Nomex Core Research Articles

Compressive testing of reinforced Nomex honeycomb at elevated temperatures

The effect of varying temperature on the compressive properties of a Nomex honeycomb core reinforced with small diameter composite tubes and rods has been investigated experimentally. Compression tests have been undertaken on a commercially-available Nomex core reinforced with tubes or rods at temperatures between 20 and 140 °C. Tests were also undertaken on unreinforced Nomex cores in order to assess the benefits associated with localized reinforcement of the core. Compression tests on the unreinforced cores have shown that the strength decreased steadily with increasing temperature, passing from 4.8 MPa at room temperature to 3.8 MPa at 140 °C. Similarly, the specific energy absorption (SEA) of the plain core samples decreased slightly from 22.3 to 18.9 kJ/kg over this range of temperatures. Reinforcing the cores with either rods or tubes had a positive effect on the compressive properties of the cores over the range of temperatures considered. For example, the room temperature compression strengths of the rod reinforced cores were 70% higher those of the plain core. Similarly, at 140°C, the rod-reinforced systems were 20% stronger than the plain core. Finally, the energy-absorbing capacity of the tube reinforced cores was higher than that of the rod-reinforced cores.

Discrete modelling of low-velocity impact on Nomex® honeycomb sandwich structures with CFRP skins

Predicting the honeycomb sandwich’s response to impact loading is a major challenge in the aeronautical industry. Several papers demonstrated the possible use of non-linear springs to model Nomex’s impact behavior. On the other hand, the authors show that the lack of transverse shear consideration constitutes a real limitation. The aim of this study is to overcome this limitation by introducing a compression/shear coupling to take into account the transverse shear. In this approach, the modelling of a low velocity impact test is performed on a sandwich composite with CFRP skins and Nomex® honeycomb. Composite skins are modeled as an elasto-plastic-damage material using a user field surbroutine (Abaqus©). The Nomex core is modeled with non-linear springs integrating a compression/shear coupling behavior. Then, the sandwich response under low speed impact is realized and compared with experimental data leading to a good correlation.

The crushing characteristics of reinforced Nomex honeycomb

A Nomex honeycomb core has been reinforced with small diameter composite rods and tubes in order to enhance its compression properties and energy-absorbing characteristics. The influence of the areal density of the rods and tubes on the strength and energy-absorbing properties of the reinforced honeycomb was investigated by introducing increasing numbers of rods/tubes in square samples with bonded composite skins. An initial series of crushing tests on arrangements of small tubes and rods resulted in a stable model of failure yielding specific energy absorption values of approximately 45 kJ/kg for the tube and rod-based structures. A subsequent observation of the failed tubes highlighted the similar failure processes to those observed previously following the tests on much larger composite cylinders. Mechanical tests on the Nomex cores have shown that the compression strength and energy-absorbing characteristics of the reinforced honeycombs increase rapidly with increasing composite reinforcement. At low and intermediate values of core density, the rod and tube-reinforced cores exhibited similar properties, in terms of their compression strengths and specific energy absorption, an effect that is likely to be due to the dominance of the heavier Nomex core in these samples. At higher densities, the rod-reinforced systems tended to out-perform their tube-reinforced counterparts. Tests at impact rates of strain have shown that the compression strength and energy-absorbing capabilities of the reinforced cores are higher under the dynamic conditions, with the rod-reinforced cores offering values of specific energy absorption as high as 78 kJ/kg.

Experimental testing on mechanical properties of sandwich structured carbon fibers reinforced composites

The various types of structural sandwich composites panels available are based either in the method of preparation or on the basis of the core material used in between the skin material. The core material is used to increase the stiffness and strength of the entire panel and can be used according to the application or the area of use. For example the various core materials available for preparing a sandwich composite are Aluminum Honeycomb, Balsa wood, High Density Polyurethane Foam, Rohacell Foam or Nomex Core. In this work carbon fiber is a reinforcement phase with various core materials such as Aluminum Honeycomb, Rohacell and HDPU Foam and epoxy resin is used as a matrix to produce various sandwich-structured composites by Vacuum bag manufacturing process. Various tests Three point bending test, tensile and compressive tests carried out in order to validate the design. After the tests are conducted the obtained results and values are compared and graphs were plotted to compare the various Bending, Tensile and Compressive Load bearing capacity of the various sandwich panels prepared with varying core materials. Foam based sandwich panels tend to have better tensile and compressive load bearing capacity as compared to Aluminium Honeycomb due to their structure and Aluminium has better flexural properties because of its core line up which can undertake bending easily.

