Large Fracture Toughness Research Articles

Twinning-governed plastic deformation in a thin film of body-centred cubic nanocrystalline ternary alloys at low temperature

The plastic deformation of FeCuNi ternary alloys of BCC nanocrystalline subjected to compressive or tensile loading is considered via molecular dynamics (MD) simulations at low temperature, to reveal the solid solution effect on BCC metals. Deformation mechanisms at nanoscale have been well revealed for FCC nanocrystalline structure, however, it remains unanswered for BCC nanocrystalline structure. The increase of solute concentration promotes the nucleation of partial dislocation due to the reduction of stacking fault energy, and suppresses the motion of dislocation owning to the severe lattice distortion, which is beneficial to improving the strength and ductility of FeCuNi alloys. There is a critical solute concentration to determine the strengthening or the softening of FeCuNi alloys. Interestingly, the deformation twinning mechanism, instead of traditional dislocation mechanism, dominates the plastic deformation of BCC ternary alloys. In addition, it is clear to observe the phase transformation from BCC to HCP, full dislocation slipping, cross-slip, grain growth, and grain boundary (GB)/twinning boundary (TB) motion. Fracture behaviors of FeCuNi alloys reveal that the addition of Cu and Ni elements results in the larger fracture toughness.

A Rigorous Hydraulic-Fracture Equilibrium-Height Model for Multilayer Formations

Summary Fracture height is a critical input parameter for 2D hydraulic-fracturing-design models, and also an important output result of 3D models. Although many factors may influence fracture-height evolution in multilayer formations, the consensus is that the so-called “equilibrium height belonging to a certain treating pressure” provides an upper limit. However, because of the complexity of the algebra involved, published height models are overly simplified and do not provide reliable results. We revisited the equilibrium-height problem, started from the definition of the fracture stress-intensity factor (SIF), considered variation of layered formation properties and effects of hydrostatic pressure, and developed a multilayer fracture-equilibrium-height (MFEH) model by use of the programming software Mathematica (2017). The detailed derivation of SIF and work flow of MFEH model are provided. The model is compared with existing models and software, under the same ideal geology condition. Generally, MShale (2013) calculated smaller height, and FracPro (2015) larger height, than the MFEH model. Most of the difference is attributable to the different interpretation of the “net pressure,” and the solving of the nonlinear equations of SIF as well. In the normally stressed case, they are both acceptable, although MShale is more reliable. The discrepancy is much larger when there is abnormally high or low stress in the adjacent layers of the perforated interval. The effects of formation rock and fluid properties on the fracture-height growth were investigated. Tip jump is caused by low in-situ stress, whereas tip stability is imposed by large fracture toughness and/or large in-situ stress. If the fluid density is ignored, the result regarding which tip will grow into infinity could be totally different. Second and even third and fourth solutions for a three-layer problem were found by Excel experiments and this model, and proved unrealistic; however, they can be avoided in our MFEH model. The full-height map with very-large top- and bottom-formation thicknesses shows the ultimate trend of height-growth map (i.e., when the fracture tip will grow to infinity) and suggests the maximum pressure to be used. To assess the potential effects of reservoir-parameter uncertainties on the height map, two three-layer pseudoproblems were constructed by use of a multilayer formation to create an outer- and inner-height envelope. The improved MFEH model fully characterizes height evolution amid various formation and fluid properties (fracture toughness, in-situ stress, thickness, and fluid density), and for the first time, rigorously and rapidly solves the equilibrium height. The equilibrium height can be used to provide input data for the 2D model, improve the 3D-model governing equations, determine the net pressure needed to achieve a certain height growth, and suggest the maximum net pressure ensuring no fracture propagation into aquifers. This model may be incorporated into current hydraulic-fracture-propagation simulators to yield more-accurate and -cost-effective hydraulic-fracturing designs.

