Crack Trapping Research Articles

Structure property relations in hot pressed 4.4 mol% calcia stabilized zirconia

4.4 Ca-TZP was hot pressed in the temperature range between 1200 and 1325 °C for 1 h at 60 MPa pressure. The material was characterized with respect to phase composition, mechanical properties and microstructure. The mechanical properties and phase composition of the Ca-TZP react very sensitively to changes in sintering temperature and average grain size. Fracture toughness continuously increases with sintering temperature from 5 MPa√m to 8.5 MPa√m while bending strength reaches an optimum of 1160 MPa combined with fracture toughness of 5 MPa√m at 1250 °C sintering temperature. The microstructure of the materials is ultrafine. The onset of monoclinic and cubic phase formation is observed at a sintering temperature of 1300 °C and a grain size of ∼140 nm. Indentation at elevated temperature was applied to suppress crack trapping which would lead to extreme overestimation of indentation toughness values.

Multiscale crack trapping for programmable adhesives.

The precise control of crack propagation at bonded interfaces is crucial for smart adhesives with advanced performance. However, previous studies have primarily concentrated on either microscale or macroscale crack propagation. Here, we present a hybrid adhesive that integrates microarchitectures and macroscopic nonlinear cut architectures for unparalleled adhesion control. The integration of these architectural elements enables conformal attachment and simultaneous crack trapping across multiple scales for high capacity, enhancing adhesion by more than 70×, while facilitating crack propagation at the macroscale in specific directions for programmable release and reusability. As adhesion strength and directionality can be independently controlled at any location, skin adhesive patches are created that are breathable, nondamaging, and exceptionally strong and secure yet remove easily. These capabilities are demonstrated with a skin-mounted adhesive patch with integrated electronics that accurately detects human motion and wirelessly transmits signals, enabling real-time control of avatars in virtual reality applications.

A regularized variational mechanics theory for modeling the evolution of brittle crack networks in composite materials with sharp interfaces

In the design of structural materials, there is traditionally a tradeoff between achieving high strength and achieving high toughness. Nature offers creative solutions to this problem in the form of structural biomaterials (SBs), intelligent arrangements of mineral and organic phases which possess greater strength and toughness than the constituents. The micro-architecture of SBs like nacre and sea sponge spicules are characterized by weak organic interfaces between brittle mineral phases. To better understand the toughening mechanisms in SBs requires simulation techniques which can resolve arbitrary interface and bulk fracture patterns.In this work, we present a modified regularization of Variational Fracture Theory (VFT) that allows for simulation of fracture in materials and structures with weak interfaces. The core of our approach is to widen the weak interfaces on a length scale proportional to that of the diffuse damage field, and assign a reduced fracture toughness therein. We show that in 2D the modified regularized functionals Γ-converge to that for sharp cracks. The resulting thin weak interfaces have fracture toughness which depends on the bulk material fracture toughness, the widened interface fracture toughness, and the ratio of the widened interface length scale to the crack regularization length scale. We next apply our modified regularization within a computer implementation of regularized VFT, which we term RVFTI. We assess the performance of RVFTI in 2D by reproducing the effective interface fracture toughness predicted by the Γ-convergence theory and simulating crack trapping at a bi-material interface. We then use RVFTI to study toughening in SB-inspired microarchitectures, namely layered materials and materials with wavy interfaces.

Toughening of intrinsically brittle materials by inserting arrays of voids

The aim of the current paper is to find optimum arrangements of voids that are able to catch and trap all possible cracks that might originate from a free surface. The idea is to investigate whether it is possible to apply this crack trapping mechanism to enhance the fracture toughness of intrinsically brittle materials. In doing so, the void volume shall be as small as possible. The investigation applies the crack trajectory interpolation (CTI) method, which is a new, computationally efficient approach to approximate crack trajectories near a single void or stiff particle. Under the assumption of linear superposition, the CTI-method is extended to predict the crack trajectories near arrangements of voids. This is done for uniaxial and biaxial loading conditions. The investigation shows that a symmetrical arrangement of voids should be avoided, since cracks near the symmetry line can bypass the array. Staggered void arrangements provide better conditions for crack trapping. By using elliptical voids with an aspect ratio of 2 instead of circular voids, it is possible to achieve a higher crack trapping efficiency and a lower void volume. A damage-based approach is used to determine the improvements in fracture toughness due to crack trapping for various void arrangements. The Python code of the CTI method is presented in the Appendix of this work.

