Fracture Toughness Of Matrix Research Articles

Micromechanics constitutive modelling and optimization of strain hardening geopolymer composite

This paper investigates the micromechanics constitutive modelling and optimization of a fiber-reinforced strain-hardening geopolymer composite (SHGC) recently developed by the authors. Micromechanical parameters of the developed fly ash-based SHGC were independently measured or deduced to compute the analytical crack bridging (σ-δ) relation of the composite. The predicted σ-δ relation was compared with the experimental test results. It was confirmed that the previously developed micromechanics-based model can reasonably predict the σ-δ relation of fly ash-based SHGCs. Using the verified model, a parametric study was then performed to evaluate the effects of fiber length, fiber surface oil-coating, and matrix fracture toughness on critical (minimum) fiber content required to exhibit saturated pseudo strain-hardening (PSH) behavior. The results indicated that the critical fiber content in fly ash-based SHGCs is mainly governed by the energy-based criterion. It was demonstrated that the fiber surface oil coating, the increase of fiber length and the reduction of matrix fracture toughness are effective approaches to reduce the critical fiber content. Using the model, it was demonstrated that fly ash-based SHGCs can be systematically optimized by proper tailoring of the material constituents to achieve saturated PSH behavior with the lowest amount of fiber, and thereby the lowest cost.

Evaluation of the geopolymer/nanofiber interfacial bond strength and their effects on Mode-I fracture toughness of geopolymer matrix at high temperature

The present article has reported the effects of several nanofiller’s aspect ratio, length and interfacial strength on Mode-I fracture toughness (KIC) of geopolymer as the matrix of continuous fibre reinforced composites. These nanofillers have been chosen based on the variations in the surface chemistry and nature of interfacial bonding with geopolymer, which include Carbon, Alumina and Silicon carbide. Geopolymer matrix was subjected to the addition of single volume fraction, 2% of each type of nanofiller with two aspect ratios, designated as nanoparticles and nanofibers. Notched beam flexure tests (SEVNB) of neat and each nanofiller reinforced samples suggest that, while baseline KIC of neat geopolymer improved with heat treatment, nanofibers with high interfacial bond strength showed maximum capability in further improving KIC. Among those nanofibers, 2 vol% Silicon Carbide Whisker (SCW) showed the largest improvement in KIC of geopolymer, which is ~164%. After heat treatment at 650 °C, SCW reinforcement was also found to be effective, with only ~28% lower than the reinforcing performance at 250 °C, while the performance of Alumina Nanofiber reinforced geopolymer notably reduced. SEM and EDS analysis suggested that the inhomogeneity in neat geopolymer and length of nanofibers control the reinforcing capability as well as crack propagation resistance of geopolymer. For instance, minimum length of nanofillers to toughen this geopolymer at 250 °C was required as ~2 μm. The results further suggested that the sample failure occurred due to the dominance of tensile failure of nanofibers over the interfacial separation.

Multiscale Characterization of Nanoengineered Fiber-Reinforced Composites: Effect of Carbon Nanotubes on the Out-of-Plane Mechanical Behavior

The mechanical properties of the matrix and the fiber/matrix interface have a relevant influence over the mechanical properties of a composite. In this work, a glass fiber-reinforced composite is manufactured using a carbon nanotubes (CNTs) doped epoxy matrix. The influence of the CNTs on the material mechanical behavior is evaluated on the resin, on the fiber/matrix interface, and on the composite. On resin, the incorporation of CNTs increased the hardness by 6% and decreased the fracture toughness by 17%. On the fiber/matrix interface, the interfacial shear strength (IFSS) increased by 22% for the nanoengineered composite (nFRC). The influence of the CNTs on the composite behavior was evaluated by through-thickness compression, short beam flexural, and intraply fracture tests. The compressive strength increased by 6% for the nFRC, attributed to the rise of the matrix hardness and the fiber/matrix IFSS. In contrast, the interlaminar shear strength (ILSS) obtained from the short beam tests was reduced by 8% for the nFRC; this is attributed to the detriment of the matrix fracture toughness. The intraply fracture test showed no significant influence of the CNTs on the fracture energy; however, the failure mode changed from brittle to ductile in the presence of the CNTs.

