Orthotropic Laminates Research Articles

In situ experimental study on possible strengthening and toughening mechanism of interlaminar pores in carbon fiber orthotropic laminates

In order to study the regulation mechanism between the internal micro-pores and the mechanical properties of carbon fiber orthogonal laminated composites. In this paper, a kind of carbon fiber orthogonal laminated composite with different types of interlaminar micro-pores was carried out in-situ loading experiments by SR-CT, and the internal fibers, pore microstructure and damage of the sample in the process of progressive failure were tracked in the whole field and in real time. Through in-situ experiments, special phenomena such as three times reduction progressive failure of the material loading curve, secondary cracking of the fiber layer, and crack arrest by porosity was observed, Furthermore, through the analysis of the two special stages of strength improvement and toughness enhancement of the loading curve, it is found that the strengthening mechanism of the orthogonal laminated structure to realize the repeated strength improvement of the material by controlling the interlaminar alternating shear action, and the toughening mechanism of crack tip passivation induced by micro-pores. It provides a possible way to realize the strengthening and toughening of composites through the design of microporous structure.

Deformation of rectangular orthotropic laminate resting on thin elastic foundation with nonlinear change of displacements over thickness under edge loading

Deformation of rectangular orthotropic laminate resting on a thin elastic orthotropic foundation with fully fixed bottom plane is studied in this paper. The laminate is subjected to an in-plane uniaxial load uniformly distributed over one of its edges. Models of joint deformation of the laminate and foundation are created based on the equations of plane elasticity for an orthotropic body. The applicability of such equations is justified by the uniformity of edge loading. A laminate model allows for the transverse shear strain, whereas a model created for the foundation considers a nonlinear character of variation of the transverse and axial displacements. A governing system of differential equationsmodelling the joined deformation of the laminate and foundation is derived using static and kinematic contact conditions. The corresponding boundary value problem is solved using the sweep method. Based on this solution, effects of laminate and foundation thicknesses, and elasticity characteristics on the stress–strain state of the jointed structure are investigated. The model created and the method of its analysis enable the strong oscillating behaviour of stresses and strains to be studied. The results obtained can be utilized for verification of solutions of similar problems found by other numerical methods, including finite-element analyses.

Anisotropic auxetic composite laminates: A polar approach

The problem of obtaining anisotropic auxetic composite laminates, i.e. having a negative Poisson’s ratio for at least some directions, is examined in this paper. In particular, the possibility of obtaining auxeticity stacking uni-directional identical plies is considered. It is shown that if the ply is composed by isotropic matrix and fibers, then it is impossible to obtain totally auxetic orthotropic laminates, i.e. auxeticity for each direction, unless at least one among matrix and fibers is auxetic itself. Moreover, it is shown what are the conditions, in terms of the mechanical properties of the constituents and of the volume fraction of the fibers, to fabricate uni-directional plies with which to realize laminates having a negative Poisson’s ratio for some directions. Several existing materials are also examined. All the analysis is done using the polar formalism, very effective for the study of plane anisotropic problems.

Multiscale simulation and prediction of mechanical properties for carbon fiber reinforced polymer orthotropic laminate

AbstractA multiscale analysis method based on numerical homogenization is presented for predicting the mechanical properties of carbon fiber reinforced polymer (CFRP) orthogonally symmetric laminate. Initially, based on the structural characteristics of CFRP orthotropic symmetric laminate, the relational equations for calculating the mechanical engineering constants from the stiffness coefficient are proposed. Subsequently, the finite element model (FEM) of orthotropic laminate is established. Corresponding subroutines are developed through ABAQUS‐Python to extract and compute the simulation results and obtain the mechanical properties of the laminate. Next, the macroscale mechanical properties are estimated by combining the micro‐scale mechanical model, classical laminate theory (CLT), and volume averaging method. Ultimately, the accuracy of the proposed equations and the feasibility of the numerical homogenization method are demonstrated by comparing the results of numerical simulation predictions, theoretical predictions, and experiments.Highlights Present the relational equations for calculating the mechanical engineering constants of CFRP orthotropic symmetric laminate from the stiffness coefficient. Establish the FEM of orthotropic laminate and develop corresponding subroutines to obtain the mechanical properties of the laminate. Analyze the mechanical properties of orthotropic laminate through microscopic modeling and CLT.

