Unbroken Fibers Research Articles

Microstructure of Milled Polyacrylonitrile-Based Carbon Fiber Analyzed by Micro-Raman Spectroscopy and TEM.

Milled polyacrylonitrile (PAN)-based Carbon Fibers (mPCFs) were prepared from PAN-based carbon fibers by using a ball milling process. The resulting structural changes in the mPCFs were analyzed by correlating the analytical results obtained by X-ray diffraction (XRD) and Raman spectroscopy and verified by transmission electron microscopy (TEM) lattice images and diffraction patterns. The crystallite size La calculated from the XRD measurements decreased as the milling time was increased to 12 h and then decreased as the milling time was further increased to 18 h. The La of both partially-milled Carbon Fiber (pmCF) and milled Carbon Fiber (mCF) calculated from the Raman spectroscopy data continuously increased as the milling time increased. The difference may be because XRD measured the entire sample regardless of pmCF and mCF, while Raman spectroscopy was limited to measuring the surface and differentiated pmCF and mCF. As the ball milling time increased, the fiber surface was firstly broken by the impact energy of the balls, decreasing crystallinity, while the La inside the unbroken fibers increased.

Developing of the optimal shape and reinforcement structure of the specimen for adequate determination of the tensile strength in unidirectional composites

Unidirectional composites exhibit the highest strength when stretched along the fibers. However, the proper determination of the strength faces great methodological difficulties. The main problems of tensile testing of polymer composites consisted in developing of the specimen shape and the method of specimen fixation which ensure the minimum impact of the stress concentration near the grips on the strength measurements. A conventional shape of the specimen with fillets is unsuitable for unidirectional polymers due to the splitting occurred in the fillet zones upon loading. Therefore, the specimens are usually standardized in the form of rectangular strips fixed using pads or special grips which provide constant transverse forces. However, with such a specimen shape, a significant stress concentration inevitably occurs at the edge of grips and the lower the ratio of the interlayer shear modulus to the longitudinal Young’s modulus, the greater the stress concentration impact. For the purpose of the most correct determination of the strength we propose to use specimens with smoothly varying dimensions at the same cross-sectional area which ensures keeping the total number of unbroken fibers in each section. The specimen thickness decreases when moving from the working part of the specimen to the gripping part, whereas the width (while maintaining the section area) grows to prevent the specimen collapsing resulting from transverse forces in standard self-tightening grips. Analytical and FEM modeling is performed to select a rational contour shape. Technological equipment has been developed and a procedure of manufacturing testing specimens has been worked out. The tensile test of specially manufactured curvilinear reinforced specimens showed higher strength values compared to standard rectangular strips or specimens with semicircular fillets.

Preparation of a novel nano-TiO2 photocatalytic composite using insoluble wood flour as bio-carrier and dissolved components as accelerant

Using NaOH/ urea solution to partially dissolve wood flour, a novel nano-TiO2 photocatalytic composite was prepared by hydrothermal method with insoluble wood flour as bio-carrier and dissolved components of wood flour as accelerant. SEM results showed that the control sample prepared without adding titanium sulfate consisted of insoluble wood flour and still possessed unbroken fibers after hydrothermal treatment. The control sample prepared without wood flour addition was only composed of TiO2 micro-particles. With assistant of the dissolve components of wood flour, nano-TiO2 with the size of 50−300 nm was well assembled on the surface of insoluble wood flour. The composite is composed of anatase TiO2 and insoluble wood flour and possess high specific surface area. The absorption edge of composite extended to the visible light range. The composite exhibited high removal rate of methylene blue, up to 97.7% within 120 min. This performance was even better than commercial TiO2 (P25) with the rate of 81.8%. The reused performance of the composite retains removal rate of 90.7% even after 5 cycles. The environment-friendly nano-TiO2 photocatalytic composite can be very useful for degrading refractory organic pollutants.

Effect of Environmental Conditions on the Resistance of Damaged Composite Materials

The present paper proposes a strength model for unidirectional composites with Lin/Epoxy. The model assumes that, a central core of broken fibers flanked by unbroken fibers which are subject to stress concentrations from the broken fibers. The approach of the model consists of using a modified shear lag model to calculate the ineffective lengths and stress concentrations around fiber breaks. In this paper, we attempt to incorporate in the proposed model the unidirectional composite property variation with temperature and moisture in order to predict even composite strength degradation. Strength degradation is often seen as a result of changes in ineffective lengths at fiber breaks. Subsequently, damage to the material can be estimated at the micromechanical scale under the effect of temperature and humidity.

