An investigation of non-linear elastic behavior of CFRP laminates and strain measurement using Lamb waves

This paper investigates the non-linear elastic behavior of unidirectional and cross-ply CFRP laminates and proposes a new method to measure tensile strain using Lamb waves. Young’s modulus was measured as a function of strain in situ using Lamb wave velocity during a tensile test. The stiffening effect of the carbon fibers on [0]₈ specimens and the softening effect of the epoxy matrix on [90]₈ specimens were accurately evaluated. Young’s modulus of the 0° ply was obtained as a quadratic function of strain. Using the function and the rule of mixture, the dependence of Young’s modulus on strain was accurately predicted for cross-ply laminates. Based on the results, the tensile strain was quantitatively correlated with the corresponding arrival time of the Lamb waves. The strains obtained from the proposed method agreed well with those from the strain gauge. Finally, the effect of transverse cracks on the in situ Young’s modulus of the cross-ply laminate under a tensile load was investigated. This method clearly detected even a small decrease in the Young’s modulus due to the transverse cracks in stiffening cross-ply laminate.     

True and apparent Young’s modulus in ferrous porous alloys

Residual porosity in ferrous powder metallurgical alloys induces the phenomenon of localized yielding, or first yielding, during tensile testing. This gives rise to the existence of a true (E₁) and apparent (E₂) Young’s modulus. The true Young’s modulus is similar to the dynamic modulus (E_d) determined by the acoustic resonance method, whereas the apparent Young’s modulus is lower than both E₁ and E_d. For alloys with hard microstructures the apparent Young’s modulus turned out to be about 6% lower than the true Young’s modulus and a negligible influence of matrix hardness and pore morphology was highlighted. However, for ferritic or ferritic–pearlitic materials this difference was higher, ranging between 14 and 31% and it decreases as pore roundness is increased. For austenitic AISI 316L alloys both E₁ and E₂ are lower than E_d because of the presence of oxides on the powder surface, which favour early decohesion at the necks during tensile testing.     

Synthesis and characterization of multiwalled carbon nanotube reinforced ultra high molecular weight polyethylene composite by electrostatic spraying technique

In the present work, multiwalled carbon nanotube (MWNT) reinforced UHMWPE composite films were prepared by electrostatic spraying followed by consolidation. X-ray diffraction and differential scanning calorimetry studies showed a decrease in the crystallinity of UHMWPE due to the nature of the fabrication process as well as addition of MWNT. Tensile test showed an 82% increase in the Young’s modulus, decrease in stress to failure from 14.3 to 12.4 MPa and strain to failure from 3.9% to 1.4% due to 5% addition of MWNT. Raman spectra showed the presence of compressive stresses in the nanotubes. Fracture surface showed presence of pullout like phenomena in the MWNT reinforced film.     

Preparation and characterization of polypropylene–wheat straw–clay composites

Preparation of polypropylene hybrid composite consisting of wheat straw and clay as reinforcement materials was investigated. The composite samples were prepared through melt blending method using a co-rotating twin-screw extruder. The composition of constituents of hybrid composite such as percentages of wheat straw, clay and maleic anhydride grafted polypropylene as a coupling agent was varied in order to investigate their influence on water absorption and flexural properties. The XRD analysis of composite samples containing clay showed shift in d₀₀₁ peak to lower 2θ indicating slight intercalation of polymer in clay sheets. The results of the study indicate that the increase in wheat straw and clay content in a composite increases the flexural modulus and reduces the resistance for water absorption. The increase in PP-MA coupling agent also increases the flexural modulus and resistance for water absorption. The morphological study by scanning electron microscope reveals that the addition of coupling agent increases the interfacial adhesion between the fibers and polymer matrix which is evidenced further from increased flexural modulus. Further, the particle size of wheat straw was analyzed before and after extrusion in order to investigate the effect of extrusion on wheat straw dimensions. The addition of clay as additional filler had no significant role on water absorption and flexural properties of the composite.     

