Modeling and damage repair of woven thermoplastic composites subjected to low velocity impact

The low velocity impact behavior of three layer thermoplastic laminates consisting of woven glass fiber and polypropylene has been investigated for two different fiber volume configurations. Panels with configurations of 50/50 and 20/80 in the warp and fill directions were subjected to low velocity impact energies between 4 and 16 J using an instrumented dropping weight impact tower. Load vs. displacement plots showed the excellent energy absorbing capabilities exhibited by the woven composites. Both configurations dissipated approximately 75% of the 16 J incident impact energy. An energy-balance model was used to successfully predict the impact response of the woven thermoplastic composites. The impact damaged plates were tested under four point bend (4 PB) loading conditions. Results showed a reduction in flexural strength and modulus as the impact energy increased. A simple compression molding damage repair process was applied to the 16 J impacted composite plates. 4 PB testing of the repaired samples revealed a significant recovery in the flexural strength and modulus of the thermoplastic woven composite with both fiber configurations.     

Flexural behavior of geopolymer composites reinforced with steel and polypropylene macro fibers

Like ordinary Portland cement concrete, the matrix brittleness in geopolymer composites can be reduced by introducing appropriate fiber reinforcement. Several studies on fiber reinforced geopolymer composites are available, however there is still a gap to understand and optimize their performance. This paper presents the flexural behavior of fly ash-based geopolymer composites reinforced with different types of macro steel and polypropylene fibers with higher aspect ratio. Three types (length-deformed, end-deformed and straight) of steel fibers and another type of length-deformed polypropylene fiber with optimum fiber volume fraction of 0.5% are studied. The effects of different geometries of the fibers, curing regimes (ambient cured and heat cured at 60 °C for 24 h) and concentration of NaOH activator (10 M and 12 M) on the first peak strength, modulus of rupture and toughness of the geopolymer composites are investigated. The quantitative effect of fiber geometry on geopolymer composite performance was also analyzed through a fiber deformation ratio. The compressive strength, splitting tensile strength and flexural toughness are significantly improved with macro fibers reinforcement and heat curing. The results also show that heat curing increases the first peak load of all fiber-reinforced geopolymers composites. End-deformed steel fibers exhibit the most ductile flexural response compared to other steel fibers in both heat and ambient-cured fiber reinforced geopolymer composites.     

混杂比对单向碳-玻层间混编复合材料0°压缩和弯曲性能的影响

本文研究了相同铺层方式下不同混杂比对单向碳-玻(碳纤维-玻璃纤维)层间混编复合材料的0°压缩和弯曲性能的影响。以碳-玻层间混编形式向纯玻纤织物中混入碳纤维,对复合材料的0°压缩强度、0°压缩模量、弯曲强度和弯曲模量均有一定的提高作用,三种碳纤混杂比不同的复合材料0°压缩强度较纯玻纤结构分别提高了22.72%、26.95%、11.43%,混杂比不同所导致的试样破坏程度也各不相同。混杂复合材料0°压缩模量随碳纤含量增加逐渐增大。三种碳纤混杂比复合材料弯曲强度基本一致,而弯曲弹性模量随碳纤含量增加逐渐增大。     

A Study on flexural properties of wildcane grass fiber-reinforced polyester composites

The main objective of this study is to introduce a new natural fiber as reinforcement in polymers for making composites. Wildcane grass stalk fibers were extracted from its stem using retting and chemical (NaOH) extraction processes. These fibers were treated with KMnO₄ solution to improve adhesion with matrix. The resulting fibers were intentionally reinforced in a polyester matrix unidirectionally, and the flexural properties of the composite were determined. The fibers extracted by retting process have a tensile strength of 159 MPa, modulus of 11.84 GPa, and an effective density of 0.844 g/cm³. The composites were formulated up to a maximum fiber volume fraction of 0.39, resulting in a flexural strength of 99.17 MPa and flexural modulus of 3.96 GPa for wildcane grass fibers extracted by retting. The flexural strength and the modulus of chemically extracted wildcane grass fiber composites have increased by approximately, 7 and 17%, respectively compared to those of composites made from fibers extracted by retting process. The flexural strength and the modulus of KMnO₄-treated fiber composites have increased by 12 and 76% over those of composites made from fibers extracted by retting process and decreased by 3 and 48% over those of composites made from fibers extracted by chemical process, respectively. The results of this study indicate that wildcane grass fibers have potential as reinforcing fillers in plastics in order to produce inexpensive materials with high toughness.     

