Reduction In Fiber Strength Research Articles

Investigation of recycling effects on the mechanical properties of short carbon and glass fiber reinforced polyetherimide composites

AbstractConsidering the increasing composite waste accumulation, this work explores the feasibility of replacing virginal carbon fibers (VCFs) and virginal glass fibers (VGFs) with recycled fibers for injection molded polyetherimide (PEI) composites. Extensive characterizations were conducted to reveal effects of sulfuric acid recycling process on mechanical properties of fibers and their composites. It was shown that strength retention rates of recycled carbon fibers (RCFs) and recycled glass fibers (RGFs) were 81.6% and 55.7%. The reinforcing factor of RCFs increased from 0.120 of VCFs to 0.147 due to the enhanced interfacial bonding while the reinforcing factor of RGFs decreased from 0.080 of VGFs to 0.065 owing to the glass fiber strength reduction. As a result, the composite tensile strength (152.8 MPa) of RCF/PEI was comparable to that (155.3 MPa) of VCF/ PEI and RGF/PEI tensile strength (102.5 MPa) was lower than that (112.3 MPa) of VGF/PEI. Moreover, the tensile modulus of RCF/PEI and RGF/PEI were also approximatively comparable to those of VCF/PEI and VGF/PEI since recycling process showed a slight effect on fiber modulus. Although there exists an impact of recycling on fiber mechanical properties, the introduction of recycled fibers (especially RCFs) by injection molding process can promisingly enhance PEI composites.

Liquid-Ammonia-Mediated Dyeing Process of Wool at a Lower Temperature

Liquid ammonia as a non-aqueous medium has many physical properties close to water, such as small molecular weight and strong permeability. It has been widely used for the ecological processing of cellulosic fibers to improve their luster, softness and dyeing properties. However, there are few reports on the dyeing of wool treated with liquid ammonia, especially at a lower temperature. Herein, a continuous liquid ammonia finishing machine was used to batch process wool followed by dyeing in a commonly-used wool dyeing machine. The results showed that many scale flakes and some cuticle cracking were seen on the fiber surface, and the disulfide bonds of cystine were broken down after liquid ammonia treatment, which promoted the diffusion of dyestuff into the fiber. Moreover, the uptakes and K/S value of wool dyed with Lanaset and Lanasol CE dyes were higher than the untreated wool, and the dyeing temperature could decrease to 85 °C, while the degree of fiber strength reduction merely decreased by 3–5%. Furthermore, for the reactive dyes, the dyeing temperature can reduce to 70 °C with the chemical auxiliaries Miralan LTD, while the degree of strength reduction decrease by 8–10%. Liquid ammonia treatment can be used for dyeing at a lower temperature than boiling temperature (100 °C), reduce energy consumption and reduce the degree of fiber strength reduction of wool. The method shows considerable to great value and is significant in providing a feasible approach for the industrial application of low-temperature dyeing technology.

Micromechanical modeling of crack-bridging relations of hybrid-fiber Strain-Hardening Cementitious Composites considering interaction between different fibers

As tensile crack-bridging constitutive relations play an important role in the multiple cracking behaviors of Strain-Hardening Cementitious Composites (SHCCs), careful control of the crack-bridging relations is the key to a successful design of the materials. This study theoretically explores the crack-bridging relations of SHCCs with fixed total volume fraction (2.5%) of hybrid polyvinyl alcohol (PVA) and steel fibers. Since a large number of experiments at the single-fiber level are needed to determine the parameters for the micromechanical model, the snubbing coefficient, fiber strength reduction factor and Cook-Gordon parameter for mono-fiber composites were theoretically calibrated rather than experimentally obtained in this study. With these calibrated parameters, the crack-bridging relations of hybrid-fiber SHCCs were then modeled and compared to the test results. The superposition principle was used to address the contributions of different types of fibers, and the interaction between PVA and steel fibers was considered through the matrix micro-spalling in the modeling. The theoretically modeled crack-bridging relations of hybrid-fiber SHCCs were in good agreement with the test curves in terms of the tensile strength and the corresponding crack opening. The findings in this study provide a better understanding of fiber hybridizations in SHCCs.

