Investigations on the compressive behavior of 3D random fibrous materials at elevated temperatures

By means of the experimental method, micromechanical model and Finite Element Method (FEM), this paper studied the compressive behaviors of the three-dimensional random fibrous (3D RF) material in the through-the-thickness (TTT) and in-plane (IP) directions at elevated temperatures. The compressive experiments showed that the fracture strength and Young's modulus of the 3D RF material in the TTT and IP directions decrease as increasing temperature. The specimens fracture through breaking the fibers under the bending deformation, while almost all the bonding zones keep intact. A simple micromechanical model and a FEM model are developed to simulate the mechanical properties of the 3D RF material. The micromechanical model ignores the randomness of the fibers, while in the FEM model special attention is drawn to the influence of the morphological characteristic. Numerical results from the micromechanical model and FEM model agree well with the observations from the compressive experiments.     

Fracture toughness of random fibrous materials with ultrahigh porosity at elevated temperatures

The fracture toughness of three‐dimensional random fibrous (3D RF) material was investigated from room temperature to 1273 K by virtue of experimental method, theoretical model and Finite Element Method (FEM) in the through‐the‐thickness (TTT) and in‐plane (IP) directions. The experiments showed that the fracture toughness in the TTT and IP directions increases (from 0.0617 to 0.0924 Mpa·m^1/2 and from 0.2958 to 0.3982 Mpa·m^1/2 for the TTT and IP directions, respectively) as the temperature until reaching a transition temperature (1123 K and 1223 K for the TTT and IP directions, respectively), then the fracture toughness decreases from 0.0924 to 0.0393 Mpa·m^1/2 and from 0.3982 to 0.3106 Mpa·m^1/2 for the TTT and IP directions, respectively. The fracture behavior was related to the bulk microstructures, the mechanical properties of fibers and the blunting of crack tip. The crack tip blunting affected the fracture toughness at elevated temperatures which was verified using the theoretical model. A FEM model with a single edge crack where special attention was drawn to the influence of the morphological characteristic was developed to simulate the fracture behavior of 3D RF material. Numerical results from the FEM modeling along with a theoretical model with crack tip blunting mechanism incorporated agreed well with the experimental results.     

Temperature effect on dynamic compression performance of random fibrous composites in the TTT and IP directions

This study examines the effect of temperature on the dynamic compressive performance of random fibrous (RF) composites at temperatures up to 1273 K in the through-the-thickness (TTT) and the in-plane (IP) directions, using an improved high-temperature split Hopkinson pressure bar (SHPB) system. The results revealed that in the IP direction, the RF composite presented a shear fracture mode below 1073 K and initiated multiple major cracks in the specimens at 1273 K. However, the composite showed a layered fracture mode in the TTT direction from 288 to 1273 K. The dynamic strength in both directions showed a consistent trend when observed under static loading below the critical temperature. The change in the strain-rate sensitivity (SRS) of the dynamic strength was insignificant for temperatures below the transition temperature of viscous-flow and brittle deformation of the RF composite. However, above the transition temperature, the SRS of the dynamic strength became significant.     

Flexural properties of S‐2 glass and TR30S carbon fiber‐reinforced epoxy hybrid composites

A study on the flexural properties of hybrid composites reinforced by S‐2 glass and TR30S carbon fibers is presented in this article. Test specimens were made by the hand lay‐up process in an intraply configuration with varying numbers of glass/epoxy laminas substituted for carbon/epoxy laminas. These specimens were then tested in the three point bend configuration in accordance with ASTM D790‐07 at a span to depth ratio of 32. The failed specimens were examined under an optical microscope, and the results show that the dominant failure mode is at the compressive side. The flexural behavior was also simulated by finite element analysis (FEA). Based on the FEA results, the flexural modulus and flexural strength were calculated. Good agreement is found between the experiments and FEA. It is shown that flexural modulus decreases with increasing percentage of S‐2 glass fibers, positive hybrid effects exist by substituting carbon fibers for glass fibers, and applying a thin layer of S‐2 glass fiber‐reinforced polymer on the compressive surface yields the highest flexural strength. The modeling approach presented will pave a way to the effective design of hybrid composites. POLYM. COMPOS., © 2012 Society of Plastics Engineers     