Developing rigid gliadin based biocomposites with high mechanical performance

We aim to produce unidirectional fiber composites with high mechanical performance based on flax fibers and a rigid gliadin matrix. As a fraction from wheat gluten, gliadin is soluble in alcohol containing media. The fabrication process did not involve any further solvents or plasticizers. Finally, samples were cooled at different rates. Overall, the cooling rate does not strongly affect the mechanical properties although slowly cooled materials contain a higher amount of non-disulfide cross-links, next to disulfide bonds within the gliadin matrix. At 40% fiber volume fraction, flax/gliadin composites with a flexural modulus and strength of respectively 21.5 GPa and 240 MPa were obtained when loaded in the longitudinal direction. These high values demonstrate that in this composite fabrication process, a good impregnation of the polymer matrix in between the fiber bundles has been achieved. However, the fiber–matrix adhesion, as measured by transverse flexural and tensile tests, was still relatively modest.     

New development of self-damping MWCNT composites

Epoxy resin modified with nanofillers cannot be used alone for high performance structural applications due to their low-mechanical properties. Therefore, the objective of this work is to hybridize unidirectional and quasi-isotropic glass fiber composite laminates with 1.0 wt% multi-walled carbon nanotubes (MWCNTs). Results from flexural and damping characterizations showed that the flexural strength and modulus, storage modulus, and damping ratio of MWCNT/E nanocomposite are improved by about 7% ± 1.5% compared to neat epoxy. The enhancement in the flexural strength of quasi-isotropic laminate (20.7%) is about ten times higher than that for unidirectional laminate (2.1%). The flexural moduli of the nano-hybridized laminates are reduced by about 7.5–10.8%. Accordingly, the ultimate failure strain and damping properties are evidently improved. The improvement in damping ratio in some cases is about 100%. The high correlation coefficient (0.9995) between flexural and storage moduli suggests using the dynamic nondestructive tests for evaluation the elastic properties of composites.     

Investigations on the thermal and flexural properties of plain weave carbon/epoxy-nanoclay composites by hand-layup technique

The matrix of carbon fiber/SC-15 epoxy composites was modified with Nanomer^® I-28E nanoclay, a surface modified montmorillonite mineral, to determine the effects of particle reinforcement on the response of these materials to flexural and thermomechanical loading. Different weight percentages of nanoclay were dispersed in SC-15 epoxy using sonication route. The nanophased epoxy was then used to manufacture plain weave carbon/epoxy nanocomposites using hand-layup process followed by vacuum bagging. Control samples of woven carbon fiber/epoxy were fabricated for comparison purposes. Effect of post curing on these samples was also investigated. 3-point bend flexure and Dynamic Mechanical Analysis (DMA) studies were carried out on 8- and 3-layered samples respectively. Results of flexural tests indicate significant improvements in flexural strength and modulus for nanoclay reinforced composites as compared to the control samples. DMA studies also showed enhancement in thermomechanical properties especially in storage modulus though no appreciable change was noticed in glass transition temperature, T _g. Scanning electron microscopy (SEM) studies were carried out to comprehend the effect of nanoclay on the microstructure and the failure modes.     

Effects of environmental aging on the mechanical properties of bamboo–glass fiber reinforced polymer matrix hybrid composites

Short bamboo fiber reinforced polypropylene composites (BFRP) and short bamboo–glass fiber reinforced polypropylene hybrid composites (BGRP) were fabricated using a compression molding method. Maleic anhydride polypropylene (MAPP) was used as a compatibilizer to improve the adhesion between the reinforcements and the matrix material. By incorporating up to 20% (by mass) glass fiber, the tensile and flexural modulus of BGRP were increased by 12.5 and 10%, respectively; and the tensile and flexural strength were increased by 7 and 25%, respectively, compared to those of BFRP. Sorption behavior and effects of environmental aging on tensile properties of both BFRP and BGRP systems were studied by immersing samples in water for up to 1200 h at 25°C. Compared to BFRP, a 4% drop in saturated moisture level is seen in BGRP. After aging in water for 1200 h, reduction in tensile strength and modulus for BGRP is nearly two times less than that of BFRP. Use of MAPP as coupling agent in the polypropylene matrix results in decreased saturated moisture absorption level and enhanced mechanical properties for both BFRP and BGRP systems. Thus it is shown that the durability of bamboo fiber reinforced polypropylene can be enhanced by hybridization with small amount of glass fibers.     

