Laminate Lay-up Research Articles

Strategies for modelling and optimization of bi-stable composite laminates actuated by embedded Shape Memory Alloy wires

Shape-adaptive or morphing capability is regarded as significant to increase aerodynamic performance of airfoils employed in aerospace, wind turbines or race cars, while at the same time reducing the number of moving parts. The inherent bistable behavior of asymmetric cross-ply composites makes them as a suitable candidate for morphing applications with a small energy input, and an efficient way to switch between the two equilibrium configurations is given by embedded shape memory alloy (SMA) wires. However, when a bi-stable composite plate is integrated with SMA wires (Shape Memory Alloy Composite, SMAC), the identification of design parameters, namely laminate size and layup, composite elastic constants, SMA wires number and thermomechanical behavior, SMAC manufacturing cycle, is not straightforward. The present work described the strategies developed by the authors to model and optimize a SMAC subjected to bi-stability and post-manufacturing deflection requirements.

Numerical and Experimental Analysis of the Suspension Connection Zone of a Formula Student Monocoque Chassis

This work focuses on the suspension attachment area of FST Lisboa’s formula student monocoque chassis, analysing two types of reinforcements, inserts and chamfers, changing some of their parameters for geometry improvement. A finite element model was built in order to set the comparison between the different geometries, first separately and then when included in a suspension quarter-car assembly. The geometries with best results were selected and a laminate layup optimization was made. Lastly, for validation of the numerical model, four specimens of each one of the two approaches were built and a pull-out test was performed on them, with a custom developed apparatus.Failure modes, similar to those previewed by theory, were achieved and a finest correlation between the experimental and numerical results was found for inserts, better than for chamfers, mostly due to manufacturing difficulties. The obtained results and conclusions were applied in the design and construction of the team’s new prototype monocoque.

Studija slučaja detekcije oštećenja vetroturbine sa lopaticom od kompozitnog materijala

In the entire wind turbine the most important part is the blade, which decides the power output by the virtue of size and shape. The design and material selection of a blade is very crucial since it constitutes the 20% of the total manufacturing cost of a whole wind turbine. To be specific, the blade material selection, manufacturing process, inspection and repair are very crucial in deciding the performance and life of the blade. Here we report the damage detection after manufacturing of turbine blade especially joints on the assembled blades by using latest techniques like infrared and ultrasonic inspection. The turbine blade is made up of glass reinforced plastic composite material manufactured by resin transfer moulding process with a length measuring up to 35.7 m. The infrared inspection method involves recording the variances in surface temperature due to heat generated by adhesive curing along bond lines. Using infrared images the type of defects like lack of adhesive webs, glue voids and uncured bonds are identified. Further inspection of adhesive joints between the blade shell and main webs and detection of its size is also carried out using ultrasonic B-scan method. The improper placing of web or hand lay-up laminate repair with improper bonding resulted signal loss. The defect thermographs and ultrasonic test scans of defects are also shown to show the accurate identification of the defects after manufacturing of a turbine blade. Finally the defects like dry laminate, delamination and wrinkles identified in the turbine blade are repaired following the appropriate methods and standards.

Flexural behaviour of corrugated panels of self-reinforced polypropylene hybridised with carbon fibre: An experimental and modelling study

The present work investigates the influence of the laminate layup on the flexural properties and failure mechanisms of corrugated panels made of self-reinforced polypropylene (SRPP) hybridised with carbon fibre. The influence of the layup on the stiffness and strength of the corrugated hybrid composite panels was much less important than the carbon fibre content. The experimental program was complemented by finite element analysis. The model revealed the importance of SRPP's non-linear behaviour and difference in tensile and compressive behaviour in controlling the flexural properties of corrugated panels. The compressive behaviour of SRPP was characterised to provide reliable input data for the model.

