Large Fracture Toughness Research Articles

A micromagnetic-mechanically coupled phase-field model for fracture and fatigue of magnetostrictive alloys

Magnetostrictive alloys are usually brittle materials with micromagnetic structures. Their structural reliability and durability depend on the complex micromagnetic-mechanical coupling at smaller length scales encompassing the evolution of micromagnetic structures. Herein we propose a micromagnetic-mechanically coupled phase-field model for fracture and fatigue behavior of magnetostrictive alloys with evolution of the micromagnetic structure. The thermodynamically-consistent model is derived from microforce theory, laws of thermodynamics, and Coleman–Noll analysis. The evolution of crack phase-field and magnetization-vector order parameters that are fully coupled is governed by history field dependent Allen–Cahn and Landau–Lifshitz–Gilbert equations, respectively. The model is extended to fatigue by introducing a degradation prefactor for the fracture energy as a function of positive elastic energy. One-dimensional analyses are then presented to anatomize the crack driving forces in terms of fully coupled micromagnetic-mechanical and pure mechanical driving force. We demonstrate the model capabilities by finite-element numerical studies on the micromagnetic domain evolution during the crack propagation and the influence of external magnetic field for type-I, type-II, and three-point bending fracture, as well as for the fracture of a single-edge notched specimen with an elliptical inclusion. The simulation result shows that depending on how micromagnetic domains are switched under micromagnetic-mechanical coupling, the magnetic field can enhance or decrease the critical load. In the presence of inclusion with larger fracture toughness, a crack is found to nucleate in the tri-junction of multi-domain micromagnetic structure owing to the high elastic strain around the tri-junction point. It is further found that a suitable magnetic field promoting magnetization-vector rotation around the crack tip could remarkably improve the fracturing load and fatigue life. The results demonstrate the model promising for the study of micromagnetic-mechanically coupled fracture and fatigue in magnetostrictive alloys.

Micromechanical characterization of zirconia and silicon nitride ceramics using indentation and scratch methods

Micromechanical characterization of brittle ceramics remains challenging due to the difficulty in sample preparation required by conventional tests. The micromechanical properties (e.g., elastic modulus EIT, indentation hardness HIT, and fracture toughness KIC) of ZrO2 and Si3N4 were investigated by indentation and scratch tests with different indenters (e.g., Vickers, Knoop, and Berkovich indenters) under various loads. The ratio of the true hardness by the Vickers indenter over that by the Knoop indenter is approximately a constant of 1.13 and independent of material. Elastic recovery of residual imprint by the Knoop indenter was also used to assess elastic modulus of ZrO2 and Si3N4, and modified equations were proposed. The length of Vickers indenter-induced cracking was found to be proportional to the applied load under large loads, and a modified formula for calculating fracture toughness was proposed for ZrO2 and Si3N4, which have relatively large fracture toughness of about 9 MPa⋅m1/2. The critical void volume fraction f* in the nanoindentation energy-based methods was found to linearly decrease with the exponent mf of the power-law equation used for curve fitting of the degradation of HIT or EIT with hmax. The determination of fracture toughness by scratch methodology was found to be affected by mechanical properties of material, indenter geometry, and data range.

Comparison of mechanical characteristics of different types of shales in the Ordos Basin

Shale gas is a very important unconventional energy. The mechanical properties of the three types of shale (laminated shale, sandwich shale and foliated shale) are different, and the difference in fracturing effectiveness is very significant. In this paper, the mineral composition, mechanical properties and conductivity of these three different types of shale were studied and compared by X-ray diffraction, triaxial mechanical experiments, and fracture conduction experiments. The study found that the foliated shale has the lowest content of rigid minerals (47.5%), lower elastic modulus and tensile strength (26.98 Gpa and 168.29 MPa, respectively), higher Poisson’s ratio (0.25), the smallest brittleness index (0.48), and larger fracture toughness (0.42). The laminated shale has a higher content of rigid minerals (68.50%), the lowest elastic modulus and tensile strength (25.77 Gpa and 122.46 MPa, respectively), the highest Poisson’s ratio (0.26), the highest brittleness index (0.56), and the lowest fracture toughness (0.18). The sandwich shale has the highest rigid mineral content (78.16%), the highest elastic modulus and tensile strength (35.31 Gpa and 197.37 MPa, respectively), the lowest Poisson’s ratio (0.24), a larger brittleness index (0.52), and larger fracture toughness (0.415). Furthermore, with the increase in the coring angle, the elastic modulus of all three shales increases. In addition, with the increase in closing pressure and the decrease in the sand laying concentration, the proppant embedding depth gradually increases and the conductivity decreases. This means that from the perspective of forming complex fracture networks, the fracturing effect of the foliated shale is unsatisfactory, while the fracturing effect of the laminated and sandwich shales is better. Moreover, it is recommended to prefer directional injection along vertical laminae or at high angles, which is conducive to the formation of complex fracture networks. For laminated shale with low strength, the sand laying concentration should be increased to ensure the conductivity of the fractured fracture. This study provides some technical guidance for the identification of different types of shale fracturing desserts and fracturing processes.