Estimating the elastic constants of orthotropic composites using guided waves and an inverse problem of property estimation

The present research focusses on estimating the elastic constants of orthotropic laminates using ultrasonic guided waves (GWs) excited through Lead Zirconate Titanate (PZT) sensors and sensed using one-dimensional laser vibrometer. The elastic constants of a material are crucial for understanding its mechanical behaviour and are typically determined through experimental testing. However, this process can be time-consuming and expensive. We formulate this problem as an inverse problem of property estimation. Thus, in this work, the simulation models with PZT transducers have been employed for generating time series (TS) GWs for the orthotropic material. Then, an inverse machine learning model is trained using a TS dataset pertaining to different elastic constants generated using the simulations. The inverse model consists of deep neural networks and designing a loss function for the specific application. Limited number of unique sets of simulations were conducted. Out of available simulation data, 30% of the sets were used for validation. To further test the model, a blind experimental test was conducted, and the corresponding elastic constants were estimated with a mean absolute percentage error (MAPE) of 12.89% and standard deviation of 5.47%. The results demonstrate that formulation of property estimation as inverse problem is capable of accurately predicting the elastic constants of a material, by using a model solely trained on simulation and a very scare amount of data. This approach has the potential to significantly reduce the computational time for predicting the elastic constants, and thereby could have wide-ranging applications in materials science and engineering.

Manipulation of acoustic wave scattering from cylindrical shells using a coating of FG magneto-electro-elastic composite materials: Active acoustic cloaking

Acoustically cloaking an object from view of a system that uses acoustic waves for detection is an attractive and practical capability. The aim of the current investigation is to hide a steel cylinder containing axial flow acoustically, which is achieved through the application of a coating of FG electro-magnetic material. This coating is employed to minimize the sound pressure that is scattered from the structure, and it comprises piezoelectric (BaTiO3) and magnetostrictive (CoFe2O4) make up the coating. Accordingly, the method presented is based on optimal control and incorporates an improved particle swarming optimization algorithm. The decision variables are the magnetic and electrical potentials on the inner and outer surfaces of the coating, whereas the cost function is the scattered acoustic pressure. In other words, what are the values of electrical and magnetic potentials on the inner and outer surfaces of the coating to minimize the scattered pressure? Subsequently, by defining the electrical and magnetic boundary conditions in a certain way, it is feasible to achieve the complete cancelation of scattered sound pressure. The governing equation of each layer is extracted using an orthotropic laminate composite model that is based on the 3-D theory of elasticity. A state-vector method is then applied to obtain the propagator matrix that links the state variables at each layer's surfaces. The results show that the proposed acoustic cloaking technique has an excellent capability for hiding the shell immersed in the fluid.

Embedding Fiber Bragg Grating Sensors in Carbon Composite Structures for Accurate Strain Measurement

Fiber Bragg Grating (FBG) sensors written by femtosecond laser pulses in polyamide-coated low bending loss optical fibers are succesfully embedded in carbon composite structures, following laminating and light resin moulding processes which optimize the size of each ply to address aesthetic, drapability and structural requirements of the final components. The sensors are interrogated by a tunable laser operating at around 1.55 μm and their response to temperature and strain variations is characterized in a thermally controlled chamber and by bending tests using suspended calibrated loads and a laser scanning system. Experimental results are in good agreement with simulations, confirming that the embedding process effectively overcomes potential issues related to FBG spectral distortion, birefringence and losses. In particular the effects of the composite material non homogeneity and FBG birefringence are investigated to evaluate their impact on the monitoring capabilities. A bi-material mechanical beam model is proposed to characterize the orthotropic laminates, pointing out better accuracy in estimating the applied load with respect to the classical homogeneous beam model. A comparative analysis, performed on different instrumented carbon composite samples and supported by theory, points out the repeatability of the FBG sensors embedding process and the effectiveness of the technology for real time accurate strain measurement. Based on such measurements damages and/or changes in local stiffness can be effectively detected, allowing for structural health monitoring of composite structures for applications in specific industrial fields like automotive and aerospace.

Experimental investigation of impact resistance and compression behavior of CF/PEEK laminates after hot‐press fusion repair with different stacking sequences

AbstractUnidirectional fiber‐reinforced composite laminates have limited resistance to out‐of‐plane impact and are prone to structural integrity damage during service due to low‐velocity impact (LVI). In this study, the hot‐press fusion method was employed to repair the impact‐damaged unidirectional carbon fiber‐reinforced poly‐ether‐ether‐ketone (CF/PEEK) thermoplastic composites. No delamination or matrix cracking induced by impact was observed in the repaired specimens, as confirmed by ultrasonic scanning and digital microscope. In addition, the hot pressing treatment enables the fibers to be covered again by the resin matrix, which eliminates fiber pull‐out and fiber/matrix debonding. Afterward, typical responses of CF/PEEK composite laminates to re‐impact and post‐repair compression are presented in a comprehensive and detailed manner, and compared with the initial impact response and compression after impact (CAI) behavior. The effects of three impact energies and two stacking sequences are considered. The results indicate that quasi‐isotropic laminates are more susceptible to localized fracture damage upon re‐impact as the impact energy increases, due to initial fiber breakage, compared to orthotropic laminates. The hot‐press treatment enhances the compression residual strength of the specimen by 20%–30% compared to its state before repair. These studies offer technical insights into the application of the hot‐press fusion method to enhance the mechanical properties of composite laminates post impact damage.

