Flexural properties of sandwich composite panels with glass laminate aluminum reinforced epoxy facesheets strengthened by SMA wires

Sandwich composite panels are widely used and significant in structural applications such as aerospace, shipbuilding and transportation, etc. This is due to their specific properties such as specific stiffness, strength and energy absorption. Still, many innovations are required to develop and upgrade their mechanical properties in various loadings and conditions, specifically in bending loads. One of the methods to enhance the properties of sandwich structures is to employ various advanced materials in these structures to change their acquired properties. In the present research work, sandwich composite panels made by fiber metal laminate like glass laminate aluminum reinforced epoxy (GLARE) as the facesheets and PVC polymer foam as the core material are investigated in flexural (bending) loading condition. To change or enhance the behaviour of sandwich panel in bending loads, shape memory alloy wires are also embedded in between glass fiber reinforced epoxy composite layers in fiber metal laminate facesheets. The shape memory wires are also pre-strained in fiber reinforced epoxy composite in sandwich panels. To study the flexural properties of sandwich panels with fiber metal laminate facesheets, the effect of shape memory alloy wires and also the effect of pre-straining of the wires, three types of sandwich panels are considered and made including panels without shape memory alloy wire, two wires with 0% tensile pre-strain, and two wires with 5% tensile pre-strain for the same cross section. Due to the importance of bending properties in structural applications, the sandwich composite specimens are subjected to flexural test according to ASTM standards. The maximum of 13% increase in maximum bending load and 84% increase in breaking load for the specimens with 0% pre-strained wires are achieved. Also, the maximum displacement and the energy absorption for the specimen with 5% pre-strain was enhanced by 26.5% and 37%, respectively. The energy absorption during the flexural test is greater in case of the specimen with pre-strained wires. Moreover, the specimens with pre-strained wires show better integrity of the structure after the failure in bending. The results represent the advantage effect of shape memory alloy wires on sandwich composite panel's behaviour in bending.     

Mechanical Properties of Regenerated Cellulose Fibres for Composites

Summary: A broad variety of regenerated cellulose fibres was subjected to single fibre tensile tests in order to determine the modulus of elasticity, tensile strength, and failure strain. The results were compared to glass fibres and flax fibres, which are considered the most important technical and natural fibres, respectively. With regard to their modulus of elasticity and tensile strength, regenerated cellulose fibres showed clearly lower values than glass fibres, even when their low density was taken into account. The average modulus of elasticity and tensile strength of regenerated cellulose fibres was also lower than the values measured for flax fibres, but when variability was considered, both fibres performed similarly. In terms of interfacial shear strength with polypropylene, lyocell fibres performed significantly less well than sized glass fibre and ramie fibre. The most important difference between regenerated cellulose fibres and both glass and flax fibres is their high failure strain and thus high work to fracture. The high work to fracture of regenerated cellulose fibres makes them particularly useful for composite applications where high fracture toughness is required.     

Effect of thermal cycling and open-hole size on mechanical properties of polymer matrix composites

This paper investigates the effects of thermal cycling on mechanical degradation of polymer matrix composites (PMCs). Un-notched and open-hole specimens are tested using developed thermal cycling apparatus and tensile test machine. In addition, the hole-size effect of open-hole tension glass/epoxy composite laminates is investigated. The tensile strength, mass loss and surface degradation of the specimens were obtained during 250 cycles. Experimental results showed that the holes diameter is the main parameter to control the thermal cycling effects on open hole structure. Also, it is found that laminates with smaller holes have higher tensile strength variation than those with larger holes. The results showed that increment of the hole diameter and number of cycles decreases the tensile strength.     

Determination of tensile strength of UHMWPE fiber-reinforced polymer composites

Quasi-static tensile test of UHMWPE fiber-reinforced composite laminate is challenging to perform due to low interlaminar shear strength and low coefficient of friction. Tensile tests proposed in the literature were conducted and limitations associated with each method led to the evolution of a new method. Tensile test of single-ply was realized as the best representative of tensile strength of a composite than tensile test of UHMWPE laminate. A fixture was developed for single-ply tests which increased friction and provided the mechanical constraint to slipping. The fixture is easy to fabricate and has provided repeatable results for eight grades of UHMWPE fiber-based (0/90) fabrics. Reported tensile strengths are in quite high range of 900–1500 MPa.     

