Damage assessment in layered composites using spectral analysis and Lamb wave

Piezo-ceramic transducers of the surface mounted type are commonly used for structural health monitoring (SHM) techniques. But, there is a disadvantage to use piezo-ceramic transducers of the surface mounted type in Lamb wave application. Due to the symmetric and antisymmetric Lamb wave modes generated by the surface mounted piezo-ceramic transducers simultaneously, the received signals are very complex and it is difficult to extract damage information from the signals.

In this paper, the practical method for SHM was proposed using piezo-ceramic transducers of the surface mounted type and Lamb wave. In order to overcome the difficulties in the signal processing of the simultaneous modes, the symmetric and antisymmetric modes were separated by using the two sensors bonded on the opposite surfaces at the same point. Also, spectral analyses of the separated symmetric and antisymmetric Lamb waves showed that each mode propagated with different frequency characteristics in the exciting frequency range.

By making use of these findings, the changes of power spectrum density in characteristic frequency band of symmetric and antisymmetric modes are proportional to the delamination size in quasi-isotropic Gr/Ep laminates. Therefore, this paper presents the damage assessment technique to extract damage information from the complicated PZT signals that could not be interpreted in time domain.     

Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves

Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.     

Imaging of porosity in fiber-reinforced composites with a fiber-optic pump–probe laser-ultrasound system

Assessing material porosity in composites is critical because pores can greatly affect material strength and lifetime. Ultrasound (US) is one of the primarily methods to quantify porosity, usually based on the relationship between US speed/attenuation and void content. However, most US approaches require a sample with plane parallel and relatively smooth surfaces to correctly measure the attenuation and speed, but such conditions cannot always be fulfilled in practice. In addition, conventional US cannot directly image porosity as X-rays can. Here we present a non-contact US method to directly image porosity that can be easily integrated with US speed/attenuation measurements. The overall approach uses ultra wideband acoustic signals generated at the surface of a composite material with a laser pulse (i.e., pump), and non-contact, point-like detection of backscattered transients (i.e., probe) with spatial resolution better than 1 ply. US-assessed porosity is compared with that measured gravimetrically.     

Interlaminar fracture micro-mechanisms in toughened carbon fibre reinforced plastics investigated via synchrotron radiation computed tomography and laminography

Synchrotron Radiation Computed Tomography (SRCT) and Synchrotron Radiation Computed Laminography (SRCL) permit 3D non-destructive evaluation of fracture micro-mechanisms at high spatial resolutions. Two types of particle-toughened Carbon Fibre Reinforced Polymer (CFRP) composites were loaded to allow crack growth in Modes I and II to be isolated and observed in standard and non-standard specimen geometries. Both materials failed in complex and distinct failure modes, showing that interlaminar fracture in these materials involves a process zone rather than a singular crack tip. The work indicates that incorporating particle/resin, fibre/interlayer and neat resin failure is essential within models for material response, since the competition between these mechanisms to provide the energetically favourable crack path influences the macro-scale toughness. The work uniquely combines the strengths of SRCT and SRCL to compare failure micro-mechanisms between two specimen geometries, whilst assessing any edge effects and providing powerful insight into the complex micro-mechanical behaviour of these materials.     

Multi-technique approach for non destructive diagnostic of structural composite materials using bulk ultrasonic waves,guided waves,acoustic emission and infrared thermography

The damage phenomenon occurring in glass fiber reinforced polymer (GFRP) laminate is very complex; it is inherently an anisotropic, multi-source and multi-scale process. In the present work, a new non destructive testing (NDT) approach is suggested using, simultaneously, multiple techniques. These are bulk and guided ultrasonic waves, acoustic emission, passive and active infrared thermography, for evaluating mechanical damage occurring in GFRP laminate under uniaxial tensile tests. The principal result of this work, other than the success of each applied technique to evaluate the created damage in the material, is the possibility to effectively combine multiple NDT techniques in order to validate or to complete the diagnosis of the structural health of the materials.     

A hybrid numerical and imaging approach for characterizing defects in composite structures

In this study, a hybrid approach coupling hyperspectral near infrared imaging with a progressive finite element method is proposed for characterization of the elastic and failure response of composites with non-uniform variations of the wrinkles profile through the thickness and across the structure dimensions. In this approach, hyperspectral near infrared spectroscopy is used to create a 3D profile of the surface resin pockets with the capability of measuring resin thickness from approximately 125 to 2500 μm. These resin pockets are directly correlated to underlying ply level wrinkling as confirmed by optical microscopy. The 3D mapped resin plane obtained from the hyperspectral imaging is used to morph a ply-by-ply finite element model of a carbon-fiber/epoxy resin laminated plate using a progressive damage failure methodology. The results show the capability of the hybrid method to predict the structural response in laminated composites containing spatially distributed and non-uniform ply-level wrinkling.     

