Crack Initiation Toughness Research Articles

The influence of circular cavity on the characteristics of dynamic crack propagation in rocks using a single notch triangle approach

We used a novel single-notch triangle (SNT) sample to conduct dynamic fracture tests and investigate the effect of a circular cavity on the dynamic fracture mechanisms that occur in fractured rock masses subjected to a single crack under dynamic loads. Dynamical loads were applied with a drop hammer weight device. A modified finite difference method code was developed to simulate the dynamic cracking behavior of SNT samples under the action of impact loading. Next, we calculated the maximum principal stress and circumferential stress around the circular cavity and investigated dynamic crack initiation toughness (DCIT) by applying both experimental and numerical approaches. We also investigated the mechanisms that occur in the circular cavity with regards to the initiation of dynamic cracking and the cracking behavior by applying numerical models. Analysis demonstrated that the circular cavity promotes cracking initiation behavior and that the most significant impact was detected at a horizontal offset distance of 15 mm. Furthermore, the crack initiation time and DCIT were used effectively to evaluate the dynamic stability of cracks. However, the DCIT is accurate parameter for investigating how a mass of fractured rock changes according to different dynamic loads. We found that the roughness of the fractured surface around the crack tip was more significant than that of the fractured surface behind the circular cavity; this demonstrated that the circular cavity enhanced the energy consumed during crack propagation.

Fracture Properties of Plastic-Steel Fiber High-Performance Concrete after High-Temperature Exposure

To study the fracture properties of plastic-steel fiber high-performance concrete at high temperatures, the load-mid-span displacement curve and load-opening displacement curve of plastic-steel fiber concrete were obtained through fracture test conducted before and after exposure to high temperature. By calculating the fracture parameters, fracture energy, and characteristic length of concrete, the relationship of fracture parameters, fracture energy, and characteristic length of concrete with temperature was comparatively analyzed against the experimental results obtained for plain concrete. According to the analytical results, the crack initiation load value, maximum load and crack initiation toughness of SFRC are significantly improved after high-temperature exposure, and the plastic-steel fiber reinforced concrete is effective in reducing the critical value of crack opening displacement for concrete. When the test temperature ranges from 200°C to 600°C, the anti-cracking effect of plastic-steel fiber on concrete is more significant relative to plain concrete. Given temperature rise, the plastic-steel fiber reinforced concrete shows decrease in brittleness but increase in toughness, with the level of fracture energy first increasing and then decreasing. Besides, there is only slight difference observed in characteristic length between plastic-steel fiber reinforced concrete and steel fiber reinforced concrete.

R-Curve Behavior of Polyhedral Oligomeric Silsesquioxane (POSS)–Epoxy Nanocomposites

Polyhedral oligomeric silsesquioxane (POSS) is a suitable nanoscale reinforcement for thermosetting polymers, such as epoxy resin systems in order to modify its mechanical, thermal and chemical properties. The inclusion of POSS in the epoxy resin at higher loading (greater than 1 wt.%); however, it introduces the ductility during the fracture behavior of these nanocomposites. Consequently, the J-integral is used to quantify the fracture behavior of these materials and characterize the crack growth resistance curve against stable crack growth. A range of nanocomposites is prepared by adding 0.5, 1, 3, 5, and 8 wt.% of glycidyl POSS into DGEBF epoxy resin cured with an amine-based curing agent. From fracture toughness experiments, the load-displacement result confirms that when the POSS reinforcement is greater than 1 wt.%, the fracture behavior of the nanocomposite changes from brittle to ductile. For both brittle and ductile nanocomposites, the addition of POSS molecules improves the crack initiation toughness. The development of POSS–POSS compliant domains, reported previously, is responsible for this change in the failure behavior. The fractured images of POSS–epoxy nanocomposites, obtained by using scanning electron microscopy, show that the increase in fracture resistance at higher values of POSS loading occurs due to the extensive shear yielding. Meanwhile, the increased fracture toughness at lower values of POSS loading occurs due to crack pinning and crack deflection.