The effects of CNT alignment on electrical conductivity and mechanical properties of SWNT/epoxy nanocomposites

The single-walled carbon nanotubes (SWNTs) filled nanocomposite SWNT/epoxy resin composite with good uniformity, dispersion and alignment of SWNTs and with different SWNTs concentrations was produced by solution casting technique. Subsequently, the semidried mixture was stretched repeatedly along one direction at a large draw-ratio of 50 for 100 times at ambient atmosphere manually to achieve a good alignment and to promote dispersion of SWNTs in the composite matrix. Composite showed higher electrical conductivities and mechanical properties such as the Young’s modulus and tensile strength along the stretched direction than perpendicular to it, and the electrical property of composite rise with the increase of SWNT concentration. The percolation threshold value of electrical conductivity along the stretching direction is lower than the value perpendicular to the SWNTs orientation. In addition, the anisotropic electric and mechanical properties results, SEM micrograph and the polarized Raman spectra of the SWNT/epoxy composite reveal that SWNTs were well dispersed and aligned in the composites by the repeated stretching process.     

New biocomposites based on thermoplastic starch and bacterial cellulose

Bacterial cellulose, produced by Acetobacter Xylinum, was used as reinforcement in composite materials with a starch thermoplastic matrix. The composites were prepared in a single step with cornstarch by adding glycerol/water as the plasticizer and bacterial cellulose (1% and 5% w/w) as the reinforcing agent. Vegetable cellulose was also tested as reinforcement for comparison purposes. These materials were characterized by different techniques, namely TGA, XRD, DMA, tensile tests, SEM and water sorption assays. All composites showed good dispersion of the fibers and a strong adhesion between the fibers and the matrix. The composites prepared with bacterial cellulose displayed better mechanical properties than those with vegetable cellulose fibers. The Young modulus increased by 30 and 17 fold (with 5% fibers), while the elongation at break was reduced from 144% to 24% and 48% with increasing fiber content, respectively for composites with bacterial and vegetable cellulose.     

Colorful and transparent poly(vinyl alcohol) composite films filled with layered zinc hydroxide salts,intercalated with anionic orange azo dyes (methyl orange and orange II)

Layered zinc hydroxide salts (zinc LHS) were intercalated with anionic orange azo dyes, namely methyl orange (MO) and orange II (OII), and co-intercalated with hydrated chloride anions. After characterization by X-ray diffraction (XRD), thermal analysis (TGA/DTA), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS), the materials were used as fillers for poly(vinyl alcohol) (PVA). Colorful transparent films were obtained by wet casting, revealing good dispersion of the material into the polymer. In the case of zinc LHS/OII, PVA was intercalated between the zinc LHS layers. Evaluation of the mechanical properties of the PVA composite films revealed that the layered colorful materials were able to increase the mechanical properties of the PVA films only when the films were stored under lower relative humidity. As expected, films with higher water content displayed reduced tensile strength and modulus because of the plasticizing effect of water. As for the films stored at 43% relative humidity, more pronounced improvement of modulus was observed for 1 and 4% zinc LHS/OII, and enhanced tensile strength was achieved for 0.5 and 1% zinc LHS/OII. This effect can be attributed to better dispersion of the layered filler and its better adhesion to the PVA matrix.     

Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti–FeMo–B prepared by spark plasma sintering

The in situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering at 800–1200 °C under 20 MPa for 5 min. The effects of sintering temperature and reinforcement volume fraction on flexural strength, Young’s modulus and fracture toughness of the composites are investigated. The titanium matrix consists of -Ti and β-Ti phases, and the volume fraction of β-Ti increases with increasing sintering temperatures. The in situ synthesized TiB reinforcements are distributed randomly and uniformly in matrix. The transverse section of TiB has a hexagonal shape aligned along [0 1 0] direction, and the crystallographic planes of the TiB needles are always of the type . The 10 vol% TiB reinforced composite sintered at 1000 °C exhibits excellent mechanical properties. The flexural strength, Young’s modulus and fracture toughness of this composite are 1560 MPa, 137 GPa and 8.64 MPa · m^1/2, respectively.     