Influence of the Valid Test Temperature Range on Master Curve Analyses of Large Fracture Toughness Data Sets

Abstract ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, standardizes the Master Curve procedure for determining the reference temperature, T0, of ferritic steels in the ductile-to-brittle transition range. In order for toughness test results to be included in the analyses, the corresponding test temperatures, T, must currently lie within the valid range defined as T0 – 50°C ≤ T ≤ T0 + 50°C. This study investigated the possibility of extending the valid test temperature range in a future revision of ASTM E1921 by assessing the consequences on the values of homogeneous (T0) and inhomogeneous (T0IN, Tm) reference temperatures calculated on 10 large “historical” data sets, already examined by the author in previous papers. The effect of expanding the valid temperature range on the macroscopical nature (homogeneous or inhomogeneous) of each data set was also considered by using the screening criterion presently proposed by ASTM E1921-22a. The results obtained appear to warrant a possible revision of the standard in this direction.

Rare-earth high-entropy aluminate-toughened-zirconate dual-phase composite ceramics for advanced thermal barrier coatings

Superb toughening is achieved by incorporating a secondary ferroelastic phase in high-entropy rare-earth zirconate 5RE2Zr2O7 (HZ). Here, we report an enhancement of 64% in fracture toughness through the addition of 30mol% high-entropy rare-earth aluminate 5REAlO3 (HA) to the HZ matrix (30HA). The aforementioned rare-earth elements RE are La, Sm, Eu, Gd, and Yb. The present dual-phase composite ceramic 30HA has a large fracture toughness of 2.77 ± 0.14 MPa m1/2, along with excellent high-temperature phase stability, resulting in good usage for potential thermal barrier coating applications. Particularly, the fracture toughness of the dual-phase composite ceramics at first increases to a maximum and then drops suddenly, as the mole fraction of HA increases from 0 to 50%. A clear definition of fitting parameters and their physical significance is provided for a better interpretation of the experimental data. The present toughening mechanism sheds light on microstructure engineering in high-entropy ceramics for excellent mechanical properties.

Solvothermal-derived multicomponent rare-earth cerate ceramics toward remarkable thermophysical properties

The multicomponent rare-earth cerate (Y0.2La0.2Nd0.2Sm0.2Eu0.2)2Ce2O7 (5RE2Ce2O7) ceramics were successfully fabricated through solvothermal method and the following calcination process. The microstructure and phase composition of the as-obtained products were systematically characterized via SEM, TEM and XRD techniques. The results showed that the as-synthesised 5RE2Ce2O7 has a single-phase fluorite-type crystal structure with the particle size of approximately 200 nm. Furthermore, the as-synthesised 5RE2Ce2O7 demonstrated lower thermal conductivity (1.9–1.26 W m−1·K−1 at 25–1000 °C), higher thermal expansion coefficients (CTEs, 12.48 × 10−6 K−1 at 1000 °C), and outstanding mechanical properties including large Young's modulus (248.0 ± 0.35 GPa) and high fracture toughness (2.4 ± 0.21 MPam1/2). The excellent properties of the as-synthesised 5RE2Ce2O7 demonstrates its potential application as a new type of next-generation TBCs.