Asymmetric toughening in the lap shear of metamaterial structural adhesives.

Metamaterial structural adhesives (MSAs), whose properties primarily rely on structural design, offer promising advantages over traditional adhesives, including asymmetric, switchable, and programmable adhesion. However, the effects of thick backing structures on the adhesion properties remain largely underexplored. Herein, we investigate a series of MSAs featuring a thin adhesive layer and an asymmetric thick beam structure terminated with a film. We conduct lap shear tests on the MSAs with varying terminated film thickness (t) and beam tilting angle (θ) while maintaining an identical adhesive layer. For MSAs with a thick terminated film (t = 2 mm), the effective adhesion energy is double that of solid samples without compromising shear strength, consistent with the theoretical predictions based on the crack trapping mechanism. Conversely, for MSAs with a thin terminated film (t = 0.5 mm), the maximum shear strength and effective adhesion energy are ∼2.8 times and ∼18.6 times those of solid samples, respectively, deviating significantly from the theoretical predictions due to new crack initiations. We further explore adhesion asymmetry by tuning the beam tilting angle (θ). For MSAs with highly tilted beams (θ = 70.3°), we achieve a maximum adhesion strength asymmetry factor of τ2/τ1 ∼ 2.2 for a thick terminated film (t = 2 mm), and a maximum adhesion energy asymmetry factor of Γ1/Γ2 ∼ 5.3 for a thin terminated film (t = 0.5 mm). Our work provides useful insights for designing metamaterial structural adhesives suitable for robotic grippers, wall-climbing robots, and wearable devices, particularly those requiring asymmetric, switchable, and stimuli-responsive adhesion, and adhesives on rough surfaces or in underwater conditions.

Fracture toughness improvement due to crack deflection and crack trapping by elliptical voids or particles

The paper investigates how voids and particles can be used to deflect and arrest cracks in order to enhance the failure stress and the fracture toughness of intrinsically brittle materials. This is done by investigating crack propagation near single elliptical voids and particles that are located in an elastic matrix material. The shapes of the inhomogeneities are varied, but their cross section is held constant. Uniaxial and biaxial loading conditions are considered and also the Young’s modulus ratio between particle and matrix material is varied. The crack trajectories near the material inhomogeneities are evaluated by applying a computationally very efficient approach, the crack trajectory interpolation (CTI) method, which has been introduced in an earlier work of the authors. With this method, critical distances are determined where an approaching crack runs into an elliptical void and gets trapped. Similarily, zones for crack arrest and crack deflection near stiff particles are found. Combinations of particles and voids are studied, too. A damage-based approach is used to investigate the improvements in failure stress and fracture toughness when a crack runs into a void. The results show that voids and particles that are elongated perpendicular to the crack plane are most useful for crack arrest. The trapping distances increase with increasing ellipticity of the inhomogeneities. However, the improvement in failure stress is highest for voids with moderate ellipticity. The results of this paper will be used to find optimum arrangements of voids or particle-void combinations for the design of structural components with increased fracture toughness.

