Ply Thickness Research Articles

Out of autoclave bonded repair process and implementation for aircraft composite structures

With the entry into service of aircrafts with a very high percentage of composite materials the necessity of efficient repairs for such structures arises. Airbus has developed an out of autoclave bonded repair process for highly loaded composite primary structures of the A350XWB. It can be used at production plants but also in the in-service operation of the aircraft. Bonded repairs eliminate the requirement of rivets in carbon fibre parts allowing flush repairs that complies with the original component functionality and requirements. The repair process includes removing the damaged plies by successive milling steps, each one equal to a single ply thickness. The repair technique consists on the application of a soft patch with specific repair materials, different from those involved in the manufacturing process and with the unique characteristic of curing at low temperature in out of autoclave conditions. The aforesaid facilitates application in plenty of circumstances and scenarios. This paper describes the coordinated investigation performed in order to provide a robust and sound out of autoclave repair system and process. The combination of feasibility trials in actual repair conditions and material characterization tests proved to be essential to understand the critical steps and to optimize the process performance.

Influence of the ply thickness and the stacking sequence on the edge stresses of quasi-isotropic symmetric laminates

An analytic approach for determining interlaminar stresses in rectangular [θ/-θ]s laminates subjected to an axial load has been adapted to the case of [θ1/θ2/θ3/θ4]s laminates. The coupling effects of each part of the laminate are compensated by the adjacent parts. In a first step, a half of the laminate is considered for determining forces and moments per unit length that prevent the global deformation of the whole symmetric laminate. Then, interlaminar stresses are computed applying the stress equilibrium equations and imposing the continuity of the stresses in the layer interfaces. The influence of the ply thickness and the stacking sequence of the laminate on the edge effect is analyzed by the analytical approach.

Optimizing the Aerothermomechanical Behavior of Sandwich Panels Using Genetic Algorithm

A multi-objective optimization of a sandwich structure is presented to maximize the fundamental frequency and the critical buckling temperature while minimizing the cost of the laminate, alongside with keeping the weight lower than a preset value. The sandwich structure was chosen to be made of a high-stiffness, relatively expensive material (carbon/epoxy), and a low-stiffness, relatively low- cost material for the core (glass/epoxy). The governing equations are derived using the principle of virtual work, based on the first-order shear deformation plate theory. MATLAB Genetic Algorithm toolbox is used to find the optimum stacking sequence and ply thicknesses to achieve the optimal solution. The ground structure was used to set the initial ply stacking sequence and then the algorithm assumes the existence or absence of each ply and their thicknesses. The optimum buckling temperature and natural frequency values are introduced for different aerodynamic pressure values and under simply supported boundary condition.

Macro-mechanical analysis of tensile strength of glass/carbon fiber reinforced plastics hybrid composites under hydrothermal environment

The material properties of composite materials are affected by changes in temperature and moisture. This study used the glass/carbon fiber reinforced plastic hybrid composite (G/CFRPHC) laminate as the research objects. The stiffness and strength of the composite lamina were expressed as a function of hydrothermal parameters. Based on classical lamination theory(CLT) and macro-mechanical analysis, using MATLAB programming, the tensile strength of G/CFRPHC laminates under a hydrothermal environment was studied. In addition, the influence of temperature, ply thickness, ply stacking sequence, and ply angle on the tensile strength of G/CFRPHC laminates under a hydrothermal environment was discussed. The results show that the tensile strength of G/CFRPHC laminates decreases with the increase of temperature and laying angle in the temperature range of 20 °C ∼ 110 °C in the hydrothermal environment (moisture absorption rate C 1 = 0.5%). Furthermore, for the G/CFRPHC laminates with laying modes of (02G/90mC)S, (04G/90mC)S, (06G/90mC)S, as m increases, their tensile strength gradually decreases. The tensile strength of G/CFRPHC laminates with the same ply angle but different ply stacking sequence is also not the same.