Effect of Z-pinning on the impact resistance of composite laminates with different layups

The paper presents the results of an experimental investigation into the effect of Z-pinning on the impact resistance of composite laminates with different multi-directional layups. Unpinned and pinned [02/902]s, [02/±45]s and [0/±45/90]s carbon/epoxy samples were subjected to low-velocity impacts resulting in damage severities ranging from barely visible damage to full laminate penetration. The responses to impact of the laminates were characterized in terms of damage evolution, load-carrying capacity, and resistance to perforation and penetration. It was found that even though Z-pins are not capable of delaying the onset of delamination, they significantly reduce delamination size for high-energy impacts. Z-pinning improves the resistance to perforation and penetration of [02/902]s, laminates, while it does not significantly affect that of [02/±45]s and [0/±45/90]s samples. The dependence of the efficacy of Z–pins on the laminate layup is attributed to the different distribution of the delaminations through the thickness of the investigated laminates.

Weight trades in the design of a composite wing box: effect of various design choices

A process to efficiently design composite wing boxes is presented. It uses analytical and semi-empirical equations for failure modes such as material strength, plate buckling, stiffener column buckling and stiffener flange or web crippling. Laminate layups for the different components are selected in accordance with basic engineering rules and guidelines and are updated as necessary to meet the local loads. The emphasis is in allowing buckling of skins at any fraction of the ultimate load and allowing local load redistribution from buckled to non-buckled panels to save weight. The design process is automated and the design can be automatically transferred over to a commercial finite-element code for detailed design and validation. The effects on weight of number of spars, ribs, and stiffeners as well as the fraction of ultimate load at which buckling is allowed are examined and insight is gained to which of these the weight is most sensitive to. In addition, the effect of minimum gage on weight was found to be a driver.

Low-velocity impact predictions of composite laminates using a continuum shell based modeling approach Part b: BVID impact and compression after impact

In this paper, a continuum shell based finite element (FE) model is introduced to predict the impact and compressive strength after impact (CSAI) response of fiber reinforced polymer matrix composite (FRPC) laminates of various traditional and non-traditional layup orientations. The FE model predicts the impact damage in the laminate. The impact results are then directly used to predict the CSAI of the structure. The model uses in-plane progressive damage and failure modeling coupled with discrete cohesive elements to capture the necessary failure mechanisms. Enhanced Schapery Theory captures the non-linearity due to matrix micro cracking as well as macro intra-lamina matrix cracking and fiber failure. Discrete cohesive elements are implemented to capture the inter-lamina failure initiation and propagation (delaminations). The numerical predictions are compared against impact and CSAI experimental data for composite laminates of various layups. A high quality post-impact inspection using ultrasound and micro-computed tomography (microCT) scanning was utilized for detailed comparison between model results and experiments. The modeling technique was seen to be highly capable of predicting the impact response and CSAI of multiple different stacking sequences using a general mesh.

A design algorithm to model fibre paths for manufacturing of structurally optimised composite laminates

Fibre steering is involved in the development of non-conventional variable stiffness laminates (VSL) with curvilinear paths as well as in the lay-up of conventional laminates with complex shapes. Manufacturability is generally overlooked in design and, as a result, industrial applications do not take advantage of the potential of composite materials. This work develops a design for manufacturing (DFM) tool for the introduction in design of the manufacturing requirements and limitations derived from the fibre placement technology. This tool enables the automatic generation of continuous fibre paths for manufacturing. Results from its application to a plate with a central hole and an aircraft structure – a windshield front fairing – are presented, showing good correlation of resulting manufacturable paths to initial fibre trajectories. The effect of manufacturing constraints is assessed to elucidate the extent to which the structurally optimal design can be reached while conforming to existing manufacturing specifications.

Strength properties of aluminium/glass-fiber-reinforced laminate with additional epoxy adhesive film interlayer

This paper studies the strength analysis of two types of fibre-metal laminates (FMLs), with a different way of preparing the adhesive coupling between 2024-T3 aluminium alloy sheets and a polymer/fibre layer. In the first variant, the 3 M structural adhesive film AF 163-2 K is used as an intermediate layer between prepreg and adherends. The adhesive layer is co-cured with prepreg and adherends. In the second FML variant, coupling between adherends was produced using epoxy resin. Two types of strength test, – i.e., tensile/shear test and 90° peel test – were used to measure the adhesion between the layers of FML composite. To measure the joint strength the tensile test was used. The fracture mode and the fracture surfaces were examined and discussed using a scanning electron microscope. The application of the adhesive film as an additional binding agent caused an increase in laminate elasticity. In this way, the strength and fatigue life of the joint increased. The main advantage of adhesive film application was a significant increase in the peel strength of laminate, which reached 289.4%. As a result of this, bonded laminates are very susceptible to normal stresses. However, the application of the additional adhesive film in the analysed 2/1 laminate lay-up increased the mass of the laminate by about 10%. Furthermore, a significant increase in the FML peel strength was achieved, with the increased cost of the additional adhesive film at about 20%.

