Unidirectional Composite Plate Research Articles

Delamination Depth Detection in Composite Plates Using the Lamb Wave Technique Based on Convolutional Neural Networks.

Delamination represents one of the most significant and dangerous damages in composite plates. Recently, many papers have presented the capability of structural health monitoring (SHM) techniques for the investigation of structural delamination with various shapes and thickness depths. However, few studies have been conducted regarding the utilization of convolutional neural network (CNN) methods for automating the non-destructive testing (NDT) techniques database to identify the delamination size and depth. In this paper, an automated system qualified for distinguishing between pristine and damaged structures and classifying three classes of delamination with various depths is presented. This system includes a proposed CNN model and the Lamb wave technique. In this work, a unidirectional composite plate with three samples of delamination inserted at different depths was prepared for numerical and experimental investigations. In the numerical part, the guided wave propagation and interaction with three samples of delamination were studied to observe how the delamination depth can affect the scattered and trapped waves over the delamination region. This numerical study was validated experimentally using an efficient ultrasonic guided waves technique. This technique involved piezoelectric wafer active sensors (PWASs) and a scanning laser Doppler vibrometer (SLDV). Both numerical and experimental studies demonstrate that the delamination depth has a direct effect on the trapped waves' energy and distribution. Three different datasets were collected from the numerical and experimental studies, involving the numerical wavefield image dataset, experimental wavefield image dataset, and experimental wavenumber spectrum image dataset. These three datasets were used independently with the proposed CNN model to develop a system that can automatically classify four classes (pristine class and three different delamination classes). The results of all three datasets show the capability of the proposed CNN model for predicting the delamination depth with high accuracy. The proposed CNN model results of the three different datasets were validated using the GoogLeNet CNN. The results of both methods show an excellent agreement. The results proved the capability of the wavefield image and wavenumber spectrum datasets to be used as input data to the CNN for the detection of delamination depth.

Influence of plate thickness on the results of residual stresses determination by through hole drilling in orthotropic composites of different fiber orientation

The main objective of this study is to characterize residual stress in composite orthotropic plates using hole drilling method. The values of hole diameter increment in the principal anisotropic directions, which were obtained from electronic speckle-pattern interferometry on the external surface of the specimen, serve as the initial experimental information. The theoretical foundation of the approach is based on S. G. Lekhnitsky's analytical solution, which describes a stress concentration along the edge of the central open hole in a rectangular orthotropic plate under tension in principal anisotropy directions. The transitional model, that is essential link for deriving residual stress components from initial experimental data, provides the unequivocally solution to the properly posed inverse problem. Firstly, the validation of the transitional model regarding prescription of residual stress values for specific plies within a specimen’s thickness is based on calibration experiments. These experiments yield data for unidirectional and cross-ply composite plates of three different thicknesses. The results demonstrate that the values of principal residual stress components for cross-ply composite material are independent of plate thickness. This means that the transition model, based on the measurements of deformation responses to through hole drilling on the external plate face, can be reliably implemented to characterize the maximal residual stress values present in the middle plane of the composite plate. An investigation into the effect of hole diamter values on the determination of residual stress is conducted for cross-ply and angle-cross-ply composites. It is shown that a hole of diameter of 3.2 mm provides the most reliable results.

Buckling optimization of M91/IM10 unidirectional composite plate by genetic algorithm method

Bu makalede, havacılık alanında kullanılan malzemeler ile karşılaştırıldığı zaman dayanımı oldukça yüksek olan M91/IM10 tek yönlü kompozit malzemesinden üretilmiş bir plakanın burkulma yük faktörünün maksimum olduğu optimum katman dizilimi genetik algoritma yöntemi ile elde edilmiştir. Söz konusu kompozit plaka 4 tarafından basit destekli ve çift eksenli düzlem içi basınç yüklerine maruz kalan bir plakadır. Genetik algoritma yöntemi kullanılarak geliştirilen bilgisayar kodu ile çeşitli yük durumu ve çeşitli plaka en boy oranı kombinasyonları için belirli fiber açısı aralıklarında ve belirli katman sayısında maksimum burkulma dayanımını sağlayan katman dizilimleri elde edilir. Laminatın simetrik ve dengeli olduğu kabul edilir. Bulunan sonuçlar literatürde çalışmaları bulunan grafit/epoksi kompozit malzemesinden elde edilen sonuçlar ile karşılaştırılmıştır. Yapılan karşılaştırma sonucunda M91/IM10 kompozit malzemesinden imal edilmiş ve grafit/epoksi kompozit malzemesinden imal edilen bir plakaya göre daha az katmana sahip bir plakanın, daha iyi burkulma yük faktörü değerini verdiği ortaya çıkmaktadır.

