Fracture Surfaces Of Fibers Research Articles

Study on the coupling effect and failure mechanism of mesh fabric reinforced polyurea coating

Mesh fabric reinforced polyurethane coating is a typical type of fiber reinforced polymer composite structure. Some designers proposed to replace the substrate material of the fabric reinforced polyurethane coating with the polyurea material. However, there is a lack of research on the analysis and design of this kind of structure. This study investigated the coupling effect between the mesh fabric and polyurea substrate and explored the mechanical behavior and failure pattern of the mesh fabric reinforced polyurea coating. The deformation behavior and load-carrying capacity of the composite coating were also analyzed compared with the traditional fiber reinforced polyurethane composite coating. The main conclusions are as follows: Fabric fiber fracture leads to instantaneous stress pulse in the polyurea substrate, fiber fracture surface, and stress-pulse-induced microcracks lead to the overall cross-section pull-off failure. The polyurea material exhibits a higher strength (10 MPa∼30 MPa). Mesh fabric induced defects lead to a strength decrease of 12.2 % and an elongation decrease of more than 90 %. The mesh fabric could undermine the strength and deformation capacity of the polyurea substrate, playing a negative role in the material's performance. The polyurethane material generally exhibits a lower strength of no more than 10 MPa. The mesh fabric could increase the tensile strength by 14 times. The mesh fabric could increase the strength of the polyurethane coating substrate, bringing a certain strengthening effect. The mesh fabric can be adopted to strengthen the substrate when the substrate material's tensile strength is lower than that of the mesh fabric. When the substrate tensile strength exceeds the strength of the mesh fabric, it is not recommended to use the mesh fabric.

Degree of Cure, Microstructures, and Properties of Carbon/Epoxy Composites Processed via Frontal Polymerization.

The frontal polymerization (FP) of carbon/epoxy (C/Ep) composites is investigated, considering FP as a viable route for the additive manufacturing (AM) of thermoset composites. Neat epoxy (Ep) resin-, short carbon fiber (SCF)-, and continuous carbon fiber (CCF)-reinforced composites are considered in this study. The evolution of the exothermic reaction temperature, polymerization frontal velocity, degree of cure, microstructures, effects of fiber concentration, fracture surface, and thermal and mechanical properties are investigated. The results show that exothermic reaction temperatures range between 110 °C and 153 °C, while the initial excitation temperatures range from 150 °C to 270 °C. It is observed that a higher fiber content increases cure time and decreases average frontal velocity, particularly in low SCF concentrations. This occurs because resin content, which predominantly drives the exothermic reaction, decreases with increased fiber content. The FP velocities of neat Ep resin- and SCF-reinforced composites are seen to be 0.58 and 0.50 mm/s, respectively. The maximum FP velocity (0.64 mm/s) is observed in CCF/Ep composites. The degree of cure (αc) is observed to be in the range of 70% to 85% in FP-processed composites. Such a range of αc is significantly low in comparison to traditional composites processed through a long cure cycle. The glass transition temperature (Tg) of neat epoxy resin is seen to be approximately 154 °C, and it reduces slightly to a lower value (149 °C) for SCF-reinforced composites. The microstructures show significantly high void contents (12%) and large internal cracks. These internal cracks are initiated due to high thermal residual stress developed during curing for non-uniform temperature distribution. The tensile properties of FP-cured samples are seen to be inferior in comparison to autoclave-processed neat epoxy. This occurs mostly due to the presence of large void contents, internal cracks, and a poor degree of cure. Finally, a highly efficient and controlled FP method is desirable to achieve a defect-free microstructure with improved mechanical and thermal properties.

