Interleaf Layers Research Articles

Ultrasonic evaluation of ply and interleaf layer properties of interleaved composite laminates

An ultrasonic nondestructive evaluation procedure is presented for the ply and interleaf layer properties of carbon/epoxy composite laminates having interleaf resin layers at their interlaminar interfaces. It is shown that the material properties of plies (density, thickness, and transversely isotropic complex elastic moduli) and elastic interleaf layers (density, thickness, Young’s modulus, and Poisson’s ratio) can be identified by best fitting the theoretical energy transmission spectra of longitudinal waves through the laminate immersed in water to those obtained in ultrasonic measurement. It is also demonstrated that compared to the mass-less and null-thickness spring-type interlaminar interface model employed in the previous works, the present finite-thickness interleaf layer model can better reproduce the experimental transmission spectra of a unidirectional interleaved composite laminate containing ultrasonic bandgaps. Furthermore, the property characterization in the case where the plies and interleaf layers are assumed to be elastic and viscoelastic, respectively, is also examined. It is also shown that the ply and interleaf layer properties determined for the unidirectional laminate can be used to reproduce the transmission spectra measured for a quasi-isotropic laminate consisting of the same plies and interleaf layers.

Influence of nonwoven interleave layer on interlaminar fracture of sandwich-structured composites

Debonding between the skins and the core is one of the critical failure modes encountered by sandwich-structured composites in highly strained conditions. The onset and growth of a debond critically hinder the load-bearing capacity of the sandwich structure. This paper investigates the effect on the debonding resistance of a sandwich-structured composite by the addition of nonwoven flax interleave veil in the skin-core interface. Mode-II fracture toughness is estimated through a proposed method based on curve fitting of numerical data to experimental results. Experimental data are obtained in a four-point bending test following the standard DIN 53 293 and Cohesive Zone Modelling (CZM) is implemented as the numerical tool in the analysis of Mode-II skin-core debonding. A series of sandwich specimens are fabricated using twill weave e-glass fabric and expanded polystyrene core with and without the addition of an interleave layer. Numerical data show a 60 % increment in Mode-II fracture toughness with the inclusion of a flax interleaf layer. Microscopic images attribute the increment to the presence of fibre bridging effect. The proposed method produces a good fit of numerical data to experimental results.

Process-induced strains measurements through a multi-axial characterization during the entire curing cycle of an interlayer toughened Carbon/Epoxy prepreg

High dimensional accuracy of thermoset composite parts is a challenging issue for manufacturers in the aeronautical industry. A deep understanding of the cure behavior is required in order to prevent internal stresses generation as being responsible of shape deformations. However, the specific challenge of experimental investigations is to cover all of the stages of the cure process including the pre-gelation region. In this paper, the characterization of the process-induced strains throughout the cure is based on using a laboratory scale bench named as PvT-HADDOC. Tests were performed on a unidirectional interlayer toughened aerospace prepreg. The device enables the measurements of the induced strains simultaneously along the out-of-plane and the in-plane directions. The consolidation of the uncured material is captured. The measured coefficients of thermal expansion are proved to be higher through the thickness direction in the liquid and cured states. The chemical shrinkage is measured before and after gelation along both transverse directions. The orthotropic behavior of IMA/M21 system is attributed to the presence of interleaf layers of resin within the prepreg plies.

A Three-Dimensional Thermo-Chemical Characterization During the Whole Curing Cycle of a Carbon / Epoxy Prepreg

Mitigation of cure-induced defects in thermoset composite parts has always been a challenging problem for manufacturers especially when it comes to high dimensional accuracy of components. Thus, it is crucial to understand the evolution of the thermo-chemical properties of these materials during the totality of the curing cycle. In this paper, a new methodology is presented to characterize the process-induced strains throughout the cure. The investigation is based on the development of an existing laboratory bench named as PvT-HADDOC. The tests were performed on an interlayer toughened aerospace carbon/epoxy prepreg. Unidirectional laminate samples (105x105 mm2) of almost 6 mm of thickness were manufactured by hand lay-up then debulked at room temperature under full vacuum. The PvT-HADDOC device allows a manufacturing process following the recommended cure cycle of epoxy composites under 7 bars pressure and a temperature up to 180°C. It enables the measurements of the process-induced strains, simultaneously, along two directions: through-thickness and in-plane. Results show a complex behavior of an assumed unidirectional composite. It exhibits a temperature and time dependent compaction behavior through the thickness only. The measured thermal expansion coefficients are proved to be higher in the thickness direction for the uncured as well as for the cured state of the material. Most of the chemical shrinkage occurs along the thickness direction. This unexpected complexity is mainly attributed to the presence of interleaf layers of resin in the laminate structure. Thus, the investigated M21/IMA material is considered fully orthotropic.

