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     

Post curing effect of poly epoxy on visco‐elastic and mechanical properties of different sandwich structures

Poly epoxy is a high performance room temperature cured epoxy system which provides excellent physical and mechanical properties. However, the effects of post curing of this resin system on the properties of different sandwich structures are unknown. This study aims to evaluate the effect of post curing (at 70°C for 2 hr) on the edgewise compressive and flexural strengths of a sandwich structure, constructed with Styrofoam and honeycomb as core materials and a plain weave carbon fabric as face sheet. Tested factors evaluated from edgewise compressive tests were as follows: peak load, compressive strength, and crash energy absorption of sandwich structures while core shear stress and bending stress of sandwich structures were determined and compared with flexural tests. It was observed that post curing affects significantly on the bending and compressive strengths of the sandwich structures. However, the data obtained for crash energy absorption suggested that the effect of post curing on the core shear strength and the total deflection was statistically insignificant. The matrix polymer was also inspected using dynamic‐mechanical thermal analysis to assess the changes in glass transition temperature and degree of conversion due to post cure. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Flexural behavior of methyl methacrylate modified unsaturated polyester polymer concrete beams reinforced with glass‐fiber‐reinforced polymer sheets

This study represents the behavior of flexural test of methyl methacrylate modified unsaturated polyester polymer concrete beam reinforced with glass‐fiber‐reinforced polymer (GFRP) sheets. The failure mode, load–deflection, ductility index, and separation load predictions according to the GFRP reinforcement thickness were tested and analyzed. The failure mode was found to occur at the bonded surface of the specimen with 10 layers of GFRP reinforcement. For the load–deflection curve, as the reinforcement thickness of the GFRP sheet increased, the crack load and ultimate load greatly increased, and the ductility index was found to be the highest for the beam with the thickness of the GFRP sheet at 10 layers (6 mm) or 13 layers (7.3 mm). The calculated results of separation load were found to match only the experimental results of the specimens where debonding occurred. The reinforcement effect was found to be most excellent in the polymer concrete with 10 layers of GFRP sheet reinforcement. The appropriate reinforcement ratio for the GFRP concrete beam suggested by this study was a fiber‐reinforced‐plastic cross‐sectional ratio of 0.007–0.008 for a polymer concrete cross‐sectional ratio of 1 (width) : 1.5 (depth). © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A study on the anchoring orientations of foam and sandwich composites with metal

Metallic anchors are used as the load transfer components for foam and sandwich composites when they are used as structural elements in design applications. The traditional method of fixation of these components is by gluing and fastening. The anchors are in the form of inserts and are imbedded in the foam during the foaming process. In this study, flexural testing was conducted on different metal anchor/foam configurations to establish typical interaction trends. The load‐deflection response, mode of failure, and fracture stresses of the structures were elucidated. Tests were conducted on foam and sandwich composites having rectangular, cylindrical, and taper geometries with different lengths. Leaf inserts were designed, manufactured, and tested inside foam and sandwich composites. Comparisons between the taper and leaf inserts are presented. Leaf inserts had better results compared with taper inserts in terms of adhesion and failure stresses. Finite element analysis (FEA) of the interactions between the inserts and the foam and sandwich composites under different loads was carried out. The FEA modeling results were very similar to the experimental results, thus validating the model. Simulations were also run with foam and sandwich composites with closeout configuration. Foam–sandwich with inserts had better adhesion and load‐bearing properties compared with same structures with closeouts. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Some mechanical properties of rigid polyurethane structural foam

The mechanical response of integral-skin rigid polyurethane foam, with an average density of 300 to 700 kg/m³, to constant rate and creep loading was determined. Sandwich specimens were modeled by layers of a core material and two skins, whose secant moduli had been determined experimentally by separate tests and approximated by linear functions of the density. The effective rigidities of the sandwich in tension and flexure were calculated and compared favorably to experimental measurements. The sandwich structure improved the flexural rigidity of homogeneous foam by a factor of more than 2.20. Tensile creep tests of sandwich specimens at relatively low stress levels (up to about 38 percent of their strength) showed that the creep was nonlinear, but a single creep curve could represent creep of specimens of various densities, provided the relative load on them was the same. A limited number of flexural creep tests led to similar conclusions, but the creep rate was smaller than in tension. Results from torsion tests of core material, compressive tests of sandwich specimens, and tension and compression tests of nonskin rigid foam are included in this article.     

