Comparison of foam core sandwich panel and through‐thickness polymer pin–reinforced foam core sandwich panel subject to indentation and flatwise compression loadings

Through‐thickness polymer pin–reinforced foam core sandwich (FCS) panels are new type of composite sandwich structure as the foam core of this structure was reinforced with cylindrical polymer pins, which also rigidly connect the face sheets. These sandwich panels are made of glass fiber–reinforced polyester face sheets and closed‐cell polyurethane foam core with cylindrical polymer pins produced during fabrication process. The indentation and compression behavior of these sandwich panels were compared with common traditional sandwich panel, and it has been found that by reinforcing the foam core with cylindrical polymer pins, the indentation strength, energy absorption, and compression strength of the sandwich panels were improved significantly. The effect of diameter of polymer pins on indentation and compression behavior of both sandwich panels was studied and results showed that the diameter of polymer pins had a large influence on the compression and indentation behavior of through‐thickness polymer pin–reinforced FCS panel, and the effect of adding polymer pins to FCS panel on indentation behavior is similar to the effect of increasing the thickness of face sheet. The effect of strain rate on indentation behavior of FCS panel and through‐thickness polymer pin–reinforced FCS panel were studied, and results showed that both types of composite sandwich panels are strain rate dependent structure as by increasing strain rate, the indentation properties and energy absorption properties of these structures are increased. POLYM. COMPOS., 37:612–619, 2016. © 2014 Society of Plastics Engineers     

High-velocity impact loading in honeycomb sandwich panels reinforced with polymer foam: a numerical approach study

The employment of lightweight structures is one of the most important goals in various industries. The lightweight sandwich panel is an excellent energy absorber and also a perfect way for decreasing the risk of impact. In this paper, a numerical study of high-velocity impact on honeycomb sandwich panels reinforced with polymer foam was performed. The results of numerical simulation are compared with the experimental findings. The numerical modeling of high-velocity penetration process was carried out using nonlinear explicit finite-element code, LS-DYNA. The aluminum honeycomb structure, unfilled honeycomb sandwich panel, and the sandwich panels filled with three types of polyurethane foam (foam 1: 56.94, foam 2: 108.65, and foam 3: 137.13 kg/m³) were investigated to demonstrate damage modes, ballistic limit velocity, absorbed energy, and specific energy absorption (SEA) capacity. The numerical ballistic limit velocity of sandwich panels, filled with three types of foam, was more than that of a bare honeycomb core and unfilled sandwich panel. In addition, the numerical results showed that the sandwich panel filled with the highest density foam could increase the strength of sandwich panel and the numerical specific energy absorption of this structure was 23% more than that of unfilled. Finally, the numerical results were in good agreement with experimental findings.

     

Investigation of sandwich structures with innovative cellular metallic cores under low velocity impact loading

Abstract

Sandwich panels are widely used for energy absorbing applications in cases of low and high velocity impacts. The core itself is capable of absorbing energy by progressive collapse, while the skins are necessary for uniformly distributing the local vertical load over the impacted area as well as for the introduction of overall panel bending resistance. In the present work, the failure response of sandwich panels with open lattice cellular cores subjected to low velocity impact is investigated. Experimental tests are performed using a mass drop testing machine. Additionally, a three-dimensional finite element model of the sandwich panels–impactor system is developed using commercial Finite Element (FE) codes. The core homogenisation is introduced in order to improve the efficiency of the FE analysis by reducing the computational time. Numerical results correlate well with experimental data, enabling detailed understanding of the parameters affecting the initiation and propagation of impact damage.     

The impact response of aluminum foam sandwich structures based on a glass fiber-reinforced polypropylene fiber-metal laminate

