Induced Crack Deflection Research Articles

From Mg(OH)2 to lightweight closed-pore MgO refractory aggregates: The role of NiO additive

MgO refractory aggregates with high closed porosity were prepared using light-burned MgO and Mg(OH)2 with adding NiO additive. The evolutions of microstructure and pores and their effects on the properties were investigated. The introduction of NiO effectively enhanced growth of MgO grains and transformation of large open pores into small closed ones, attributing to the lattice distortion caused by MgxNi1-xO solid solution formation. Due to the occurrence of more closed pores and lattice distortion, thermal insulation performance was improved significantly. Moreover, the formed circular closed pores and MgxNi1-xO could absorb thermal stress, lower thermal expansion coefficient and induce crack deflection, thus effectively enhancing thermal shock resistance. The specimen with 4.0 wt% NiO showed the best performance, with a median pore diameter of 5.69 μm, closed/apparent porosity of 9.4 %/3.6 %, thermal conductivity of 9.9 W/(m·K) (500 °C), flexural strength of 68.5 MPa, and residual flexural strength of 10.8 MPa after thermal shock.

Effect of sintering temperature on mechanical properties of WC-cBN-SiCw composites with multi-scale and multi-morphology toughening strategy

The binderless WC cemented carbide was synthesized with multi-scale and multi-morphology toughening strategy by Spark Plasma Sintering. The influence of sintering temperature on the mechanical properties of WC-cBN-SiCw was investigated. The result showed that the Vickers hardness and fracture toughness initially increase and then decrease with the increase of sintering temperature. As the sintering temperature reaches 1300℃, the best Vickers hardness and fracture toughness can be obtained, 12.8 GPa and 10.6 MPa.m1/2 respectively. The toughening mechanism can be attributed to the strong and weak mixed interface characteristic constructed by micro and nano particles, which induce crack deflection and crack bridging. Meanwhile, the adhesion of nanoparticles on SiC whisker enhances the resistance of whisker pull out, which promotes the consumption of fracture energy.

Non-Covalent Functionalization of Graphene Oxide with POSS to Improve the Mechanical Properties of Epoxy Composites.

Phenyl polyhedral oligomeric silsesquioxane (POSS) is modified onto the GO surface by using the strong π-π coupling between a large number of benzene rings at the end of the phenyl POSS structure and the graphite structure in the GO sheet, realizing the non-covalent functionalization of GO (POSS-GO). The POSS-GO-reinforced EP (POSS-GO/EP) composite material is prepared using the casting molding process. The surface morphology of GO before and after modification and its peel dispersion in EP are examined. Furthermore, the mechanical properties, cross-sectional morphology, and reinforcement mechanism of POSS-GO/EP are thoroughly examined. The results show that the cage-like skeleton structure of POSS is embedded between the GO layers, increasing the spacing between the GO layers and leading to a steric hindrance effect, which effectively prevents their stacking and aggregation and improves the dispersion performance of GO. In particular, the 0.4 phr POSS-GO/EP sample shows the best mechanical properties. This is because, on the one hand, POSS-GO is uniformly dispersed in the EP matrix, which can more efficiently induce crack deflection and bifurcation and can also cause certain plastic deformations in the EP matrix. On the other hand, the POSS-GO/EP fracture cross-section with a stepped morphology of interlaced "canine teeth" shape is rougher and more uneven, leading to more complex crack propagation paths and greater energy consumption. Moreover, the mechanical meshing effect between the rough POSS-GO surface and the EP matrix is stronger, which is conducive to the transfer of interfacial stress and the strengthening and toughening effects of POSS-GO.

Performance enhancement of free CaO containing magnesia by in situ intergranular CaAl2O4 and MgAl2O4

Magnesia refractory raw material with free CaO has inherent weaknesses of low resistance to hydration, thermal shock and slag penetration. In this work, micro-sized Al2O3 powder was added into such magnesia with the purpose of improving its comprehensive performances. The results showed that with increasing Al2O3 content, CaAl2O4 and MgAl2O4 phases were sequentially formed at the MgO grain boundaries, which enlarged the MgO grains and reduced the apparent porosity. Owing to the transformation of free CaO to CaAl2O4, reduction of apparent porosity and covering of intergranular MgAl2O4 on MgO grains, hydration of the specimens was inhibited effectively by introducing the Al2O3 addition. The formed CaAl2O4 and MgAl2O4 phases lowered the thermal expansion coefficient and induced crack deflection, and therefore residual flexural strength ratio after thermal shock tests increased remarkably from 15.1% to 33.2% with increasing Al2O3 addition from 0 wt% to 7 wt%. Moreover, slag penetration resistance of the specimens was improved effectively with raising the Al2O3 content to 3 wt%, attributing to the increase in MgO grain size, decrease in apparent porosity and formation of high stable intergranular CaAl2O4 phase. However, further increasing Al2O3 content would dramatically accelerate the slag penetration owing to the decrease in grain size and formation of more unstable intergranular MgAl2O4 phase.