Scaling effect on the tensile properties of [±45/0/±45/0/±45] Polypropylene/Twaron laminates

The effects of scaling on the mechanical response under tension of balanced nonsymmetrical laminates were investigated for a thermoplastic composite: Polypropylene reinforced with Twaron^® fibers. The composite baseline was an 8-ply laminate which consisted of unidirectional plies arranged in the sequence [±45/0/±45/0/±45]. The influence of specimen size on the tensile properties was studied for one (thickness), two (in-plane) and three (volume) dimensional scaling. The stress-strain curves suggested some variation in laminate behavior owing to the dimensional scaling; nevertheless, a further analysis with the classical lamination theory demonstrated that the observed effect was due to small variations in the fiber volume fraction of the laminates. It was concluded that the mechanical properties of these thermoplastic laminates do not exhibit scaling effects. The failure mechanism of the laminates was studied at macroscopic level; a scale effect of the fracture mechanism was observed.     

Short predicting the strength and toughness of fiber composites

Predictions of strength and toughness for short-fiber-reinforced plastic systems are complex but industrially crucial problems. In this contribution, we utilize a new approach which accounts for the large stress concentration penalties in a perfectly aligned short-fiber composite. Although empirical, the approach permits calculation of a strength reduction factor which can then be utilized with an appropriate failure criterion to calculate the strength of a wide range of short-fiber composite systems. A similar approach is used for the toughness problem, with the fracture toughness of an isotropic, random-in-a-plane, short-fiber composite being expressed in terms of the fracture toughness of single unidirectionally oriented plies with cracks oriented along the two orthotropic axes. For both strength and toughness, a laminate analogy approach is used. The short-fiber system is thought of as being composed of several plies or layers, each containing uniaxially aligned short fibers. The plies are oriented in the laminate to replicate the actual system fiber orientation distribution, and the linear stress-strain or fracture toughness properties are calculated by analyzing the individual ply responses to the overall applied stress. Comparison with experimental data for random-in-a-plane fiber orientation and for biased in-plane orientations at practical fiber volume loadings shows good agreement. In the case of fracture toughness, there was qualitative agreement between theory and experiment, but the data scatter precluded a rigorous comparison.     

Refined fixture design for effective essential work of fracture toughness characterization of m-LLDPE thin films

Characterization of Mode-I fracture toughness of ductile polymeric thin films is nontrivial. Proper specimen preparation and experimental procedures are required to ensure in-plane tensile loading. In this study, a custom-built double-edge notched tensile test fixture was employed to characterize the Mode-I essential work of fracture (EWF) toughness of metallocene linear low-density polyethylene (m-LLDPE) films. Effects of specimen geometry, strain rate and film orientation on the specific essential work of fracture, w_e, and the specific non-essential work of fracture, w_p, were investigated. Results indicate both EWF parameters are independent of the crosshead speed, gauge length (distance between upper and lower clamps) and specimen width within the ranges tested. w_e is significantly higher for thinner films and for crack propagation perpendicular to the blown film machine direction (MD). The usefulness of EWF for evaluating m-LLDPE fracture toughness is discussed.     

Liquid crystal polymers. I. Preparation and properties of p-hydroxybenzoic acid copolyesters

High molecular weight copolyesters were prepared by the acidolysis of poly(ethylene terephthalate) with p-acetoxybenzoic acid and polycondensation through the acetate and carboxyl groups. The mechanical properties of the injection-molded copolyesters containing 40–90 mole-% p-hydroxybenzoic acid (PHB) were highly anisotropic and dependent upon the PHB content, polyester molecular weight, injection-molding temperature, and specimen thickness. As the injection-molding temperature increased and the specimen thickness decreased, the tensile strength, stiffness, and Izod impact strength increased when measured along the direction of flow of the polymer melt, and the coefficient of thermal expansion was zero. In some compositions these properties were superior to those of commercial glass fiber reinforced polyesters. Maximum tensile strengths, flexural moduli, notched Izod impact strengths, and minimum melt viscosities were obtained with polyesters containing 60–70 mole-% PHB. Higher oxygen indicies (39–40) and heat deflection temperatures (150–220°C) were obtained with 80–90 mole-% PHB.     