三维机织复合材料单胞模型各向异性的有限元分析

使用三维绘图软件Pro/E绘制出三维浅交弯联机织复合材料数字化结构模型,借助大型有限元分析软件ANSYS模拟单胞模型承受不同方向压缩载荷作用下的力学性能。探究在不同方向的压缩载荷作用下复合材料单胞模型的应力分布情况,并借此分析复合材料单胞模型的各向性能;以承受X方向压缩载荷的单胞模型为例,分析复合材料中纤维与树脂的受力情况。结果表明:三维机织复合材料受到压缩载荷时,表现出明显的各向异性,表现为X方向压缩性能最好,Z方向压缩性能最差;纤维作为主要承载体,承担较多载荷作用,树脂作为次要承载体,承担较少载荷作用。     

Bioactive monetite‐containing whisker‐like fibers reinforced chitosan scaffolds

The aim of this work was to develop bioactive chitosan scaffolds reinforced with monetite‐containing whisker‐like fibers. The fibers synthesized by homogeneous precipitation were characterized as monetite/hydroxyapatite short fibers (MAFs), using XRD, FTIR and SEM. The pure chitosan and MAFs/chitosan composite scaffolds were produced by freeze‐drying, and characterized with respect to porosity, pore size, swelling behavior, compressive strength and modulus, and in vitro bioactivity. The incorporation of MAFs in chitosan matrices led to increase the pore size, according to the evaluation by FE‐SEM, and decrease the porosity of composite scaffolds. The swelling ratio decreased as MAFs content of scaffolds increased. The compressive strength and modulus of scaffolds were improved by an increase in MAFs content. The noncross‐linked scaffolds with a chitosan: MAFs weight ratio of 1:1 (CW3) showed a porosity of 75.5%, and the strength and modulus of 259 kPa and 2.8 MPa in dry state, respectively. The crosslinking by glutaraldehyde resulted in improved mechanical properties. The strength and modulus of cross‐linked CW3 scaffolds in wet state reached to 345 kPa and 1.8 MPa, respectively. The in vitro bioactivity of the reinforced scaffolds, evaluated by FE‐SEM/EDS, XRD, and ATR‐FTIR, was confirmed by the formation of a carbonated apatite layer on their surfaces when they soaked in simulated body fluid (SBF). The results of this initial study indicate that the monetite‐containing whisker‐like fibers may be an appropriate reinforcement of chitosan scaffolds.     

Morphology development of high‐density rigid polyurethane foam upon compression by on‐line scanning electronic microscope

The high‐density rigid polyurethane foam (RPUF) was obtained by airtight cast molding. The morphology development during the compressive fracture process of RPUF was observed on‐line by a scanning electronic microscope (SEM). In the early stage of loading, the deformation of samples came from the tiny shape change in the cells' windows. As the load increases, some creases were formed in some cell windows in the vertical direction of loading, and the creases enlarged and resulted in the cracks across the whole cells. Moreover, during the deformation process, a failure band was formed in the weakest position. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Compressive test and numerical simulation of center‐notched composite laminates with different crack configurations

Experimental and computational studies of the composite laminates with thin center notches under axial compressive loading are carried out. A series of compressive testing of the composites with different crack lengths and angles between the loading vector and 0° fiber direction were conducted. The damage mechanisms as well as load–displacement curves are obtained from the test to analyze the effects of crack dimensions on stress distribution and ultimate load. It was shown that the compressive strength of composites drastically reduces when the crack angle goes from 0° to 90°. By studying the fracture surfaces of the tested specimens, all initial cracks within the laminates are found to extend without a straight crack path until fibers fracture simultaneously. Cases that involve crack propagation are modeled for different crack dimensions with a 3D progressive damage finite element analysis using the Abaqus. Numerical simulations qualitatively reproduce the general observations made in the laboratory experiments. POLYM. COMPOS., 38:2631–2641, 2017. © 2015 Society of Plastics Engineers     

Deformation kinetics of pH‐sensitive hydrogels

Polymeric gels can undergo large deformation when subjected to external solutions of varying pH. It is imperative to understand the deformation process of pH‐sensitive hydrogels for the effective application of these attractive materials in the biomedical and microfluidic fields. In the modeling of these multi‐phase materials, finite element (FE) modeling is a useful tool for the development of future applications, and it allows developers to test a wide variety of material responses in a cost‐effective and efficient manner, reducing the need to conduct extensive laboratory experiments. Although a FE user‐defined material model is available for the equilibrium state, the transient response of pH‐sensitive gels has not been effectively modeled. Based on our recent work using the heat transfer analogy to tap into the readily available coupled temperature–displacement elements available in the commercial FE software ABAQUS for simulation of the transient swelling process of neutral hydrogels, the transient swelling process of a pH‐sensitive hydrogel is studied and a FE model is further developed to simulate the transient phenomena. Some benchmark examples are investigated to demonstrate the model's capabilities in the simulation of nonlinear deformation kinetics relevant to several applications of pH‐sensitive hydrogels. © 2013 Society of Chemical Industry     