The effect of silane treated- and untreated-talc on the mechanical and physico-mechanical properties of poly(lactic acid)/newspaper fibers/talc hybrid composites

This paper evaluates the effect of the addition of silane treated- and untreated- talc as the fillers on the mechanical and physico-mechanical properties of poly(lactic acid) (PLA)/recycled newspaper cellulose fibers (RNCF)/talc hybrid composites. For this purpose, 10 wt% of a talc with and without silane treatment were incorporated into PLA/RNCF (60 wt%/30 wt%) composites that were processed by a micro-compounding and molding system. PLA is utilized is a bio-based polymer that made from dextrose, a derivative of corn. Talc is also a natural product. The RNCF and talc hybrid reinforcements of PLA polymer matrix were targeted to design and engineer bio-based composites of balanced properties with added advantages of cost benefits besides the eco-friendliness of all the components in the composites. In this work, the flexural and impact properties of PLA/RNCF composites improved significantly with the addition of 10 wt% talc. The flexural and impact strength of these hybrid composites were found to be significantly higher than that made from either PLA/RNCF. The hybrid composites showed improved properties such as flexural strength of 132 MPa and flexural modulus of 15.3 GPa, while the unhybridized PLA/RNCF based composites exhibited flexural strength and modulus values of 77 MPa and 6.7 GPa, respectively. The DMA storage modulus and the loss modulus of the PLA/RNCF hybrid composites were found to increase, whereas the mechanical loss factor (tan delta) was found to decrease. The storage modulus increased with the addition of talc, because the talc generated a stiffer interface in the polymer matrix. Differential scanning calorimetry (DSC) thermograms of neat PLA and of the hybrid composites showed nearly the similar glass transition temperatures and melting temperatures. Scanning electron microscopy (SEM) micrographs of the fracture surface of Notched Izod impact specimen of 10 wt% talc filled PLA/RNCF composite showed well filler particle dispersion in the matrix and no large aggregates are present. The comparison data of mechanical properties among samples filled with silane-treated- and untreated- talc fillers showed that the hybrid composites filled with silane treated talc displayed the better mechanical prosperities relative to the other hybrid composites. Talc-filled RNCF-reinforced polypropylene (PP) hybrid composites were also made in the same way that of PLA hybrid composites for a comparison. The PLA hybrid bio-based composites showed much improvement in mechanical properties as compared to PP-based hybrid counterparts. This suggests that these PLA hybrid bio-based composites have a potential to replace glass fibers in many applications that do not require very high load bearing capabilities and these recycled newspaper cellulose fibers could be a good candidate reinforcement fiber of high performance hybrid biocomposites.     

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.     

Processing,characterization, and modeling of carbon nanotube-reinforced multiscale composites

Carbon fiber-reinforced epoxy composites modified with carbon nanotubes (CNTs) were fabricated and characterized. High-energy sonication was used to disperse CNTs in the resin, followed by infiltration of fiber preform with the resin/CNT mixture. The effects of sonication time on the mechanical properties of “multiscale” composites, which contain reinforcements at varying scales, were studied. A low CNT loading of 0.3 wt% in resin had little influence on tensile properties, while it improved the flexural modulus, strength, and percent strain to break by 11.6%, 18.0%, and 11.4%, respectively, as compared to the control carbon fiber/epoxy composite. While sonication is an effective method to disperse CNTs in a resin, duration, intensity, and temperature need to be controlled to prevent damages imposed on CNTs and premature resin curing. A combination of Halpin–Tsai equations and woven fiber micromechanics was used in hierarchy to predict the mechanical properties of multiscale composites, and the discrepancies between the predicted and experimental values are explained.     

Mechanical Properties of Thermoplastic and Thermoset Composites Reinforced with 3D Biaxial Warp-knitted Fabrics

In this study, two types of thermoplastic matrices (low melting point polyethylene terephthalate (LPET) fiber and polypropylene (PP) fiber) and glass fiber/epoxy resin/multi-walled carbon nanotubes (MWCNTs) were used to fabricate the thermoplastic and thermoset composite materials with 3D biaxial warp-knitted fabrics. Thermoplastic and thermoset composites were fabricated using hot-press and resin transfer molding (RTM) methods. The fabricated samples were tested with tensile and three-point flexural tests. In thermoplastic composites, samples in the 90° direction and LPET matrix showed the best tensile and flexural properties with an improvement of 39 and 21% tensile modulus and strength, 16 and 8% flexural modulus and strength compared to the PP samples in the same direction. In thermoset composites, samples in the 90° direction and MWCNTs showed the best improvement of the flexural modulus and strength with 97 and 58% compared to the samples without MWCNTs. This improvement can most likely be attributed to an increase in interfacial adhesion due to the presence of the carbon nanotubes.     

