Effect of the fiber orientation on the microcrack formation in a fibrous polymer composite during friction

Triboluminescence is used to study the microcrack formation during dry sliding friction on steel of polyphenylene sulfide (PPS) and a composite material consisting of a PPS matrix reinforced with a carbon fiber fabric. It is found that, when fabric layers are parallel to the slip plane, the number and linear sizes of microcracks are larger than those in PPS. In contrast, when fabric layers are normal to the slip plane, the number and linear sizes of microcracks are smaller than those in PPS. These effects are explained by the formation of boundary layers in the matrix in which molecules are oriented parallel to the axis of fibers in the fabric.     

Effects of carbon fibers on the transcrystalline interphase in poly(phenylene sulfide) of different molecular weights

Composite films of different molecular weight poly(phenylene sulfide) (PPS) and three types of carbon fibers (Pitch, PAN, and Rayon-based fibers) have been studied by optical microscopy and wide-angle X-ray diffraction. Transcrystallization of growing spherulites on carbon fibers is found under all thermal conditions of growth on Rayon and Pitch-based carbon fibers for all types of matrices. For composite films with PAN carbon fibers transcrystallization of growing spherulites is not uniform and sometimes is not found at all. Existence of b axial orientation of twisted lamellae for transcrystalline zone of PPS is demonstrated by X-ray diffraction technique and compared with orientation of the stretched sample. The new induction time quantitative approach is applied to the transcrystalline growth of PPS spherulites on the surface of carbon fibers. The interfacial free energy difference for fiber/crystallite and heterogeneities/ crystallite systems in the melt that is defined from growth and nucleation studies are calculated and compared. The relative tendency for a polymer to crystallize at the fiber surface rather than in the bulk is demonstrated.     

Correlations among micro- and macromechanical properties of composite materials based on the interphase concept

In this paper an attempt was made to define microstructural properties of carbon fiber/PP composites, with respect to fiber surface chemistry and morphology. In order to define the effects of the fiber surface sizings and morphology on the polymer microstructure, the interphase and mechanical properties of the composites, carbon fibers with similar, but not identical surface chemistry (CH and CT) were used. Characterization was performed by several techniques: SEM, POM, reflection microscopy, DSC, FTIR, XPS, contact angle measurements. For microstructural analysis, the geometrical method, method of intercept and DIF method were used. It was found that both carbon fibers have a strong influence on the nucleation mechanism and crystallization as well as on the microstructural parameters in the model and macro composites. Nucleation efficiency of the fibers has been confirmed by the nucleation parameter Q, measured by Muchova–Lednicky method and by the interfacial energy parameters. Microstructural analysis based on the photographs obtained by POM, SEM and reflection microscopy has shown that in the CH/PP model and macrocomposites the sieve-grain network was formed, which indicates better mechanical properties. The results obtained for the macromechanical properties of PP composites reinforced with CH and CT have confirmed the prediction based on micostructural analysis.     

Some aspects of nucleation and evolution of microscopic and mesoscopic cracks and quasi-brittle fracture of homogeneous materials

The dynamics of nucleation of microcracks in the region of a developing macroscopic crack has been studied using scanning electron microscopy during in situ experiment and the acoustic emission method. An explosive-like nucleation of microcracks and the influence of dissipative properties of a material on the size and the rate of redistribution of local stresses of the microcracks have been established. Each of nucleations of microscopic and mesoscopic defects is considered as an act of local testing of the dissipative ability of the system, and the interaction between microcracks is determined by relaxation processes when the ability is sufficient. With deteriorating dissipative properties (with increasing residence time under loading, deformation rate, etc.), an elastic linear interaction becomes possible, and the macroscopic fracture occurs. In the microcrack dynamics, a specific ductile-brittle transition is determined by a time deterioration of the dissipative properties of the material under the action of a mechanical load. Generally, as a large latent energy is stored during deformation, a macroscopic fracture can be caused by any little discrete structural transformation.     