Alkali treatment on nettle fibers

This work deals with alkali treatment on nettle fibers. The first part of this work examines structure and properties of alkali-treated nettle fibers, while the second part reports on optimization of alkali treatment to improve tensile properties of nettle fibers. In this part, the alkali-treated nettle fibers were examined for their chemical, structural, physical, and mechanical characteristics and compared to untreated fibers. The alkali treatment appeared to remove lignin from the fiber, thereby resulting in increase in its crystallinity, improvement in its appearance, and decrease in its width and linear density. The untreated fiber was quite strong but less flexible and less extensible. A mild alkali treatment resulted in increase in tensile strength and elongation at break, without much changing initial modulus. On the other hand, a severe alkali treatment resulted in reduction in fiber strength as well as initial modulus without much change in elongation at break.

A micromechanics-based fatigue dependent fiber-bridging constitutive model

Fiber-reinforced cementitious composites (FRCC) represent a large group of construction and building materials. While numerous experimental studies have been conducted on fatigue of FRCC, predicting FRCC fatigue performance remains difficult. This paper proposes a novel multi-scale analytical model to capture the fatigue dependency of fiber bridging constitutive law in FRCC. On the micro-scale, a new analytical model to predict the post-fatigue single-fiber pullout behavior (P-u curve) is established based on the understanding of the fatigue dependency of fiber and fiber-matrix interface. On the macro-scale, the fatigue-induced fiber strength reduction was considered and probabilistics is introduced to describe the randomness of fiber location and orientation so that the fatigue dependent fiber-bridging constitutive law can be predicted. The model proposed in this paper is the first analytical model that is able to capture the effects of fatigue cycle as well as the fatigue loading level on deterioration of fiber bridging in FRCC.

Characterization and biological depectinization of hemp fibers originating from different stem sections

The wide variation of mechanical properties of natural fibers limits their applications in matrix composites. The aim of this study is to evaluate the properties of hemp fibers from different stem sections (top, middle and bottom) and to assess fungal retting pretreatment of hemp from different stem sections with the white rot fungi Phlebia radiata Cel 26 and Ceriporiopsis subvermispora. For the untreated hemp fibers, no apparent difference in tensile behavior for fiber bundles from different stem sections was observed, and more than 90% tested samples demonstrated plastic flow behavior. Fiber strength and stiffness were highest for the fibers from the top and middle stem sections. These properties were related to the compositional make up and morphological properties of hemp fibers, notably the secondary fiber cell contents. In fungal retting, there was a strong dependence of depectinization selectivity on stem section, which decreased from bottom to top presumably due to the significantly higher lignin content in the bottom section than in the top section (middle section was in between). Consequently, the fungal retting caused a lower reduction in strength of fibers from the bottom section than in those from the top stem section, and essentially reversed the influence of stem section on fiber tensile strength through depectinization selectivity. At whole hemp stem level, the fungal retting with P. radiata Cel 26 exhibited better mechanical properties with an ultimate tensile strength, strain and stiffness of 736MPa, 2.3% and 42GPa, respectively, while fibers treated with C. subvermispora exhibited lower mechanical properties of 573MPa, 1.9% and 40GPa, respectively. The study thus also showed that less variable and high strength fibers may be produced using the dependence of depectinization selectivity on stem section for composite application.

Circumventing the Mechanochemical Origins of Strength Loss in the Synthesis of Hierarchical Carbon Fibers

Hierarchical carbon fibers (CFs) sheathed with radial arrays of carbon nanotubes (CNTs) are promising candidates for improving the intra- and interlaminar properties of advanced fiber-reinforced composites (e.g., graphite/epoxy) and for high-surface-area electrodes for battery and supercapacitor architectures. While CVD growth of CNTs on CFs has been previously shown to improve the apparent shear strength between fibers and polymer matrices (up to 60%), this has to date been achieved only at the expense of significant reductions in tensile strength (~30-50%) and stiffness (~10-20%) of the underlying fiber. Here we demonstrate two approaches for growing aligned and unaligned CNTs on CFs that enable preservation of fiber strength and stiffness. We observe that CVD-induced reduction of fiber strength and stiffness is primarily attributable to mechanochemical reorganization of the underlying fiber when heated untensioned above ~550 °C in both hydrocarbon-containing and inert atmospheres. We show that tensioning fibers to ≥12% of tensile strength during CVD enables aligned CNT growth while simultaneously preserving fiber strength and stiffness even at growth temperatures >700 °C. We also show that CNT growth employing CO2/acetylene at 480 °C without tensioning-below the identified critical strength-loss temperature-preserves fiber strength. These results highlight previously unidentified mechanisms underlying synthesis of hierarchical CFs and demonstrate scalable, facile methods for doing so.

