Strength Of Individual Fibers Research Articles

Chemo-mechanical ageing of paper: effect of acidity, moisture and micro-structural features

A multi-scale modeling framework is proposed for the prediction of the chemo-mechanical degradation of paper, with the particular aim of uncovering the key factors affecting the degradation process. Paper is represented as a two-dimensional, periodic repetition of a fibrous network unit cell, where the fibers are characterized by a moisture-dependent chemo-hygro-mechanical constitutive behavior. The degradation of paper occurs primarily as a result of the hydrolysis of cellulose, which causes a reduction of the degree of polymerization and a consequent decrease of the effective mechanical properties, ultimately leading to fiber embrittlement and a loss of material integrity. The interplay between the acidity of the paper, the ambient environmental conditions, and its chemo-mechanical degradation behaviour is a complex process. In the model, these interactions are accounted for by determining the coupled temporal evolution of the degree of polymerization, the acidity of the paper, and the moisture content, from which the time-dependent tensile strength of the paper is calculated. The internal stresses developing in the fibrous network under a change in moisture content lead to brittle fiber fracture once they reach the fiber tensile strength. The successive breakage of individual fibers results in damage development in the fibrous network, altering its effective constitutive properties. The temporal evolution of the effective hygro-mechanical properties of the fibrous network is calculated by employing asymptotic homogenization. For obtaining accurate model input, the strength and stiffness properties of individual fibers and the degree of polymerization of paper samples are measured at different ageing times by carrying out dedicated experiments. Subsequently, a series of numerical simulations is performed to analyze the chemo-mechanical degradation process of paper, highlighting the influence of the time-evolving acidity and moisture content. The numerical study further considers the effects of micro-structural features (i.e., the anisotropy of the fibrous network orientation and the fiber longitudinal elastic modulus) on the macroscopic degradation response of paper. The results of this work may help conservators of cultural heritage institutions determining optimal environmental conditions to limit or delay the time-dependent degradation of valuable historical paper artefacts.

Kraft pulp viscosity as a predictor of paper strength: Its uses and abuses

For bleached kraft pulps, two factors govern paper strength: the individual fiber strength, and the bond strength that adheres the individual fibers together in the paper matrix. Inherent fiber strength is related to the length of the carbohydrate polymers, also known as the degree of polymerization (DP). Average DP (DP) is inferred by performing pulp viscosity measurements. Under certain circumstances during kraft pulping and bleaching, the average polymer lengths can be shortened, resulting in lower pulp viscosity, and may indicate fiber damage. Fiber damage typically manifests itself as a reduction in tear strength for well-bonded handsheets. This paper will review the literature on how pulp viscosity can predict paper/fiber strength and how it can be used as a diagnostic tool. It can be a means to monitor pulp quality during pulping and bleaching, as well as to alert when such operations approach a critical threshold. However, viscosity losses must be carefully and judiciously analyzed. Like most diagnostic tools, viscosity measurements can be misused and abused, which can lead to incorrect inferences about intrinsic fiber strength. This review will also cover these misuses. The overall goal is to provide the papermaker a better understanding of what pulp viscosity is, how it correlates to potential sheet strength, and what its limitations are. It will be illustrated that when pulp viscosity drops below a critical value, it will indicate an appreciable deterioration in the paper’s tear and tensile strength.

EFFECTS OF BUTYLAMINE TREATMENT ON CELLULOSE FIBERS DURING RECYCLING OF OLD CORRUGATED CONTAINERS (OCC)

This study aimed to determine the effects of butylamine treatment on cellulose fibers during the recycling processes. Three recycling stages have been carried out and two different butylamine (Ba) treatments (5.0% and 7.5%) were applied to old corrugated containers (OCC) papers during recycling. After each recycling process, papers were produced and the mechanical and optical properties of these papers were measured. SEM images and FTIR spectra were taken and the crystallinity index of the cellulose fiber was calculated by the XRD peak height method. Some improvement (11.0-15.2%) in paper brightness with the Ba treatment was realized in the third recycling stage, while yellowness values typically decreased by 1-2 points. Chemical treatments resulted in definite differences in water absorptiveness (Cobb value) for papers. The highest water absorptiveness of 160.0 g/m2 was observed with a 7.5% Ba treatment in the second recycling stage (75Ba2), indicating a value about 15.6% higher than that of the control. The highest tensile and burst indices were observed with 7.5% Ba treatment in the third recycling stage, indicating approx. 28.7% higher tensile and 34.5% burst indices, compared to the control at similar recycling stages. In contrast, the highest tear strengths of 4.54 Nm2/g, followed by 3.86 Nm2/g, were observed for untreated samples in the second and first recycling stage, respectively. The butylamine treatment in the recycling processes increased the cellulose crystallinity more (1.3%), compared to the control. It seems that the strength properties of recycled paper, such as tensile and burst, are closely related to the individual fiber strength and fiber bonding potential, which are typically reduced in recycling, but could be improved by Ba treatment.

