Crack Bridging Mechanisms Research Articles

Reinforcing Mechanisms of Graphene and Nano-TiC in Al2O3-Based Ceramic-Tool Materials.

Graphene and nano-TiC, which have good reinforcing effects on Al2O3-based ceramic-tool materials, are generally used as additive phases for ceramics. In this study, nine kinds of samples were sintered, to investigate the effects of graphene and nano-TiC on the reinforcing mechanisms of Al2O3-based ceramics. The experimental results indicated that adding 0.5 vol% graphene and 10 vol% nano-TiC can obtain the optimum flexural strength, fracture toughness, and Vickers hardness, which were 705 ± 44 MPa, 7.4 ± 0.4 MPa m1/2, and 20.5 ± 0.8 GPa, respectively. Furthermore, the reinforcing mechanisms of crack bridging, pull-out of graphene, and pull-out of nano-TiC are identified, which are contributed to improving the mechanical properties of ceramics. Meanwhile, other reinforcing mechanisms induced by graphene (graphene break, crack guiding, and 3D propagation) and nano-TiC (crack branching, crack deflection, and peeling) are discussed. These reinforcing mechanisms are coupled together, while decoupling is hard to work out. Thus, further quantitative studies of reinforcing effects of graphene and nano-TiC on Al2O3-based ceramic-tool materials are necessary to be carried out.

Energy-absorbing wood composite for improved damage tolerance inspired by mollusc shells

The crossed lamellar structure (CLS) found in mollusc shells is an excellent example for nature’s ability to form complex hierarchical microstructures with a remarkable balance between strength and toughness. The CLS has become the subject of numerous studies focusing on the replication of the unique microstructure using synthetic composites. The present study proposes a wood composite replicating the CLS’ middle layer microstructure and investigates the mechanical properties using three-point bending tests. The morphology of the failure mechanisms is recorded using digital microscopy and the experimental data are compared to those from ply- and solid woods. The results show a successful replication of the dominating failure mechanisms of crack deflection and crack bridging. While strength decreased significantly by ∼60%, toughness increased remarkable by ∼70% compared to plywood and was in the range of solid wood. The small data scattering from the wooden CLS samples compared to solid wood further hints on a stable failure mechanism and uniform energy-absorption. The results document that wood can be used to design an energy-absorbing composite based on the CLS-inspired ductile microstructure.

Effects of SiC content on thermal shock behavior and elastic modulus of cordierite–mullite composites

Cordierite (Mg2Al4Si5O18) is a commercially available ceramic with low fracture toughness that hampers its broad industrial applications. Although several studies have reported the mechanical improvement of cordierite using various reinforcements, modulating its mechanical and thermal shock characteristics is not explored precisely. In the present research, we investigated the manufacturing of cordierite–mullite ceramics and the role of SiC on their thermomechanical properties. The in-situ formed mullite particles were obtained by mixing andalusite-talc-alumina and addition of SiC. It was found that thermal shock behavior and elastic moduli are dependent on SiC content and retained porosity. Furthermore, the addition of SiC to cordierite-based ceramics could enhance the thermal shock resistance via proper activation of the crack bridging mechanism in the matrix of the prepared composite.

Experimental Investigation Into Banana Fibre Reinforced Lightweight Concrete Masonry Prism Sandwiched with GFRP Sheet

Abstract This paper presents the stress-strain behaviour of Natural Banana microfibre reinforced Lightweight Concrete (LWC) prisms under axial compression. The compressive strength of masonry is obtained by testing stack bonded prisms under compression normal to its bed joint. LWC blocks of cross-sectional dimensions 200 mm x 150 mm were used to construct the prism with an overall height of 630 mm. Three series of specimens were cast; (a) prism without Banana fibre (control), (b) prism with Banana microfibres, (c) prism with Banana microfibres sandwiched with Glass Fibre Reinforced Polymer (GFRP) sheets. Natural Banana fibres were used as structural fibre reinforcement at different volume fractions (VF). The results indicate that the presence of fibres helps to improve the strength, stiffness, and ductility of LWC stack bonded prisms under compression. The test results also indicate that banana fibre reinforcement provides an improved crack bridging mechanism at both micro and macro levels. The GFRP sandwiched prism specimens exhibited excellent ductility and load-carrying capacity resulting from improved plastic deformation tolerance under compression and bonding between the LWC block and GFRP sheet.