Optimization of surface treatment to enhance fiber–matrix interface and performance of composites

Oxidation treatment with concentrated HNO3 was employed to the carbon fabric (CF) for various time intervals (30–180min) to observe the effect of treatment on two simultaneous processes involved viz. improvement in its adhesion with the matrix and reduction of fiber strength which in turn is responsible for change in the performance properties of composites. Seven composites with untreated and acid treated CF were developed based on the polyetherimide (PEI) matrix and evaluated for adhesive wear properties under various loads (200–600N) against mild steel disc. 90min treated CF composite indicated the best tribological properties and showed 30% reduction in specific wear rate (K0) and 23% in coefficient of friction (μ) respectively at 600N load. Treatment beyond this time proved detrimental for improvement in properties. Field emission scanning electron microscopy (FE-SEM) showed increase in roughness with treatment time, while atomic force microscopy (AFM) studies indicated substantial increase in roughness value. Scanning electron microscopy (SEM) of worn surfaces supported the wear mechanisms and improvement in adhesion between fiber and matrix.

Improving the mechanical properties of chitosan-based heart valve scaffolds using chitosan fibers

Chitosan is being widely studied for tissue engineering applications due to its biocompatibility and biodegradability. However, its use in load-bearing applications is limited due to low mechanical properties. In this study, we investigated the effectiveness of a chitosan fiber reinforcement approach to enhancing the mechanical properties of chitosan scaffolds. Chitosan fibers were fabricated using a solution extrusion and neutralization method and incorporated into porous chitosan scaffolds. The effects of fiber/scaffold mass ratio, fiber mechanical properties and fiber length on scaffold mechanical properties were studied. The results showed that incorporating fibers improved scaffold strength and stiffness in proportion to the fiber/scaffold mass ratio. A fiber-reinforced, heart valve scaffold achieved leaflet tensile strength values of 220±17 kPa, comparable to the radial values of human pulmonary valve leaflets. Additionally, the effects of 2 mm fibers were found to be up to threefold greater than 10 mm fibers at identical mass ratios. Heparin crosslinking of fibers produced a reduction in fiber strength, and thus failed to produce additional improvements to fiber-reinforced scaffold properties. Despite this reduction in fiber strength, heparin-modified fibers still improved the mechanical properties of reinforced scaffolds, but to a lesser extent than unmodified fibers. The results demonstrate that chitosan fiber reinforcement can be used to achieve porous chitosan scaffold strength approaching that of tissue, and that fiber length and mechanical properties are important parameters in defining the degree of mechanical improvement.

Tensile properties of carbon fibres and carbon fibre–polymer composites in fire

The effect of fire on the tensile properties of carbon fibres is experimentally determined to provide new insights into the tensile performance of carbon fibre–polymer composite materials during fire. Structural tests on carbon–epoxy laminate reveal that thermally-activated weakening of the fibre reinforcement is the dominant softening process which leads to failure in the event of a fire. This process is experimentally investigated by determining the reduction to the tensile properties and identifying the softening mechanism of T700 carbon fibre following exposure to simulated fires of different temperatures (up to 700°C) and atmospheres (air and inert). The fibre modulus decreases with increasing temperature (above ∼500°C) in air, which is attributed to oxidation of the higher stiffness layer in the near-surface fibre region. The fibre modulus is not affected when heated in an inert (nitrogen) atmosphere due to the absence of surface oxidation, revealing that the stiffness loss of carbon fibre composites in fire is sensitive to the oxygen content. The tensile strength of carbon fibre is reduced by nearly 50% following exposure to temperatures over the range 400–700°C in an air or inert atmosphere. Unlike the fibre modulus, the reduction in fibre strength is insensitive to the oxygen content of the atmosphere during fire. The reduction in strength is possibly attributable to very small (under ∼100nm) flaws and removal of the sizing caused by high temperature exposure.