Mechanical properties and morphology of intumescent flame retardant filled polypropylene composites

The intumescent flame retardant (IFR) filled polypropylene (PP) composites were prepared using a twin‐screw extruder. The tensile and impact fracture behavior of the composites were measured at room temperature. It was found that the Young's modulus increased roughly, while the tensile strength decreased slightly with increasing the IFR weight fraction; the toughening effect of the filler on the PP resin was significant. Both the V‐notched Izod impact strength and the V‐notched Charpy impact strength of the PP/IFR composites showed a nonlinear increase with increasing the filler weight fraction (φ_f) as φ_f was less than 20%, then it decreased. The limited oxygen index of the composites increases nonlinearly with increasing φ_f. The relationship between them obeyed a quadratic equation. The impact fracture surface was observed by means of a scanning electronic microscope to understand the toughening mechanisms for the composite systems. Copyright © 2014 John Wiley & Sons, Ltd.     

Cellulose man-made fibre reinforced polypropylene—correlations between fibre and composite properties

A series of viscose fibres from the tyre cord type varying in mechanical parameters and titre were compounded with polypropylene to produce fibre reinforced composites. Single fibre strength is analysed in detail and conclusions are drawn with respect to effective strength values in composite applications. Composites were analysed in terms of tensile and impact properties. Correlations between single fibre and composite properties are studied. High fibre elongation leads to favourable composite impact properties via high composite elongations at break. Using water as a plastisizing agent increasing fibre elongation, notched Charpy impact strength can be improved by more than 50%. Using a modified rule of mixtures and a shear lag model for the composite modulus it was shown how a titre reduction improves the composite stiffness by an increased interfacial area. A direct fibre-composite strength correlation was not found.     

Effect of ageing of sisal fibres on properties of sisal - Polypropylene composites

A composite laminate based on natural sisal fibre and polypropylene was prepared by compression moulding. The mechanical properties of the composite were assessed under tensile, flexural and impact loading. Changes in the stress-strain characteristics, yield stress, tensile strength, and tensile (Young's) modulus, due to ageing have been analysed. Important findings with the fresh and aged fibres and their behaviour in composites have been reported and analysed.     

Effect of molecular orientation on the fracture behavior of carbon black-filled natural rubber compounds

The fracture behavior of carbon black-filled natural rubber compounds, differing in filler content, was studied performing tensile tests in biaxial loading conditions, using a central notched cross-shaped specimen. The test consisted of two steps: a drawing step was initially performed loading the specimen in the direction parallel to the notch plane, up to different draw ratios, and then the specimen was loaded in the direction normal to the notch plane up to fracture. Using a fracture mechanics approach, the fracture toughness was evaluated as a function of the draw ratio applied in the drawing step. A correlation between the fracture phenomenology observed and molecular orientability and orientation was attempted. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1509–1515, 2010     

环氧树脂/液晶聚合物体系的形态、力学性能和热稳定性

合成了一种端基含有活性基团的热致性液晶聚合物 (LCPU) ,用其改性环氧树脂CYD 12 8 4 ,4′ 二氨基二苯砜 (DDS)固化体系 ,对改性体系的冲击性能、拉伸性能、弯曲性能、弹性模量、断裂伸长率、玻璃化转变温度Tg、热失重温度TG与LCPU含量的关系进行了探讨 ,对不同种类液晶化合物改性CYD 12 8 DDS体系效果进行了比较 ,用扫描电镜 (SEM)研究了材料断面的形态结构 .结果表明 ,LCPU的加入可以使固化物的力学性能和热稳定性提高 ,改性后材料断裂面的形态逐渐呈现韧性断裂特征     