Experimental and numerical studies of stepped-scarf circular repairs in composite sandwich panels

This paper investigates the static mechanical performance of bonded circular repairs on sandwich panels made with carbon-epoxy composite skins and a Nomex core. First, the mechanical behaviour of pristine, open-hole and repaired sandwich panels under edgewise compressive loading is studied. Next, pristine and repaired sandwich beams are tested under four-point bending with the circular repair loaded in tension. Then, finite element analyses are performed to predict the strength of the repaired sandwich panel. The adhesive film was considered as an elastic-plastic material with a shear failure criterion. The honeycomb core is assumed to behave as a linear elastic material while for the composite skins, a progressive damage model for woven fabric composites is used to predict the skin behavior until rupture. The good agreement between stiffness and strength levels obtained for both experimental measurements and finite element predictions, for pristine, open-hole and repaired sandwich panels, indicates that an effective analysis tool for the mechanical behavior of the repaired panels has been set-up.

Mechanical performance of repaired sandwich panels: Experimental characterization and finite-element modelling

This paper describes the static performance of adhesively bonded repairs on sandwich panels made with carbon-epoxy composite skins and a Nomex core. First, the mechanical behaviour of pristine and repaired panels under tensile loading was studied. All tests were conducted under room temperature conditions. Then, finite-element analyses were performed to predict the behaviour of repaired panels. Two material models were developed for the adhesive joint: one was linear elastic and the second was elastic-plastic with a shear failure criterion. For the composite skins, an orthotropic linear elastic model was used. Numerical model predictions are in good agreement with the experimental results. It was found also that the strength recovery of the repaired structure increases with the decrease of the scarf angle.

Experimental Validation of the Transverse Shear Behavior of a Nomex Core for Sandwich Panels

This work deals with determination of the transverse shear moduli of a Nomex® honeycomb core of sandwich panels. Their out-of-plane shear characteristics depend on the transverse shear moduli of the honeycomb core. These moduli were determined experimentally, numerically, and analytically. Numerical simulations were performed by using a unit cell model and three analytical approaches. Analytical calculations showed that two of the approaches provided reasonable predictions for the transverse shear modulus as compared with experimental results. However, the approach based upon the classical lamination theory showed large deviations from experimental data. Numerical simulations also showed a trend similar to that resulting from the analytical models.

Impact resistance of Nomex honeycomb sandwich structures with thin fibre reinforced polymer facesheets

In order to investigate the impact resistance of the Nomex honeycomb sandwich structures skinned with thin fibre reinforced woven fabric composites, both drop-weight experimental work and meso-mechanical finite element modelling were conducted and the corresponding output was compared. Drop-weight impact tests with different impact parameters, including impact energy, impactor mass and facesheets, were carried out on Nomex honeycomb-cored sandwich structures. It was found that the impact resistance and the penetration depth of the Nomex honeycomb sandwich structures were significantly influenced by the impact energy. However, for impact energies that cause full perforation, the impact resistance is characterized with almost the same initial stiffness and peak force. The impactor mass has little influence on the impact response and the perforation force is primarily dependent on the thickness of the facesheet, which generally varies linearly with it. In the numerical simulation, a comprehensive finite element model was developed which considers all the constituent materials of the Nomex honeycomb, i.e. aramid paper, phenolic resin, and the micro-structure of the honeycomb wall. The model was validated against the corresponding experimental results and then further applied to study the effects of various impact angles on the response of the honeycomb. It was found that both the impact resistance and the perforation depth are significantly influenced by the impact angle. The former increases with the obliquity, while the latter decreases with it. The orientation of the Nomex core has little effect on the impact response, while the angle between the impact direction and the fibre direction of the facesheets has a great influence on the impact response.

Transverse Shear Behavior of a Nomex Core for Sandwich Panels

The out-of-plane transverse shear characteristics of a Nomex honeycomb core have been studied. Finite-element analyses were performed to find the equivalent transverse shear moduli of the honeycomb core by using a unit-cell-based modeling approach with account of the orthotropic nature of Nomex paper. The results obtained are compared with those of three theoretical approaches. The differences between the numerical and theoretical results are attributed to the isotropic behavior of the basic core material considered in the theoretical approaches.

Identification and prediction of cyclic fatigue behaviour in sandwich panels

The work describes the development of analytical model to identify and predict the cyclic fatigue behaviour of composite sandwich panels subjected to cyclic fatigue loading under 3-point bending conditions. Sandwich samples made from CFRP skin and Nomex core have been loaded with a mean displacement corresponding to 60% of the failure deformation, and subsequently subjected to cyclic loading under displacement control with different loading levels. The fatigue tests show that the stiffness degradation over the number of cycles is characterised by three different phases according to the loading level used. The evolution of the energy dissipated per unit volume versus the number of cycles has also been considered. The cyclic history of the stiffness degradation is developed here following an alternative approach to the one currently adopted. The approach involves the use of interpolation through empirical functions of the experimental data, with the coefficients of the interpolation based on the material properties of the sandwich structure and the type of loading. The alternative modelling approach presented in this work allows the prediction of the fatigue behaviour in sandwich structures without using a large number of test data currently needed in fatigue testing of sandwich panels.

