Tooling geometry optimization for compensation of cure-induced distortions of a curved carbon/epoxy C-spar

The respect of the manufacturing tolerances is a challenging issue due to the complex distortions caused by the curing process, being sometimes a major obstacle to an increasing use of composites in aeronautics. The cure-induced distortions are modeled and mitigated owing to the development of a computational mold compensation strategy. An in-house surrogate-based optimizer coupled to three-dimensional curing simulations, is used to iteratively alter the shape of the mold in order to minimize the discrepancy with respect to the nominal geometry. Two parametrization strategies are proposed and applied to a generic curved C-spar geometry. One strategy consists in the characterization of the major distortion modes caused by the curing process, and in the parametrization of the geometry to compensate for each mode. The other strategy is to select a number of control points which can move freely in space to find the optimal configuration. Each method performs well but in a different manner, and the optimal choice depends on the industrial specifications of the problem.     

A semi-analytical simulation strategy and its application to warpage of autoclave-processed CFRP parts

The manufacturing of composite structures is accompanied by fabrication induced deformations. Those deformations are undesirable and lead to transgression of geometric tolerances in the finished parts. In order to get the part within aspired dimensional tolerances, geometrical compensation of the tool is necessary. This often iterative conducted tooling-rework is commonly time consuming and costly. This paper presents an shell element based. semi-analytical simulation approach focusing on warpage deformations due to tool part interaction, in order to account for manufacturing induced deformations within the tool design process. Deviation measurements on test specimen level serve as inputs for the calculation of equivalent coefficients of thermal expansion according to the proposed analytical model. Thus, ‘warpage properties’ of different prepreg – tool–material combinations are determined. The use and the practicability of the developed approach is demonstrated by means of the calculation of a warpage compensated tool surface.     

Optimising the energy consumption on pultrusion process

This study is based on a previous experimental work in which embedded cylindrical heaters were applied to a pultrusion machine die, and resultant energetic performance compared with that achieved with the former heating system based on planar resistances. The previous work allowed to conclude that the use of embedded resistances enhances significantly the energetic performance of pultrusion process, leading to 57% decrease of energy consumption. However, the aforementioned study was developed with basis on an existing pultrusion die, which only allowed a single relative position for the heaters.In the present work, new relative positions for the heaters were investigated in order to optimise heat distribution process and energy consumption. Finite Elements Analysis was applied as an efficient tool to identify the best relative position of the heaters into the die, taking into account the usual parameters involved in the process and the control system already tested in the previous study. The analysis was firstly developed based on eight cylindrical heaters located in four different location plans. In a second phase, in order to refine the results, a new approach was adopted using sixteen heaters with the same total power. Final results allow to conclude that the correct positioning of the heaters can contribute to about 10% of energy consumption reduction, decreasing the production costs and leading to a better eco-efficiency of pultrusion process.     

Finite element simulation of the braiding process for arbitrary mandrel shapes

An approach to simulate the two-dimensional braiding process using a commercial explicit finite element software is presented. Preforms with generic shapes are analyzed. A procedure is given to determine the boundary conditions of the braiding mandrel including the extraction of necessary geometry information. The friction coefficients needed as input parameters are determined in separate tests. The simulation results are processed with an algorithm that derives the braiding angle and the axial spacing of the yarns. For validation, a generic mandrel geometry is overbraided and a method to compare simulation and experiment is presented. The preform is analyzed using an optical sensor. The measurements are filtered and averaged. The simulation model is validated by comparing the braiding angle of simulation and experiment. A good agreement between simulation and experimental results is achieved.     

High performance composite nozzle for the improvement of cooling in grinding machine tools

Although grinding is one of the most versatile machining operations that can be used to produce surface finish up to the micrometer level; it often induces thermal damage to a ground surface and higher power consumption if careful selection of grinding parameters are not made. The main aim of this study is to investigate the effect of a newly developed composite structure with enhanced coolant delivery system for optimizing grinding processes in comparison to the nozzles commercially available. The current investigation further aims to correlate the effect of tool geometry developed on grinding process conditions through experiment and modeling by means of Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA). The results show that with the use of the newly developed composite nozzle, a 30% decrease in coolant waste was achieved. Besides, it was found that the new nozzle yielded approximately 60–80% percentage pump pressure and power reduction compared to commercially available nozzles.     