On thermal-expansion properties of more-orthotropic prepreg laminates with and without interleaf layers

More-orthotropic laminates are increasingly used in today’s carbon-fiber-epoxy composite airframe structures. A recent study reported on a considerable discrepancy between measured and calculated coefficients of thermal expansion (CTEs) for M21E/IMA prepreg laminates, which was found to increase with the laminates’ degree of orthotropy ξ=Ex/Ey. In this paper it is analyzed whether the observed effect is related to more-orthotropic laminate stackings in general or to the particular inner laminate architecture of M21E/IMA laminates, which is characterized by discrete interleaf layers. Therefore, a quasi-isotropic and three more-orthotropic laminates, made from 8552/AS4 and M21E/IMA prepreg, were analyzed by means of thermo-mechanical analysis (TMA) and classical-laminate theory (CLT). Calculated and measured CTEs are compared and the effect of orthotropy is quantified. Potential affectations of TMA measurements and CTE determination due to edge effects and sample size are examined by means of finite-element analyses.

How free-edge effects impede CTE determination for composite laminates

The determination of in-plane coefficients of thermal expansion (CTEs) of full-stack composite laminates by means of thermo-mechanical analysis (TMA) is affected by free-edge effects. A numerical study has been performed, which is focussed on the quantification of the effects of TMA sample size, laminate layup and the presence of discrete interleaf layers on the determined laminate CTEs. Fine finite-element (FE) meshes have been used to visualize and to examine free-edge effects. The study results call for carefulness when CTEs of full multi-angle laminates shall be determined by means of TMA. When the typical sample dimensions of a few millimeter are used CTEs will likely be overestimated.

A controlled recipient evacuation process to form composite profiles from flat multi-angle prepreg stacks – infrastructure and C-profile verification

Single-diaphragm forming is a cost saving alternative to labor-intensive ply-by-ply layup. This paper reports on forming of flat uncured multi-angle prepreg stacks into C shape. The main focus is on the forming-process profile , which has attracted little attention in previous studies on the topic. Hexcel’s M21E/IMA prepreg is examined within the study to analyze the particular effect of the prepreg’s interleaf layers on the forming process and vice versa. Specimens with different multi-angle stackings were formed at 40, 60 and 80 °C, on male tools with 4, 6 and 8 mm radii. It is shown that the composed infrastructure setup allows for a precise control of the recipient pressure profile. The forming status is monitored based on a resistance-measurement-based approach, whose application suggests a two-phase forming process characteristic. Recipient pressure levels of 60 and 510 mbar below ambient pressure were identified as practical for gentle forming. It could be shown that interleaf layers of M21E/IMA specimens are not harmed considerably by the forming procedure. Overall, the proposed forming process led to prepreg preforms of adequate quality, suitable for series production.

Modeling the influence of interleaf layers in composite materials on elastic properties, thermal expansion, and chemical shrinkage

Within this paper, it is shown that the assumption of transversal isotropy usually applied to unidirectional layers is not valid, if interleaf layers are introduced to increase the impact performance of the material. An approach is suggested to consider the meso-structure of composite materials with interleaf layers in the calculation of the elastic properties, the thermal expansion coefficients, and the chemical shrinkage coefficients. The approach uses known rules of mixture in a first step, to determine the properties of the carbon fiber–reinforced polymer layer as well as those of the interleaf layer and combine them in a second step by new meso-scale rules of mixture. The approach is discussed using known material data and shows to provide plausible results. The effects of the material properties influenced by the meso-scale rules of mixture on the process induced distortions are then discussed exemplary considering the thermally induced spring-in. It is shown that the meso-structure has a considerable influence on the process-induced distortions and cannot be neglected in the prediction of it.

Ultrasonic bandgaps in composite laminates: Experimental observation and Floquet-Bloch wave analysis

The ultrasonic bandgap characteristics of fiber-reinforced composite laminates are studied experimentally and theoretically. The transmission spectra of obliquely incident ultrasonic waves are obtained experimentally for 16-ply carbon/epoxy composite laminates having interleaf layers at interlaminar interfaces. Compared to the conventional composite laminates, these interleaved composite laminates exhibit clearer bandgaps in the transmission spectra and enable closer examination of their features. The transmission spectra for different incident angles are favorably reproduced by the theoretical plane wave transmission analysis based on the spring-type interlaminar interface model. The Floquet-Bloch wave analysis for the corresponding periodic structures is also employed to elucidate the ultrasonic bandgap mechanism in the composite laminates.