Effect of stacking sequence on the mechanical performance of the composite structure under slamming impact

Composite materials are attracting increasing interest from many industrial sectors, and their use is becoming more widespread. The naval industry, for example, remains a large field of investigation for composite materials due to their mechanical properties, able to reach very interesting performance levels. Composites are often composed of plies arranged according to loading directions and forming structures called laminates; these directions can have a great effect on the structural behavior of the composite. For this, the study carried out in this article, consists in determining orientations and stacking sequences effect on the mechanical performance of a glass/epoxy composite panel, with a thickness of 8 mm containing 12 plies of the same thickness and different orientations, inclined at an angle of 10° and impacting the water at a velocity of V = 8 m/s.

Analysis of a Rectangular FML composite subjected to coupled thermo-mechanical multi-axial loading

Glare composites are a new set of materials with a combination of metal- fibre layers where glass fibre/epoxy and aluminium alloy are used as its constituent elements. By varying the constituent proportions and types the behaviour of the material changes. In the present study, Glare 4B combination is considered with specific overall thickness as constant and varying the ply thickness by changing the number of plies in each set as 8, 12, 16, 20, and 24 plies. The effect of number of layers and aspect ratios over the failure mechanical properties when subjected to coupled thermo – mechanical multi – axle loading conditions is under study. As number of layers increases each individual layer thickness goes on reducing and the effect of it over failure loads is presented. As the aspect ratio increases the relative comparable factor value is diminishing. Aspect ratio of 1 is considered to be better choice as S1 & S2 at bottom and top layers are inhibiting the stress limits. When it is required for an alternative value as per design considerations it is good to choose aspect ratio of 1.25.

Effects of ply thickness and 0°-layer ratio on failure mechanism of open-hole and filled-hole tensile tests of thin-ply composite laminates

Thin-ply technology provides increasing possibilities for composite structures, expanding the design space for ply thickness and laminate layup. This study examines the effects of ply thickness and the 0°-layer ratio on the strength and damage behavior of thin-ply composite laminates. We conducted open-hole tensile (OHT) and filled-hole tensile (FHT) tests on laminates with various ply thicknesses and 0°-layer ratios, observing the internal damage via X-ray radiography. It was confirmed that the damage retardation effect of the thin-ply reduced both the OHT and FHT strengths. Furthermore, the results revealed that laminates with a higher 0°-layer ratio have higher strengths and the failure of laminates with a higher 0°-layer ratio transitioned to splitting-dominated failure. Finite element analysis of the fastener restraint indicated that the fastener restraint alleviated the stress concentration in the 0°-layer around the hole, potentially leading to a difference in the OHT and FHT strengths.

Stress Redistribution Near a Crack in Maraging Steel using Composite Patch

The presence of a crack significantly reduces the load bearing capacity of a structure made of fracture prone material. The conventional process of repairing a defect is gouging and filling the gouged location by rewelding. It is not only time consuming but also constrained by the number of repairs that can be done as material properties degrade with each round of rewelding. In an attempt to overcome the limitation of the conventional repair process, repairing a defect using composite patch is proposed. The study is carried out on Maraging steel (M250) and the defect considered is a crack. Stress intensity factor (SIF), being an important parameter in fracture-based design, it is evaluated in Abaqus. Extended finite element method is used to model the crack. SIF is used to predict the failure load of a surface cracked tension specimen and the same is compared with the values reported in literature. Composite patch is modelled using woven ply properties. A separate layer of adhesive is also modelled to predict the properties adhesive properties. Failure analysis of each component namely, the Maraging steel plate, the composite patch and the adhesive is carried. It was observed that the addition of a composite patch completely nullifies the presence of a crack. The patch with thickness 1mm and woven ply properties is having minimal damage initiation and likely to survive. The adhesive properties required is also obtained from the finite element analysis. Thus, it was observed that a composite patch with woven ply properties and thickness 1mm is able to completely nullify the effect of a crack when bonded with a suitable adhesive as predicted by the analysis.

