Aramid Honeycomb Core Research Articles

Aramid Honeycomb Cores under Constant Pressure: Unveiling the Out-of-Plane Compression Deformation.

The primary challenge during the secondary bonding process of full-height honeycomb sandwich structures is the aramid honeycomb core's height shrinkage. This paper systematically investigated the height evolution behavior of the honeycomb core by using a creep testing machine. The results showed that the out-of-plane compression deformation curve of aramid honeycomb cores is mainly divided into three stages: the dehumidification stage, the pressurization stage and the creep stage. Under conditions of high temperature and pressure, height shrinkage was attributed to the dehydration caused by moisture infiltration, and the compression creep resulted from the slippage of polymer molecular chains. Dehydration shrinkage is stable, whereas compression creep reflects typical viscoelastic polymer characteristics. By employing the viscoelastic Burgers mechanical model and applying the nonlinear surface fitting method, the total height shrinkage deformation behavior of the aramid honeycomb core during the curing process can be accurately predicted by summing the above three stages. This research contributes valuable insights for the manufacturing process of honeycomb sandwich structures.

Structural analysis of cylindrical composite structures for high velocity applications: A parametric optimization study

This research explores the significance of structural modifications on the energy absorption and ballistic response of a cylindrical composite structure exposed to high velocity impact. The composite structure comprises an aramid honeycomb core supported by cylindrically shaped aluminum facesheets on both sides. Ballistic impact tests were performed by firing cylindrical projectiles, namely conical and hemispherical, using a pneumatic gun. Three-dimensional models of the composite structure with aluminum facesheets and an aramid honeycomb core were created using ABAQUS/Explicit. By utilizing numerical models, impact phenomena such as facesheet failure, core crushing, facesheet debonding, etc. were analyzed, which showed a high degree of similarity with a real impact scenario, thus validating the model. Furthermore, a parametric analysis was carried out using this model on different parameters affecting the composite structure. The study also examined the effect of these parameters on the ballistic responses of the cylindrical composite structure against both projectiles using a grey-based Taguchi analysis method. The investigation focused on multi-response optimization of the cylindrical composite structure's characteristics using Minitab software. The proposed grey-based Taguchi method proved effective in solving multi-response problems.

A case study on the performance of repaired sandwich composite aeronautical structures in different failure characteristics under bending loading

It is important to develop composite repair methods and investigate their performance against in-service and severe conditions due to high cost of composites, current environmental conscience and difficulties in composite recycling. In this study, the performance of various repair methods applied to two different types of polymer sandwich composite panels made of identical aramid honeycomb core, which are actively used in aerospace structures, is investigated. It is aimed to obtain different failure characteristics and to investigate the effects of these failure modes on the performance of repair methods for two different facesheet types. Three-point bending tests are configured for the flexural performance of repaired sandwich structures in the case of damage modes of core-facesheet adhesive bonded joint delamination and core damage and facesheet failure. The load carrying capacity is measured by calculating the ultimate value of core shear stress and facing stress. The results show that the configured three-point bending procedure is effective to analyse different span regions and to have various failure characteristics. The repaired samples succeed between 80% and 112% in the load capacity. It is proved that repaired specimens have higher load carrying capacity if facesheet based failure due to facing ultimate normal stress is occurred against delamination and core failure due to core shear stress. In addition to this, a modified epoxy filling/patch repair method for sandwich structures made of honeycomb core which can increase the load carrying capacity significantly is presented.

Sandwich nanoarchitectonics of heterogenous CB/CNTs honeycomb composite for impedance matching design and microwave absorption

Sandwich structure composite is one of the most potential and effective materials to comprehensively realize microwave absorption in wide frequency range and structural lightweight function. Here, honeycomb (HC) composites were fabricated by immersion method, consisting of a carbon nanotubes/carbon black/polyurethane (CNTs/CB/PU) absorbing layer and an aramid HC core. The using CNTs and CB were both modified by improved Hummers method with a heterogeneous structural surface to enhance the dielectric properties of the carbon materials. The sandwich honeycomb (SHC) absorber showed excellent broadband absorption performance with an effective absorption bandwidth (RL<−10 dB) of 15.5 GHz. Furthermore, the compressive stress of the absorber reached 3.69 MPa. With the integrated high microwave absorption and load-bearing performance, the as-prepared SHC absorber demonstrated promising future for applications in avionics.