Experimental and theoretical study on mechanical behaviors of CFRP–steel interface

The performance of the CFRP–steel interface is crucial to the reinforcement efficiency of the steel structures strengthened by using externally pasted CFRP sheet method; while it is influenced significantly by adhesive properties and geometrical characteristics of joints. The present study aims to optimize the adhesive layer of the CFRP–steel interface and establish a theoretical model of predicting the interfacial behaviors of the optimized CFRP–steel joints. A commercially available adhesive was modified by addition of Nano-SiO2 ranged from 0 to 7wt% to ascertain the optimal content of Nano-SiO2. A series of tensile tests were carried out on dog-bone adhesive specimens, single-lap shear specimens and CFRP–steel double-lap joints to obtain the mechanical properties of the adhesive at different content of nanofiller and the corresponding bond–slip laws in the unmodified and modified CFRP–steel interfaces. Accordingly, theoretical models were derived to predict the load capacity of the joints and the bond stress distribution along the CFRP–steel interface during different loading phases. The results show that the mechanical properties of the adhesive are improved with the addition of nano-SiO2 up to 1.4 wt%. When the content of nano-SiO2 rises from 1.4 to 7 wt%, the tensile and shear strengths as well as the tensile modulus decline gradually whereas the ductility keeps growing with a lower growth rate. These indicate the nano-SiO2 content of 1.4 wt% is favorable. Both the peak stress and the load capacity of the CFRP–steel joint increase with the adhesive thickness varying from 0.5 to 1 mm. The bond–slip law of the unmodified CFRP–steel interface can be simplified to be bilinear, regardless of the adhesive thickness and curing duration. In contrast, the modified interface presents a trilinear curve due to the special plastic plateau caused by the filling of nano-SiO2. Consequently, a larger fracture toughness and a higher load capacity can be reached by the optimized joints. The theoretical results are in agreement with the experimental phenomena and results. The modified adhesive is beneficial to utilize the CFRP material better and improve the reinforcement efficiency.

Nacre-liked material with tough and post-tunable mechanical properties

• A universal fabrication methodology for post-tunable mechanical properties materials. • Wide tunable range without the need for specific functional groups. • Stable and reversible ‘brick and mortar’ structure. • Superior fracture toughness. Biomaterials, often imparted time-dependent mechanical properties, which are promising in fields ranging from sensors to robotics. Here, a facile method was proposed to fabricate post-tunable mechanical properties composites based on hydrogels and ceramic nanofiller. The wide tunable range of Young's modulus (27.3 kPa to 3.5 GPa) and ultimate stress (173 kPa to 102 MPa) can be achieved by combining solvent absorption and evaporation process with platelets reinforcement effect. Additionally, a large fracture toughness (∼32,000 J m - 2 ) is obtained as a result of the nacre-liked “brick and mortar” structure introduced by shear force during fabrication. The superior flexibility and designability of this material were demonstrated via actuators, portable structure, and metamaterials. Above all, this study provides a new thought to fabricate tough materials with post-tunable mechanical properties.

Eye Protection for Mitigating Soccer-Related Ocular Injuries: A Finite Element Approach

Abstract In this work, eye protection for mitigating the soccer ball ocular injuries were delineated with computational models. The retinal injuries and the risk of globe rupture were evaluated in terms of the stress in the posterior retinal layer and the intraocular pressure (IOP) measurement at the center of the vitreous. The mechanism of the eye protection with goggles was quantified in terms of impact indentation and velocity caused at the contact location in the eyeball. Results have shown that both polycarbonate and acrylic goggles can significantly reduce the stress in the retina and IOP at the center of the vitreous by more than 50%. The polycarbonate (PC) goggles remained intact and bounced backward during impact, leading to smaller impact indentation and velocity in the contact region in the eyeball, compared to the fractured acrylic goggles. These findings may be due to the larger fracture toughness modulus of PC compared to acrylic, indicating that the fracture toughness modulus could be an important parameter for the material selection in protection design. The soccer ball-goggles interaction indicate that the local deformation of soccer ball, i.e., backward bending, plays an important role in the eye protection. These mechanistic understandings could serve as a guidance for the eye protection design.