Ion‐Exchanged Lithium Aluminosilicate Glass: Strength and Dynamic Fatigue

Sodium for lithium and potassium for lithium ion‐exchanges of a lithium aluminosilicate glass were conducted and the resulting strength and dynamic fatigue characteristics were studied. Four‐point bend mechanical tests revealed that greater strengthening can be achieved by the potassium for lithium ion‐exchange, compared to the sodium for lithium ion‐exchange, and that the dynamic fatigue tendency is strongly suppressed by both exchanges. This suppression of dynamic fatigue characteristics of ion‐exchange strengthened glass was explained by the ability of the surface compressive layer to delay the onset of slow crack growth. Bulk stresses continue to increase in magnitude while the crack is arrested in the surface compressive stress region. Upon offsetting the surface compressive stress, the crack rapidly propagates into a high‐magnitude tensile stress field until the fracture toughness is reached, resulting in minimal crack growth prior to material failure. A slow crack growth model utilizing a fracture mechanics weight function was developed to simulate the experiments. Dynamic fatigue characteristics of the as‐received glass, without ion‐exchange treatment, were also measured and simulated for comparison.     

Influences of loading rate and preloading on the mechanical properties of dry elasto‐plastic granules under compression

To ensure high quality of granular products post‐industrial operations, it is necessary to precisely define their micro–macro mechanical properties. However, such an endeavor is arduous, owing to their highly inhomogeneous, anisotropic and history‐dependent nature. In this article, we present the distributed granular micromechanical and macromechanical, energetic and breakage characteristics using statistical distributions. We describe the material behavior of elastoplastic zeolite 4AK granules under uniaxial compressive loading until primary breakage, and localized cyclic loading up to different maximum force levels, at different displacement‐controlled loading rates. The observed force‐displacement behavior had been approximated and further evaluated using well‐known contact models. The results provide the basis for a detailed analysis of the viscous behavior of zeolite 4AK granules in the moist and wet states, indicating that higher compressive loads are required at higher displacement‐controlled loading rates to realize equivalent deformation and breakage probability achieved by loads at lower displacement‐controlled loading rates. © 2014 American Institute of Chemical Engineers AIChE J 60: 4037–4050, 2014     

Experimental study on the mechanical behavior and failure mechanism of 3d MWK carbon/epoxy composites under quasi‐static loading

Quasi‐static tensile, out‐of compression, in‐plane compression, three‐point‐bending and shear tests were conducted to reveal the mechanical behavior and failure mechanisms of three‐dimensional (3D) multiaxial warp‐knitted (MWK) carbon/epoxy composites. The characterization of the failure process and deformation analysis is supported by high‐speed camera system and Digital Image Correlation. The results show that tensile, bending, out‐of‐plane compression, in‐plane compression stress–strain response exhibit obvious linear elastic feature and brittle fracture characteristics, whereas the shear response exhibits a distinct nonlinear behavior and gradual damage process. Meanwhile, 3D MWK carbon/epoxy composites have good mechanical properties, which can be widely used in the fields of engineering. In addition, the failure for tension behaves as interlayer delaminating, 90/+45/−45° interface debonding and tensile breakage of 0° fibers; the damage for out‐of‐plane compression is mainly interlaminar shear dislocation together with local buckling and shear fracture of fibers; the failure pattern for in‐plane compression is 90° fiber separating along fiber/matrix interface as well as 0/+45/−45° fiber shear fracture in the shear plane. The main failure for bending is fiber/matrix interface debonding and fibers tearing on the compression surface, 0° fibers breakage on the tension surface as well as fiber layers delaminating. Although the shear behavior is characterized by a gradually growing shear matrix damage, 90/+45/−45° interface debonding, +45/−45° fibers shear fracture, and final 0° fiber compression failure. POLYM. COMPOS., 37:3486–3498, 2016. © 2015 Society of Plastics Engineers     

Antibacterial functionalization of cotton fabrics by electric‐beam irradiation

An N‐halamine precursor monomer, 2,2,6,6‐tetramethylpiperidinyl acrylate (TMPA), was synthesized and successfully grafted onto cotton fibers via an impregnation process (IP) and electron‐beam irradiation (EB). The grafted cotton fibers could provide antibacterial efficacy after chlorination through a dilute sodium hypochlorite solution. The antibacterial efficacy was challenged against Staphylococcus aureus and Escherichia coli. The cotton fibers grafted with TMPA and acrylic acid by EB inactivated all of the bacteria within 30 min of contact, whereas the samples grafted with TMPA via an IP could not completely kill the bacteria with 60 min. The breaking strength and UVA light stability also improved significantly. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42023.     