Investigation of the applicability of single cantilever beam test for the evaluation of fracture toughness of sandwich composites

Sandwich composites are widely used in load-carrying structures and there is a need for a testing method that will allow for qualitative and quantitative assessment of the bond between the facesheets and the core. The presented research investigates the applicability of the single cantilever beam (SCB) test to the evaluation of the mode I dominated fracture toughness of such structures under both quasi-static and fatigue loading.The SCB method uses a relatively simple fixture, the debond front loading conditions are independent of the debond length and the propagation takes place near the facesheet-to-core interface. It allows for the determination of the initiation and the propagation fracture toughness and can also be used for fatigue testing.A sandwich composite, made of carbon-epoxy facesheets and an aramid honeycomb core, was tested in quasi-static and fatigue loading conditions. The initiation values of the Energy Release Rate (ERR) were determined using the Modified Beam Theory (MBT) approach for different definitions of the initiation point. The propagation tests were conducted along with the visual observation of the debond growth. The propagation fracture toughness was determined using two approaches – MBT and the Area Method. The fatigue testing was realized, using the same setup, by cycling the specimens through a range of ERR values. The debond length was measured during the test which allowed for the determination of a relationship between the ERR and the debond length growth rate.The research confirmed that the SCB method can be used for the quantitative measurement of the fracture toughness of sandwich structures.

Mechanical properties of selected polymer sandwich composites

The aim of this work was to determine the influence of material and geometric factors of selected sandwich composites on their mechanical properties. The first pair of sandwich materials under consideration were made by the industrial infusion method and consisted of epoxy resin reinforced with 7 layers of glass fabric (skin) with core made of PVC foam or aramid honeycomb. The second pair of materials was prepared manually and consisted of polyester resin reinforced with glass mat (skin) with aramid honeycomb core of varying thicknesses. Mechanical properties were determined in static bending, compression and tensile tests. In each case, the failure mode of the tested composite materials was determined. In the case of composites with epoxy resin skins application of aramid honeycomb core resulted in higher tensile and edgewise compressive strength. For materials with polyester resin skins and honeycomb cores it was found that increase of core thickness yielded higher bending stiffness but the tensile and bending strength were reduced.

Determination of Paris parameters from S−N curve in the simulation of fatigue in Nomex honeycomb sandwich structures

Determination of S-N and Paris parameters form the two primary approaches as far as fatigue testing is concerned. With many economic factors and complexities involved in experimentation, these approaches are not related to each other and require implementation of separate test standards for both of them. In this work, an attempt is made on the interconvertibility of these experimental parameters and a numerical technique based on the virtual crack closure technique (VCCT) is proposed for the evaluation of Paris parameters from S−N curve, corresponding to four-point bending end notched flexural (4ENF) sandwich specimen made up of Nomex® aramid honeycomb core with glass fibre reinforced composite (GFRP) facesheets. VCCT was effectively utilised for the estimation of strain energy release rates (SERR), in both 2D plane stress/strain and 3D conditions, which were compared with each other, and the resultant was incorporated with the S−N data for the determination of parameters of the Paris curve. These determined parameters were employed for the simulation of fatigue delamination growth by using ‘Direct Cyclic’ step and compared with the results obtained from the literature. The comparison revealed a good agreement between the two, thereby validating the proposed technique.

A comparative study on the ballistic performance of aramid and aluminum honeycomb sandwich structures

This study investigated the perforation phenomenon of aramid and aluminium honeycomb sandwich structures under high-velocity impacts and compared the impact responses experimentally and numerically. The dimension of the face sheets used was 1 mm in thickness and 220 mm in diameter, and for both the honeycomb cores, the cell thickness of 0.05 mm and the cell size of 3.2 mm was used. Ballistic impact tests were conducted by employing a pneumatic air gun to study damage behavior and failure modes under different impact velocities between 35 m/s and 150 m/s using conical shaped and hemispherical shaped projectiles.Three-dimensional finite element models were implemented in ABAQUS/Explicit to model sandwich panels.The numerical model was validated with experimental data and, subsequently, the numerical study was further extended to study the effects of different geometrical parameters affecting the impact response of sandwich structures. The results showed that for the ballistic limit, sandwich structure having an aluminium honeycomb core performed better than the aramid honeycomb core by 3.09% and 2.5% against conical and hemispherical projectiles, respectively. However, the aramid honeycomb structure was proven to be more efficient than the aluminum honeycomb structure in terms of energy absorption and specific energy absorption.

