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     

Hybrid composites from coir fibers reinforced with woven glass fabrics: Physical and mechanical evaluation

Sandwich composites based on coir fiber nonwoven mats as core material were manufactured by Vacuum Assisted Resin Transfer Molding technique. Mechanical and physical properties of produced coir/polyester and coir‐glass/polyester composites were assessed. Samples were evaluated according to their reinforcement contents, resin contents, areal density, and thickness. Tests on physical properties revealed that coir‐glass/polyester sandwich structure has the lowest values of thickness swelling, water absorption and moisture contents compared with coir/polyester composite. Mechanical tests such as tensile strength, open‐hole tensile strength, and flexural strength were also performed on all samples. Coir‐glass/polyester sandwich structure showed significant increase in tensile strength of 70 MPa compared with 8 MPa of coir/polyester composite. Introducing two skins of fiber glass woven roving to coir/polyester increased its flexural strength from 31.8 to 131.8 MPa for coir‐glass/polyester. POLYM. COMPOS., 38:2212–2220, 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     

Water absorption of hemp fiber/unsaturated polyester composites

The water absorption of hemp fiber/unsaturated polyester composites was determined by immersing the samples in water or by exposing them to air with a relative humidity of 94%. The water absorption increased with increasing fiber content. By using images obtained with a magnetic resonance imaging (MRI) system, the moisture absorption process was shown to follow a diffusion mechanism and to be more important in the longitudinal than in the transverse direction. The longitudinal diffusion coefficient was computed to be about 3 × 10^–11 m²/s. Composite samples immersed in water reached saturation levels after about eight months and showed no signs of cracking due to swelling. Fibers reached the same saturation limit whether submerged in water or exposed to saturated air when fiber content was less than 21 vol%. Various fiber treatments were tested but none resulted in a substantial increase of the resistance to water absorption. The most effective technique to enhance moisture resistance was to properly seal all the fibers within the matrix. POLYM. COMPOS. 26:509–525, 2005. © 2005 Society of Plastics Engineers     

湿热循环对PMI泡沫/3218-1环氧树脂夹层结构性能影响的研究

通过对PMI泡沫/3218-1环氧树脂夹层结构进行湿热处理,测定夹层复合材料的吸湿、脱湿曲线,研究湿热对夹层复合材料的吸湿性能的影响,同时对湿热循环处理前后的夹层复合材料试样进行力学性能和介电性能的测试,以研究湿热循环对两种PMI泡沫/3218-1环氧树脂夹层结构的吸湿性能、力学性能及介电性能的影响。     

Moisture absorption by PVC plastisol components

Moisture can be absorbed from the air by PVC plastisols and by the fused product. Water in plastisols can contribute to defects in films, sheet goods and foam products, usually in the form of depressions or craters. Water absorbed by thermoformable foam PVC sheets can result in defects in the molded product, often appearing as wrinkles. Correlation between the sum of the water absorbed by individual PVC plastisol components and the water absorbed by a fused thermoformable foam sheet suggests that moisture sensitivity of the product can be minimized by selecting plasticizers and PVC resins having low moisture absorption.     

A study on the compressive characteristics of co‐cured foam‐composite sandwich structures according to the crimp angle variations

A foam‐composite sandwich column composed of two‐ply carbon/epoxy fabric prepreg as a skin and PVC foam for a core material was investigated to determine the effect of crimp angle variations of the fabric prepreg on the compressive characteristics of the sandwich column. The crimp angle of the composite skin was observed and measured in terms of the forming pressure and the foam density as correlated with the foam deformation regimes. End compression tests for the foam‐composite sandwich columns were carried out to obtain the values of the compressive modulus and strength depending on the crimp angle variations. From the tests and observation results, it was found that the crimp angle and compressive strength were highly correlated; as the crimp angle increased, the compressive strength decreased as a large‐crimp angle caused the composite skin to fail due to simple microbuckling of the longitudinal tows. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

