Preform Quality Research Articles

Parametric study on the effect of material properties, tool geometry, and tolerances on preform quality in wind turbine blade manufacturing

Increasing throughput in wind turbine blade production can be achieved by separately manufacturing pre-shaped binder-stabilised dry preforms, and subsequently placing them in the blade mould. To avoid manufacturing defects, a trade-off between the formability and the handleability of the preform is necessary. In this paper, an experimentally validated preform model is used to study how variations in material properties, tool geometry, and placement tolerances influence defect generation. The results from three studies are presented. In the first study, a preform is formed over a ramp transition with variations in geometry. The results from this study indicate that a short ramp promotes transverse shearing of the preform. In the second study, the material properties of the preform are varied. The results indicate that a high mode I cohesive law of the binder and a high bending stiffness of the fabric promote transverse shearing and remove wrinkles. In the last study, placement tolerances for a pre-shaped preform are studied. The results show that if the preform can shear between the preform edge and the tool edge, it can conform to the mould even with large placement offsets. Process engineers and blade designers can readily use these results to help reduce forming-induced wrinkles.

Characterisation of process-induced variability in wrinkle defects during double diaphragm forming of non-crimp fabric

Experiments and numerical simulations are employed to determine the critical process variables that affect the quality of a two-layer biaxial non-crimp fabric (NCF) preformed using the double diaphragm forming (DDF) process. To prevent wrinkling defects, the process variables must be controlled, including the vacuum pressure, diaphragm tension, and ply-tool alignment. Inter-ply friction plays a critical role in the deformation mechanism, while the frictional interaction between the diaphragm and tool has a limited impact. The study also reveals that geometrical features have an impact on the variability induced by the forming process. Preforms deformed to tools with geometrical features of higher asymmetry or Gaussian curvatures reduce the uncertainty of the forming process by maintaining the same preform quality despite changes in forming variables.

Analyses of wrinkling mechanisms during forming of multi-layer woven fabric reinforced thermoplastic prepregs

Thermoplastic composites have made their way into the aerospace industry. The forming of prepregs with different ply orientations induces both intra-plane shear and inter-ply slippage behaviors, and the wrinkling risk is strongly increased by improper parameter control. This paper aims to investigate the deformation behavior of prepreg with different ply orientations and of wrinkling mechanisms during forming. A simulation approach of multi-layer thermoplastic prepreg based on a variable friction model with Stribeck curves and a non-orthogonal constitutive model is developed. The relationship between the local fiber deformation and the development of wrinkle defects is revealed. The results show that the balance between the three deformation mechanisms of shear, bending, and slippage behaviors determines the preform quality. Fiber compression induced by inter-ply friction is the main mechanism of wrinkle defects in the forming process of multi-layer thermoplastic prepregs.

Surfactant effect on DLP fabrication of silica fibre preforms

Three-dimensional (3D) printing based on digital light processing (DLP), for its great advantage in dealing with material and structural complexities, is being engaged for the fabrication of custom-designed silica optical fibre preforms. Resin preparation and printing are key processes critical to DLP fabrication of optical fibre preforms with high silica loading. In this work, the surfactant effect on preparation of resin and DLP printing of optical fibre preforms for higher silica loading is investigated. Based on our experimental studies of the rheological and photocuring properties of resins and the quality of printed silica fibre preforms, we find that, in the case where 2-phenoxyethanol (POE) is usually added as the surfactant, the resin has higher viscosity and results in poorer preform quality. By removing POE from resin ingredients and designing a multi-step resin processing with appropriate settling and degassing processes, fibre preforms with high silica loading up to 0.8 (w/w) have been successfully fabricated.

Large Scale Forming of Non-Crimp Fabrics for Aerostructures

The increased production rate targets of the aerospace industry have driven the development of dry fibre infusion processes. Biaxial Non-Crimp Fabrics (NCFs) are considered in this work due to their potential high deposition rates and higher mechanical performance to woven fabrics. Forming is an integral step prior to infusion and curing. Understanding the forming behaviour of NCFs at scale is therefore key to achieving high quality parts at high rates. To investigate the draping and shearing behaviour of NCFs, geometries with complexities associated with the composite structure are used. This study presents an experimental campaign on two large scale (2 metres in span) geometries with complexities seen in primary aerostructures. The combination of features such as ramps and curvature with corner radii leads to distinctive out-of-plane wrinkling. The relationship between geometry, material and resulting preform quality is observed through the use of 3D scans. Results show differing preform quality in terms of wrinkling phenomena, showing the importance of geometry of choice for material drapability tests at an industrial scale.