A comparative study of the incorporation effect of SWCNT-OH and DWCNT with varied microstructural defects on tensile and impact strengths of epoxy based nanocomposite

This study presents an investigation of the influence of Single-Walled Carbon Nanotubes (SWCNTs) and Double-Walled Carbon Nanotubes (DWCNTs) on tensile and impact strengths of epoxy based nanocomposites. Two different CNTs weight percentages were employed including 0.5 and 0.75 wt.% in order to verify their effects on microstructures and mechanical properties. Scanning Electron Microscopy (SEM) and Field Emission Scanning Electron Microscopy (FESEM) were used to analyze the microstructural characteristics of the CNT doped epoxy including presence of defects such as air bubbles, tiny pores and aggregates. DWCNTs/epoxy nanocomposites presented higher amount of pores due to lack of functionalization, while SWCNTs/epoxy ones showed more entanglement mostly at high CNT loading due to its larger aspect ratio. Results showed that tensile strength was slightly enhanced (6 %) for 0.5 wt.% SWCNT, whereas for other nanocomposites it reduced in comparison with the neat epoxy, due to presence of manufacturing defects in the material. Impact strength significantly improved by 51 % and 31 % at 0.5 wt.% SWCNTs and 0.5 wt.% DWCNTs, respectively, resulting from activation of crack bridging and CNTs pull-out mechanisms. Different behaviours between tensile and impact tests are related to homogeneous stress distribution and localized stress concentration in tensile and impact tests, respectively.

Mechanical Performance of Cementitious Composites by MWCNTs Addition for Structural Applications

This paper presents the investigation on the behaviour of a prism reinforced with multi walled carbon nanotubes (MWCNTs) to study mainly the strength and durability aspects for structural applications. Prisms were made by varying the MWCNT content from 0.1 to 0.5% by weight of cement. Ultrasonic energy was employed to disperse carbon nano tubes (CNTs) in water. For evaluating mechanical property such as load-de?ection, tests were performed on CNT admixed prisms under ?exure. The outcomes were then compared with plain mortar prisms. An attempt has also been made to explore the optimum concentration of MWCNT additions that will give ideal performance with respect to mechanical and durability properties. Flexural and compressive strength is enhanced by 25% and 19%, respectively compared to control prisms at 28 days when CNT was used in the cementitious matrix. Both porosity and water absorption are reduced by about 25% at 28 days. Based on the parametric study, a tentative optimum CNT concentration (0.3% by weight of cement) has been proposed. SEM images shows perfect crack bridging mechanism; several of the CNTs were shown as crack arrestor across the fine cracks along with some CNTs breakage.

Ti and SmAlO3 co-affected Al2O3 ceramics: Microstructure, electrical and mechanical properties

This work aims to investigate the co-reinforcing effects of Ti and Sm 2 O 3 on Al 2 O 3 ceramics for optimizing electrical and mechanical properties and study the role of elongated SmAlO 3 in toughening. For this purpose, 20 vol% Ti dispersed Al 2 O 3 composites were chosen as matrix and rare earth element oxide of Sm 2 O 3 (0–2.0 at%) was used as the reinforcement to fabricate Sm 2 O 3 added Al 2 O 3 /Ti composites and toughening mechanism caused by elongated SmAlO 3 was discussed. Results indicated that elongated SmAlO 3 with rod-like shape due to the reaction between Al 2 O 3 and Sm 2 O 3 was identified by XRD and EDX analysis. When Sm 2 O 3 content was below 2.0 at%, all composites exhibited excellent electrical conductivity of ∼10 −2 Ωcm. Owing to strengthening and toughening effects caused by SmAlO 3 , 1.5 at% Sm 2 O 3 added composites showed the highest flexural strength and fracture toughness with the values of 567 MPa and 5.53 MPa cm 1/2 . Elongated SmAlO 3 phase contributed greatly to toughening by crack bridging and particle pull-out mechanisms which were determined by aspect ratio of SmAlO 3 particles varying with Sm 2 O 3 content. • Al 2 O 3 ceramics were significantly toughened by the combined addition of Sm 2 O 3 and Ti. • Elongated SmAlO 3 contributed greatly to fracture toughness by crack bridging and particle pull-out mechanisms. • The role of SmAlO 3 in toughening is depended on the aspect ratio varying with Sm 2 O 3 content.