蒸馏水含量对三明治结构硬质聚氨酯泡沫性能的影响

采用异氰酸酯、聚酯多元醇、发泡剂（水）等原料通过一体发泡成型技术制备出一种新型的三明治泡沫夹心复合材料。利用热重分析、扫描电子显微镜等对不同水含量（质量分数分别为0、0.5 %和1.0 %）的硬质聚氨酯泡沫材料的泡孔直径、密度、热导率、压缩性能、三点弯曲和热力学性能等做了研究,进而确定提高硬质聚氨酯性能的最佳工艺。结果表明,随着水含量的增加,硬质聚氨酯泡沫材料泡孔直径增大,密度变小,热导率降低,保温性能提高,而压缩性能和三点弯曲却呈下降趋势;综合考虑硬质聚氨酯泡沫材料泡孔结构和良好的保温隔热及弯曲等力学性能,其最佳含水量为0.5 %。     

Barrier properties for short‐fiber‐reinforced epoxy foams

The barrier properties of short‐fiber‐reinforced epoxy foam are characterized and compared with unreinforced epoxy foam in terms of moisture absorption, flammability properties, and impact properties. Compression and shear properties are also included to place in perspective the mechanical behavior of these materials. Compared with conventional epoxy foam, foam reinforced with aramid fibers exhibits higher moisture absorption and lower diffusion, while glass‐fiber‐reinforced foam is significantly stiffer and stronger. In addition, the polymeric foam composites studied present superior fire‐resistance compared with conventional epoxy foam systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3266–3272, 2006     

Migration resistant polymeric plasticizer for poly(vinyl chloride)

Flexible films of poly(vinyl chloride) (PVC) and linear or branched poly(butylene adipate) (PBA), synthesized from 1,4‐butanediol and adipic acid or dimethyl ester of adipic acid, were aged in an aqueous environment for 10 weeks to study how branching, molar mass, and end‐group functionality affect the leaching of polyester plasticizer from thin films. Principal component analysis was applied to reveal patterns and correlations between mechanical properties, material characteristics, and aging behavior. Introduction of branches in the polyester structure increased the miscibility between PVC and the polyester, resulting in improved mechanical properties and lower water absorption. Methyl ester end‐group in PBA polyester stabilized the polymeric plasticizer toward hydrolysis, and reduced the formation and migration of monomeric degradation products from the blends during aging in water. The combination of branched structure with methyl ester end‐groups resulted in a migration resistant polymeric plasticizer. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2458–2467, 2007     

Low speed impact behaviour of adhesively bonded foam-core sandwich T-joints

This paper investigates the effects of foam core density and aluminum skin plates on the low speed impact behaviour of adhesively bonded sandwich T-joints having a PVC foam core and aluminum face-sheets. The dynamic response of adhesively bonded sandwich T-joints was analyzed by the explicit finite element method. Two different material models were implemented to the foam core material: a hyperelastic model and a crushable foam material with ductile damage whereas the aluminum face-sheets were modelled as an elasto-plastic material. The cohesive response of adhesive interfaces was included using three dimensional cohesive element based on cohesive zone model. Adhesively bonded sandwich T-joint specimens were manufactured and tested to validate the numerical model. A very good agreement between the experimental and FE results were achieved. The density of the foam core material of adhesively bonded sandwich T-joint played important role on the joint failure mechanism. The joint having a stiffer foam core experienced more damage in both stiffener panel and adhesive layers.     

