Low-energy electron beam cured tape placement for out-of-autoclave fabrication of advanced polymer composites

The autoclave curing process for advanced polymer composites is labor and capital intensive, with the curing cost increasing dramatically by the growth in part size. In order to develop an out-of-autoclave (OOA) fabrication process for advanced polymer composites, a new process integrating automated tape placement with low-energy electron beam radiation curing was explored, by which in situ layer-wise curing of advanced composites can be achieved with the tape placement process. The irradiation process was optimized to get a homogenous curing, by tuning the electron beam dose-depth distribution in the prepreg material. Besides, the curing characteristics of the prepreg material by the low-energy electron beam irradiation was investigated and effect of exposure dose and post curing on curing degree, glass transition temperature (Tg) and interlaminar shear strength (ILSS) were characterized experimentally.     

A simulation study on the effects of particle size on the consolidation of polymer powder impregnated tapes

The powder prepreg technique offers a low-cost, high-speed method of manufacture of composites. Consolidation of the prepreg tows is a key step in the manufacture of a final `useful' part. The efficiency of the consolidation for any polymer powder prepreg tape produced from powder prepreg processes would conceivably depend on the material properties of the polymer, particle size, temperature, consolidation pressure and volume fraction of the tape. A mathematical process model that identifies and describes a set of parameters to predict the consolidation conditions of a given polymer powder prepreg tape has been developed. A simulation study that identifies the different parameters and compares the advantages and limitations of polymer physical properties and particle size has been performed. An integrated Flow Resistance (FR) index has been developed to mathematically describe the consolidation process. The results show that small powder particle sizes are beneficial to the process.     

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.     

A method for the direct measurement of the fibre bed compaction curve of composite prepregs

A method to measure the fibre bed compaction curve directly from composite prepreg is presented. The method was used to measure the compaction curve of unidirectional and quasi-isotropic AS4/3501-6 carbon–epoxy prepregs. Similar compaction curves were obtained in all cases. The compaction curve obtained was used by a finite element process model, COMPRO, to simulate the uniaxial compaction of 8 and 16 ply laminates at different temperatures. The force–displacement response predicted by the model closely matched the experimental results. The method which can be used on both tape and fabric prepregs, has the major advantages of being a direct measure of the prepreg behaviour, and requires no special preparation of the sample.     

Effects of Processing Parameters on the Forming Quality of C-Shaped Thermosetting Composite Laminates in Hot Diaphragm Forming Process

In this study, the effects of processing temperature and vacuum applying rate on the forming quality of C-shaped carbon fiber reinforced epoxy resin matrix composite laminates during hot diaphragm forming process were investigated. C-shaped prepreg preforms were produced using a home-made hot diaphragm forming equipment. The thickness variations of the preforms and the manufacturing defects after diaphragm forming process, including fiber wrinkling and voids, were evaluated to understand the forming mechanism. Furthermore, both interlaminar slipping friction and compaction behavior of the prepreg stacks were experimentally analyzed for showing the importance of the processing parameters. In addition, autoclave processing was used to cure the C-shaped preforms to investigate the changes of the defects before and after cure process. The results show that the C-shaped prepreg preforms with good forming quality can be achieved through increasing processing temperature and reducing vacuum applying rate, which obviously promote prepreg interlaminar slipping process. The process temperature and forming rate in hot diaphragm forming process strongly influence prepreg interply frictional force, and the maximum interlaminar frictional force can be taken as a key parameter for processing parameter optimization. Autoclave process is effective in eliminating voids in the preforms and can alleviate fiber wrinkles to a certain extent.     

Flow modeling of the VARTM process including progressive saturation effects

Modeling of vacuum based liquid composite molding methods (e.g., VARTM) relies on good understanding of closely coupled phenomena. The resin flow depends on the preform permeability, which in turn depends on the local fluid pressure; the preform compaction behavior, and the membrane stresses in the vacuum bag. It has also been shown that for many preforms there is a significant unsaturated region behind the flow front, and that the flow in this region affects the overall flow behavior of the process. Studies of preform compaction have shown that the preform stiffness, as well as being non-linear and exhibiting significant hysteresis, is dependant on the fluid saturation. For this reason most researchers model the preform compaction based on the pressure-compaction behavior of saturated preforms during unloading. This assumption leads to an effective discontinuity in preform thickness at the flow front, which is not observed in actual experiments. In this paper an improved compaction model incorporating the saturation dependence of the compaction pressure in the partially saturated region, is used in a one-dimensional model of the VARTM process. The model gives physically more realistic results for the thickness in the flow front region, and an improved model for the consolidation of the preform at the end of infusion.     

