch due to tensile stresses, and delaminations of the interface planes between the plies of woven glass fiber rovings due to shear stresses. Specifically, energies absorbed by the specimen over the entire failure process and values of maximum load occurring during the impact process are measured over a large range of temperatures. The data are correlated with the observed failure phenomena. The high level of information obtained in characterizing the failure behaviour by means of a test which requires limited technical effort proves the instrumented Charpy impact test to be a simple but effective tool for quantifying the quality of a composite in practical applications, as e.g. in surveillance programs for controlling processes such as manufacturing or aging of the material.     

Fabrication and characterization of potassium-sodium niobate piezoelectric ceramics by spark-plasma-sintering method

High density (Na_1−xK_x)NbO₃ (x = 0.5, 0.6, and 0.7) ceramics were successfully prepared by spark-plasma-sintering (SPS) method. The dielectric and piezoelectric properties of the SPS samples were investigated and compared to that of hot-pressed samples. It is found that, the SPS-sintered (Na_1−xK_x)NbO₃ samples show higher room temperature dielectric constant, higher coercive fields, lower remnant polarizations and lower electromechanical coefficients than that of the hot-pressed (Na_1−xK_x)NbO₃ samples. The dielectric and piezoelectric property differences between the SPS-sintered and hot-pressed samples have been attributed to grain size effects.