Tensile failure of crystalline polymers

In the creep experiment the brittle fracture of the unoriented semicrystalline polymers at very small and very high tensile load with the intermediate ductile region may be explained by the competition between crazing and shear band formation during the microcrack growth phase. The former type of microcrack growth leads to brittle fracture while the latter type yields necking which transforms the original lamellar structure into the final fibrous structure. The actual fate of the strained sample depends on the growth time of the craze, t_g, and of the shear band formation time, t_s. If t_g<t_s, the material will break in a brittle manner, and if t_g > t_s, the material will deform plastically. The failure of the fibrous material seems to occur when the ratio between the average distance and diameter of the microcracks reaches a value about 3. The microcracks seem to form primarily at defects of the microfibrillar structure, i.e., at the ends of microfibrils where the axial connection of subsequent crystal blocks through the amorphous layers by a great many taut tie molecules is either completely interrupted or at least drastically reduced. The stress concentration resulting from the opening of these defects into microcracks may rupture also some of the adjacent microfibrils. Such nucleation and subsequent lateral growth of the microcrack ruptures the taut tie molecules in its path. The ruptured molecules yield free radicals which can be monitored by electron spin resonance.     

Dynamics of microcracks and time dependences of surface deformation of a heterogeneous body (granite) under an impact

The dynamics of fractoluminescence flashes and the time dependences of surface deformation of granite with different sizes of feldspar grains under an impact on samples by a metal pin have been studied with a 10 ns resolution. A band at ??1.9 eV has been observed in the fractoluminescence spectra, which means that, under the influence of mechanical stresses, Si-O-Si bonds are broken and ?? SiO^?? free radicals are formed. The fractoluminescence has the form of flashes with a duration of ??10 ns. It has been assumed that each of them corresponds to the nucleation of a microcrack. From the flash intensities and the elastic wave velocity, the linear size of microcracks has been estimated to be from ??8 to 30 ??m. Microcracks are mainly generated during passage of a deformation wave through feldspar grains. An impact causes the appearance of eigenvibrations of the entire sample, and cracking of grains gives rise to eigenvibrations of grains.     

Multimaterial preform coextrusion for robust chalcogenide optical fibers and tapers

The development of robust infrared fibers is crucial for harnessing the capabilities of new mid-infrared lasers. We present a novel approach to the fabrication of chalcogenide glass fiber preforms: one-step multimaterial extrusion. The preform consists of a glass core and cladding surrounded by a built-in, thermally compatible, polymer jacket for mechanical support. Using this approach we extrude several preform structures and draw them into robust composite fibers. Furthermore, the polymer cladding allows us to produce robust tapers with submicrometer core diameter.     

Effect of transcrystallization in carbon fiber reinforced poly(p-phenylene sulfide) composites on the interfacial shear strength investigated with the single fiber pull-out test

The effect of transcrystallinity in carbon fiber reinforced poly(p-phenylene sulfide (PPS) composites on the apparent shear strength was investigated with the single fiber pull-out test. Transcrystalline zones around the reinforcing fibers do not seem to improve the adhesion level significantly. Neighbor fibers hinder the formation of the transcrystalline zone and a ductile fracture behavior can be observed. However, the apparent strength level is slightly higher for composites containing such reinforcing neighbor fibers compared with single fiber composite samples. During annealing a brittle interface can be formed in the multifiber composite yielding a higher level of the apparent shear strength.     

Wear Performance and Mechanisms of Polyphenylene Sulfide/Polytetrafluoroethylene Wax Composite Coatings Reinforced by Graphene

Polyphenylene sulfide (PPS) composite coatings reinforced by graphene were prepared through a spraying method. Wear performance of the composite coatings were evaluated using a block-on-ring test rig, and the results showed that the wear life of the composite coatings were over seven times higher than that of a pure PPS coating. Wear mechanisms of PPS composite coatings reinforced by graphene are discussed. It was concluded that adhesive wear was the major wear mechanism of the pure PPS coating but the wear form of the composite coatings was dominated by abrasive wear due to the graphene filler that has high mechanical strength. In addition, fatigue wear appeared for composite coatings with higher content of graphene. The formation of a uniform thin transfer film on the counterpart ring and fine wear debris for the composites coatings during abrasion were consistent with the improvement of wear performance. The 3D morphology of the surface of the counterpart ring was also used to discuss the wear mechanism of PPS composite coatings.     

Emission kinetics of light,sound, and radio waves from single-crystalline quartz after impact on its surface

A steel striker impacting the surface of single-crystalline quartz generates strain waves and their related acoustic and electromagnetic emissions. Simultaneously, microcracks with free excited SiO• radicals at their edges appear in the single crystal. The relaxation of the electronic excitation causes bursts of fractoluminescence. The intensity of the bursts is proportional to the microcrack surface area. It is found that the linear sizes of microcracks vary from 15 to 70 μm. Cracking changes the slope of the time dependences of the acoustic and electromagnetic emission intensities. The microcrack size distribution obeys a power law with an exponent of about two.     

