Crack Deflection Effect Research Articles

Experimental method for determining the fracture toughness of concrete based on the modified two‐parameter model and DIC technique

AbstractAn experimental method, based on the modified two‐parameter model and digital image correlation technique, was proposed for determining the mixed mode fracture toughness (KI+II) of concrete. Three‐point bending tests on notched concrete beam specimens of different sizes, maximum aggregate sizes, and aggregate volume contents were conducted, and the values of KI+II were evaluated considering the crack deflection. The values of KI+II and KI under mode I loading were compared. The results show that KI+II increases with the specimen size and gradually stabilizes. In addition, KI+II decreases with the maximum aggregate size, but the aggregate volume content has little effect on KI+II. Higher values of KI+II are presented after considering the effect of crack deflection. Only a monotonic loading process is required for the proposed method to determine KI+II.

High‐temperature fatigue crack propagation mechanism of orthogonal 3‐D woven amorphous SiC Fiber/SiC/YSi2‐Si matrix composites

AbstractTo elucidate degradation mechanisms attributable to high‐temperature fatigue crack propagation, a study was conducted of 3‐D woven SiCf/SiC CMC in which amorphous SiC fiber was used as a reinforcement material and in which a matrix was formed through low‐temperature melt infiltration. From a high‐temperature fatigue test conducted at 1373 K in the atmosphere with stress of 142 MPa or more, the fracture lifetime of newly developed SiCf/SiC CMC was found to be longer than that of SiCf/SiC CMC, which uses crystalline SiC fiber. Furthermore, repeatedly applying high temperatures during high‐temperature fatigue tests and using X‐ray computed tomography, fatigue cracks were found to propagate in a direction across 0‐degree fiber bundles that undergo stress. Electron mapping of regions with crack propagation revealed that oxidation eliminates boron nitride (BN), which has a crack deflection effect. The SiC fibers and matrix are fixed through the formation of oxides. Cracks propagate because of the consequent decrease in toughness of the SiCf/SiC CMC. In regions without crack propagation, fracture surfaces were not covered with oxides. These regions underwent forcible fracture in the final stage of the high‐temperature fatigue tests. From the test results presented above, SiCf/SiC CMC is considered to undergo fracture when the effective cross‐sectional area is reduced because of crack propagation accompanying oxidation and when the test load exceeds the tensile strength of the residual cross‐sectional area. However, some cracks in the matrix produced by a low‐temperature melt infiltration process were closed by oxides derived from YSi2. Because of crack closing, crack propagation is presumed to be avoided. Also, LMI‐CMC showed excellent high‐temperature fatigue properties at pressures higher than 150 MPa, which exceeds the proportional limit.

Fatigue crack deflection and branching behavior of low carbon steel under mechanically large grain condition

In situ SEM observation of fatigue crack growth behavior of low carbon steel under mechanically large grain condition, and EBSD analysis were carried out to investigate mechanisms of crack deflection and branching and effects of crystal orientation on them. It is found that the crack propagates along the activated slip bands in the present material and the crack deflection can be formed when a new activated slip band is created in a different direction from the growing crack. The crack branching can be formed by the following process: (1) the slip band along the crack cannot be activated further due to the restraint by such as grain boundary or strain hardening, (2) the crack cannot propagate further in the same direction, and (3) the secondary slip plane is activated to form a new slip band behind the crack tip. Not the stress intensity factors Kdef for deflected crack and ΔKeff for crack closure but rather the ΔKeff,tip is the intrinsic fracture mechanics parameter for controlling fatigue crack growth behavior of the material with crack deflection and branching.

Hybrid multilayer graphene and SiC whisker reinforced TiB2 based nano-composites by two-step sintering

Advances in the TiB2 nano-composites reinforced with hybrid of SiC whisker (SiCw) and multilayer graphene (MLG) have been thoroughly investigated. Hybrid reinforcements, combining the advantages of SiCw as well as MLG, resulted in further enhanced mechanical properties in TiB2-based ceramic nano-composites. Excellent mechanical properties were achieved for hybrid MLG/SiCw reinforced TiB2–TiN–MgO samples with 2.0 wt% SiCw and 0.4 wt% MLG through two-step sintering with a hardness of 21.2 GPa, a flexural strength of 1006.3 MPa and a fracture toughness of 8.8 MPa mm1/2. Hybrid addition of MLG and SiCw performed effectively on improving the densification and inhibiting grain growth of TiB2-based ceramics. Furthermore, hybrid addition of MLG/SiCw could significantly improve the dispersion of MgO nano-particulates in TiB2 matrix. The synergistic effect of crack deflection, branching and bridging as well as MLG/SiCw pullout dramatically enhanced the toughness of TiB2–TiN–MgO ceramic nano-composites.