The Role of Bias Extension Testing to Guide Forming of Non-Crimp Fabrics

The increased production rate targets of the aerospace industry has driven the development of dry fibre processes and biaxial Non-Crimp Fabrics (NCFs). Understanding the forming behaviour of NCFs at scale is key to achieving high quality parts at high rates. The bias extension test is commonly used to characterise the shear behaviour of dry fabrics, for example as input to drape forming models. In industry, it is still often used as a standalone material selection tool as part of initial material selection processes. It is however well known that the boundary conditions of this test are often not representative of full-scale forming trials. A direct comparison with wrinkles observed during a forming experiment is carried out to show that the bias extension test overpredicts wrinkle height. Overall, the bias extension is considered unsuitable on its own for predicting preform quality in an NCF forming process where excess length is generated due to part geometry. If the bias extension is to be used as an NCF material ranking tool, wrinkle height data should be captured alongside force shear data, as shown in this study.

Modeling and analysis of tension system used in rotating yarn compensation carrier

The leveraged carrier makes the yarn have a certain tension through the lever. However, the length of the lever greatly limits the yarn storage capacity of the carrier, especially for some large end braiding machines. When the carrier moves from the outer horn gear to the inner one, the distance between the carrier and the braiding ring is greatly reduced, and the yarn storage is difficult to satisfy the requirement, which seriously affects the preform quality. On the other hand, the yarn tension control of leveraged carriers is not stable, there are many sudden change points, which is easy to cause yarns abrasion, fuzzing and even fracture. To solve these problems, a rotating yarn compensation carrier is designed, and the dynamic model of the yarn system is established. The motion law of carriers on the four-layer end braiding machine is studied, and the kinematic model of the yarn traction system is established. Moreover, the yarn tension of carriers in the feeding and recovering stages are analyzed. The curves of the yarn tension and key structure movements are drawn by MATLAB software to study the influence of yarn winding number on the yarn tension. The numerical analysis results show that the yarn storage capacity of the rotating yarn compensation carrier can meet the working requirements of the 4-layer end braiding machine, and the yarn tension can be maintained with stable changes by increasing the number of winding turns of the rods.

Work Envelope Expansion and Parametric Optimization in WAAM with Relative Density and Surface Aspect as Quality Constraints: The Case of Al5Mg Thin Walls with Active Cooling

The successful and efficient production of parts with specific features by Wire + Arc Additive Manufacturing (WAAM) strongly depends on the selection of proper and typically interrelated deposition parameters. This task might be particularly challenging in the making of thin walls, which might be highly impacted by processing conditions and heat accumulation. In this context, this study aims at expanding the work envelope and optimizing the parametric conditions in WAAM with relative density and surface aspects of the preforms as quality constraints. The experimental approach was based on the deposition of thin Al5Mg walls by the CMT process on its standard welding setup and with an active cooling technique to enhance the deposition robustness. Internal voids were estimated by Archimedes’ method. The surface quality of the walls was assessed through the visual aspect and the surface waviness by cross-section analysis. All the conditions presented relative density higher than 98%. The upgrade of the standard welding hardware to WAAM purposes through the addition of a supplementary shielding gas nozzle to the torch and the intensity of the heat sinking from the part significantly expanded the process work envelope, with its applicability being successfully demonstrated with multi-objective optimization. To sum up, a decision-making procedure is presented towards achieving intended preform quality.

Concept and validation of an active cooling technique to mitigate heat accumulation in WAAM

This work aimed at introducing and exploring the potential of a thermal management technique, named as near-immersion active cooling (NIAC), to mitigate heat accumulation in Wire + Arc Additive Manufacturing (WAAM). According to this technique concept, the preform is deposited inside a work tank that is filled with water, whose level rises while the metal layers are deposited. For validation of the NIAC technique, Al5Mg single-pass multi-layer linear walls were deposited by the CMT® process under different thermal management approaches. During depositions, the temperature history of the preforms was measured. Porosity was assessed as a means of analyzing the potential negative effect of the water cooling in the NIAC technique. The preform geometry and mechanical properties were also assessed. The results showed that the NIAC technique was efficient to mitigate heat accumulation in WAAM of aluminum. The temperature of the preforms was kept low independently of its height. There was no measurable increase in porosity with the water cooling. In addition, the wall width was virtually constant, and the anisotropy of mechanical properties tends to be reduced, characterizing a preform quality improvement. Thus, the NIAC technique offers an efficient and low-cost thermal management approach to mitigate heat accumulation in WAAM and, consequently, also to cope with the deleterious issues related to such emerging alternative of additive manufacturing.

