Unidirectional Prepreg Research Articles

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.

Influence of cooling rate on the properties of carbon fiber unidirectional composites with polypropylene, polyamide 6, and polyphenylene sulfide matrices

The longitudinal and transverse strength of three unidirectional thermoplastic prepreg systems: carbon fiber/polypropylene (CF/PP), polyamide 6 (CF/PA6), and polyphenylene sulfide (CF/PPS) are studied and analytical formulas are proposed for the estimation of matrix and fiber/matrix interface properties from composites properties. Since the matrices are semi-crystalline thermoplastics, the influence of cooling rate on the strength is statistically evaluated. While the 0º tensile strength is found to be independent of the cooling rate, the 90º tensile strength is strongly influenced by the matrix type and cooling rate. The matrix modulus increases as the cooling rate is decreased; the degree of crystallinity also increases. The matrix residual stress, interfacial shear strength, and mode II interlaminar fracture toughness are also found to depend on the cooling rate, with the trends different for different matrices.

Effects of material and process parameters on void evolution in unidirectional prepreg during vacuum bag-only cure

Void reduction during composites manufacturing is critical for successful processing. In this study, we perform a parametric study to determine the mechanisms of interply void evolution in unidirectional prepregs during vacuum bag-only cure and to identify the key factors that affect interply air removal. We employ an in situ visualization setup for direct, real-time observation of air removal for prepregs during cure. Results showed that super-ambient dwell at 50℃ effectively promoted interply air removal in unidirectional prepregs, reduced vacuum quality (80% vacuum) had negligible effects on part quality, and that an increase in moisture content of the laminate notably increased void content. Prepreg moisture content was tracked by the inspection of laminate water content at different times during the cure cycle, and the data was combined with a diffusion-based analytical model to predict void size and to improve the understanding of void evolution mechanisms. Results indicated that moisture content of the laminate decreased markedly as cure progressed, providing insights into bubble behavior (expansion and shrinkage) observed during cure. The modified model predictions aligned with experimental data, especially during the second stage, confirming that the observed void growth results from moisture diffusion.

Temperature Dependency of the Deformation Behavior of Hybrid CFRP/Elastomer/Metal Laminates under 3-Point Bending Loads

Fibre-metal-elastomer laminates offer the possibility of using material combinations which often have to deal with premature delamination, for example due to different coefficients of thermal expansion or galvanic corrosion due to different electronegativities. The present study deals with laminates made of layers of CFRP and aluminum, each of which is bonded together by an elastomer layer. The shear-soft elastomer also allows the much stiffer aluminum and CFRP layers to be sheared off against each other under bending stress. This leads to complex deformation behavior. The shear of the elastomer also plays a crucial role in the damping behavior of the laminate. Due to large shear deformations in the elastomer layer, the combination of rigid layers and soft elastomer layers shows very good damping behavior according to the principle of constrained layer damping. Since bending vibrations that occur during normal use usually have only small amplitudes, the deformation behavior is of particular interest in the elastic range. Since this deformation behavior is strongly dependent on the shear modulus of the elastomer used and this in turn is strongly influenced by temperature, the deformation behavior is characterized at different temperatures. Within the scope of this investigation, quasi-static 3-point bending tests are carried out on different laminate lay-ups in the temperature range from -40 °C to +80 °C. The laminates are consolidated by compression molding and contain two different EPDM elastomers in varying layer thicknesses, unidirectional CFRP prepreg in biaxial layer lay-up and aluminum 2024 sheets. The deformation behavior is analyzed by digital image correlation. This is used to measure both the bending line of the overall composite and strains over the layer thickness. In particular, the shear in the elastomer layers is evaluated and set in relation to the bending lines. Finally, the ability of the laminate lay-up to damp bending vibrations is evaluated.