Crack mediated dislocation activities in Al/Ti nanolayered composites: an atomistic study

In this work, to understand crack propagation in Al/Ti nanolayered composites, a series of molecular dynamic simulations were performed with crack in different layers of the nanolayered composites and subjected to mode I loading. Nanolayered composite with a crack in Al layer, and monolithic Al show ductile fracture behavior that occurs by nucleation of Shockley partial dislocation at the crack tip. On the other hand, the fracture behavior in nanolayered composites with a crack in Ti shows crack bowing which is similar to the brittle fracture, and subsequent crack trapping at the interface. However, monolithic Ti shows typical cleavage fracture followed by activation of basal and pyramidal slip that blunts the crack leading to ductile fracture. When the crack is in the Ti layer, the other Ti layers in a nanolayered composite deform by prismatic and pyramidal slip. However, the Ti layer deforms only via slip on prismatic planes when the crack is in the Al layer. Critical strain energy release rate G c based continuum analysis predicts the fracture mode in monolithic Ti correctly, but it fails to predict the fracture mode in monolithic Al and nanolayered composites with crack in the Al layer. It is found that the G c determined based on external loading is marginally higher when the crack is in the Al layer as compared against the case when the crack is in the Ti layer. The G c value for the basal and pyramidal slip in Ti is higher than the G c value for cleavage. This poses an interesting phenomenon since the G c in monolithic Al is found to be much lower than that of monolithic Ti. The reason is attributed to the constrained plasticity in the presence of an Al/Ti interface.

Phase Continuity, Brittle to Ductile Transition Temperature, and Creep Behavior of a Eutectic Mo–20Si–52.8Ti Alloy

Mo–Si–Ti alloys, like eutectic Mo–20Si–52.8Ti (at%), have previously been intensely investigated, owing to their excellent oxidation and creep resistance. To better understand high‐temperature mechanical behavior, a holistic assessment of microstructural features is necessary. Correspondingly, 3D‐focused ion beam tomography is carried out in Mo–20Si–52.8Ti. The results indicate a severely interconnected network of Mo solid solution (MoSS) and intermetallic (Ti,Mo)5Si3. Both phases retain similar network connectivity, lamellar sizes, etc. The brittle to ductile transition temperature (BDTT) is then determined through a series of bending tests and interpreted using the microstructural information. The BDTT is found to be ≈1100–1150 °C, different from Mo–9Si–8B with a continuous MoSS network. The BDTT is an immediate consequence of the continuous network of both MoSS and (Ti,Mo)5Si3. The MoSS network is instrumental in crack trapping and bridging, indicating that the present phase distribution maximized the mechanical performance over (Ti,Mo)5Si3. Having determined the network microstructure and BDTT, tensile creep behavior is evaluated and compared to previously published compressive creep results. The results show consistency in terms of strain rate, stress exponent, and microstructural features indicating a reliably good creep resistance for the network microstructure of Mo–20Si–52.8Ti regardless of loading direction.

Double-stage hardening behavior of a lightweight eutectic high entropy alloy in the course of low cycle fatigue

A unique crack growth behavior was detected during room temperature cyclic loading of a eutectic high entropy alloy, which caused the occurrence of two-stage cyclic hardening under the low cycle regime in tensile and compressive half cycles. This was attributed to the crack trapping between the dendritic branch, which caused retardation of the growth stage until the crack sheared and pass through the dendrite. The mechanical fragmentation of the dendrite and distribution through the microstructure could effectively resist crack propagation. This led to higher stress levels during cyclic loading compared with monotonic loading under both tensile and compressive modes of deformation.

Snap-back instability of double cantilever beam with bridging

Adhesive bonding community shows a continued interest in using bridging mechanisms to toughen the interface of secondary bonded joints, especially in the case of laminated composites. Due to snap-back instability that occurs during fracture, confusions may exist when identifying the toughening effect experimentally. The true toughening effect may be overestimated by lumping all energy contributions (kinetic energy included) in an overall effective toughness. Here, fundamentals for bridging to enhance fracture resistance are explored through the theoretical analysis of the delamination of a composite double cantilever beam (DCB) with bridging. Specifically, we establish a theoretical framework on the basis of Timoshenko beam theory and linear elastic fracture mechanics to solve the fracture response of DCB in the presence of discrete bridging phases. We elucidate the crack trapping and the snap-back instability in structural response during the crack propagation. We identify the contribution to the overall toughness observed numerically/experimentally of both the physical fracture energy and other types of dissipation. The associated toughening mechanisms are then unveiled. Furthermore, we study the effects of property of the bridging phases on the snap-back instability, based on which, we propose a dimensionless quantity that can be deployed as an indicator of the intensity of snap-back instability. Finally, we identify the role of geometrical properties, i.e. the substrate thickness and the arrangement spacing of the bridging phases, in the snap-back instability and the macroscopic fracture toughness of a DCB. This work provides, from a theoretical point of view, an essential insight into the physics related to the structural response of DCB with discrete toughening elements.