In-situ observations of abrasion mechanisms of nonwoven fabric

Experiments were conducted to observe non-woven fabrics in-situ during abrasion, with a view to elucidating the mechanism of abrasive damage. The initial stage involves the generation of fuzziness, characterized primarily by unbroken fibers being pulled out of the fabric with or without detachment from a bond site. Use of higher bonding temperatures during manufacture reduces this form of damage, and leads to better abrasion resistance. The morphology of the pills that were formed has been correlated to the abrasion conditions. For example, long pills are formed preferentially upon abrasion with a smooth surface, while ball pills are generated upon abrasion with another fibrous material.

Effective stress transferring interface and mechanical property enhancement of poly(l-lactide)/multi-walled carbon nanotubes fibers

The poly(l-lactide) grafted multi-walled carbon nanotubes (MWNT-g-PLLA) was firstly prepared, and a maximum grafting amount of 31 wt% was obtained by optimizing grafting reaction conditions. Then the PLLA/MWNT-g-PLLA composite fibers were fabricated by melt spinning, and showed significant enhancement of tensile strength and elongation at break, comparing with the unmodified PLLA/MWNTs or pure PLLA ones. Structure analysis of as-spun fibers revealed uniform dispersion of MWNT-g-PLLA in PLLA matrix on account of good interfacial compatibility, which could prevent premature interface damage and ensure the continuous deformation of fibers under stress. The crystal morphology of strain hardening before unbroken fibers were expressly observed, suggesting a possible stretching induced formation of nano-hybrid shish-calabash similar to structure in which the PLLA lamellae growing onto the MWNT-g-PLLA axis. This interface crystallization greatly improved the interfacial stress transferring between PLLA and MWNT-g-PLLA, thus resulted in notable promoted tensile strength and elongation at break.

Comparison of the fracture resistances of glass fiber mesh- and metal mesh-reinforced maxillary complete denture under dynamic fatigue loading.

PURPOSEThe aim of this study was to investigate the effect of reinforcing materials on the fracture resistances of glass fiber mesh- and Cr–Co metal mesh-reinforced maxillary complete dentures under fatigue loading.MATERIALS AND METHODSGlass fiber mesh- and Cr–Co mesh-reinforced maxillary complete dentures were fabricated using silicone molds and acrylic resin. A control group was prepared with no reinforcement (n = 15 per group). After fatigue loading was applied using a chewing simulator, fracture resistance was measured by a universal testing machine. The fracture patterns were analyzed and the fractured surfaces were observed by scanning electron microscopy.RESULTSAfter cyclic loading, none of the dentures showed cracks or fractures. During fracture resistance testing, all unreinforced dentures experienced complete fracture. The mesh-reinforced dentures primarily showed posterior framework fracture. Deformation of the all-metal framework caused the metal mesh-reinforced denture to exhibit the highest fracture resistance, followed by the glass fiber mesh-reinforced denture (P<.05) and the control group (P<.05). The glass fiber mesh-reinforced denture primarily maintained its original shape with unbroken fibers. River line pattern of the control group, dimples and interdendritic fractures of the metal mesh group, and radial fracture lines of the glass fiber group were observed on the fractured surfaces.CONCLUSIONThe glass fiber mesh-reinforced denture exhibits a fracture resistance higher than that of the unreinforced denture, but lower than that of the metal mesh-reinforced denture because of the deformation of the metal mesh. The glass fiber mesh-reinforced denture maintains its shape even after fracture, indicating the possibility of easier repair.

Monitoring self-sensing damage of multiple carbon fiber composites using piezoresistivity

The change of electrical resistance in small bundles of multiple carbon fibers and multiple unidirectional carbon fiber/epoxy composites with applied tensile strain has been investigated. The electrical resistance of bundles initially increases relatively slowly in a stepwise manner with increasing strain due to fracture of peripheral fibers. This regime corresponds to the linearly increasing part of the load-strain curve. At higher strain, a progressive fracture of inner fibers in the bundle associated with flat region of load-strain curve leads to concomitant sudden rise of resistance. When the whole sample undergoes major failure, the slope of the load-strain curve becomes negative while the relative resistance increases abruptly to infinity. In strands of carbon fibers slightly impregnated with epoxy the change of resistance is affected by the thickness of epoxy layer surrounding the fibers. We demonstrate that volume fraction of fibers as well as initial number of fibers in the epoxy determines the piezoresistance properties of the specimen. Broken fibers can come into electrical contact with unbroken fibers and thus participate to the overall resistance of the specimen. As a result the dependency of relative resistance versus increasing applied strain presents stepwise behaviour also in the high strain region that is attributed to fiber fracture. In contrast to the bundles of bare carbon fibers, the stress-strain curves of the composites demonstrate monotonous linear increase in both low and high strain regions. The relative resistance goes to infinity when all remaining unbroken fibers undergo fracture.