Deployable, shape memory carbon fibre composites without shape memory constituents

Trials have been conducted to investigate the shape memory capability of an interleaved composite consisting of carbon fibre reinforced epoxy laminae and polystyrene interleaf layers. It has been shown that the composite can be readily re-shaped by deforming it at an elevated temperature and then cooling the composite in the deformed state. On re-heating, the composite almost fully returns to its original shape. One potential application of the shape memory capability of the interleaved composite is in deployable structures and a simple structure has been manufactured to demonstrate this possibility.

Improving the ply/interleaf interface in carbon fibre reinforced composites with variable stiffness

Polystyrene-interleaved carbon fibre reinforced epoxy composites exhibiting controllable stiffness have been manufactured. These composites undergo reductions in flexural stiffness of up to 99% when heated above the glass transition temperature Tg of the interleaf layers. Potential applications for such materials include their use in morphing and deployable structures. Flexural tests at room temperature indicated that improvements in adhesion between the polystyrene and CFRP layers are required to prevent premature failure of the composites at low shear stresses. Here we investigate how modification of the interleaf layer improves the interlaminar shear strength of the laminates without affecting the stiffness loss at elevated temperatures. Two poly(styrene-co-maleic anhydride) (SMA) films with different maleic anhydride content were prepared and used as interleaf films. Thick adherend shear tests showed that the adhesion strength more than doubled, while flexural tests showed that composites containing SMA interleafs had more than twice the apparent flexural strength of composites containing pure polystyrene layers at 25 °C and yet still undergo significant reductions in stiffness at elevated temperature.

106 層間強化型CFRP積層板の落錘衝撃損傷挙動の解明(材料力学I)

106 層間強化型CFRP積層板の落錘衝撃損傷挙動の解明(材料力学I)

OS8-34 Fatigue Behavior of Open-Holed Unidirectionally Arrayed Chopped Strand CFRP Laminates with Interleaf Layers(CFRP,OS8 Fatigue and fracture mechanics,STRENGTH OF MATERIALS)

OS8-34 Fatigue Behavior of Open-Holed Unidirectionally Arrayed Chopped Strand CFRP Laminates with Interleaf Layers(CFRP,OS8 Fatigue and fracture mechanics,STRENGTH OF MATERIALS)

One-step metal electroplating and patterning on a plastic substrate using an electrically-conductive layer of few-layer graphene

Few-layer graphene (FLG) was investigated as an electrically-conductive interleaf layer for one-step electroplating and patterning of metal on nonconductive polymer substrates without using multiple and toxic pretreatment processes in traditional electroplating. An individual FLG (5–10 nm of thickness with 6.4% of oxygen content) was obtained by expanding graphite with microwave followed by exfoliating the expanded graphite with sonication in N-methyl-pyrrolidone. Stacking FLG in the in-plane direction, a robust FLG film was obtained by the vacuum-assisted filtering and drying methods, and transferred to a polyethylene terephthalate (PET) substrate via an intermediate transfer to the water surface. The sheet resistance of the FLG film on the PET substrate was 0.9 kΩ/sq with a thickness of 80 nm and the root-mean-square roughness of 29 nm. In the electroplating of nickel on the FLG film, hemisphere-shape metal seeds appeared in the early stage of electroplating and they subsequently grew up to 200–480 nm, which became connected to form a continuous nickel layer. The thickness of the continuous nickel layer increased linearly with electroplating time. The developed electroplating method demonstrated its capability of selective patterning on nonconductive substrates using a simple masking technique.

An experimental study of interlaminar shear fracture toughness of a nanotube reinforced composite

This study demonstrated that fluorine functionalized carbon nanotubes (f-CNTs) can be used to reinforce in the midplane of a fiber reinforced epoxy composite laminate for increasing the interlaminar shear fracture toughness. A spraying methodology was used to deposit the f-CNTs on the midplane carbon fabric to produce this through-thickness interlaminar reinforcement. The four-point end notch flexure (4ENF) specimen was used in the experiments to calculate the critical strain energy release rate, G IIc , to quantify a value for fracture toughness. Compared to the neat case (0.0 wt% CNTs) there was a 23% increase in the average initiation and 27% increase in the average propagation G IIc with only 0.5 wt% f-CNTs dispersed on the fabric and in the resin of the midplane region. High resolution microscopic examinations of the specimen fracture surfaces suggest that the functionalized carbon nanotubes are covalent bonding nanoscale tethers between the fibers and the epoxy resin, resulting in toughened fiber–matrix interfacial strengths to achieve these improved through-thickness mechanical properties for this nanocomposite laminate system. This nanotube technology could displace the need for using cross-fabric stitching or pinning, or using thickening interleaf layers at the midplane region of laminates in order to improve the interlaminar shear fracture toughness.