Low temperature characteristics of asphalt mixture based on the semi-circular bend and thermal stress restrained specimen test in alpine cold regions

Low-temperature crack resistance performance for preventing and controlling asphalt pavement cracking plays an important role in alpine regions. This study is to evaluate the low temperature characteristics of asphalt mixture in alpine cold regions. Three kinds of asphalt binders (SK90, SBS modified asphalt, SRA modified asphalt-an independent R & D type for the alpine cold regions) and three types of gradations (AC-13, AC-16, SMA-13) were selected. The fracture characteristics of asphalt mixtures based on the energy perspective were evaluated based on the semi-circular bend (SCB) test. The influences of asphalt binder type, gradation type, temperature and notch depth on the stress–strain curve were investigated. In addition, the thermal contraction coefficient of different kinds of asphalt mixtures were also analyzed. The temperature stress curves of asphalt mixtures were constructed based on the thermal stress restrained specimen test (TSRST). The results showed that the distribution characteristics of the coarse aggregate had a greater impact on the cracking path of the specimen. The asphalt mixture specimen exhibited brittle fracture at −15 °C and −35 °C, while it exhibited ductile failure at 15 °C based on the stress–strain curves. The unit reference fracture energy can be used as an effective index to evaluate the crack resistance. SRA asphalt mixture specimens had the largest fracture toughness and unit reference fracture energy. The higher the notch depth, the smaller the fracture energy; the fracture energy and the incision had a good linear correlation. The thermal contraction coefficients of SRA, SBS and SK90 asphalt mixtures were 10.46 × 10-6 ∼ 18.65 × 10-6/°C, 10.66 × 10-6 ∼ 22.85 × 10-6/°C, 13.45 × 10-6 ∼ 28.80 × 10-6/°C, respectively; they were closely related to the asphalt type and temperature, it changed exponentially under different temperatures. The freeze-break temperature was within a range and the difference was within 5 °C. SMA-13 had the strongest low-temperature crack resistance, followed by AC-16, and AC-13.

Friction control by tailoring deformation mechanism of interfacial grains in metals

Micro-scale stick-slip motion of metals is closely related to interfacial plasticity. However, previous studies about the effect of interfacial plasticity on the stick-slip motion only focused on the role of asperity plasticity while ignored the role of grain size of interfacial grains. This study focuses on the stick-slip motion of coarse-grain (CG), ultrafine grain (UFG), nanograin (NG), and heterogeneous grain structure Ti–6Al–4V alloys. Micro-scale stick-slip motion controlled by tailoring grain size of interfacial grains was discovered for the first time. When interfacial materials are coarse-grains (CGs) and ultrafine grains (UFGs), the friction mode is mixed regime (stick-slip competed). When interfacial material is nanograins, the friction mode shows slip regime (slip dominated). When interfacial material is heterogeneous grain structure, the friction mode displays partial slip regime (stick dominated). Notched micro-cantilever tests show that dislocation slip in CGs contributes to large plasticity and high fracture toughness, and that grain boundary migration in NGs results in limited plasticity and low fracture toughness. The plasticity and fracture toughness affected by deformation mechanism play an important role in micro-scale stick-slip motion. The nature of the stick-slip motion control by tailoring grain size of interfacial grains is that the change of grain size can affect the deformation mechanism. This study may provide a new design strategy of micro-scale stick-slip motion control by tailoring deformation mechanism of interfacial grains in metals.

Intrinsic and extrinsic effects on the fracture toughness of ductile metallic glasses

The potential application of metallic glasses (MGs) as structural materials demands accurate measurements of their toughness and the understanding of the underlying factors affecting it. Currently, it is challenging to precisely measure the toughness of MGs, especially ductile MGs. Measured toughness values are widely scattered, even for MGs with identical compositions. That is attributed to the combined effect of intrinsic and extrinsic factors including processing, sample geometries, and loading conditions. A fundamental understanding of the influences of these elements is thus of great significance. In the present study, molecular dynamics simulations are performed to investigate the influence of intrinsic and extrinsic effects on the fracture toughness of CuZr MGs. In particular, focused is placed on the effects of cooling rate, notch shape and size, strain rate, and temperature. The results indicate that the fracture toughness of a MG scales with the cooling rate used to prepare it. This is attributed to increased free volume content generated at high cooling rates, which enhances plastic deformation and amplifies the associated energy dissipation during plastic deformation events. Fracture toughness also increases with the strain rate, arguably due to strain rate-induced crack extension delay. Overall, the results demonstrate that the largest fracture toughness are achieved when MG samples are fabricated at high cooling rates and subjected to high strain rate deformation. In addition, results suggest that the fracture toughness decreases with increasing temperature, due to the significant decrease in strength. The correlations revealed between these crucial intrinsic and extrinsic parameters and the calculated MG fracture toughness support the development of a framework to understand the root of the discrepancies in the measurement of the toughness of MGs and provide insights into the design of tough MGs for structural applications.