Mechanical energy dissipation using carbon fiber polymer–matrix structural composites with filler incorporation

Continuous carbon fiber composites with enhanced mechanical energy dissipation (vibration damping) under flexure are provided by incorporation of fillers between the laminae. Exfoliated graphite (EG) as a sole filler is more effective than carbon nanotube (SWCNT/MWCNT), halloysite nanotube (HNT), or nanoclay as sole fillers in enhancing the loss tangent, if the curing pressure is 2.0 (not 0.5) MPa. The MWCNT, SiC whisker, and HNT as sole fillers are effective for increasing the storage modulus. The combined use of a storage modulus-enhancing filler (CNT, SiC whisker, or HNT) and a loss tangent-enhancing filler (EG or nanoclay) gives the best performance. With EG, HNT, and 2.0-MPa curing, the loss modulus is increased by 110%, while the flexural strength is decreased by 14% and the flexural modulus is not affected. With nanoclay, HNT, and 0.5-MPa curing, the loss modulus is increased by 96%, while the flexural strength and modulus are essentially not affected. The filler incorporation is more effective for crossply than unidirectional composites. The highest fraction of mechanical energy dissipated is 11%. The loss tangent enhancement is primarily contributed by the innermost interlaminar interfaces, indicating shear deformation dominance in damping. The filler incorporation increases the interlaminar interface thickness, which remains below ~10 μm.     

Flexural and impact response of woven glass fiber fabric/polypropylene composites

Impact tests with a falling dart and flexural measurements were carried out on polypropylene based laminates reinforced with glass fibers fabrics. Research has shown that the strong fiber/matrix interface obtained through the use of a compatibilizer increased the mechanical performance of such composite systems. The improved adhesion between fibers and matrix weakly affects the flexural modulus but strongly influences the ultimate properties of the investigated woven fabric composites. In fact, bending tests have shown a clear improvement in the flexural strength for the compatibilized systems, in particular when a high viscosity/high crystallinity polypropylene was used. On the contrary, the low velocity impact tests indicated an opposite dependence on the interface strength, and higher energy absorption in not compatibilized composites was detected. This result has been explained in terms of failure mechanisms at the fiber/matrix interface, which are able to dissipate large amounts of energy through friction phenomena. Pull-out of fibers from the polypropylene matrices have been evidenced by the morphological analysis of fracture surfaces after failure and takes place before the fibers breakage, as confirmed by the evaluation of the ductility index.     

单向连续竹青纤维填充量对不饱和聚酯树脂力学性能及微观形貌的影响

以单向连续竹青纤维（OBF）和不饱和聚酯树脂（UP）制备了单向OBF/UP复合材料,研究了OBF含量对OBF/UP复合材料纵向静态力学性能及动态力学性能的影响,并采用SEM观察了复合材料拉伸断面处界面结合情况。结果表明:随着OBF含量的增加,OBF/UP复合材料静态力学性能呈先增加后减小趋势,当OBF含量为50wt%时,复合材料拉伸、弯曲性能最优,拉伸强度、拉伸模量、弯曲强度、弯曲模量分别达到285.52 MPa、16.06 GPa、359.80 MPa、27.32 GPa;OBF/UP复合材料存储模量随OBF含量增加呈先增加后减小趋势,当OBF含量为50wt%时,OBF/UP复合材料存储模量最大,且随着OBF含量的增加,OBF/UP复合材料玻璃化转变温度向低温方向移动,损耗峰变宽;断面处微观形貌表明,OBF含量为50wt%时,复合材料界面结合强度较好。制备的OBF/UP复合材料力学性能优良,有潜力取代玻璃纤维增强树脂复合材料在风电叶片材料、公路防护栏材料、船舶材料等领域的应用。      

