Fracture Toughness Of Samples Research Articles

Experimental study on mechanical properties and evolution of fracture failure of granite considering initial temperature damage and prefabricated fissure length

With the increasing mining depth of deep mineral resources, the influence of high temperature on the fracture failure of surrounding rock with fissures becomes increasingly serious. To investigate the fracture mechanical properties and the evolutionary characteristics of fracture process of fissured rock experiencing initial temperature damage, the three-point bending experiment was conducted on the small sized granite samples with different initial damaging temperatures and prefabricated fissure lengths by applying a μTS mesoscopic testing system. Based on the digital image correlation methods, the evolution of fracture process zone of sample was revealed. Moreover, the macroscopic fracture failure of sample was predicted by the evolution of time-varying acoustic emission b-value. The results show that with the increase of initial damaging temperature and prefabricated fissure length, the peak load of granite sample has a decreasing trend and the fracture characteristics of sample experiencing initial temperature damage gradually transform from brittleness fracture to ductility fracture, resulting in a decrease in the fracture toughness of sample. The crack tip opening displacement, the critical opening displacement and the length of fracture process zone of sample all show an increasing trend. The time interval from the time when time-varying acoustic emission b-value continues to decline to the appearance moment of macroscopic cracks is selected as the warning time of fracture failure of sample, which can conclude that the warning time increases with increasing the initial damaging temperature and prefabricated fissure length, indicating an increase in ductility and a reduction in failure suddenness of sample.

Accelerated laboratory evaluation of the effect of deicing condition on fracture characteristics of Silane-based modified asphalt mixtures

Winter maintenance of roads with inappropriate deicers causes much damage to road infrastructures. Researchers have always looked for suitable alternatives to salts or methods to increase the resistance of asphalt mixtures against chloride-based deicers. In this research, an accelerated laboratory method was used to investigate the effect of different deicers on nano-modified silane-based asphalt mixtures. Asphalt mixture samples were made by gyratory compaction method, and after core extraction, they were exposed to moisture and a solution of salt, Calcium Magnesium Acetate (CMA), and Potassium Acetate (KAc) for 96 h at 60 °C. Afterward, their fracture characteristics were measured at low and intermediate temperatures, and the results were statistically analyzed using two-way ANOVA and Tukey test. Statistical analysis showed that although there is no significant difference between the fracture toughness of asphalt mixtures at 0 °C in the presence of distilled water, CMA, and KAc, the reduction in the fracture toughness of samples is much greater in the presence of brine solution so that the fracture toughness of this mixture experienced a 30% reduction compared to the dry sample. Submerging the specimens in the brine solution reduces the resistance of asphalt mixture to cracking at both low and intermediate temperatures, while KAc has the least damaging effect on the fracture toughness of asphalt mixtures. Based on visual inspection and test results, CMA causes additional stress and cracking in asphalt specimens and reduces their resistance to fracture at intermediate temperatures. The silane-based anti-stripper, maintains the fracture toughness of asphalt mixture samples submerged in distilled water and the deicers at 0 °C at almost constant levels but reduces the Jc parameter in all samples. Also, conditioning in the deicers has a greater effect than binder type on the fracture toughness of asphalt mixtures at 0 °C. Besides, there is a strong interaction between binder and deicer type on stress intensity factor, K1c, of asphalt mixtures.

Durability of graphene-modified epoxy vinyl resin served as matrix phase of composite bar in simulated concrete environment

The polymer resin is usually used as an adhesive for repairing and strengthening concrete structures or served as the matrix phase of composite bar. Because the alkaline environment of concrete makes polymer resin degradation with time, the toughening modification of polymer resin has great significance. Herein, the effects of nanofiller content and exposure time on the durability of graphene-modified epoxy vinyl resin in the simulated ordinary concrete environment were evaluated by the accelerated aging method at 60 °C. The results indicated that the addition of 0.3 wt% graphene made the tensile, compressive, flexural strengths and fracture toughness of epoxy vinyl resin samples improve by 18.5%, 2.6%, 29.7% and 22.1% compared to the plain resin samples, respectively. Meanwhile, a novel multi-stage compressive constitutive model was proposed to describe the stress-strain response of polymer-based composite. Additionally, a series of tests illustrated that the durability of graphene-modified epoxy vinyl resin samples was enhanced after exposure to the simulated concrete environment. The degradation degrees of tensile, compressive, flexural strength and fracture toughness of the samples with a 0.3 wt% incorporating content of graphene declined by 7.9%, 10.4%, 5.5% and 10.4% after twenty-eight days of exposure compared to those of the exposed plain resin samples, respectively.