Fracture of monolayer boronitrene and its interface with graphene

We investigate through molecular dynamics simulations the fracture properties of boronitrene (BN), graphene, and their interfaces. Four types of interfaces between boronitrene and graphene are considered. It is found that the fracture toughness of graphene is highest among the examined models and is 3.61 and 4.24 $$\hbox {MPa}\sqrt{\hbox {m}}$$ in the armchair and zigzag directions, respectively. Compared to graphene, boronitrene exhibits approximately 12 and 21% smaller values of the fracture toughness in the armchair and zigzag directions, respectively. In the armchair direction, the fracture toughness of the interface between boronitrene and graphene with B–C bonds in the interface is weakest and is about 2.49 $$\hbox {MPa}\sqrt{\hbox {m}}$$ , while the interfacial fracture toughness with C–N bonds in the interface is very close to that of graphene. In the zigzag direction, the interfacial fracture toughness is close to that of BN sheet. Under tension in the zigzag direction, a centered crack, which is initially perpendicular to the tensile direction, kinks at both tips in graphene and boronitrene regions. Since graphene has larger fracture toughness than that of boronitrene, an initial crack in their interface is forbidden to penetrate the graphene region; i.e., the crack can only propagate in the boronitrene region or along their interface of the hybrid BN/graphene sheets. The crack shape in the hybrid BN/graphene sheets depends on the arrangement of B–C–N atoms around the interface and the initial crack tip region.

Fracture Energy of Woven Fabric Kenaf Composite Plates with Different Fiber Orientations

Natural fibers are potentially used as composite reinforcement in polymeric materials, but lacking of research exploration such as kenaf fibers has limited implementation in Malaysia. The advantages of natural fibers are renewable, less hazardous during fabrication and handling process and relatively cheaper as compared to synthetic fibers. The objectives of current project are to determine the fracture energy of woven fabric kenaf composite (KFRP) plates with various fiber orientations. The experiment framework includes a variation of fiber orientations as designated in testing series by using single-edge notch (SEN) testing technique. The experimental results demonstrated that testing coupons with woven fabric with 90o fiber orientation has largest fracture toughness Gc value compared to other fiber orientations understudied. Good correlations and findings were found in other parametric studied.

High pressure infiltration sintering behavior of WC-Co alloys

ABSTRACTIn this paper, two average tungsten carbide particle sizes of 2, 0.5 μm are placed respectively, in contact with a WC-16Co substrate, pressed at the pressure of 4.5–5.5 GPa, and heated to temperatures ranging from 1350°C to 1500°C in a large-volume cubic press. During the process Co was forced out of the WC-16Co substrate into the compressed powder. The resulting infiltrated samples were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), Vickers hardness and cutting performance tests. The results of XRD confirmed that the sintered bulks have WC and Co phases. The scanning electron microscopy (SEM) analysis reveals that the WC grains in well-sintered alloys are round in shape and cobalt with lower content is uniformly dispersed in the WC grain boundaries. The sintered sub-micron WC-Co alloy with a cobalt content of 3.8 wt% exhibits a prominent combination of high hardness value of 23.1 GPa and a large fracture toughness value of 8.6 MPa m½. The high-speed cutting tests indicating its cutting performance is significantly superior to the commercial YG6X (WC-6 wt%Co with WC grain size of 0.5 μm).