Buckling of woven fibre and graphene platelet reinforced nanocomposite laminates

Buckling of a hybrid laminate combining woven glass fibres with graphene platelets in a polymer matrix is the subject of the present study. Multiscale composites provide improved stiffness and light weigh laminates and as such they have become the materials of choice in several applications. The elastic properties of 3-phase materials are determined by using micromechanical relations. In the present study, the elastic constants of the woven fabric are computed first and the second step involves extending the micromechanical computations to determine the properties of the 3-phase woven fabric-graphene reinforced matrix. The objective is to improve the buckling capacity of the woven fabric laminates with high stiffness and light weight graphene reinforcement. Analytical results are given for the buckling load for a simply supported and orthotropic laminate, taking into account the complex pattern of woven fabrics consisting of weft and warp fibres. Numerical results illustrate the effect of the design parameters such as the layer thicknesses, fibre and graphene contents and aspect ratio on the buckling load. In particular, the effectiveness of the graphene reinforcement on improving buckling load is illustrated by means of contour plots.

Tension failure assessment of composite bolted joints under bearing-bypass load interaction using analytical means

Bolted joints are often used in thin parts such as plates or shell-like components in lightweight structures, e.g. air- and spacecraft. This is also motivated by an inexpensive manufacturing and the ability to disassemble. To make these joints, holes need to be drilled and stress concentrations arise. Leading to the instantaneous destruction of the connection, attention should be drawn to fatal tension failure. The efficient and precise prediction of the corresponding failure stresses using analytical methods is the focus of this paper. Usually, rows of bolts are placed. Then, the load is partly introduced into one bolt while the rest stays in the plate. This setting is also referred to as bolted joint under combined bearing-bypass load, which shall be idealised as a linear 2D plate problem. The corresponding characteristic stresses are discussed for orthotropic laminates showing good agreement to Finite Element values. Then, failure analysis is conducted using Finite Fracture Mechanics. This part is dedicated to quasi-isotropic laminates. The size effect and in this context the failure stress reduction with increasing bolt diameter is analysed. Primary physical effects due to finite dimensions and the ratio of bearing and total load are identified. The higher this ratio and the larger the bolt diameter, the lower the sustained failure stress. Failure envelopes for the graphical determination of the critical bearing and bypass stresses are provided. All in all, a comprehensive and efficient framework for tension failure assessment of composite bolted joints under bearing-bypass load interaction is developed.

The effect of laminate sequences on tuft bridging in GFRP laminates under tensile loading

This conference paper reports the tuft bridging behaviour of a single tuft when used in an orthotropic or a quasi-isotropic laminate. Laminates are subjected to pull-out experiments with a full release film placed in the mid-layer of laminates. The force-separation diagrams and damaged samples obtained by tensile tests are analysed to compare two different laminate. The results show that the larger amount of resin present around the tuft in orthotropic laminate reduces by 60% the ultimate tensile strength of the tuft when compared to quasi-isotropic laminates. The tuft tends to fail in the mid-layer of quasi-isotropic laminate with no pull-out, while the tuft has a bit pulling out in orthotropic laminate, causing an increase in the fracture energy.

Orthotropic vs quasi-isotropic: How the tape orientation influence the performance of disentangled polyethylene laminates

Disentangled ultrahigh molecular weight polyethylene (DPE) is a high-tenacity and high-modulus material having great potential for impact resistance applications. This research presents a comprehensive experimental study on the effect of orientation on the tensile and low-velocity impact performance of laminates produced from DPE tapes. The laminates were constructed by laying the DPE tape at different orientations followed by compression moulding. Quasi-isotropic laminate showed good tenacity in all directions whereas cross-ply orthotropic laminate showed the maximum tenacity in both the principal directions (0° and 90°). The impact energy absorption by the orthotropic laminate was 61.1% higher compared to that of quasi-isotropic laminate. The results imply that cross-ply laminate is the most suitable one for ballistic applications.