Optimum Electrode Configurations for Two-Probe, Four-Probe and Multi-Probe Schemes in Electrical Resistance Tomography for Delamination Identification in Carbon Fiber Reinforced Composites

Internal damage in Carbon Fiber Reinforced Polymer (CFRP) composites modifies the internal electrical conductivity of the composite material. Electrical Resistance Tomography (ERT) is a non-destructive evaluation (NDE) technique that determines the extent of damage based on electrical conductivity changes. Implementation of ERT for damage identification in CFRP composites requires the optimal selection of the sensing sites for accurate results. This selection depends on the measuring scheme used. The present work uses an effective independence (EI) measure for selecting the minimum set of measurements for ERT damage identification using three measuring schemes: two-probe, four-probe and multi-probe. The electrical potential field in two CFRP laminate layups with 14 electrodes is calculated using finite element analyses (FEA) for a set of specified delamination damage cases. The measuring schemes consider the cases of 14 electrodes distributed on both sides and seven electrodes on only one side of the laminate for each layup. The effectiveness of EI reduction is demonstrated by comparing the inverse identification results of delamination cases for the full and the reduced sets using the measuring schemes and electrode sets. This work shows that the EI measure optimally reduces electrode and electrode combinations in ERT based damage identification for different measuring schemes.

Fatigue life assessment of uni‐ and multidirectional carbon fiber reinforced plastics with regard to stiffness degradation

AbstractCarbon fibre reinforced plastics are widely used in the automotive industry. In this paper a short introduction into the theoretical damage mechanisms and fatigue assessment approaches applicable for carbon fibre reinforced plastics is provided. The fatigue behaviour of uni‐ and multi‐directional carbon fibre laminates is shown for various laminate orientations and layups under axial loading. In addition to constant amplitude loading, experimental results using multiple staged block loading are presented. Online stiffness measurement during fatigue life is conducted. Supplementary investigations using in‐situ X‐ray and local strain measurements give further insight in the damage mechanisms. Furthermore a phenomenological residual stiffness approach is presented in order to describe the fatigue behaviour of the carbon fibre reinforced plastic tested in this project. Numerical evaluations are performed using Siemens Virtual.Lab Durability software and its composite fatigue solver. The computed stiffness degradation curves and corresponding fatigue life results are compared to the obtained test data under constant amplitude (SN curve and stiffness degradation) as well as multiple staged block loading.

Modelling the deformation of continuous fibre-reinforced thermoplastics subjected to hygrothermal loading

For the application of continuous fibre-reinforced thermoplastics in car body structures it has to be ensured that they can pass through the automotive paint shop. There are substantial chemical, hygroscopic and thermal impacts throughout this process that can affect the shape of such components. The aim of this study is to model the deformation behaviour of continuous fibre-reinforced thermoplastics under hygrothermal loading. Relaxation tests were performed to assess temperature-dependent and viscoelastic behaviour. Input parameters for temperature and moisture distribution were determined experimentally and computed using finite difference method. A straightforward approach based on classical laminate theory is presented that comprises anisotropic viscoelasticity as well as hygroscopic, thermal and polymer-chemical effects. The introduced phenomenological model was successfully used to describe the deformation of five different laminate layups both dry and fully saturated with moisture.

In situ monitoring and analysis of void evolution in unidirectional prepreg

In the layup of prepreg laminates, air is inevitably entrapped between adjacent prepreg plies, yet the removal of this inter-ply air under vacuum bag cure conditions is not well understood. In this study, an in situ visualization technique was developed to dynamically observe inter-ply air removal during the cure of an out-of-autoclave prepreg. The technique was used to investigate mechanisms of air removal and void evolution in unidirectional prepreg. Prepreg impregnation was also tracked by inspection of laminate cross-sections prepared at different times during the cure cycle. From these data, a three-stage air removal mechanism was documented based on the relationships between void content, resin properties, and tow impregnation as functions of time. Furthermore, a positive correlation was observed between the rate of evacuation and bubble elongation. Though discussed here in the specific context of voids in unidirectional laminates, the in situ observation technique developed for this work has broad potential to enhance understanding of processing phenomena associated with out-of-autoclave prepregs.