Metaheuristic Optimization of Laminated Composite Plates with Cut-Outs

The stacking sequence optimization of laminated composite plates while maximizing the structural performance or minimizing the weight is a subject investigated extensively in the literature. Meanwhile, research on the optimization of laminates with cut-outs has been relatively limited. Cut-outs being an indispensable feature of structural components, this paper concentrates on the stacking sequence optimization of composite laminates in the presence of circular cut-outs. The buckling load of a laminate is used as a metric to quantify the structural performance. Here the laminates are modeled as carbon fiber-reinforced composites using the finite element analysis software, ABAQUS. For the optimization, the widely used harmony search algorithm is applied. In terms of design variables, ply thickness, and fiber orientation angles of the plies are used as continuously changing variables. In addition to the stacking sequence, another geometric variable to consider is the aspect ratio (ratio of the length of the longer sides to the length of the shorter sides of the plate) of the rectangular laminates. The optimization is carried out for three different aspect ratios. It is shown that, by using dispersed stacking sequences instead of the commonly used 0°/±45°/±90° fiber angle stacks, significantly higher buckling loads can be achieved. Furthermore, changing the cut-out geometry is found to have a significant effect on the structural performance.

Effects of hybridization and ply thickness on the strength and toughness of composite laminates

This work presents the results of an experimental study performed in carbon/epoxy composite materials manufactured using a ply-level hybridization technique. The aim of the study is to investigate the potential of such hybridization technique to promote pseudo-ductile failure, and to enhance fracture toughness. Two thin-ply carbon-epoxy systems and three different carbon-carbon hybrid lay-ups are considered. Both strength and fracture tests are performed on the manufactured laminates and the properties of the hybrid materials are compared to those of the baseline non-hybrid composites. Digital Image Correlation and post-mortem X-ray imaging are used to analyze the fracture process of the different materials. The comparison of the mechanical response of the different materials demonstrates that, by means of thin ply hybridization, a pseudo-ductile failure in tension can be obtained, associated with fibre fragmentation. However, the hybridization seems not to be responsible for the increased fracture toughness that is ascribable to the ply-thickness effect.

Effective bending modulus of thin-ply composites with non-uniform fibre spacing

Composite materials are gaining increasing interest for design purposes in many industries such as aeronautical and automotive, not only because of their reduced weight, but also due to their enhanced mechanical properties. The heterogeneity inherent in the nature of composites arises not only because of the mismatch in fibre/matrix properties but also due to the fibre arrangement within the matrix. Previous studies have shown that classical homogeneous elasticity models can either underestimate or overestimate stiffness properties for small ply thicknesses. As such, both positive (stiffer) and negative (more compliant) size effects become increasingly prevalent with decreasing ply thickness, when considering regular distributions of fibres within a matrix material. Current work addresses the effects of irregular fibre spacing on size effects and identifies a characteristic length that can be used to represent the heterogeneity in an equivalent homogeneous micropolar model. Results show that, based on the type of irregular fibre spacing, both kinds of positive and negative size effect arise in a composite ply. As a result, the bending characteristic length embedded in micropolar theory can be defined even for the difficult case whereby fibres are stiffer than matrix depending on the nature of the irregular fibre spacing.

Design and discrete optimization of hybrid aluminum/composite drive shafts for automotive industry

Fiber reinforced composites have been widely used in automotive industry since they offer significant weight reduction, low manufacturing and tooling cost, and better integration of parts compared to metal counterparts. In this study, design optimization of a hybrid aluminum/composite drive shaft subjected to torsion was carried out using ANSYS Workbench with ACP module. The numerical validation of finite element (FE) model was carried out by means of theoretical, experimental, and numerical studies in the literature. The ply material, lay-up orientations, and thickness of aluminum layer were considered as design variables. The geometric parameters in design were the length and inner diameter of the drive shaft. Two important design constraints, the minimum first mode natural frequency and design torque, were considered to satisfy the design requirements of a rear-wheel drive shaft used in automotive industry. The optimum design variables were determined by using screening method. The optimum design parameters (length, inner diameter, ply angle, and material) were presented in tabular form. Compared to nonoptimized scenario, the optimized solution reduced the cost of the hybrid composite drive shaft about 30% without ignoring the design requirements.