High‐toughness mullite ceramics fabricated by hot‐press sintering of pyrophyllite and AlOOH at low temperatures

AbstractHigh‐toughness mullite ceramics were fabricated through hot‐press sintering (HPS) of pyrophyllite and AlOOH, which were wet‐milled and well mixed using a planetary ball mill. The impacts of sintering temperatures and contents of AlOOH on mullite phase formation, densification, microstructure and mechanical properties in ceramic materials were investigated through XRD, SEM and mechanical properties determination. The results indicated that high‐toughness mullite ceramics could be successfully prepared by HPS at temperatures higher than 1200°C for 120 min. Increasing the sintering temperature from 1000 to 1300°C significantly enhanced the flexural strength and fracture toughness of samples. The highest flexural strength of 297.97±25.32 MPa and fracture toughness of 4.64±0.11 MPa⋅m1/2 were obtained for samples sintered at 1300°C. Further increase of temperature to 1400°C resulted in slight decrease of flexural strength and fracture toughness. Compared with the mullite ceramics prepared only using pyrophyllite as raw material, incorporation of AlOOH into raw material significantly increased the mechanical properties of final mullite ceramics. And stoichiometric AlOOH and pyrophyllite as starting material gave the best performance in fracture toughness. The high‐toughness of mullite ceramics were ascribed to the high mullite phase content, fine mullite whiskers and in situ formed, intertwined three‐dimensional network structure obtained through HPS at a low temperature of 1300°C.

Rapidly fabricating Y2O3 transparent ceramics at low temperature by SPS with mesoporous powder

AbstractFine‐grained and dense highly transparent Y2O3 ceramics have been successfully prepared using high sintering activity mesoporous Y2O3 powders without any additive by spark plasma sintering (SPS). The influences of the sintering temperature on microstructure, density, optical, and mechanical properties of SPS‐sintered Y2O3 ceramics were studied in detail. As results, the optimal Y2O3 ceramics with high relative density of 99.90% and fine average grain size of 140 nm were obtained at a low sintering temperature of 1140°C and a moderate load pressure of 60 MPa for 5 min. Meanwhile, the dense Y2O3 ceramics with 1 mm thickness after annealing show a high linear transmittance of 78% (close to 94% of the theoretical value) at 2.4–3 µm wavelength. In additions, the Vickers hardness and fracture toughness of samples can reach 8.48 GPa and 1.45 MPa m1/2, respectively. This result proves that the high activity of mesoporous Y2O3 is considered to be an important means for preparing high‐performance fine Y2O3 ceramics at low sintering temperature.

High Mechanical Properties of Stretching Oriented Poly(butylene succinate) with Two-Step Chain Extension.

The structure, morphology, fracture toughness and flaw sensitivity length scale of chain-extended poly(butylene succinate) with various pre-stretch ratios were studied. PBS modification adopted from a multifunctional, commercially available chain-extension containing nine epoxy groups (ADR9) as the first step chain extension and hydroxyl addition modified dioxazoline (BOZ) as the second step. Time-temperature superposition (TTS) studies show that the viscosity increased sharply and the degree of molecular branching increased. Fourier transform infrared spectroscopy (FT-IR) confirm successful chain extension reactions. The orientation of the polymer in the pre-stretch state is such that spherulites deformation along the stretching direction was observed by polarized light optical microscopy (PLOM). The fracture toughness of sample (λfix = 5) is Γ ≈ 106 J m-2 and its critical flaw sensitivity length scale is Γ/Wc ≈ 0.01 m, approximately 5 times higher than PBS without chain-extension (Γ ≈ 2 × 105 J m-2 and Γ/Wc ≈ 0.002 m, respectively). The notch sensitivity of chain-extended PBS is significantly reduced, which is due to the orientation of spherulites more effectively preventing crack propagation. The principle can be generalized to other high toughness material systems.