Skin‐core debonding in resin‐infused sandwich structures

Techniques for enhancing the interfacial fracture properties of foam‐based sandwich structures are investigated. Prior to manufacture, holes were drilled into a PET foam core in order to facilitate resin flow during the subsequent resin‐infusion process. Glass fibers were then inserted into the perforations in an attempt to increase the interfacial fracture toughness of the sandwich structure. The results from these tests are compared to data generated from similar tests on a plain PET core, as well as on samples in which no fiber reinforcement was incorporated into the vertical holes. The inclusion of fibers in the through‐thickness holes served to increase the skin‐core interfacial fracture toughness of the sandwich structures. Here, it was noted the highest values of fracture toughness were more than three times the value measured on the plain foam system. POLYM. COMPOS., 37:2974–2981, 2016. © 2015 Society of Plastics Engineers     

Mechanical and morphological properties for sandwich composites of wood/PVC and glass fiber/PVC layers

This work manufactured sandwich composites from glass fiber/poly(vinyl chloride) (GF/PVC) and wood/PVC layers, and their mechanical and morphological properties of the composites in three GF orientation angles were assessed. The effects of K value (or viscosity index) of PVC and Dioctyl phthalate (DOP) loading were of our interests. The GF/PVC was used as core layer whereas wood/PVC was the cover layers. The experimental results indicated that PVC with low K value was recommended for the GF/PVC core layer for fabrication of GF/WPVC sandwich composites. The improvement of PVC diffusion at the interface between the GF and the PVC core layer was obtained when using PVC with K value of 58. This was because it could prevent de‐lamination between composite layers which would lead to higher mechanical properties of the sandwich composites, except for the tensile modulus. The sandwich composites with 0° GF orientation possessed relatively much higher mechanical properties as compared with those with 45° and 90° GF orientations, especially for the impact strength. Low mechanical properties of the sandwich composites with 45° and 90° GF orientation angles could be overcome by incorporation of DOP plasticizer into the GF/PVC core layer with the recommended DOP loadings of 5–10 parts per hundred by weight of PVC components. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Collapse Mechanism of Foam Cored Sandwich Structures Under Compressive Load

In this work the moisture absorption capability, compressive properties, collapse modes of various types of composite sandwich structures are reported. The tested sandwich structures were constructed with varieties of hybridized skin materials and different compositions of the core materials. The moisture absorption, Flatwise compression and Edgewise compression tests are conducted for core as well as sandwich structures. Comparisons of results have been between the hybridized and non-hybridized sandwich structures. Two modes of collapse were noticed in the Edgewise compressive test, one of which being progressive end-crushing of the sandwich structure featured by significant crash energy absorption. This feature was highly desired for the parts of transportation vehicles. Microscopic analysis has been carried out to know the nature of failure under compressive loads. It has been observed that with increasing the debonding strength of the core–face interface, the failure mode changes from unstable collapse mode stable progressive crushing.     

Near infrared (NIR) spectroscopy compared with thermogravimetric analysis as a tool for on‐line prediction of water diffusion in polyamide 6,6

NIR spectroscopy in the transmission mode and thermogravimetric analysis were used to predict diffusion of water into polyamide 6,6 samples immersed in water at 40, 60, 75 and 90 °C for different periods of time. The sorption curves between 40 and 75 were sigmoidal indicating that the surface concentration was time dependent. The sorption curves were readily fitted by the use of a time‐dependent surface concentration and a water‐concentration‐dependent diffusivity. The zero‐concentration water diffusivity decreased non‐linearly and the activation energy of diffusion increased from 24 to 58 kJ mol^?1, with decreasing temperature. The surface concentration relaxation time decreased rapidly. The sorption of water in thick polyamide samples was readily characterized by FT‐NIR spectroscopy. The accuracy and feasibility of this method was similar to conventional thermogravimetric methods. The greatest advantage of FT‐NIR, however, is the possibility of detecting and monitoring the moisture concentration on‐line and in a non‐destructive way. © 2002 Society of Chemical Industry     

格构增强型复合材料夹层结构的制备与受力性能

真空导入成型工艺是一种新型的适合大型/异型复合材料结构件成型的技术.选用H-60 PVC泡沫、四轴向玻璃纤维布以及乙烯基酯树脂,通过在泡沫芯材上、下表面开槽,同时沿芯材厚度方向剖开,采用真空导入成型工艺制备出在结构上具有创新构型的格构增强型复合材料夹层结构.研究结果表明,真空导入成型工艺充模速度快、成型效益高;格构增强型复合材料夹层结构的剪切、平压与抗弯性能均较传统夹层结构得以提高;其格构腹板可有效抑制泡沫芯材剪切裂纹的扩展,避免面板与芯材的剥离破坏;阐明了格构增强型复合材料夹层结构的受弯极限承载能力.     