Hygromechanical properties of composites of crosslinked allylglycidyl-ether modified starch reinforced by wood fibres

In a previous work a new family of thermoset composites of allylglycidyl ether modified starch as matrix, an ethylene glycol dimethacrylate as cross-linker and a wood fibre as reinforcement were prepared. The aim of the present work was to study the hygromechanical properties of the new composites including density, dimensional stability in water, water uptake, stiffness, and ultimate strength in three-point bending. It was shown that the samples with a starch matrix of a high degree of substitution (DS = 2.3), HDS, absorbed less water, were more stable in water and had also higher stiffness and strength than corresponding composite samples with a starch matrix of low degree of substitution (DS = 1.3), LDS. Overall, the fibre addition improved water stability. An increased fibre content from 40 to 70% by weight had a negligible impact on the water uptake. An increase in fibre content did, however, improve the mechanical properties. The HDS-sample with highest fibre content, 70% by weight showed the highest Young’s modulus (3700 MPa) and strength (130 MPa), which are markedly higher compared with the samples based on the pure HDS matrix (Young’s modulus of 360 MPa and strength of 15 MPa). The measured Young’s modulus and tensile strength values were roughly one order of magnitude higher than earlier reported cellulosic fibre reinforced natural polymer composites.     

Preparation and properties of microfibrillated cellulose polyvinyl alcohol composite materials

This work reports the preparation of MFC–PVA composite films, and the thermal and mechanical properties of these films. Microfibrillated cellulose (MFC), which was separated from kraft pulp by a mechanical process, was used as the reinforcement in polyvinyl alcohol (PVA) matrix. This MFC reinforcement has an interconnected web-like structure with fibrils having a diameter in the range of 10–100 nm, as observed by TEM. MFC–PVA composite films were created by casting from a water suspension to produce a homogeneous dispersion of MFC in the polymer matrix. DMA shows an increase of the storage modulus in the glassy state with increasing MFC content, but a more significant increase in modulus is detectable above the glass transition temperature. There is a steady increase in both the modulus and strength of the composite films until a plateau is reached at 10 wt% MFC. The thermal stability of the PVA composite films is slightly increased with the addition of MFC.

As a result of this research, it has been shown that MFC is an excellent reinforcement comparable to cellulose nanowhiskers. Furthermore, by combining MFC with PVA in addition to good mechanical properties, this composite has good chemical resistance and biodegradability. The water soluble characteristics of PVA combined with a water dispersion of MFC are also easily processable.     

Natural fiber-reinforced thermoplastic starch composites obtained by melt processing

Thermoplastic starch (TPS) from industrial non-modified corn starch was obtained and reinforced with natural strands. The influence of the reinforcement on physical–chemical properties of the composites obtained by melt processing has been analyzed. For this purpose, composites reinforced with different amounts of either sisal or hemp strands have been prepared and evaluated in terms of crystallinity, water sorption, thermal and mechanical properties. The results showed that the incorporation of sisal or hemp strands caused an increase in the glass transition temperature (T_g) of the TPS as determined by DMTA. The reinforcement also increased the stiffness of the material, as reflected in both the storage modulus and the Young’s modulus. Intrinsic mechanical properties of the reinforcing fibers showed a lower effect on the final mechanical properties of the materials than their homogeneity and distribution within the matrix. Additionally, the addition of a natural latex plasticizer to the composite decreased the water absorption kinetics without affecting significantly the thermal and mechanical properties of the material.     

Two-dimensional and axisymmetric unit cell models in the analysis of composite materials

Unit cell models have been widely used for investigating fracture mechanisms and mechanical properties of composite materials assuming periodically arrangement of inclusions in matrix. It is desirable to clarify the geometrical parameters controlling the mechanical properties of composites because they usually contain randomly distributed particulate. To begin with a tractable problem this paper focuses on the effective Young’s modulus E of heterogeneous materials. Then, the effect of shape and arrangement of inclusions on E is considered by the application of FEM through examining three types of unit cell models assuming 2D and 3D arrays of inclusions. It is found that the projected area fraction and volume fraction of inclusions are two major parameters controlling effective elastic modulus of inclusions.     