Review of Hybrid Fiber Based Composites with Nano Particles-Material Properties and Applications.

The present review article provides an overview of the properties of various natural and synthetic fibers for the fabrication of pure natural composites and the combination of both natural/synthetic fibers-based hybrid composites, bio-based resins, various fabrication techniques, chemical and mechanical properties of fibers, the effect of chemical treatment and the influence of nanoparticles on the composite materials. Natural fibers are becoming more popular and attractive to researchers, with satisfactory results, due to their availability, ease of fabrication, cost-effectiveness, biodegradable nature and being environmentally friendly. Hybrid composites made up of two different natural fibers under the same matrix material are more popular than a combination of natural and synthetic fibers. Recent studies relevant to natural fiber hybrid composites have stated that, due to their biodegradability and the strength of individual fibers causing an impact on mechanical properties, flame retardancy and moisture absorption, natural fibers need an additional treatment like chemical treatment for the fibers to overcome those drawbacks and to enhance their better properties. The result of chemical treatment on composite material properties such as thermal, mechanical and moisture properties was studied. Researchers found that the positive influence on overall strength by placing the filler materials (nanoparticles) in the composite materials. Hybrid composites are one of the fields in polymer science that are attracting consideration for various lightweight applications in a wide range of industries such as automobile, construction, shipping, aviation, sports equipment, electronics, hardware and biomedical sectors.

Salidroside mitigates skeletal muscle atrophy in rats with cigarette smoke-induced COPD by up-regulating myogenin and down-regulating myostatin expression.

Objectives: The present study aimed at investigating the therapeutic effect of Salidroside on skeletal muscle atrophy in a rat model of cigarette smoking-induced chronic obstructive pulmonary disease (COPD) and its potential mechanisms.Methods: Male Wistar rats were randomized, and treated intraperitoneally (IP) with vehicle (injectable water) or a low, medium or high dose of Salidroside, followed by exposure to cigarette smoking daily for 16 weeks. A healthy control received vehicle injection and air exposure. Their lung function, body weights and gastrocnemius (GN) weights, grip strength and cross-section area (CSA) of individual muscular fibers in the GN were measured. The levels of TNF-α, IL-6, malondialdehyde (MDA), superoxide dismutase (SOD), glutathione (GSH) in serum and GN tissues as well as myostatin and myogenin expression in GN tissues were measured.Results: In comparison with that in the healthy control, long-term cigarette smoking induced emphysema, significantly impaired lung function, reduced body and GN weights and CSA values in rats, accompanied by significantly increased levels of TNF-α, IL-6 and MDA, but decreased levels of SOD and GSH in serum and GN tissues. Furthermore, cigarette smoking significantly up-regulated myostatin expression, but down-regulated myogenin expression in GN tissues. Salidroside treatment decreased emphysema, significantly ameliorated lung function, increased antioxidant, but reduced MDA, IL-6 and TNF-α levels in serum and GN tissues of rats, accompanied by decreased myostain, but increased myogenin expression in GN tissues.Conclusion: Salidroside mitigates the long-term cigarette smoking-induced emphysema and skeletal muscle atrophy in rats by inhibiting oxidative stress and inflammatory responses and regulating muscle-specific transcription factor expression.

Functional Properties of a Pitch-Based Carbon Fiber-Mortar Composite as a Thin Overlay for Concrete Pavement.