Analysis of fracture state and fatigue life of high modulus carbon fiber wrapped composite barrel

High-modulus Carbon Fiber-reinforced Polymer composites are widely used in critical industries due to their exceptional mechanical properties. However, the fatigue and fracture of high modulus carbon fiber materials are currently poorly studied. For this purpose, the NOL ring fatigue tensile testing is referenced and improved to investigate the performance and service life of high-modulus carbon fiber wound barrels. NOL ring specimens, made of M40 carbon fiber material, are subjected to quasi-static tensile and fatigue tests. The fracture state and characteristics of high modulus materials during fatigue are analyzed. Two-parameter Weibull distribution function is used to analysis statistically the fatigue life results of composite samples. Then, the life curve is drawn for different stress amplitudes. Additionally, the fiber fracture surface is observed under a high-power microscope. The fiber fracture model under brittle fracture is established. The results indicate that the fracture characteristics and mechanisms of high-modulus materials differ from those of high-strength materials, necessitating different fatigue life assessment methods. The fatigue failure of high modulus fibers exhibits brittle fracture similar to quasi-static loss, with flat fracture surfaces and minimal cumulative damage. The failure mechanism is accompanied by brittle fractures with a small amount of fiber pulled out.

Mechanical and morphological properties of unsaturated polyester resin composites reinforced with recycled PET fibers of varying lengths

PurposeThis study aims to deal with both the development and mechanical investigations of unsaturated polyester matrix (UPR) composites containing recycled polyethylene terephthalate (PET) fibers as new fillers.Design/methodology/approachUPR/PET fibers composites have been developed as mats by incorporating 5, 8, 13 and 18 parts per hundred of rubber (phr) of 6-, 10- and 15-mm length PET fibers from the recycling of postconsumer bottles. The mechanical and physical properties of the composites were investigated as a function of fiber content and length. A significant increase in stress at break and in ultimate stress (sr) were observed for composites reinforced with 5 and 8 phr of 15-mm length PET fibers. The Izod impact strength of UPR/mat PET fiber composites as a function of fiber rate and length showed that the 5 and 8 phr composites for the 15-mm length PET fiber have the optimal mechanical properties 13.55 and 10.50 Kj/m2, respectively. The morphological study showed that the strong adhesion resulting from the affinity of the PET fiber for the UPR matrix. The ductile fracture of materials reinforced with 5 and 8 phr is confirmed by the fiber deformation and fracture surface roughness.FindingsThis study concluded that the PET fiber enhances the properties of composites, a good correlation was observed between the results of the mechanical tests and the structural analysis revealing that for the lower concentrations, the PET fibers are well dispersed into the resin, but entanglements are evidenced when the fiber content increases.Originality/valueIt can be shown from scanning electron microscopy micrographs that the fabrication technique produced composites with good interfacial adhesion between PET fibers and UPR matrix.

The hole entrance damage in longitudinal torsional ultrasonic helical milling of CFRP composites

Carbon fiber reinforced polymer (CFRP) is prone to surface damage such as delamination, uncut fibers and tear during the process of hole-making in aerospace field. Helical milling greatly improves the exit damage of CFRP holes, but the entrance damage will further deteriorate. To further diminish hole entrance damage and enhance hole machining quality, this paper proposed a longitudinal torsional ultrasonic assisted helical milling method to investigate the formation mechanism of hole entrance damage based on a cutting fracture mechanism with a fiber angle of 0° to 180°. The differences of hole entrance damage between longitudinal torsional ultrasonic helical milling (LTUHM) and traditional helical milling (THM) were analysed by a series of comparative experiments. The results showed that longitudinal torsional ultrasonic machining significantly reduced the damage of CFRP holes compared to THM. The delamination damage factor and uncut fibers factor of the hole entrance are reduced to 24.92% and 20.28%, respectively, and the fiber fracture surface is flatter under LTUHM. The study provides a production guide for efficient hole-making of CFRP.