On-line consolidation of thermoplastic composites using laser scanning

A novel technique to process thermoplastic composites by laser-assisted tape winding has been developed and evaluated. Previous investigations were limited to a stationary beam and consolidation of narrow prepreg tapes. The technology was extended to process wider prepreg tapes using a galvanometer-based scanning system. The energy distribution along the consolidation line is affected by the triggering signal used to position the scanning mirror. This distribution is predicted using a geometrical model and the results are confirmed experimentally with imprints of the scanned beam in Plexiglas. It was found that the most homogenous heating of the surface is achieved if a triangular triggering signal is used. Density and short beam shear tests are used to evaluate the bond quality that can be achieved using the beam scanning system in comparison to parts fabricated with a stationary beam. Tests have been performed on parts processed with different laser power settings and scanning frequencies. The shear strength increases with increasing laser power to an optimum value after which it decreases due to the degradation of the thermoplastic matrix. The scanning frequency in contrast had no influence on the shear strength. The possibility to process wider tapes using the scanning system was demonstrated by consolidating a one-inch wide PPS/Carbon prepreg tape.     

A non-local void filling model to describe its dynamics during processing thermoplastic composites

A model is developed to describe the void dynamics within thermoplastic composite tape during the tape placement process. The model relates the volatile pressure in voids, the applied compaction load, fiber bed response and the resin pressure due to squeeze-flow of resin from resin-rich regions to fill void regions. This model relies on some geometric simplifications, but incorporates the relevant physical phenomena.This void consolidation model was implemented in a numerical code which predicts the void development during the process. The initial void geometry can be introduced either manually, using a random generation algorithm or from actual processed tape micrographs.The model predicts that the final void content depends on the original void content but also on the initial void distribution. Presented results analyze the influence of void distribution on tape consolidation. Limitations of the consolidation process rate by the resin squeeze flow pressures are clearly demonstrated.     

Interstrand void content evolution in compression moulding of randomly oriented strands (ROS) of thermoplastic composites

Compression moulding of randomly oriented strands (ROS) of thermoplastic composite is a process that enables the forming of complex shapes with keeping final properties close to that of continuous fibre composites. During forming several deformation mechanisms occur. In this paper we focus on the interstrand void content (ISVC) reduction: the squeezing of each single strand during compression enables filling of the gaps between strands. A modelling of this deformation mechanism was developed. The compaction is ruled by an ordinary differential equation that was solved numerically. The model was validated experimentally using an instrumented hot press with Carbon-PEEK prepreg strands. The model accurately predicted ISVC in four characteristic cases. Using the proposed model, the influence of several process and material parameters were investigated. Finally, a design chart giving the final ISVC for a wide range of pressure, strand geometry and part thickness, was constructed.     

A stochastic approach to model material variation determining tow impregnation in out-of-autoclave prepreg consolidation

Material variations are always present even though out-of-autoclave prepregs are machine-made. They strongly determine the consolidation and may eventually lead to voids within the final part, depending on applied process conditions. To capture any contingencies, stochastic differential equations are derived to describe various interacting phenomena in OoA consolidation. In a second step the probabilistic space is discretized using the Karhunen–Loève truncation and the Probabilistic Collocation method is applied in order to use deterministic solvers for flow and compaction problems. The initial degree of impregnation is represented by an Ornstein–Uhlenbeck process and calibrated with CT-images.     