The dual role of TRIP effect on ductility and toughness of a medium Mn steel

In this study, the tensile and fracture behaviors of a medium Mn steel fabricated by two thermomechanical processes, namely intercritical annealing (IA) and room-temperature quenching and partitioning (RT Q&P), were investigated. The IA steel consists of ultrafine-grained, fully recrystallized ferrite and austenite, while the RT Q&P steel is comprised of martensite matrix and retained austenite. The austenite in the IA steel is less stable due to its lower carbon content. High-resolution micro-digital image correlation (micro-DIC) results reveal moderate strain localization at boundaries in the RT Q&P steel. On the contrary, strain is highly localized in the austenite of the IA steel, making austenite transform into fresh martensite quickly and promoting the work hardening rate, i.e., transformation-induced plasticity (TRIP) effect. However, this intensive TRIP leads to premature decohesion at boundaries and sudden fracture without post-elongation. The J-integral-based resistance curves (J-R curve) were also measured for both steels. The crack-initiation toughness of the IA steel is 22% less due to its frequent martensite cracking and intergranular decohesion. In contrast, the hierarchical microstructure in the RT Q&P steel suppresses brittle fracture and enables significant crack tip blunting, resulting in a high fracture toughness. The present work illustrates the dual role of TRIP on ductility and fracture toughness, namely, intensive TRIP is useful for tensile strength and uniform elongation, but is harmful to fracture toughness, which contradicts the common belief that TRIP is beneficial for both ductility and toughness.

Experimental investigation of fracture process zone of rock in dynamic mode I fracturing and its effect on dynamic crack initiation toughness

Fracture process zone (FPZ) is critical to the applicability of linear fracture mechanics (LEFM) in rock fracturing, but little attention has been given to quantify the FPZ under dynamic loading condition in previous studies. This study presents the experimental results of notched semi-circular bending (NSCB) tests undertaken by split Hopkinson pressure bar (SHPB) to investigate the FPZ evolution for red sandstone under dynamic mode I fracturing and its effect on dynamic crack initiation toughness. A hybrid displacement–strain calibration method was proposed to characterize the FPZ with high-speed digital image correlation (DIC) technique. Results show that the FPZ initiation in dynamic fracturing is much earlier than that in quasi-static cases, and the FPZ length fully develops when crack initiation taking place at the pre-peak stage. The FPZ in dynamic fracturing is of a semi-elliptical shape with a width independent from the loading rate. The opening displacement exhibits a nonlinear decreasing trend along the FPZ boundary. LEFM theory underestimates the inherent dynamic crack initiation toughness, and the corrected dynamic crack initiation toughness was approximately 1.6 ∼ 2.1 times the uncorrected one within the loading rate ranging from 8.5 ∼ 78.2 GPa•m1/2/s in present study. The FPZ length in dynamic mode I fracturing lies in between the values predicted by Schmidt model and Irwin model, which indicates a non-linear cohesive stress with higher gradient near the FPZ tip is distributed along the FPZ.

Study on the dynamic fracture properties and size effect of concrete based on DIC technology

The fracture properties of concrete are highly influenced by the size effect and strain rate effect. To explore the size effect on the dynamic fracture properties of concrete, a fracture experiment on the three-point bending beam was carried out in this paper under four strain rates (10−5/s, 10−4/s, 10−3/s and 10−2/s) and three span to height ratios (2, 2.5 and 3). Then, the fracture characteristic parameters and mechanism were analyzed based on the experimental data combined with the digital image correlation (DIC) technology. The results indicate that the fracture properties of concrete have an obvious strain rate effect and size effect. The fracture load, fracture energy and crack initiation toughness all increase with an increasing strain rate, but decrease with an increasing span to height ratio. On the contrary, the unstable fracture toughness shows no obvious size effect or strain rate effect. Based on the DIC technology, the evolution of crack propagation length in different loading stages was calculated. It is shown that, under the strain rate of 10−5/s, 10−4/s, 10−3/s and 10−2/s, the rapid propagation of crack occurs in the period from Pre-70% to Pre-90%, from Peak load to Post-90%, from Post-90% to Post-80%, and from Post-90% to Post-80%, respectively. More specifically, the crack propagates most rapidly under the strain rate of 10−2/s. The rapid propagation period tends to be delayed with the increase of strain rate, and the strain rate effect is gradually weakened with the increase of span to height ratio. By carrying out in-depth research on the dynamic fracture failure mechanism of concrete, this paper provides reference for the future studies on the dynamic fracture properties of concrete.