Performance Evaluation of Out-of-Autoclave Sandwich Structures with K-COR and Nomex Core

Performance Evaluation of Out-of-Autoclave Sandwich Structures with K-COR and Nomex Core

Performance Evaluation of Out-of-Autoclave Sandwich Structures with K-COR and Nomex Core

Composite sandwich structures are being increasingly employed in the aerospace, marine as well as other applications which require high strength, high in-plane and flexural stiffness coupled with a reduction in weight. In the aerospace industry sandwich composites are usually manufactured using the autoclave process which is expensive and time consuming. A viable alternative is the Out-of-Autoclave (OOA) process whose main advantage is the low cost of manufacturing. One of the most dominant failure modes in sandwich composites is the debonding that occurs at the facesheet-to-core interphase. A well-formed adhesive bond at the skin to core interphase and improved adhesion strength are significant factors in preventing bond failure. In this paper honeycomb and K-COR sandwich composites are manufactured using the Out-of-autoclave process. The adhesive bonding strength in each of the manufactured panels is quantified by evaluating the flatwise tensile strength in accordance with ASTM standards. Core-facesheet debonding was observed in K-COR sandwich while core failure was observed in the Nomex sandwich structures. Finite element modeling and simulation of Nomex and K-COR sandwich structures subjected to flatwise tension is conducted using comprehensive three-dimensional finite element models. Cohesive damage modeling approach is used to simulate delamination failure in both Nomex and K-COR sandwich structures. Finite element results are compared with the experimental results for model validation. The ultimate flatwise tensile strength predicted by simulation was found to be in good agreement with the experimental results.

Bird impact at aircraft structure – Damage analysis using Coupled Euler Lagrangian Approach

Numerical bird strike damage prediction procedure has been applied on the very detailed large airplane secondary structure consisting of sandwich, composite and metallic structural items. The impacted inboard flap finite element model is modelled using 3D, shell and continuum shell elements, coupled with appropriate kinematic constraints. The bird has been modelled using Coupled Euler Lagrangian approach, in order to avoid the numerical difficulties connected with mesh distortion. Various failure modes, such as Carbon Fibre Reinforced Plastics (CFRP) face layer rupture, failure of composite matrix, damage initiation / evolution in the sandwich structure Nomex core and elastoplastic failure of a metallic structure have been investigated. Besides, general contact has been applied as to efficiently capture the contact between Eulerian bird material and the structure, as well as large deformations of the different structural components. Compared to the classic Lagrangian modelling of the bird, the analysis has proven to be more stable, and the results, such as damage areas, physically more realistic. The impact has been applied in the area that is the most probably subjected to the impact damage during exploitation.

Vacuum excitation in shearographic NDT

Materials of composite nature, such as for example carbon fibre skin, sandwiching Nomex core and other honeycomb structures, are increasingly being utilised in the aerospace industry due to the benefits of reduced weight, the ease of formability and good mechanical properties that they offer. Composite materials are difficult to inspect for flaws such as delaminations and disbanding using traditional NOT techniques, however digital shearography is demonstrably well-suited for the purpose and particularly when applying vacuum stressing. The paper reports on developments regarding the challenge of transferring shearographic inspection from the confines of the laboratory to the industrial environment, the development of a portable vacuum chamber which would be placed on the surface to be inspected, and guidelines on the technique's sensitivity or resolution in attempting to detect hidden flaws of differing reduced magnitude. The results indicate that vacuum stressing is particularly desirable when inspecting laminates for disbanding or delaminations because its nature dramatically highlights the flaw, with the added benefit of allowing the detection of relatively small magnitude defects. Notwithstanding the fact that the shearographic method is an optical interferometric technique, an additional optical element, such as the window of a vacuum chamber, interposed between the shearography optics and the surface of the component under test, did not affect the procedure in any manner. This fact provided the confidence to proceed in developing the portable vacuum chamber, in that the current digital shearograph _ > : system's versatility would be conserved by not transforming it to a turnke _ v solution for only composites vacuum testing.

Impact Damage and Strength Reduction Behavior of Honeycomb Sandwich Structure Subjected to Low Velocity Impact

The goals of the paper are to identify the impact damage and strength reduction behavior of sandwich structure, composed of carbon/epoxy laminates skin and Nomex core with two kinds of thickness (10 and 20mm). For these, low velocity impact tests were conducted using the instrumented impact-testing machine and damages are inspected by SAM. And then, subsequent static tests are conducted under flexural loading to identify the strength reduction behavior of the impacted sandwich structures. The impact damages are mainly delamination in carbon/epoxy skin and their behavior is mostly independent of core thickness. Also, their energy absorbing behavior is identified through calculating the energy absorbed by impact damage. Finally, the strength reduction behavior is evaluated through Caprino’s model, which was proposed on the unidirectional laminates.