脆性断裂的统计理论

本文试图从位错理论出发来探索晶体脆性断裂的统计理论。脆性断裂过程,实质上是微裂缝在极小的范性形变过程中形成长大和传播的随机过程。本文导出了描述这种随机过程的微分方程,利用微裂缝形成长大的位错机理,解出了微裂缝大小的统计分布函数。文中给出了范性形变、加工硬化和活动位错源数目与微裂缝数目和大小之间的函数关系。过去研究脆性断裂时,范性变形只是含糊地包括在有效表面能之内,而加工硬化和活动位错源数目则一向被略去。从微裂缝大小的统计分布函数和微裂缝的传播条件,导出了强度的统计分布函数,从而求得了脆性断裂判别式、脆性断裂强度及脆性-范性转变温度。     

Dynamics of fractoluminescence and electromagnetic and acoustic emission upon an impact against the marble surface

The blow of a steel striker against the marble surface induces strain waves and electromagnetic emission. Simultaneously, microcracks appear in the marble single crystal with excited free CO ₂ ^? radicals at the microcrack banks. Relaxation of electronic excitation leads to the emergence of fractoluminescence bursts. The burst intensity is proportional to the area of the microcrack surfaces. Measurements show that the linear size of the microcracks varies from ～2 to ～47 μm.     

Nanosecond dynamics of destruction of an inhomogeneous solid (granite) induced by an impact on its surface

Dynamics of microcrack formation and deformation of a granite-sample surface after a striker impact have been studied with a time resolution of 2 ns. The impact excites the sample’s natural vibrations that lead to formation of clusters of microcracks with linear dimensions of ~2 to 10 μm in feldspar grains. The formation of microcracks, in turn, excites natural vibrations of the grains.     

Morphology and fracture of drawn crystalline polymers

Drawn crystalline polymers, fibers and films, exhibit a characteristic fibrous structure with the long and narrow microfibril as the basic element. The highly oriented microfibrils originating from the same stack of parallel lamellae of the starting material form the fibril which, as a rule, differs a little in drawing ratio from adjacent fibrils. As a consequence of this variation and the location of the ends of microfibrils on the outer boundary of the fibril, this is the area of reduced strength where the fracture phenomena start. The free ends of microfibrils are point vacancy defects of the microfibrillar superlattice which, under applied stress, open and form microcracks long before the sample fails. The radial crack propagation first breaks the microfibrils adjacent to the microcrack at the point vacancy and produces a great many radicals as a consequence of the rupture of tie molecules in at least one amorphous layer of each broken microfibril. Longitudinal crack propagation proceeds along the boundary between adjacent microfibrils and creates very few radicals because there are very few interfibrillar tie molecules which can be ruptured during this process. The coalescence of microcracks by radial and axial growth finally leads to a crack of critical size which by catastrophical growth leads to sample failure. If the longitudinal growth is sufficiently enhanced, the fracture surface is fibrous with individually broken fibrils which are separated from each other by longitudinal voids. Such an effect occurs in a torsional test, at the end of cold drawing, and very often in cyclic loading tests.     

Polyaniline-Containing composites based on highly porous carbon cloth for flexible supercapacitor electrodes

Composites based on commercially available carbon cloth Busofit T-040 and conductive polymer polyaniline are fabricated using the electrochemical polymerization of aniline on the surface of carbon-cloth fibers. The sequence of technological operations for obtaining the composite is optimized; the procedure of preliminary modification of the carbon-cloth surface by electrochemical etching is worked out; and the capacitive characteristics of the obtained composites for use as flexible supercapacitor electrodes are studied. It is found that the introduction of polyaniline into the composition of composite electrode structures leads to an increase in the capacitance by 2–2.5 times compared to the initial carbon cloth due to the pseudocapacitance of polyaniline while maintaining a high electrical conductivity and efficiency. For a composite based on etched carbon cloth, the specific capacitance is 267 F/g (8.9 F/cm² per unit of the geometric surface of the electrode) with a charge efficiency of 97–99%. The specific surface area of the composite, determined by the BET method, is 548 m²/g.     