Bioinspired PcBN/hBN fibrous monolithic ceramic: High-temperature crack resistance responses and self-lubricating performances

The high strength and toughness of natural materials are mainly determined by a combination of mechanisms operating at different length scales, which can be used as a strategy to reduce the intrinsic brittleness of ceramics. Inspired by the architectures of bamboo, the polycrystalline cubic boron nitride/hexagonal boron nitride (PcBN/hBN) fibrous monolithic ceramics with a long fiber arrangement structure was constructed with PcBN fiber cells and hBN cell boundaries, and its crack resistance responses and tribological performances were investigated. The composite ceramic failed in a non-brittle manner with the rising resistance curve (R-curve) behavior, which was attributed to multiscale crack effects in the hierarchical architecture. The maximum crack growth toughness was extremely high (approximately 21 MPa·m1/2), corresponding to a 270% increase over the crack initiation toughness. Excellent fracture resistance could be retained even above 1000 °C. Moreover, the composite ceramic exhibited low and stable friction coefficients (approximately 0.33) when paired with a Si3N4 pin at high temperature (1000 °C), owing to the lubrication function of hBN cell boundaries with weak van der Waals forces and a small amount of liquid B2O3 produced. As a result, a synergistic improvement of mechanical and tribological properties at high temperature (1000 °C) was realized by combining bionic structure and tribological design. It provides important theoretical and technical support for expanding the application of self-lubricating composite ceramics in harsh environments.

Fracture resistance of 3D nano-architected lattice materials

Exploiting small scale material effects and structural topology, nano-architected lattices represent a recent novel class of mechanical metamaterials, which exhibit unprecedented combination of mechanical properties. Together with scarce resistance to fracture and catastrophic failure, understanding of the fracture characteristics and properties of 3D nano-architected lattices still represents a limiting factor for the design and realization of future engineering applications. Here, using a combination of in-situ tensile fracture experiments and finite element simulations, we first show the possibility to reach stable crack growth in nano-architected materials harnessing only the intrinsic plastic toughening mechanism. Exploring the effect of lattice topology on the fracture properties, we then demonstrate similar performance between the octet and 3D kagome architecture (along one direction). Based on the experimental and numerical results, a power-scaling law of normalized crack initiation toughness with relative density ρ¯ (i.e., fraction of material per unit volume) KIC/σyL∝ρ¯1.11, ρ¯1.17−1.27 is exhibited by the octet and 3D kagome topology, respectively, given the yield strength σy and the unit cell size L. Owing to the combination of the parent material’s size effect and plasticity (3D-printed photo-resist polymer), the fracture initiation toughness (considering σy=27MPa) of our octet nano-architected lattices is ∼ 8 times that of previously realized macroscopic octet titanium structures. After crack initiation, the two architectures manifest rising (in average ∼ 18%) fracture resistance curves (i.e., R-curves), without catastrophic failure. In addition, we find that the fracture toughness of architected lattices, measured by means of compact tension specimens, seems not to be dependent on the sample’s thickness, forcing to re-think the plain strain toughness definition for this class of materials. Our results uncover the basic fracture characteristics of 3D architected materials exhibiting stable crack growth, providing insights for the design of light-weight, tough materials, with implications for future macro-scaled structural applications.