Hygrothermal degradation of the composite laminates from woven carbon/SC‐15 epoxy resin and woven glass/SC‐15 epoxy resin

The degradation mechanism for hygrothermal aging of woven carbon‐epoxy and woven glass‐epoxy composite laminates was investigated in the micro‐scale. Interlaminar shear and cross laminar flexural tests were performed on notched and unnotched specimens to know the mechanical performance of the composite laminates. The Interlaminar Shear Stress (ISS) for both the composites was also evaluated and correlated with the number of hygrothermal cycles. Four‐point bending and tensile or compression shear loading configurations were also used. The stress at the onset of delamination (Delamination Damage Tolerance, DDT) was identified from the load‐deflection curve of the flexural specimens and correlated with the number of hygrothermal cycles. It was found that both the ISS and DDT decrease with the exposure time. Dimensional stability was almost unchanged throughout the aging process, although there was a very little moisture absorption (∼1.3%) in glass‐epoxy and carbon‐epoxy composite laminates. SEM photomicrographs of the delaminated surface show that failure occurs suddenly in a macroscopically brittle mode by crack initiation and propagation method. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.     

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。     

Sandwich structures with composite inserts: Experimental studies

Experimental studies are presented on the performance of insert assemblies of the sandwich structures under localized through‐the‐thickness compressive loading. Through‐the‐thickness and partially inserted fully potted inserts are studied. Insert materials considered are: aluminum and 3D woven composite. Experimental results are compared with the analytical predictions. It is observed that the specific strength of 3D woven composite inserts is more than that of aluminum inserts. Further, it is observed that the specific strength of through‐the‐thickness inserts is more than that of partially inserted fully potted inserts. Delamination between upper face plate and core material and sliding of attachment/insert within the core are the main modes of failure initiation. Quantitative results are presented for typical cases. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Behavior of sandwich beams with functionally graded rubber core in three point bending

The three‐point bending behavior of sandwich beams made up of jute epoxy skins and piecewise linear functionally graded (FG) rubber core reinforced with fly ash filler is investigated. This work studies the influence of the parameters such as weight fraction of fly ash, core to thickness ratio, and orientation of jute on specific bending modulus and strength. The load displacement response of the sandwich is traced to evaluate the specific modulus and strength. FG core samples are prepared by using conventional casting technique and sandwich by hand layup. Presence of gradation is quantified experimentally. Results of bending test indicate that specific modulus and strength are primarily governed by filler content and core to sandwich thickness ratio. FG sandwiches with different gradation configurations (uniform, linear, and piecewise linear) are modeled using finite element analysis (ANSYS 5.4) to evaluate specific strength which is subsequently compared with the experimental results and the best gradation configuration is presented. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

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.     

Effects of alkaline and silane treatments on the water‐resistance properties of wood‐fiber‐reinforced recycled plastic composites

The water‐resistance properties of wood‐fiber‐reinforced recycled plastic composites (WRPCs) prepared from postconsumer high‐density polyethylene (HDPE) and wood fibers from saw mills were studied. Three methods consisting of an alkaline method (AM), a silane method (SM), and a combination of the alkaline and silane methods (ASM) were used to modify the wood fibers. The effects of fiber/matrix mix ratio and surface treatment on the moisture content, thickness swelling, and flexural strength change of the WRPCs, before and after immersion in 60°C water for 8 weeks, were studied and analyzed. The flexural fractured surfaces of the WRPCs before and after immersion in hot water were examined, and the fracture mechanism of the WRPCs was discussed. The results showed that the different surface treatments of the wood fibers had significant effects on the moisture content, thickness swelling, and flexural strength of the WRPCs after a long immersion time in hot water. For WRPCs treated by ASM, the moisture content was the lowest, the thickness swelling was at a minimum, and the flexural strength was the highest. Higher water absorption of composites with fiber treated by the AM or SM methods, as compared to those treated by ASM, could be attributed to the incomplete adhesion and wettability between the wood fibers and the polymer matrix, which may have caused more gaps and flaws at the interface. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers.     

Interlaminar shear test by hybrid sandwich specimen under distributed load

A new method is proposed for the determination of the interlaminar shear strength of composites. The method is particularly pertinent to composites of high interlaminar shear strengths, where the ratio of tensile (compressive) strength to shear strength is relatively low. In such materials, including unidirectional composites with improved fiber/matrix bond strength and angle-ply laminates, an analysis based on a short beam interlaminar shear test is highly problematic and may, in fact, be erroneous. The test method is based on the use of a sandwich composite structure with a core made of layers of the tested composite and skins made of an elastic, strong unidirectional composite. A proper design procedure determines the choice of the skin material and of the relative thicknesses, so that flexural testing under distributed load leads to the intended core failure in shear. Calculations of the stress profile in a hybrid sandwich beam in bending and of the stress ratios under distributed load are presented. Also presented are experimental results recorded with sandwich hybrids made of unidirectional carbon-fiber-reinforced epoxy skins and a ±θ aramid-fiber-reinforced epoxy angle-ply core.     

