Class Of Laminates Research Articles

Optimization of composite aeronautical components by Re-designing with double-double laminates

This paper introduces a novel approach to optimize aircraft structural design for enhanced performance and environmental sustainability. Traditional constraints on composite laminate stacking sequences, such as the symmetrical and balanced nature of classical quasi-isotropic laminates, often lead to over-dimensioning of aeronautical components. To overcome these limitations, the adoption of a new class of laminates known as Double-Double laminates, originally introduced by Prof. S.W. Tsai, is proposed. Unlike traditional laminates, Double-Double laminates offer flexibility in layer orientations, small building blocks, easy homogenization, and tapering capability.This study presents various case studies of increasing complexity to assess the effectiveness of the proposed methodology. First, a multicopter frame is redesigned to reveal the methodology's ability to reduce component weight while ensuring strength requirements under typical service loads. Then, the “DD Automated Design Tool”, a newly developed Finite Element Method tool interfacing with the Lamsearch engine, is introduced. This tool facilitates the rapid identification of optimal composite double-double laminate layups based on material properties, applied loads, and boundary conditions. By iteratively interacting between the Finite Element Method software and the Lamsearch engine, the tool efficiently identifies the best laminate angles to meet specific strength requirements.To validate the weight-saving capability of the proposed methodology, the optimizations on a composite fuselage section and a real composite wing-box under realistic loading conditions have been carried out. Both cases resulted in significant weight reductions compared to initial configurations, demonstrating the effectiveness of Double-Double laminates in achieving structural efficiency and meeting performance demands.

Geometric measure theory and differential inclusions

In this paper we consider Lipschitz graphs of functions which are stationary points of strictly polyconvex energies. Such graphs can be thought as integral currents, resp. varifolds, which are stationary for some elliptic integrands. The regularity theory for the latter is a widely open problem, in particular no counterpart of the classical Allard’s theorem is known. We address the issue from the point of view of differential inclusions and we show that the relevant ones do not contain the class of laminates which are used in [23] and [26] to construct nonregular solutions. Our result is thus an indication that an Allard’s type result might be valid for general elliptic integrands. We conclude the paper by listing a series of open questions concerning the regularity of stationary points for elliptic integrands.

On the use of non traditional stacking to maximize critical buckling loads in flat composite panels

Traditional lay-ups in laminated composite structural parts have largely focused on using quasi-isotropic laminates made of permutations of 0°,90° and ±45° laminae. These laminates are referred to as legacy quad laminates (LQL). In this study, three different lay-ups of symmetric laminates to compute critical buckling loads under uniaxial in-plane compressive loads is first investigated. An optimization problem is solved to determine the lay-ups that maximize the buckling performance. The resulting laminates are compared against a new class of laminates, referred to as double-double (DD), which have distinctive advantages for thickness tapering and therefore manufacturability. Both, symmetric and unsymmetric DD laminates are studied, while in the latter case, a thermal cure cycle followed by the application of in-plane loads is used to determine the structural performance in compression. To further verify the versatility of DD laminates, biaxial buckling loads are computed for quad, DD and optimized lay-ups. It is concluded that DD laminates, because of their manufacturability advantages, are promising candidates for several aerostructural applications.

Effect of rotational restraints on the stability of curved composite panels under shear loading

The present work focuses on the effect of rotational restraints on the shear buckling of symmetrically laminated curved composite panels. The Sanders–Koiter shell theory and a first-order shear deformation scheme were used for the mathematical representation of the deformation kinematics of cylindrical shells. The eigenvalue buckling equations were obtained using the principle of minimum total potential energy and by employing the Ritz method. The solution for the deformed shape was approximated as a series of trigonometric functions compatible with the essential boundary conditions of the problem. The effect of the rotational and torsional springs was incorporated by adding their corresponding potential energy to the total potential energy of the panel-loading system. Using the developed formulation, the effect of the influential parameters such as aspect ratio, panel curvature and restraint stiffness on the buckling strength of a specific class of laminates was studied extensively. To present the results in a more insightful manner, non-dimensional parameters were used in parametric studies. To normalize the effect of torsional elements, a new non-dimensional parameter was introduced.