Technical agave fiber tensile performance: The effects of fiber heat-treatment

Natural fiber composites are a lightweight, sustainable alternative to high-density synthetic fiber composites in semi-structural automotive applications. Heat-treatment of natural fibers prior to compounding has been shown to improve composite thermal and dimensional stability while reducing moisture absorption and odor. Previous investigation into the performance of heat-treated blue-agave fiber composites found that heat treatment of fibers at 180 ℃ increased fiber crystallinity but did not significantly alter the mechanical performance of the resulting agave fiber composites. Therefore, this study seeks to better understand the effects of heat treatment on blue-agave fiber mechanical performance (single-fiber tensile) and morphology (SEM) prior to compounding. Heat-treatment of fibers at 180 ℃ resulted in an increase in fiber secondary modulus by approximately 150 % and a reduction in strain at break by approximately 80 %. Morphological analysis of fiber fracture surfaces showed that heat-treated fibers failed in a more brittle manner than untreated fibers, which is consistent with an increase in fiber crystallinity and stiffness. Additionally, this work evaluated the ability to predict agave fiber behavior using a modified 3-parameter Weibull distribution model. The resulting volume-based model was found to be a suitable predictor for agave fiber strength.

Study on the surface formation mechanism in scratching test with different ultrasonic vibration forms

Rotary ultrasonic machining (RUM) has been considered as an effective approach in the manufacturing of advanced materials such as ceramic matrix composite (CMC). However, the distinct removal mechanisms in RUM with different vibration forms has not been elucidated, which hinders the adoption of RUM technology in the field of material processing. In this research, the ultrasonic-assisted scratching tests of CMC were conducted to reveal the fundamentals of material removal and surface formation. Based on the kinematics of RUM processes, the vibration effects exerted in cutting areas were clarified as impact-separation and reciprocating-polishing, thus two types of diamond indenters have been specially designed for the intermittent and continuous scratching tests, respectively. The scratching force, friction factor and grooves morphology have been analyzed and compared with that of the conventional scratching without vibration. For the intermittent scratching with vertical vibration, it was found that the high frequency impact-separation between indenter and workpiece increased the critical penetration depth of ductile-brittle transition, which can help restrain the cracking defects of CMC. For the continuous scratching with tangential vibration, the overlapping rate was firstly proposed to evaluate the effect of reciprocating polishing. It was shown that higher overlapping rate and larger dynamic shear angle can be achieved by matching the scratching velocity with vibration frequency, which is conducive to obtaining even fiber fracture and better surface integrity. Due to the minimal volume of material removed in each vibration cycle, the scratching resistance was significantly lowered with smaller fluctuation under both vibration forms. Moreover, the diminishing of vibration effectiveness at certain scratching parameters was also discussed. This research work brings in a better understanding on the material removal mechanisms under different effects of ultrasonic vibration, which can provide a guidance for the design and optimization of the vibration assisted processing of advanced materials.

The cut resistant characteristics of organic high-performance yarns and STF/yarns

This paper mainly investigated the cut resistant property of shear thickening fluid enhanced organic high-performance yarn. Cut tests of neat yarn and shear thickening fluids/yarn were performed with two cutting angles. External forces involved in the cutting were analyzed. A simple theoretical relation was established based on the principle of the energy conversion. Two types of shear thickening fluids were prepared. Compared to neat yarn, the shear thickening fluids/yarn exhibited extremely high cut resistant property, especially, shear thickening fluids/yarn with graphene, indicating a synergistic effect. Fracture surfaces of fibers after yarns cut off were initially studied, which verified the cut resistant characteristics of organic high-performance yarns and shear thickening fluids/yarn.