Damage-mechanics mesoscale modeling of composite laminates considering diffuse and discrete ply damages: Effects of ply thickness

Thin-ply composites are gaining significant attention owing to their high damage resistance and their ability to enhance design flexibility in a composite structure. Herein we proposed a damage mechanics model considering diffuse and discrete ply damages in thin-ply composite laminates for mesoscale simulation. To investigate the effects of ply thickness on damage evolution, loading and unloading tests were conducted using cross-ply, angle-ply, and quasi-isotropic laminates with several ply thicknesses. The damage evolution was quantitatively evaluated using continuum damage mechanics. The evolution laws of damage were formulated considering the ply thickness dependency. A damage simulation using the proposed model demonstrated that the retardation and suppression effects of thin-ply were captured by the proposed model. The simulation results also validated that the stiffness degradation and failure strength of laminates with three different ply thicknesses were predicted with good accuracy by considering both diffuse and discrete damages.

Layup optimisation of laminated composite tubular structures under thermomechanical loading conditions using PSO

Use of laminated tubular composite structures in applications such as pressure vessels and rocket nozzles provides several advantages including significant weight reduction over traditional monolithic materials. This study investigates the use of Particle Swarm Optimisation (PSO) for the layup design optimisation of a generic tubular geometry under simultaneous thermal and mechanical loading conditions. Existing analytical, closed-form 3D elasticity solutions were used to predict the performance of laminated tubular structures with a specified layup; including implementation of ply-specific fibre-volume fraction. Use of the PSO algorithm is established as a suitable method for optimising laminated tube layup designs by improving upon the weight reduction obtained by a comparative study. Consideration of additional variables, in particular the ply count, is shown to enable increased weight reduction to be achieved. The effect of increasing temperature on the optimised layup design and associated weight reduction was also explored. Significant increases were observed for the weight reduction obtained, facilitated by corresponding reductions in ply thickness and fibre-volume fraction, while ply angle is shown to widen to maintain specified structural requirements. Use of PSO will have a significant impact on the design of laminated tubular composite structures under thermomechanical loading, enabling significant weight reduction over current designs.

Investigation of the Deformation Behaviour and Resulting Ply Thicknesses of Multilayered Fibre–Metal Laminates

Multilayered fibre–metal laminates (FMLs) are composed of metal semifinished products and fibre-reinforced plastics, and benefit from the advantages of both material classes. Light metals in combination with fibre-reinforced thermoplastics are highly suitable for mass production of lightweight structures with good mechanical properties. As the formability of light metal sheets is sometimes limited at room temperature, increasing the process temperature is an appropriate approach to improve formability. However, the melting of thermoplastic materials and resulting loss of stiffness limit the processing temperature. Since single-ply layers have different through-thickness stiffnesses, the forming process changes the ply thickness of the multilayered laminate. In the present study, the deformation behaviour of multilayered FMLs was investigated using a two-dimensional finite-element model assuming plane strain. The thermoelastic-plastic finite-element analysis made investigation of the variation in thickness made possible by incorporating sufficient mesh layers in the thickness direction. The results indicate that a thermoelastic-plastic finite-element model can predict the delamination of plies during deformation, as well as in the final product. Additionally, the predicted changes in thickness of the plies are in good agreement with experimental results when a temperature-dependent friction coefficient is used.

On the source of the thermoelastic response from orthotropic fibre reinforced composite laminates

In thermoelastic stress analysis (TSA) of orthotropic laminated polymer composites, heat transfer influences the measured stress induced temperature change, or ‘thermoelastic response’. The composite constituents, including different fibre types, fibre geometry, ply thickness and resin systems, in combination with the manufacturing process means that, even for nominally identical materials, different conditions are generated for heat transfer. Hence, definitively identifying the ‘source’ of the thermoelastic response for a general composite laminate has remained elusive. A procedure based on the simultaneous application of digital image correlation (DIC) and TSA is devised that enables the source of the thermoelastic response to be established categorically. In glass fibre laminates, it is shown that heat conduction cannot take place so the thermoelastic response emanates from the surface resin rich layer. In similar carbon fibre laminates, adiabatic conditions are only met at higher frequencies with the response emanating from the orthotropic surface ply.