Fabrication,process simulation and testing of a thick CFRP component using the RTM process

The present article introduces the case of a CFRP con-rod beam, and describes many aspects regarding its production with the Resin Transfer Moulding (RTM) process.The objective was to find the best process parameters of the injection and curing stages in order to manufacture the 20 mm thick CFRP part. The results are analysed in terms of the aesthetic aspect, the porosity and the mechanical properties of the final component.For the resin injection stage, results obtained from production experiences are presented, which have been performed with different set-ups, and simulations of the resin flow are used to analyse them. The results show that the resin flow during injection could be rather unpredictable, probably because of the fibre rearrangement and race tracking effects. Improvements in terms of aesthetic aspect and porosity of the part could be achieved by a process which included final compaction of the cavity by means of compressed air.Regarding the curing stage, the article presents the simulation results of a curing cycle, and it’s validation through DSC analysis of specimens obtained from the finished component.Finally, results of tensile mechanical tests are provided, performed on finished components produced by RTM and compared to others produced with the method of hand lay-up of pre-impregnated plies and curing in autoclave (Prepreg + Autoclave). The results confirm that it is possible to achieve components through RTM with comparable mechanical performance to those produced with the Prepreg + Autoclave process.     

Cure simulation with resistively in situ heated CFRP molds: Implementation and validation

In composite processing of parts with varying cross-sections, homogeneous cure is sought but poses a significant challenge. Electrically heated molds for resin transfer molding (RTM) processes offer the potential to locally introduce heat and, thus, achieve more homogeneous cure and enhanced part quality. However, low conductivity of CFRP poses a risk of uncontrolled exothermic reactions. To target this potential, an appropriate and efficient numerical method is presented in this study to simulate part cure governed by resistive heated CFRP molds. A numerical control algorithm for 3D finite element cure simulations is developed, which uses the reaction flux of a temperature boundary condition to calculate the arising tool temperature field. The capability of this method to predict non-uniform tool temperatures of self-heated CFRP molds with close to thermocouple accuracy during the cure process is shown by means of numerical verification and experimental validation on a self-heated CFRP plate.     

A progressive fracture model for carbon nanotubes

An atomistic-based progressive fracture model for simulating the mechanical performance of carbon nanotubes by taking into account initial topological and vacancy defects is proposed. The concept of the model is based on the assumption that carbon nanotubes, when loaded, behave like space-frame structures. The finite element method is used to analyze the nanotube structure and the modified Morse interatomic potential to simulate the non-linear force field of the C–C bonds. The model has been applied to defected single-walled zigzag, armchair and chiral nanotubes subjected to axial tension. The defects considered were: 10% weakening of a single bond and one missing atom at the middle of the nanotube. The predicted fracture evolution, failure stresses and failure strains of the nanotubes correlate very well with molecular mechanics simulations from the literature.     

The effects of heat input on adjacent paths during Automated Fibre Placement

One key aspect of the quality of parts attainable from the thermoset Automated Fibre Placement process is the impact of the heat source. For most industrial applications, an infrared heater is used and suitable process windows are still defined by trial-and-error approaches. Within this study, the need for robust thermal prediction tools is shown as well as the need for thermal management of the process. The influence of the radiation distribution on adjacent paths is presented experimentally and numerically, highlighting the non-uniform temperature distribution of tool and component. A multi-angle flat plate component was laid up, and bulk temperatures measured point-wise by a grid of thermocouples, as well as surface temperatures captured using a thermography camera. Finally a parameterised 3D Finite Element model was developed as a basis for precise prediction of the thermal history during the complete layup process.     

Design, testing, and FEM simulation of interlaced fiber composite structures

Design, testing, and FEM simulation of FRP structural parts manufactured using continuous fiber and light-curing resin are presented in this study. The structures can be described as formed by girders spaced by skew ribs. The study was limited to plane structures with two girders and 45° ribs between girders. Roving was positioned along the girders and the ribs so that fibers were interlaced at the joints between the ribs and girders. Several deposition paths were designed. They differ by interlacing, and proportions of fiber in the girders and ribs. Overall stiffness and strength of the structures were investigated in bending, and local deformations were measured with the strain gauges. Finite element modeling of four-point bending test for some test pieces were carried out using commercial code ANSYS to predict the deflection and the strains.     