Durability study on engineered cementitious composites (ECC) under sulfate and chloride environment

The lack of durability of concrete hydraulic structures, especially under sulfate and chloride environment, has become a worldwide problem. The present paper investigated the feasibility of applying ductile engineered cementitious composites (ECC) as an alternative to conventional concrete in hydraulic structures to improve their durability performance. Specifically, the durability of ECC under sulfate and combined sulfate-chloride conditions were studied. Compressive and tensile behavior of ECC after long-term exposure to Na2SO4 and Na2SO4+NaCl solutions was experimentally characterized. Subsequently, micromechanical study was adopted to investigate the microscopic mechanisms underlying the composite level behavior of ECC under those aggressive environments. The research findings demonstrated that ECC remains durable and maintains its high mechanical performance even after 200days of exposure to concentrated sulfate and combined sulfate-chloride environments. In addition, the micromechanics-based study quantitatively characterized the influence of sulfate and sulfate-chloride solutions on the micromechanical parameters including matrix fracture toughness and fiber/matrix interfacial bond, providing insights into the underlying mechanisms of the composite level behavior of ECC under those environments.

Designing ductile CuZr-based metallic glass matrix composites

The CuZr-based metallic glass matrix composites (MGMCs) with different volume fractions of crystalline phases were designed by doping of nickel. Minor addition of nickel element can change the glass-forming ability of the resultant composites. Large compressive plasticity accompanied by strong work-hardening capacity was achieved in the Cu47Zr48Al4Ni1 composite with a volume fraction of crystalline phases of 33%. The excellent compressive properties were mainly attributed to the inhibition for the propagation of shear bands by the ductile crystals and deformation induced martensitic transformation of the B2-CuZr phase. However, no obvious global tensile ductility was obtained, due to the mode I fracture toughness and small plastic-zone size of glass matrix. To uncover the shear-band evolution during deformation, finite element simulation was conducted, revealing that the ductile B2 phase can tune the shear-stress distribution and consequently initiate and retard shear bands, which stimulates the multiplication of shear bands. Accordingly, the spacing of B2 CuZr particles is a vital factor dominating the plasticity of CuZr-based MGMCs, especially upon tension.

Simulation and test of flexural performance of polyvinyl alcohol-steel hybrid fiber reinforced cementitious composite

The flexural performance of polyvinyl alcohol-steel hybrid fiber reinforced engineered cementitious composite with characteristics of low drying shrinkage special focus on impacts of steel fiber content and matrix strength has been investigated in both experimental and theoretical aspects in this paper. Four matrix types with water to binder ratio of 0.25, 0.35, 0.45, and 0.55 and three additional steel fiber contents in the composite with polyvinyl alcohol fiber content of 1.7% in volume were used in the test program. The experimental results show that cracking and flexural strength of the composites are increased with the addition of steel fiber. This enhancement becomes more and more pronounced with decreasing of water to binder ratio of the composites. Meanwhile, fracture mechanics-based flexural model is used to simulate the flexure performance of the polyvinyl alcohol -steel hybrid fiber reinforced engineered cementitious composite with characteristics of low drying shrinkage. The model results show that a double peak load is expected of the composites under bending load. The first peak is controlled by the fracture toughness of matrix or cracking strength of matrix, and the second peak is governed by the fiber bridging. The effect of addition of steel fiber in engineered cementitious composite with characteristics of low drying shrinkage on the first peak is unapparent. The impact of steel fiber on the second peak is significant. This enhancement of additional steel fiber gradually decreases with the decrease of water to binder ratio of the matrix, which coincides well with the experimental findings. The test results are compared to the model and reasonable agreement is found.