Fracture, fatigue, and creep of nanotwinned metals

Abstract

Thermoelectric and mechanical properties of ZnSb/SiC nanocomposites

Intermetallic compound ZnSb is a promising thermoelectric (TE) material with the advantages of low toxicity, abundance, and low cost; however, the relatively low figure of merit ZT and the brittleness of ZnSb limit its applications in TE devices. In this study, ZnSb/SiC nanocomposites were synthesized in order to improve both the TE and mechanical properties of ZnSb. ZnSb-based nanocomposites with x vol% SiC nanoparticles (x = 0, 0.3, 0.5, and 0.7) were prepared by mechanical alloying and spark plasma sintering. The power factor of ZnSb/SiC nanocomposite with 0.3 vol% SiC is increased. The thermal conductivity of all ZnSb/SiC nanocomposites is decreased due to the increase of interface scattering for phonons. More importantly, the fracture toughness of the nanocomposites is enhanced due to the addition of SiC. The largest ZT value and fracture toughness are found in the sample with 0.7 vol% SiC. The maximum ZT value of 0.68 is obtained at 400 °C, which is 35 % higher than that of the reference sample without SiC. The largest fracture toughness is 0.64 MPa m1/2, which is 31 % larger than that of the reference sample. The experimental data demonstrate that ZnSb/SiC nanocomposites with simultaneous enhancement of TE and mechanical properties are favorable for practical TE applications.

Fracture toughness of the interface between Ni-Cr/ceramic, alumina/ceramic and zirconia/ceramic systems

Few studies have focused on the interface fracture performance of bi-layered structures, which have an important role in dental restorations, using ceramic materials. The purpose of this study is to evaluate the fracture mechanics performance of the Ni–Cr/ceramic, alumina/ceramic and zirconia/ceramic interfaces by investigating the propagation of an interfacial crack under a wide range of mode mixities. The effect of the mechanical properties of the base materials and the interface, on the crack initiation and crack path, will also be studied. The finite element method (FEM) was used to calibrate the production of the experimental specimens, allowing to obtain the minimum dimensions and amounts of material needed to correctly characterize the fracture event. The specimens were tested until failure using a three-point bending test machine. The interface fracture parameters were obtained using the FEM. For all specimens, the cracks propagated into the ceramic. The results suggest that, in Ni–Cr/ceramic, alumina/ceramic and zirconia/ceramic bi-layered structures, the ceramic is weaker than the interface, which can be used to explain the clinical phenomenon that the ceramic chipping rate is larger than interface delamination rate. Consequently, a ceramic material with a larger fracture toughness is needed to decrease the failure rate of ceramic restorations.

Temperature and Loading-Rate Dependence on a Local Criterion for Cleavage Fracture in the Transition Region of the Tempered Martensitic Steel Eurofer97.

Finite element calculations of stationary cracks were performed in the ductile to brittle transition region of the reduced activation tempered martensitic steel Eurofer97, for which a large fracture toughness database was already available. Two models were run in this work: i) compact tension specimens simulated in 3D, and ii) small scale yielding boundary layer model in 2D plane strain conditions. The analysis was focused on the unusual low fracture toughness of Eurofer97 on the lower shelf, with respect to other "ferritic" steels, as well as in the lower transition. The analysis of the near tip stress field suggested that the minimum stress intensity factor on the lower shelf represents the minimum loading condition to have the local fracture stress act over a distance of the order of the prior austenite grain size. The effect of loading rate on the tip stress fields were also investigated by considering a strain rate dependence of the flow stress explicitly in the finite element calculations. These effects were shown to be non-negligible and to depend on the ratio between the peak stress in the fracture process zone and the local fracture stress.

Zr 46.9 Cu 45.5 Al 5.6 Y 2.0 金属玻璃含B2-CuZr相内生复合材料的制备及其力学性能 *

Bulk metallic glass (BMG) composites containing B2-CuZr phase are of interest due to they behave large plastic strain and apparent work hardening in tension. Nevertheless till now most BMG composites containing B2-CuZr phase are based on Cu47.5Zr47.5Al5 or Zr48Cu47.5Al4Co0.5 BMG, which has limited glass forming ability (GFA). The prepared sample size is small, which restricts their potential engineering structural applications. In this work, Zr-Cu-Al-Y quaternary system is selected due to its high GFA. By tuning composition close to CuZr alloy in Zr-Cu-Al-Y quaternary system, Zr46.9Cu45.5Al5.6Y2.0 BMG is selected because it has proper GFA (critical diameter Dc=5 mm) and relatively large fracture toughness (KQ=(49±3) MPa·m). By decreasing the cooling rates of the melt via increasing diameter of casting rods, large-sized in situ Zr46.9Cu45.5Al5.6Y2.0 BMG composites containing 13% and 25% volume fractions spherical B2-CuZr phase were prepared in the casting rods with 6 and 7 mm in diameters, respectively. In compression testing, the in situ BMG composites containing 25%B2-CuZr phase promote multiple shear bands within glass matrix and remarkable global plastic deformation, accompanied by a large compressive plastic strain as 6.5%. Nevertheless in tension testing no obvious global ductility was achieved, which attri*国家自然科学基金资助项目51171180 收到初稿日期: 2015-03-11,收到修改稿日期: 2015-07-04 作者简介:沈勇,男, 1976年生,助理研究员 DOI: 10.11900/0412.1961.2015.00140 第1407-1415页 pp.1407-1415