Effect of tailored fiber deposition in 3D printed composites: application of an anisotropic phase field model

Continuous Fiber 3D printing is a relatively new technology which can allow for tailored reinforcement of critical regions in structural components, i.e., stress concentrations, following principal stress lines. The influence the fiber deposition path has on the mechanical and failure behavior of such components is assessed using an anisotropic phase field model. A comparison with experimental results for notched unidirectional composite plates, available in the literature, demonstrates the ability of the method to produce satisfactory predictions for unidirectional reinforcement paths. The analysis is then extended to Open-Hole and Double Edge-Notched tension coupons of both unidirectional and variable stiffness reinforcement patterns. It is observed that the strength obtained for the components made with a reinforcement pattern that follows the principal stress lines is markedly higher than that for the equivalent unidirectionally reinforced ones. It is highlighted that the improvement in strength deriving from the tailored fiber deposition cannot be deduced solely by the analysis of the stress concentration factor but an analysis taking damage into account is necessary. In addition, the effect of the reinforcement strategy on the size effect was also explored, highlighting how the tailored fiber path leads to an increase in the failure load attainable by the specimens for all the dimensions analyzed.

Theoretical and Experimental Analysis of the Group Velocity for Lamb Wave on Unidirectional Composite Plates

Theoretical and Experimental Analysis of the Group Velocity for Lamb Wave on Unidirectional Composite Plates

Influence of Line Processing Parameters on Properties of Carbon Fibre Epoxy Towpreg

This paper explores the performance of low-cost unidirectional carbon fibre towpregs with respect to line production speed and fibre volume fraction. Using an automated production line, towpregs were produced at different production speeds, resulting in modified fibre volume fractions. The towpregs were used to manufacture unidirectional composite plates, which were then tested to evaluate mechanical performance. The fibre straightness and interfacial void ratio of the composite plates were determined by statistical analysis of the samples’ optical micrographs. The results demonstrate that adjusting the line production speed enables targeted fibre volume fractions (FVF) to be reached, resulting in the composites having different mechanical performances (2039 MPa and 2186.7 MPa tensile strength, 1.26 and 1.21 GPa flexural strength for 59.8% and 64.4% FVF, respectively). It was shown that at lower production speeds and FVF, composites exhibit good consolidation and low porosity, which is highlighted by the better interlaminar shear strength performances (8.95% increase), indicating the limitations of manufacturing very high FVF composites. Furthermore, it was concluded that fibre straightness plays a key role in mechanical performance, as samples with a lesser degree of fibre straightness showed a divergence from theoretical tensile properties.

3D progressive damage analysis of a unidirectional multi-layered composite for mechanical joints

ABSTRACT Due to the presence of holes in composite mechanical joints, stresses around the holes are increased, and they may cause failure in the composites. Analysis of the composite behaviour after damage initiation around the holes is important, complex, and mostly three-dimensional. In order to achieve accurate results about the composite failure modes, damage analysis using a modern criterion is needed. In this research, to investigate the damage behaviour of a unidirectional multi-layered composite plate of a mechanical joint under pure tension, three codes are prepared in the USDFLD, VUSDFLD and VUMAT subroutines based on the Hashin criterion and implemented in ABAQUS software. These codes, using the methods of suddenly and gradually degradation in composite properties, progressively simulate the three-dimensional damage behaviour of the composite plate after damage initiation. The results show that, when a mechanical joint is under pure tension, the adequate distance from the edge of a hole to the edge of a composite plate is about two times the hole diameter. Furthermore, it is found that to rapidly investigate and analyse a mechanical joint in the composite structures, the USDFLD subroutine, which uses suddenly decreased material properties after damage initiation, is reasonable and more efficient.