Crack modes and toughening mechanism of a bioinspired helicoidal recursive composite with nonlinear recursive rotation angle-based layups

The rotation angle is an important parameter affecting the performance of helical structures, and helical structures with nonlinearly increasing rotation angles have been studied. The fracture behavior of a 3D-printed helicoidal recursive (HR) composite with nonlinear rotation angle-based layups was investigated by performing quasistatic three-point bending experiments and simulations. First, the crack propagation paths during the loading of the samples were observed, and the critical deformation displacements and fracture toughness were calculated. It was found that the crack path that propagated along the soft phase increased the critical failure displacement and toughness of the samples. Then, the deformation and interlayer stress distribution of the helical structure under static loading were obtained by finite element simulation. The results showed that the variation in the rotation angle between the layers caused different degrees of shear deformation at the interface between adjacent layers, resulting in different shear stress distributions and thus different crack modes of the HR structures. The mixed-mode I + II cracks induced crack deflection, which slowed the eventual failure of the sample and improved the fracture toughness.

Microstructural characteristics and mechanical response of diffusion bonding Inconel 617 superalloy

This study focuses on the microstructural characteristics and mechanical response of the diffusion-bonded joints of Inconel 617 superalloy. The joints exhibited two types of interfacial microstructure involving the straight interface (i.e., bond line) and the interface with partial migration of grain boundaries (GB). The straight interface was composed of the GB of γ-Ni, M23C6 carbide and α-Al2O3 oxide. The M23C6 carbide was precipitated with a semi-coherence of 33̅3M23C6//111̅γ−Ni, and an amorphous interface was detected between the α-Al2O3 and γ-Ni phase. The migration of GB across the interface was mainly caused by dynamic recrystallization and strain-induced GB migration. In addition, the α-Al2O3 oxide was remained inside the migrated grain. The formation of the M23C6 and α-Al2O3 was governed by diffusion mechanism, and the γ′-Ni3Al phase was formed surrounding the α-Al2O3 due to the abundant diffusion of Al. The straight interface, particularly embedded with the hard particles (i.e., the M23C6 carbide and α-Al2O3 oxide), restricted GND slipping, resulting in concentration of stresses and initiation of cracks. The migrated interface, in contrast, effectively prevented crack propagation and induced crack deflection, supporting more continuous and uniform plastic deformation. The elongation of 51.2 % was obtained for the joint with partial migrated interface, reaching 71 % of the base metal. Nevertheless, the optimum impact toughness of the joints (20.38 J/cm2) was merely 24 % of the base metal, mainly affected by the remained straight interface.

Design of crack deflection induced high toughness laminated Si3N4 ceramics based on hollow oriented one-dimensional microstructure

A laminated silicon nitride (Si3N4) ceramic material with a hollow, oriented, one-dimensional microstructure was successfully prepared based on the tape casting and sacrificial template method. The results show that hollow, oriented, one-dimensional microstructures can effectively induce crack deflection. Different arrangements of the structural design layer and dense layer will have different effects on the material. In particular, bulks with a single-layer orthogonal arrangement of the structural design layer possess high toughness and obvious crack deflection during the fracture process. A kind of multiscale crack deflection mode was realized. Compared with the fracture toughness of the monolithic Si3N4 ceramic bulk (5.55 MPa m1/2), the fracture toughness can reach 8.73 MPa m1/2, and the flexural strength can still reach 391.47 MPa with only a slight decrease.