This experimental study investigated the utility of a pitch-based carbon fiber–mortar composite, which could replace polyacrylonitrile carbon fiber, as a thin overlay for concrete pavement. The objective was to explore the utility of the low-cost carbon fiber, which was produced via a melt-blown method, i.e., blowing at high pressure after melting the pitch residue following crude oil purification. The mechanical properties, durability, and thermal properties of the pitch-based carbon fiber were explored to maximize strength, durability, functionality, and economy by using micro-sized fibers that are closer in size to the constituents of cementitious materials. Melt-blown pitch-based carbon fiber has low individual fiber strength but generally excellent thermal conductivity. Thermal conductivity tests were conducted on mortar panels (560 mm × 560 mm; thickness = 25, 40 or 60 mm) containing 0, 0.4, 0.5 or 0.6 wt % pitch-based carbon fiber. The absolute thermal conductivity tended to improve with higher wt % of pitch-based carbon fiber, in the range of 9~11 W/°C. However, thermal conductivity tended to be lower under the 0.6 wt % condition, possibly due to the effect of dispersion. Compressive strength degradation was tested over 350 cycles of freezing and thawing: the strength of the 0.4, 0.5 or 0.6 wt % samples was 91, 89, and 82%, respectively, relative to the control specimen (0 wt %). Thus, all specimens had a compressive strength of 80% or more after 350 cycles compared to the control specimen. To test the adhesion performance for new thin overlays and old concrete surfaces, concrete cylinders (100 × 200 mm; thickness = 10 mm) were cut at an angle of 46 degrees, and the pitch-based carbon fiber-mortar composite was used to bond the various sections. The bond strength of the test specimens was more than twice that of the reference specimen.

Effect of mechanically induced micro deformations on extensibility and strength of individual softwood pulp fibers and sheets

Tensile tests of individual bleached kraft pulp fibers and paper sheets of industrial origin were conducted in order to investigate the effect of refining and small-scale fiber deformations on the extensibility of fibers and paper. The shape of the tensile curves of most of the fibers was concave upward (i.e., increasing slope) and consisted of two or three phases suggesting that the fibrillar structure and disordered regions in the fiber wall were straightened out during straining. Only a few of the individual BSKP fiber tensile curves were apparently linear. Elongation of the individual kraft fibers varied from 8 to 32% and the average elongation was not increased by high consistency refining. Tensile test results of laboratory sheets made of the same BSKP pulp suggested that the fiber bonding not only governs paper strength, but also is highly relevant for the elongation of fiber networks. The key conclusion related to this investigation and freely dried sheets was that the increased network elongation and strength after refining is mainly due to increased inter-fiber bonding and a higher shrinkage tendency of the fiber network and not due to the increased elongation or strength of individual fibers.Graphical abstract

The impact of fiber maturity on estimating the number of cotton (Gossypium hirsutum L.) fibers per seed surface area

The number of fibers per seed surface area (fiber density) is a selection criterion used to improve cotton fiber quality and yield simultaneously in cotton breeding programs. However, the parameters utilized to estimate fiber density are calculated from fiber quality attributes that are sensitive to environmental variations, especially fiber maturity. Fiber maturity is one of the most important fiber properties that influences other fiber properties such as individual fiber strength and length. In order to investigate the impact of within-plant fiber maturity on the estimates of fiber density, field experiments were conducted at Lubbock, Texas during the 2012, 2013 and 2014 growing seasons. A set of twelve upland cotton cultivars popular in the Texas High Plains were grown in a randomized complete block design with three field replications. Bolls were box picked at harvest to provide samples from each cultivar representing a range of fiber maturity. Fuzzy seeds obtained after roller ginning were acid-delinted, scanned on a flatbed scanner, and the WinSeedle Pro software was used to estimate the seed surface area (SSA). The number of fibers per seed was estimated using the fiber quality parameters provided by AFIS (Advanced Fiber Information System), the lint weight, and the number of seeds in the sample. The number of fibers per seed surface area was obtained by dividing the number of fibers per seed by the estimated SSA. In this study, cultivars with relatively less stable fiber maturity across the fruiting branches of the cotton plant tend to have more variable number of fibers per seed surface area estimates, suggesting that the calculation of the number of fibers per seed surface area may be biased. It is likely that the source of bias is fiber maturity (immature fibers are weak and tend to break during mechanical processing). Therefore, fiber maturity should be considered when screening lines based on estimated fiber density because low fiber maturity can lead to unreliable estimates of fiber density.