Progressive matrix cracking master curves of mid and outer off-axis plies in CFRP laminates

An energy based criteria is developed to predict the propagation of mid and outer-ply matrix cracks in laminates of type 0n/θms and θm/0ns with θ=90,75,60 subject to uniaxial tensile loading. Numerous specimens with 14 different layups are tested and optical microscopy is implemented to detect matrix cracks during tensile tests. Matrix cracking master curve is plotted for over 300 data points obtained from experimental observation of all tested layups. A single critical energy release rate Gmc is derived for off-axis and transverse ply cracking in contrast to previous studies which only included transverse plies. Crack densities of all tested layups are analytically predicted and compared to experimentally obtained crack densities. For the first time in the literature, critical energy release rate is confirmed to be a material property independent of laminate layup as far as analytical predictions of matrix crack densities are in accordance with experimental observations.

Non-orthogonal bistable laminates for energy harvesting

This paper presents experimental results of non-orthogonal, [0/θ], bistable laminates for broad-band energy harvesting which are characterized by subjecting them to harmonic excitation of varying frequency and acceleration. The root mean squared (RMS) power as a function of frequency and vibration level is used as a basis of comparison and analysis and harvester performance is assessed in terms of peak power, cumulative power, and full half-power bandwidth. Nonlinear features in the frequency response are noted for different laminate geometries. The relationship between natural frequency and θ indicates that as the ply angle is reduced, the mismatch in thermal strain is decreased and the stiffness and level of bistability is diminished; the leads to laminates with a low θ exhibiting non-linear effects and snap-through from one stable state to another at lower vibration levels. The variation of the laminate lay-up is therefore an attractive method to tailor the response of broadband energy harvesters based on bistable laminates.

Predicting the Compression-After-Impact (CAI) strength of damage-tolerant hybrid unidirectional/woven carbon-fibre reinforced composite laminates

The evaluation of Compression-After-Impact (CAI) strength is of great significance in the design of composite aerostructures. This paper presents a model for the numerical simulation of Compression-After-Impact (CAI) of hybrid unidirectional (UD)/woven carbon-fibre reinforced composite laminates. This three-dimensional damage model is based on Continuum Damage Mechanics (CDM) and Linear Elastic Fracture Mechanics (LEFM), implemented as a user defined material subroutine (VUMAT) in Abaqus/Explicit. This model, which accounts for interlaminar and intralaminar damage, and load reversal, incorporates a non-linear shear profile to account for matrix plasticity. Two different composite laminate lay-ups with varying extent of initial impact damage were tested to validate the computational model and enable a quantitative study of the influence of using woven plies on the surfaces of a laminate. Woven surface plies are often used in composite aerostructures to mitigate damage during drilling and constrain the extent of damage during low velocity impact. Good correlation was obtained between physical testing and simulation results, which establishes the capability of this damage model in predicting the structural response of composite laminates. The fully validated model was used to compare the CAI strength of an equivalent UD-only carbon-fibre reinforced composite laminate. The results showed that the hybrid unidirectional (UD)/woven laminate had a marginally higher strength (+3.3%) than the equivalent unidirectional (UD)-only laminate.

