Unidirectional Prepreg Tape Research Articles

Experimental and numerical investigation on the compression after high-velocity impact behavior of composite laminates

Considering the differences in damage mechanism between high-velocity impact (HVI) and low-velocity impact (LVI), this study conducted HVI tests and compression-after-impact (CAI) tests to reveal the post-impact compression behavior of laminates manufactured by unidirectional prepreg tape and exposed a completely different mechanical response in residual strength compared to that of LVI. By controlling the impact velocity within the range of 200 m/s ̃ 380 m/s, three kinds of impact results were obtained, including un-penetrated and penetrated conditions. Different CAI damage morphologies can be observed in these laminates. In order to fully understand the CAI damage mechanism induced by different HVI events, a surface-based cohesive behavior and a CDM intralaminar damage model were implemented. In addition, a more complete picture of the compression behavior after HVI was observed through the simulation results, which is of great help in estimating the residual strength of post-impact laminates. However, the HVI damage was introduced through the spherical steel projectiles, and the sample size was determined based on ASTM D7137, which might be different factors from the application conditions.

Influence of carbon fiber non-crimp fabrics stitching parameters on the out-of-plane permeability in liquid composite molding process

The widespread use of carbon-fiber-reinforced plastics (CFRP) has attracted many industries such as defense, and aerospace as well as in sailboats, automotive, and rotor-blades, where high strength-to-weight ratios are required. For the aerospace and defense customer today, the importance is no longer just on minimum weight and therefore performance, but primarily on cost of ownership. Textile-manufactured composites, particularly of non-crimp fabric (NCF) type offer significant cost savings in terms of reduced labor time and higher deposition rates over the unidirectional prepreg tape which is the most traditional method. Vacuum vacuum-assisted infusion method is the most used production technic in CFRP parts which also offers high fiber ratios. The most important goal is the impregnation of carbon fiber package in a through-the-thickness direction to get good quality parts in infusion. The behavior of the impregnability depends mainly on the out-of-plane permeability of the fabric. The fiber type, orientation of the fibers, and the stitching parameters, mainly influence the permeability of the fabric. In this study, the influence of textile parameters on the out-of-plane permeability of UD (unidirectional) carbon fiber NCF was investigated. The stitching yarn parameters (stitch pattern, stitch tension, and stitch length) were varied to see the influence on the out-of-plane permeability. The out-of-plane permeability was measured using a tailored visual flow front monitoring method by measuring resin flow distance on the X and Y axes of the fabric during infusion. The results show that the out-of-plane permeability of the UD (0°) carbon fiber NCF is significantly affected by the stitch pattern, stitch tension, and stitch length.

Experimental characterisation of large in-plane shear behaviour of unidirectional carbon fibre/epoxy prepreg tapes for continuous tow shearing (CTS) process

The continuous tow shearing (CTS) process is an advanced tape placement technique with the ability to steer unidirectional prepreg tapes by in-plane shear deformation without the presence of tape buckling, gaps and overlaps. However, the inherent fibre misalignment within the tape can induce fibre waviness during the shearing process, which is further affected by processing parameters such as temperature, shear rate and fibre tension. In this paper, the effect of those parameters on the shear behaviour of two different unidirectional carbon/epoxy prepreg tapes was experimentally investigated. A bespoke test fixture was designed to shear the tape specimens at various shear rates and fibre tensions. Digital image correlation (DIC) was used to obtain full-field strains of the specimens to investigate the fibre realignment during shearing. Experimental results showed that both temperature and fibre tension significantly affect the uniformity of the fibre realignment. Moreover, the responses of the two prepreg tapes under the same shearing condition were different due to their different impregnation characteristics.

Processing of post-industrial unidirectional prepreg tapes using SMC equipment

The aerospace industry has an issue of disposing of waste, unused carbon fiber unidirectional prepreg tapes. This paper explores producing chips using sheet-molding compounding equipment (SMC) for conversion into products via compression molding, which are stronger than traditional glass-reinforced SMC parts due to the carbon fiber and high strength epoxy resin. Without a means to convert the continuous tape prepreg into chips, it will not be possible to convert the prepreg into a useful product and provide a way to reduce flow to the landfill and produce less expensive, lighter, and stronger parts for the transportation industry. The cure state of the epoxy resin in the prepreg tapes, which is affected by the amount of time the material has spent stored at elevated or room temperatures, significantly changes the ability of the material to be chopped into short fiber chips. In addition, equipment-related factors affect the material’s ability to be chopped by the SMC equipment. This paper accounts for each of these factors and uses statistical methods to design experiments to investigate the impact each of these material-related and equipment-related factors on the success of the cutting portion of the process. The level of cure, as defined by endothermic peak, glass-transition temperature (Tg), or heat of reaction/degree of cure, has the greatest impact on the cutting process. For material with a moderate level of cure, the sharpness of the blades in the cutting roller also has a major impact on the success of the process. The cutting pressure, roller speed, and tape tension also have an impact on the success of the process, but impact of these factors lessens for feedstock prepreg with a sufficiently high level of cure. An optimal setup was determined, and general rules of thumb for selecting material and equipment settings were established.