Equivalent stiffness calculation of composite hat stiffened laminate

The stiffness is the main reason that restricts the large-scale of composite ships, the basis to solve this problem is to solve the stiffness of composite ships’ skeleton structure. Based on classical laminates theory, this paper calculated the equivalent in-plane stiffness and bending stiffness of composite hat stiffened laminate using equivalent stiffness principle. The hat stiffened laminate can be simplified and equivalent to orthotropic laminates using this solution idea, which solves the problem that it is difficult to obtain the analytical solutions of deformation, stress and strain of stiffened laminate. The research results can be used to calculate the stiffness of composite stiffened laminate and the local or global stiffness of the hull girder with such a skeleton structure.

Analytical and Experimental study on FVA of Isotropic and Orthotropic Laminates with FEM Validations

Several approaches such as two-dimensional shear deformation and higher order theories, three-dimensional exact elasticity method and 2-D, 3-D finite element procedures are available in predicting the natural frequencies of composite laminates. In all, a 3-D FEM assists in developing a model for structures with complex constraints, materials and complex geometries. But FE directs an approach that obtains an approximation in solution that depends on discretization type and the polynomials method desirable for field variable limitations. Hence, a convergence test is performed in addition to FE in order to improvise the finite element results accuracy and validating with other benchmark outcomes. The current investigation involves the solving for several boundary conditions of a thick rectangular plate’s free vibration analysis [FVA] for isotropic and orthotropic materials by the application of 3-D finite element methods and convergence results are compared with available analytical results. In addition, experiments are conducted on thick isotropic and orthotropic materials along with experimental modal analysis in order to verify 3-Dimensional finite element process. From this study it can be known that, the values of finite element method highly converges with various experimental and analytical results of isotropic material. When comparing 3-D FEM with the experimental results of the orthotropic sample, a deviation of 5-8% was observed.

A higher-order plate theory for the analysis of vibrations of thick orthotropic laminates

In this contribution, flexural vibrations of linear elastic laminates composed of thick orthotropic layers, such as cross-laminated timber, are addressed. For efficient computation, an equivalent single-layer plate theory with eight kinematic degrees of freedom is derived, both in terms of equations of motion at the continuum level and in terms of a finite element representation. The validity of the plate theory is demonstrated by comparing natural frequencies of a simply supported plate over a wide frequency range for which an analytical solution is available. Furthermore, the influence of individual material properties on the accuracy of the plate theory is investigated, demonstrating its broad applicability. The influence of material orientation on the accuracy of the plate theory is examined by comparative finite element simulations. It is shown that, for cross-ply laminates, the plate theory is valid for elements oriented at any angle to the material principal axes.

Experimental and multiscale modeling investigations of cryo-thermal cycling effects on the mechanical behaviors of carbon fiber reinforced epoxy composites

In cryotank applications, carbon fiber reinforced epoxy composites are required to face the challenge of extreme cryo-thermal cycles while rare relevant research work has been conducted. This paper reports cryo-thermal cycling effects on the mechanical behaviors of carbon fiber reinforced epoxy composites via experimental and multiscale finite element method (FEM) investigations. Four types of cryo-thermal cycling are designed to explore the roles of the cycling number and the cycling period under normal and severe conditions. Unidirectional and orthotropic laminates are prepared to demonstrate intralaminar and interlaminar expansion mismatch phenomena. For unidirectional laminates, the transverse tensile strength shows a dramatic initial decrease of 18.7%, 21.2%, 4.1% and 17.4% while the interlaminar shear strength is lowered by 4.9%, 14.2%, 5.1% and 14.0% after cryo-thermal cycling for four cycle types, respectively. Meanwhile, it is exhibited that the interlaminar shear strength of orthotropic laminates is reduced by 17.1%, 18.1%, 13.1% and 17.9%, respectively. Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) measurements are executed to detect the secondary curing. Based on the constitutive relationship affected by temperature and moisture absorption, a microscale unit cell with random fibers and a specimen-sized model with cohesive elements are established to reveal the intralaminar and interlaminar damage evolutions during cryo-thermal cycling. After elucidating typical damage modes, such as interface debonding, matrix microcracks, surface frost heave and interlaminar gap generating, the multiscale FEM model is modified to evaluate mechanical degradations quantitatively, which gives reasonable explanations for experimental results and provides an efficient and convenient method for composite design.