Research on Acoustic Absorption Properties of Aramid Honeycomb Composite Material Reinforced by Polyimide Foam

To endue sound absorption behavior for aramid honeycomb core (ARHC), isocyanate‐based polyimide foam (IBPIF) with excellent thermal insulation and acoustic properties produced by a new formulation of foaming slurry is filled into ARHC by the free foaming route. As a core factor to adjust the cellular structure of IBPIF, the effect of water dosage on the acoustic behavior of IBPIF and IBPIF filled ARHC (IBPIF/ARHC) is studied, and the optimal formula to produce IBPIF/ARHC is filtered out. Results suggest that water dosage effectively regulates the cellular structure of IBPIF. When water dosage is 7.2 g, IBPIF has a large windows opening rate, which plays a positive role in sound absorption performance. IBPIF/ARHC‐3 has the best sound absorption property, which possesses a maximum sound absorption coefficient (α) of 0.9 at 1500 Hz. Its average α increases to 0.78 and shows an increase of 680% compared to ARHC. Meanwhile, the density of IBPIF/ARHC is only about 59 kg m−3. In 300–600 Hz range, IBPIF‐3 and ARHC show obvious synergy effects in sound absorption behavior. The average α of IBPIF‐3 and ARHC is only 0.3 and 0.06, respectively, and that of IBPIF/ARHC‐3 reaches up to 0.42. The thermal insulation performance of ARHC is also improved, and thermal conductivity decreases from 0.067 to 0.047–0.049 W (m K)−1.

Effect of nanopowders (TiO2 and MMT) and aramid honeycomb core on ablative, thermal and dynamic mechanical properties of epoxy composites

The present work investigates the ablative, thermal and dynamic mechanical properties of epoxy resin modified with nano-titanium dioxide and organomodified monmorillonite. The composites properties with aramid honeycomb core as stiffness reinforcement were compared with those without honeycomb. Samples have been subjected to combustions gases at a temperature above 1900 °C during 120 s. The best thermal protective properties, i.e. the lowest: temperature of the rear surface area and ablative weight loss were exhibited by the composites containing 3.75% of oMMT and 1.25% of TiO2 with aramid honeycomb core. Dynamic mechanical thermal analyser measurements were carried out on an Instrument DMA SDTA861. The tests were performed by using a compression deformation mode in the temperature range from 20 °C to 140 °C. The glass transition temperature (Tg) was determined from the position of the maximum value of tan δ. This temperature reached values from about 74 °C (with tan δ = 0.59) to 79 °C (for tan δ = 0.66). The highest values of the complex modulus M* were in the range from 70 MPa to 465 MPa, for all phase compositions, but at temperature of 140 °C the complex modulus M* had much lower values in the range of 20–25 MPa.

Numerical simulation and experimental validation of effective thermal conductivity coefficient of hexagonal aramid honeycomb used in wing skin of tourist class reusable spaceplane

Lightweight aramid honeycombs are promising cores for sandwich structures in aerospace applications due to their high specific properties. The determination of the in-plane and through-plane thermal conductivity of the aramid honeycomb is a sophisticated problem due to combined heat transfer inside the honeycomb cell. The aim of the present study is to investigate the thermal conductivity of aramid honeycomb core used in the wing structure of the suborbital tourist class spaceplane via heat transfer modelling of the representative volume element of the material. The analytical and experimental methodology is developed to validate the results of numerical modelling. The methodology includes the experimental obtaining of time dependences of the thermal conductivity coefficients of the wing skin element on the contact heating installation. The subsequent solution of the inverse heat conduction problem is followed by finding in-plane and through-plane thermal conductivity of honeycomb core. Overall, the experimental results have a good agreement with the numerical simulations. The findings in this study provide an important foundation for the subsequent thermal and structural design of the spaceplane wing.