Influence of consolidating process on the properties of composites from thermosetting carbon fiber reinforced tapes

AbstractCarbon fiber reinforced tapes are a feasible option to tailor material properties of composite structures and to reduce waste due to cut‐offs. But composite properties are not only influenced by the selected material, but also via the applied curing process. The aim of this experimental investigation is a comparison of the mechanical properties of thermosetting carbon fiber reinforced tapes, which are cured in different curing processes. In the composites industry the autoclave curing process is considered to be the highest quality process for handling preimpregnated semifinished‐products, so called prepregs. Alongside the autoclave process there are several alternative routes, named “out‐of‐autoclave”–processes, which can also be chosen for prepreg curing. One option is prepreg hot‐pressing, a process which is intended for large part quantities and easy process control. The development of fast curing thermosetting resin systems with the possibility of complete curing in the range of minutes introduced snap‐cure prepregs, recommended to be used solely in hot presses. For this reason this type of prepreg, in form of unidirectional tapes, was used for the following experiments. Two additional curing processes were investigated for process‐comparison, namely oven‐curing and in‐situ consolidation. The placement of the prepreg tapes for the laminate build‐up was done by an automated fiber placement unit followed by the curing processes as mentioned before. During the characterization of the material properties the general accepted statement of the excellent autoclave laminate quality could be confirmed. Strengths of around 930 MPa and even above were achieved with three‐point coupon bending tests for laminates cured in the autoclave. The fiber volume fraction was slightly above 58% and the laminate porosity was 0.5%. In comparison oven cured and hot pressed laminates strength values were lower at around 675–775 MPa. Hot pressed laminate porosity content was about 1.3% and the fiber volume fraction of these laminates was 52%. Oven cured laminates showed a higher porosity content above 4% and a fiber volume fraction of 51%. The new approach for prepreg curing, the in‐situ consolidation process provided fairly good results which were comparable to those of the oven cured laminates. In this process the unidirectional prepreg tape is directly cured at the surface of a heatable table during automated fiber placement without subsequent curing under a vacuum bag. The positive testing outcome of in‐situ consolidated laminates shows a big potential for future research and development of this process.

Flexural and shear strength properties of unidirectional carbon fiber reinforced polymer composite interleaved with recycled carbon fiber and short virgin aramid fiber non-woven mats

Carbon fiber reinforced polymer composites (CFRPs) are one of the most widely used composite types and wastes associated with them (CFRPs) get generated through either their manufacturing or end-of-service-life. Predominately due to environmental concerns and governmental regulations, recycling these CFRPs is needed and to make use of the recycled carbon fibers (rCFs), a wet paper-making technique was used to convert the rCFs into a 60 g/m2non-woven mat. For comparison purposes, the same technique was used to convert short virgin aramid fibers (vAFs) into a 60 g/m 2 non-woven mat. Each mat was sandwiched with two resin films and then interleaved with 12-ply unidirectional (UD) prepreg tapes (carbon/epoxy). The assemblage was molded into composite laminates using a vacuum bagging assisted compression molding technique, and the samples for the tests were cut using a waterjet machine accordingly. Compared with the control, the results indicate an increment in the flexural modulus, and the specific flexural modulus for the CFRPs with non-woven mats: the flexural modulus increased by approximately 8.2% and 12.0% for the CFRP with rCF and vAF mats, respectively; the specific flexural modulus increased around 9.5% and 13.3%, respectively for the CFRP with rCF and vAF mats. On the other hand, the shear strength approximately decreased by 6.4% and 6.0% for the CFRP with rCF and vAF mats, respectively. The negative shear strength performances of the composite laminates with non-woven mats reflected on their flexural strength performances: the flexural strength increased about 1.1% and decreased by approximately 7.9% for the CFRP with vAF and rCF mats, respectively. To resolve the negative shear strength performances, it is recommended that the surfaces of the mats be treated with a coupling agent to improve their interfacial adhesions.