The high strain rate tension behaviour of pseudo-ductile high performance thin ply composites

In this study the effect of strain rate in tension on two different types of thin-ply pseudo-ductile laminates is reported. These comprised Unidirectional (UD) and quasi-isotropic (QI) hybrids made from low strain thin-ply carbon/epoxy and high strain glass/epoxy prepreg and all-carbon laminates made from thin-ply [±ϴ/0]s laminates referred to as angle-ply (AP) composites. Laminates were designed and subjected to a range of cross-head speeds from 2 mm/min (0.0003 s−1) to 10 m/s (100 s−1). The designed laminates were made from single or hybrids of low strain and high strain unidirectional prepregs, i.e. thin-ply Carbon/epoxy and Glass/epoxy, respectively. Dynamic stress-strain plots and details of the damage mechanisms were obtained for each sample and compared with the quasi-static test results. The investigated thin-ply UD and QI hybrid composite laminates showed excellent pseudo-ductility at different strain rates, whereas the AP laminates were strain rate dependent and pseudo-ductility decreased with increasing strain rate.

Buckling and free vibration study of variable and constant-stiffness cylindrical shells

The ability to steer carbon fibre tapes, varying the tow angle, can widen the designs possibilities of cylindrical shells that are one of the main components of aerospace structures. This research presents experimental and numerical investigation of two carbon fibre reinforced plastic cylindrical shells – a cylinder with conventional layup made of unidirectional prepreg and a variable-stiffness cylinder manufactured by applying fibre placement technology. The shells were tested in compression until buckling and later subjected to a vibration analysis. Load-shortening curves and buckling shapes were acquired during the compression tests, while the natural frequencies and the mode shapes were measured during the vibration tests. Both tests provide a useful data set of the mechanical response of the cylinders which can be applied for further validation of models. The acquired experimental results were compared to a simple, approximated numerical model of the variable-stiffness cylinder showing good correlation with the test results.

Comparison of MSC.Software Failure Models MAT8A, MATD054, and MATD058 of Composite Materials

The MSC Dytran and Nastran progressive failure models MAT8A, MATD054, and MATD058 composite materials were investigated. The main objective of the investigation was understanding the particularities of the models, their advantages, disadvantages, and limits of application. The material chosen for simulations was a Toray T700GC-12K-31E/#2510 unidirectional carbon-epoxy prepreg. During a vast number of singleelement simulations, each model was calibrated in accordance with experimental stress–strain curves. Based on correlation errors and simulation experience, conclusions about the material models are made.

Effect of nano-OMMTs on mode I and mode II fracture toughness of continuous glass fibre reinforced polypropylene composites

The influence of nano-organic montmorillonites (nano-OMMTs) on mode I and mode II interlaminar fracture toughness of continuous glass fibre reinforced ammonium polyphosphate polypropylene (CGF/PP/APP) composites was experimentally studied in this paper. Nano-OMMTs modified CGF/PP/APP unidirectional prepreg composites were fabricated by hot-melt impregnation process. To investigate the effect of nano-OMMTs on the interlaminar fracture toughness of composites, double cantilever beam (DCB) test and end-notched flexure (ENF) test were conducted to obtain the mode I initiation interlaminar fracture toughness (GIC init.), mode I propagation interlaminar fracture toughness (GIC prop.), and mode II initiation interlaminar fracture toughness (GII C init.) of samples, respectively. Through comparison against to the neat CGF/PP/APP composites, with the addition of 6.5 wt% nano-OMMTs into PP matrix, the GIC init., GIC prop., and GII C init. of composites can be enhanced from 0.502 kJ/m2, 0.746 kJ/m2 and 0.623 kJ/m2 to 0.712 kJ/m2, 1.119 kJ/m2, and 1.195 kJ/m2, respectively. Finally, it was found that nano-OMMTs in matrix can act as “pins”, which could absorb energy in the forms of nano-particles pullout and enhance the interlaminar fracture toughness values in DCB and ENF tests.

Experimental investigation on spring-back deformation during autoclave curing of parabolic antenna reflectors

High dimensional fidelity is the primary challenge for the large size carbon fiber reinforced polymer (CFRP) antenna reflectors used in space applications. Selection of crucial process parameters governs dimensional control of a composite product. In this paper, the influence of the mould materials, curing cycle, lay-up sequence and laminate thickness on spring-back deformation during autoclave curing is investigated by fabricating a large number of parabolic reflectors using the unidirectional prepreg system. The non-contact type accurate 3D scanning technique is used to measure the dimensions of the parabolic antenna reflectors. In the present work, an experimental approach is adopted for the determination of a combination of process parameters which gives minimum spring-back deformation. The results reveal that the spring-back deformation of parabolic reflectors is significantly affected by the mould material and laminate thickness, whereas the lay-up sequence and curing cycle do not have a significant effect.