Mode-I, mode-II and mixed-mode I+II fracture behavior of composite bonded joints: Experimental characterization and numerical simulation

The fracture behavior of composite bonded joints subjected to mode-I, mode-II and mixed-mode I + II loading conditions was characterized by mechanical testing and numerical simulation. The composite adherents were bonded using two different epoxy adhesives; namely, the EA 9695 film adhesive and the mixed EA 9395-EA 9396 paste adhesive. The fracture toughness of the joints was evaluated in terms of the critical energy release rate. Mode-I tests were conducted using the double-cantilever beam specimen, mode-II tests using the end-notch flexure specimen and mixed-mode tests (three mixity ratios) using a combination of the two aforementioned specimens. The fracture behavior of the bonded joints was also simulated using the cohesive zone modeling method aiming to evaluate the method and point out its strengths and weaknesses. The simulations were performed using the explicit FE code LS-DYNA. The experimental results show a considerable scatter which is common for fracture toughness tests. The joints attained with the film adhesive have much larger fracture toughness (by 30–60%) than the joints with the paste adhesive, which exhibited a rather brittle behavior. The simulation results revealed that the cohesive zone modeling method performs well for mode-I load-cases while for mode-II and mixed-mode load-cases, modifications of the input parameters and the traction-separation law are needed in order for the method to effectively simulate the fracture behavior of the joints.

Fracture mechanics analyses of ceramic/veneer interface under mixed-mode loading

Few studies have focused on the interface fracture performance of zirconia/veneer bilayered structure, which plays an important role in dental all-ceramic restorations. The purpose of this study was to evaluate the fracture mechanics performance of zirconia/veneer interface in a wide range of mode-mixities (at phase angles ranging from 0° to 90°), and to examine the effect of mechanical properties of the materials and the interface on the fracture initiation and crack path of an interfacial crack. A modified sandwich test configuration with an oblique interfacial crack was proposed and calibrated to choose the appropriate geometry dimensions by means of finite element analysis. The specimens with different interface inclination angles were tested to failure under three-point bending configuration. Interface fracture parameters were obtained with finite element analyses. Based on the interfacial fracture mechanics, three fracture criteria for crack kinking were used to predict crack initiation and propagation. In addition, the effects of residual stresses due to coefficient of thermal expansion mismatch between zirconia and veneer on the crack behavior were evaluated. The crack initiation and propagation were well predicted by the three fracture criteria. For specimens at phase angle of 0, the cracks propagated in the interface; whereas for all the other specimens the cracks kinked into the veneer. Compressive residual stresses in the veneer can improve the toughness of the interface structure. The results suggest that, in zirconia/veneer bilayered structure the veneer is weaker than the interface, which can be used to explain the clinical phenomenon that veneer chipping rate is larger than interface delamination rate. Consequently, a veneer material with larger fracture toughness is needed to decrease the failure rate of all-ceramic restorations. And the coefficient of thermal expansion mismatch of the substrates can be larger to produce larger compressive stresses in the veneer.