Numerical and experimental studies of the influence of curing and residual stresses on buckling in thin-walled, CFRP square-section profiles

The study provides the first comprehensive experimental and numerical study of thin-walled, carbon fibre square-section columns subjected to static compression. The profiles were manufactured from unidirectional prepreg tape of Hexcel AS4 high-strength carbon fibres in HexPly® 8552 thermoset resin with ply angles [45/−45/45/−45]s. The dimensions of the columns were: (height × width × length) 80 mm × 80 mm × 240 mm and the wall thickness was 0.92 mm.Advanced Finite Element Models (FEM) of the residual stresses generated during production were used as a basis for simulations of static compression. The deformation predicted by these models showed excellent agreement with 3D scans of the columns. These results also highlighted the residual stress concentrations generated at sample corners during production.It was found that the residual stresses from the autoclaving process significantly enhance the buckling load performance (by ~35%) and that the models provide an effective comparison with the experimental results (<10% error). This demonstrates the importance including residual stresses in FEM of buckling, but also highlights residual stress tailoring as a route to significantly enhance the buckling load capacity of carbon fibre systems used in industrial applications.

Gap filling mechanisms during the thin ply Automated Tape Placement process

Automated Tape Placement (ATP) is one of the attractive automated composite manufacturing processes as it reduces labor cost, increases production rate and improves repeatability. In this method, the part is built by consolidating one unidirectional prepreg tape layer at a time with a placement head using a compaction roller. Defects such as gaps between neighboring tapes during lay up are inevitable. This challenge becomes even more critical in the ATP of the thin ply prepreg tapes as the ratio of the gap width to the tape thickness increases. The filling process of such gaps during the process cycle results in resin-rich areas and fiber waviness. In this paper, we present a model to numerically simulate the gap-filling process by coupling the squeeze flow behavior within the tapes to fill the gap with the deformation of the top layers into the hollow space within the gap. First, the experimental results clarify the microstructural physics of the gap-filling process. This is followed by proposing a model to predict the flow front of resin that moves to fill the gap and the gap thickness. The predictions of the model are compared with experimental results and validated by the micrographs of fabricated samples.

Analysis of contact area in a continuous application-and-peel test method for prepreg tack

The relationship between prepreg tack and the degree of intimate contact (DoIC) between prepreg and a rigid substrate was explored in the context of a continuous application-and-peel test method. Tack for a unidirectional prepreg tape was characterised for different surface combinations and varying test parameters (material feed rate, temperature) at a constant compaction pressure. Application of the prepreg to a transparent rigid substrate (glass), was carried out at matching test conditions to the prepreg tack measurements. Optical microscopy was utilised to acquire images of the contact area at the prepreg-glass interface. Image analysis of the micrographs enabled quantification of the contact area. The time- and temperature-dependent viscoelastic behaviour of the resin was explored directly on the prepreg using oscillatory parallel plate rheometry, and time-temperature superposition was applied to construct both tack and DoIC master curves. The shifted DoIC data showed that true contact area increases with decreasing shifted feed rates, until maximum contact area is achieved. Similarly, tack increases with decreasing shifted feed rates. However, at a critical feed rate, the bond failure mechanism switches from adhesive to cohesive failure. In cohesive failure, tack decreases with decreasing feed rate despite the high levels of DoIC.

Comparing Test Methods for the Intra-ply Shear Properties of Uncured Prepreg Tapes

Abstract To achieve cost-efficient manufacturing and a high part quality in Thermoset Automated Fiber Placement (TS-AFP), knowledge about the interaction between material and process parameters is of special interest. Material properties of prepregs are well known at the cured state of the resin. However, there are no standardized test procedures for the mechanical behavior of the uncured prepreg tapes. To investigate the intra-ply shear deformation behavior of uncured unidirectional prepreg tapes, we compared several measurement procedures and conducted experiments for rheometer based tests using 8552/AS4 material. We identified a rotational parallel platens rheometer test method and a torsion bar rheometer test method to be suitable. Experiments using both methods revealed that the Torsion Bar Test has a higher repeatability and the analysis is less complex. Furthermore, first results show that changes in material properties caused by aging can be analyzed using this method. In the future,we will use the Torsion Bar Test to characterize changes in deformation behavior due to material aging as well as material modifications. By this, we will be able to provide data for the material modeling thus enabling the prediction of lay-up defects such as buckling due to steering.