Synergistic Effect of Multi-Walled Carbon Nanotubes and Graphene Nanoplatelets on the Monotonic and Fatigue Properties of Uncracked and Cracked Epoxy Composites.

The fatigue properties of the polymer nanocomposites reinforced with a hybrid nano-filler system have seldom studied before. Accordingly, epoxy nanocomposites with various multi-walled carbon nanotube (MWCNT)/graphene nanoplatelet (GNP) filler ratios were prepared to study comprehensively the synergistic effect of the hybrid nano-fillers on the monotonic and cyclic mechanical properties of the nanocomposites. The quasi-statically tensile properties and fatigue-life curves were experimentally determined using uncracked bulk specimens. Additionally, pre-cracked specimens were utilized to study the fracture toughness and fatigue crack growth rate of the nanocomposites. A synergistic index based on the properties of the nanocomposites with individual types of filler was proposed to evaluate the synergistic effect of two employed nano-fillers on the studied properties. The index was verified to be a highly discriminatory tool to evaluate the synergistic effect of hybrid nano-fillers on the studied mechanical properties. The experimental results show that the composites with a MWCNT:GNP ratio of 1:9 have the higher monotonic and fatigue properties than those with other filler ratios. Adding appropriate amount of CNTs can prevent the agglomeration of GNPs. The flexible CNTs bridge adjacent GNPs to constitute a favorable network for load transfer. Moreover, there is a linear relationship between the static and fatigue strengths of the studied nanocomposites. Integrated analysis of experimental data and a fracture surface study reveals that the dispersion of nano-fillers influences the mechanical properties significantly. The crack deflection effect due to the path bifurcation caused by encountering the filler cluster and the filler bridging effect are the main reinforcement mechanism of the studied properties.

Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al2O3–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al2O3–TiC composites synergistically reinforced with MWCNTs and GNPs.

Study of the Mechanical Properties and Toughening Mechanism of Nano-NbC Particles Toughened Si₃N₄-Based Ceramics.

In this article, nano-NbC particles toughened Si₃N₄-based ceramics were prepared by injection moulding and their mechanical properties along with toughening mechanism were studied. An increase of nano-NbC content, gradually homogenizes microstructure of the Si₃N₄-based ceramics along with increase in its density. However, the fracture toughness and flexural strength first increases and then decreases. The Si₃N₄-based ceramics demonstrate good comprehensive properties at the 15 wt% nano-NbC content and sintering temperature of 1550 °C (where the density is 85.3%, the flexural strength is 845 MPa, and the fracture toughness is 9.3 MPa·m1/2), Backscattered electron imaging shows that nano-NbC particles can be well dispersed in the Si₃N₄ ceramic matrix by injection moulding and ceramics are toughened by crack deflection and microcracking effects. It was also found that increasing sintering temperature makes the β-Si₃N₄ grain distribution more uniform by reducing the porosity.

A composite structural high-temperature-resistant adhesive based on in-situ grown mullite whiskers

The effect of whisker growing catalyzed by different amounts of AlF3 on the various properties of an epoxy/silicone resin-based high-temperature-resistant adhesive was comprehensively investigated. Although the addition of AlF3 promoted the oxidation reactions, accelerated the premature formation of mullite whiskers, increased the production of SiC ceramic, and reduced the thermal expansion of bonding layer, the excessive use of AlF3 induced many more large holes in the bonding layer, caused the structural heterogeneity, and made mullite whiskers overgrow, thus finally led to the weaker mechanical properties of adhesives. The best dosage of AlF3 in this work was 3 % (mass fraction), a uniform composite structure that whisker weaved adhesive frame kept steady from 1100 °C to 1500 °C, and an impressive bonding strength of 63 MPa was achieved at 1100 °C, 80 % higher than that of the blank sample. Under the synergistic effect of whisker repair, whisker bridging, whisker drawing and crack deflection, a multi-stage fracture phenomenon happened during the shear test, and the fracture displacement of 1100 °C-calcined adhesive with 3 % AlF3 increased by 61 %, compared with that of the black sample.

In situ TiB-network-reinforced Al2O3/Ti6Al4V joints

Abstract A three-dimensional network architecture of TiB whiskers were in situ synthesized in Al2O3/Ti6Al4V joints by using Ni–B coated Cu foam interlayer. Boron was extracted from Ni–B alloy at low braze temperature to form a thin flexible boride layer comprised by TiB and Ni3B nano-particles. Subsequently, a network structure of TiB whiskers was developed from the interlaced boride layers at high temperature. The underlying mechanisms were addressed based on microstructural evolution. The uncoordinated deformation of the network microstructure, stress adjustments and crack-deflection effects of TiB whiskers contributed to the high shear strength of joints.