热压罐零吸胶工艺树脂压力在线测试及其变化规律

针对碳纤维缎纹布/环氧914预浸料热压罐零吸胶工艺,采用热压成型过程树脂压力在线测试系统监测树脂压力的大小与分布,分析了真空、外加气压对树脂压力的作用规律,通过显微观察研究了真空及外加气压对孔隙缺陷的影响。实验结果表明,所采用的在线测试系统可以定量分析真空在铺层内的作用程度和树脂压力的变化;零吸胶工艺树脂承担了大部分外压且沿层板厚度及面内方向分布均匀;真空通过铺层内的气路通道排出夹杂空气,其作用程度受到树脂黏流状态和铺层密实程度的影响;不同压力条件下复合材料层板孔隙状况与树脂压力的测试结果相吻合。     

Time–temperature equivalence in the tack and dynamic stiffness of polymer prepreg and its application to automated composites manufacturing

A recently developed peel test designed to simulate the automated tape lay-up (ATL) process was used to measure tack and dynamic stiffness of newly developed ATL prepregs. Resin was extracted from the prepreg process before impregnation of the fibres. Isothermal small amplitude frequency sweeps were carried out in shear rheology to determine time–temperature superposition parameters in the form of Williams–Landel–Ferry equation. Gel permeation chromatography and differential scanning calorimetry demonstrated that the resin was not significantly changed during the prepregging process. The WLF parameters were used to transpose isothermal tack and dynamic stiffness results with excellent agreement found. This relationship offers manufacturers using composite prepreg a method to maximise and maintain tack levels at different feed rates by appropriate changes in temperature. This is of significant importance in improving the reliability of automated composite lay-up processes such as AFP and ATL, whose feed rate must vary to accommodate lay-up operations.     

碳纤维/环氧树脂复合材料-铝合金层合板深拉成型特性

碳纤维增强树脂复合材料（Carbon fiber reinforced plastic,CFRP）因其轻质高强的特点,越来越多地应用到汽车轻量化设计和制造中。为研究CFRP板件及CFRP-Al层合板深拉成型影响因素,加速CFRP零部件产业化进程,本文通过DSC测试分析了CFRP预浸料的固化放热过程,以此为依据,用热压罐制备了不同后固化温度下成型的CFRP板材及单向、编织两种预浸带铺层的CFRP-Al层合板,用Inspekt table 100材料试验机对上述两种板材分别做了深拉试验。考虑到提高制备效率,用打磨、打磨+涂覆硅烷偶联剂、阳极氧化+涂覆硅烷偶联剂三种方式对铝合金板进行表面处理,不经热压罐固化成型,直接和正交对称铺层的单向预浸带一起在Inspekt table 100材料试验机的环境箱中混合温深拉,固化成形。并通过金相显微镜、SEM进行显微组织观察,验证后固化温度、深拉环境温度、预浸带的种类对CFRP板材及CFRP-Al层合板深拉成型性能的影响及铝合金板表面处理方式对CFRP叠层预浸带、铝合金板材混合温深拉成型性能的影响。结果表明,适当降低后固化温度、提高深拉环境温度有利于板材二次深拉成型。编织预浸带较单向预浸带能更好地承受压力,深拉成型质量更优。阳极氧化+涂覆硅烷偶联剂的表面处理方式一方面能在铝合金板材表面形成致密、均匀的微孔,另一方面硅烷偶联剂能很好地促进铝合金板材和CFRP的界面结合,有利于深拉成型。      

Forming induced wrinkling of composite laminates with mixed ply material properties; an experimental study

One disadvantage of multi-layer forming of unidirectional (UD) prepreg tape is the risk of out-of-plane wrinkling. This study aims to show how mixed ply material properties affect global wrinkling behaviour.An experimental study was performed using pre-stacked UD prepreg on a forming tool with varying cross sections. Parameters studied include local interply friction, effects of co-stacking and fibre stresses in critical fibre directions. Experimental evaluation was performed on out-of-plane defect height, type and location. The study shows that fibre stresses in some fibre directions were crucial for the shearing required to avoid wrinkling. The same fibre stresses may cause wrinkling if the lamina is stacked in a non-beneficial order. Changing the friction locally, or reducing the number of difficult combinations of fibre angles, improves the forming outcome slightly. However, in order to make a significant improvement, co-stacking or different fibre stacking is required.     