Effect of Defects Part II: Multiscale Effect of Microvoids, Orientation of Rivet Holes on the Damage Propagation, and Ultimate Failure Strength of Composites

Material properties at the vicinity of the cut-outs in composites are not entirely defect-free. The nteraction of multiple cutouts like rivet holes, the repercussion of their configuration on crack propagation, and ultimate strength were predicted using Peridynamic method and the results are reported in this article. The effect of microscale defects at the vicinity of the cutouts on macroscale damage propagation were shown to have quantifiable manifestation. This study focused on two to four holes in unidirectional composite plates with 0°, 45°, and 90° fiber directions, while the vicinity of a hole was considered degraded. Numerical results were validated using quantitative ultrasonic image correlation (QUIC) and the tensile test. Both the experimental and numerical results confirmed that the strength of the horizontal configuration is higher than the vertical in the plates with two holes. Furthermore, the square configuration was found to be stronger than the diamond configuration with four holes. When the effect of microscale defects was considered, the prediction of ultimate strength was better compared to the experimental results. The predictive model could be reliably used for progressive damage analysis.

A machine learning model for predicting the ballistic impact resistance of unidirectional fiber-reinforced composite plate

It has been a vital issue to ensure both the accuracy and efficiency of computational models for analyzing the ballistic impact response of fiber-reinforced composite plates (FRCP). In this paper, a machine learning (ML) model is established in an effort to bridge the ballistic impact protective performance and the characteristics of microstructure for unidirectional FRCP (UD-FRCP), where the microstructure of the UD-FRCP is characterized by the two-point correlation function. The results showed that the ML model, after trained by 175 cases, could reasonably predict the ballistic impact energy absorption of the UD-FRCP with a maximum error of 13%, indicating that the model can ensure both computational accuracy and efficiency. Besides, the model’s critical parameter sensitivities are investigated, and three typical ML algorithms are analyzed, showing that the gradient boosting regression algorithm has the highest accuracy among these algorithms for the ballistic impact problem of UD-FRCP. The study proposes an effective solution for the traditional difficulty of the ballistic impact simulation of composites with both high efficiency and accuracy.

Novel Analytical Model of Stress Concentration around Broken Fibers of Unidirectional Composite Plate

During the fiber fracture of unidirectional composite a distribution of stress around the neighbored fibers happens, this mechanism is called the local redistribution efforts. Referring to the "shear lag" method, the researches wanted to predict the stress concentration in the surrounding area of broken fibers as well as the longitudinal resistance of the unidirectional composite which presents a fiber breaking. The goal of this paper is to develop a new probabilistic model of unidirectional composite plate to calculate the stress concentration at the broken fibers and their neighboring fiber intact. The "shear lag" method has been generalized to see the broken fibers interference on the stress concentration factor variation of surrounding sound fibers.

Nondestructive determination of local material properties of laminated composites with the impulse excitation technique

An innovative experimental methodology for the nondestructive assessment of local elastic properties of composite materials is proposed in this paper. A local region of rectangular shape is isolated on the investigated laminate with a proper clamping system. The first resonant frequency of the retained region with clamped boundaries is determined through the Impulse Excitation Technique. In order to take into account the anisotropy of the composite laminate, values of the resonant frequency are measured after rotating the clamping system with respect to the composite plate. The material constants are finally obtained from the measured frequencies through an inverse optimization algorithm based on Finite Element analysis.The proposed methodology is validated on a unidirectional composite plate with known elastic properties. Boundaries of the investigated region are clamped by pressing two rectangular frames to the plate through a testing machine, thus localizing the measure of the resonant frequencies. The first resonant frequency is measured at different angles (0°, 30°, 60° and 90°) between fibers direction and main axis of the rectangular region. Dynamic elastic properties are finally assessed through the proposed inverse optimization algorithm. The small differences between the nominal and the estimated mechanical properties prove the effectiveness of the proposed methodology.

A loading assisted diffuse wave inspection of delamination in a unidirectional composite