Microcrack localization using a collinear Lamb wave frequency-mixing technique in a thin plate

A novel Lamb wave frequency-mixing technique is proposed for locating microcracks in a thin plate,which does not require the resonance condition of Lamb wave mixing and can accurately locate the microcracks through only one-time sensing.Based on the bilinear stress-strain constitutive model,a two-dimensional finite element(FE)model is built to investigate the frequency-mixing response induced by the interaction between two primary Lamb waves and a microcrack.When two primary Lamb waves of A0 and S0 modes with different frequencies excited on the same side of the plate simultaneously impinge on the examined microcrack,under the modulation of the contact acoustic nonlinearity,the microcrack itself can be deemed as the secondary sound source and it will radiate the Lamb waves of new combined frequencies.Based on the time of flight of the generated A0 mode at difference frequency,an indicator named normalized amplitude index(NAI)is defined to directly locate the multi-microcracks in the given plate.It is found that the number and location of the microcracks can be intuitively visualized by using the NAI based frequency-mixing technique.It is also demonstrated that the proposed frequency mixing technique is a promising approach for the microcrack localization.     

Surface and interfacial morphologies of PAN-CVI carbon–carbon composite

Scanning electron microscopy (SEM), polarized light microscopy (PLM), and transmission electron microscopy (TEM) techniques have been used to characterize the normal surface and flank surface microstructure of a polyacrylonitrile (PAN)-based carbon fiber reinforced chemical vapor infiltrated (CVI) matrix carbon–carbon composite. Optical and SEM results indicate that the CVI deposit consists of two structures: an isotropic phase is present in the fiber bundle-bundle junctions and a second highly oriented lamellar structure is present in the intrabundle matrix. TEM shows that matrix platelets are highly parallel to the fiber axis and the crystallites of the matrix near the fiber surface exhibit better alignment than those farther away from fibers.     

多脉冲激光对K9玻璃的表面损伤实验研究

 实验研究了波长为1 064 nm、脉宽为10 ns、重复频率为1 Hz的激光脉冲对K9玻璃的表面损伤特点,给出了脉冲透过能量随激光脉冲作用次数变化的规律。采用3维立体显微镜对损伤形貌进行观察,发现K9玻璃的损伤表面呈环状分布,分为烧蚀区、微裂纹区和断裂区。随着激光脉冲个数的增加,损伤由点状破坏演变为损伤区,微裂纹逐渐增长,损伤面积逐渐增大。基于激光支持的爆轰波理论分析,激光与脆性材料的相互作用可引起微裂纹的大量增长。在多脉冲激光的作用下,K9玻璃损伤的累积效应明显,表面损伤阈值明显降低,表面裂纹增长明显,损伤面积逐渐增大;但随着激光脉冲的继续增加,这种损伤趋于稳定。     

Effect of dislocation distribution in twin boundaries on the nucleation of microcracks at the twin tip

The effect of dislocation distribution in the boundaries of an arrested twin on the nucleation of microcracks at its tip is investigated. The twin is simulated by a double step pileup (cluster) of twinning dislocations located in adjacent slip planes. The equilibrium equations for dislocations are solved numerically. Clusters with different total numbers of dislocations and with different ratios of the numbers of dislocations at the upper and lower twin boundaries are considered. The formation of microcracks as a result of coalescence of head dislocations according to the force and thermally activated mechanisms is analyzed. The equilibrium configurations of a single twin boundary and of the twin are calculated. It is found that the condition for microcrack formation at the twin tip considerably depends on the ratio of the numbers of dislocations in twin boundaries. In the limit, this condition coincides with the condition of crack formation at the tip of a single twin boundary with the same total number of dislocations. It is shown that thermally activated formation of a microrack corresponds to lower values of the critical stress.     

Composited BCN/carbon fibers prepared by hot-filament chemical vapor deposition

The combined BCN/carbon fibers with porous configuration have been successfully prepared by hot-filament chemical vapor deposition (HF-CVD). The composited materials consist of carbon fiber inside covered by the cylindrical BCN films. The differences in the surface morphology and the diameter of the composite fibers are related to the different reactant gases. It is demonstrated that the elements of B, C, and N are chemically bonded with atomic-level BCN hybrid in the composite fibers. The resistance of the composite fibers is about 300 Ω which is 10 times higher than that of the isolated carbon fibers (27.5 Ω). When the applying voltage increases up to 8-15 V, the BCN films have been broken down and the resistance of composite fibers decreases to the typical value of the carbon fibers. The composite fibers with porous configuration have the strongly capacity to adsorb oxygen. The findings suggest that the combined BCN/carbon fibers are favorable for achieving high performance nano-optoelectronic and sensor devices.