Bioinspired nacre-like 2024Al/B4C composites with high damage tolerance

The bio-inspired 2024Al/B4C composites with a laminate-reticular hierarchical architecture were constructed by squeeze casting of 2024Al into loose freeze-cast ceramic scaffolds. This pressurized infiltration process provided a clean and well-bonded interface without physical gaps. By regulating the initial suspension concentration (20, 25, 30 and 35 vol%), the effects of different ceramic content on the microstructure, damage-tolerance behavior and toughening mechanisms of the composites parallel and perpendicular to the ice-growth direction were investigated. The strength and toughness in the longitudinal direction were greater than that in the transverse direction. The 2024Al/20 vol% B4C composite in the longitudinal direction yielded the highest flexural strength of 658 MPa, crack-initiation toughness (KIc) of 18.4 MPa m1/2 and crack-growth toughness (KJc) of 27.5 MPa m1/2. The unique damage-tolerant properties were attributed to multiple toughening mechanisms, including crack deflection, branching and blunting, ductile-ligament bridging and multiple-crack propagation, as evidenced by the stable crack growth and rising R-curve behavior during fracture. The significantly decreased damage tolerance in the transverse direction was mainly due to inadequate toughening tools. On the other hand, both the flexural strength and fracture toughness reduced remarkably as the ceramic content increased. The 2024Al/35 vol% B4C composite fractured in a single-crack mode and the crack growth path was almost straight, showing a relatively low flexural strength (502 MPa) and crack-initiation toughness (9.1 MPa m1/2). The toughening mechanism was discussed in terms of the relationship between structural characteristics and cracking mode.

Investigation on model-I fracture toughness of sandstone with the structure of typical bedding inclination angles subjected to three-point bending

The fracture mechanics behaviors of rock with bedding plane structure have considerable influences on rock mass engineering, among which crack initiation toughness (KIini) and unstable fracture toughness (KIun) play a significant role. This work first adopts the classical equivalent elastic fracture theories from different perspectives to precisely determine KIun. Then the double-K fracture criterion (DKFC) and the double-G fracture criterion (DGFC) are employed to inversely compute KIini through cohesive fracture toughness (KIc). The experimental results indicate that the crucial strength and deformation parameters show obvious weakening trends as the bedding direction varies from horizontal to vertical for stratified sandstone under standard three-point bending with a loading rate of 0.0001 mm/s. Furthermore, there are little differences in KIun among three common calculation methods, including the ASTM standard, the Guinea proposed method, and the energy release rate. The average values of KIun are 8.20 MPa·mm1/2, 7.38 MPa·mm1/2, 4.76 MPa·mm1/2, and 4.49 MPa·mm1/2 for the tested specimens with bedded orientation angles 0°, 30°, 60°, and 90°, respectively. However, there is a remarkable difference between KIiniK evaluated by the DKFC and KIiniG evaluated by the DGFC, and two relationships are satisfied as KIiniK= 0.42KIun and KIiniG= 0.30KIun. Additionally, the applicability of the DKFC and the DGFC is validated by the acoustic emission (AE) technique. The comparisons manifest that the crack initiation toughness evaluated by the AE counts and cumulative AE counts is consistent with that of the DGFC. The leading significances for this study are to explore the application of various fracture mechanics theories to three-point bending specimens and provide fracture mechanics parameters for underground engineering and tunnel engineering.

Impact of test environment on the fracture resistance of cortical bone

Water is a crucial component of bone, affecting the interplay of collagen and minerals and contributing to bone's high strength and ductility. Dehydration has been shown to significantly effect osseous mechanical properties; however, studies comparing the effects of various dehydrating environments on fracture toughness of bone are scarce. Accordingly, the crack resistance curve (R-curve) behavior of human and sheep cortical bone was characterized in a bio-bath, in ambient pressure air, and in scanning electron microscopes (SEMs) under three different environmental conditions (water vapor pressure, air pressure, and high-vacuum). The aim of this work was to better understand the impact of test environment on both intrinsic and extrinsic toughening and hence crack initiation toughness, K0 and crack growth resistance, dK/dΔa. Results show significantly lower K0 values for samples that were tested inside SEMs combined with pronounced extrinsic toughening through microcracking and crack path deflections out of the mode I plane. Importantly, all three SEM test environments gave similar results, and thus it does not matter which type of SEM is used. Ex situ testing of hydrated samples revealed similar K0 for both environments but elevated crack growth resistance for testing in ambient air relative to the bio-bath. Our data reveals the experimental difficulties to directly observe microscale crack propagation in cortical bone that resembles the in vivo situation. Ex situ testing immersed in Hanks' Balanced Salt Solution (HBSS) with subsequent crack path analysis, while tedious, is thought to presents the most realistic picture of the in vivo structure-fracture property relations in biological tissue.