Fracture behavior in electrically poled alkaline bismuth- and potassium- based lead-free piezoceramics using Vickers indentation

The fracture behavior of unpoled and electrically poled Ka0.5Na0.5NbO3 (KNN)- and Na1/2Bi1/2TiO3 (NBT)- based lead-free piezoceramics were characterized using the Vicker's indentation technique and compared with soft Pb(Zr,Ti)O3 piezoceramics. In the unpoled state, NBT-based piezoceramics demonstrate the largest fracture toughness in comparison with KNN-based and soft PZT piezoceramics due to its large intrinsic fracture toughness, which is in accordance with the previous reports. In the electrically poled state, KNN- and NBT- based piezoceramics exhibit large fracture toughness along the poling direction as opposed to perpendicular to the poling direction, with aspect ratios of 1.2 and 1.1, significantly smaller than that for PZT (1.8). The variations of the remanent strain and the process zone, which are related to the texture and the material properties during crack propagation were used to rationalize the results.

Thermal shock resistance of double-layer thermal barrier coatings

Abstract

Evolutionary characteristics of mode-I fracture toughness and fracture energy in granite from different burial depths under high-temperature effect

For urgent solutions to existing major engineering problems such as deep mining and deep burial of high level radioactive nuclear wastes, exploring the mechanical behaviors of rocks under the synergistic effect of the burial depth of rocks and temperature is of great significance. The mode-I fracture characteristics of granite specimens in the semi-circular bending (SCB) test under the effects of different temperatures and burial depths were experimental investigated. The test results showed that the mineral compositions and fracture characteristics of granite both varied significantly at different burial depths in Daliuhang Gold Mine in Yantai, Shandong Province, China. The high-temperature effect would remarkably weaken the influence of the burial depth on the fracture characteristics of granite; moreover, increasingly growing temperature gradually became a dominant factor influencing the fracture characteristics of granite. As the temperature rose from 20 °C to 900 °C, the peak strength, fracture toughness and fracture energy of granite at different burial depths all declined with the temperature rise. Additionally, a significant quadratic polynomial function relationship was found in temperature with peak strength and fracture toughness. Additionally, it was macroscopically found that the fracture energy linearly increased with the fracture toughness, implying that the specimens with larger fracture toughness need to consume more energy during the failure. However, the specific proportions and rates of energy consumption differed in different fracture stages of granite.

Transition from source- to stress-controlled plasticity in nanotwinned materials below a softening temperature

Nanotwinned materials have been widely studied as a promising class of nanostructured materials that exhibit an exceptional combination of high strength, good ductility, large fracture toughness, remarkable fatigue resistance, and creep stability. Recently, an apparent controversy has emerged with respect to how the strength of nanotwinned materials varies as the twin thickness is reduced. While a transition from hardening to softening was observed in nanotwinned Cu when the twin thickness is reduced below a critical value, continuous hardening was reported in nanotwinned ceramics and nanotwinned diamond. Here, by conducting atomistic simulations and developing a theoretical modeling of nanotwinned Pd and Cu systems, we discovered that there exists a softening temperature, below which the material hardens continuously as the twin thickness is reduced (as in nanotwinned ceramics and diamond), while above which the strength first increases and then decreases, exhibiting a maximum strength and a hardening to softening transition at a critical twin thickness (as in nanotwinned Cu). This important phenomenon has been attributed to a transition from source- to stress-controlled plasticity below the softening temperature, and suggests that different hardening behaviors may exist even in the same nanotwinned material depending on the temperature and that at a given temperature, different materials could exhibit different hardening behaviors depending on their softening temperature.

Experimental and numerical analysis of mode-I interlaminar fracture of composite pipes

A common industrial production process for composites is filament winding, widely used for the production of axially symmetric components. In these composite components, delamination is a predominant failure mechanism. The current work focuses on investigating experimentally the effect of the initial crack and fiber bridging length on the mode-I delamination resistance curve (R-curve) behavior of various double cantilever beam (DCB) specimens. For this purpose, the magnitudes of initiation and propagation fracture toughness (GIC-init and GIC-prop) and the compliance C are calculated. DCB specimens with a stacking sequence of [± 50]6 and various initial crack lengths of a0 = 33, 37, 59 and 70 mm are manufactured from a real composite pipe using filament winding process. In order to evaluate the critical energy release rate in mode-I, various fracture tests are conducted on these specimens. The greatest bridging zone length dues to good penetration of two adjacent layers of the delamination interface. Moreover, the results indicate that the fiber bridging length has a significant effect on the GIC and the largest value of fiber bridging causes a large fracture toughness value. Finally, numerical simulations are performed using finite element (FE) method and GIC-init measurements obtained experimentally are compared to the numerical findings. The numerical displacements and GIC-init, calculated from the numerical displacements, are found to be in good agreement compared to the experimental results.