Integrative analysis for prediction of process-induced, orientation-dependent tensile properties in a stochastic prepreg platelet molded composite

Prepreg platelet molded composite (PPMC) is derived from compression molded pieces of chopped unidirectional prepreg tape. The properties of a stochastic PPMC arise from the meso-structure that develops during processing. This paper describes an integrated methodology for analysis of stochastic PPMC to develop process-structure-property relationships. Flow-induced fiber orientation distributions were predicted using an anisotropic viscous constitutive model implemented in a nonlinear, explicit finite element (FE) solver. The predicted orientation state was validated by CT-based orientation measurements and optical microscopy. A computational framework for simulation of tensile property distributions of a stochastic PPMC by progressive failure analysis is presented. The probabilistic simulation results were statistically validated against experimental data. A FE analysis was developed with an explicitly modeled platelet meso-structure wherein the platelets are treated as a homogeneous orthotropic medium using continuum damage mechanics to model the intraplatelet failure and a cohesive zone model for interlaminar disbonding. Panels were molded using partial charge coverage to induce flow alignment of the fibers. Tensile coupons were excised from the molded panels both along (parallel) and transverse (perpendicular) to the preferential fiber direction to compare the composite effective properties with those reported in the literature for planar random orientation states. The composite tensile properties were found to be strongly dependent on the global orientation state.

Novel filler materials for composite out-of-plane joints

In the manufacture of the out-of-plane joints of composite stiffened panels, such as the connection between skin and T, I, omega shaped stiffeners, a filler material is needed to fill the void between the flanges, web and skin. The most common filler is a rolled unidirectional prepreg tape (“noodle”), which is not only expensive to manufacture, but also has low strength that can lead to premature failure of the loaded joint. In this work, eight novel filler concepts are introduced and experimentally validated against the baseline noodle via T-joint tensile tests. Polyamide nonwoven interleaved joints increase the damage tolerance of the structure and nonwoven nanofibres increase the failure initiation load. 3D printed fillers have lower strength but demonstrate the possibility of thermoplastic-thermoset hybrid structures. Fillers made of chopped prepreg match the strength of the baseline noodle and can serve as a low cost replacement. Another low cost, resin infused braided concept has lower strength, but its counterpart using multiple individual braids has the same strength as the unidirectional noodle. Moreover, the latter concept shows that different resin systems can be cured together without causing a knockdown in strength, and can serve as a basis for a range of novel applications.

Characterisation of tack for uni-directional prepreg tape employing a continuous application-and-peel test method

Employing a test method with coupled application and peel phases, tack was characterised for a UD prepreg tape. Different aspects of tack were explored by varying test parameters and material condition. In addition, different surface combinations were studied. In general, the test parameters, feed rate and temperature, affect the balance between cohesion within the resin and adhesion between resin and substrate. Exploring a range of parameters is required to understand the effect of viscoelastic resin properties on tack. The application pressure determines the true contact area between prepreg and substrate and hence affects tack. Changes in molecular mobility in the resin related to specimen conditioning, i.e. ageing or moisture uptake, result in maximum tack to occur at lower or higher feed rates, respectively. Differences in tack for different material combinations can be attributed to different molecular interactions at the contact interfaces and different resin distributions on the prepreg surfaces.

Experimental investigation of friction between prepreg tape and compaction roller for prepreg tape hoop winding

As an inevitable issue in the filament winding technology, the process-induced friction has gained increasing attention. The purpose of this study is to illustrate the experimental investigation of friction between prepreg tape and hot compaction roller in prepreg tape hoop winding system. Based on the modified version of Howell’s equation, the friction model is established to offer an indirect method for measuring the frictional coefficient during the actual composite fabricating process. Three types of prepreg tapes, namely T300/Y69 unidirectional prepreg tape, T300/DFQS-3 and BWT260-82/DFQS-3 orthogonal prepreg tapes are selected to discuss the effect of process parameters on friction under different processing conditions. The experimental results reveal that the friction behavior transfers from resin-dominated friction to fiber-governed friction as the temperature rises. Besides, acceleration of the spindle speed on the frictional coefficient can be markedly improved due to the increase of resin strain rate. Meanwhile, the fiber has a stronger ability to increase friction than resin. It is also observed that the friction plays a role in amplifying the winding tension by comparing the winding tension with the outgoing winding tension.