The effect of Cr2O3 additions on sinterability and mechanical properties of liquid-phase sintered SiC ceramics

In this study, the effect of using Cr2O3 with Al2O3 and Y2O3 additives on physical and mechanical properties of SiC-matrix composites was investigated. The samples were sintered at 1900 °C for 1.5 h in argon atmosphere. The phases, microstructure and chemical composition of the samples were studied using X-ray diffraction analysis (XRD), scanning electron microscopy (FESEM) and X-ray diffraction spectroscopy (EDX). The results showed that addition of Cr2O3 nanoparticles up to 5 wt % results in densification improvement of samples. Due to weak interface of SiC grains and grain boundary phase, crack propagation path is mainly grain boundaries. The highest density, elastic modulus, hardness and indentation fracture resistance were respectively 97.45%, 403 GPa, 28.19 GPa and 6.2 Mpa.m1/2, attributed to specimen containing 5 wt % Cr2O3. Microscopic studies showed that, formation of microcracks, crack deflection, crack bridging and crack branching mechanisms are the most important mechanisms in indentation fracture resistance improvement of composites.

Crack growth rates in lithium disilicates with bulk (mis)alignment of the Li2Si2O5 phase in the [001] direction

Processing-induced alignment of Li2Si2O5 crystals in biomedical lithium disilicate glass-ceramics have been shown to increase the monotonic resistance to fracture and better simulate the mechanical behavior of natural dental tissues. The (mis)orientation of the [001] direction in the Li2Si2O5 crystal phase relative to the crack growth plane was induced by injection-molding of a high aspect ratio composition in order to determine the effect of crystal alignment. The role of particulate geometry was assessed by comparing two compositions having randomly oriented Li2Si2O5 crystals of similar phase fraction but dissimilar crystal aspect ratios. Static and cyclic fatigue experiments in varying frequencies and R-ratios were conducted to clarify the degradation of crack bridging mechanisms induced by the crystal phase during repetitive opening and closure of the crack faces. Both crystal alignment and aspect ratio showed to play important roles in affecting the subcritical crack growth resistance the lifetime of lithium disilicate biomedical components.

Phase assemblage and properties of a nonoxide composite fabricated by a two-step gas-pressure sintering

In this work, a composite composed of [Formula: see text]-Sialon (Z[Formula: see text]=[Formula: see text]4) and ZrN has been fabricated by a two-step gas-pressure sintering method, and the effects of ZrN content and applied pressure on the phase behavior, densification and mechanical properties have been investigated. The phase behaviors were mainly dependent on the ZrN content and the applied pressure. The composites composed of [Formula: see text]-Sialon (Z[Formula: see text]=[Formula: see text]4), ZrN, 15R-Sialon (0.4 MPa) and 12H-Sialon (0.7 MPa) as major phases, with different intermediate phases depending on the ZrN content. It is revealed that with the two-step sintering technique, a higher applied gas pressure has a positive effect on mass loss, and significantly improved the mechanical properties. The addition of ZrN particles greatly helped the densification behavior, reduced the mass loss, and increased fracture toughness of the composites, but decreased hardness due to formation of intermediate phases and grain coarsening. The addition of ZrN increased the fracture toughness due to the toughening mechanisms of crack branching, crack deflection and crack bridging.