Revealing toughening mechanism for alternating multilayered polypropylene/poly(ethylene-co-octene) sheets

Alternating multilayered (16, 32, and 128 layers) polypropylene/poly(ethylene-co-octene) (PP/POE) sheets were prepared via multilayer co-extrusion. Impact (−20°C), tensile, and dynamic rheological tests were carried out on the prepared multilayered samples. The toughening mechanism for the multilayered samples was revealed via investigating their fracture surface morphology and analyzing the work of fracture. The results showed that the impact energy for the notched multilayered samples is mainly absorbed by the continuous thin ductile POE layers due to cavitation, whereas for the unnotched multilayered samples the continuous thin POE and PP layers both contributed significantly to the total work of fracture due to multiple crazing, cracking and distinct plastic deformation in both PP and POE layers, and thus significantly extended crack propagation paths. Hence, the multilayered samples possessed much higher unnotched impact strength than notched impact strength. Moreover, the multilayered samples exhibited slightly increased notched impact strength and obviously increased unnotched impact strength with increasing layer number. Interestingly, the multilayered samples exhibited lower notch sensitivity than the PP sample. In addition to significantly improved low temperature impact toughness, the multilayered samples maintained the strength and stiffness as well as having superior extensibility to that of the PP sample.     

Viscoelastic response and interlaminar delamination resistance of epoxy/glass fiber/functionalized graphene oxide multi-scale composites

Graphene oxide (GO) was functionalized using three different diamines, namely ethylenediamine (EDA), 4,4′-diaminodiphenyl sulfone (DDS) and p-phenylenediamine (PPD) to reinforce an epoxy/glass fiber (EP/GF) composite laminate, with the aim of improving the overall composite mechanical performance. Different mechanical characterization techniques were used to determine the mechanical performance, including: tensile stress strain, double cantilever beam (DCB) mode-I fracture toughness and dynamic mechanical thermal analysis (DMTA). Scanning electron microscopy (SEM) was used to support the results and conclusions. The results demonstrated remarkable enhancements in the mechanical performance of EP/GF composite laminates by incorporation of functionalized graphene oxide (FGO) nanofiller, whilst the mechanical performance of the GO reinforced composite only improved marginally. Finally, the mechanical performance of the EP/GF/FGO multi-scale composites was found to be dependent on the type of FGO functional groups; of which EDA exhibited the highest performance. These investigations confirmed that the EDA-FGO-reinforced EP/GF composites possess excellent potential to be used as multifunctional engineering materials in industrial applications.     

Self‐reinforced polypropylene composites based on low‐cost commercial woven and non‐woven fabrics

In the present paper, different self‐reinforced polypropylene (PP) composites based on low‐cost commercial woven (w) and non‐woven (nw) fabrics were obtained. Hot compaction (HC) and film stacking (FS) followed by compression molding were used to prepared the composites. The fracture and failure behavior of the different materials was determined under different testing conditions through quasi‐static uniaxial tensile tests, Izod impact experiments and by means of fracture mechanics tests on mode I double‐edge deeply notched tensile specimens. In the case of the composite obtained by film stacking + compression molding (rPP/nw/w‐FS) and the hot‐compacted composite (nw/w‐HC) containing simultaneously woven and non‐woven fabrics, the acoustic emission technique was applied in situ in the tensile tests to determine their consolidation quality and to identify the failure mechanisms responsible for their fracture behavior. It was observed that both composites exhibited relatively similar high consolidation quality. However, the hot‐compacted composite presented a more uniform distribution of failure mechanisms (debonding and fiber fracture) than the film‐stacked composite. The hot‐compacted composite containing both types of reinforcements exhibited the best combination of mechanical (tensile, impact, and fracture) properties. Therefore, this composite appeared as the most promising for structural applications among the different composites investigated.     