Experimental and numerical studies of process variabilities in biaxial carbon fiber braids

This paper investigates the manufacture of biaxial carbon fiber braids and the influence that different machine settings have on variability of the textile architecture produced. In parallel, numerical simulations of the braiding process with these different machine settings have been conducted. For these studies yarn tension and process speed are varied to generate cylindrical biaxial braids with an average braid angle of±45°. The overall preform quality is characterized by means of variability in braid angle, yarn width, cover factor and fiber damage, using a variety of experimental techniques. Furthermore, from the final infused composite variations in yarn cross-section dimensions have been measured. A method is presented to transfer braid process simulation results to a detailed three dimensional finite element model of the architecture using a technique based on thermal expansion and compaction simulation. This method also allows the possibility to introduce experimentally observed variability in yarn cross-section dimensions. Such a model provides a valuable starting point for mesoscopic infusion or mechanical analysis of the textile composite. A comparison between experimental and numerical results shows that the process simulation can well reproduce the real braid angles in terms of mean value and scatter under different machine configurations and that the meso-scale textile model gives a good reproduction of the true textile architecture.

Mechanical characterisation of the shear, bending and friction behaviour of bindered woven fabrics during the forming process

AbstractA critical process step within the liquid composite moulding constitutes in the preforming of two‐dimensional textile material into a near‐net‐shape and load‐capable fibre structure by stamp forming. This paper presents fundamental material experiments on the shearing behaviour, bending properties and friction behaviour considering the binder quantity and forming temperature. In addition, the influence of the process time on the resulting stability of the preform is examined. The findings indicate that the quantity of the binder as well as the forming temperature and the process time have a great influence on the preform quality.

Influence of punch sequence and prediction of wrinkling in textile forming with a multi-punch tool

Liquid composite moulding (LCM) processes show a high potential in automated, large scale production of continuous fibre-reinforced plastics (FRP). One of the most challenging steps is the forming of the two-dimensional textile material into a complex, three-dimensional fibre structure. In this paper, a multi-punch forming process is presented. The upper mould of a generic part geometry is divided into 15 independently controllable punches. Depending on the different punch sequences, draping effects as well as defects related to wrinkling and shearing of the textile material are investigated. It has been shown that the sequence of the punches has a significant influence on the final preform quality. To predict the resulting regions of wrinkling and shearing, a finite-element based simulation model is set up. Forming tests and simulations with different punch-sequences are then performed and evaluated for validation purposes. To make a statement about the global preform quality, different objective functions regarding wrinkling are presented and analysed.

Optimisation of local in-plane constraining forces in double diaphragm forming

Rigid blocks (risers) were introduced in the double diaphragm forming (DDF) process to change the local in-plane strain distribution in the diaphragms, aimed at reducing wrinkling defects in the production of fabric preforms. A two-step optimisation method was developed to determine the position and dimension of each riser. In Step I, optimisation of the riser position was conducted using a simplified finite element (FE) model coupled with a genetic algorithm (GA). The height of each riser was optimised in Step II using a detailed FE model with the optimised riser positions from Step I. For demonstration, a hemisphere preform was manufactured by DDF using the optimum riser arrangement established by the optimisation routine. Results indicate that the optimum riser pattern (shape and position relative to the component boundary) can dramatically improve the preform quality through reduction of out-of-plane wrinkles, validating the feasibility of the two-step routine.

Process design and modelling methods for automated handling and draping strategies for composite components

Fibre-reinforced-thermoplastics (FRT) have excellent weight-specific properties compared to conventional engineering materials. However, a wider dissemination of this technology into existing plant technologies is restrained by the low degree of automation. Complex FRT component geometries pose special challenges to gripper design and handling strategies in automated preform processes regarding limp material behaviour and fast cooling time. The preform quality is influenced by the component geometry, reinforcing fabric, and preform process. This paper presents the development of an automated handling and draping strategy, which is validated by finite-element-analysis and experimental testing to meet the requirements of large-scale preforming processes for complex geometries.