The fracture properties and impact response of a series of aluminum foam sandwich structures with the glass fiber–reinforced polypropylene-based fiber-metal laminate (FML) skins have been studied. Initially, the manufacturing process for producing the FML skins was optimized to obtain a strong bond between the composite plies and the aluminum layers. The degree of adhesion between the composite plies and the aluminum was characterized by conducting single cantilever beam tests. Here, it was found that the composites could be successfully bonded to the aluminum using a simple short stamping procedure. A detailed examination of the fracture surfaces indicated that crack propagation occurred within the composite ply in the fiber-metal laminates and along the composite-aluminum foam interface in the sandwich structures. The low velocity impact response of the FMLs and the sandwich structures was investigated using an instrumented drop-weight impact tower and a laser-Doppler velocimeter. The energy absorption characteristics of the sandwich structures were investigated along with the failure processes. Finally, a series of tensile tests on the damaged FMLs and thermoplastic sandwich structures showed that both systems offer promising residual load-bearing properties. Here, shear failure in the aluminum foam was observed in the sandwich structures, indicative of a strong bond between the FML skins and the aluminum core. Polym. Compos. 25:499–509, 2004. © 2004 Society of Plastics Engineers.     

Impact resistance and bonding capability of sandwich panels with fibre–metal laminate skins and aluminium foam core

Sandwich panels with aluminium foam core and fibre–metal laminate (FML) skins for enhanced impact resistance have been designed and manufactured during this research activity. The FML skins are made of a combination of aluminium sheets and E-glass fibre/epoxy-laminated plies. Drop-weight impact tests are conducted on two groups of sandwich panels with aluminium foam core bonded to the aluminium sheet in Group 1 panels, and aluminium foam core bonded to the E-glass/epoxy ply in Group 2 panels to allow an investigation of the bonding capability between the aluminium foam and the FML skins under impact and the impact resistance of the sandwich panels. The delamination and debonding ranges, the maximum deformed height in the impact area are measured and the deformed volumes of the sandwich panels after drop-weight tests are evaluated. via comparison of these parameters for the two groups of sandwich panels, it is found that the bonding between aluminium foam to the E-glass/epoxy surface provides a better resistance to impact than that between aluminium foam bonded to the aluminium sheet of the FML facing, and that overall Group 2 panels exhibit better bonding capability and impact resistance with less facing delamination and core/facing debonding than the Group 1 panels.     

Permanent indentation and damage creation of laminates with different composite systems: An experimental investigation

Quasi‐static indentation (QSI) and low‐velocity impact tests have been conducted to laminated composite specimens made from different composite systems. C‐scan and thermal deply techniques were used to observe the internal damage characteristics of the laminates. The test results show the effect of fibers and matrices on the permanent indentation and damage behaviors of composite laminates. Fiber breakage plays an important role in the formation of permanent indentation and the match between the fiber and matrix also has a significant influence on the permanent indentation and damage behaviors. A comparison shows the correlation between the peak load energy of the QSI tests and the knee point energy of the impact tests, based on which a condition for the prominent indentation formation was proposed for the impact tests. POLYM. COMPOS., 35:872–883, 2014. © 2013 Society of Plastics Engineers     

修复方式对蜂窝夹芯结构弯曲性能的影响

针对夹芯结构经常出现的三种损伤方式:单面板损伤、单面板和蜂窝损伤以及穿透性损伤,利用真空袋压工艺,采用高强玻璃布补片和Nomex蜂窝芯作为修补材料,通过复合材料胶接与挖补修复工艺对损伤结构进行修复,主要探讨了修复前后不同损伤孔径对其弯曲性能的影响。结果表明,三种损伤方式对蜂窝夹芯结构的最大载荷和弯曲刚度都有很大影响;三种损伤方式的夹芯结构的最大载荷和弯曲刚度都随着损伤孔径的增大而降低;用复合材料胶接与挖补工艺对三种损伤方式的夹芯结构进行修复,能大大提高受损结构的弯曲性能;修复后的最大弯曲载荷达到完好夹芯结构的80%以上,修复后的弯曲刚度达到完好夹芯结构的85%以上。     

An Investigation of Low Velocity Impact Properties of Rotationally Molded Skin–Foam–Skin Sandwich Structure