Numerical investigation to prevent crack jumping in Double Cantilever Beam tests of multidirectional composite laminates

During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) or to grow asymmetrically, invalidating the tests.The aim of this study is to check the feasibility of defining the stacking sequence of Double Cantilever Beam (DCB) specimens so that these undesired effects do not occur, leading to meaningful onset and propagation data from the tests. Accordingly, a finite element model using cohesive elements for interlaminar delamination and an intralaminar ply failure criterion are exploited here to thoroughly investigate the effect of specimen stiffness and thermal residual stresses on crack jumping and asymmetric crack growth occurring in multidirectional DCB specimens.The results show that the higher the arm bending stiffness, the lower the tendency to crack jumping and the better the crack front symmetry. This analysis raises the prospect of defining a test campaign leading to meaningful fracture toughness results (onset and propagation data) in multidirectional laminates.     

Micro-mechanical modelling of shear-driven fibre compressive failure and of fibre kinking for failure envelope generation in CFRP laminates

A micro-mechanical Finite Element (FE) model is used to investigate the failure mechanisms and generate failure envelopes for fibre reinforced composites under combined in-plane shear and longitudinal compressive loading. The results show that the failure envelopes are defined by two regions corresponding to different failure mechanisms: (i) shear-driven fibre compressive failure and (ii) kinking/splitting. The FE model is also used to reproduce and give insight into different experimental trends typically reported in the literature.     

Effect of interphase and thermal environment on the effective properties of Macro-Fiber Composites (MFC)

A simplified analytical model is devised based on equivalent layered approach using the concepts of ‘rule of mixtures’ and ‘series and parallel capacitance theory’ to find the effect of bonding layer thickness and the thermal environment on the effective properties of Macro-Fiber Composites (MFC). A thermo–electro–mechanical analysis is performed based on finite element calculations using unit-cell method accounting for the geometric properties of constituents and non-uniform electric field distribution across the electrodes. Also, experiments are performed on commercially available MFCs under electrical load in various thermal environments to evaluate the coupling constants. The predictions based on the proposed models are validated with experimental results and data from the manufacturer.     

Simulation of curing process of carbon/epoxy composite during autoclave degassing molding by considering phase changes of epoxy resin

Strain monitoring of a carbon/epoxy composite cross-ply laminate ([0₅/90₅]_s) during thermoforming was conducted by using fiber Bragg grating (FBG) sensors. The entire process was simulated by employing finite element analysis (FEA) by taking into consideration the phase changes of the epoxy resin. For the precise simulation of the curing process, a dielectrometry sensor was used to detect the epoxy-resin dissipation factor, which in turn was used to identify the curing point. To investigate the phase changes and consolidation of the composite laminate by employing FEA, modulus changes with temperature were measured by dynamic mechanical analysis (DMA), and the permeability was estimated by measuring the fiber volume fraction according to the curing temperature. As the epoxy resin changed from a liquid to solid phase, the strain generated along the carbon fibers dynamically changed, and the analysis results generally predicted the strain variation quite well. To apply this simulation technique to practical structures, a composite-aluminum hybrid wheel was analyzed and experimentally verified.     

Composites with needle-like inclusions exhibiting negative thermal expansion: A preliminary investigation

In this work a simple cylindrical structure with a stiff needle-like inclusion embedded within a much softer matrix is presented and analysed with the aim of obtaining a system with tunable thermal expansion properties. It is shown that by the correct combination of the thermal and mechanical properties of the matrix and inclusion, it is possible to design a system which can be tailor-made to exhibit particular values of the coefficient of thermal expansion (CTE) in the radial direction and also negative thermal expansion (NTE). In particular an analytical model to quantify the radial strain with changes in temperature is derived and verified through finite element analysis. The model is used to find correct property combinations which lead to particular values of thermal expansion which could also be negative or zero.     

On the characterisation of transverse tensile properties of molten unidirectional thermoplastic composite tapes for thermoforming simulations

The development of Finite Element (FE) thermoforming simulations of tailored thermoplastic blanks, i.e. blanks composed of unidirectional pre-impregnated tapes, requires the characterisation of the composite tape under the same environmental conditions as forming occurs. This paper presents a novel approach for the characterisation of transverse tensile properties of unidirectional thermoplastic tapes using a Dynamic Mechanical Analysis (DMA) system in a quasi-static manner. The relevance of the presented method is assessed by testing, under the same environmental conditions, a control material with both a universal testing machine and a DMA system. For simulation purposes, a unidirectional thermoplastic tape is characterised under environmental forming conditions using the presented test method. Experimental results, which include stress–strain behaviour and transverse viscosity, are eventually used to identify, via an inverse approach, simulation parameters of a thermo-visco-elastic composite material model (MAT 140, PAM-Form, ESI Group). Comparisons between simulated and experimental results show good agreement.     