Relationship between Process Conditions and Mechanical Properties of Unidirectional Prepregs Prepared by Melt Impregnation

Relationship between Process Conditions and Mechanical Properties of Unidirectional Prepregs Prepared by Melt Impregnation

Numerical Analysis of Two Types Polymeric Fibre Composite Materials with Different Reinforcement Architecture for Creation of Innovative Snap-fit Joints

In this study we analyzed application of two types reinforcements of the polymer matrix composites (PMC): (1) fabric prepreg, (2) unidirectional prepreg and their hybrids for production of modern snap–fit joints. The numerical calculations were made in Abaqus program. Moreover, different layer systems and geometries were proposed to get the most effective joints. For each kind of joints the opening forces were assessed and Tsai-Hill strength criterion for estimation of a damage parameter inside of the PMC was applied. The proposed methodology leads to designing of more optimal joints for various engineering structures.

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 Model of Impact Damage for Guided Wave-Based Inspection of Composites

The objective of this work is to assess to which extent the interaction of antisymmetric ultrasonic guided waves with impact damage can be captured with an experimental model consisting of a single artificial delamination in composite structures. The structures of interest are composed of unidirectional prepreg carbon fiber-reinforced polymer (CFRP) with a quasi-isotropic layup. The artificial delamination is introduced into the laminate using two circular Teflon tapes during manufacturing and the realistic damage is simulated by impacting the samples at two energy levels. Two colocalized rectangular piezoceramics are used to generate an antisymmetric mode and noncontact measurement is performed using a three-dimensional (3D) laser Doppler vibrometer (3D-LDV) to extract the required information for evaluation of the reflection, transmission, as well as the scattering behavior of the antisymmetric mode. The corresponding coefficients as a function of frequency, incident angle, and type of damage are extracted. It is found that the amplitude of the coefficients and directivity patterns of scattered waves are barely affected by incident angle but significantly by the impact energy. In light of the results, design guidelines are proposed for efficient guided wave inspection of composite structures submitted to impacts.

Effect of novel intumescent flame retardant on mechanical and flame retardant properties of continuous glass fibre reinforced polypropylene composites

In this study, the influence of a novel intumescent flame retardant (IFR) on the mode I interlaminar fracture toughness and flame retardant properties of continuous glass fibre reinforced polypropylene (CGF/PP) composites is presented. The novel IFR system that consists of ammonium polyphosphate/poly (ethanediamine-1, 3, 5 triazine-o-4-amion-2, 2, 6, 6- tetramentylniperidine) (APP/PETAT) was introduced into the CGF/PP unidirectional prepregs (UD) composites by hot-melt impregnation process. Double cantilever beam (DCB) tests were performed to evaluate the mode I interlaminar fracture toughness (GIC) by using modified beam theory (MBT). It was found from the experimental results that compared to the neat CGF/PP composite, the mode I fracture toughness initiation toughness (GIC init.) and propagation toughness (GIC prop.) of CGF/PP/12.5 wt% IFR composites were increased by an average of 99.7% and 119.11% respectively. Meanwhile, samples with 12.5 wt% IFR could self-extinguish with an oxygen concentration of 32.4% and exhibit lower fire spread speed in UL-94 test. Furthermore, the peak of heat release rate (PHRR), total heat release (THR), fire performance index (FPI), and mass loss rate (MLR) tests indicated that IFR could dramatically enhance the flame retardancy of the composites.