Interleaving methodology for property tailoring of CFRP laminates

The effect of interleaf layers incorporated between laminae on the mechanical properties of quasi-isotropic carbon/epoxy laminates was investigated. Two kinds of laminates, with and without interlayers, were compared in this study. Static and fatigue tests in tension, and vibration damping tests were performed on both laminates. It is shown that introducing the interleaf layers remarkably improved the ultimate tensile load and tension fatigue resistance of the laminates. The interlayers delayed the onset of interlaminar delamination from the free edge and impeded the progression of interlaminar delamination until final failure. It is also shown that the vibration damping of CFRP laminates can be remarkably improved by introducing compliant interlayers between composite plies. The loss factors of interleaved CFRP were 5 through 50 times as large as those for conventional CFRP in the case of quasi-isotropic laminate configuration, which depend upon the location of interlayers between composite plies. Discussion is also made on the optimum interleaf width in the case when the interleaf films were placed only in a limited local area near free edges, in order to tailor the interfacial properties and thus improve the fatigue resistance of quasi-isotropic carbon/epoxy laminates.

Flexural failure of unidirectional carbon/epoxy composites: Effects of interleaving and flexural rate

The effects of a low-modulus high-strain interleaf layer, introduced at the midplane, on unidirectional carbon/epoxy laminates under flexural loading were investigated. The apparent energy-to-fail of the laminates increased significantly whereas the maximum load decreased slightly upon interleaving. Real-time microscopic observations during static flexure tests at the lowest deflection rate indicated a dramatic change of failure mode from the dominantly compressive fracture of the baseline laminate to the dominantly tensile fracture process upon interleaving. Failure modes of interleaved and baseline laminates showed no significant changes with test rate in the deflection rate range of 10−6 to 100 m/sec.

Effect of interleaving on the impact response of a unidirectional carbon/epoxy composite

Instrumented drop-weight impact response of a unidirectional carbon/epoxy composite and that of the corresponding laminate interleaved with poly(ethylene- co-acrylic acid) at the midplane were studied. The impact penetration energy of the unidirectional composite beam increased strongly upon interleaving. Realtime observations using a microscope in a static flexure test indicated a dramatic change of failure mode from the dominantly compressive fracture of the baseline laminate to the dominantly tensile fracture process upon interleaving, in accordance with the corresponding fractographic features of the impact specimens. These observations were attributed to the change in stress distribution upon introduction of the interleaf layer. The relevance of the present observations to the role of high-strain, low-modulus adhesive layers in the toughening of composites is discussed.

Vibration Damping of Interleaved Carbon Fiber-Epoxy Composite Beams

Vibration-damping behavior of unidirectional and symmetric angle-ply carbon fiber-epoxy laminates as well as their interleaved counterparts with a layer of poly(ethylene-co-acrylic acid) (PEAA) at the mid-plane was examined. The introduction of the PEAA layer significantly improved the damping capability. The effectiveness of interleaving increased with the flexural modulus of the outer layers. In the case of unidirectional laminates, calculations based on a sandwich structure of isotropic layers quantitatively reproduced this trend. In the case of angle-ply laminates, however, the model predicted only part of the improvement experimentally observed. This was explained in terms of the bending of the angle-ply laminates in the transverse direction which would induce additional deformations in the interleaf layer and was not accounted for by the present model.

Suppression of interlaminar damage in carbon/epoxy laminates by use of interleaf layers

Carbon fiber reinforced plastics (CFRP) have been widely used as a structural material. In general, fiber orientation angle of each lamina in these laminates is variously chosen in order to taylor a material which meets the particular requirement for the material properties in arbitrary direction of laminate. Quasi-isotropic lamination, in which the laminate consisted of laminae with fiber orientation of 0[degree], +45[degree], [minus]45[degree] and 90[degree] to loading axis, is most commonly employed in the actual application. However, quasi-isotropic carbon/epoxy laminates are known to develop the interlaminar stress concentrations near the free edge region. These laminates have a strong tendency to delaminate near the edges when subjected to axial in-plane loading. Such a free edge delamination under loading in the plane of the laminate is a unique problem to laminated composites. This paper summarizes the author's investigation which was performed to reduce the free edge interlaminar stresses in the laminate by incorporating interleaf films between plies and thus to improve the mechanical properties of these materials. In their previous work, the authors have shown that these laminates exhibit a high vibration damping capability. Loss factor values for these CFRP/interleaf laminates which were measured in cantilever beam tests, are 5 to 50more » times as large as that for conventional CFRP. In this paper, discussion will be provided on the mechanical properties of the interleaved quasi-isotropic carbon/epoxy laminate, [0/[+-]45/90][sub s], with a special emphasis on the optimum design of interply locations to incorporate the interleaf films for the particular requirement such as static strength, elastic modulus, fatigue resistance and so on.« less