熔融沉积成型木塑复合夹层板的弯曲性能

采用熔融沉积成型(FDM)制造方法,以木塑复合线材为原料,利用3D打印软件Ultimaker Cura的“填充结构”功能设计网格、直线、三角形等13种芯层结构(二维6种、立体7种),并将其与纸板粘接得到木塑复合夹层板。利用三点弯曲测试,研究不同夹层板的破坏失效形式与弯曲性能。结果表明:木塑夹层板的失效模式主要有弹性变形、面板起皱、芯子剪切和芯子压溃。在13种芯层结构中,立体的同心3D芯层结构夹层板弯曲性能最佳,弯曲模量和静曲强度分别为159.56 MPa和4.85 MPa,分别是网格芯层结构夹层板的5.4倍和2.3倍,具有较强的抗弯曲变形能力,适合于设计制造轻质高强度制品。     

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     

剪切对泡沫夹层结构梁弯曲性能的影响

本文以受剪后横截面仍为一平面但与轴线不再垂直为基本假设,采用能量法建立了一种对泡沫夹层结构梁的弯曲性能进行分析的方法。通过对比试验数据以及有限元的计算结果,得到用该方法可较为准确地预测泡沫夹层结构梁的挠度。通过分析,得到了剪切对泡沫夹层结构梁挠度的影响程度随着梁的跨高比的增大而减小,同时讨论了梁横截面正应变及正应力的分布情况。     

Mechanical properties of honeycomb prepared from aromatic polyimide film

Aromatic polyimides have many advantages such as low thermal expansion coefficient, good electrical insulation, and self-extinguishing properties. We tried to use polyimide film for honeycomb structure. Polymide honeycomb core and sandwich panel were prepared from polyimide film, i.e., UPILEX R, and adhesive by the expansion method. Mechanical properties, i.e., compressive, crushed, shear, and flexural strengths, were evaluated for this core and panel. Compressive and crushed properties increased largely with the density of the honeycomb, whereas shear and midspan flexural properties did not vary so much with the density, because these failures occured in the adhered interface. Strong adhesion is required for improving the latter properties. © 1993 John Wiley & Sons, Inc.     

Flexural behavior of fiber and nanoparticle reinforced concrete at high temperatures

In this study, the flexural tests were conducted to investigate the effects of temperature, steel fiber, nano‐SiO₂, and nano‐CaCO₃ on flexural behavior of concrete at high temperatures. The load‐deflection curves of fiber and nanoparticle reinforced concrete (FNRC) were measured both at room and high temperatures. Test results show that the load‐deflection curves become flatter, and the flexural strength, peak deflection, and energy absorption capacity decrease seriously with the increase of temperature. Both steel fiber and nanoparticles could significantly improve the flexural behavior of the concrete at room and high temperatures. The energy absorption capacity of FNRC before the peak point increases with the increase of steel fiber volume fraction. The improvement of nano‐SiO₂ on flexural strength of FNRC at high temperature is better than that at room temperature, but the enhancement on energy absorption capacity is reverse. Nano‐SiO₂ is more effective than nano‐CaCO₃ in improving flexural behavior of concrete both at room and high temperatures.     

Effect of wheat straw ash on mechanical properties of autoclaved mortar

This study investigates the effect of wheat straw ash (WSA) on the mechanical strength of autoclaved mortar. The mechanical properties studied include compressive, tensile, and flexural strengths of mortar. Mortar mixes were prepared using natural silica, wadi (local sand), and crushed limestone fine aggregates at a w/c ratio of 0.6. Mortar specimens were exposed to autoclave for 2.5 h at a pressure of 2 MPa. Three percentages of WSA replacement levels (3.6%, 7.3%, and 10.9%) by weight of sand were utilized in the study. The study showed that the replacement of sand by WSA increases the mechanical strength of autoclaved mortar. Mortar specimens containing limestone aggregate with 10.9% WSA replacement level showed an average increase in compressive, tensile, and flexural strength by 87%, 67%, and 71%, respectively, compared to control mortar specimens. Scanning electron micrographs for autoclaved paste specimens containing 7.3% WSA replacement level revealed a more packed structure compared to control paste specimens.     

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.     

Study on mechanical properties and material distribution of sandwich plaques molded by co‐injection

In the sandwich injection molding process (co‐injection), two different polymer melts are sequentially injected into a mold to form a part with a skin/core structure. Sandwich molding can be used for recycling, improving barrier and electrical properties, or producing parts with tailored mechanical properties. In this study the evaluation of flexural modulus and impact strength of co‐injected plaques have been investigated. Virgin and short glass fiber reinforced (10 and 40%) polypropylene were used in six different combinations of sandwiched layers. The skin and core thicknesses were measured by optical microscopy and used to calculate the theoretical flexural modulus, which was compared to the experimentally measured modulus. Fiber orientation states were also observed by scanning electronic microscopy (SEM) at some specific locations and their effect on mechanical properties discussed. The experimental results indicate that an important improvement in transverse modulus, near the gate, is obtained when the virgin polypropylene (PP) is used as a skin and 40% short glass fiber polypropylene (PP40) as core. When both skin and core are made of PP40, the flexural moduli are slightly higher than conventionally injected PP40. POLYM. COMPOS. 26:265–275, 2005. © 2005 Society of Plastics Engineers.