Effect of polyurea coating on the ductility of aluminum foam

The closed-cell metal foams are utilized in various industrial applications due to their impact and energy absorption capacity, noise cancelation, and lightweight properties. Additionally, the manufacturing methods of stochastic foams with blowing agents or inert gas injection are well known. However, the poor mechanical strength and high brittleness are restricting the application of these materials. A method of increasing the ductility, plastic deformation, and fracture toughness of these materials is the integration of elastomer coating. Polyurea is widely used in applications where water-proofing and high ductility are aimed. In this study, two types of polyurea coatings as MS950 and MS955 are sprayed over the aluminum foam with three densities as 0.508, 0.557, and 0.624 g/cm3 to evaluate the effect of coating on the ductility and fracture toughness of samples considering the tensile behavior and 4 densities as 0.508, 0.538, 0.557, and 0.624 g/cm3 for the flexural bending test. The MS955 polyurea coating elevated the ultimate tensile strength and elasticity modulus of all specimens with a maximum of 239% and a minimum of 70%. However, the MS950 coating did not result in a significant rise in tensile strength. The main advantage of the MS950 coating is in the amount of strain increment with 374% in maximum and 181% in minimum resulting in higher ductility and fracture toughness. Additionally, the bending characteristics of MS950 coating demonstrate a drastic increase in both load and strain which recommend the usage of this coating in bending and impact applications. Finally, an analytical evaluation of the coated and uncoated specimens in the elastic region and the elastoplastic region is provided using linear and power-law interpolation. In tensile tests, the MS955 coating resulted in higher elasticity modulus and fracture toughness. However, in bending applications, the MS950 coating demonstrated higher load strength with elevated ductility.

Unveiling the brittleness of hybrid organic–inorganic 0-D histammonium zinc chlorometallate by nanoindentation

Hybrid organic–inorganic halometallates, with different organic and inorganic components, can provide a wide array of tunable physical properties. While many optoelectronic phenomena are being explored, research on the mechanical properties of this class of materials, especially fracture toughness, is lacking, resulting in conclusions on material flexibility being drawn from their elastic modulus and hardness alone with an implicit assumption that these properties correlate with material flexibility. In this Letter, we report nanoindentation results on the elastic modulus, hardness, and fracture toughness of single crystal samples of hybrid organic–inorganic histammonium chlorozincate, HistZnCl4 along the [001] axis. We find that the elastic modulus is 12.078 ± 1.034 GPa, and the hardness is 0.611 ± 0.089 GPa. Moreover, the fracture toughness of this sample is measured to be 0.098 MPa m12. Although these materials have a hardness to modulus ratio similar to that of metals, they fracture like brittle materials, demonstrating the importance of conducting studies on a material fracture toughness before determining their applicability in flexible device applications.

Experimental investigation and multi-objective optimization of fracture properties of asphalt mixtures containing nano-calcium carbonate

The low temperature fracture is one of the most important challenges in asphalt mixtures, which, if not paid, will lead to high maintenance costs. Therefore, researchers are looking for different materials to enhance the fracture behavior of mixtures. As the asphalt surface is affected by different types of loading throughout their operational lifetime, this study explores the impact of nano-calcium carbonate (NCC) on the fracture behavior of mixtures in different fracture modes. For this purpose, semi-circular bending (SCB) tests were applied to specify the fracture toughness. Two bitumen types of PG 64-22 and PG 58-28 were modified with 1%, 3%, 5% and 7% NCC. Finally, the fracture toughness of samples in pure mode-I, pure mode-II and mixed-mode I/II was investigated at −10 °C. Moreover, the prediction models of multivariate regression (MVR), group method of data handling (GMDH) and genetic programming (GP) were provided to present the best model with higher accuracy in order to obtain optimum NCC content with two objectives of KIf and KIIf (the fracture toughness in modes I and II). The results indicated that NCC had a notable influence on the fracture toughness. However, in each mode and bitumen, the additive percentage that is associated with the highest fracture toughness was different. From the fracture tests, the most optimal percentage of NCC was determined between 3% and 7% and 3% to 5% for mixtures made with bitumen types of PG 64-22 and PG 58-28, respectively. Among the various models, GMDH had the greatest R2 so that the R2 amount of GMDH for KIf and KIIf was 98.68% and 99.02%, respectively. The two-objective optimization results showed that 4.17%, 3.62% and 6.29% NCC were the best optimal amounts to maximize KIf and KIIf amounts simultaneously for all mixtures and mixtures made with bitumen types of PG 64-22 and PG 58-28, respectively.