Compression fatigue properties and damage mechanisms of a bioinspired nacre-like ceramic-polymer composite

Fatigue resistance is invariably critical for structural materials, but is rarely considered in the development of new bioinspired materials. Here the fatigue behavior and damage mechanisms of a nacre-like ceramic (yttria-stabilized zirconia) - polymer (polymethyl methacrylate) composite, which resembles human tooth enamel in its stiffness and hardness, were investigated under cyclic compression to simulate potential service conditions. The composite has a brick-and-mortar structure which exhibits a staircase-like fracture behavior; it displays a transition in cracking mode from the fracture of the ceramic bricks to separation along the inter-brick polymer phase with increasing stress amplitude. The nacre-like structure functions to induce crack deflection, increase the roughness of the crack surfaces, and promote the mutual sliding between bricks during fracture; this results in high fatigue resistance, which enhances the potential of this composite for dental applications.

Compression Fatigue Properties and Damage Mechanisms of a Bioinspired Nacre-Like Ceramic-Polymer Composite

Fatigue resistance is invariably critical for structural materials, but is rarely considered in the development of new bioinspired materials. Here the fatigue behavior and damage mechanisms of a nacre-like ceramic (yttria-stabilized zirconia) - polymer (polymethyl methacrylate) composite, which resembles human tooth enamel in its stiffness and hardness, were investigated under cyclic compression to simulate potential service conditions. The composite has a brick-and-mortar structure which exhibits a staircase-like fracture behavior; it displays a transition in cracking mode from intergranular splitting of ceramic bricks to separation along the inter-brick polymer phase with increasing stress amplitude. The nacre-like structure functions to induce crack deflection, increase the roughness of crack surfaces, and promote the mutual sliding between bricks during fracture; this results in high fatigue resistance, which enhances the potential of this composite for dental applications.

Bio-inspired, epoxy-based lamellar composites with superior fracture toughness by delignified wood scaffold

Inspired by the wood sponge and lamellar structure, delignified wood was introduced into epoxy as a scaffold to improve its fracture toughness. By chemically removing lignin and hemicellulose from natural wood and subsequently freeze-drying, a delignified wood template with a grid-like structure was obtained. The isotropic and anisotropic bulk epoxy/delignified wood biocomposites with 8 wt% delignified wood were prepared. The anisotropic biocomposite exhibits a KJC of 6.74 MPa m1/2, 8.31 times that of neat epoxy resin. Further studies reveal that the wood layer can generate micro-cracks and induce crack deflection through debonding and plastic deformation, causing fracture toughness to increase. Considering the strength and stiffness of delignified wood template, the biocomposites show unexpectedly excellent mechanical properties. The tensile strength and Young's modulus of the anisotropic materials are as high as 88.5 MPa and 3.5 GPa, respectively.

Fretting wear mechanism of plasma-sprayed CuNiIn coating on Ti-6Al-4V substrate under plane/plane contact

The fretting wear mechanism of plasma-sprayed CuNiIn coating on Ti-6Al-4V substrate under plane/plane contact conditions was investigated systematically using a bench level test. The obtained experimental results showed that the fretting wear mechanism of the CuNiIn coating can be divided into three stages: (I) In the initial stage, the asperities on the rough surface were flattened to form debris and fill the nearby pits. The contact surface gradually flattened, which was accompanied by slight abrasive wear. (II) With increasing number of fretting cycles, the fretting wear process of the CuNiIn coating entered the delamination wear stage. Under the action of alternating shear loads, the lamellar structure and pores in the CuNiIn coating rapidly cracked and propagated to the surface. In addition, some fatigue cracks propagated into the matrix along the direction of maximum shear stress. (III) Eventually, the CuNiIn coating entered a stable wear state under the lubrication of a third body, which was accompanied by local delamination. The lamellar structure greatly enhanced the fretting wear resistance of the CuNiIn coating. In the process of fretting wear, the lamellar structure of the CuNiIn coating effectively hindered fatigue crack growth, guided crack deflection along the lamellar interface, and induced crack bifurcation. In addition, the pores can blunt the crack tip. • A new self-balancing device was developed to improve geometrical adaptation of friction pairs with plane/plane contact. • The fretting wear resistance of CuNiIn coating was better than that of Ti-6Al-4V. • The wear rate of the CuNiIn coating was non-monotonic with increasing number of cycles. • The lamellar structure of CuNiIn coating was conducive to delamination wear. • The lamellar structure of CuNiIn coating could induce crack deflection, stop crack propagation, and guide crack bifurcation.