Effect of the environmental conditions on the ultimate load of damaged unidirectional of natural hemp fiber/epoxy composite

Natural continuous natural fibres (hemp, flax, etc.) are gaining popularity in composite materials because they can advantageously replace glass fibers. It is found that intrinsic properties of unidirectional (UD) composites are almost equivalent to those of unidirectional glass fiber composites. Unfortunately it is difficult to get repeatable results because of the inherent variability in the properties of natural fibers compared to glass fibers. Their quality is largely affected by the weather conditions, the extraction location along the plant and the techniques used to extract them (retting, bleaching, etc.). The present paper proposes a strength reliability model for unidirectional composites with natural fibers in a hexagonal array. The model assumes that, a central core of broken fibers flanked by unbroken fibers which are subject to stress concentrations from the broken natural fibers. Thermal and hygroscopic residual stresses are neglected because they haven’t more effect when the composite was subjected to tensile than transverse loading. The approach of the model consists of using a modified shear lag model to calculate the ineffective lengths and stress concentrations around the broken fibers. In this paper, we attempt to incorporate in the proposed model the unidirectional composite property variation with temperature and moisture in order to predict even composite strength degradation. Strength degradation is often seen as a result of changes in ineffective lengths at natural fiber breaks and the corresponding stress concentrations in intact neighboring fibers.

TU-CD-207-08: Intrinsic Image Quality Comparison of Synthesized 2-D and FFDM Images

Purpose: With the combined interest of managing patient dose, maintaining or improving image quality, and maintaining or improving the diagnostic utility of mammographic data, this study aims to compare the intrinsic image quality of Hologic’s synthesized 2-D (C-View) and 2-D FFDM images in terms of resolution, contrast, and noise. Methods: This study utilized a novel 3-D printed anthropomorphic breast phantom in addition to the American College of Radiology (ACR) mammography accreditation phantom. Analysis of the 3-D anthropomorphic phantom included visual assessment of resolution and analysis of the normalized noise power spectrum. Analysis of the ACR phantom included both visual inspection and objective automated analysis using in-house software. The software incorporates image- and object-specific CNR visibility thresholds which account for image characteristics such as noise texture which affect object visualization. T- test statistical analysis was also performed on ACR phantom scores. Results: The spatial resolution of C-View images is markedly lower (at least 50% worse) than that of FFDM. And while this is generally associated with the benefit of reduced relative noise magnitude, the noise in C-View images tends to have a more mottled (predominantly low-frequency) texture. In general, for high contrast objects, C-View provides superior visualization over FFDM; however this benefit diminishes for low contrast objects and is applicable only to objects that are sufficiently larger than the spatial resolution threshold. Based on both observer and automated ACR phantom analysis, between 50–70% of C-View images failed to meet ACR minimum accreditation requirements – primarily due to insufficient (unbroken) fiber visibility. Conclusion: Compared to FFDM, C-View offers better depiction of objects of certain size and contrast, but provides poorer overall resolution and noise properties. Based on these findings, the utilization of C-View images in the clinical setting requires careful consideration, especially if considering the discontinuation of FFDM imaging.

Stochastic mechanical degradation of multi-cracked fiber bundles with elastic and viscous interactions.

The mechanics of fiber bundles has been largely investigated in order to understand their complex failure modes. Under a mechanical load, the fibers fail progressively while the load is redistributed among the unbroken fibers. The classical fiber bundle model captures the most important features of this rupture process. On the other hand, the homogenization techniques are able to evaluate the stiffness degradation of bulk solids with a given population of cracks. However, these approaches are inadequate to determine the effective response of a degraded bundle where breaks are induced by non-mechanical actions. Here, we propose a method to analyze the behavior of a fiber bundle, undergoing a random distribution of breaks, by considering the intrinsic response of the fibers and the visco-elastic interactions among them. We obtain analytical solutions for simple configurations, while the most general cases are studied by Monte Carlo simulations. We find that the degradation of the effective bundle stiffness can be described by two scaling regimes: a first exponential regime for a low density of breaks, followed by a power-law regime at increasingly higher break density. For both regimes, we find analytical effective expressions described by specific scaling exponents.