Mechanical properties of thermally-treated and recycled glass fibres

This paper investigates the effects of temperature, heating time and atmosphere on the tensile modulus and strength of thermally-treated E-glass fibres. The heating conditions that were investigated are identical to those used in thermal recycling of waste polymer matrix composite materials, and therefore this study determines the effects of the recycling process conditions on the properties of reclaimed fibreglass. The loss in fibre strength is dependent on the temperature and time of the thermal process, and large strength loss occurs under the heating conditions used for high temperature incineration of polymer composites. A phenomenological model is presented for the residual fibre strength for the temperatures and heating time of the thermal recycling process. The reduction in fibre strength is dependent on the thermal recycling atmosphere under low temperature or short heating time conditions, but at high temperatures the strength loss is the same, regardless of furnace atmosphere (ambient air, dry air or inert gas). Quantitative fractographic analysis of the fibres shows that fracture for all heat treatments is caused by surface flaws. The strength loss is most probably due to structural relaxation during thermal annealing and a secondary effect of adsorbed surface water attacking the glass by thermally-activated stress-corrosion. It is shown that large reductions in fibre strength due to thermal recycling are not recovered during composite manufacture, therefore resulting in composite materials with significantly lower strength. The reduced strength of the composite matches the reduced fibre strength following thermal recycling.

Strength degradation of NiAl coated sapphire fibres with a V 2AlC interlayer

The tensile strength of sapphire fibres coated with NiAl containing a V 2AlC interlayer was investigated. The effect of the deposition processes as well as the influence of temperature experienced during diffusion bonding onto the tensile strength was analysed. Strength degradation mechanisms were identified by chemical and morphological analysis. The investigated material systems show significant strength degradation at room temperature. This may be due to thermal stress induced twin formation as well as formation of fracture mirrors. Thermal stresses arise upon cooling after the deposition and the consolidation process, respectively. With the introduction of a V 2AlC interlayer chemical reactions between fibre and matrix could be prevented and the population of morphological features on the fibre surface due to Al 2O 3 surface diffusion acting as stress concentration sites could be reduced. The extensive reduction in fibre strength can be understood based on thermal stress induced twinning and fracture mirror formation.

Fabrication of ultrafine powder from eri silk through attritor and jet milling

Fibroin protein derived from silk fibres has been extensively studied with exciting outcomes for a number of potential advanced biomaterial applications. However, one of the major challenges in applications lies in engineering fibroin into a desired form using a convenient production technology. In this paper, fabrication of ultrafine powder from eri silk is reported. The silk cocoons were degummed and the extracted silk fibres were then chopped into snippets prior to attritor and air jet milling. Effects of process control agents, material load and material to water ratio during attritor milling were studied. Compared to dry and dry–wet attritor milling, wet process emerged as the preferred option as it caused less colour change and facilitated easy handling. Ultrafine silk powder with a volume based particle size d(0.5) of around 700 nm could be prepared following the sequence of chopping ➔ wet attritor milling ➔ spray drying ➔ air jet milling. Unlike most reported powder production methods, this method could fabricate silk particles in a short time without any pre-treatment on degummed fibre. Moreover, the size range obtained is much smaller than that previously produced using standard milling devices. Reduction in fibre tenacity either shortened the milling time even further or helped bypassing media milling to produce fine powder directly through jet milling. However, such reduction in fibre strength did not help in increasing the ultimate particle fineness. The study also revealed that particle density and particle morphology could be manipulated through appropriate changes in the degumming process.