Effect of Stacking Sequence on Interlaminar Shear Strength of Multidirectional GFRP Laminates

Abstract Glass fiber reinforced polymer matrix composites are widely used in many applications such as aerospace, automobile, marine, electrical etc. because of its superior mechanical property coupled with minimum weight requirements of the component. The performance of a composite part gets mostly affected by the residual stresses, stacking sequences due to relation with in-plane stresses. This study investigates performance of multidirectional laminates using bi-directional plain-weave glass fabric, the alteration in stacking sequence is done in, in-between plies, keeping inner and outer ply orientation constant. Laminates are made by hand layup and compression moulding technique. Five class of laminate, keeping stacking sequence of [0°/(0°)2/0°]S, [0°/(15°)2/0°]S, [0°/(30°)2/0°]S, [0°/(60°)2/0°]S, [0°/(75°)2/0°]S are manufactured and tested for interlaminar shear stress using ASTM D2344-16 standard on fully computerised UTM with strain rate of 1 mm/s. It is seen that laminate performance exhibits dependency on orientation of in between plies. The Laminates with all plies oriented in 0° position depicts lowest ILSS whereas the laminate having 60° ply in between possesses highest ILSS

Ballistic impact response of an UHMWPE fiber reinforced laminate encasing of an aluminum-alumina hybrid panel

The impact response of an ultrahigh molecular weight polyethylene (UHMWPE) fiber reinforced polymer matrix composite laminate has been investigated. The laminate encapsulated an aluminum alloy sandwich panel whose corrugated core was filled with prismatic alumina inserts. The laminate encased hybrid core target could sustain ceramic prism base impacts by a spherical, 12.7 mm diameter steel projectile with velocities in excess of 2.7 km s−1. This was 150% higher than the ballistic limit of an equal areal density, similarly encapsulated aluminum plate target. By contrast, when the projectile impacted a hybrid core target at the apex of a ceramic prism insert, failure of the UHMWPE laminate on the rear face occurred at a lower impact velocity. High-speed imaging, three-dimensional digital image correlation and x-ray tomography measurements are used to show that upon impact the projectile and the ceramic insert fragment. These fragments then load the UHMWPE laminate on the rear face with a significantly reduced pressure compared to the impact pressure of the projectile on the front surface of the target. The loading area on the inner surface of the rear laminate was highest for a prism base impact and lowest for a prism apex impact. The inability to penetrate the rear laminate of the base impacted samples is consistent with the recent identification of an impact pressure controlled mechanism of progressive penetration in this class of laminate.

The polar analysis of the Third-order Shear Deformation Theory of laminates

In this paper the Verchery’s polar method is extended to the conceptual framework of the Reddy’s Third-order Shear Deformation Theory (TSDT) of laminates. In particular, a mathematical representation based upon tensor invariants is derived for all the laminate stiffness matrices (basic and higher-order stiffness terms). The major analytical results of the application of the polar formalism to the TSDT of laminates are the generalisation of the concept of a quasi-homogeneous laminate as well as the definition of some new classes of laminates. Moreover, it is proved that the elastic symmetries of the laminate shear stiffness matrices (basic and higher-order terms) depend upon those of their in-plane counterparts. As a consequence of these results a unified formulation for the problem of designing the laminate elastic symmetries in the context of the TSDT is proposed. The optimum solutions are found within the framework of the polar-genetic approach, since the objective function is written in terms of the laminate polar parameters, while a genetic algorithm is used as a numerical tool for the solution search. In order to support the theoretical results, and also to prove the effectiveness of the proposed approach, some new and meaningful numerical examples are discussed in the paper.

Crack Opening Displacement and the Associated Response of Laminates with Varying Constraints

This paper reports an experimental study of the opening displacement behavior of 90-ply cracks in a class of composite laminates where the constraint to cracking is varied systematically. The study is motivated by a synergistic approach to damage mechanics proposed by Talreja (1996) where the crack opening displacement forms a key micromechanically determined parameter in a continuum damage formulation of the composite response. It is found that although the opening displacement shows scatter from point to point along a crack length, its average over the crack length forms a robust parameter. This parameter, when introduced as the damage influence coefficient in a continuum description of damage, allows satisfactory predictions of the elastic moduli changes due to transverse cracks in the considered class of laminates with varying constraints.