Reduced-graphene-oxide-reinforced boron carbide ceramics fabricated by spark plasma sintering from powder mixtures obtained by heterogeneous co-precipitation

Abstract Reduced graphene oxide (rGO) sheets were uniformly dispersed in boron carbide ceramics by a heterogeneous co-precipitation method. This approach was used to improve the fracture toughness of boron carbide ceramics and to address the problem of agglomeration of graphene in the boron carbide matrix. Cetyltrimethyl ammonium bromide was used as a heterogeneous co-precipitation reaction initiator to prepare a homogeneously dispersed graphene oxide/boron carbide (GO/B4C) mixture. Reduced graphene oxide/boron carbide (rGO/B4C) powder mixtures with good dispersion were obtained by high temperature heat treatment. Dense rGO/B4C composite ceramics were fabricated by spark plasma sintering at 1800 °C and 50 MPa. The fracture toughness and flexural strength of the rGO/B4C with an rGO content of 2 vol% composite increased by 42% (from 3.43 to 4.88 MPa·m1/2) and 28% (from 372 to 476 MPa) compared with those of pure B4C, respectively. The markedly improved fracture toughness and flexural strength of the boron carbide ceramics were attributed to the effect of crack bridging and crack deflection by graphene sheets, graphene interface sliding, and pulling out of graphene.

Enhanced Fatigue Characteristics of Topologically Optimized Biomedical Titanium Porous Structure by Selective Laser Melting

Fatigue property is a critical consideration in the porous structures and most existing porous samples have unsatisfactory performance due to lack of structural optimization. This work finds that a topologically optimized structure of α-type commercial-purity titanium (CP-Ti) produced by selective laser melting presents excellent fatigue properties with an ultra-high normalized fatigue life of ~0.65 at 106 cycles at a low density of 1.3 g/cm3. All factors affecting fatigue have been studied, including material microstructure and porous structure stress analysis. The topologically optimized structure can effectively increase the fatigue life by reducing stress concentrations. The cyclic ratcheting effect plays a dominant role in fatigue crack initiation for the topologically optimized structure. As a results of twinning occurred during the fatigue process, the porous CP-Ti sample exhibits a higher ductility than the Ti-6Al-4V sample with the same structure, which delayed the fatigue crack initiation and therefore produced a higher fatigue life. In addition, the fatigue crack propagation rate was reduced significantly because of the large plastic zone ahead of the fatigue crack tip and the effect of fatigue crack deflection and bifurcation.

Stress Intensity Factor Solutions for CTS Mixed Mode Specimen

The Compact Tension Shear (CTS) specimen is used to study fracture and fatigue under mixed mode I/II loading conditions. Existing K solution was developed for fracture studies and it does not consider the effect of crack deflection. The aim of present work is to develop KI and KII empirical solutions for cracks with different crack length, loading angle and crack orientation. A total number of 1120 cracked geometries were studied numerically by the finite element method and solutions were fitted to numerical predictions. An average difference of 0.53 % was found between numerical predictions and the solution proposed for KI. For KII the difference is higher, but the equivalent stress intensity factor showed a difference of only 1% because KII is lower than KI. Experimental work was developed to study fatigue crack growth in CTS specimens. The cracks always adopted a direction approximately normal to loading direction, i.e., tend to propagate under mode I.

Microstructural evolution and growth/degradation behavior of in situ TiB whiskers in ZrB2-SiC joints using Ti/Ni/Ti filler

An in situ short fiber reinforced brazing technique by taking advantage of Ti/Ni contact reactions has been developed to join ZrB2-SiC ceramics. Heating the sample above 961 °C involves a modest combustion by melting of Ti layer. The TiB whiskers are in situ synthesized along ceramic interface while a continuous TiNi layer is formed at the seam center. Further increasing temperature above 1030 °C results in complete diffusion of Ni. The degradation of TiB whiskers is observed at this stage and the possible dissolution mechanism is discussed. Additionally, a new brittle phase Ti(Zr)49Ni33Si18 found at high temperature is detrimental to joint stability and strength. The accommodation to the coefficients of thermal expansion by TiB whiskers is evaluated. The mechanical properties of joints are characterized by microhardness and shear tests. The pull-out and crack deflection effects of TiB are observed in the joint fracture.

