Conventional Prepreg Research Articles

Sustainable composite manufacturing from non-expiring carbon fiber/epoxy prepregs based on a vitrimeric matrix

Despite the benefits of manufacturing composite materials from prepregs, its use is nowadays limited to high-production rate industries due to the high costs associated with their limited shelf life. Once this time passes, the material is considered as expired and disposed of as a non-usable waste. To avoid shortening its shelf life, prepregs are stored in refrigerators, increasing energy consumption, and thus magnifying the negative environmental impact. For this reason, the objective of this paper is to develop a new lifelong prepreg material without an expiration date that can be consolidated as a laminate after surpassing the gelation time of the resin, thus allowing composite materials processing technology based on prepregs without the need of freeze storage. To prove this concept, the study is carried out using a vitrimeric resin composed of an epoxy monomer (DGEBA) and 2-Aminophenyl disulfide (AFD) as a hardener. Two prepregs are manufactured and stored for 30 days at different conditions: environmental conditions, considered as the aged or non-conventional prepreg; and at −18 °C in a freezer, to replicate the conventional prepreg conditions. The results of the cured composites show stable glass transition temperatures and curing degrees between the two laminates. Concerning the mechanical properties, it has been proved that the gelation phenomenon of the non-conventional prepreg does not have any negative effect, showing a 11 % and a 21 % improvement of the flexural strength and failure strain, respectively, together with a 10 % increase of the interlaminar shear strength (ILSS) in comparison with the conventional prepreg, proving the potential of the proposed sustainable prepregs.

In situ resin age assessment using dielectric analysis and resin cure map for efficient vacuum infusion

The physical state of epoxy resin designed for vacuum infusion was assessed in situ by immersing a dielectric sensor into the resin pot. The measured ion viscosity of aged resin was directly converted to a degree-of-cure metric using a resin cure map constructed by correlating cure kinetics and dielectric analysis data. Next, an age-adjusted infusion process map was employed to define a nominal infusion window and to identify key process metrics. Finally, process simulations and flow contour maps were used to validate and refine the process map and to guide adjustment of infusion process parameters based on resin age and part size/geometry. The study describes a pathway to more efficient use of aged resin using in situ process diagnostics, cure map design, and process simulation. The methodology employed to evaluate resin age and to adjust process parameters accordingly can be extended to other composite manufacturing processes, including conventional prepreg processing.

Effects of heavy lightning strikes on pristine and repaired carbon composite structures

The effects of particularly heavy lightning strikes on representative carbon fibre reinforced plastics composite airframe structures, specifically with epoxy matrix systems, which are common within the aerospace industry, have been investigated in this study. The applied action integrals of the lightning strikes significantly exceed the requirements for airworthiness. All tests were performed with conventional prepreg materials and resin transfer moulding/non-crimp fabric materials with high lightning strike current ratings. The pristine panels exhibit major damage zones around the impact points. The results of non-destructive investigations show that the surface damage is predominantly superficial. Only small zones were considerably damaged where extensive repair was necessary. Carbon fibre reinforced plastics panels featuring repair patches were also investigated. Lightning strikes were placed above the scarf and the damage was analysed by various non-destructive investigation methods including micro-computed tomography. In contrast to the pristine panels, the repaired panels reveal different damage behaviour. The damaged zone on the surface was relatively small. In the tapered zone of the patch, electric flashovers between the patch and the base material were observed. Additional microscopy investigations show that these electric sparks also occur inside within the adhesive layers between the patch and the base material. After enhancing the electrical conductivity of the adhesive by adding carbon nanotubes, these difficult-to-detect electric sparks within the layers disappear.

An embedded trace FCCSP substrate without glass cloth

Coreless embedded trace has attracted interest from mobile device, in few metal layer Flip-Chip Chip Scale Package (FCCSP) substrate design, for electrical performance, high density, and thickness reduction. However, the mainstream Prepreg (PP) dielectrics with glass-cloth utilized in coreless embedded trace substrate (ETS) are insufficient to fulfill future requirements of warpage behavior, RF performance, miniaturization and even cost. This research is targeted on developing an alternative 2-layer coreless ETS technology platform, without glass-cloth, to make up the above shortage. The total solution from substrate fabrication to package verification had been studied. With the design for manufacturability and reliability approaches, a pioneering 120μm thin 2-layer coreless ETS, by Ajinomoto Buildup Film-like dielectric without glass-fabric, was developed for FCCSP, with both experimental and simulation efforts. Compared with the conventional PP with glass-cloth, a cost effective substrate featured with 20% thinner and 20% less package warpage deformation was gained. The new material scheme also allows better compatibility with fine pitch design and RF transmission. This technology can be an extended process platform to higher multi-layer (≥3) advanced coreless substrate for flip chip BGA & module assemblies.