Experimental investigation on interply friction properties of thermoset prepreg systems

A comprehensive novel investigation into the characterisation of interply friction behaviour of thermoset prepregs for high-volume manufacturing was conducted. High interply slipping rate and normal pressure typically used for high-volume manufacturing present challenges when preforming carbon fibre reinforced plastics. The study involved multiple reinforcement architectures (woven and unidirectional with the same rapid-cure resin system) which were characterised using a bespoke interply friction test rig used to simulate processing conditions representative to press forming and double diaphragm forming. Under prescribed conditions, woven and unidirectional prepregs exhibit significantly different frictional behaviour. Results demonstrated the unidirectional material obeys a hydrodynamic lubrication mode. For the woven material, a rate-dependent friction behaviour was found at low normal pressure. At higher normal pressure however, the woven material exhibited a friction behaviour similar to that of a dry reinforcement and significant tow displacement was observed. Post-characterisation analysis of test-specimens showed significant resin migration towards the outer edges of the plies, leaving a relatively resin-starved contact interface. The findings generate new knowledge on interply friction properties of thermoset prepreg for high-volume manufacturing applications, yet reveal a lack of understanding of the influence of tow tensions as well as the pre-impregnation level for a range of processing conditions.

Characterization of fibre alignment in as-received aerospace grade unidirectional prepreg

Analysis of the fibre dominated properties of composite materials typically assumes, often implicitly, the perfect alignment of fibres. Increasingly, however, it is acknowledged that waviness (in-plane misalignment) and wrinkling (out-of-plane misalignment) play an important role in both manufacturing and structural response. In this paper, a previously developed experimental method to measure fibre misalignment has been improved and extended to also determine fibre excess length. Results are presented for two unidirectional aerospace grade prepregs (8552 and 8552–1 resins reinforced by AS4 fibre). The fibre alignment distributions are well fitted by an analytical model that allows for a distribution of excess lengths; the fibre excess length is not constant at the end of the prepregging process. It is shown that only with a distribution of excess lengths can the experimental unimodal misalignment distributions be analytically modelled. Misalignment distributions presented in this paper are consistent with those reported elsewhere, and furthermore it is shown that the fibres were significantly more aligned out-of-plane than in-plane. Finally, contrary to the best working hypothesis in the literature, samples prepared using a thermal cure process saw no correlation between fibre misalignment and initial location on a prepreg roll.

Analysis of the continuous mode conversion of Lamb waves in fiber composites by a stochastic material model and laser vibrometer experiments

The complex internal structure of modern laminate composite materials, for example, carbon fiber reinforced plastics (CFRP), has a pronounced influence on the propagation of elastic guided waves (GWs). Along with the well‐known anisotropy‐conditioned amplitude and dispersion directivity of source‐induced wavefields and their remarkable attenuation due to the polymer matrix viscosity, continuous mode conversion (CMC) of GWs in multilayered composites fabricated from unidirectional prepregs has been recently observed experimentally. The latter means that the symmetric fundamental mode continuously converts into the antisymmetric one without passing a damage, that is, even in an intact structure. Being strongly related to the inhomogeneous CFRP microstructure and possessing remarkable intensity for specific propagation directions, this phenomenon should be accounted for in ultrasonic nondestructive testing and structural health monitoring systems. In this work, we investigate the CMC phenomenon in an important class of laminates, namely cross‐ply ones, experimentally as well as numerically. In the computational model it is addressed through the concept of spatially varying material properties, and the finite element method (FEM) is employed for the GW propagation simulation. Experimental measurements are performed for piezoelectrically excited GWs with scanning laser Doppler vibrometry technique, allowing clear visualization of the CMC phenomenon and confirming theoretically predicted dependence of its intensity from propagation direction.

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.

In situ monitoring and analysis of void evolution in unidirectional prepreg

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

Numerical and experimental approach for improving quasi-static and fatigue testing of a unidirectional CFRP composite laminate

Mechanical response of CFRP unidirectional prepreg was investigated under tensile quasi-static and fatigue loading. Firstly, the findings of an initial investigation on the effects of the end tabs and gripping condition are presented. A dialogue between numerical simulations and experimental tests led to avoid premature splitting failures in the clamps. Proper tensile failure is assessed in the gage length, at enhanced strength levels for the material considered. Consequently, the unidirectional material was successfully tested under tension-tension fatigue, at high load levels. Fatigue damage mechanisms arise at high stress amplitude, revealing that also a UD laminate can suffer from fatigue degradation along the fibres direction.