Fibrous composite matrix characterisation using nanoindentation: The effect of fibre constraint and the evolution from bulk to in-situ matrix properties

The variation of the in-situ matrix properties of a carbon-fibre composite has been investigated using nanoindentation. The aerospace carbon-fibre composite material (HTA/6376) and the bulk matrix (6376) have been co-cured to produce specimens ideal for matrix characterisation. The in-situ matrix has been characterised using fifty indentations in matrix pockets of many different sizes. The fibre constraint effect on in-situ matrix indentations has been characterised experimentally using the continuous stiffness measurement (CSM) technique, showing good correlation with finite element results from a previous study. The co-cured specimens allow the evolution of property change in the matrix material to be observed. The in-situ matrix modulus increases with decreasing matrix pocket size, and is up to 19% greater than the bulk matrix. This property change occurs outside the normal range of the interphase region for CFRP materials, and is statistically significant relative to the experimental scatter associated with the nanoindentation technique.     

Mode III interlaminar fracture of carbon/epoxy laminates using the Six-Point Edge Crack Torsion (6ECT)

The recently proposed Six-Point Edge Crack Torsion (6ECT) test was used to evaluate the mode III interlaminar fracture of carbon/epoxy laminates. Plate specimens with starter delaminations in 0/0, 0/90 and 0/45 interfaces were tested. Data reduction was performed with an effective crack scheme validated in a previous numerical study. The tests allowed the evaluation of fairly unambiguous initiation G_IIIC values and of subsequent R-curves. Examinations of specimen cross-section showed considerable lengths of pure interlaminar propagation in specimens with starter delaminations in 0/90 and 0/45 interfaces. The latter specimens had the lowest initiation G_IIIC values.     

Numerical method for optimizing design variables of carbon-fiber-reinforced epoxy composite coil springs

To successfully reduce a vehicle's weight by replacing steel with composite materials, it is essential to optimize the material parameters and design variables of the structure. In this study, we investigated numerical and experimental methods for determining the ply angles and wire diameters of carbon fiber/epoxy composite coil springs to attain a spring rate equal to that of an equivalent steel component. First, the shear modulus ratio for two materials was calculated as a function of the ply angles and compared with the experimental results. Then, by using the equation of the spring rate with respect to the shear modulus and design variables, normalized spring rates were obtained for specific ply angles and wire diameters. Finally, a finite element model for an optimal composite coil spring was constructed and analyzed to obtain the static spring rate, which was then compared with the experimental results.     

A hybrid S-N formulation for fatigue life modeling of composite materials and structures

A semi-empirical S-N formulation for the modeling of the constant amplitude fatigue behavior of composite materials and structures is introduced in this paper. The new S-N formulation is based on the commonly used exponential and power law fatigue models. It is a hybrid formulation combining the two existing models in order to improve their modeling accuracy in the low and high cycle fatigue regions. This formulation was applied to a number of fatigue databases for different composite materials and structural elements in order to simulate their fatigue behavior. The modeling accuracy of the hybrid model was compared to the accuracy of commonly used S-N models for composite materials. As proved, the hybrid model performs better in the majority of the examined cases and is able to overcome the disadvantages of previously developed models without introducing any complexity in the fitting procedure.     

Matrix failure in composite laminates under compressive loading

The failure envelope of the matrix in composite laminates under compressive loads has not received much attention in literature. There are very little to no experimental results to show a suitable failure envelope for this constituent found in composites. With increasing popularity in the use of micromechanical analysis to predict progressive damage of composite structures which requires the use of individual failure criteria for the fibre and matrix, it is important that matrix behaviour under compression is modelled correctly.In this study, off-axis compression tests under uniaxial compression loading are used to promote matrix failure. Through the use of micromechanical analysis involving Representative Volume Elements, the authors were able to extract the principal stresses on the matrix at failure. The results indicated that hydrostatic stresses play an important role in the failure of the matrix. Thus, Drucker–Prager failure criterion is recommended when modelling compressive matrix failure in composite structures.     

Behaviour of woven hybrid basalt-carbon/epoxy composites subjected to laser shock wave testing: Preliminary results

This study addresses the effect of basalt fibre hybridization on the damage tolerance of carbon/epoxy laminates subjected to laser shock wave tests. Interply hybrid specimens with two different stacking sequences (sandwich-like and intercalated) were tested at different laser intensities and residual post-shock properties of the different configurations have been characterized by quasi-static three point bending tests monitored by acoustic emission. Results indicate that the best compromise in terms of both quasi-static properties (2% reduction in flexural strength compared to all carbon laminates) and damage tolerance appears to be the sandwich-like structure with basalt fibre skins. In particular, this configuration exhibited the highest damage tolerance among the hybrids, with a percent decrease in flexural strength of about 5% compared to 15% in the case of all carbon laminates. Damage induced by laser shock testing in carbon-basalt woven fabric/epoxy composites is mainly inter-ply delamination. This study also highlights the tougher behaviour of basalt plies in response to a sudden application of load compared to carbon layers with a favourable hybridization effect.     