Renewable resource based biocomposites from natural fiber and polyhydroxybutyrate-co-valerate (PHBV) bioplastic

Renewable resource based green biocomposites were prepared using a bacterial polyester i.e., poly(hydroxybutyrate-co-valerate) (PHBV) and natural bamboo fiber. Fabrication of the biocomposites was carried out by injection molding following extrusion compounding of PHBV and bamboo fiber with 30 or 40 wt.% fiber. The mechanical, thermo-mechanical and morphological properties of the biocomposites were evaluated. Little variation in the thermo-mechanical and impact properties was observed when the fiber content was varied. The tensile modulus of biocomposites at 40 wt.% fiber improved by 175% as compared to that of neat PHBV. The theoretical tensile modulus of the biocomposites was calculated using Christensen’s equations and compared with the experimental results. It was found to be in near approximation to the experimental data. The storage modulus was affected slightly by the variation of fiber content from 30 to 40 wt.% in biocomposites. The heat deflection temperature of PHBV increased by 9 °C at 40 wt.% of fiber reinforcement. Morphological aspects and thermal stability were studied using scanning electron microscopy and thermo-gravimetric analysis, respectively. In addition, a comparative analysis of bamboo fiber–PHBV with wood fiber–PHBV biocomposites was performed. Statistical analysis of both biocomposites was carried out by performing a two-way ANOVA on their tensile and flexural moduli in order to evaluate the effect of fiber type and content in the PHBV matrix.     

An analytical study of the effect of slamming pressures on the interlaminar behaviour of composite panels

This paper discusses the effect of material properties on the interlaminar behaviour of ship panels made of composite materials under stress waves caused by slamming loads. First, a brief reference to the analytical model developed to simulate the propagation of stress waves caused by slamming impact on composite materials which has been recently published in Composite Structures is made. Then, some parametric studies are done, discussing the influence of material Young’s modulus on the interlaminar peak stresses. The influence of strain rates is also discussed.     

复合材料螺旋弹簧刚度及其影响因素分析

该研究制备了一种与中空圆截面簧丝轴线成±45°铺层的新型复合材料圆柱螺旋压缩弹簧,并对该弹簧进行准静态压缩试验。将纤维复合材料平板的剪切模量等效替代各向同性材料弹簧刚度表达式中的剪切模量,从而理论导出该类复合材料弹簧的刚度表达式。针对该表达式分析影响弹簧刚度的各种因素及其影响程度。结果表明:弹簧的刚度系数随着纤维弹性模量、纤维体积分数、簧丝外直径的增加而急剧增加,随它们的下降而显著下降;基体弹性模量与复合材料泊松比对弹簧刚度的影响十分微小;簧丝内直径在一定范围内变化时对弹簧刚度的影响几乎可以不计,但却能显著降低弹簧质量,超出该范围后,弹簧刚度随内直径增加而急剧下降;刚度系数随弹簧有效圈数和弹簧圈平均直径的增加而剧烈下降,随着它们的下降而急剧增加。     

镁合金表面TaC/TaC-Mg/Mg梯度涂层的 残余热应力分析

采用间歇式挤出发泡工艺制备淀粉/PVA复合发泡材料。在淀粉含量固定、甘油作为增塑剂的情况下,研究偶氮二甲酰胺（AC发泡剂）与聚乙烯醇（PVA）的含量对发泡材料的表观密度、发泡倍率、相对硬度、吸水性能、回弹性能以及压缩性能等的影响。结果表明：随着PVA含量的升高,复合材料的表观密度和压缩模量减小,回弹性能变好。PVA含量对吸水率影响不明显,吸水率稳定在20%左右。吸水至饱和状态后,相对硬度随着PVA含量的增加而不断增加。随AC发泡剂含量的升高,复合材料的表观密度减小,相对硬度降低,发泡倍率和回弹率增加,材料的泡孔孔径逐渐增大但是发泡孔径的均匀性变差。当PVA、AC发泡剂的添加质量分数分别为30%, 1%时,复合材料性能最优。     

Improvement of plant based natural fibers for toughening green composites—Effect of load application during mercerization of ramie fibers

Effect of mercerization to tensile properties of a ramie fiber was explored. Load application technique during mercerization has been employed in order to improve mechanical properties of the fiber. A chemical treatment apparatus with tensile loading portion for applying monofilaments was newly developed. The ramie fiber was alkali-treated by 15% NaOH solution with applied loads of 0.049 and 0.098 N. The results showed that tensile strength of the treated ramie fiber was improved, 4–18% higher than that of the untreated ramie fiber, while Young’s modulus of the treated fibers decreased. It should be noted that fracture strains of the treated ramie fiber drastically increased to 0.045–0.072, that is, twice to three times higher than those of the untreated ramie fiber. It was considered that such property improvements upon mercerization were correlated with change of morphological and chemical structures in microfibrils of the fiber. Finally, the plastic deformation behavior and fracture mechanism of the mercerized fibers under tensile loading process was explained using a schematic model.     