Characteristics of CF/PEI tape winding process with on-line consolidation

The effects of processing conditions on consolidation quality for the tape winding process of thermoplastic composites with on-line consolidation have been investigated. Composite rings were manufactured using carbon fibre (CF)/polyetherimide (PEI) tape for both cases with and without an insulated ring. Based on heat transfer and intimate contact/autohesion mechanisms, a steady-state finite element method (FEM) model was developed to analyze consolidation quality and overheating in composite tapes. The processing windows with the upper bound for good consolidation and the lower bound for overheating were established for winding speed as a function of processing temperature, when the compaction pressure was kept constant. Good correlation was obtained between the steady-state FEM model and experimental data.     

Modelling slit tape buckling during automated prepreg manufacturing: A local approach

This paper investigates the local buckling of slit tape in the Automated Fibre Placement process for a circular fibre path using an analytical approach that accounts for prepreg tack and stress distribution. Curing effects are not considered in this study. The main objective is to define the critical buckling load and therefore the minimal steering radius for which no tow wrinkling occurs. Simulations are performed for the buckling of the first ply, using a tack parameter governing the interaction between tool and material. A sensitivity analysis of the most relevant parameters of the model is performed. Numerical results show that lower values of the steering radius are obtained for lower tow width values and higher prepreg tack values. Experimental investigations show that the estimation of wrinkling occurrence in steered fibres could be improved by taking into account certain additional parameters, such as lay-down rate, temperature and the dynamic behaviour of the resin.     

The impact of air evacuation on the impregnation time of Out-of-Autoclave prepregs

Most Out-of-Autoclave prepregs (OoA) are only partially impregnated with resin. Their impregnation completes during the cure cycle, solely driven by the difference between atmospheric and vacuum pressure. Increased part length leads to an impregnation time gradient caused by the transient air flow inside the fibrous medium. In this work, a novel numerical approach capable of predicting the local impregnation time of a fibrous domain with resin, at isothermal conditions, under the influence of transient air flow, is proposed (delayed air evacuation). Sensitivity studies prove the robustness of the numerical scheme, for a large range of flow time-scales. The same approach is used to predict the local impregnation time of a commercial OoA prepreg tow, for a wide range of part lengths. It is demonstrated that for manufacturing long parts OoA, accurately capturing the influence of the air pressure on the local impregnation state of the tow, is important for quantifying the risk for residual tow porosity.     

The effect of processing temperature and placement rate on the short beam strength of carbon fibre–PEEK manufactured using a laser tape placement process

The ability of a modern near infra-red laser tape placement system to produce high-quality laminates is investigated by performing short beam strength tests on samples manufactured at different process temperatures from 400 °C to 600 °C at placement rates of 100 mm/s and 400 mm/s. The temperature history in tape placement is highly dynamic and the correlation between the process control temperature, laser power and the consolidation temperature is not well understood. The complete temperature history was therefore estimated with a previously developed optical-thermal model and validated using long wave infra-red imaging. Short beam strengths equivalent to conventional manufacturing methods were found for placement rates of 400 mm/s. Failure modes of the samples were elucidated by scanning electron microscopy of the fracture surfaces. Signs of degradation were observed on samples prepared with a 600 °C process temperature at 100 mm/s, however none was evidenced at 400 mm/s for the same process temperature.     

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.     

Surface porosity during vacuum bag-only prepreg processing: Causes and mitigation strategies

In this study, we employ a parametric approach coupled with surface analysis to identify the source(s) of surface porosity and to develop effective mitigation strategies. Results confirmed that surface porosity was primarily associated with air that was trapped at the tool–prepreg interface during layup. The magnitude and distribution of surface porosity was affected by multiple parameters, including vacuum hold time, freezer and out time, and material and process modifications that affect air evacuation. The results indicated that prepreg out time (and thus tack) and vacuum quality were the primary drivers of surface porosity; for example, surface porosity decreased by 83% after just four days of out time and by 99% after 14 days of out time. These factors were used to formulate guidelines to mitigate surface porosity by (a) increasing the driving force for air evacuation and/or (b) increasing the permeability of the tool–prepreg interface.