Biodegradability of all-cellulose composite laminates

The high mechanical properties of single-polymer composites based on degradable non-derivatised cellulose, aka all-cellulose composites, have recently captured the attention of researchers. All-cellulose composites possess the intriguing combination of high strength and biodegradability. However, the biodegradation behaviour of all-cellulose composites has so far not been reported. In this work, soil burial experiments were carried out to compare the biodegradation behaviour of all-cellulose composites with conventional biocomposites in order to investigate the end-of-life disposal of this relatively new class of bio-based composite materials. All-cellulose composites are characterised by exceptional biodegradability with mass losses of up to 73% following a soil burial time of 70 days. An investigation of the mechanisms of biodegradation of all-cellulose composites is undertaken for the first time.     

Meso-scale modeling of hypervelocity impact damage in composite laminates

ObjectivesThis paper presents an approach to numerical modeling of hypervelocity impact on carbon fiber reinforced plastics (CFRP).MethodsThe approach is based on the detailed meso-scale representation of a composite laminate. Material models suitable for explicit modeling of laminate structure, including fiber-reinforced layers and resin-rich regions, are described. Two numerical impact tests with significantly different impact energies were conducted on thermoplastic AS4/PEEK materials with quasi-isotropic layups. Simulations employed both SPH and Finite element methods.ResultsResults of simulations are verified against experimental data available from the literature and demonstrate good correlation with the experiments.ConclusionsDeveloped modeling approach can be used in simulations where post-impact damage progression in composite material is of the main focus.     

Experimental investigations in embedded sensing of composite components in aerospace vehicles

This paper summarizes the experimental investigations for smart embedded sensing in rotorcraft composite components. The overall objective of this effort was to develop smart embedded sensor technologies for condition based maintenance (CBM) for composite components in army rotorcraft. This paper presents the results of experimental investigations related to development and maturation of different types of embedded sensing solutions for structural health monitoring of composite components including Fiber Bragg Grating (FBG) sensors, phased and discrete piezoelectric sensor arrays. A discussion is provided relative to embedment of optical fibers into composites, and the results from embedded FBG sensors in a rotorcraft flexbeam subcomponent test specimen with seeded delamination subjected to dynamic loading. Likewise, results are analyzed of surface mounted phased array and embedded smart piezoelectric sensors in the flexbeam subcomponent test specimen with embedded delamination, subjected to fatigue cyclic loading. The paper also summarizes the lessons learned from efforts to nucleate and propagate delamination within composite components under dynamic cyclic loading.     

Smart soft composite actuator with shape retention capability using embedded fusible alloy structures

This work presents a new kind of shape memory alloy (SMA) based composite actuators that can retain its shape in multiple configurations without continuous energy consumption by changing locally between a high-stiffness and a low-stiffness state. This was accomplished by embedding fusible alloy (FA) material, Ni-chrome (Ni–Cr) wires and SMA wires in a smart soft composite (SSC) structure. The soft morphing capability of SMA-based SSC structures allows the actuator to produce a smooth continuous deformation. The stiffness variation of the actuator was accomplished by melting the embedded FA structures using Ni–Cr wires embedded in the FA structure. First, the design and manufacturing method of the actuator are described. Then, the stiffness of the structure in the low and high-stiffness states of the actuator were measured for different applied currents and heating durations of the FA structure and results show that the highest stiffness of the actuator is more than eight times that of its lowest stiffness. The different shape retention capability of the actuator were tested using actuators with one or two segments and these were compared with a numerical model.     

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.     

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.     

Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber and its fiber Bragg grating (FBG) sensor for embedment inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. First, a brief summary is presented for applications of small-diameter FBG sensors to damage monitoring in composite structures. Then, we propose a new damage detection system for quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by an FBG sensor attached on or embedded in the laminate using a newly developed high-speed optical wavelength interrogation system. This system was applied to detect interlaminar delamination in CFRP cross-ply laminates. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element analysis. From the changes in the amplitude ratio and the arrival time of the new mode depending on the delamination length, it was found that this system could evaluate the delamination length quantitatively. Furthermore, small-diameter FBG sensors were embedded in a double-lap type coupon specimen, and the debonding progress could be evaluated using the wavelet transform.     

Acoustic emission analysis of composite pressure vessels under constant and cyclic pressure

The use of acoustic emission (AE) for the detection of damage in carbon fibre composite pressure vessels was evaluated for constant and cyclic internal gas pressure loading conditions. AE was capable of monitoring the initiation and accumulation of damage events in a composite pressure vessel (CPVs), although it was not possible to reliably distinguish carbon fibre breakage from other microscopic damage events (e.g. matrix cracks, fibre/matrix interfacial cracks). AE tests performed on the carbon fibre laminate used as the skin of pressure vessels revealed that the development of damage is highly variable under constant pressure, with large differences in the rupture life and acoustic emission events at final failure. Numerical analysis of the skin laminate under constant tensile stress revealed that the high variability in the stress rupture life is due mainly to the stochastic behaviour of the carbon fibre rupture process.