Molecular dynamics simulations of dopant effects on lattice trapping of cracks in Ni matrix* *Project supported by the National Natural Science Foundation of China (Grant No. 11604237), the Natural Science Foundation of Hebei Province of China (Grant Nos. E2019105073 and E2015105079), the Scientific Research Foundation of Tangshan Normal University of China (Grant Nos. 2016A06 and 2020C03), and the Department of

Molecular dynamic analysis was performed on pure and doped (by Re, Ru, Co or W) Ni at 300 K using the embedded-atom-method (EAM) potentials to understand the crack formation of the doped Ni matrix in the (010)[001] orientation. When Ni was doped with Re, Ru, and W, the matrix demonstrated increased lattice trapping limits and, as a result, improved the mechanical properties. Consequently, this prevented the bond breakage at the crack tips and promoted crack healing. The average atomic and surface energy values increased when Re, Ru, and W were added. Analysis of these energy increase helpedus to understand the influence these elements had on the lattice trapping limits. The fracture strength of the Ni matrixat 300 K increased because of the formation of the stronger Ni–Re, Ni–Ru, and Ni–W bonds. At the same time, doping the Ni matrix with Co did not demonstrate any strengthening effects because of the formation of Co–Ni bonds, which are weaker than the Ni–Ni bonds. Out of all dopants tested in this work, Ni doping with W showed the best results.

Enhanced fracture resistance of thermoset/thermoplastic interfaces through crack trapping in a morphology gradient

The hybrid assembling of thermoplastics with thermosets through controlled interdiffusion is an attractive approach towards integrated structural composite bonding. The fracture toughness of the interphase between the high Tg thermoplastic polyetherimide and the RTM6 epoxy is found to be four times higher than the corresponding RTM6 value under specific curing cycle. The origin of this remarkable cracking resistance is elucidated by analyzing the crack path in relationship with the morphology and local elastoplastic and fracture properties of the interphase. The crack is trapped inside a morphological gradient involving a relatively low PEI concentration. Uniaxial compression and nanoindentation tests on homogenous blends exhibit an unexpected softening in this PEI content window. This efficient crack trapping mechanism originates thus from a synergistic coupling between a lower local strength and higher local fracture toughness. This finding opens to many new options for interface toughening beyond the system addressed in this research.

The Role of Hydrogen on the Behavior of Intergranular Cracks in Bicrystalline α-Fe Nanowires.

Hydrogen embrittlement (HE) has been extensively studied in bulk materials. However, little is known about the role of H on the plastic deformation and fracture mechanisms of nanoscale materials such as nanowires. In this study, molecular dynamics simulations are employed to study the influence of H segregation on the behavior of intergranular cracks in bicrystalline α-Fe nanowires. The results demonstrate that segregated H atoms have weak embrittling effects on the predicted ductile cracks along the GBs, but favor the cleavage process of intergranular cracks in the theoretically brittle directions. Furthermore, it is revealed that cyclic loading can promote the H accumulation into the GB region ahead of the crack tip and overcome crack trapping, thus inducing a ductile-to-brittle transformation. This information will deepen our understanding on the experimentally-observed H-assisted brittle cleavage failure and have implications for designing new nanocrystalline materials with high resistance to HE.