Stress corrosion cracking and brittle failure in a fiber-reinforced plastic (FRP) insulator from a 400 kV transmission line in humid environment

Abstract A fiber-reinforced plastic (FRP) suspension insulator belonging to the Israel Electric Corporation (IECo) failed catastrophically on a 400 kV transmission line during service at the humid south Israeli coastline. The failure occurred during the summer of 2016 after 15 years of service. The suspension had a declared life expectancy of 30 years. The failure resulted in a local outage at a high cost. An investigation was commissioned. The insulator includes a FRP rod with a housing and weather-sheds made of silicon rubber (SiR); the FRP rod is fitted into a metal connector at its end. The fracture occurred 30–40 mm above the metal fitting/grading ring area near the first weather-shed. Macroscopically, the fracture surface was partially characterized by the features of brittle fracture mode in FRP, while the rest of the fracture surface featured severe fiber pullout and debonding. It was also possible to observe that a large crack in the SiR housing led directly to the fracture surface area that included stained and crushed fibers. EDS analysis revealed that the housing around the crack and the stained fibers themselves were polluted with a significant amount of nitrogen, in addition to other foreign pollutants. After surveying similar cases, it was strongly suggested that the source of nitrogen was nitric acid formed as a result of corona discharges on the insulator's surface. The acid caused the housing, fiber and resin to undergo corrosion. SEM imaging revealed fiber fracture surfaces with a morphology typical of stress corrosion cracking, showing that stress was part of the failure mechanism. The fracture of fibers and loss of resin led to a decrease in FRP rod strength and to failure.

Investigation of Transcrystalline Interphases in Polypropylene/Glass Fiber Composites Using Micromechanical Tests

In composites, a strong interphase between the components is essential for mechanical properties. By using a suitable sizing (i.e., surface modification) of the fiber, the interphase may be varied, e.g., by suppressing or promoting heterogeneous nucleation of a thermoplastic matrix. In the latter case, three-dimensional transcrystallized interphases with properties differing from those of the bulk matrix are formed. Polypropylene-glass fiber composites are prepared as single-fiber model composites with (a) sizings either inducing or suppressing a transcrystalline interphase, (b) different amounts of modifier maleic acid anhydride grafted polypropylene, and (c) different molecular weights of the matrix polymer. These are studied in quasi-static or cyclic load tests. Static tests permit insights in the interfacial characteristics such as critical interface energy release rate, adhesion strength and frictional stress. Cyclic tests on these model composites can be used to study the nature of dissipative processes and the damage behavior. Atomic Force Microscopy (AFM) investigations of the fiber fracture surfaces provide supplementary information. The transcrystalline layer can indeed improve the mechanical parameters (a 70–100% increase of strength and a 25 or 125% increase in toughness, depending on the molecular weight (MW) of the matrix polymer at low modifier concentration). However, the effect is partially neutralized by an opposing effect: high nucleation in the bulk in samples with commonly used concentrations of modifier.

Mechanical and Thermal Performances of Roselle Fiber-Reinforced Thermoplastic Polyurethane Composites

ABSTRACTThe aim of this paper is to present research findings on the measurements of mechanical, morphological, and thermal properties of Roselle fiber-reinforced thermoplastic polyurethane composites. The Roselle fiber/thermoplastic polyurethane composites were prepared with fibers of different sizes such as 125 µm and lower, 125–300 and 300–425 µm by internal mixer and hot press at 170°C. The results show that mechanical properties (tensile, flexural, and impact properties) of the composites were improved with the increase in fiber sizes. The highest tensile (10.45 MPa), flexural strength (6.93 MPa), and impact strength (20.22 kJ/m2) was obtained from composites with 300–425 µm fiber size of Roselle fiber/thermoplastic polyurethane composites. Morphological properties of dispersion fiber and tensile fracture surfaces were studied using scanning electron microscope. Thermal properties of the composites were studied using thermogravimetric analyses and results showed that the thermal decomposition effect was almost similar for all compositions.

Fractal study on fracture surface of PAN-based carbon fiber

ABSTRACTPAN-based carbon fiber is a type of brittle material whose fracture surface has a structure peculiar to brittle materials. The tensile strength of a carbon fiber is determined by the strength of weak links in the fiber to a large extent. Since fracture surface is a type of weak link, the fracture surfaces of PAN-based carbon fibers were investigated in this article. SEM (Scanning Electron Microscopy) micrographs of selected carbon fibers with different tensile strengths were analyzed on computer and the fractal dimensions of fracture surfaces were obtained on the basis of fractal theories. The relationship between the tensile strength of PAN-based carbon fiber and the fractal dimension of its fracture surface was demonstrated.