Strength, fatigue strength and toughness of dissimilar Ti17–Ti64 linear friction welded joints: Effect of soft surface contamination and depletion of α precipitates

The effect of microstructure of three dissimilar Ti17–Ti64 linear friction welded joints on the strength, fatigue strength and fracture toughness was studied. A special attention was paid to role of soft contaminants and α precipitate depletion. Three joints were produced: one in the as machined state after electrical discharge machining, one ground prior to welding, and one welded in the as machined state and post-weld heat treated. The microstructure of the weld centre zone (WCZ) exhibited an acicular entangled α' martensite with a Widmanstätten morphology on the Ti64 side and fine equiaxed distorted β grains on the Ti17 side. A depletion of the strengthening α precipitates was observed in the WCZ and the thermo-mechanically affected zone (TMAZ) on the Ti17 side. For the studied welding parameters, the LFW self-cleaning mechanism failed to extrude the contaminants in the weld interface of the joint welded in the as machined state. These contaminants were related to prior to welding machining and were denominated soft contaminants due to their relatively low melting point. A defect layer was formed that led to a pseudo-brittle fracture in at the WCZ during tensile testing, cyclic loading and fracture toughness testing. During tensile testing of the joint that was ground prior to welding, failure occurred after plastic deformation sometimes at the Ti64 parent material (PM) and sometimes at the Ti17 TMAZ. Fatigue cracks initiated at the latter weak zone with a reduced fatigue strength. Compared to the PM, fracture toughness of samples even without fatigue pre-crack was also reduced. Failure at the weak zone highlighted unexpected colonies of elongated dimples that nucleated on transgranular deformation lines.

Effect of build orientation on fracture and tensile behavior of A357 Al alloy processed by Selective Laser Melting

Selective Laser Melting (SLM) is an additive manufacturing technology that allows producing components through local melting of a metallic powder bed using high-power laser beam in accordance with 3D digital CAD models. Cast Al-Si-Mg alloys are broadly used due to their good processability by SLM and mechanical properties. This work investigates the mechanical behavior of A357 aluminum alloy produced by SLM in the as built and artificially aged conditions. In particular, the tensile properties and fracture toughness of the alloy were investigated according to different specimen orientations on the build plate in the first case and different crack propagation planes in the second. In addition, microstructural and fractographic analyses were performed in order to investigate crack propagation paths and fracture mechanisms. The results showed that building orientation has a strong influence on material response, mainly with regards to the fracture toughness. The melt pool boundaries act as preferential paths for crack propagation and lead to a marked drop in fracture toughness of samples with cracks perpendicular to the building direction.

Concrete fracture toughness increase by embedding self-healing capsules using an integrated experimental approach

An alternative solution to repair cracks in concrete has been recently established, namely autonomous healing by embedding repair agent into concrete during casting. The agent is included into small-size capsules that break as cracks form. As the agent is released into the crack void, it solidifies fast permitting crack sealing and mechanical restoration. Most of literature studies focus on the healing system design and the repair efficiency assessment. However, limited knowledge exists on the effect of the macro-capsules themselves on concrete mechanical behaviour. This is a crucial subject as the healing mechanism should not compromise the material mechanical properties. The paper investigates the effect of capsules on damage resistance by considering fracture constitutive models. Fracture toughness of samples with capsules increases up to 35% compared to the reference. Acoustic emission is applied to assess the capsules effect on fracture processes. Events localization indicates fracture process zone expansion by 40%. It is proven that the capsules beneficially contribute as local reinforcement perturbating the crack path and forming multiple microcracks (additionally verified by Digital Image Correlation). In summary, an integrated experimental protocol is developed to highlight the concrete toughness enhancement by adding capsules.