A new compression-molding approach using unidirectionally arrayed chopped strands

In this study, we propose a new compression-molding approach using unidirectionally arrayed chopped strands (UACS). UACS is a sheet made by introducing slits into a conventional prepreg. We demonstrate that complexly shaped components, such as rib structures, can be fabricated by stacking UACS plies and curing them by hot pressing. Moreover, the layer structure is maintained after the processing, and there are few resin-rich regions in it. We also demonstrate the superior mechanical properties of UACS laminate by comparing them with those of sheet molding compound (SMC) and propose that the strength of UACS can easily be deduced by a simple model based on fracture mechanics. As a result, we can clarify the contribution of slits introduced into the UACS to the final failure.

Compression testing of pultruded carbon fibre-epoxy cylindrical rods

The fibre waviness inherent in conventional prepreg laminatessignificantly reduces their compressive strength. This waviness canbe reduced through the use of unidirectional fibre rods. In thiswork, the development of a new test procedure and specimen design isreported that was used to determine the compressive properties ofpultruted T300/828 and IM7/828 carbon fibre-epoxy unidirectionalrods at room temperature. The IM7/828 system demonstrates a highercompressive strength than the T300/828 composite due to strongerfibres used and fewer manufacturing defects. Since the fibres as intension primarily carry the compressive load, the final fracture ofthe rods occurs when the fibres fail. Post-failure examinationreveals that failure of the fibres is microbuckling-induced. This isa bending failure as a consequence of buckling. Other events such asfibre-matrix debonding (splitting) and matrix yielding do not bythemselves cause the final failure, but they facilitate fibrebuckling by reducing the lateral support for the fibres.Microbuckling failure models are used to predict the compressivestrength of the carbon fibre rods; agreement between theory andexperiment is acceptable.

A comparison of the properties of hot compacted gel-spun polyethylene fibre composites with conventional gel-spun polyethylene fibre composites

In this paper we present an investigation of the ballistic and low strain rate mechanical properties of gel-spun polyethylene (GSPE) fibre composites. Composites were made using two different processing routes, a conventional prepreg route and the hot compaction process developed at the University of Leeds. Within these two groups different matrices and fibre arrangements were also investigated. Composites manufactured by the hot compaction process were found to have ballistic performance equivalent to that of the composites manufactured by the conventional prepreg route. The mechanisms of energy absorption in the two types of composite were, however, different. The hot compacted materials absorbed energy by fibre failure and back-face delamination while the composites made by the conventional prepreg route absorbed energy by fibre failure only. Interfacial strength played a very important role in the energy absorption of composites that underwent back-face delamination upon ballistic impact.

Mechanical properties and damage tolerance of multiaxial warp-knit composites

Multiaxial warp-knit fabric is a relatively new textile material form that is being considered as an alternative to biaxial woven and unidirectional broadgoods. This paper presents the results of an experimental characterization of composite materials reinforced with four different knitted carbon fabrics and compares the results with conventional prepreg tape and uniweave fabric composites. Tension and compression mechanical property tests, open hole tension and open hole compression tests, and compression after impact tests were conducted on fabric-reinforced carbon/epoxy composite specimens to assess material performance. The fabrics were produced on Liba and Mayer multiaxial warp-knitting machines. Two different generations of Liba machines were used by Hexcel and Saerbeck and a Mayer machine was used by Milliken to produce 4-ply multiaxial fabrics. Composite test laminates were fabricated by two resin transfer molding methods with two different epoxy resin systems. Test results indicate that high quality knitted carbon fabrics can be produced and resin transfer molding can be used to produce aerospace quality composite materials. Tension and compression strength reductions for the Hexcel and Milliken fabric composites ranged from 20 to 30% compared to prepreg tape laminates. However, the knitted fabric composites exhibited compression after impact strengths up to 80% higher than the strength of comparable prepreg tape laminates. The performance of the Saerbeck knitted fabric was comparable to uniweave fabric for all the tests conducted.

Braided to Net Section Graphite/Epoxy Composite Shapes

The fabrication of braided-to-net-shape graphite/epoxy composites has been demonstrated for channel, cruciform, Tee, and hat section geometries. Resin impregnation, cure, and tooling methods were devised, which successfully combined dry-braided AS4 fiber preforms with neat 3501-6 resin to produce the various shapes. The local buckling, post-buckling, and crippling behavior of short thin-walled braided compression members were investigated. The braided composites displayed large scatter in their test data, which may be characteristic of this form of construction. Stiffener pull-off-strength of the braided tee-section approached the transverse strength of the braided composite. Handling during fabrication was much easier with the braid than with conventional prepreg. The braid required only ¼ the time used to layup and bag a similar channel specimen using identical tooling. The high cost of the orthogonal braiding process remains the major barrier to serious structural applications.