Uncertainties of unidirectional composite strength under tensile loading and variation of environmental condition

A probabilistic strength model is developed for unidirectional composites with fibers in hexagonal arrays. The model assumes that, a central core of broken fibers surrounded by unbroken fibers which are subjected to unidirectional tensile loading. The proposed approach consists in using a modified shear lag model to calculate the ineffective lengths and stress concentrations around fiber breaks. The main feature in the model lies in incorporating the variation of composite properties due to temperature and moisture, in order to predict degradation of fibers and matrix characteristics. The strength degradation is often seen as a result of changes in ineffective lengths at fiber breaks, leading to stress concentrations in intact neighboring fibers. As fiber breaks are intrinsically random, the variability of input data allows us to describe the probabilistic model by using the Monte-Carlo method. The sensitivities of the mechanical response are evaluated regarding the uncertainties in design variables such as Young’s modulus of fibers and matrix, fiber reference strength, shear yield stress, fiber volume fraction and shear parameter defining the shear stress in the inelastic region.

Influence of fiber failure on fatigue hysteresis loops of ceramic matrix composites

An analytical methodology has been developed to investigate the influence of fiber failure on fatigue hysteresis loops of ceramic matrix composites in this article. During fatigue loading, matrix cracking, interface debonding and fiber failure occur upon first loading to the fatigue maximum stress. Matrix cracking space and interface debonding length are obtained by matrix statistical cracking model and fracture mechanics interface debonding criterion. Based on the assumption of global load-sharing criterion for the load distribution between the unbroken and broken fibers, an approach to determine fiber failure probability during fatigue loading for the degradation of interface shear stress and fiber strength is developed in this analysis. Upon unloading and subsequent reloading, stress—strain hysteresis loops develop as fiber sliding relative to matrix in the interface debonded region. The unloading interface counter slip length and reloading new slip length are obtained by the fracture mechanics interface debonding criterion. The effects of characteristic fiber strength, fiber Weibull modulus, and fatigue maximum stress on fiber failure during fatigue loading, and then on the shape, location, and area of the fatigue hysteresis loops are investigated. The fatigue hysteresis loops of three different ceramic composites corresponding to different cycles are predicted and agree well with the experimental data.

Simulation of crack propagation in fiber-reinforced bulk metallic glasses

Based on a phase-field model for deformation in bulk metallic glasses (BMGs), shear band formation and crack propagation in the fiber-reinforced BMG are investigated. Ideal unbroken fibers embedded in the BMG matrix are found to significantly influence the shear banding and crack propagation in the matrix. The crack propagation affected by fibers’ length and orientation is quantitatively characterized and is described by micromechanics models for composite materials. Furthermore, fractures in some practical fiber-reinforced BMG composites such as tungsten-reinforced Zr-based BMG are simulated. The relation between the enhanced fracture toughness and the mechanical properties of fiber reinforcements is determined. Different fracture modes of BMG-matrix composites are identified from the systematic simulation studies, which are found to be consistent with experiments. The simulation results suggest that the phase-field modeling approach could be a useful tool to assist the fabrication and design of BMG composites with high fracture toughness and ductility.

Analysis of Cracks with Bridged Zone of Large Scale

AbstractABSTRACT. The direct boundary element method is used to analyze of the stress–strain state and fracture parameters of fiber–reinforced and layered composites with bridged interface cracks under transient thermal and mechanical loadings. It is supposed that unbroken fibers make up a bridged zone along of whole or part of a delamination region between layers of composite materials. For modelling purposes a bridged zone is considered as a part of a crack between joined materials and supposed that distributed spring–like bonds with given nonlinear deformation law link and constrain crack surfaces. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Bridged interface cracks under transient thermal loading

AbstractThe direct boundary element method is used to analyze of the stress‐strain state and fracture parameters of fiber‐reinforced layered composites structures with interface bridged cracks under transient thermal and mechanical loadings. It is supposed that unbroken fibers make up bridged zone along of whole or part of a delamination region between layers of composite materials. For modeling purposes the bridged zone is considered as a part of the interface crack between of joined materials and supposed that distributed nonlinear spring‐like bonds link and constrain the crack surfaces. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