Experimental and numerical studies on the impact response of damage-tolerant hybrid unidirectional/woven carbon-fibre reinforced composite laminates

Abstract A woven five-harness satin (5HS) weave with AS4 carbon fibres, and unidirectional high strength IMS60 carbon fibres were used to manufacture hybrid laminates, using resin infusion, to assess their performance in low velocity impact tests. Load/energy-time curves and load-displacement curves were extracted from the experimental data, and non-destructive C-scanning was performed on all pre- and post-impacted specimens to quantify the extent of damage incurred. A finite element-based computational damage model was developed to predict the material response of these hybrid unidirectional/woven laminates. The intralaminar damage model formulation, by necessity, consists of two sub-models, a unidirectional constitutive model and a woven constitutive model. The built-in surface-based cohesive behaviour in Abaqus/Explicit was used to define the interlaminar damage model for capturing delamination. The reliability of this model was validated using in-house experimental data obtained from standard drop-weight impact tests. The simulated reaction-force and absorbed energy showed excellent agreement with experiment results. The post-impact delamination and permanent indentation deformation were also accurately captured. The accuracy of the damage model facilitated a quantitative comparison between the performance of a hybrid unidirectional/woven (U/W) laminates and a pure unidirectional (PU) carbon-fibre reinforced composite laminates of equivalent lay-up. The hybrid laminates were shown to yield better impact resistance.

Getting to grips with automated prepreg handling

Automated manufacturing of composite parts based on prepreg material is receiving increased interest with the rising use of composite materials for high-performance applications. The two main automation alternatives for prepreg layup, automated tape layup and fiber placement, are not cost-effective for all types of products, and manual labor is common for the manufacturing of complex parts in low manufacturing volumes. Alternatives to the two dominant automation solutions have been pursued, but so far these have had a limited impact in industrial applications. This paper presents four different solutions for automated layup of prepreg plies to flat laminates that can be formed in a subsequent forming process, which decrease the system complexity. The solutions target products where the layup of flat laminates today is done manually due to technical limitations or cost considerations. The layup solutions must manage challenges caused by material properties such as low material rigidity and tack, and be able to handle a high number of ply shapes. All four solutions are designed for prepreg that is covered with a stiff backing paper. The aim of the paper is to analyze and compare the four solutions. It can be concluded that the most versatile solution is a dual-arm robot equipped with simple end effectors. The dual-arm solution presents a possibility to control the pick-up and lay-down motions that make peeling motions possible, which is advantageous when picking material that has tacked to the pickup surface.

The effect of laminate lay-up on the multi-axial notched strength of CFRP panels: Simulation versus experiment

The effect of laminate lay-up upon the evolution of damage and upon the multi-axial failure locus of notched carbon fibre reinforced polymer (CFRP) panels is investigated experimentally and numerically. The proportion of 0° plies and ±45° plies is varied, and combined tensile and shear loading (and direct compression) is generated by a modified Arcan test rig. The failure surface is measured for panels containing a central hole or sharp notch, and the evolution of sub-critical damage is observed by X-ray CT and sectioning of the specimens. Three distinct damage modes are evident, depending upon the applied stress state, lay-up and geometry of the stress-raiser. The open-hole and open-notch values of tensile and compressive strength increase with the proportion of 0° plies, whereas the shear strengths do not scale with the proportion of ±45° plies. Inter-fibre splitting of the main loading bearing plies reduces the stress concentration, but the degree of splitting varies with loading direction and lay-up. The Finite Element (FE) method is used to predict not only damage development within each ply but also inter-ply delamination by the use of cohesive interfaces using a traction-separation law. The predicted failure envelopes and damage evolution are in good agreement with the observations, provided the mesh aligns with the fibre direction. This supports the adopted ply-by-ply FE strategy as a useful predictive tool for composite failure.

Distributed optical sensing in composite laminates

An optical fiber–based full-field strain measurement technique was used to investigate delamination growth in laminated composites. An experimental setup to load the test samples under idealized modes of delamination was used to investigate the ability to capture the shape and location of the delamination front. It is envisioned that the demonstrated approach has significant field applications in controlled laboratory settings where delaminations have to be located accurately. Furthermore, the ability of this measurement system to provide full-field strain measurements at any given pre-implanted location through the thickness overcomes the surface strain measurements obtained by digital image correlation. In order to demonstrate the technique, distributed fiber optic sensing is used to monitor the propagation of delaminations under pure mode I and II loading. Optical fibers were embedded one ply from the crack plane of both double cantilever beam and end notch flexure specimens. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers during the laminate layup process were established. Specimens with and without embedded fibers were tested to verify the fibers did not affect measured fracture toughness values. Crack lengths measured with the optical fibers compared well with true crack lengths, and measured strain distributions compared well with results from finite element analysis.