Effects of Imperfections in Heat Straightening Repair of Steel Beam Bridges

A 40 ft. long two-span continuous steel bridge with two composite beams was constructed in the laboratory and subjected to damage followed by heat straightening repair. A36 steel section (W30 × 90) was used for the main girders (beams). Four spans (specimens) of the test bridge were statically damaged at each midspan using a hydraulic actuator, and subsequently repaired by applying Vee heats and restraining forces in the damaged region. Restraining force magnitude (corresponding to 0.4 Mp: 6.2 kips and 0.6 Mp: 9.5 kips), maximum heating temperature (800°F, 1200°F, and 1400°F), and the number of multiple damage-repair cycles (one and three cycles) were considered as the test parameters. The steel material properties were measured by taking samples from the repaired areas, and compared with undamaged steel material properties. Samples taken from specimens subjected to overheating (up to 1400°F) had similar structural properties and fracture toughness values as those taken from specimens subjected to normal heating (up to 1200°F). Specimens repaired with overstraining (0.6 Mp) combined with underheating (up to 800°F) required the largest number of heating cycles to fully repair the same damage. The fracture toughness of samples taken from specimens subjected to multiple (three times) damage-repair cycles was lower (decreased to about 84%) than the fracture toughness of samples taken from specimens subjected to only one damage-repair cycle. Therefore, multiple heat straightening repairs of a damaged beam should be performed with caution. With reference to serviceability performance for AASHTO HL-93 live load, the midspan deflections of beam specimens subjected to damage and heat straightening repair were comparable to those of undamaged beam specimens.

Effect of radial hydride fraction on fracture toughness of CWSR Zr-2.5%Nb pressure tube material between ambient and 300 °C temperatures

Pressure tube spools fabricated from Zr-2.5%Nb alloy, were subjected to gaseous hydrogen charging and stress reorientation treatment to form radial hydrides in a specially designed fixture. Curved Compact Tension specimens of 17 mm width containing 100 wppm of hydrogen were made using electro discharge machining. Some of the samples were annealed at 300, 325 and 350 °C followed by furnace cooling. Metallographic examination of the samples revealed 100% radial hydride in the samples to which stress reorientation treatment was imparted. The radial hydride fraction decreased with increase in the temperature of annealing that followed stress reorientation. Fracture toughness of samples with variable radial hydride fraction was evaluated as per ASTM E1820-13 procedure between 25 and 300 °C. All the samples exhibited a sharp ductile to brittle transition behavior and the transition temperature was observed to increase with increase in the radial hydride fraction.

Phase morphology, fracture toughness and failure mechanisms in super-toughened PLA/PB-g-SAN/PMMA ternary blends: A quantitative analysis of crack resistance

The effect of poly(methyl methacrylate) (PMMA) content on the fracture resistance, failure mechanisms and micromechanical deformation of poly(lactic acid) (PLA)/polybutadiene-g-poly(styrene-co-acrylonitrile) (PB-g-SAN) blend was studied. The fracture toughness of samples was characterized by the essential work of fracture (EWF) method. The specific EWF (we) linearly increased with PMMA content up to 30 wt%, indicating improved fracture toughness of ternary blends with PMMA content. The specific plastic work (βwp) increased at first with PMMA content up to 15 wt% and then decreased at higher loadings of PMMA. The energy dissipated during yielding (we,y) and subsequent tearing (we,t) stages was determined by the energy partitioning approach. Both we,y and we,t components monotonically increased with PMMA content in ternary blends, suggesting enhanced material resistance against crack initiation and subsequent crack propagation. Fracture behavior and toughening mechanisms were studied. Suitable interfacial adhesion, cavitation of rubber particles and irreversible plastic growth of micro-voids caused massive shear yielding of matrix material in the super-toughened blends.

Effect of TiO2 addition on the properties of Ti3Si(Al)C2 based ceramics fabricated by reactive melt infiltration

In this paper, Ti3Si(Al)C2 based ceramics were fabricated by reactive melt infiltration (RMI) of TiC/TiO2 preforms with liquid silicon. The microstructure, phase composition, and mechanical properties of the Ti3Si(Al)C2 based ceramics have been investigated to understand the effect of phase composition of the preforms on the formation mechanisms of Ti3Si(Al)C2. The preforms with different content of TiO2 infiltrated at 1500°C with liquid silicon for 1h were composed of Ti3Si(Al)C2, Al2O3, TiC, TiSixAly and residual Al. The prior generated Al2O3 phases inhibited the dispersion of Ti3Si(Al)C2 phases, resulting in the drastically grain growth of Ti3Si(Al)C2. Subsequently, the microstructure with gradually increasing Ti3Si(Al)C2 grain size resulted in the decrease of the bending strength and fracture toughness of samples. When the content of TiO2 reached 20wt%, the bending strength reached the maximum, 326.6MPa. The fracture toughness attained the maximum, 4.3MPam1/2, when the content of TiO2 was 10wt%.