Blast performance of a bio-mimetic panel based on the structure of nacre – A numerical study

Nacre, the tough protective layer of a mollusk seashell, has a fracture toughness that is several orders of magnitude higher than brittle aragonite, a ceramic that accounts for 95% of its composition. As such, it possesses characteristics that may be highly beneficial for protective structural applications. In this research, several structural characteristics from nacre’s brick and mortar-like microstructure are mimicked with the goal of enhancing the stiffness and fracture toughness of a monolithic ceramic panel under blast loading. These features include the mineral bridges connecting the adjacent brick-like tablets for enhancing the stiffness of the panel, the multi-layered structure for enhancing its toughness via crack bridging mechanisms and the growth bands between the nacreous tablet layers for deflecting cracks. The results from the numerical simulations showed that the nacre-like panel possesses superior energy dissipation over the monolithic ceramic panel, which thereby reduces the reaction forces transmitted to the supports and mitigates catastrophic failure. This has positive implications in terms of the capability of fine-tuning the structural characteristics of an armor system for defeating impulsive loads, by employing the principles adopted in the microstructure of a natural armor system.

An Experimental Investigation of Fracture Toughness and Volume Resistivity of Symmetric Laminated Epoxy/Glass Fiber/CNT multiscale composites

In this work an attempt is made to improve the fracture toughness and electrical conductivity of epoxy/glass fiber based laminates by the inclusion of carbon nanotube (CNT) fillers. The fiber orientation of the epoxy/glass fiber (GF) fabric laminates was optimized based on estimation of mechanical properties. The carboxylic acid functionalized CNTs were incorporated into epoxy matrix by ultra-sonication method. The nano filled epoxy resin was used to prepare laminates with 30/45 GF fabric orientation. The CNT content was varied and its effect on the tensile properties was determined. The fracture toughness of multiphase composites was estimated using single edge notch bend (SENB) test. The presence of CNTs improved the fracture toughness by a crack bridging mechanism. The volume resistivity of multiphase composites was found to be superior to the conventional epoxy/CNT composite. The presence of glass fabric reduces the number of inter-tube contacts contributing to the reduction in volume resistivity.

Atomically Thin Polymer Layer Enhances Toughness of Graphene Oxide Monolayers

Summary During the last decade, two-dimensional (2D) materials have emerged as versatile building blocks for the next generation of engineered materials. However, the intrinsically brittle behavior of 2D materials has thus far delayed their adoption in applications such as sensors and structural materials. Herein, we demonstrate a strategy for toughening graphene oxide (GO) through synergistic interfacial interactions between GO monolayers and ultrathin layers of strongly interacting poly(vinyl alcohol) (PVA). By creating GO-PVA and PVA-GO-PVA nanolaminates, we demonstrate a 2-fold increase in GO toughness, which translates into dramatic increases in energy dissipation and piercing resistance. Atomistic simulations show that this remarkable behavior arises from a polymer chain crack-bridging mechanism, resulting from a synergistic combination of interdomain reinforcements across the GO monolayer and extensive GO-polymer interfacial hydrogen-bonding interactions. The reported findings highlight the potential for achieving engineered 2D materials with superior mechanical properties by incorporating deformation and failure-resistant mechanics arising from tailored chemical interactions between constituents.

Fracture modes and hybrid toughening mechanisms in oscillated/twisted plywood structure