Flame retardancy and toughening properties of epoxy composites containing ammonium polyphosphate microcapsules and expanded graphite

Ammonium polyphosphate microcapsules (BM (polybenzoxazine modified) APP) were prepared through the in situ ring-opening polymerization of allyl group containing benzoxazine monomers on the surfaces of ammonium polyphosphate (APP), and they were significantly hydrophobic than the APP. A flame retardant system of epoxy (EP) resin was prepared with BMAPP and expanded graphite (EG). Flame retardancy, the thermal degradation behavior, a mechanical property of EP and EP/BMAPP/EG composites was investigated through limited oxygen index, vertical burning test, cone calorimetry (CONE), and the thermogravimetric analysis (TGA). The flame retardancy tests indicated that the EG could improve the thermal performance, promote the charring, and enhance the char quality of EP/BMAPP. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze the morphology and composition of the char residue formed during CONE testing, and to understand the mechanism of char formation. The results of TG-FTIR confirmed the possible mechanism of flame retardancy of EP/BMAPP/EG in the gas phase during combustion. The EG content effects on Young’s modulus, the tensile strength, and the fracture toughness ( KIC) of the EP/BMAPP composites were also investigated. The KIC of the composites containing 1% of EG and 10% of BMAPP increased by approximately 76% and 153%, respectively, compared to the neat matrix and EP/BMAPP-10%. The SEM images of the fractured surface indicated that the enhanced toughness of EP/BMAPP/EG composites mainly attributed to the debonding of the BMAPP and the subsequent plastic void growth of the matrix, as well as the crack deflection effect of the BMAPP/EG.

Quantitative analysis of toughening effect of crack deflection on Si3N4/Si2N2O composites through improved indentation method

In this study, Si3N4/Si2N2O composite ceramics prepared by hot pressing were used as an example, and the material fracture morphology and fracture mechanism were analyzed. Based on the formula of fracture toughness measured by an indentation method, a quantitative computation method was proposed to determine the toughened effect of ceramic materials resulting from the crack deflection by the second phase. The grain size and sintering density are increased with the increase of sintering temperature. The toughening effects resulting from the crack deflection is increased, and the main mode of fracture is transformed into the transgranular fracture. The Si2N2O grains can play a role in the toughening process because these grains can hinder the cracks extending along the radial direction. However, when the cracks extend in the axial direction, the toughening effect of Si2N2O grains is not obvious because of the internal stacking faults in the axial direction. The improved indentation method can quantitatively analyze the toughening effect of the second phase of composite ceramics, and the validity of this method are verified by comparing the fracture toughness of Si3N4/Si2N2O and fine grained β- Si3N4 ceramics.

Evaluation of effects of crack deflection and grain bridging on toughening of nanocrystalline SiO2 stishovite

Nanocrystalline SiO2 stishovite has a fracture toughness higher than 10MPam1/2 due to the toughening mechanism by fracture-induced amorphization. In order to identify other toughening mechanisms which may operate simultaneously, we evaluated effect of crack deflection on fracture toughness. The median deflection angle of nanocrystalline stishovite was lower than polycrystalline ceramics such as silicon nitride and Y-TZP. While the crack deflection can contribute to fracture toughness to some extent, it cannot be the major origin of high fracture toughness of nanocrystalline stishovite. We discussed also the role of grain bridging from the relation between the fracture toughness and grain size.

Synergistic effects of processing and nanofiber reinforcement on the mechanical and ferroelectric performance of geopolymer matrix composites

This study involved the evaluation of mechanical and ferroelectric properties of a new class of nanofiller infused inorganic polymer (geopolymer, GP). To evaluate the mechanical performance, compressive strength and fracture resistance of neat and nanofillers infused GP were studied at various treatment temperatures. It was found that, addition of 5vol% alumina nanofiber (ANF), increased compressive strength and modulus by over 30% and 60%, respectively, while it increased fracture toughness (KIC) by over 60% compared to the baseline specimens. Simultaneously, ferroelectric properties were investigated at various treatment temperatures (250°C, 650°C and 870°C). Remarkably, higher ferroelectric hysteresis was observed with the GP treated at 870°C and remnant polarization increased with the addition of alumina nanofiber. Scanning Electron Microscopy confirmed that neat materials are composed of particles embedded into the poly-condensed matrix, where particle nature existed until the treatment temperature reached above 870°C. X-ray diffraction analysis suggests that, baseline geopolymer started becoming crystalline while the particle nature gradually disappeared with heating at or beyond 870°C. The bonding between the polymer and alumina nanofiber tends to be stronger with increasing treatment temperature. The increase in KIC with the addition of 2vol% and 5vol% alumina nanofibers (ANF) is due to homogeneous dispersion of high interfacial strength nanofillers, which essentially create strong crack bridging and crack deflection effect. The increase in ferroelectric hysteresis is potentially due to the formation of hierarchical order and domain reorientation of the materials.