Internal-structure-model based simulation research of aramid honeycomb sandwich panel subjected to intense impulse loading

An experimental investigation on the dynamic response of composite sandwich panels, comprising two titanium alloy plates and an aramid honeycomb core, subjected to intense impulse loading was carried out with the electric gun method. And brief results were presented within this paper. Based on the experiments, simulations were conducted with SPH-based internal structure model using the AUTODYN software. Here, the elasto-plastic constitutive was adopted to describe the mechanical behaviour of the core and corresponding matrix constitutive parameters were obtained quantitatively. An optimal particle size was determined by comparing the effects of different particle sizes on computational efficiency and accuracy. Afterwards, the proposed SPH model was validated with the experiments, making it feasible for following studies. To study the weakening effect of the core, a stress decay coefficient was proposed, and the results proved that the aramid honeycomb core played an important role in weakening the reflected tensile wave. Finally, the impact resistance assessment, in which three different forms of panels were compared, indicated that the aramid honeycomb sandwich panel was superior to the other two.

NCCR - Aramid Sandwich Insulator for Cryogenic Applications

Structural integrity of cryogenic propulsion system in which fuel and oxidizer tanks are connected using truss tubes was well established. The critical issue in the design is the temperature constraint so that temperature is limited to within K for LH2 tank as against K for the LOX. In the present work, based on transient heat transfer analysis for 600s, polyimide foam filled aramid honeycomb core sandwich insulator is designed for common bulkhead instead of truss tubes each for cryogenic and semi cryogenic systems. Non conducting cryo compatible resin (NCCR) is used to bond the skin and core. Comparison of the back wall temperature for different heat flux and different thermal specifications obtained from test and prediction shows a good agreement. The study shows that a small increase in core height of foam filled sandwich insulator considerably controls the temperature increase of LH2. Test data on flat - wise tensile strength at 77K of NCCR resin used in the honeycomb sandwich insulator shows the values in the acceptable range.

Effects of aramid honeycomb core on the flame retardance and mechanical property for isocyanate‐based polyimide foams

ABSTRACTIn this work, composite materials containing aramid honeycomb core (ARHC) reinforced isocyanate‐based polyimide foams (IBPIFs) were prepared by a simple free‐foaming process. The flame retardance, smoke performance, and mechanical property of the composite materials along the transverse (W) and longitude (L) directions of ARHC were studied. Results of limiting oxygen index and cone calorimeter reveal that the space partition action of ARHC on IBPIFs, barrier action of honeycomb structure on fire, and generation of straight, flurry and porous char layer in the honeycomb cells during combustion effectively enhanced the flame retardance of IBPIFs. Mechanical property test shows that IBPIFs and ARHC presented a positive synergetic effect, and ARHC increased the compression strength of IBPIFs along the W and L directions of ARHC by approximately 300%. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45041.

OOA composite structures applicable in railway industry

Composite sandwich structures offers several advantages over conventional structural materials such as lightweight, high bending and torsional stiffness, superior thermal insulation and excellent acoustic damping. In the aerospace industry, sandwich composites are commonly manufactured using the autoclave process which is associated with high operating cost. Out-of-autoclave (OOA) manufacturing has been shown to be capable of producing low cost and high performance composites. In this paper we present results of experimental testing of various sandwich materials according various standards and actual requirements in transport industry. We compared the different types of surface and paint systems, because these layers are the most important in contact with the surrounding environment and load conditions. In the experimental measurements were used various materials. For the core of the sandwich structure were selected aluminium honeycomb, aramid honeycomb and PET (Polyethylene terephthalate) foam core. Support layers were chosen two kinds of predimpregnated materials. The conditions of measurements were requirements for strength and rigidity, safety - flame resistance and reflectivity resistance. The samples were tested at the 3 - point bending test according to standard EN ISO 178, by modified test to determine the force required to rapture threaded insert, by test of reflectivity according to UIC CODE 844-4 R and according to standard EN 45545-2 fire protection of railway vehicles.

Vacuum bag only co-bonding prepreg skins to aramid honeycomb core. Part I. Model and material properties for core pressure during processing

A process model was developed for the honeycomb core pressure during sandwich panel manufacturing. The model predicted the inflow of moisture from the cell walls into the cell void space using by Fick’s law, and the outflow of moist air through the bag-side skin was governed by Darcy’s law. The model required the core moisture content and honeycomb skin through-thickness air permeability to be experimentally measured. A partially saturated out-of-autoclave woven prepreg was used as the skin material in this study. In-order to accurately apply Darcy’s law during elevated temperature processing, an incremental temperature measurement technique was proposed to maintain constant skin thickness during air permeability characterization. Micro-CT imaging was performed on cure quenched samples to confirm that the proposed characterization technique maintained the impregnation dynamics of the target cure cycle. Model predictions using the characterized material properties are compared to in-situ core pressure measurements in Part II.