Investigation of Mixed-Mode I/II Fracture under Impact Loading Using Split-Hopkinson Pressure Bar

Mixed-mode fracture of construction building materials under impact loading is quite common in civil engineering. The investigation of mixed-mode crack propagation behavior is an essential work for fundamental research and engineering application. A variable angle single cleavage semi-circle (VASCSC) specimen was proposed with which the dynamic fracture test was conducted by using a Split-Hopkinson pressure bar (SHPB). Notably, the mixed-mode crack propagation velocity could be detected by the synchronized crack velocity measuring system. With experimental results, the dynamic initiation stress intensity factors KI and KII were calculated by the experimental-numerical method. Additionally, the crack path of mixed-mode I/II fracture can be predicated precisely by using numerical method. Thus, a comprehensive approach of investigation on mixed-mode I/II fracture under impact loading was illustrated in this paper. The study demonstrates that the mixed-mode I/II crack would transform from complicated mode I/II to pure mode I during crack propagation, and several velocity decelerations induced crack deflection. The dynamic initiation fracture toughness of mixed-mode crack was determined by the experimental-numerical method. The VASCSC specimen has a great potential in investigating mixed-mode fracture problems with the SHPB device.

Two-stage interface enhancement of aramid fiber composites: Establishment of hierarchical interphase with waterborne polyurethane sizing and oxazolidone-containing epoxy matrix

Hydroxy-terminated waterborne polyurethane (WPU) and oxazolidone-containing epoxy (OXEP) matrix were designed to realize two-stage interface enhancement of WPU sized aramid fiber/oxazolidone-containing epoxy (WAF/OXEP) composite, and the interfacial properties and interphase reinforcing mechanism were investigated. Compared with commercial aramid fiber (CAF), the surface roughness and energy of WAF were increased, and the fracture toughness and elongation at break of OXEP matrix were improved relative to those of general epoxy (GEP) matrix. In contrast to CAF/GEP and CAF/OXEP composites, the flexible interphase with high bonding capability from WPU sizing was constructed in WAF/GEP composite, which released interphase stress through deformation and improved interfacial adhesion. The hierarchical interphase encompassing inner modulus intermediate layer and outer flexible layer was established in WAF/OXEP composite, which could prevent crack propagation onto fiber surface and induce crack deflection through the enhancement of modulus and deformability of hierarchical interphase.

The hidden structure of human enamel

Enamel is the hardest and most resilient tissue in the human body. Enamel includes morphologically aligned, parallel, ∼50 nm wide, microns-long nanocrystals, bundled either into 5-μm-wide rods or their space-filling interrod. The orientation of enamel crystals, however, is poorly understood. Here we show that the crystalline c-axes are homogenously oriented in interrod crystals across most of the enamel layer thickness. Within each rod crystals are not co-oriented with one another or with the long axis of the rod, as previously assumed: the c-axes of adjacent nanocrystals are most frequently mis-oriented by 1°–30°, and this orientation within each rod gradually changes, with an overall angle spread that is never zero, but varies between 30°–90° within one rod. Molecular dynamics simulations demonstrate that the observed mis-orientations of adjacent crystals induce crack deflection. This toughening mechanism contributes to the unique resilience of enamel, which lasts a lifetime under extreme physical and chemical challenges.

Compressive residual thermal stress induced crack deflection in carbon nanotube-doped carbon/carbon composites

Abstract Introducing carbon nanotubes (CNTs) by electrophoretic deposition (EPD) is a promising method to improve the strength and toughness of carbon/carbon (C/C) composites. Herein, a new reinforcing mechanism called “compressive residual thermal stress (RTS) induced crack deflection” has been reported. Concretely, CNTs, with different loading content, were introduced by EPD method. Results showed that the CNT content had little influence on CNT-induced matrix refinement. However, the strength of the CNT-doped C/C composites increased with the rising content of CNTs and cracks could only deflect when the CNT interface reached a certain thickness. A theory based on compressive RTS induced crack deflection was built to interpret this discrepancy. Tensile stress existed at the interface in pure C/C composites, while compressive stress occurred and increased with the rising thickness of the CNT interface, which were verified by finite element analysis and Raman test. Calculation revealed that compressive stress exceeded 30 MPa at the crack tip could make the crack deflection happen more easily since it released more strain energy than penetration.