How xylan effects the breaking load of individual fiber–fiber joints and the single fiber tensile strength

Bleached and unrefined softwood kraft fibers were treated with aqueous solutions of xylans and potassium hydroxide (KOH) to vary the amount of xylan present on the fiber surface and within the fiber wall. This was done in order to directly measure the influence of xylan on the joint strength of individual fiber–fiber joints as well as the tensile strength of individual fibers to determine the elastic modulus. The results were compared with two unbleached and unrefined softwood kraft pulps (Kappa 42 and 54). Additional xylan had a statistically significant effect on the mean values of the breaking load of individual fiber crossings. Moreover, the breaking load was strongly affected by the cooking time during pulping. However, no correlation between the optical bonded area and the joint strength was observed. The modulus of elasticity of individual fibers was not influenced by additional xylan or alkaline extraction. The elastic modulus was influenced more by chemical pulping and bleaching. The results indicate that additional xylan on the fiber surface has a strength-enhancing effect on the joint strength individual fiber crossings which also depends on the distribution of xylan. On the other hand, the elasticity of a fiber seems to be more influenced by the morphology and the composition of the fiber walls, especially in the secondary S2 wall.

Tensile strength of handsheets from recovered fibers treated with N‐Methylol melamine and 1,3‐dimethylol‐4,5‐dihydroxyethyleneurea

ABSTRACTThe main objective of this study was to assess the effect of two amino resins, N‐methylol melamine (NMM) and 1,3‐dimethylol‐4,5‐dihydroxyethyleneurea (DMDHEU), on the intrafiber and interfiber strengths and water absorption of two types of waste paper categories, office paper (OP), and old corrugated containers (OCCs). The tensile strength of individual fibers measured at zero span was reduced by increases in the resin concentrations. The dry tensile strengths of the recovered handsheets measured at a finite span were enhanced with increases in the weight percentage gain of the resins. The increasing of the resin concentration also significantly improved the intrabonding of the OP and OCCs in moist measuring conditions. The water absorption of the handsheets considerably decreased at the higher concentration of the thermosetting resins, especially with NMM. The results are promising for the use of NMM‐ and DMDHEU‐treated recovered fibers as an alternative fiber resource for the production of laminated paper and also for the use of DMDHEU as a new N‐methylol compound for laminated paper. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41290.

Preparation and characterization of multilayered ZrO2 coatings on silicon carbide fibers for SiC/SiC composites

We have studied the surface morphology, phase composition, and oxidation resistance of multilayered tetragonal zirconia coatings produced on silicon carbide fibers by a sol-gel process and measured the tensile strength of individual fibers as a function of the number of layers in the coating. SiC-fiber-reinforced silicon carbide minicomposites have been prepared through pyrolysis of an organosilicon polymer, and their fracture surfaces have been examined. Using microindentation, we have determined the critical fiber-matrix debonding stress. The results demonstrate that the ZrO2 coating on the fibers has the form of uniform, weakly bonded layers. The presence of a multilayered ZrO2 interphase alters the fracture behavior of the SiC/SiC composites. The fiber debond stress in the composites markedly decreases with an increase in the number of layers in the interphase.

Mechanical properties of individual electrospun PA 6(3)T fibers and their variation with fiber diameter

Young’s modulus and yield strength of individual electrospun fibers of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) have been obtained in uniaxial extension. The Young’s modulus is found to exhibit values in excess of the isotropic bulk value, and to increase with decreasing fiber diameter for fibers with diameter less than roughly 500 nm. The yield stress is also found to increase with decreasing fiber diameter. These trends are shown to correlate with increasing molecular level orientation within the fibers with decreasing fiber diameter. Using Ward’s aggregate model, the correlation between molecular orientation and fiber modulus can be explained, and reasonable determinations of the elastic constants of the molecular unit are obtained.

Determination of Wet Fiber Strength

This study presents a method for measuring the strength of wet pulp fibers. It is derived from the breaking stress of wet paper strips at a zero span and the number of fibers bearing the load. Measurement was car-ried out with thermomechanical and chemical pulps under various levels of refining. In the results we obtained, there is a significant correlation between the strength of individual fibers and the degree of treatment, while the tra-ditional zero-span tensile method does not indicate this. The calculated strength could be interpreted as a measure of the resistance of fibers to damage under mechanical treatment and development of fibers in the refining process.