Quasi-static mode II fracture tests and simulations of Z-pinned woven composites

Experimental results from quasi-static mode II ENF (end notch flexure) fracture tests for Z-pinned woven composites are used to investigate the influence of Z-pin density and Z-pin diameter on Mode II fracture toughness. The experimental results show that the 2% Z-pin composite and the 3% Z-pin composite have larger fracture toughness than the 1% Z-pin composite and a composite without Z-pins. Crack propagation changes from unstable propagation to stable propagation (crack arrest) as the Z-pin density increases. The additional fracture toughness provided by Z-pins prevents the primary crack to further propagate, enabling secondary and tertiary cracks to consume additional energy. While the primary crack is held from propagating by the frictional forces provided by Z-pins, the matrix in the composite is driven into the plastic state and enables the composite to absorb more energy. For numerical simulations, a discrete cohesive zone model (DCZM) is used to perform simulations. The simulations in conjunction with experimental results provide critical fracture toughness values.

Lead-Free Piezoelectric MEMS Energy Harvesters of (K,Na)NbO3 Thin Films on Stainless Steel Cantilevers

We fabricated piezoelectric MEMS energy harvesters (EHs) of lead-free (K,Na)NbO3 (KNN) thin films on microfabricated stainless steel cantilevers. The use of metal substrates makes it possible to fabricate thin cantilevers owing to a large fracture toughness compared with Si substrates. KNN films were directly deposited onto Pt-coated stainless steel cantilevers by rf-magnetron sputtering, thereby simplifying the fabrication process of the EHs. From XRD measurement, we confirmed that the KNN films on Pt-coated stainless steel cantilevers had a perovskite structure with a preferential (001) orientation. The transverse piezoelectric coefficient e31f and relative dielectric constant εr were measured to be -3.8 C/m2 and 409, respectively. From the evaluation of the power generation performance of a KNN thin-film EH (length: 7.5 mm, width: 5.0 mm, weight of tip mass: 25 mg), we obtained a large average output power of 1.6 µW under vibration at 393 Hz and 10 m/s2.

Effect of NiO additive on microstructure, mechanical behavior and electrical properties of 0.2PZN–0.8PZT ceramics

A NiO-added Pb((Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80)O3 system is prepared and investigated. The results reveal that Ni doping induces a phase transformation from the morphotropic phase boundary to the tetragonal phase side. Above the solubility limit of 0.3 wt% in NiO form, excess Ni ions segregate at the grain boundaries and triple junctions, which facilitate the formation of a liquid phase with excess PbO and lead to remarkable grain growth. The mechanical behavior (Vickers hardness (Hv) and fracture toughness (KIC)) can be tailored by controlling the content of additive; this is accompanied by a transition in the fracture mode changed from transgranular without NiO additive to intergranular with 1.0 wt% NiO additive. Moreover, the NiO addition weakens the dielectric relaxor behavior and improves the piezoelectric properties simultaneously. The 0.2PZN–0.8PZT with 0.5 wt% NiO addition shows good transduction coefficient (d33·g33 = 10,050 × 10−15 m2/N) and large fracture toughness (KIC = 1.35 MPa m1/2).

Ultrahard nanotwinned cubic boron nitride

Cubic boron nitride (cBN) is a well known superhard material that has a wide range of industrial applications. Nanostructuring of cBN is an effective way to improve its hardness by virtue of the Hall-Petch effect--the tendency for hardness to increase with decreasing grain size. Polycrystalline cBN materials are often synthesized by using the martensitic transformation of a graphite-like BN precursor, in which high pressures and temperatures lead to puckering of the BN layers. Such approaches have led to synthetic polycrystalline cBN having grain sizes as small as ∼14 nm (refs 1, 2, 4, 5). Here we report the formation of cBN with a nanostructure dominated by fine twin domains of average thickness ∼3.8 nm. This nanotwinned cBN was synthesized from specially prepared BN precursor nanoparticles possessing onion-like nested structures with intrinsically puckered BN layers and numerous stacking faults. The resulting nanotwinned cBN bulk samples are optically transparent with a striking combination of physical properties: an extremely high Vickers hardness (exceeding 100 GPa, the optimal hardness of synthetic diamond), a high oxidization temperature (∼1,294 °C) and a large fracture toughness (>12 MPa m(1/2), well beyond the toughness of commercial cemented tungsten carbide, ∼10 MPa m(1/2)). We show that hardening of cBN is continuous with decreasing twin thickness down to the smallest sizes investigated, contrasting with the expected reverse Hall-Petch effect below a critical grain size or the twin thickness of ∼10-15 nm found in metals and alloys.