Delamination localization and characterization are critical for the structure integrity of fiber reinforced polymer composites with various fiber distributions and geometries. Different to direct wave based methods, the diffuse wave inspection is promising for rapid structure interrogation with only a few ultrasonic transducers required. The angle dependant diffusivity and dissipation are first measured in a unidirectional composite plate manufactured from hot press. Both coefficients represent an orthotropic behavior and can be predicted through rotation matrix calculation. 27% signal energy variation is found between the 0.1 mm thick delamination zone and the intact zone. The measurement along the fiber direction gives the best signal-to-noise ratio among all directions. No coefficient difference exists under amplitude normalization. A compressive loading is then applied to have different acoustic responses from different zones. Based on the acoustoelastic effect, the decorrelation coefficient variation is negligible in the intact zone with the strain of 10−3. Similar to the clapping effect, the loading triggered decorrelation coefficient variation becomes important as the transducer pairs approach the delamination zone. This has also been validated by the energy variation method. In addition, when the loading is centered in the delamination zone, the decorrelation coefficient variation area that can be captured by the diffuse wave is 45 × 210 mm2. Based on the Locadiff imaging method, the delamination geometry is finally characterized through Bayesian inference together with the numerical defect sensitivity kernel and the decorrelation coefficients. According to this loading-delamination-diffuse wave interaction analysis, the loading unveiled delamination geometry is found to center at [146,138] mm with an area of 3.5 × 10−4 ~ 11 × 10−4 m2 compared to the ground truth of [145,150] mm and 5.2 × 10−4 m2 respectively. It is promising for early-stage defect inspection in advanced composite structures.

Numerical and experimental investigation of delaminations in a unidirectional composite plate using NDT and SHM techniques

In this paper, the non-destructive testing (NDT), structural health monitoring (SHM), and scanning laser Doppler vibrometer (SLDV) techniques were presented to quantify three simulated delaminations inserted at different depths of a unidirectional composite plate. First, the RollerFORM and Omniscan equipment were sufficiently used to identify the delaminations. Second, in conjunction with guided waves, the developed imaging method was successfully used to detect and quantify the interested delaminations. The tuning curves were determined experimentally to define the dominant Lamb wave modes of incident waves. Third, multi-physics three-dimensional finite element simulations of propagating and interacting Lamb waves with delaminations were implemented to extract the wavefield data for wavenumber analysis. The experimental part was conducted to validate the numerical results using SLDV. The effect of the delamination depth on the trapped waves generated over the delamination region was studied numerically and experimentally. The results showed that trapped waves could be affected by the delamination depth. Both numerical and experimental results demonstrated that the near surface delamination has strong trapped waves over the delamination region while the far surface delamination has weak trapped waves. The energy distribution maps of numerical and experimental wavefields data sufficiently quantified the interested delaminations. A good agreement was achieved between the numerical and experimental results.

A layerwise multiscale analysis method for composite laminated plates

In the two-dimensional shear deformation theories (SDT) of composite laminates, the three-dimensional plates or shells are transferred to a two-dimensional problem in the reference surface by introducing displacement fields and stress resultants along the thickness direction, the constitutive relationship is a resultant form. The multiscale analysis methods developed from SDT can not describe the periodicity along the thickness direction in composite laminated plates and shells, so it is almost impossible to accurately analyze the local stress fields surrounding the fibers based on the two scale analysis scheme. A two scale layerwise multiscale analysis method (LMAM) is established based on the Reddy’s layerwise theory (RLWT) and O(1) homogenization method for the composite laminated plates, considering the periodicity in in-plane and along the thickness direction simultaneously. The macroscopic problem of laminated plate is discretized by RLWT, while the unit cells consisted by fiber and matrix are modeling by three-dimensional finite elements. In numerical examples, the proposed LMAM is verify by using direction numerical simulation (DNS) and the O(1) homogenization method based on three-dimensional macroscopic finite elements. The unidirectional and woven composite plates are studied by the proposed LMAM; the macroscopic and microscopic stress distributions along the thickness are investigated.

Dynamic and fluid-structure interaction simulations of a ceramic matrix composite plate

This study presents numerical simulations of a unidirectional ceramic matrix composite (CMC) plate under different loads considering the effect of fluid-structure interaction (FSI). The two-scale method was used in the dynamic simulation of CMC plate. The shear-lag model considering the damage modes of CMCs was used to simulate the constitutive behavior in the micro-scale. The explicit dynamic finite element method was used to simulate the dynamic response in the macro-scale. The dynamic responses of CMC plate under impulse and harmonic load were simulated. The incompressible Navier-Stokes equations were employed in the simulation of fluid. The dynamic response of CMC plate with FSI was solved based on the staggered method. The effect of fluid on the dynamic response of CMC plate was discussed.