Relaxation dynamics in strained fiber bundles

Under an applied external load the global load-sharing fiber bundle model, with individual fiber strength thresholds sampled randomly from a probability distribution, will relax to an equilibrium state, or to complete bundle breakdown. The relaxation can be viewed as taking place in a sequence of steps. In the first step all fibers weaker than the applied stress fail. As the total load is redistributed on the surviving fibers, a group of secondary fiber failures occur, etc. For a bundle with a finite number of fibers the process stops after a finite number of steps, tf. By simulation and theoretical estimates, it is determined how tf depends upon the stress, the initial load per fiber, both for subcritical and supercritical stress. The two-sided critical divergence is characterized by an exponent -1/2, independent of the probability distribution of the fiber thresholds.

Characterizing TEMPO-mediated oxidation of ECF bleached softwood kraft pulps

Elementally chlorine-free (ECF) bleached softwood (SW) kraft pulp was catalytically oxidized with the 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO)–KBr–NaClO system. The carboxyl group content of the fibers was improved with increasing NaClO charge whereas, the carbonyl group content of fibers approached a maximum when a charge of 0.85 mmol NaClO/g o.d. fibers or higher was employed. The degree of polymerization of fibers drastically decreased from 2416 to 688 depending on the increase of NaClO charge during the TEMPO-mediated oxidation. Scanning electron microscopy (SEM) images of the original fibers and the oxidized fibers showed that TEMPO-mediated oxidation is able to remove the primary wall of SW kraft fibers. The optimum reaction temperature and pH for TEMPO-mediated oxidative generation of carboxyl groups was found to be 23 °C at a pH of 9.10. Oxidized fibers were shown to exhibit 62.9% higher water retention values (WRV) than the original fibers. Due to the enhanced carboxyl group content after oxidation, physical strength studies of paper test sheets indicated that the tensile index of the oxidized fibers is 13.8% greater than that of the original fibers. The individual fiber strength of the oxidized fibers was 17.0% lower than that of the original fibers as determined by zero-span strength measurements.

Measurement of the Elastic Modulus of Single Bacterial Cellulose Fibers Using Atomic Force Microscopy

The ability of the atomic force microscope to measure forces with subnanonewton sensitivity at nanometer-scale lateral resolutions has led to its use in the mechanical characterization of nanomaterials. Recent studies have shown that the atomic force microscope can be used to measure the elastic moduli of suspended fibers by performing a nanoscale three-point bending test, in which the center of the fiber is deflected by a known force. We extend this technique by modeling the deflection measured at several points along a suspended fiber, allowing us to obtain more accurate data, as well as to justify the mechanical model used. As a demonstration, we have measured a value of 78 +/- 17 GPa for Young's modulus of bacterial cellulose fibers with diameters ranging from 35 to 90 nm. This value is considerably higher than previous estimates, obtained by less direct means, of the mechanical strength of individual cellulose fibers.

Minimum Number of Genes Controlling Cotton Fiber Strength in a Backcross Population

Textile mills in the USA require strong cotton fiber (Gossypium hirsutum L.) for modern high‐speed textile operations. The primary objective of this study was to determine the inheritance of strength descending from FTA 263‐20 (FTA). FTA was developed by introgression into G. hirsutum from G. arboreum L., G. thurberii Todaro, and G. barbadense L. Five backcrosses (BCs) into ‘Deltapine 16’ (DP 16) followed by six BCs into DP 90ne were made. In 2001, three sets of 64 BC6 F2:F3 progenies were evaluated for strength. Significant variability for F2:F3 strength (F = 2.79), yield, three yield components, and four other fiber traits were detected. From a three replication test, strength gene number(s) estimates ranged from 1.10 to 1.29 and combined over sets was 1.23 genes. Average strength for the three BC5 parents was 10.3% greater than DP 90ne and yield was 16.9% less. Strength was highly correlated with lint percentage, boll weight, seed weight, and 2.5% span length. Gene numbers for these correlated traits ranged from 0.02 for micronaire to 1.04 for yield. A separate study involving the BC5 parents, ‘Deltapine 90’ (DP 90) and DP 90ne was used to determine the major physical components of strength. Fineness and individual fiber strength had no effect. Short fiber content significantly impacted strength as the three BC5 parents average short fiber was 6.7 versus 8.7% for the DP 90s. The BC5 parents average strength was 11% higher, 240 vs. 219 kN m kg−1, and its yield was 9.0% lower than DP 90ne. Probably a single major gene or closely linked cluster of genes resulted in increased fiber strength.