Treatment of Raw Cotton Fibers with Cellulases for Nonwoven Fabrics

Enzymatic treatment of fabrics is known to have a negative impact on the fabrics' tensile characteristics. The strength loss problems would be more severe in the case of nonwovens, since cellulase could attack bonded areas of the fabric. To prevent the fabric strength loss, a study was undertaken to treat cotton fibers as opposed to the final fabric. Raw cotton fibers were hydrolyzed with Cellusoft L and endoglucanase without a cellulose— binding domain. Factors such as the reducing end group, fiber length and tenacity were followed throughout the enzymatic hydrolysis. The results showed that if the concentration of Cellusoft L cellulase mixture was minimized to 0.25 % owf, extensive weakening of cotton fibers could be prevented. Endoglucanase demonstrated a moderate reduction in fiber strength; most of the reduction occurred at the beginning of hydrolysis. The concentration of reducing ends was observed to have a maximum for both enzyme solutions. The location of enzymatic attack and changes in surface morphology were monitored by Congo red staining and scanning electron microscope, respectively.

The Role of Thioredoxin Reductases in Brain Development

The thioredoxin-dependent system is an essential regulator of cellular redox balance. Since oxidative stress has been linked with neurodegenerative disease, we studied the roles of thioredoxin reductases in brain using mice with nervous system (NS)-specific deletion of cytosolic (Txnrd1) and mitochondrial (Txnrd2) thioredoxin reductase. While NS-specific Txnrd2 null mice develop normally, mice lacking Txnrd1 in the NS were significantly smaller and displayed ataxia and tremor. A striking patterned cerebellar hypoplasia was observed. Proliferation of the external granular layer (EGL) was strongly reduced and fissure formation and laminar organisation of the cerebellar cortex was impaired in the rostral portion of the cerebellum. Purkinje cells were ectopically located and their dendrites stunted. The Bergmann glial network was disorganized and showed a pronounced reduction in fiber strength. Cerebellar hypoplasia did not result from increased apoptosis, but from decreased proliferation of granule cell precursors within the EGL. Of note, neuron-specific inactivation of Txnrd1 did not result in cerebellar hypoplasia, suggesting a vital role for Txnrd1 in Bergmann glia or neuronal precursor cells.

Ultra-fine silk powder preparation through rotary and ball milling

Three commercial silk varieties, namely mulberry, muga and eri, were used to prepare ultra-fine silk particles. Degummed silk fibres were chopped into short snippets and then pulverised using rotary and planetary ball milling. The effects of degree of degumming, size of milling media, water and lubricant on particle refinement were studied. Before milling, single fibre strength tests were conducted on silk fibres degummed under different conditions. The results indicate that while reducing fibre strength via harsh degumming could cut milling time drastically, too severe a reduction in fibre strength is actually detrimental to achievable minimum particle size due to increased particle aggregation. Water played an important role in affecting the performance of ball milling. With the milling processes used in this study, a volume based median particle size ( d(0.5)) of around 200 nm was achieved, which is much smaller than previously reported results. To achieve a similar particle size, mulberry silk required more milling time, even though the overall milling behaviour was quite similar for the three silk varieties examined. SEM observations revealed axial spitting and fragmentation of micro and nanofibrillar architecture of silk fibres due to milling. Unlike ball milling, which produced particles with variable shapes, rotary milled particles remained fibrous through the size reduction process.

Evaluation of hygrothermal effects on the shear properties of Carall composites

Fiber metal laminates are the frontline materials for aeronautical and space structures. These composites consists of layers of 2024-T3-aluminum alloy and composite prepreg layers. When the composite layer is a carbon fiber prepreg, the fiber metal laminate, named Carall, offers significant improvements over current available materials for aircraft structures. While weight reduction and improved damage tolerance characteristics were the prime drivers to develop this new family of materials, it turns out that they have additional benefits, which become more and more important for today's designers, such as cost reduction and improved safety. The degradation of composites is due to environmental effects mainly on the chemical and/or physical properties of the polymer matrix leading to loss of adhesion of fiber/resin interface. Also, the reduction of fiber strength and stiffness are expected due to environmental degradation. Changes in interface/interphase properties leads to more pronounced changes in shear properties than any other mechanical properties. In this work, the influence of moisture in shear properties of carbon fiber/epoxy composites and Carall have been investigated by using interlaminar shear (ILSS) and Iosipescu tests. It was observed that hygrothermal conditioning reduces the Iosipescu shear strength of CF/ E and Carall composites due to the moisture absorption in these materials.