Experimental characterization and numerical simulation of the humidity absorption process in glass reinforced composites under dissymmetric exposure conditions

This article investigates the dissymmetrical water absorption process in E‐glass fiber reinforced polyester composites containing fillers and low profile additives. Two different cases of dissymmetrical exposures to water were considered. In the first case, only one side of the aged specimen was exposed to water at different temperatures, the other side was exposed to air. In the second case, both sides were exposed to water, but at different temperatures on each side. The moisture diffusivity and the maximum moisture content reached by the composites were determined using the gravimetric test method. In the first case of the dissymmetrical immersion, the temperature inside the aged specimen was found to be almost constant and the kinetic of the water diffusion was found to obey perfectly the one‐dimensional Fick's second law. In the second case, the specimen temperature varies from one side to the other, thus preventing the use of Fick's laws. Alternatively, the use of the finite element code ABAQUS provided an excellent agreement between the experimental and simulated results in both cases of dissymmetrical immersions, as well as in the case of symmetrical immersion where the two sides were exposed to the same environmental conditions. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

聚苯乙烯泡沫作为风电叶片夹芯材料的可行性研究

讨论了聚苯乙烯(PS)泡沫在风电叶片中的应用,通过对比研究聚氯乙烯(PVC)泡沫和PS泡沫本体以及泡沫玻璃钢夹芯结构的力学性能,通过有限元模拟夹芯结构悬臂梁在加载后发生的位移值来判断PS泡沫作为夹芯结构的芯材对整体刚度的影响,并通过实验测量和有限元模拟泡沫夹芯结构。结果显示,PS泡沫本体力学性能较PVC低;PS泡沫作为夹芯结构的芯材可以使用在叶片的剪切腹板中,在替代PVC泡沫使用在壳体蒙皮的夹芯层中,若要保证蒙皮的刚度不变则需要增加PS泡沫的厚度。     

Non‐fickian three‐dimensional hindered moisture absorption in polymeric composites: Model development and validation

A new, three‐dimensional, anisotropic non‐Fickian diffusion model is developed to characterize moisture absorption in polymeric composites. The new hindered diffusion model extends the classical Fickian theory to include the effects of the interaction of diffusing molecules with the chemical and physical structure of polymeric composites. The numerical solution of the hindered diffusion model is obtained for a three‐dimensional, anisotropic domain by using a forward‐time, centered‐space finite difference technique. The moisture weight gain over time predicted by the model is shown to mimic a wide variety of anomalous absorption behavior, often exhibited by anisotropic composite laminates. The accuracy of the numerical solutions is verified by comparing the results to known analytical solutions of a one‐dimensional, “Langmuir‐type” diffusion model and for the limiting case of the three‐dimensional Fickian model. The utility of the proposed hindered diffusion model is demonstrated by accurately recovering the absorption behavior of three different material systems reported in literature. First, it is shown that the hindered diffusion model can accurately predict the moisture absorption data for unidirectional glass‐reinforced epoxy plates of varying dimensions exposed to a relative humidity of 80%. Second, the one‐dimensional version of the model is applied to experimental moisture absorption data for isotropic epoxy resin samples of different thicknesses. Anomalous effects due to sample thickness reported in the original article are accurately captured. Third, the proposed model is shown to be more accurate than a two‐stage diffusion model applied to moisture absorption data obtained from a woven 3‐ply carbon fiber reinforced bismaleimide composite. POLYM. COMPOS., 34:1144–1157, 2013. © 2013 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.