立体多向编织结构对复合材料性能的影响

本文采用1×1,1×2和1×3三种不同的编织结构对轴向增强和非增强三维多向编织复合材料的性能进行了研究。对编织复合材料的拉伸强度、刚度和弯曲强度、刚度进行了实验分析。结果表明;三维编织复合材料具有良好的性能。编织结构对复合材料性能有较大的影响。纤维表面编织角和纤维体积比是影响复合材料性能的重要结构参数。通过轴向加入非编织增强纤维,使编织复合材料的拉伸强度和模量,弯曲强度和模量有了较大改善。     

Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites

Natural fiber reinforced polymer composites became more attractive due to their light weight, high specific strength, and environmental concern. However, some limitations such as low modulus, poor moisture resistance were reported. This study aimed to investigate the effect of glass fiber hybridization on the physical properties of sisal–polypropylene composites. Polypropylene grafted with maleic anhydride (PP-g-MA) was used as a compatibilizer to enhance the compatibility between the fibers and polypropylene. Incorporating glass fiber into the sisal–polypropylene composites enhanced tensile, flexural, and impact strength without having significant effect on tensile and flexural moduli. In addition, adding glass fiber improved thermal properties and water resistance of the composites.     

Flexural properties loss of unidirectional epoxy/fique composites immersed in water and alkaline medium for construction application

Epoxy fique composites were evaluated for construction applications and compared with conventional wood used in construction. The composites studied were made with fique fibers treated using Na(OH) solution at 18 w/v%, untreated fique fibers were also used. The matrices were epoxy and epoxy with 5 wt.% of chemically modified C30B montmorillonite. Unidirectional composites of 90 mm × 20 mm × 4 mm were elaborated by pultrusion processing technique. The flexural properties loss occurred over 20 days of composites submitted to three types of environments: (i) water, (ii) saturated calcium hydroxide solution and (iii) mortar with w/c ratio of 0.45 and 540 kg/m³ of cement, cured in a saturated solution of lime stone at 50 °C. Results showed that fiber treatment and montmorillonite addition improved the flexural modulus and strength of composites in 40% and 34% respectively. Moreover the flexural properties of composites before and after ageing resulted comparable or even better than conventional wood used in construction.     

Flexural properties loss of unidirectional epoxy/fique composites immersed in water and alkaline medium for construction application

Epoxy fique composites were evaluated for construction applications and compared with conventional wood used in construction. The composites studied were made with fique fibers treated using Na(OH) solution at 18 w/v%, untreated fique fibers were also used. The matrices were epoxy and epoxy with 5 wt.% of chemically modified C30B montmorillonite. Unidirectional composites of 90 mm × 20 mm × 4 mm were elaborated by pultrusion processing technique. The flexural properties loss occurred over 20 days of composites submitted to three types of environments: (i) water, (ii) saturated calcium hydroxide solution and (iii) mortar with w/c ratio of 0.45 and 540 kg/m³ of cement, cured in a saturated solution of lime stone at 50 °C. Results showed that fiber treatment and montmorillonite addition improved the flexural modulus and strength of composites in 40% and 34% respectively. Moreover the flexural properties of composites before and after ageing resulted comparable or even better than conventional wood used in construction.     

The effect of alkaline treatment on mechanical properties of kenaf fibers and their epoxy composites

In this work, kenaf fibers were pre-treated in a NaOH solution (6% in weight) at room temperature for two different periods (48 and 144 h). The chemical treatment of kenaf fibers for 48 h allowed to clean their surface removing each impurity whereas 144 h of immersion time had detrimental effect on the fibers surface and, consequently, on their mechanical properties.Untreated and NaOH treated kenaf fibers (i.e. for 48 h) were also used as reinforcing agent of epoxy resin composites. The effect of the stacking sequence (i.e. using unidirectional long fibers or randomly oriented short fibers) and the chemical treatment on the static mechanical properties was evaluated showing that the composites exhibit higher moduli in comparison to the neat resin. As regards the strength properties, only the composites reinforced with unidirectional layers show higher strength than the neat resin. Moreover, the alkali treatment increased the mechanical properties of the composites, due to the improvement of fiber–matrix compatibility.The dynamic mechanical analysis showed that the storage and the loss moduli are mainly influenced by the alkali treatment above the glass transition temperature. Moreover, the alkali treatment led to a notable reduction of tan δ peaks in addition to significant shifts of tan δ peaks to higher temperatures whereas the stacking sequence did not influence the trends of storage modulus, loss modulus and damping of the composites.