Bolted joints in quasi-unidirectional glass-fibre NCF composite laminates

In this study, crossply and quasi-isotropic quasi-unidirectional (quasi-UD) non-crimp fabric (NCF) glass/epoxy laminates were manufactured by using vacuum assisted resin transfer moulding (VARTM) and tested according to the ASTM bearing test standard D5961. The specimen geometry was kept constant in all specimens with the ratios of e/d=3, w/d=6 for d=6mm. Similar damage modes and damage propagation trends were observed between the classical unidirectional prepreg tapes and the current material system. Furthermore, the bolted joint strength of quasi-UD NCF glass/epoxy laminates was estimated by a three-dimensional (3-D) progressive damage model (PDM) to evaluate the capabilities of a widely used finite element (FE) program, ANSYS. The experimental data and predicted results were in a good agreement and the experimental failure modes were successfully predicted.

Experimental investigations of thin-walled GFRP beams subjected to pure bending

The results of the experimental buckling and postbuckling behaviour analysis of thin-walled, square cross-section tubes subjected to pure bending are presented. The beams under investigation were manufactured from a glass fibre reinforced polymer unidirectional prepreg tape in an autoclaving procedure. The experimental results for six different layups of the eight-layer laminate were considered. In order to collect all necessary data, strain gauges and an ARAMIS Digital Image Correlation (DIC) system produced by the GOM company were employed. The strain gauges measured strains at one point, whereas the DIC system was able to generate maps of displacement fields. On the basis of the data from an Instron universal testing machine, it was possible to determine buckling loads and equilibrium paths in order to compare the results. Moreover, grips designed especially for pure bending tests are presented.

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.

Experimental investigation of failure process in compressed channel-section GFRP laminate columns assisted with the acoustic emission method

The experimental investigations of thin-walled channel section columns subjected to static compression are described. The columns under consideration were made of eight layers of a glass/epoxy unidirectional prepreg tape, with the autoclaving technique. Six different symmetrical layer arrangements were taken into account. The tests were performed on a Zwick Z100/SN3A universal testing machine with special self-aligning grips. The columns under investigation were loaded with the force values from zero to the maximal load destroying the profile, which allowed one to observe the column behaviour till its collapse. The strain gauges measurement technique was employed to determine the equilibrium paths, the buckling loads and the post-buckling behaviour. During all the tests, AMSY-5 AE equipment made by the Vallen company was applied to detect acoustic emission phenomena. The acoustic emission method allowed to investigate the behaviour of composite structures in the phase prior to their collapse. The results of the experimental investigations were employed to validate the proposed FE model in order to analyse the post-buckling behaviour and to determine the failure load with the well-known failure criteria.

Numerical and experimental investigations of the post-buckling behaviour of square cross-section composite tubes

The buckling and post-buckling behaviour of thin-walled square cross-section tubes subjected to static compression is presented. The columns under consideration were made with the autoclaving technique of eight layers of a glass fibre reinforced polymer unidirectional prepreg tape. Six different arrangements of layers were taken into account. The following experimental data were collected: force–displacement relations obtained from a Zwick/Roel universal testing machine, strains at chosen points of the tube measured with a strain gauge technique, a map of deflections obtained with a non-contact 3D optical method (Aramis System, made by GOM company). All those results were used to determine buckling load and equilibrium paths in the pre- and post-buckling range. A numerical model of the analysed tube was prepared with the ANSYS software. A linear and nonlinear buckling analysis was performed. The experimental results were used to validate the proposed FEM model. An influence of the layer arrangement on the buckling and post-buckling behaviour, differences between the numerical and experimental results and the methods employed for buckling load determination are discussed.

Investigation on Compressive Strength after Impact Damage of Carbon/Epoxy Laminate

Development of the Korean small scale aircraft (KC-100) has been carried out by KAI (Korea Aerospace Industry Ltd.) for the BASA(Bilateral Aviation Safety Agreement) program with FAA. The KC-100 aircraft adopted the environmental friendly whole composite structure concept due to low fuel consumption owing to its light weight. However the composite structure has a disadvantage. They are very weak against impact damage due to FOD (Foreign Object Damage). This study is to investigate the residual compressive strength of the carbon/epoxy fabric and UD (Unidirectional prepreg tape) laminate due to impact damages. This residual compressive strength test follows fully the ASTM standard test procedure. Experimental investigation results reveal the residual strengths and the damage mode and size of impact damage specimens. Through these tests, it is found that the residual strength of the impact damage specimens is greatly reduced in comparison to the undamaged specimens depending on increase of both the indentation depth and the impact energy. The size and shape of damage zone greatly affects the reduction of the compressive strength.