Effect of Microstructures on Fatigue Crack Growth Behavior of Friction Stir Processed NiAl Bronze Alloy

Friction stir processed (FSPed) NAB alloy exhibits inhomogeneous microstructures that can be divided into three subregions from the top surface to the bottom according to α phase morphologies: Widmanstatten α subregion, banded α colonies, and stream-like α colonies. In this study, a constant stress intensity range (ΔK) was used for each sample to study the effect of microstructures on the fatigue crack growth rate (FCGR) of FSPed NAB alloy. The results show that α phase in banded and stream-like α colonies experiences completely dynamic recrystallization and forms equiaxed α grains during FSP. The FCGR of FSPed NAB alloy continuously decreases from the top surface to the bottom. In the subregion with stream-like α colonies, the alloy containing a higher content of equiaxed α grains and fine κ iv phase, and less retained β (β′) phase exhibits the best FCG resistance. The equiaxed α grains deflect the main crack and increase crack tortuosity effect, which make a main contribution to FCG resistance of FSPed NAB alloy, while martensite β′ phase produced during FSP accelerates its fatigue crack growth. Compared to matrix alloy, FSPed NAB alloy exhibits better FCG resistance only at high ΔK levels. At low ΔK levels, the crack deflection effect caused by coarser κ phase in the matrix alloy obviously improves its FCG resistance. With the increasing ΔK, the aforementioned crack deflection effect gradually diminishes and fatigue crack prefers to propagate in a flat way, resulting in higher FCGR of matrix alloy.

Properties of CNTs/MoSi2 composites prepared by spark plasma sintering

Molybdenum disilicide (MoSi2) based composites with various contents of carbon nanotubes (CNTs) were fabricated by spark plasma sintering (SPS) in vacuum under a pressure of 25 MPa. The composites obtained under a sintering temperature of 1500 °C and time of 10 min exhibited optimum mechanical properties at room temperature in terms of fracture toughness and transverse rupture strength. MoSi2 based composite with 6.0% CNTs (volume fraction) had the highest fracture toughness, transverse rupture strength and hardness, which were improved by about 25.7%, 51.5% and 24.4% respectively, as compared with pure MoSi2. A Mo4.8Si3C0.6 phase was detected in CNTs/MoSi2 composites by both X-ray diffraction (XRD) method and microstructure analysis with scanning electron microscopy (SEM). It is believed that the fine grains and well dispersed small Mo4.8Si3C0.6 particles had led to a higher hardness and strength of CNTs/MoSi2 composites because of their particle pullout, crack deflection and micro-bridging effects.

Toughness enhancement in graphene nanoplatelet/SiC reinforced Al2O3 ceramic hybrid nanocomposites

This paper elucidates the effect of silicon carbide nanoparticles (SiCNP) and graphene nanoplatelets (GNPs), on their own and together, on the densification behavior and fracture toughness of alumina (Al2O3) ceramic matrix. This was investigated by using the high-frequency induction heat sintering (HFIHS) process. While the addition of each nanostructure caused varying degrees of grain refinement and enhancement of mechanical properties, the incorporation of as little as 0.5 wt.% GNPs along with 5.0 wt.% SiCNP promoted uniform dispersion of the latter due to the lateral surface area of the graphene nanosheets with their two-dimensional morphology. There was an associated reduction in grain size from 1500 to 300 nm upon the addition of both types of nanoscale reinforcements. Extensive electron microscopy of the as-produced nanocomposites indicated the presence of SiCNP within, as well as at, the grain boundary areas whereas the 2D GNPs anchored between neighboring grains. Fractography of the samples revealed a transition from a mixed intergranular/transgranular mode for SiCNP or GNP-reinforced nanocomposites to transgranular fracture mode for the hybrid nanocomposites with improvements in fracture toughness and microhardness by 160 and 27%, respectively, largely due to the synergic role of the nanostructured reinforcements and their distinctly different toughening mechanisms. A new toughening model is proposed for the hybrid nanocomposites by taking into consideration crack deflection and pull-out effects due to SiCNP and the atomic level slip-stick driven GNPs inter-layer slithering. It was found that the addition of GNPs facilitates SiCNP dispersion that subsequently develops dense, fine-grained microstructures after a short-cycle, pressure-assisted consolidation process.