Gradual failure in high-performance unidirectional thin-ply carbon/glass hybrid composites under bending

This paper introduces new composite architectures using carbon and glass fibre-reinforced epoxy prepregs to achieve gradual failure under bending. The concept is based on a technique developed by the authors to design hybrid composites with gradual failure in tension combined with beam theory to identify and control the failure sequence of the plies in the layups. Two layups are designed based on standard ply thickness S-Glass and hybrid sub-laminates made out of intermediate and high modulus thin-ply carbon prepregs. The layups are tested under 4-point bending loading where a gradual failure alongside high values of flexural displacement are achieved. No catastrophic failure is observed throughout the whole loading process. The gradual layer-by-layer failure of the surface layers on the tensile side produces a brush-like appearance. Microscopy observations from interrupted tests verified fragmentation of the high-modulus carbon layer followed by gradual failure of the intermediate modulus carbon layer and delamination on the tensile side, as well as stable shear cracks of the high-modulus carbon layer on the compression side.

A new structural similitude method for laminated composite cylinders

Similitude theory has been applied to design scaled models in many fields of engineering. For laminated composite cylinders, the ply thickness is too thin to manufacture in scaled models, leading to similitude distortion. Meanwhile, this distortion problem cannot be solved with the traditional similitude method. The present study aims to propose a new similitude method to correct the distortion caused by non-scalability of ply thickness and to establish high-precision similitude of the free vibration of laminated composite cylinders. Firstly, the complete and partial similitude relationships are deduced while the similitude distortion derived from the non-scalability of ply thickness is discussed. Then, with the analysis of similitude for energy, a new similitude method named the energy similitude correction method is proposed. Finally, this method is verified through numerical examples of various cylinders. The results show that the proposed method can solve the similitude distortion of ply thickness with high accuracy, and it can be applied to various cylinders with different ply thickness, stacking sequences and materials.

Effects of ply thickness deviation and ply angle misalignment on the surface accuracy of CFRP laminates

This study examines two factors that affect the surface accuracy of symmetric carbon-fibre-reinforced plastic (CFRP) laminates: ply angle misalignment and ply thickness deviation. First, based on classic laminate theory, theoretical expressions for the effects of these factors are deduced. Then, COMSOL (FEM software) is used to analyse the relationship between these factors and the surface accuracy, and Zernike polynomials are used to fit the surface deformation of the CFRP laminates. It proves that ply angle misalignment and ply thickness deviation can cause astigmatic and power aberrations. Based on the integral constraint method in the form of Lagrangian multipliers, the rigid body displacement problem of the free thermal expansion simulation of CFRP laminates has been solved. Finally, three groups of experimental tests show that ply thickness deviation does occur. A ZYGO interferometer is used to observe the surface accuracy of the laminates, and it shows that astigmatism is the dominant aberration.

Approach for analysing off-axis tunnelling cracks in biaxially loaded laminates

A novel approach is presented on how the growth of off-axis tunnelling cracks in a composite laminate subjected to biaxial loads can be studied on an equivalent laminate with an another off-axis angle under uniaxial load. The primary objective is to determine the off-axis angle of the equivalent laminate such that the mode mixity of tunnelling cracks is identical to that of the tunnelling cracks in the original biaxially loaded laminate. The situation of steady state cracking is considered for both the laminates and the stress state at the crack front is expressed in terms of the mode mixity and the steady state energy release rate, both averaged across the cracked ply thickness. The approach is based on an energy accounting method that uses the crack opening displacements, obtained from a finite element analysis, and the principle of superposition to account for biaxial loads. Results are presented in terms of average energy release rates and average mode mixity for an isolated crack in an arbitrary oriented off-axis ply subjected to uniaxial and biaxial loads. The design of the equivalent uniaxially loaded laminate is analysed with reference to the general off-axis laminates subjected to a range of biaxial load ratios. With respect to testing, the presented approach can replace a complex biaxial fatigue test method with a more simple uniaxial fatigue test.