Variation of Transparency of Zeolite-Derived Silica Glass Prepared by Spark Plasma Sintering

A series of zeolite-derived silica glasses have been fabricated through the order-disorder transition process of zeolites using spark plasma sintering (SPS). The samples were characterized by UV-VIS-NIR transmittance, Raman spectra and Vicker’s hardness. Results showed that the transmittance of zeolite-derived silica glass samples increased with the sintering temperature increasing. The transmittance of the sample prepared at 1300°C was lower than 5%. The highly transparent samples could be obtained when the sintering temperature was above 1350°C. The transmittance of these samples was higher than 80% in the range of 780~1700nm and 60% in the UV-vis range of 250~780nm. One absorption band at 300nm can also be observed in the spectrum. The reason of this phenomenon could be ascribed to a few left crystallite fragments of ZSM-5. The Raman spectra showed that the bands at 378 and 292cm-1of ZSM-5 were significantly weakened and then disappeared with increasing the sintering temperature and the bands at 489 and 600cm-1 of silica glass became more apparent. The Raman spectra of the samples obtained above 1350°C were the same as that of the fused silica glass. The Vicker’s hardness of as-prepared samples increased with the rising of the sintering temperature. The maximum hardness of 7.31 GPa was achieved for the sample sintered at 1400°C. The fracture toughness of samples increased from 0.83 MPa·m1/2 to 2.37 MPa·m1/2 with the sintering temperature increasing.

Test of subcritical crack growth and fracture toughness under water-rock interaction in three types of rocks

The subcritical crack growth and fracture toughness in peridotite, lherzolite and amphibolite were investigated with double torsion test. The results show that water-rock interaction has a significant influence on subcritical crack growth. With water-rock interaction, the crack velocity increases, while the stress intensity factor declines, which illustrates that water-rock interaction can decrease the strength of rocks and accelerate the subcritical crack growth. Based on Charlse theory and Hilling & Charlse theory, the test data were analyzed by regression and the correlation coefficients were all higher than 0.7, which shows the correlation is significant. This illustrates that both theories can explain the results of tests very well. Therefore, it is believed that the subcritical crack growth attributes to the breaking of chemical bond, which is caused by the combined effect of the tensile stress and the chemical reaction between the material at crack tip and the corrosive agent. Meanwhile, water-rock interaction has a vital effect on fracture toughness. The fracture toughness of samples under atmospheric environment is higher than that of samples immersed in water. And water-rock interaction has larger influence on fracture toughness in amphibolite than that in peridotite and lherzolite.

Effect of Rare-Earth Oxides Additive on Liquid Phase Sintered SiC

The densification of α-SiC occurred by liquid-phase sintering mechanism with AlN-RE2O3(RE=Nd, Gd, Y, Lu) was studied. The total additive content was fixed at 15 wt%. Cold isostatically pressed samples were sintered at 1800-1950 °C under N2atmosphere for 1 h. The linear shrinkage and weight loss of the samples were about 17-20% and 2-5%, respectively. The mechanical properties and microstructure of sintered samples were investigated. The experimental results showed that the fracture toughness of samples was 6-8 MPa·m1/2, the hardness was in the range of 18-21 GPa and the bending strength was in the range of 400-500 MPa. It was found that a decrease in the cationic radius of the rare-earth oxides was accompanied by an increase in hardness and flexural strength of the SiC ceramics, whereas the fracture toughness was improved by incorporating rare-earth oxides of larger cationic radius. The morphology (SEM) of sintered sample showed a fine grained microstructure with equiaxed grains. Fracture mode was intergranular fracture.

Fracture Toughness of Treated OPEFB Filled Polymer Nanocomposites at Different Clay Loading

Two different clay loadings were added to OPEFB/PP/MAPP which was 7 phr and 10 phr in order to see its effect on the fracture toughness. Standard dimension of samples was prepared for 180 μm, 355 μm and 355 μm treated with 1% NaOH. The result showed that 7 phr clay contents had the highest fracture toughness of 1.16 MPa.m1/2. Alkali treatment on OPEFB fiber gave 7% improvement of fracture toughness for samples with 7 phr PPnanoclay loading. SEM images show the improvement of bonding between the fibers and the matrix upon alkali treatment.