Development and Characterization of the Midrib of Coconut Palm Leaf Reinforced Polyester Composite

In this paper, midrib of coconut palm leaves (MCL) was investigated for the purpose of development of natural fiber reinforced polymer matrix composites. A new natural fiber composite as MCL/polyester is developed by the hand lay-up method, and the material and mechanical properties of the fiber, matrix and composite materials were evaluated. The effect of fiber content on the tensile, flexural, impact, compressive strength and heat distortion temperature (HDT) was investigated. It was found that the MCL fiber had the maximum tensile strength, tensile modulus flexural strength, flexural modulus and Izod impact strength of 177.5MPa, 14.85GPa, 316.04MPa and 23.54GPa, 8.23KJ/m² respectively. Reinforcement of MCL enhanced the mechanical properties of pure polyester, including that of tensile strength (by 26%), tensile modulus (by 356%), flexural strength (by 41.81%), flexural modulus (by 169%) and Izod impact strength (by 23 times), but the compressive strength was adversely affected. HDT decreased due to fiber loading, but increased with weight fraction of fiber content. Moreover, the experimental results were compared with theoretical model (Rule of mixture) and other natural fiber /polyester composites.     

Mechanisms and impact of fiber–matrix compatibilization techniques on the material characterization of PHBV/oak wood flour engineered biobased composites

Fully biobased composite materials were fabricated using a natural, lignocellulosic filler, namely oak wood flour (OWF), as particle reinforcement in a biosynthesized microbial polyester matrix derived from poly(β-hydroxybutyrate)-co-poly(β-hydroxyvalerate) (PHBV) via an extrusion injection molding process. The mechanisms and effects of processing, filler volume percent (vol%), a silane coupling agent, and a maleic anhydride (MA) grafting technique on polymer and composite morphologies and tensile mechanical properties were investigated and substantiated through calorimetry testing, scanning electron microscopy, and micromechanical modeling of initial composite stiffness. The addition of 46 vol% silane-treated OWF improved the tensile modulus of neat PHBV by 165%. Similarly, the tensile modulus of MA-grafted PHBV increased 170% over that of neat PHBV with a 28 vol% addition of untreated OWF. Incorporation of OWF reduced the overall degree of crystallinity of the matrix phase and induced embrittlement in the composites, which led to reductions in ultimate tensile stress and strain for both treated and untreated specimens. Deviations from the Halpin–Tsai/Tsai–Pagano micromechanical model for composite stiffness in the silane and MA compatibilized specimens are attributed to the inability of the model both to incorporate improved dispersion and wettability due to fiber–matrix modifications and to account for changes in neat PHBV and MA-grafted PHBV polymer morphology induced by the OWF.     

仿竹结构单丝玻璃纤维增强多孔聚醚砜基复合材料

竹子是一种以竹纤维为增强体、多孔木质素为基体而组成的天然复合材料。本文借鉴竹子的结构特征,采用高性能热塑性聚合物浸没沉淀相转化法在玻璃纤维(GF)表面沉积梯度孔径分布的多孔聚醚砜(PES)基体,制备仿竹结构单丝玻璃纤维增强多孔聚醚砜基复合材料(GF/PES),并对其微观形貌、拉伸力学性能和“温度-模量”智能响应性进行了研究。结果表明,基于梯度多孔PES基体良好的吸能作用及其对玻璃纤维表面微小缺陷的修复作用,GF/PES的拉伸强度和断裂伸长率最高可分别比GF提高39.11%和58.1%。此外,多孔聚合物基体还可作为各类功能材料的载体,例如在其多孔结构中填充水,当水随着温度变化发生相变时,可赋予GF/PES显著的模量变化,从而制备出“温度-模量”智能响应复合材料。