Experimental analysis of influence of pouring interval on fracture performance of concrete structures with cold joints

The pouring interval will cause cold joints in concrete components, which reduce the safety and reliability of structures. This study conducted a three-point-bending test on concrete members with cold joints under different pouring intervals, and analyzed the influence of different pouring intervals on the fracture performance of concrete members with cold joints from four aspects: the crack expansion width, fracture surface characteristics, P-CMOD curve, and fracture parameters of the double-K fracture model. The tests results indicate that the crack expansion width shrank as the pouring interval was extended, the expansion path gradually converged to the vicinity of the cold joint, and the roughness of the fracture surface and the value of the fractal dimension decreased with the extension of the pouring interval. The cohesion of the cold joints’ interface was the main reason for the difference in the fracture width and fracture surface characteristics. The failure mode of a concrete structure with cold joints changed from the step-wise cracking mode to the brittle cracking mode as the pouring interval increased. The cohesive toughness, crack initiation toughness, and instability toughness of the components decreased as the pouring interval was extended. The maximum decrease of the ultimate bearing capacity, cohesive toughness, and instability toughness of the components occurred after the pouring interval exceeded the initial setting time. The drop rates are 78%, 80%, 80%, respectively. The crack initiation toughness decreased linearly as the pouring interval was extended.

Experimental Investigation on Fracture Performance of Short Basalt Fiber Bundle Reinforced Concrete

In this paper, a notched three-point bending test is used to study the fracture performance of the short basalt fiber bundle reinforced concrete (SBFBRC). To compare and analyze the enhancement effect of different diameters and different content of basalt fiber bundles on the fracture performance of concrete, some groups are set up, and the P-CMOD curves of each group of specimens are measured, and the fracture toughness and fracture energy of each control group are calculated. The fracture toughness and fracture energy are two important fracture performance parameters to study the effect and law of the new basalt fiber bundles on the fracture performance of concrete. The research results show that the diameter and content of the new basalt fiber bundles have a certain effect on the fracture performance of concrete. With the increase of the content of basalt fiber bundles, the peak load, crack initiation toughness, instability toughness and fracture energy of SBFBRC are greatly improved compared with the benchmark group. When the fiber bundle diameter is 0.2 mm, the peak load increases by 69.5% compared with the reference group. The instability toughness reaches its maximum value at 0.2 mm diameter, which is 59.7% higher than the benchmark.

Nacre-like alumina composites reinforced by zirconia particles

Nacre-like alumina is a class of bio-inspired ceramic composite manufactured by field-assisted sintering of green bodies made primarily of alumina platelets with an anisotropic microstructure. Here we investigate the addition of zirconia particles to enhance the mechanical properties of the composite. The resulting structure is a nacre-like anisotropic structure which features deflection and reinforcement during crack propagation. Monoclinic zirconia has no impact on the mechanical properties of the composite while tetragonal zirconia improves its fracture resistance properties. Both types of zirconia seem to slow down grain growth during sintering. The addition of zirconia stabilised in the tetragonal phase is thus a good option to obtain a composite with a fine microstructure and higher mechanical properties than a standard nacre-like alumina, with a flexural strength of 626 ± 39 MPa and a crack initiation toughness of 6.1 ± 0.6 MPa.m0.5.