In this study, the low velocity impact properties of rotationally molded skin–foam–skin sandwich structures were investigated experimentally since there is a need for a greater understanding of the impact behavior of these composites in service to extend the range of their applications. Polyethylene rotationally molded sandwich structures were manufactured at various skin and core layer thickness combinations and tested using an instrumented low velocity drop weight impact testing machine at 20–100 J impact energy levels, at room temperature. This allowed the identification of the impact response, failure mode, and the effects of the skin and core layer thickness on impact resistance. Force–deflection curves, maximum force, contact time, maximum deflection versus impact energy curves were analyzed. Samples were seen to fail due to the indentation dart piercing the upper and lower skins, with crushing and consolidation seen in the core foamed layer. Delamination at the core/skin interface was not observed. It was found that fracture initiates from the lower skin and then continues to grow to the upper skin via the foamed core layer. The impact resistance was noted to increase with increasing skin and core layer thickness; though an increase in skin layer thickness had a greater contribution than an increase in the core layer thickness. POLYM. ENG. SCI., 60: 387–397, 2019. © 2019 Society of Plastics Engineers     

Analysis on low velocity impact damage of laminated composites toughened by structural toughening layer

The intralaminar and interlaminar damages of U3160/3266 laminated composites toughened by polyamide nonwoven fabric (PNF) under low velocity impact are investigated through a numerical model which considers both the three‐dimensional continuum damage mechanics (CDM) and the bilinear cohesive zone model (CZM). The analysis of the intralaminar damage is implemented by the ABAQUS/Explicit finite element code coupled with a user‐defined subroutine VUMAT where the longitudinal failure, transverse matrix cracking, and nonlinear shear of the material are taken into account. Then the effects of the thickness and strength of PNF/3266 interlayer on the damage of composites are numerically analyzed. The results reveal that damage morphology can be simulated qualitatively compared to the experimental counterparts. With the decreasing interlayer thickness or the increasing interlayer strength, the damage area is effectively reduced. This work provides an effective model to predict the low velocity impact damage of composites, and is helpful for the optimization of interlayer toughened composites. POLYM. COMPOS., 38:1280–1291, 2017. © 2015 Society of Plastics Engineers     

Behavior of sandwich structures and spaced plates subjected to high‐velocity impacts

This work evaluates the behavior of sandwich and spaced plates subjected to high‐velocity impacts. The sandwich structures were made of glass/polyester face‐sheet and a PVC foam core. The spaced plates were made of two plates of the same material of the sandwich face‐sheet at a distance equal to the core thickness. The residual velocity, the ballistic limit, and the damage area were selected to compare the response of both structures. The residual velocity and ballistic limit was very similar in both cases. Nevertheless, the damage area of sandwich structures and spaced plates differed due to the dissimilar properties between the sandwich core and the air inside of the spaced plates. An analytical model, based on energy criteria, was applied to estimate the residual velocity of the projectile, the absorbed energy by each face‐sheet, and the ballistic limit in the spaced plates. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

A new improved high‐order theory for analysis of free vibration of sandwich panels

A new improved high‐order theory is presented to investigate the dynamic behavior of sandwich panels with flexible core. Shear deformation theory is used for the face sheets, whereas the three‐dimensional elasticity theory is used for the core. Displacements in the core are assumed as polynomial with unknown coefficients. Inertia forces, moments of inertia and shear deformations in the core, and the face sheets are taken into consideration. Unlike the previous improved theory, the in‐plane normal and shear stresses in the core are considered. The governing equations and the boundary conditions are derived by Hamilton's principle. Closed form solution is achieved using the Navier method and solving the eigenvalues. The numerical results of present analysis compared with the available results in the literature. It indicates that the present new modified theory is more accurate than the other developed theories for sandwich panels. In this study, the variation of the nondimensional natural frequency with respect to the various parameters is presented. POLYM. COMPOS., 31:2042–2048, 2010. © 2010 Society of Plastics Engineers     

玻璃钢蜂窝夹层结构板热导率研究

玻璃钢蜂窝夹层结构板除高比强度、比刚度的优点外,还具有低热导率的特点,是一种隔热保温材料,在航天、航空、建筑方面得到广泛应用。本文对蜂窝夹层结构板的热导率进行了理论上的研究,分别对传导、对流、辐射进行分析,最后得到可供设计使用的热导率计算公式及曲线。计算结果与试验值很符合。     

Experimental and Numerical Investigations of Honeycomb Sandwich Composite Panels With Open-hole Damage and Scarf Repair Subjected to Compressive Loads