Hygrothermal effects on the shear properties of carbon fiber/epoxy composites

The environmental factors, such as humidity and temperature, can limit the applications of composites by deteriorating the mechanical properties over a period of time. Environmental factors play an important role during the manufacture step and during composite’s life cycle. The degradation of composites due to environmental effects is mainly caused by chemical and/or physical damages in the polymer matrix, loss of adhesion at the fiber/matrix interface, and/or reduction of fiber strength and stiffness. Composite’s degradation can be measure by shear tests because shear failure is a matrix dominated property. In this work, the influence of moisture in shear properties of carbon fiber/epoxy composites (laminates [0/0]s and [0/90]s) have been investigated. The interlaminar shear strength (ILSS) was measured by using the short beam shear test, and Iosipescu shear strength and modulus (G12) have been determinated by using the Iosipescu test. Results for laminates [0/0]s and [0/90]s, after hygrothermal conditioning, exhibited a reduction of 21% and 18% on the interlaminar shear strenght, respectively, when compared to the unconditioned samples. Shear modulus follows the same trend. A reduction of 14.1 and 17.6% was found for [0/0]s and [0/90]s, respectively, when compared to the unconditioned samples. Microstructural observations of the fracture surfaces by optical and scanning electron microscopies showed typical damage mechanisms for laminates [0/0]s and [0/90]s.

Processing and hygrothermal effects on viscoelastic behavior of glass fiber/epoxy composites

Fiber reinforced epoxy composites are used in a wide variety of applications in the aerospace field. These materials have high specific moduli, high specific strength and their properties can be tailored to application requirements. In order to screening optimum materials behavior, the effects of external environments on the mechanical properties during usage must be clearly understood. The environmental action, such as high moisture concentration, high temperatures, corrosive fluids or ultraviolet radiation (UV), can affect the performance of advanced composites during service. These factors can limit the applications of composites by deteriorating the mechanical properties over a period of time. Properties determination is attributed to the chemical and/or physical damages caused in the polymer matrix, loss of adhesion of fiber/resin interface, and/or reduction of fiber strength and stiffness. The dynamic elastic properties are important characteristics of glass fiber reinforced composites (GRFC). They control the damping behavior of composite structures and are also an ideal tool for monitoring the development of GFRC’s mechanical properties during their processing or service. One of the most used tests is the vibration damping. In this work, the measurement consisted of recording the vibration decay of a rectangular plate excited by a controlled mechanism to identify the elastic and damping properties of the material under test. The frequency amplitude were measured by accelerometers and calculated by using a digital method. The present studies have been performed to explore relations between the dynamic mechanical properties, damping test and the influence of high moisture concentration of glass fiber reinforced composites (plain weave). The results show that the E’ decreased with the increase in the exposed time for glass fiber/epoxy composites specimens exposed at 80∘C and 90% RH. The E’ values found were: 26.7, 26.7, 25.4, 24.7 and 24.7 GPa for 0, 15, 30, 45 and 60 days of exposure, respectively.

Residual strength of PIP-processed SiC/SiC single-tow minicomposite exposed at high temperatures in air as a function of exposure temperature and time

Abstract Tensile behavior and residual strength at room temperature of the undirectional PIP (polymer impregnation and pyrolysis)-processed SiC/SiC single-tow minicomposite exposed at 823–1673 K for 3.6×10 2 –3.6×10 5 s in air were studied experimentally and analytically. In the early stage of oxidation, the residual strength remained to be the same as that of original one (Stage I). Then it decreased sharply (Stage II). In the late stage, it decreased more but the decreasing rate with respect to exposure time became low (Stage III). The decrease in strength of the composite could be attributed mainly to the reduction in fiber strength caused by the extension of the crack made by the premature fracture of the SiO 2 layer into the fiber. Based on the fracture mechanical approach combined with the kinetics for the growth of SiO 2 layer, a simple model was presented to describe the variation of residual strength as a function of exposure temperature and time. The experimental results could be described well by the presented model.