Vacuum-bag-only co-bonding prepreg skins to aramid honeycomb core. Part II. In-situ core pressure response using embedded sensors

Miniature pressure sensors were embedded into the honeycomb core of sandwich panels featuring both tool-side and bag-side skins. The pressure response was measured throughout the vacuum hold and elevated temperature processing stages of both oven and autoclave manufacturing. The elevated temperature processing measurements validated the honeycomb core pressure model presented in Part I, confirming that gas flow primarily occurs through the bag-side skin in semi-infinite panels. Aramid core panels showed much higher honeycomb core pressures than aluminum cores during elevated temperature processing. Higher core pressures during processing led to more gas flow and as a result, cured skins with interconnected porosity. The best sealed aramid core honeycomb skins were processed under one atmosphere of positive pressure with a vented vacuum bag, avoiding the continuous extraction of gas through the skins by the vacuum pump while the resin was highly mobile.

Nonlinear dynamic behavior of viscoelastic sandwich composite plates under non-uniform blast load: Theory and experiment

In this paper, the dynamic behavior of a viscoelastic sandwich composite plate subjected to the non-uniform blast load is investigated. The theoretical and experimental study is carried out. The plate examined has carbon/epoxy face sheets and an aramid honeycomb core. In the theoretical study, the sandwich plate is modeled using first order shear deformation theory. Because of the large deformations which occurred after the blast load in some tests, geometrical nonlinearities are considered in the derivations. The shear and large deflection effects are considered. The equations of motion are derived by the use of virtual work principle for the sandwich plate. The clamped boundary conditions are considered for all edges of the plate. The viscoelastic properties of carbon/epoxy coupons and sandwich coupons are determined using Dynamic Mechanical Analyzer. The viscoelastic behavior is modeled by using the Kelvin-Voigt linear viscoelastic theory. The equations of motion are obtained in the time domain using Galerkin's method. The nonlinear coupled equation system is solved by Mathematica Software. A special experimental setup is used to obtain the blast pressure for the blast test. The experimental, theoretical and finite element analysis results are compared and the vibration characteristics, peak points, vibration frequencies are found to be in an agreement.

Design and testing of a deformable wind turbine blade control surface

Wind tunnel tests were conducted on a 1.3 m chord NACA 63–418 blade section fitted with an adaptive trailing edge flap. The 20% chord flap had an aramid honeycomb core covered with a silicone skin and was actuated using servo motors. The honeycomb core had a high stiffness in the thickness direction but was compliant in chordwise bending. These anisotropic properties offer a potential solution for the conflicting design requirements found in morphing trailing edge structures. Static and dynamic tests were performed up to a Reynolds number of 5.4 × 106. The tests showed that deflecting the flap from − 10° to + 10° changes the blade section lift coefficient by 1.0 in non-stalled conditions. Dynamic tests showed the flap to be capable of operating up to 9° s−1 using a 15 V power supply. A two-dimensional static aeroelastic model of the morphing flap was developed to analyse strains, predict actuator requirements and study fluid–structure interaction effects. The model was used to conduct parametric studies to further improve the flap design. Potential applications include wind turbine blade load alleviation and increased wind energy capture.

Bending Fatigue Behaviour of Laminated Sandwich Beams

In this study, the bending fatigue behaviour of laminated sandwich beams, which are made of carbon/epoxy face sheets and aramid honeycomb core, has been investigated experimentally. The wet hand lay-up technique is used and curing is processed on the heated vacuum table at an elevated temperature to manufacture the sandwich beams. The experimental set-up for bending fatigue test provides a cantilever in one end and a cyclic load at the free end with constant displacement amplitude at room temperature. The load applied to the beam is measured using a load cell during the bending test. Different displacement amplitudes are performed. Mechanical properties, bending stiffness and free vibration frequency of the sandwich beam are investigated. The bending test of the beams and vibration identification test using a vibration analyzer, a hammer and an accelerometer are performed to measure the bending stiffness and determination of free vibration frequency of the clamped sandwich beams before starting and after completion of the fatigue tests. The bending stiffness and free vibration frequencies before and after the fatigue tests are compared to understand the effect of repeated loadings on the mechanical performance of the laminated sandwich beams.