Effects of raster angle on the mechanical properties of PLA and Al/PLA composite part produced by fused deposition modeling

This paper studies the mechanical properties of polylactic acid (PLA) and aluminum fiber–reinforced PLA composite (Al/PLA) specimens fabricated by fused deposition modeling (FDM) process. The effect of raster angle (0°, 90°, 45°, 0°/90°, and ± 45°) on dynamic mechanical thermal property and tensile property of FDM‐printed PLA and Al/PLA has been studied. The results show reduced tensile strength and Young's modulus in Al/PLA composite specimen in comparison with pure PLA specimen. However, the elongation‐at‐break increases, which is due to Al fiber with the higher elasticity and lower tensile strength than PLA. The addition of Al fibers improves the dynamic mechanical thermal property of pure PLA because of the good interaction of the PLA matrix with the surrounding Al fibers. Raster angle plays an important role in FDM process. All specimens printed with 0° raster angle show highest tensile strength and dynamic mechanical properties, while specimens printed with 90° raster angle have the lowest values. Fractured surfaces indicate that the failure of the specimen with 0° raster angle is due to breaking of individual layers, while for 90° raster angle, specimen fails under separation of the adjacent raster layers.     

Mechanical property evaluation of glass–jute fiber reinforced polymer composites

The natural fibers such as jute, coir, hemp, sisal etc. are randomly used as reinforcements for composite materials because of its various advantages such as low cost, low densities, low energy consumption over conventional fibers. In addition, they are renewable as well as biodegradable, and indeed wide varieties of fibers are locally available. In this study, glass–jute fiber reinforced polymer composite is fabricated, and the mechanical properties such as tensile, flexural and impact behavior are investigated. The materials selected for the studies were jute fiber and glass fiber as the reinforcement and epoxy resin as the matrix. The hand lay‐out technique was used to fabricate these composites. Fractured surface were comprehensively examined in scanning electron microscope (SEM) to determine the microscopic fracture mode. A numerical procedure based on the finite element method was then applied to evaluate the overall behavior of this composite using the experimentally applied load. Results showed that by incorporating the optimum amount of jute fibers, the overall strength of glass fiber reinforced composite can be increased and cost saving of more than 30% can be achieved. It can thus be inferred that jute fiber can be a very potential candidate in making of composites, especially for partial replacement of high‐cost glass fibers for low load bearing applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Ultimate tensile behavior of linear polyethylene solids

The influence of the elongation rate and temperature on the ultimate tensile properties of melt‐crystallized linear polyethylene solids was investigated, with a double‐edge‐notched specimen to avoid necking, in which uniform deformation could be assumed throughout the experiment. The data on ultimate properties such as the tensile strength and elongation at break for different temperatures could be superimposed, by shifts along the elongation rate axis, to give a master curve as a function of the time to rupture. The shift factors obtained from the superpositioning of both the tensile strength and ultimate strain took the form of the Williams–Landel–Ferry equation. As a result, the ultimate data provided a failure envelope curve that made it possible to predict rupture times when the tensile tests were conducted under any experimental conditions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2018–2026, 2002     

Effect of Al2O3 nanoparticles on the steel-glass/epoxy composite joint bonded by a two-component structural acrylic adhesive

This paper reports a study on the effect of Al₂O₃ nanoparticles on the adhesion strength of steel-glass/epoxy composite joints bonded by a two-component structural acrylic adhesive. The addition of Al₂O₃ nanoparticles to the two-component acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strength of the adhesive joints increased by addition of Al₂O₃ up to 1.5 wt%, which decreased by further addition of the nanofiller. Introduction of the nanoparticles caused a reduction in the peel strength of the joints. DSC analysis revealed that the glass transition temperature (Tg) of the adhesives rose by increasing the nanofiller content. The advancing water contact angle was decreased for adhesives containing nanoparticles. SEM micrographs indicated good dispersions of the Al₂O₃ nanoparticles within the acrylic matrix in the specimens with up to 1.5 wt% Al₂O₃ and revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.