Effect of Fluid Compressibility on Toughness-Dominated Hydraulic Fractures With Leakoff

Summary Hydraulic fracturing using supercritical carbon dioxide (CO2) has a recognized potential to grow in importance for unconventional oil and gas reservoirs. It is characterized by higher compressibility than traditional liquid-phase hydraulic-fracturing fluids. Motivated by the larger compressibility of supercritical CO2, this paper considers the problem of a hydraulic fracture in which a compressible fluid is injected at a constant rate to drive a hydraulic fracture in a permeable and brittle rock. The two cases of a plane-strain fracture and a penny-shaped fracture are considered. It is shown that for many practical cases, the formation has a large enough fracture toughness that the propagation is in a regime for which the pressure inside the hydraulic fracture can be treated as spatially uniform (“toughness dominated”). Both numerical simulations and analytical solutions for the relevant limiting regimes show that fluid compressibility affects fracture shape only at the very beginning period, which corresponds to the storage regime, and has little effect on fracture growth in the leakoff regime. Overall, because the transition from the storage regime to the leakoff regime is expected to often take place in a short time after the fracture starts propagating, the influence of compressibility in the storage regime is very brief and can be quickly ignored. Therefore, even relatively sizable fluid compressibility has almost no effect on fracture growth in the toughness-dominated regime when leakoff is taken into account.

In Situ Acoustic Diagnostics of Particle-Binder Interactions in Battery Electrodes

Summary The high phase-transformation strain developed upon intercalation in the host particles of a composite battery electrode affects the polymeric binder network mechanically, deteriorating the electrode cycling performance. Here, electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is used to demonstrate a new strain-accommodation mechanism, in high-strain NaFePO 4 /PVdF electrodes, via relaxation of the binder network surrounding the intercalation particles. Complete mechanical degradation of the polymer network occurs during long-term cycling of NaFePO 4 electrodes in aqueous solutions (hard and tough behavior). In contrast, in aprotic solutions, a softened binder easily accommodates the high transformation strain, ensuring excellent electrode cycling performance (soft and tough behavior). Quantification of the high-frequency viscoelastic properties of an operating composite electrode linked to the binder's fracture toughness ensures fast and facile screening of the optimal polymeric binder/electrolyte solution combinations. This methodology should be extremely important for optimization of cycling performance of Li-Si anodes undergoing huge volume changes during cycling.

High damage-tolerance bio-inspired ZL205A/SiC composites with a lamellar-interpenetrated structure

Novel bio-inspired ZL205A/SiC composites with a lamellar-interpenetrated structure were successfully fabricated via gas-pressure infiltration of a commercial ZL205A alloy into freeze-cast porous SiC scaffolds. The influences of initial ceramic content (20, 30 and 40vol%) on the microstructure and damage-tolerance behavior of the resultant composites as well as their toughening mechanism were investigated. With an increase in the ceramic content in the composite, the flexural strength and fracture toughness in the longitudinal direction gradually decreased; whereas, in the transverse direction they showed a first increase and then decrease but their elastic modulus increased. The composites displayed significant damage-tolerance anisotropy, with higher strength and toughness in the longitudinal direction (parallel to the lamellae) than that in the transverse direction (perpendicular to the lamellae). In the longitudinal bending, the ZL205A/20vol%SiC composite exhibited the maximum flexural strength (760MPa) and the largest fracture toughness (33.0MPam1/2). The excellent damage tolerance of the composites was attributed to multiple toughening mechanisms such as plastic deformation in the matrix alloy, crack blunting, deflection and branching, and uncracked-ligament bridging of the ductile alloy layers. However, in the transverse bending, the crack propagated along the ceramic layer in the composites and thus greatly weakened their damage-tolerance capability.