Stress analysis of finite orthotropic composite containing an elliptical hole

Although the mechanical integrity of a member can be highly influenced by associated stresses, determining the latter can be very challenging for finite orthotropic composites containing cutouts. This is particularly so if the external loading is not well known, a common situation in practical situations. Acknowledging the above, a finite elliptically-perforated orthotropic tensile laminate is stress analyzed by combining measured displacement data with relevant analytical and numerical tools. Knowledge of the external loading is unnecessary. Results are verified independently and the concepts are applicable to other situations. The developed technology can provide important design-type information for orthotropic composites. In particular, the ability to apply analyses for perforated composite structures which assume infinite geometry to finite geometries is demonstrated.

Influence of a flame-retardant on the fire-behaviour and the residual mechanical properties of C/PEKK composite laminates exposed to a kerosene flame

This work examines the influence of a phosphorus-based flame-retardant additive (Exolit) on the fire-behaviour and the residual mechanical properties of composite laminates manufactured by a thermo-consolidation process from unidirectional (UD) carbon fibers (C) and a PEKK (Polyether-ketone-ketone) thermoplastic associated with or without Exolit. Depending on the laminates lay-up (orthotropic or quasi-isotropic), the influence of Exolit on the thermal degradation, resulting from a kerosene flame exposure (116 kW/m2 and 1100 °C), on the composites structural integrity was examined. The changes in the residual tensile properties (axial stiffness and strength) were compared with respect to the virgin materials (experiencing no prior flame exposure) in room temperature (RT) conditions post fire exposure. The discussions on fire- and mechanically-induced damage mechanisms are supported by fractographic analysis of specimens. It is therefore possible to better understand how the fire-induced damages within the laminates micro- and meso-structures modify the mechanical behaviour of flame-exposed laminates. Thus, the axial stiffness and strength decrease moderately (−6% and −18%, respectively) in quasi-isotropic laminates with respect to virgin specimens. In orthotropic laminates, the axial stiffness decreases by 15%, whereas the axial strength dramatically decreases (−55%). The obtained results suggests that the flame-retardant additive not only delays the thermal degradation of C/PEKK laminates but also efficiently contributes to maintain the structural integrity of the composites, hence resulting in good mechanical properties. The positive effect of Exolit is particularly noticeable in orthotropic laminates whose mechanical behaviour is driven by 0° carbon fibers.

Prediction of the spring-in of cylindrically orthotropic media and cross-ply laminates

The equation for prediction of the spring-in angle of a cylindrically orthotropic segment is shown to be independent of all material properties except for the anisotropic coefficients of thermal expansion and a stress-free state is insured for the corresponding unconstrained deformation. In contrast, the complete cylindrical geometry is shown to provide constraint to thermal deformation and thereby induce thermal residual stresses in the form of a moment. The method of superposition is demonstrated whereby traction-free conditions yield stress-free cylindrical elements with corresponding angular displacements at the element free boundaries. The first derivation of the spring-in equation is attributed to Radford, in contrast to the widely accepted view that the equation was first developed by Spencer et al. Finite-element methods, combined with the superposition approach, further validate the accuracy of the Radford equation for cylindrically orthotropic segments and explore its limitations for multiaxial composite laminates.

Kerosene flame behaviour of C/PEKK composite laminates: Influence of exposure time and laminates lay-up on residual mechanical properties

This work examines the influence of a kerosene flame on the residual mechanical tensile properties of composite laminates manufactured by thermo-consolidation process from unidirectional (UD) carbon fibers and a Polyether Ketone Ketone (PEKK) thermoplastic. Considering laminates with two stacking sequences (orthotropic or quasi-isotropic - QI), the influence of a kerosene flame exposure (116 kW/m2 and 1100 °C) on the composites structural integrity was examined as a function of exposure time (5-10-15min). The discussions on fire- and mechanically-induced damage mechanisms are supported by fractographic analysis of specimens. Not surprisingly, longer exposures to kerosene flame lead to gradually increasing damages within the laminates at different scales (micro-meso-macro), ultimately resulting in degrading the bearing capabilities of the material. With respect to virgin specimens, the axial strength decreases by almost 60 and 40% (after a 15min exposure) in orthotropic and QI laminates, respectively. At the same time, the axial stiffness is about 30 and 20% lower (after a 15min exposure). Contrary to orthotropic laminates, two well-defined areas are observed through the thickness in QI laminates: a severely degraded one and a virtually undamaged one. The first one corresponding to the exposed surface side is significantly damaged (fibers oxidation and pyrolysis of the PEKK matrix leading to an extensive delamination). The second one corresponding to the back-surface side is characterized by two plies with very little damage. When loaded in tension, the “undamaged” plies are characterized by the breakage of 0° fibers. This failure mechanism shows that these “undamaged” plies are capable to bear significant portions of the tensile loading contrary to orthotropic specimens in which extensive delamination prevails through the whole thickness.