The mechanical analysis of adhesive repair of honeycomb sandwich composite plate is more complicated than that of the laminated plate repair, because not only the laminated damage, but also the honeycomb buckling should be considered. Experimental and numerical investigations are carried out to understand the damage propagation and ultimate strength of both open-hole and repair plates under compressive load. The results of numerical models are verified by experimental data and they are found in good agreement. It is revealed from the results that the strength of open-hole plate is about 34% of the strength of intact plate, while the strength of repair plate could resume to 76% of that of intact plate after the repair process. It is found that the structure strength increases as the scarf repair angle decreases due to the decline of shear stress in the adhesive. We also present the optimum ply sequence and the optimum number of overlays in considering the improvement of structure strength. This research will be useful in improving the design and analysis techniques for scarf patch repair of sandwich structures.     

Numerical analysis of adhesively bonded T-joints with structural sandwiches and study of design parameters

Adhesively bonded T-joints are extensively used in assembling sandwich structures. The advantage of adhesive bonded joints over bolted or riveted joints is that the use of fastener holes in mechanical joints inherently results in micro and local damages to the composite laminate during their fabrication. One type of adhesive joint in such structures is the T-joint between sandwich panels. The aim of this research paper is to study, by numerical analysis, the effect of fillet geometry and core material of sandwich panels on the performance of T-joints. The base angle of the core triangle (fillet) is the most important geometry parameter of the triangular T-joint. Nine geometrical models with different base angles of the core triangle are made to investigate the effect of the base angle on the performance of the T-joints. It should be mentioned that the base angle in the triangular foam is changed, so that the final volume of the filler is kept constant in all the cases. Different foams with different stiffness are used to model the core of the panels to study the effect of the core material of sandwich panels. To model the adhesive between joint components, contact elements and cohesive zone material models are used. Therefore, failure of adhesive and separation of joint elements can be modeled. Damage and core shear failure of the base panel are modeled by using a written macro-code in the ANSYS finite element method (FEM) program. The ultimate strength of the joint in each case is calculated by modeling adhesive failure and core shear failure of the sandwich panels. Finally, the results of FEM are validated by experimental results available in the literature. In general, the failure load predicted by the FEM is within 5% of the experimental results. The best angle of the core triangle was found to be 45°. Also, the results showed that by changing the core material of the sandwich panel, the joint failure load is also changed.     

Experimental investigation into glass fiber/epoxy composite laminates subjected to single and repeated high-velocity impacts of ice

Many engineering components in aerospace structures which are made from polymer composite materials are often damaged during service life due to hail ice and bird impact. This study examines the damage which may be incurred by a single and repeated high-velocity impact of 11.7 g cylindrical-shaped ice on glass fiber/epoxy laminated composite panels carried out on a 20-mm diameter smooth barrel gas gun. The laminates were made from E-glass fiber/epoxy resin with 0/90, ±45, chopped strand mat (CSM) and unidirectional fiber orientation and in different stacking sequence. The impact velocity was in the range of 130–140 m/s and the resulting damage extension zones from ice projectile impacts were measured. Damage extension was successfully identified in all specimens subjected to high-velocity ice projectile impact. Results showed specimens with ±45 orientation and CSM fiber exhibited the lowest damage extension. The results also revealed that specimens with plain weave 0/90 lay-up of glass woven roving show the highest damage extension. Extended damages were observed in composite panels under repeated ice projectile impacts. Study of the stacking sequence effect indicated significant role played by presence of ±45 reinforcement in reducing the damage extension in the laminated plates. Delamination constituted the major damage mechanism for most specimens tested followed by matrix and fiber fracture.     

Reaction‐to‐fire properties of polymer matrix composites with integrated intumescent barriers

The integration of an intumescent barrier between the plies of prepreg based polymer matrix composite and sandwich panels is investigated in detail with regard to reaction‐to‐fire properties. Incident heat flux, panel thickness and insertion depth within the panel were varied systematically. Fire retarding effects are compared to the application of an intumescent and top coating on the surface. All tests were carried out with a commercial material: HexPly® 8552/IM7 by Hexcel. Design rules for an effective improvement of reaction‐to‐fire properties are derived. Two practical applications were identified not interfering with mechanical properties: A metal mesh as support for the intumescent material underneath a single top ply and the one‐sided integration in a sandwich with the possibility to expand into the honeycomb. Degradation mechanisms are characterized by cone calorimetry and temperature development throughout the specimens. Copyright © 2014 John Wiley & Sons, Ltd.     