Monitoring of temperature and cure-induced strain gradient in laminated composite plate with FBG sensors

Unidirectional laminated composite plate composed of AS4/8552 prepregs was manufactured with autoclave processing, and the bare and encapsulated FBG sensors were longitudinally embedded into the composite plate along the through-thickness direction to monitor the development of temperature and strain gradient. Before the measurement, the coefficient of temperature sensitivity and axial strain sensitivity of FBG sensors were calibrated, and the results reveal that the coefficient of axial strain sensitivity is a constant but coefficient of temperature sensitivity depends on the magnitude of temperature. Subsequent data analyses on the monitored results by encapsulated FBG sensors show that temperature distribution is relatively uniform inside composite plate but temperature gradient still exists in the through-thickness direction during curing. Meanwhile the monitored results by bare FBG sensors indicate that strain gradient also occurs during curing, and further investigation demonstrates that tool-part interaction is the major cause for the formation of strain gradient within the unidirectional composite plate.

A Local Sensitivity-Based Multiscale Stochastic Stress Analysis of a Unidirectional Fiber-Reinforced Composite Material Considering Random Location Variation of Multifibers

This paper describes an efficient computational method for estimating the probabilistic properties of the maximum microscopic stresses in a unidirectional fiber-reinforced composite material against microscopic random variations of fibers locations. Some microscopic geometrical random variations will cause a large variation of the microscopic stresses, even if the influence on the homogenized elastic properties is small. The random variation of the microscopic stresses will have a significant influence on the apparent strength of composites, and therefore, estimation of the random variation will be important for reliability-based design of a composite structure. Further, for more precise analysis, a unit cell containing many inclusions should be employed. When the number of random variables becomes large, a multipoint approximation-based approach will not be appropriate. Therefore, a computational approach with a local surrogate constructed by a successive sensitivity analysis is proposed in this paper. The realizations of the microscopic stresses are estimated with the successive sensitivity-based local surrogate, and the probabilistic properties of the stresses are estimated with using the approximated realizations in the Monte Carlo simulation. As an example, the multiscale stochastic stress analysis of a unidirectional fiber-reinforced composite plate under unidirectional tensile load along the transverse direction is performed with considering randomness in fibers locations. For this problem, probabilistic properties as the expectation and coefficient of variation of the maximum microscopic stresses in resin are estimated. From comparisons between the direct Monte Carlo simulation and the proposed method, validity and effectiveness of the proposed approach are discussed.

Experimental Investigation of Fatigue and Mechanical Properties of Unidirectional Composite Plates Filled Nanoparticles

Experimental Investigation of Fatigue and Mechanical Properties of Unidirectional Composite Plates Filled Nanoparticles

Modeling of fiber circumplacement around a hole using a streamline approach

The insertion of holes into laminates can be done by producing a fiber reinforced composite plate and, subsequently, drilling the borehole. Alternatively, we can bypass the fibers around the final hole before injecting the matrix material. In the first case, the spatial distribution of the axis of anisotropy, and the structural tensor concerned, are spatially constant. In the second case, i.e. the fiber circumplacement around the hole, a space-dependent anisotropy has to be considered. Instead of the common approach of defining region-wise constant fiber orientations, we propose a continuous formulation of fiber orientations using streamlines. To estimate the final stress and strain state for a unidirectional composite plate, three-dimensional finite element simulations are performed, where spatially constant transverse isotropy is compared to inhomogeneously distributed fiber orientation around the hole. It will turn out that the resulting stress states lead to both reduced stress amplitudes in loading direction as well as compressive strains in lateral direction. A detailed mathematical derivation of the basic equations accompanies the investigations.

Strain gradient elasticity and size effects in the bending of fiber composite plates

The present work aims to examine whether strain gradient elastic theory is able to capture both positive and negative size effects appearing in the bending of unidirectional fiber composite plates. To this goal, the deflections of fiber composite plates subjected to a bending loading under plane strain conditions are numerically evaluated and compared to the corresponding ones provided by the bending of the same plates homogenized as gradient elastic materials. All the plates have the same dimensions and fiber volume fraction, while the radius of the embedded fibers gradually changes. Plates with up to 2352 periodically and randomly distributed cylindrical elastic fibers, embedded in a matrix medium, has been considered. The fibers are either harder or softer than matrix medium while the case of cylindrical cavities is also considered. Each of the considered problems is solved numerically through a Boundary Element method (BEM) supported by Adaptive Cross Approximation (ACA) techniques so that to be able to solve large-scale problems involved in the present study. The obtained results are demonstrated and commented.