Active forming manipulation of composite reinforcements for the suppression of forming defects

For composite applications in automotive serial production, reinforcement textiles are brought into the desired shape by forming processes. The preform quality depends on the interacting factors of tool geometry, textile material, and forming process. The primary cause of defects in multilayer draping is relative movements of the plies, and the occurrence of wrinkles. Thus, the reduction of the interactions between plies is crucial to enhance preform quality. This was achieved with active metal sheets between the fabric layers. Those intermediate layers are additionally stimulated with piezo actors to reduce friction. Additional local and ply-specific clamping of layers was achieved with tension rods and segmented actuators. Defects and wrinkles in the preform could be eliminated or reduced significantly and fiber orientation could be controlled. Thus, a forming process providing high-quality preforms from multiple fabric layers was developed. Furthermore, automation complexity could be reduced significantly by utilizing rigid interlayers.

太赫兹聚合物光子晶体光纤关键制备工艺研究

The technologies of fabricating polymer photonics crystal fiber to suit the application needs of terahertz transmission were studied, which were related to material selecting, fiber preform fabrication and fiber drawing. According to the analyzation of optical polymers' properties and the experimental verification, ZEONEX has low absorption of less than 3 cm-1in Terahertz waves, low water absorption of less than 0.01%, high glass transition tempreture and decomposition temperature of 136 and 420 respectively. As for fiber preform fabrication and drawing, the model system was improved based on injection moulding, and drawing technology of Pascal level pressure auto-control was initially invented. The controlled value oscillations is no more than 1.5 Pa in the range of 10~200 Pa. Therefore the preform quality and reliability are promoted and fiber microstructure is effectively controlled. With the proposed technology it is hopeful of producing high air filling factor polymer photonics crystal fiber. © 2017, Science Press. All right reserved.

Analyses of interaction mechanisms during forming of multilayer carbon woven fabrics for composite applications

The use of textile reinforcing structures provides enormous possibilities in the design of lightweight composites. However, the physical mechanisms during fabric forming are complex and far from being fully understood especially in multilayer draping. The aims of this study are the analyses of interaction mechanisms of individual textile layers during the forming operation and of defects arising from interactions. In basic experiments of the carbon woven fabric, friction properties and the fabric integrity were investigated. In single and multilayer draping experiments the findings were transferred to the composite preforming process. Interaction defects are characterised as interdependency between the acting inter-ply shear forces and the structural integrity of the fabric. The defects resulting from the interactions depend on the configuration of the fabric (e. g. shearing) and the acting normal forces. The reduction of friction is crucial for the preform quality but is opposite to an actively force-controlled material manipulation.

Properties of SiCp/6061-Al metal matrix composites prepared by infiltrating molten aluminum into ternary packing of SiC preforms

SiC particles reinforced aluminum matrix composites (SiCp/Al) with SiC content of more than 60%, without particle segregation for mirror substrate applications, are prepared by using the improved process. The improved process contains sintering process design, preform quality traceability, and equipment designing. The maximum SiC volume fraction of 70.4% is obtained by optimizing the selection of the following proportion: 71.4% coarse, 7.1% medium, and 21.5% fine particles. The variation of SiC volume fraction is correlated with the percentage of fine particles by using regression analysis. The preforms with the compressive strength above 10.34 MPa are prepared by sintering at 1600℃ for 2 h. Results showed that no defects in the scale of more than 10 µm are observed in the preforms. Due to the generation of a SiO2 layer during the sintering process, the formation of Al4C3 phase was inhibited. As the SiC particle volume fraction increased from 65.6% to 70.4%, the bending strength increased from 483.2 MPa to 585.2 MPa. The elastic modulus also increased from 174.2 GPa to 206.1 GPa, while the coefficient of thermal expansion between 50℃ and 350℃ decreased from 11.02 × 10−6/℃ to 9.53 × 10−6/℃.

Modified Rod in Tube Technique for Fabrication of Tellurite Glass Preforms

A modified rod in tube technique for tellurite glass preform fabrication is reported, and the effects of the fabrication process on the preform quality are discussed in terms of different processing factors. The conditions for obtaining good quality of reproducible preforms were experimentally investigated. Preforms with a good interface between core and cladding were obtained by this technique, then drawn to fiber.

Gas Flow Test of Braided Preform Quality for Resin Transfer Moulding

Gas Flow Test of Braided Preform Quality for Resin Transfer Moulding