Twisted or oscillated plywood structure can be often found in biological composites such as claws of lobsters, bone of mammals, dactyl club of mantis shrimps, and exoskeleton of beetles, which exhibits a combination of high stiffness, high fracture toughness and low density. However, there lacks a quantitative understanding of the relationship between the fracture toughness of the composite and its internal geometry. In this article, we propose that a combination of crack tilting and crack bridging determines the effective fracture toughness of the fiber-reinforced composite with the plywood structure. During the fracturing process, a crack plane initially propagates in the matrix-fiber interface following the twisted fiber alignment. Such crack tilting mechanism can significantly enlarge the area of cracking surface and thus enhance the effective fracture toughness of the composite. With the propagation of the tilted crack plane, the local energy release rate becomes too small to maintain the growth of the tilted crack plane, leading the crack to grow into the matrix, crossing the fibers. Because of the high strength of the fiber, a few fibers can maintain unbroken behind the crack tip, corresponding to crack bridging mechanism. Based on our quantitative analysis, it is found that the effective fracture toughness of the composite can be maximized for a certain pitch angle of the oscillated/twisted plywood structure, which agrees well with experiments. Statements of SignificanceFiber-reinforced composites can be widely found in nature and engineering applications. Recently, it has been discovered that many fiber-reinforced composites in biology have twisted or oscillated plywood structure with high fracture toughness, high mechanical stiffness and low density. Detailed experiments have indicated that an optimal pitch angle may exist for the plywood structure to maximize the fracture toughness of the composites. However, there lacks a quantitative model of revealing such pitch angle-dependent fracture toughness. In this work, we propose a hybrid toughening mechanism and predict the optimal pitch angle in twisted/oscillated plywood structure for maximizing the fracture toughness. Our predictions agree reasonably well with experimental results. As such, the theory may help the design of better fiber-reinforced composites.

Room temperature R-curve and stable crack growth behaviour of ZrB2–SiC ceramic composites

ABSTRACTR-curve and controlled stable crack growth behaviour of ZrB2–17vol.-%SiC and ZrB2–45vol.-%SiC ceramic composites was studied on V-notched samples using four-point bending at room temperature. The rising K1R behaviour was determined as a function of the crack extension Δa with a crack bridging mechanism being dominant in such behaviour. Significant differences in crack growth rates were found within the same composition of ceramics simply as the crack length varied during crack growth processes. These differences are indicative of the significant influence of microstructural parameters of the ceramics on crack propagation. The peculiarities of stress intensity factor K1 and the crack growth-specific behaviour in ZrB2–SiC particulate ceramic composites are discussed.

Novel engineered high performance sugar beetroot 2D nanoplatelet-cementitious composites

In this paper, we show for the first time that environmentally friendly nanoplatelets synthesized from sugar beetroot waste with surface area and hydroxyl functional groups similar to those of graphene oxide (GO) can be used to significantly enhance the performance of cementitious composites. A comprehensive experimental and numerical simulation study was carried out to examine the performance of the bio waste-derived 2D nanoplatelets (BNP) in cementitious composites. The experimental results revealed that the addition of BNPs decreased the workability of the cement pastes due to their high surface area and dominant hydrophilic functional groups. The experimental results also revealed that the BNP sheets altered the morphology of the hydration phases of the cementitious composites. At 0.20-wt%, the BNP sheets increased the content of the C-S-H gels. At higher concentrations (i.e., 0.40-wt% and 0.60-wt%), however, the BNP sheets increased the content of the calcium hydroxide (Ca(OH)2) products and altered their sizes and morphologies.The flexural results demonstrated that the 0.20-wt% BNPs produced the highest flexural strength and modulus elasticity and they were increased by 75% and 200%, respectively. The numerical simulations were in good agreement with the fracture test results. Both results showed that the 0.20-wt% BNPs optimal concentration significantly enhanced the fracture properties of the cementitious composite and produced mixed mode crack propagation as a failure mode compared to Mode I crack propagation for the plain cementitious composite due to combined crack bridging and crack deflection toughening mechanisms. Because of this, the fracture energy and the fracture toughness were increased by about 88% and 106%, respectively.

Improved trade-off between thermoelectric performance and mechanical reliability of Mg2Si by hybridization of few-layered reduced graphene oxides

Nanocomposites can simultaneously enhance the thermoelectric and mechanical properties of thermoelectric materials. Here, we fabricated bulks of Mg1.96Al0.04Si0.97Bi0.03 with monodispersed few-layered reduced graphene oxides utilizing ultrasonic-based wet chemical pulverizing-mixing and spark plasma sintering to improve unfavorable trade-off between thermoelectric performance and mechanical reliability, which is important for commercialization. An unexpected high fracture toughness of ~1.88 MPa m1/2 was observed due to the synergetic effect of the deflection of crack propagation, bridging, and sheet pull-out mechanisms, and a high thermoelectric figure of merit ~0.6 was obtained even for a high content (3 vol.%) of reduced graphene oxides.