Decoupling Microstructural and Residual Stress Effects on Glass-Ceramic Toughening

Barium disilicate (BaO.2SiO 2 = BS2) is one of the very few stoichiometric glasses that allows for accurate control of the desired microstructure via thermal treatment, making it a (scarcely studied) model system for microstructure-property studies. Here, we performed a systematic research work on the variation in hardness, elastic modulus, fracture strength, and toughness as a function of the crystalline volumetric fraction, crystal size, and residual stresses in BS2 glass-ceramics (GC). These microstructural features were independently modified; the average crystal diameter varied from 5 to 100 μm and the crystallized volumetric fraction from 0 to 68%. The internal residual stresses in the crystals, are tensile in this system . Samples with an average spherulite size above 30 µm showed spontaneously fractured crystals due to the residual stresses. The samples with 5 and 10 µm spherulites have not cracked because the spherulites have a high volume fraction of residual glass (74%) and moderate internal stresses (40-70 GPa). The fracture toughness, K IC , increased with the spherulite size and volume fraction. However, the residual glass inside the spherulites rendered crack bowing, bridging, and trapping ineffective. The variation of K IC with the crystallized volume fraction is similar for GCs with different crystal sizes. Also, a comparison with a lithium silicate glass-ceramic showing compressive residual stresses yielded similar results. These combined findings indicate that the crystallization of a tougher phase is the crucial parameter controlling fracture toughness in these materials. The results with this model material can be extended to design novel strong and tough glass-ceramics.

Optimizing the microstructure and mechanical behaviors of bimodal Mo-Si-B alloy by heat treatment

Previous studies revealed that developing bi-modally structured α-Mo grains in the fine-grained Mo-12Si-8.5B-0.57 wt% La2O3 alloy could improve room-temperature fracture toughness without sacrificing strength dramatically. However, the toughness is expected to be enhanced further for engineering application of alloy. Here, the optimization of the bimodal α-Mo microstructure was achieved by annealing treatment at 1700–1800 °C. After annealing, the semi-continuous or isolated α-Mo colonies were transformed into the continuous α-Mo matrix meanwhile the volume fraction of α-Mo was improved slightly. Besides, the fine-grained and coarse-grained regions were coarsened concurrently and the distribution of them became more homogenous. The 1800 °C-annealed bimodal alloy exhibited an optimal microstructure consisting of dispersive Mo3Si/Mo5SiB2 (~1.72–2.00 μm) particles distributed in a continuous bimodal fine-grained (~2.03 μm)/coarse-grained (~6.36 μm) α-Mo matrix. Also, this alloy presented a significantly improved fracture toughness (13.41 MPa·m1/2) and high compression strength (2682 MPa) and hardness (841 HV) simultaneously. Toughening mainly originated from crack trapping in the coarsening bimodal α-Mo structure and forming intragranular micro-cracks in the coarsening α-Mo and intermetallic phases induced by La2O3. High compression strength was attributed to the coarseness of α-Mo phase improving the plastic deformation capacity of alloy.

Effects of Zirconium and Yttrium Oxide on Mechanical and Oxidation Properties of Mo–3Si–1B–1Zr–1Y2O3 (wt.%) Alloy

Mo–3Si–1B alloys with zirconium (1 wt.%) and yttrium oxide (1 wt.%) additives were fabricated by vibrating sintering techniques. The doped Mo–3Si–1B alloys consisted mainly of α-Mo, Mo3Si, and Mo5SiB2 (T2) phases. It was found that the grains were reduced, and the intermetallics particles were dispersed more homogeneously after the addition of Zr and Y2O3. The optimization in microstructure induced corresponding improvements in both fracture toughness and oxidation resistance. The predominant strengthening mechanisms were fine-grain strengthening and particle dispersion strengthening. In addition, fracture toughness test showed that the additions could improve the toughness of Mo–3Si–1B alloys, for which the toughening mechanism involved a crack trapping by α-Mo phases and extensive small second phase particles in the alloys. What should be paid attention to is the satisfactory oxidation resistance, both at medium-low temperature (800 °C) and high temperature (1200 °C) with doped additives.