Preparation and property of epoxy resins-penetrated aligned carbon nanotube bundle hybrid microcapsules for self-healing polymers

The epoxy resins-penetrated aligned carbon nanotube bundle (ACNTsB) hybrid system (epoxy@ACNTsB) was prepared by soaking ACNTsB in the acetone solution of epoxy resins. Epoxy@ACNTsB was encapsulated in aqueous solution containing an amine curing agent to prepare hybrid microcapsules (MCs). The structure and morphology of MCs were characterized by Fourier transform infrared spectroscopy, scanning electronic microscopy, transmission electron microscopy, and laser scanning confocal microscopy. The thermal properties of MCs were carried out by differential scanning calorimetry and thermogravimetric analysis. The solvent resistance of MCs in acetone was also investigated. The results show that MCs exhibit high initial decomposition of temperatures (266°C). The content of epoxy resin in MCs is about 74% and the polymer film thickness of MCs is from 1 to 2 μm. MCs show high thermal and chemical stability below 200°C and excellent solvent resistance. MCs embedded in cyanate ester (CE) resins can significantly toughen the matrix. The fracture MCs can release the epoxy resins under heating condition, and the released epoxy resins can react with the reactive –OCN group/triazine rings in the CE matrix to rebond the crack surfaces. When the temperature schedule of 100°C/1 h + 200°C/2 h is applied, 10–15 wt% MCs can recover from 82.7% to 99.8% of the original fracture toughness of CE matrix.

Study of the effect of amino-functionalized multiwall carbon nanotubes on dry sliding wear resistance properties of carbon fiber reinforced thermoset polymers

This work investigates the effect of multiwall carbon nanotubes (MWCNTs) on the mechanical and tribological behavior of a fiber reinforced composite (FRC). Fiber reinforced composites and nano-engineered FRCs are manufactured by resin transfer molding. In-plane tensile tests, in-plane shear tests and through-thickness compression tests are used to assess the influence of MWCNTs on the material mechanical behavior. Pin on disk dry sliding tests are used to quantify the effect of MWCNTs on the friction coefficient and the specific wear rate. It was determined that (1) MWCNTs have an influence on the improvement on both the through-thickness compression strength and the specific wear rate, and (2) they do not influence the material stiffness, in-plane tensile and shear strengths and the friction coefficient. It is assumed that the observed improvements are due to the demonstrated positive influence of the MWCNTs effect on the matrix/reinforcement interfacial strength and on the matrix fracture toughness.

Matrix design of strain hardening fiber reinforced engineered geopolymer composite

The feasibility of developing a fiber reinforced engineered geopolymer composite (EGC) exhibiting strain hardening behavior under uni-axial tension has been recently demonstrated. The effect of different alkaline activators on the matrix and composite behavior of such EGC has also been evaluated to enhance its compressive and tensile strengths with relatively low concentration activator combinations. The focus of this study, as a follow up investigation, is to evaluate the quantitative influence of geopolymer matrix properties on the strain hardening behavior of the recently developed fly ash-based EGC with the aim of selecting the appropriate type of geopolymer matrix to manufacture the strain hardening EGC with enhanced elastic modulus while maintaining the tensile ductility behavior of the composite. The effects of water to geopolymer solids ratio, sand size and sand content, as the most significant matrix-related parameters, on the matrix properties including workability, compressive strength, elastic modulus, fracture toughness and crack tip toughness, and the uni-axial tensile performance of the composite were evaluated. Experimental results revealed that lowering the water to geopolymer solids ratio and the addition of sand enhanced the elastic modulus of the geopolymer matrix and composite in all cases. However, the excessive use of fine sand and the use of coarse sand adversely affected the strain hardening behavior of the developed EGC due to the increase of the matrix fracture toughness and the first-crack strength of the composite. Only geopolymer matrices with suitable fracture toughness, as defined by the micromechanics design model, maintained the desirable tensile ductility of the developed fly ash-based EGC.