Bio-inspired hierarchical design of composite T-joints with improved structural properties

The biological principle of hierarchical (multi-scale level) design was used at the structural and laminate levels to design a novel carbon/epoxy T-joint with improved structural properties for potential use in light-weight aircraft structures. The bio-inspired structural modification mimics tree branch–trunk joints by embedding the stiffener flange into skin plies. This design concept results in increased fracture toughness due to crack branching and deflection. Simultaneously, bio-inspired ply angle optimisation was used to mimic the tailored arrangement of cellulose micro-fibrils observed in the wood cells contained within tree branch joints. The optimisation procedure minimises the interlaminar stress concentration in the T-joint radius bend and increases strength while maintaining similar global laminate stiffness properties. The hierarchical joint resulted in a significantly improved tensile strength compared to a conventionally designed T-joint. The new design additionally exhibited higher absorbed strain energy to failure load for bending and tension loading. Additionally, the hierarchical T-joint had a significantly reduced critical joint cross-sectional area (weight) due to the embedded design.     

A progressive damage simulation algorithm for GFRP composites under cyclic loading. Part I: Material constitutive model

A life prediction algorithm and its implementation for a thick-shell finite element formulation for GFRP composites under constant or variable amplitude loading is introduced in this work. It is a distributed damage model in the sense that constitutive material response is defined in terms of meso-mechanics for the unidirectional ply. The algorithm modules for non-linear material behaviour, pseudo-static loading-unloading-reloading response, Constant Life Diagrams and strength and stiffness degradation due to cyclic loading were implemented on a robust and comprehensive experimental database for a unidirectional glass/epoxy ply. The model, based on property definition in the principal coordinate system of the constitutive ply, can be used, besides life prediction, to assess strength and stiffness of any multidirectional laminate after arbitrary, constant or variable amplitude multi-axial cyclic loading. Numerical predictions were corroborated satisfactorily by test data from constant amplitude fatigue of glass/epoxy laminates of various stacking sequences.     

Dynamic saturation curve measurement in liquid composite molding by heat transfer analysis

In Liquid Composite Molding (LCM) processes the saturation of the reinforcement by the resin may induce the creation of porosity in the preform affecting the final properties of the composite. The purpose of this work concerns the development of an experimental protocol and the associated modeling to identify the dynamic saturation curve during filling by taking advantage of sharp contrasts of thermal properties existing between dry and fully-saturated reinforcement. To identify saturation, several injections were performed with a laboratory RTM mold for which thermal design allows accurate control of heat transfer. Several heat flux sensors were used to identify the saturation curve. Sensitivity analysis proves the feasibility of the method. The results are compared with a conductometric method with good agreement. Evolution of residual voids identified for several flow rates are also consistent with those expected according to the capillary number.     

Numerical calibration of bond law for GFRP bars embedded in steel fibre-reinforced self-compacting concrete

An experimental program was carried out at the Laboratory of Structural Division of the Civil Engineering Department of the University of Minho (LEST-UM) to investigate the bond behaviour of glass fibre reinforced polymer (GFRP) bars embedded in steel fibre reinforced self-compacting concrete (SFRSCC) for the development of an innovative structural system. Thirty-six pull-out-bending tests were executed to assess the influence of the bond length, concrete cover, bar diameter and surface treatment on the bond of GFRP bars embedded in SFRSCC. This paper reports the results of a numerical study aiming to identify an accurate GFRP–SFRSCC bond–slip law. Thus, the above mentioned pullout bending tests were simulated by using a nonlinear finite element (FE) constitutive model available in FEMIX, a FEM based computer program. The bond–slip relationship adopted for modelling the FE interface that simulates the interaction between bar and concrete is the key nonlinear aspect considered in the FE analyses, but the nonlinear behaviour of SFRSCC due to crack initiation and propagation was also simulated. The evaluation of the values of the relevant parameters defining such a bond–slip relationship was executed by fitting the force versus loaded end slip responses recorded in the experimental tests. Finally, correlations are proposed between the parameters identifying the bond–slip relationship and the relevant geometric and mechanical properties of the tested specimens.