Texture Effect on Fatigue Crack Propagation Behavior in Annealed Sheets of an Al-Cu-Mg Alloy

The effects of texture, including Brass, Goss + Cube and Cube, on the fatigue crack growth behavior in annealed Al-Cu–Mg alloy sheets were characterized in this investigation. Results showed that the Goss + Cube sheet possessed the greatest fatigue threshold value and the lowest fatigue crack propagation (FCP) rate, and the Brass sheet presented the smallest threshold value and the greatest FCP rate among the three textured sheets. SEM results showed more slip bands and more tortuous crack path formed in Goss + Cube sheet during cyclic loading, and EBSD results showed that numerous dramatic crack deflections occurred when fatigue crack propagated across Goss-grain boundary in Goss + Cube sheet, indicating that the grain orientations including Goss and Cube had positive effect on enhancing the FCP resistance. On the one hand, Goss and Cube grains have more high Schmid factor slip systems than Brass grain, promoting persistent slip bands formation and externally applied energy consumption, consequently giving rise to a high resistance to FCP. On the other hand, Goss grains could induce crack deflection with large twist angles, resulting in more energy consumption during the cracking process and thereby enhance the FCP resistance.

Preparation, microstructure and toughening mechanism of superhard ultrafine-grained boron carbide ceramics with outstanding fracture toughness

Dense ultrafine-grained B4C ceramics with superhigh hardness and outstanding fracture toughness were fabricated via spark plasma sintering at 1700 °C and 80 MPa. This process used the ultrafine B4C powder prepared by high-energy ball milling as starting powder. During sintering, densification began earlier at 1050 °C and the maximum densification rate appeared earlier at 1500 °C under the influence of high pressure and high sintering activity of the powder. The B4C ceramics possessed ultrafine grains, with average grain size of 370 nm, which contributed to the superhigh hardness. Many small B4C grains with size of less than 200 nm were individually dispersed among large B4C grains or accumulated to form congregated areas. The small grains induced crack deflection, particle pullout and crack bridging, thereby enhancing the toughness of the ceramics. The relative density, Vickers hardness and fracture toughness of the B4C ceramics reached 99.4 ± 0.49%, 41.1 ± 0.9 GPa and 4.57 ± 0.12 MPa m1/2, respectively.

Anisotropy in fatigue crack propagation behavior of Al-Cu-Li alloy thick plate

Anisotropy in fatigue crack propagation (FCP) behavior of Al-Cu-Li-T87 alloy thick plate is investigated by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD) in the present work. Results show that FCP rates of the three orientations show distinct difference. Grain size and boundary, coarse inclusion particles, precipitates and grain orientations are capable of affecting the fatigue performance, but coarse inclusion particles and T1 precipitates are found to be the main factors in determining the fatigue performance. Grain boundaries of pan-caked grain structure fail to impede the fatigue crack growth effectively because of the coarse inclusion particles and precipitates along grain boundaries. Fatigue cracks easily propagate along the grain boundaries, therefore result in a low FCP resistance. In addition, Brass grains can induce crack deflection due to the relative large twist angle of grain boundaries between Brass grain and neighboring grain when considerable T1 precipitates are observed in Al-Cu-Li alloy.

Transformation induced crack deflection in a metastable titanium alloy and implications on transformation toughening

The deflection of cracks in grain interior was observed in a metastable β-phase Ti alloy (Ti-24Nb-4Zr-7.9Sn) with transmission electron microscopy during high cycle fatigue. This peculiar phenomenon is induced by the β → α″ martensitic transformation in front of crack tips in grain interior when cracks propagate along the {011}β slip planes with acute angles between α″ lamellae and cracks approximately of 45°. The α″ product phase has lattice dilation and contraction along two main transformation directions, thus introduces compressive and tensile strains perpendicular and parallel to crack propagating planes, respectively. This exerts beneficial effect on the crack deflection during the subsequent cyclic-loading.

Impurity-Induced Grain Boundary Strengthening in Polycrystalline Graphene

First-principles density functional theory (DFT) calculations were performed to investigate the effects of boron (B) and nitrogen (N) doping on the fracture behaviors of a series of symmetric graphene grain boundaries (STGBs) under biaxial straining. Doping was found to generally enhance the fracture strength of STGBs, which was shown to be attributed to dopants mechanically lowering the local stretching energy or chemically strengthening the critical bond. We also showed that doping may also induce crack deflection to further improve the fracture resistance of graphene grain boundaries (GBs). Furthermore, we showed that the presence of multiple B and N dopants in the pentagon-heptagon ring(s) can contribute to promoting intergranular fracture in the presence of a small prestrain along the GBs. Our findings clarify the role of B and N dopants in GBs strengthening of polycrystalline graphene.