Mechanical performance of ECC with high-volume fly ash after sub-elevated temperatures

Engineered cementitious composites (ECC) are known for their strain-hardening behavior under tension, and have been increasingly applied in engineering practice. However, the lack of understanding about the performance of ECC exposed to elevated temperatures limits their application in some special fields. Therefore, the residual mechanical performance of ECC containing very high volume fly ash (FA/C=4.4) and polyvinyl alcohol fibers (HVFA-ECC) was investigated after temperature exposures of 20°C, 50°C, 100°C and 200°C, considering the melting temperature of polyvinyl alcohol fiber is about 230°C. The test results indicated that HVFA-ECC maintains its unique multiple cracking and pseudo strain hardening characteristics after temperature exposure within this range. The tensile properties, including the ultimate tensile strength and tensile strain capacity, increased after the 50°C and 100°C treatments, but diminished after the 200°C exposure. To better understand the impacts of thermal exposure, tests were carried out on the fiber tensile strength, fiber/matrix interfacial bond and matrix fracture toughness. It was found that the fiber’s tensile strength retained its room temperature value up to 100°C exposure, but dropped significantly after undergoing the 200°C heating. The interface parameters (chemical bonding Gd and frictional bonding τ0) and the strain-hardening index (Jb′/Jtip) have a similar trend as the composite tensile properties, which explained the variation of composite response with thermal treatment. The test results indicates that HVFA-ECC can resist a sub-elevated temperature (⩽200°C) exposure, and a moderate temperature treatment (⩽100°C) may actually enhance ECC’s tensile properties. This study provides a scientific basis for further development of ECC for elevated temperature applications and also a possible technique to improve ECC’s mechanical properties.

Quantification of crack area in ceramic matrix composites at single-fiber push-out testing and influence of pyrocarbon fiber coating thickness on interfacial fracture toughness

For mechanical characterization of interfacial properties in fiber-reinforced ceramic matrix composites by single-fiber push-out tests, a determination of the relevant crack area is required. In established evaluation methods, the relevant crack area is approximated by the total cylindrical fiber surface of the pushed fiber. This concept disregards that stable crack propagation, which is relevant for prediction of macromechanical behavior, may occur on just part of the fiber-matrix interface area. In the present publication, a new approach to quantify the relevant crack area is presented, enabling a more reliable determination of the interfacial fracture toughness of ceramic matrix composites.The new concept is applied to SiC-fiber reinforced SiC-matrix composites with pyrocarbon fiber coatings (SiC/PyC/SiC) produced via chemical vapor infiltration technique. The occurrence of stable and unstable crack growth, as predicted in literature, can be verified experimentally. A strong correlation between PyC fiber coating thickness and interfacial fracture toughness is found.

Development and Property Evaluation of Chilled MMC Prepared with LM25 as Matrix and SiO<sub>2</sub> (Glass) Particulates as Dispersoids

Main objective of the present work is to investigate the effect of dispersiod content and the effect of chill on the mechanical properties of chilled MMC with LM25 as matrix and SiO2 as dispersoid. Investigation is carried out to evaluate Ultimate Tensile Strength (UTS), Fracture toughness, Hardness, and microstructure of chilled aluminum matrix and Glass particulate composite. The glass (SiO2) particles ranging from 30-to100µm were chosen as dispersiod and added, ranging from 3to 12wt% in steps of 3%. The composite was prepared by stir-casting technique and poured into the sand molds incorporated with non-metallic and metallic chills. Test result showed that this MMC was greatly influenced by the dispersiod and chills. Fracture toughness & UTS of the composite are found to depend on the wt% of the dispersiod and chilling medium. It is observed that chill has influenced hardness of the composite. Volumetric heat capacity (VHC) of the chill is found to increase the amount of heat absorbed. Microstructure analysis has reveled uniform distribution of the dispersiod, which results in improved properties of the particulate reinforced metal matrix composite.

Influence of Carbon Nanotube Inclusion on the Fracture Toughness and Ballistic Resistance of Twaron/Epoxy Composite Panels