Structure and properties of biaxial‐oriented crystalline polymers by solid‐state crossrolling

The effect of biaxial orientation by solid‐state crossrolling on the morphology of crystalline polymers including polypropylene (PP), high density polyethylene (HDPE) and Nylon 6/6 was investigated with polarized optical microscopy, atomic force microscopy, wide‐angle X‐ray scattering, and small‐angle X‐ray scattering techniques. It was found that crossrolling gradually changed the initial spherulitic structure into a biaxially oriented crystal texture with chain axis of crystals becoming parallel to the rolling direction for all three polymers. The effect of microstructure change on the macromechanical properties was studied in tension at both ambient temperature and ?40°C. In tension at room temperature, the localized necking deformation of HDPE and PP control changed upon orientation into homogeneous deformation for the entire sample length. This was attributed to that the oriented crystal morphology eliminated the stress concentration, which existed in the original spherulitic structure from lamellae orientation in the polar and equatorial regions. At ambient conditions, the elastic moduli of HDPE and PP were found to decrease slightly with orientation whereas the modulus of Nylon 6/6 increased with increasing orientation. This was due to the fact that the amorphous chains of HDPE and PP are in a rubbery state and orientation increased the shear relaxation in the orientation direction but the amorphous chains of Nylon 6/6 are in the glassy state inhibited the shear relaxation. Both the yield stress and strain hardening exponent increased with increasing orientation for all three polymers. In tension at ?40°C, orientation changed the failure mechanism of all three polymers from brittle fracture into ductile failure, as the original spherulitic structure was changed into an oriented structure with chain axis of crystals becoming parallel to the tension direction, which allowed chain slip deformation of crystals and resulted in oriented samples showing ductile failure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Fatigue performance of an injection‐molded short E‐glass fiber‐reinforced polyamide 6,6. I. Effects of orientation,holes, and weld line

This article presents the experimental results of stress‐controlled fatigue tests of an injection‐molded 33 wt% short E‐glass fiber‐reinforced polyamide 6,6. The effects of specimen orientation with respect to the flow direction, hole stress concentration, and weld line on the fatigue life have been considered. In addition, the effect of cyclic frequency has been examined. In addition to the modulus and tensile strength, the fatigue strength of the material was significantly higher in the flow direction than normal to the flow direction, indicating inherent anisotropy of the material caused by flow‐induced orientation of fibers. The presence of weld line reduced the modulus, tensile strength, failure strain, and fatigue strength. The fatigue strength of specimens with a hole was lower than that of un‐notched specimens, but was insensitive to the hole diameter. At cyclic frequencies ≤ 2 Hz, failure was due to fatigue, and fatigue life increased with frequency. However, at cyclic frequencies > 2 Hz, the failure mode was a mixture of fatigue and thermal failures, and fatigue life decreased with increasing frequency. POLYM. COMPOS., 27:230–237, 2006. © 2006 Society of Plastics Engineers.     

Strain‐rate and temperature effects on the deformation of polypropylene and its simulation under monotonic compression and bending

Monotonic compressive loading and bending tests are conducted for solid polypropylene (PP) under constant or time‐varying strain‐rates and temperatures of 10, 25, 40°C. The observed compressive stress‐strain responses under constant conditions have revealed that the inelastic deformation behavior is remarkably dependent on loading rates and temperatures of normal use. The examination of such inelastic behavior has indicated that the strain‐rate effects correspond with the temperature effects based on the concept of time‐temperature equivalence. The viscoplastic constitutive theory based on overstress (VBO) has successfully reproduced the experimental responses with stress‐jumping phenomena using the equivalent time. Four‐point bending tests are performed under monotonic loading and holding for PP beams at three different temperatures. The observed deformation behavior has shown that the Bernoulli‐Euler hypothesis is valid. The VBO model and beam bending theory has generated the basic equations for PP beams, showing an analogy with the uniaxial one. In the numerical analysis, the equations are transformed into nonlinear ordinary differential equations with use of Gaussian quadrature for the spatial integrals. The comparison of numerical and experimental results has suggested some modifications for actually loaded moment taking the effect of deflection and friction into consideration. Finally, the numerical calculation has simulated the experimental time‐histories of curvatures fairly well.     