Direct observation of nanoparticle migration in epoxy based fiber reinforced composites using fluorescence microscopy

The preferential accumulation of nanoparticles at carbon fiber surfaces, induced by the addition of a thermoplastic “migrating agent” to an epoxy resin, was monitored via in situ fluorescence microscopy. (3-Glydidyloxypropyl)trimethoxysilane functionalized fluorescent silica nanoparticles (GFCSP) were synthesized by a modified Stöber method to track the spatiotemporal abundance of nanoparticles. Single carbon fibers were embedded in an uncured epoxy mixture consisting of tetraglycidyl-4,4-diaminodiphenylmethane and 4,4’-diamnodiphenyl sulfone, as well as thermoplastic migrating agent, poly(ether sulfone), and GFCSP. A heated microscope stage was used to monitor the fluorescence in the local vicinity of the fiber as the epoxy begins to cross-link and solidify upon heating. Our results show that the synthesized GFCSP accumulate at fiber surfaces only in the presence of poly(ether sulfone), as verified using scanning electron micrographs of Mode I fiber fracture surfaces.

Low-Voltage SEM of Natural Plant Fibers: Microstructure Properties (Surface and Cross-Section) and their Link to the Tensile Properties

In this study, the microstructure of different natural plant fibers (flax, jute, ramie, and sisal fibers) were characterized by using low-voltage Scanning Electron Microscopy (LV-SEM). The LV-SEM observations indicated that jute and sisal fibers exhibit less variation in terms of the fiber cross-sectional area, internal lumen shape and size, and cell wall thickness in comparison to flax and ramie fibers. We find that this is also reflected in the tensile properties of the fibers. The tensile properties of single ramie fibers and their fracture behavior was investigated in detail. The stress-strain behavior showed two distinctive regimes. For linear curves, the tensile strength varies from 648-1086 MPa whereas nonlinear curves result in much lower values (177-452) MPa. This variation was linked to differences in the microstructure of the fibers. The LV-SEM of the tensile fracture surfaces of ramie fibers revealed details on the cell wall structure and its fracture behavior under tensile load. Moreover, the SEM images confirm that the collapse of the primary cell wall generally leads to a non-linear stress-strain curve for single ramie fibers.

The Effect of Plasma Treatment on Tensile Strength of Ensete Ventricosum Fibres

This study was focused on an effect of the plasma treatment on a mechanical behaviour of false banana's fibres (Ensete Ventricosum). The aim of the experiment was to describe the tensile strength of Ensete Ventricosum fibres which were modified by the plasma surface treatment. The fibres of Ensete Ventricosum, originally from Ethiopian region Hawassa, were used for this experiment. The fibres were prepared in the length of 100 mm. The samples were modified by the plasma treatment. Plasma was generated from a plasma generator (Plasma Reactor KPR 200 mm RM 54) while supplying the reaction gas (oxygen) and maintaining the reactor's pressure at 0.1 Torr with the use of a vacuum pump. To determine the properties that depend on the discharge power and treatment time, the plasma treatment was conducted in the power range 200-350 W for 10-50 s. The maximum tensile force was measured immediately after the plasma treatment to determine the ultimate strength. The ultimate strength and the deformation were determined by destructive tests using the tensile testing equipment (Labortech, MPTest 5.050, sensing unit AST type KAF 50 N, Czech Republic) with a rate of deformation 5 mm min-1. Fracture surfaces of fibres were studied using a scanning electron microscope (Tescan Mira 3, Czech Republic). The samples were covered with a thin layer of gold using a sputter coater (Quorum Q150R ES, United Kingdom) before SEM observation.