The objective of this study is to investigate the effect of multi walled carbon nanotube (MWCNT) inclusion on the fracture toughness and the ballistic resistance properties in terms of energy absorption. The determination of fracture toughness of the epoxy/MWCNT matrix was carried out by using a single edge notch bending (SENB) method according to the ASTM D5045-99. Four different weight percentage (wt %) of multi-walled carbon nanotubes (MWCNTs) contents were used, which were 0wt.%, 0.1wt.%, 0.55wt.% and 1.0wt.%. Epoxy binder and MWCNTs were mixed by using a mechanical stirrer for 10minutes at 1500rpm speed and was further sonicated for 30minutes at 30Hz amplitude in order to enhance the homogeneity of MWCNTs in the matrix. The compositepanel comprised of Twaron fabrics with epoxy resin filled with MWCNTs was fabricated using hand layup and vacuum bagging assist method. The Twaron/epoxy/MWCNT composite panels weresubjected toa ballistic test using 9mm Full Metal Jacket bullet at different impacting velocities. From the SENB results, it can be reported that MWCNT inclusion up to 1.0% w.t content shows significantinfluence towards increment of fracture toughness value.MWCNT also improves the ballistic resistance capability of the composite panel where it was found that the impact energy absorption value was proportionally increasewith theepoxy/MWCNT matrix's fracture toughness properties.

Comparison between Commercial and Synthesized Hyperbranched Polyesters Regarding Fracture Toughness of Epoxy Matrix

A hyperbranched poly (butylene adipate) (HPBA) polymer was compared with a commercial dendritic polyol (Boltorn H311) as toughening agent for a commercial one-part epoxy resin, Cycom 890 RTM. Both modifiers were added in a weight percentage 1, 3, 5 and 10 %. Blends obtained were characterized through DMA and impact resistance (KIC). SEM morphology were also evaluated. The toughness improvement was achieved without substantial impairment on thermomechanical properties. Fractography analysis has evidenced the fracture standard changing from brittle to ductile by modifiers addition.

ZrO2 fiber-matrix interfaces in alumina fiber-reinforced model composites

Properties of the fiber/matrix interface in monoclinic ZrO2-coated alumina fiber-reinforced composites were investigated using model composites with dense alumina matrix and single alumina fibers. The fiber-matrix porous interface was produced via dip-coating single fibers in monoclinic ZrO2 suspensions and two different particle sizes were used in order to vary the coating porosity. The interfacial sliding stress was determined via fiber pushin test; crack deflection behavior was analyzed from cracks created via Vickers indentation and via fracture surface. Fracture toughness and elastic modulus of interface, matrix and fibers were evaluated and used to predict the crack deflection behavior using the model of He and Hutchinson. Results showed that monoclinic zirconia interfaces are suitable for dense alumina composites, due to the low fiber/interface chemical interaction. A uniform coating can be produced via dip coating and tailored with using different monoclinic zirconia particle sizes. The sliding stress values obtained with the pushin test were about 120MPa.

Effect of matrix properties on the fatigue damage initiation and its growth in plain woven carbon fabric vinylester composites

The influence of matrix properties including the fracture toughness and carbon fiber (CF)/vinylester (VE) adhesion on the static tensile properties and tension–tension fatigue properties of CF/VE composites has been investigated. An appropriate chemical modification of VE resin improved the tensile strength up to 37.3% and extended the fatigue life of CF/VE composites greatly by several ten times than that using regular VE resin as matrix. Under static loading different failure modes appeared and strain at failure changed due to chemical modification of VE altering the fracture toughness and CF/VE adhesion. Based on TSA (thermoelastic stress analysis) and SEM (scanning electron microscopy) observation, it was revealed that the fracture toughness of matrix dominated the initiation and growth of matrix cracks in CF/VE composites at fatigue initial stage. Transverse cracks formed by linkage of individual matrix crack occurred at interface or matrix that depended on CF/VE adhesive strength. At fatigue middle stage, local delamination tended to initiate early in composites fabricated by resin with lower fracture toughness. High CF/VE adhesive strength also brought a positive effect on the resistance to delamination growth resulting in remarkable extension in fatigue life of composites. The coupling effect of fracture toughness of matrix and CF/VE adhesion on the mechanical properties especially fatigue performance of CF/VE composites was investigated.