Thermal Deicing of Polymer Composite Helicopter Blades

ABSTRACTThis paper presents the results of an investigation to determine if thermal deicing methods (hot air, hot water, hot glycol, or radiative heat) would damage the composite materials in helicopter blades. Samples made from Blackhawk helicopter blades were thermally cycled in the regime of the temperatures of thermal deicing methods and then mechanically load tested. The strength reduction was compared with the number of thermal cycling and the temperatures of thermal cycling. The strength results and the modes of failure indicate that damages indeed do develop in the composites, especially along the bond line of the composite skin and the nomex core of the blade structure, and mechanical strength is reduced.

Numerical modeling of impact-damaged sandwich composites subjected to compression-after-impact loading

Semi-empirical numerical models are developed for predicting the residual strength of impact-damaged sandwich composites comprised of woven fabric carbon epoxy facesheets and Nomex honeycomb cores subjected to compression-after-impact loading. Results from non-destructive inspection and destructive sectioning of damaged sandwich panels are used to establish initial conditions for damage (residual facesheet indentation, core crush dimensions, etc.) in the numerical analyses. Honeycomb core crush test results are used to establish the non-linear constitutive behavior for the Nomex core. The influence of facesheet property degradation on the strain redistribution in damaged sandwich panels is examined. Facesheet strains from material and geometric non-linear finite element analyses correlate relatively well with experimentally determined values. Moreover, numerical predictions of residual strength are consistent with experimental observations. Similar calculations may prove useful in the development of a damage tolerance plan for sandwich composites that accounts for relevant details of damage morphology and provide conservative estimates of residual strength.

Scaling and Curvature Effects on the Damage Tolerance of Impacted Composite Sandwich Panels

An experimental program was conducted to investigate the effects of specimen width and specimen curvature on the residual compression strength of composite sandwich panels after low-energy impact. Flat and curved (1.1 m radius) specimens of three widths (83, 152, and 305 mm) were manufactured from thin-gage (four plies) graphite/epoxy facesheets and 25.4-mm-thick Nomex core. A drop mechanism was used to create barely visible impact damage on one facesheet of the panels (convex face of the curved panels) maintaining the energy level as constant in all cases. All specimens were then loaded axially in compression to failure. Above a certain panel width, the panel width and curvature had noeffect on the ultimate strength of the panel. Although width and curvature increased the size of the damage caused by an identical energy impact, the indentation growth was delayed with respect to farfield strain (stress). Because indentation growth is chiefly a local phenomenon, the delay was probably due to the variations in the damage state. The baseline specimen (flat, 83 mm in width) serves as an adequate coupon-level test even though it is conservative when compared to wider plates.

Response of Indented Sandwich Panels

A study was conducted of the effect on the geometric eccentricity due to the resulting indentation from impact. To prevent the interaction of any other damage, uncured sandwich panels were indented with a 25-mm-diameter steel up to a depth of 1.6, 3.2, and 4.8 mm and then cured, resulting in an indented facesheet. A total of 12 specimens with IM7/8552 [90/0]S facesheets cocured to 25-mm thick Nomex core were tested. All specimens were 83 mm in width and 356 mm in length. The specimens were loaded in uniaxial compression and strain measurements were made. The strain gage data indicated that although the manufactured indentations displaced deeper into the core, little growth transverse to the load was observed. In a few cases, no growth was observed prior to ultimate failure. In all cases the ultimate strength of the specimens with manufactured indentations was greater than comparable specimens with impact damage. Also, the ultimate failure of the indented specimens initiated away from the indentation. Thus...

Indentation failure analysis of sandwich beams

Failure of sandwich honeycomb structures under indentation loading is considered. A failure criterion for Nomex honeycombs subjected to combined compressive and shear stresses is determined using biaxial tests. By combining this with a theoretical calculation of the stress distribution in the core due to indentation loading, found from a high-order sandwich beam theory (HOSBT), the indentation failure load of the sandwich beam due to core failure can be predicted. It is assumed that both the skin and core are elastic up to failure, which is a reasonable approximation for the GFRP skins and Nomex cores considered. Short beam 3-point bending tests are used to validate the theoretical predictions, using beams made with GFRP skins and Nomex cores with densities between 29 and 128 kg/ m 3 . Theoretical predictions of indentation failure load are in excellent agreement with measured values. Inclusion of shear stresses in the failure criterion significantly improves the predictions, correctly modelling the observed stronger behaviour of cores with a longitudinal ribbon direction.