Constraint-corrected fracture mechanics analysis of nozzle crotch corners in pressurized water reactors

This paper presents fracture mechanics analysis results for various cracks located at pressurized water reactor pressure vessel nozzle crotch corners taking into consideration constraint effect. Technical documents such as the ASME B&PV Code, Sec.XI were reviewed and then a fracture mechanics analysis procedure was proposed for structural integrity assessment of various nozzle crotch corner cracks under normal operation conditions considering the constraint effect. Linear elastic fracture mechanics analysis was performed by conducting finite element analysis with the proposed analysis procedure. Based on the evaluation results, elastic-plastic fracture mechanics analysis taking into account the constraint effect was performed only for the axial surface crack of the reactor pressure vessel outlet nozzle with cladding. The fracture mechanics analysis result shows that only the axial surface crack in the reactor pressure vessel outlet nozzle has the stress intensity factor exceeding the low bound of upper-shelf fracture toughness irrespectively of considering the constraint effect. It is confirmed that the J-integral for the axial crack of the outlet nozzle does not exceed the ductile crack initiation toughness. Hence, it can be ensured that the structural integrity of all the cracks is maintained during the normal operation.

Dual-gradient structure leads to optimized combination of high fracture resistance and strength-ductility synergy with minimized final catastrophic failure

Nature-inspired gradients can be implemented in metallic materials to achieve a synergy of strength and ductility. However, due to the small (often microscale) size of the gradient structured samples, their fracture properties have remained relatively unexplored. By fabricating centimeter-sized gradient-structured pure nickel samples using direct-current electroplating technique, we demonstrate that a dual-gradient architecture in pure nickel, comprising grain-size transitions from coarse grains to nano grains and then back to coarse grains (CG→NG→CG), achieves an optimized combination of strength-ductility synergy and exceptional fracture resistance – a crack-initiation toughness exceeding 300 MPa m½ – while minimizing the problem of final unstable catastrophic failure. Significantly, this dual-gradient CG→NG→CG structure can effectively arrest any brittle fracture in the nano grains by inducing a stable rising R-curve with an enhanced crack-growth toughness exceeding 350 MPa m½. We believe that this dual-gradient CG→NG→CG structure provides a promising prototype for designing multi-layer graded structures with exceptional combinations of mechanical properties which can be readily tuned to meet the advanced requirements of safety-critical applications.

Investigation of tensile and fracture mechanical properties of bituminous coal at different strain rates

Coal is one of the most important energy sources in China, and bituminous coal accounts for 80% of China's total coal. However, there are still some deficiencies in the previous understanding of the failure and mechanical characteristics of bituminous materials under external loads of different strain rates (especially high strain rates). To investigate the tensile and fracture mechanical properties of bituminous coal at different strain rates, the GCTS multifunctional rock mechanical loading device and Hopkinson bar dynamic loading system were employed. The crack extension process and crack distribution after damage of bituminous coal specimens were also analyzed by high-speed camera and CT scanning instrument. The results show that the peak stress distribution of the Brazilian disc (BD) bituminous coal samples ranged from 3.5 MPa to 5 MPa. The main crack expansion path of the internal splitting is not straight due to the non-homogeneity of the coal sample itself. And the secondary cracks of the specimen are more developed and mostly located in the middle region with transverse spreading. The average peak load of the notched semicircular bending (NSCB) coal samples tends to decrease with increasing depth of the notch. In addition, dynamic crack initiation toughness of NSCB bituminous coal specimen decreases continuously as the initiation time increases. The relationship between fracture toughness and crack initiation time is an approximate exponential function. The normalized dynamic fracture toughness test value of bituminous coal is higher than that of steel material, but lower than that of granite.

Influence of circular hole defect on dynamic crack propagation behavior under impact loads