Modeling Radiation Effects on the Fracture Process in Simplified Nuclear Glass

Experimentally, the evolution of several mechanic properties (hardness, density, Young’s modulus, fracture toughness) is observed in nuclear glasses under irradiation. In this work, classical molecular dynamics calculations are performed to better understand fracture mechanisms in simplified nuclear glasses at atomistic scale and to explain the radiation effects. Fractures are simulated in more disordered glasses, representative of irradiated samples, to reveal radiation effects. We observe a lower elastic limit and a greater plasticity in the irradiated glass that can explain its larger fracture toughness.

Micromirror with large-tilting angle using Fe-based metallic glass

For enhancing the micromirror properties like tilting angle and stability during actuation, Fe-based metallic glass (MG) was applied for torsion bar material. A micromirror with mirror-plate diameter of 900 μm and torsion bar dimensions length 250 μm, width 30 μm and thickness 2.5 μm was chosen for the tilting angle tests, which were performed by permanent magnets and electromagnet setup. An extremely large tilting angle of over -270° was obtained from an activation test by permanent magnet that has approximately 0.2 T of magnetic strength. A large mechanical tilting angle of over -70° was obtained by applying approximately 1.1 mT to the mirror when 93 mAwas applied to solenoid setup. The large-tilting angle of the micromirror is due to the torsion bar, which was fabricated with Fe-based MG thin film that has large elastic strain limit, fracture toughness, and excellent magnetic property.

Development of the Large Scanning Mirror Using Fe-Based Metallic Glass Ribbon

In order to achieve the large scanning angle without mechanical failure during actuation, the micro-mirror structure was fabricated using Fe-based metallic glass (MG) ribbon. High values of mechanical strength and elastic strain limit are desired for the torsion bar for providing high performance of the mirror, including large tilting angle and good stability. MG materials guarantee superior value of elastic strain limit and fracture toughness resulting from its amorphous structure without line defects like dislocations and grain boundaries. We chose Fe-base MG material for mirror actuation according to its good soft magnetic properties and simple structure without any further step of integration of magnetic material. Millimeter-size mirror device was successfully fabricated by laser machining techniques from Fe-based MG ribbon. Extremely large optical tilting angle of exceeding 100° was obtained on activation of the mirror by magnetic force when electrical current of 100 mA was applied. Large tilting angle of the mirror is due to the torsion bar which was fabricated with Fe-based MG material that has large elastic strain limit and high fracture toughness.

Development of the Large Scanning Mirror Using Fe-Based Metallic Glass Ribbon

In order to achieve the large scanning angle without mechanical failure during actuation, the micro-mirror structure was fabricated using Fe-based metallic glass (MG) ribbon. High values of mechanical strength and elastic strain limit are desired for the torsion bar for providing high performance of the mirror, including large tilting angle and good stability. MG materials guarantee superior value of elastic strain limit and fracture toughness resulting from its amorphous structure without line defects like dislocations and grain boundaries. We chose Fe-base MG material for mirror actuation according to its good soft magnetic properties and simple structure without any further step of integration of magnetic material. Millimeter-size mirror device was successfully fabricated by laser machining techniques from Fe-based MG ribbon. Extremely large optical tilting angle of exceeding 100° was obtained on activation of the mirror by magnetic force when electrical current of 100 mA was applied. Large tilting angle of the mirror is due to the torsion bar which was fabricated with Fe-based MG material that has large elastic strain limit and high fracture toughness.