Coupled thermo‐mechanical analysis for plastic thermoforming

An FEM software ARVIP‐3D was developed to simulate the process of 3‐D plastic thermoforming. The coupled thermo‐mechanical analysis, thermal stress and warpage analysis for plastic thermoforming was carried out by means of this software. Rigid visco‐plastic formula was adopted to simulate the deforming process. During this process, the method of comparing velocity, time and area was adopted as the contact algorithm at different nodes and triangular elements. Sticking contact was assumed when the nodes become in contact with tool surface. The Arrhenius equation and the Williams equation were employed to ascertain the temperature dependence of material properties. In order to analyze the temperature field of plastic thermoforming, the Galerkin FEM code and the dynamic heat conduction boundary condition were adopted; latent heat and deformation heat were treated as dynamic internal heat sources. Based on the above, the model of coupled thermomechanical analysis was established. Assuming that the thermal deformation occurs under elastic conditions, the thermal stress and the warpage following the cooling stage were estimated. Experiments of plastic thermoforming were made for high‐density polyethylene (HDPE). An infrared thermometer was used to record the temperature field and a spiral micrometer was used to measure the thickness of the part. Results of numerical calculation for thickness distribution, temperature field and warpage were in good agreement with experimental results.     

Initiation and development of the heat‐affected zone in the vibration welding of polyvinylidene fluoride and its copolymers

Using a servo hydraulically controlled vibration‐welding machine, the temporal and spatial development of hierarchical structure in the heat‐affected zone of Poly(vinylidene fluoride) homopolymer and a copolymer of vinylidene fluoride and hexafluorpropylene are investigated. The dynamical changes early in the welding process are rather complex, and start at highly localized regions that then develop rapid adhesive character. In subsequent stages, these regions coalesce and a wave pattern establishes with wave normals oriented in the vibration direction. The crests of the waves near the side surfaces then begin to extrude fibrillar structure. These fibers are also collected and extruded in the vibration direction with their long axes normal to the vibration direction. The production of such features is partly attributed to the high melt elasticity of these polymers that results in instability resulting in their production. X‐ray analysis indicates that the fibers are unoriented in their crystalline regions but electron microscopy studies show that a spiral orientation develops in the extruded fibers indicating that they underwent extension plus twisting deformation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3377–3388, 2002     

Improvement in the bonding strength of laminated composites by using air‐textured core‐and‐effect glass yarns

Delamination is the most common failure mode in laminated composites due to the reduced strength in the through‐the‐thickness direction. Air‐jet texturing was used to produce bulk and loops in the yarn, which provides more surface contact between the fibers and the resin. The development of core‐and‐effect textured glass yarns and the effect of texturing parameters were presented in the previous article. This article describes the effect of texturing on the mechanical properties including tensile properties, flexure properties, interlaminar shear strength (ILSS) and fracture toughness (Mode I) of glass laminated composites. The composites of plain and twill weave fabrics were developed from both the textured and nontextured yarns. It was observed that the tensile properties decreased and the flexure properties remained unchanged after texturing. However, significant improvement was observed in ILSS and the Mode I fracture toughness of the composites after texturing. The bulkier, loopy structure of the textured yarn provided more surface contact between the fiber and the resin and significantly improved the bonding strength. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Flexural and tensile moduli of polypropylene nanocomposites and comparison of experimental data to Halpin‐Tsai and Tandon‐Weng models

This research focuses on the reinforcing efficiency of nanomateterials and the role of the reinforcement's dispersion and orientation on the nanocomposite's flexural and tensile moduli. Polypropylene‐based composites reinforced with (i) exfoliated graphite nanoplatelets, xGnP?, (ii) vapor grown carbon fibers, (iii) PAN‐based carbon fibers, (iv) highly structured carbon black and (v) montmorillonite clay were fabricated by extrusion and injection molding. It was found that graphite platelets are the best reinforcement in terms of flexural modulus whereas PAN‐based carbon fibers cause the largest improvement in the tensile modulus. The difference in the reinforcing efficiency during the flexural and tensile testing is attributed to (i) the degree of fiber alignment along the flow direction during injection molding, which is higher in the thinner tensile specimens than in the flex specimens; and (ii) the different deformation modes of the two tests. The importance of good dispersion of the reinforcements within the polymer matrix and of perfect contact between the two phases is emphasized comparing the experimental modulus data to theoretical predictions made using the Halpin‐Tsai and the Tandon‐Weng models. POLYM. ENG. SCI., 47:1796–1803, 2007. © 2007 Society of Plastics Engineers