VISCOELASTIC ANALYSIS OF PROCESSING-INDUCED RESIDUAL STRESSES IN THICK COMPOSITE LAMINATES

Residual stresses induced during the processing of thick composite laminates are analyzed. A two-dimensional thermochemical analysis is used to obtain temperature and degree of cure distributions during cure. A two-dimensional finite-element model is developed to predict the residual stress history in rectangular laminated plates. A cure-dependent visco-elastic material model is used in the analysis. Both unidirectional and cross-ply laminates are investigated. Unidirectional laminates are studied because residual stresses in this class of laminates is driven purely by thickness effects. Regardless of the layup,for moderately thick (2.54-cm) laminates the nonuniformities in temperature and degree of cure are mild. As a result, the development of residual stress is very similar to the analysis of thin laminates. For laminates of large thickness (7.5 cm), stronger variations in temperature and degree of cure develop during the cure cycle. The development of residual stress in this case is more complex. The transverse stress distribution in unidirectional laminates is tensile at the center and compressive on the edge at the end of cool-down. The profile is inverted twice during the cure cycle as competing mechanisms of thermal expansion, chemical shrinkage, and chemical hardening interact. The transverse residual stress for 7.5-cm thick unidirectional laminates was shown to reach a value of 3.5 MPa after cure, compared to a value of only 0.65 MPa for 2.54-cm-thick laminates. In all of the cases analyzed, the laminates exhibited a center-to-surface cure behavior. Thicker laminates and laminates constructed of materials with different cure kinetics could show a surface-to-center cure behavior. In such cases the residual stress development will be significantly changed from the results presented in this article.

Optimization of robotic arms made of composite materials for maximum fundamental frequency

The paper deals with the design of a composite arm, to be employed in the architecture of industrial, measuring robots. Based on the real working conditions, the arm is reduced to a cantilever beam, shaped as a thin walled tube, supporting a mass at the free end. An optimization study is carried out, aiming to determine the laminate properties capable of providing the maximum fundamental frequency (FF) in bending. The optimization relies on an approximate solution, obtained by modifying the Rayleigh energy method to account for the shear effect. It results in a closed-form formula, explicitly correlating FF with composite elastic properties. Two classes of laminates, namely (± θ) s and ( 0 ± 45 n ) s , are examined in order to assess their ability to fulfil the requirement of a maximum FF value. Moreover, the advantages and drawbacks of composites compared to metals are highlighted for the application under concern. Finally, the validity of the proposed solution is demonstrated by comparison with the results of a finite element analysis.

Analysis of a symmetric laminate with mid-plane free edge delamination under torsion: theory and application to the edge crack torsion (ECT) specimen for mode III toughness characterization

A shear deformation theory is developed to analyze a symmetric laminate with mid-plane edge delamination under torsion loading. The theory is based on an assumed displacement field which includes shear deformation. The governing equations and boundary conditions are obtained from the principle of virtual work. Closed-form solutions for the compliance and strain energy release rate are obtained. The theory is used to analyze a mid-plane Edge Crack Torsion (ECT) specimen used for characterizing the mode III delamination behavior of composites. The prediction of the present theory for a [90, (±45)n, (±45)n, 90]s class of laminates is compared with existing test data, and good agreement is observed. In addition, the Classical Lamination Theory (CLT) is found to be less accurate for the analysis of the edge crack torsion specimen. The layup effect on the behavior of some ECT specimens is analyzed using the present theory.

Influence of free-edge delaminations on the extension-twist coupling of elastically tailored composite laminates

A closed form solution is developed in order to analyze the influence of free-edge delaminations on elastically tailored composite laminates. The analysis is based upon a shear deformation theory and a sublaminate approach. The solution is applied to predict the influence of free-edge delamination on the behavior of a class of laminates exhibiting extension-twist coupling. Comparison of analytical predictions with test data from laminates with mid-plane and off mid-plane free edge delaminations is performed. The results indicate that free-edge delamination can have a significant influence on extension-twist coupling.

Deformations in Composite Shells of Revolution under Axisymmetric and Thermal Loads

Numerous papers have been published dealing with stresses and deforma tions in composite cylinders. However, none of them included thermoelastic analysis and spherical geometry. In the present paper, deformations in balanced laminated shells of rev olution subjected to internal pressure and thermal loads were studied using the membrane theory. The results for a special case of spherical shells show that membrane theory is sufficient in predicting the deformed shapes of laminated shells with identical axial and transverse stiffnesses, which are common in many engineering applications. Deformations in laminates with different axial and transverse stiffnesses become unbounded at the poles which indicate that, for such class of laminates, the stress buildup at the tip point cannot be balanced by membrane forces alone.

Ply stacking sequences for laminated plates having in-plane and bending orthotropy

It is shown that selection of a suitable ply stacking order for uncoupled, orthotropic laminates enables orthotropy in bending, as well as in-plane, to be achieved. In addition, a class of laminate is described which lacks ply symmetry about the mid-plane but which exhibits bending and in-plane orthotropy together with no elastic coupling between in-plane and bending effects.