Anchorage in steel fiber reinforced concrete – concept, experimental evidence and design recommendations for concrete cone and concrete edge breakout failure modes

A large number of studies confirm the beneficial effects of the steel fiber reinforcement in concrete structures. The addition of steel fibers to the concrete mix leads to better mechanical and physical concrete properties including higher fracture energy, reduced crack widths, higher impact and abrasion resistance, increased durability. Due to the increasing popularity of using anchorages in steel fiber reinforced concrete (SFRC), it is essential to understand the behavior of anchorages in SFRC and to validate the applicability of the current provisions for the design of anchorages for use in structural concrete. However, there is only little research available on the behavior of anchorages in SFRC. This paper presents the results of experimental investigations on tension and shear loaded steel anchors in normal-strength plain concrete (PC) and in SFRC. The comprehensive test program includes 62 pull-out and shear loading tests on single anchors and on anchor groups. The test results are discussed in detail to emphasize the influence of steel fiber reinforcement on the load-displacement behavior of fastening systems. The results indicate that the fiber content has a positive effect on the load-displacement behavior of the anchorages, in general. A better utilization of fastening systems can be attained due to the more ductile behavior and due to the crack bridging mechanism of the SFRC. Furthermore, in certain applications and parameter combinations the ultimate load in case of concrete failure may be higher in SFRC compared to PC.

Tougher and harder zirconia toughened alumina (ZTA) composites through in situ microstructural formation of LaMgAl11O19

Advanced ceramic materials are highly demanded for various modern industrial and practical engineering applications. Nonetheless, the requirement of attainment hard and simultaneously tough materials is a crucial matter for industrial sectors. To accomplish this target, the strategy of designing a novel microstructure of Zirconia Toughened Alumina (ZTA) ceramics was utilised through in situ formation of LaMgAl11O19 (LMA) based on the stoichiometry ratio of constitute oxides inside ZTA composites during sintering process. ZTA Samples were conventionally sintered at 1600 °C for 2 h to ensure fully formation of LMA while their characteristics were analysed using SEM-EDX and XRD. The LMA elongated grains significantly enhanced ZTAs' Indentation Fracture Resistance (KIFR) due to crack deflection and crack bridging mechanisms combined with transgranular and intergranular fracture modes. Additionally, higher hardness was obtained due to high LMA density, creation of low porous ZTA and strong interface bonding between LMA and ZTA grains. The KIFR was stable while the hardness was steadily declined due to more porosity accompanied by LMA in situ formation. Consequently, sustainable high hardness and enhanced KIFR of ZTA ceramics were achieved. The optimum outcomes were superior KIFR (7.8 MPa.m^1/2) and hardness (1660 HV) than the pure ZTA (5.9 MPa.m^1/2, 1568 HV). The investigation provides a new hint for future design and fabrication of economic and high performance ceramics through in situ formation of LMA according to the stoichiometric ratio of the reinforcing oxides.

Microstructure and mechanical properties of SiC joint with an in-situ formed SiC-TiB2 composite interlayer

Diffusion bonding of SiC ceramic with in-situ formed SiC-TiB2 composite interlayer by reactive spark plasma sintering with B4C-Ti3SiC2-Si powder mixture was investigated. It was found that a dense composite interlayer between the SiC ceramic substrate could be obtained at 1600 °C for 10 min. Granular-shaped SiC with submicron-size and elongated TiB2 with micro-size were uniformly distributed in the bonding layer. The joint strength and hardness of the composite interlayer increased as the joining temperature increased up to 1600 °C due to the improved relative density and the higher fraction of elongated TiB2 phase. The grain growth and formation of micro-cracks deteriorated the mechanical strength of the 1700 °C joint. The maximum shear strength of 128.0 ± 7.3 MPa was achieved for the joint bonded at 1600 °C. The toughening mechanisms of crack deflection, crack bridging and crack branching, and hardening mechanism of dual phase strengthening were observed in the morphology of indentation cracks.