Failure due to fatigue in fiber bundles and solids.

We consider first a homogeneous fiber bundle model where all the fibers have got the same stress threshold (sigma(c)) beyond which all fail simultaneously in absence of noise. At finite noise, the bundle acquires a fatigue behavior due to the noise-induced failure probability at any stress sigma. We solve this dynamics of failure analytically and show that the average failure time tau of the bundle decreases exponentially as sigma-->sigma(c) from below and tau=0 for sigma>or=sigma(c). We also determine the avalanche size distribution during such failure and find a power law decay. We compare this fatigue behavior with that obtained phenomenologically for the nucleation of the Griffith cracks. Next we study numerically the fatigue behavior of random fiber bundles having simple distributions of individual fiber strengths, at stress sigma less than the bundle's strength sigma(c); (beyond which it fails instantly). The average failure time tau is again seen to decrease exponentially as sigma-->sigma(c); from below and the avalanche size distribution shows similar power law decay. These results are also in broad agreement with experimental observations on fatigue in solids. We believe, these observations regarding the failure time are useful for quantum breakdown phenomena in disordered systems.

Use of TOF-SIMS for the analysis of surface metals in H2O2-bleached lignocellulosic fibers

Abstract Oxidation of cellulose, caused by metal-catalyzed hydrogen peroxide decomposition, is one of the main potential reactions that can occur during the alkaline peroxide bleaching of pulp. This oxidation decreases the individual fiber strength and thus the sheet strength. The bleaching reaction should be controlled to limit cellulose degradation. The goal of this project was to characterize the fiber surface metal distribution using time-of-flight secondary ion mass spectrometry (TOF-SIMS) on laboratory-generated hydrogen peroxide-bleached pulps. For this study, a single black spruce was chosen and kraft pulped. Peroxide bleaching was conducted via benchtop polyethylene bag bleaching in a temperature-controlled waterbath. The overall metals content was determined using inductively coupled plasma (ICP), whereas the surface metals were determined with TOF-SIMS. Many fundamental approaches to study metal identity and content in fibers have been undertaken, including ICP, electron spectroscopy for chemical analysis (ESCA), X-ray fluorescence, and UV­vis. None of these methods can provide the localized density distribution and metal-mapping ability that is available through TOF-SIMS. It has spectacular imaging capabilities that were exploited for evaluating metals on the fiber sheets used in this work. The samples were subjected to other sheet and fiber testing. Tests conducted on select samples included brightness and viscosity. Factors in this experiment included metal addition, chelation, and peroxide charge.

Intermediate‐Temperature Stress Rupture of a Woven Hi‐Nicalon, BN‐Interphase, SiC‐Matrix Composite in Air

Woven Hi‐Nicalon™‐reinforced melt‐infiltrated SiC‐matrix composites were tested under tensile stress‐rupture conditions in air at intermediate temperatures. A comprehensive examination of the damage state and the fiber properties at failure was performed. Modal acoustic emission analysis was used to monitor damage during the experiment. Extensive microscopy of the composite fracture surfaces and the individual fiber fracture surfaces was used to determine the mechanisms leading to ultimate failure. The rupture properties of these composites were significantly worse than expected compared with the fiber properties under similar conditions. This was due to the oxidation of the BN interphase. Oxidation occurred through the matrix cracks that intersected the surface or edge of a tensile bar. These oxidation reactions resulted in strong bonding of the fibers to one another at regions of near fiber‐to‐fiber contact. It was found that two regimes for rupture exist for this material: a high‐stress regime where rupture occurs at a fast rate and a low‐stress regime where rupture occurs at a slower rate. For the high‐stress regime, the matrix damage state consisted of through‐thickness cracks. The average fracture strength of fibers that were pulled out (the final fibers to break before ultimate failure) was controlled by the slow‐crack‐growth rupture criterion in the literature for individual Hi‐Nicalon fibers. For the low‐stress regime, the matrix damage state consisted of microcracks which grew during the rupture test. The average fracture strength of fibers that were pulled out in this regime was the same as the average fracture strength of individual fibers pulled out in as‐produced composites tested at room temperature.