Cavities and crack defects usually coexist in deep earth rock mass structures, which together affect the structural safety and stability of rock masses. In order to study the effect of circular cavity on crack propagation behavior in rock mass under dynamic loads, circular opening specimens with straight crack cavity (COSSCC) specimen were proposed in this study, and a large-scale drop hammer impact device was applied to conduct impact tests. Crack propagation gauges were implemented to measure fracture mechanics parameters, such as dynamic crack initiation time and crack propagation velocity. Then a modified finite difference code Autodyn was applied to carry out the numerical simulation analysis of crack propagation path and stress field around the circular hole. The traditional finite element code Abaqus was also used to calculate the dynamic initiation toughness and dynamic propagation toughness. The results indicate that: (1) when the inclination θ of the circle hole is less than 10°, the crack propagation path deflects and passes through the surface of the circle hole; when the inclination θ of circle hole is 20° and 30°, the crack propagation paths deflects in the direction of the hole but does not pass through the circular hole, indicating that the circular hole has obvious guiding effect on crack propagation; when the inclination θ of circle hole is 40° and 50°, crack propagation path does not deflect, and the guiding effect of the circular hole is obvious weaken. (2) When the crack propagation path reaches the vicinity of the circular hole, the tensile stress zone at the crack tip coincides with the tensile stress zone at the edge of the circular hole. At this time, the crack propagation speed increases significantly, and the dynamic fracture toughness of the crack decreases significantly. (3) The deflection direction of the crack is basically the same as the direction of the maximum circumferential stress at the crack tip. (4) The dynamic fracture toughness of the crack is always smaller than the crack initiation toughness, and the dynamic fracture toughness of the crack has a linear relationship with the dynamic crack growth rate. The larger the dynamic crack growth rate, the smaller the dynamic fracture toughness of the crack.

V-notch crack toughness of PMMA by strain gages and its comparison with caustics results under impact loading

Strain gages and dynamic caustics were used in this experiment to measure the V-notch crack initiation toughness and propagation toughness of polymethyl methacrylate (PMMA) specimens. Furthermore, a method of pasting strain gages on V-notch PMMA specimens for three-point bending test was proposed. The specimens were divided into five groups according to the opening angle (ω), and each group corresponded to two pasting methods: acute-angle pasting method and obtuse-angle pasting method. The results of notch stress intensity factors (NSIFs) obtained by strain gage measurement were consistent with the results calculated using the dynamic caustics method. The concept of the dimensionless notch stress intensity factor (DNSIF) was introduced for a better analysis of the effect of opening angles on cracking. The experimental results revealed that the initiation of V-notch crack in the specimens were inhibited more when ω was larger than 90° than for small opening angles. Under this condition, the specimens displayed higher cracking initiation toughness. Furthermore, the time required for crack initiation was longer, and the cracking was instantaneous. An analysis of the vicinity of the V-notch crack tip showed that with the increase in ω, the stress component decreased, and the rate of decrease became higher. This also explains the phenomenon wherein the specimens transformed from being easy to crack to not being so as ω increased. The experimental results showed that the crack propagation velocity and propagation toughness were also affected by the opening angle of the V-notch crack.

High damage-tolerance bio-inspired B4C/2024Al composites with adjustable mechanical performance by tuning ceramic thickness

Freeze casting is a promising approach for assembling lamellar metal-ceramic composites with an exceptional combination of strength and toughness. Although these mechanical properties can be optimized by regulating the microstructure of porous ceramic structures, the effect of lamellar thickness is rarely mentioned, especially for B4C/Al composites. Herein, by controlling the freezing temperature, we used freeze casting to create nacre-like B4C scaffolds with identical ceramic content yet different lamellar thicknesses and then infiltrated them with 2024Al alloy. The effects of lamellar thickness on the damage-tolerance behavior and toughening mechanisms are discussed. The refinement of lamellae decreases the probability of observing catastrophic flaws in ceramic layers, increasing strength from 534 ± 14 to 578 ± 15 MPa and increasing crack-initiation toughness (KIc) from 9.2 ± 0.6 to 11.4 ± 0.2 MPa m1/2. These composites exhibit higher damage tolerance resulting from several toughening mechanisms, such as plastic deformation, crack deflection and blunting, and the uncracked-ligament bridging of ductile metal layers, which is reflected in the stable crack propagation during fracture and rising R-curve behavior. Importantly, coarsening of the structure of composites allows the fracture behaviors transform from single to multiple crack propagation, thus absorbing much more fracture energy, with the valid crack-growth toughness (KJc) enhancing markedly from 17.1 ± 2.4 to 29.8 ± 2.3 MPa m1/2.