Fabrication process and mechanical properties of C/SiC corrugated core sandwich panel

Carbon fiber reinforced SiC composite is a kind of promising high-temperature thermal protection structural material owing to the excellent oxidative resistance and superior mechanical properties at high temperatures. In this work, a novel design and fabrication process of lightweight C/SiC corrugated core sandwich panel will be proposed. The compressive and three-point bending of the C/SiC corrugated sandwich panels are conducted by experiment and numerical simulation. The relative density of as-prepared C/SiC sandwich panel and the density composite material are 1.1 and 2.1 g/cm³, respectively. As the density of the C/SiC sandwich panel is only 52.3% of the bulk C/SiC, suggesting that lightweight characteristic is realized. Moreover, the C/SiC sandwich panel manifests itself as linear-elastic behavior before failure in compression and the strength is as high as 15.1 MPa. The failure mode is governed by the core shear failure and panel interlayer cracking. The load capacity under the three-point bending C/SiC composite sandwich panel is 1947.0 N. The main failure behavior is core shear failure. The stress distribution under the compression and three-point bend was simulated by FE analysis, and the results of numerical simulations are in accordance with the experimental results.     

Impact behavior of unidirectional polyethylene–glass fibers: Resol/vinyl acetate‐2‐ethylhexylacrylate interpenetrating polymer network systems

Resol was solution blended with vinyl acetate‐2‐ethylhexylacrylate (VAc–EHA) resin in an aqueous medium at a 90‐10 w/w ratio with hexamethoxymethylmelamine (HMMM) as crosslinker. Here we aimed to study the impact behavior of unidirectional laminates cast from (Resol/VAc–EHA/HMMM)/glass fiber (GF), (Resol/VAc–EHA/HMMM)/polyethylene fiber (PEF), and (Resol/VAc–EHA/HMMM)/GF/PEF (hybrid) and to study the role of PEF ply/plies in hybrid laminates toward the impact behavior, as dependent on the relative position of the ply/plies. A brittle failure mode was observed in the GF‐reinforced laminates, which tended to the ductile failure mode, with the incorporation of PEF ply/plies. Again, the impact fracture mode of GF was minimized by the placement of PEF ply/plies at the impacted side of the hybrid laminates. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 339–342, 2004     

双向格构腹板增强泡沫夹层复合材料梁弯曲性能试验

复合材料夹层结构具有比强度高、比刚度高、可设计性强、耐腐蚀等特点,以聚氨酯泡沫为芯材,以玻璃纤维增强复合材料为面板和格构腹板,采用真空导入成型工艺,制备双向格构腹板增强泡沫夹层复合材料梁。对无格构泡沫夹芯复合材料梁,不同腹板高度、腹板间距双向格构增强泡沫夹层复合材料梁进行三点弯曲试验,研究其破坏模式和机理。基于泡沫填充矩形蜂窝芯材的等效十字模型,预估试件的抗弯刚度和挠度,计算值与试验值吻合较好。     

Nanoclay Addition and Core Materials Effect on Impact and Damage Tolerance Capability of Glass Fiber Skin Sandwich Laminates

This study explores the effects of modified (OMMT) nanoclay and core material on low velocity impact behavior and damage tolerance capability of glass fiber reinforced (FRP) polyester resin – polystyrene foam (PS) sandwich laminates. The FRP and sandwich laminates are prepared by a compression molding technique for investigation. Low velocity impacts are carried out on all the fabricated laminates by using a instrumented drop weight impact tower with the energy level of 30 J and load–energy–time plots were recorded using data acquisition software. Post impact flexural tests have been conducted to evaluate the damage tolerance capability of the fabricated composite laminates. X-ray Diffraction (XRD) results have been obtained for the samples, where the nanoclay has indicated that intergallery spacing of the layered clay increases with the matrix. Scanning Electron Microscopy (SEM) has given the morphological picture of the nanoclay dispersion in the polymer fracture samples. The results of the study show that the impact properties and damage tolerance capability of the 4% nanoclay polyester sandwich have been greatly increased.