A rapid FIB-notch technique for characterizing the internal morphology of high-performance fibers

This work introduces an innovative technique to characterize the internal morphology of high-performance fibers by using a focused ion beam (FIB) sample preparation method and subsequent atomic force microscopy (AFM). A FIB is used to mill opposing notches that facilitate direct failure along a longitudinal shear plane, and expose the internal surface of the fiber. By exposing this surface via longitudinal shear, distortion of the cleaved surface is minimal, which is an advantage over surface preparation methods such as microtoming. After cleaving the notched fibers, an AFM technique is used to generate modulus maps of the fiber fracture surfaces. These modulus maps provide qualitative and quantitative microstructural information. Initial results obtained from Kevlar KM2 and Dyneema SK76 fibers are presented and a brief analysis of the observed internal features is provided. Extending the technique to image internal features in other materials is also discussed.

Strength improvement in injection-molded jute-fiber-reinforced polylactide green-composites

The mechanical properties of green-composites based on polylactic acid (PLA) with jute fibers were investigated. A long fiber pellet was developed to obtain a high aspect ratio of residual fiber after injection molding. Comparative studies were carried out, where shorter fiber pellets were compounded by different screw configurations using a twin-screw extruder. To interpret the results of our mechanical tests, the fiber geometry, dispersion state, and fiber fracture surfaces after tensile testing were analyzed. We found that the composites made of short fiber pellet (which suffer high compound intensity), exhibited optimal mechanical performance. Although, compounding with a twin-screw extruder decreased the overall aspect ratio of residual fibers, we observed that it significantly facilitated both the dispersion of the jute yarn to jute bundle and the decohesion of jute bundle to elementary fibers. This fiber separation caused by high intensity mixing led to efficient load transfer from matrix to fiber, and improvement of interfacial strength. These findings provide us with an insight into the critical parameters required to develop a high performing jute/PLA composite.

Effect of Natural Fiber Reinforced Polypropylene Composite Using Resin Impregnation

In this paper, we deals with mechanical performance of resin impregnation with natural fiber and fiber reinforced composites. The effect of the addition of a rein impregnation process on static strength of the injection molded composites was investigated by carrying out tensile and banding tests, followed by Scanning electron microscopy (SEM) observation of fiber surface and fracture surface of composites. The tensile strength of natural fiber and natural fiber reinforced composites with resin impregnation method increases with Polyvinyl alcohol (PVA) impregnation. In addition, Phenol resin impregnation recovers fiber tensile strength after alkali treatment. Resin impregnation causes decrease in contact surface area; however, it does not cause decrease in mechanical properties. Our results suggest that the using rein impregnation method has better effect on the mechanical properties of natural fiber reinforced Polypropylene (PP) composites.

Failure mechanisms of carbon nanotube fibers under different strain rates

Strong and uniform carbon nanotube (CNT) fibers with tensile strength around 1.2GPa were prepared from vertically aligned CNT arrays, and their mechanical properties were studied using a wide range of tensile strain rates. The cyclic load/unload process, polarized Raman measurements, and fiber fracture surfaces were also used to study the failure mechanism of the CNT fibers. It is found that the fibers exhibit a strain-rate strengthening effect, and have different failure mechanisms at high and low strain rates. The key factors that limit the mechanical properties of the CNT fibers were then investigated based on a failure mechanism analysis: inter-tube slippage happens at low strain-rates, and “cascade-like” breaking dominates at high strain-rates. The maximum strength of the fibers appears at high strain rates, and is mainly determined by the CNT alignment.

Vector potential photoelectron microscopy

A new class of electron microscope has been developed for the chemical microanalysis of a wide range of real world samples using photoelectron spectroscopy. Highly structured, three-dimensional samples, such as fiber mats and fracture surfaces can be imaged, as well as insulators and magnetic materials. The new microscope uses the vector potential field from a solenoid magnet as a spatial reference for imaging. A prototype instrument has demonstrated imaging of uncoated silk, magnetic steel wool, and micron-sized single strand tungsten wires.