The effect of matrix fracture toughness on the plastic deformation of the metallic glassy composite

In the present study, two kinds of bulk metallic glasses (Zr55Cu30Al10Ni5 and Cu46Zr42Al7Y5) with high strength and no plastic strain were adopted as matrixes. Both of them were reinforced by ductile crystalline TiNb with the same volume fraction and particle size. The compression results showed that the plastic strain of TiNb/Zr55Cu30Al10Ni5 composite was higher 292% than that of TiNb/Cu46Zr42Al7Y5 composite. It should be related to the fracture toughness of the glassy matrix. For the glassy matrix with high toughness, the formation of shear bands and micro-crack at the front of the crack reduced the energy of main crack development. It resulted in a large plastic strain. On the other hand, for the brittle metallic glassy matrix, crack developed quickly through the cross section so that no more shear bands was formed. It resulted in the failure of the sample with smaller plastic strain.

Thermal and mechanical properties of electrospun nano-celullose reinforced epoxy nanocomposites

Although thermoset polymers have been widely used for engineering components, adhesives and matrix for fiber-reinforced composites, due to their good mechanical properties compared to those of thermoplastic polymers, they are usually brittle and vulnerable to cracking. Therefore, ductile materials such as micro-sized rubber or nylon particles added to thermoset polymers are used to increase their fracture toughness, which might decrease their strength if micro-sized particles act like defects. In this work, in order to improve the fracture toughness of epoxy matrix, nano-cellulose with diameter 250 nm produced via electrospinning was used as filler in epoxy matrix. The effect of different contents of electrospun nano-cellulose from cellulose acetate (ECA) on the morphology and thermo-mechanical properties of the prepared nanocomposites was examined. The pronounced effect of the presence of ECA in the matrix is reflected by the increase of flexural strength and flexural modulus by ∼20 and ∼17%, respectively. The results showed that addition of ECA enhanced mechanical and thermal properties as well as fracture resistance of nanocomposites compared to those without ECA.

Fabricating hollow turbine blades using short carbon fiber-reinforced SiC composite

The development of hollow turbine blades fabricated by nickel-base superalloy is limited by its high density and low operating temperature (<1,100 °C). A new technology for fabricating hollow turbine blades of fiber-reinforced SiC composite was put forward. The chemical corrosion process of DSM Somos 19120 resin mold was investigated, and the pyrolysis of phenolic resin and the infiltration of liquid silicon were analyzed by using scanning electron microscope and X-ray diffraction. The influence of carbon fiber content on fracture strength and fracture toughness of fiber-reinforced SiC composite was investigated using the measurement of three-point flexural strength test. Results showed that stereolithography molds of photosensitive resin were corroded perfectly using KOH alcohol water solution. The porous carbon preforms were obtained after phenolic resin was pyrolyzed from 400 to 800 °C, which were then infiltrated with molten silicon to gain SiC matrix at the temperature of 1,450 °C in 1 h. The fracture strength and fracture toughness of SiC ceramic matrix were enhanced with the increase of short carbon fiber content. The maximum fracture strength and fracture toughness of samples were about 270 MPa and 5.1 MPa m1/2, respectively. Finally, hollow turbine blades were successfully fabricated using short carbon fiber-reinforced SiC composite.

Unprecedented simultaneous enhancement in strain tolerance, toughness and strength of Al2O3 ceramic by multiwall-type failure of a high loading of carbon nanotubes

Carbon nanotubes (CNTs) have a remarkable load-bearing ability. Recently, however, multi-walled CNTs (MWCNTs) have been shown to possess dramatically higher load-bearing ability when intimately embedded in an oxide ceramic (Al2O3), because the load could be transferred not to only their outermost walls but also their generally unloaded inner walls via the strong interwall shear resistance originating from residual compressive stresses. This phenomenon is characterized by an uncommon, highly energy-dissipating, multiwall-type failure of individual MWCNTs during hybrid fracture, with no evidence of pullout. Here, we demonstrate that this nanoscale in-MWCNT load-transfer process, at an optimized, high loading of MWCNTs (10 vol%) and in a pore-free and uniform platform, leads to unprecedented, dramatic simultaneous enhancement in strain tolerance (81%), fracture toughness (52.2%), and flexural strength (22%) of the Al2O3 ceramic matrix. The extent of toughening by this mechanism is also the highest ever reported. This unprecedented performance by using a high loading of functional MWCNTs, namely, toughening, strengthening, softening and lightening, simultaneously and at this level, has implications for many functional and structural applications.