A higher-order theory for crack growth in fiber-metal laminates under generalized plane-stress conditions

Fiber-metal laminates (FML) are hybrid materials that consist of alternating layers of metal and fiber-reinforced prepreg. The classical plane-stress theory has difficulty in dealing with the fatigue fracture of such materials where the crack only grows in the metal layers, while the prepreg layers remain intact. In this paper, a new theoretical treatment is given to FML under generalized plane-stress conditions. The new theory introduces a harmonic anti-plane-stress potential p to describe the interlaminar stresses near the crack tips and the “bridging” effect of the unbroken fibers along the crack wakes. An analytical solution is derived for GLARE-3(3/2) containing collinear cracks with length 2a0 (the initial crack length) in the prepreg and length 2a in the aluminum layer. The effective stress intensity factor is obtained in a closed form, and the theoretical prediction is compared with the experimental behavior obtained from fatigue crack growth testing of center-notched specimens.

Stress transfer and damage evolution simulations of fiber-reinforced polymer–matrix composites

The stress transfer from broken fibers to unbroken fibers in fiber-reinforced thermosetting polymer–matrix composites and thermoplastic polymer–matrix composites was studied using a detailed finite element model. In order to check the validity of this approach, an epoxy–matrix monolayer composite was used as thermosetting polymer–matrix composite and a polypropylene (PP)–matrix monolayer composite was used as thermoplastic polymer–matrix composite, respectively. It is found that the stress concentrations near the broken fiber element cause damage to the neighboring epoxy matrix prior to the breakage of other fibers, whereas in the case of PP–matrix composites the fibers nearest to the broken fiber break prior to the PP matrix damage, because the PP matrix around the broken fiber element yields. In order to simulate composite damage evolution, a Monte Carlo technique based on a finite element method has been developed in the paper. The finite element code coupled with statistical model of fiber strength specifically written for this problem was used to determine the stress redistribution. Five hundred samples of numerical simulation were carried out to obtain statistical deformation and failure process of composites with fixed fiber volume fraction. The result indicates that the moderate interfacial shear sliding strength can weaken the adverse effect of fiber breaks on composite strength, resulting in the increase of the average strength of composites.

Investigation of progressive failure in composites by combined simulated and experimental photoelasticity

The description of progressive failure of unidirectional fibre reinforced composites using, e.g., fibre bundle models depends on realistic assumptions for load redistributions from broken to unbroken fibres. Therefore, gaining qualitative and quantitative insight in real fibre–matrix and fibre–fibre interactions is essential, however, available techniques have serious drawbacks in applicability or in significance of results. This paper presents a non-destructive method for investigating stress conditions in model samples using combined experimental and simulated photoelasticity. The approach provides a powerful basis of comparison between straight experiments and comprehensive finite element analysis (FEA). Its result is an experimentally based fully tensorial 3D stress field in the matrix and fibres. Hence the method is a reliable tool to study fibre, matrix and interface properties including shear transmission and debonding. The presented application on polymer model specimens containing up to five parallel glass fibres loaded at angles from 0° to 15° proves the capability of the approach. It provides deeper insights into angle-dependent successive failure events.

Fatigue damage and fracture behavior of tungsten fiber reinforced Zr-based metallic glassy composite

The fatigue life, damage and fracture behavior of tungsten fiber reinforced metallic glass Zr 41.25Ti 13.75Ni 10Cu 12.5Be 22.5 composites are investigated under cyclic push–pull loading. It is found that the fatigue life of the composite increases with increasing the volume fraction of tungsten fibers. Similar to crystalline metals, the regions of crack initiation, propagation and overload fracture can be discerned on the fracture surface of the specimen. Fatigue crack normally initiates in the metallic glass matrix at the outer surface of the composite specimen and propagates predominantly in the matrix. Different crack front profile around the tungsten fibers and fiber pullout demonstrate that fatigue crack may propagate around the fiber, leading to bridging of the crack faces by the unbroken fiber and hence improved fatigue crack-growth resistance. Locally decreased effective stiffness in the region where fiber distribution is sparse may provide preferential crack path in the composite. A proposed model was exercised to elucidate different tungsten fiber fracture morphologies in the fatigue crack propagation and overload fracture regions in the light of Poisson's ratio effect during fatigue loading.