Hierarchical levels of organization of the Brazil nut mesocarp

Aiming to understand Nature´s strategies that inspire new composite materials, the hierarchical levels of organization of the Brazil nut (Bertholletia excelsa) mesocarp were investigated. Optical microscopy, scanning electron microscopy (SEM), microtomography (MicroCT) and small-angle X-ray scattering (SAXS) were used to deeply describe the cellular and fibrillary levels of organization. The mesocarp is the middle layer of the fruit which has developed several strategies to avoid its opening and protect its seed. Fibers have a different orientation in the three layers of the mesocarp, what reduces the anisotropy of the structure. Sclereids cells with thick cell walls fill the spaces between the fibers resembling a foam-filled structural composite. The mesocarp has several tubular channels and fractured surfaces which may work as sites for crack trapping and increase toughness. The thick and lignified cell wall of sclereids and fibers and the weak interface between cells can promote a longer and tortuous intercellular crack path. Additionally, fibers with high strength and stiffness due to microfibrils oriented along the main cell axis (µ = 0° to 17°) were identified in the innermost layer of the mesocarp. Such an understanding of each hierarchical level can inspire the development of new cellular composites with improved mechanical behavior

Guiding and Trapping Cracks With Compliant Inclusions for Enhancing Toughness of Brittle Composite Materials

Abstract The problem of toughening heterogeneous materials with a stiff matrix and compliant inclusions is investigated through numerical simulations and experiments. Specifically, the problem of optimizing a combination of effective toughness and effective elastic modulus in the context of a square array of compliant inclusions in a stiff matrix is explored. Crack propagation in the heterogeneous material is simulated using a variational phase-field approach. It is found that the crack can meander between or get attracted to and trapped in the inclusions. Composite specimens with a stiff matrix and compliant circular inclusions were 3D printed, and their fracture toughness was measured using a specially designed loading fixture. The experimental results show agreement with the numerical predictions by demonstrating the attraction and trapping of cracks in the inclusions. This study demonstrates the potential for significant enhancement of toughness through elastic compliance contrast between the matrix and the inclusion without notably compromising the effective elastic modulus of the composite material.

Enhanced fracture toughness and toughening mechanisms of fine-grained Mo–12Si–8.5B alloy with a bi-modally structured α-Mo grain

Fine-grained Mo–Si–B alloys possess superior strength and oxidation resistance at high temperature, but they usually exhibit poor toughness at room temperature. Improvement of toughness is critical to the practical application of alloys. Here, a series of fine-grained Mo–12Si-8.5B-0.57 wt% La2O3 alloys with bi-modally structured α-Mo grains were fabricated by hot pressing sintering of a mixture of ball milled Mo–Si–B–La2O3 powders and unmilled Mo–La2O3 powders in select proportion. When the mass fraction of unmilled Mo–La2O3 powders was 15%, the sintered alloy showed an optimal microstructure consisting of a continuous bimodal fine-grained (~0.62 μm)/coarse-grained (~3.06 μm) α-Mo matrix with embedded Mo3Si/Mo5SiB2 (~0.52–0.54 μm) particles. This bimodal-structural alloy displayed high yield strength (2460 MPa) and compression strength (2561 MPa) together with significant enhanced fracture toughness (12.5 MPa m1/2). The high strength resulted from the contributions of fine α-Mo grains and nanometer La2O3 particles, meanwhile the improved toughness might be attributed to (1) coarse-grained α-Mo regions promoting crack trapping, (2) the formation of intragranular microcracks within the coarse-grained α-Mo regions induced by La2O3, (3) occurrence of the α-Mo/intermetallic interface debonding.

High toughness and strength of Mo-12Si-8.5B-ZrB2 alloy resulting from a bimodal α-Mo grain structure

Designing a bimodal α-Mo grain structure in Mo-12Si-8.5B-ZrB2 alloy was achieved via mechanical alloying followed by hot pressing. This bimodal structure consisted of a major of nano-scaled α-Mo grains and partial micron-scaled α-Mo grains. The effective cooperation of these fine and coarse α-Mo in the Mo-12Si-8.5B-ZrB2 alloy was very beneficial for improving its toughness as well as strength. The high toughness was mainly attributed to the in-situ toughening and crack trapping effects originating from the coarse α-Mo. While the high strength was owing much to the grain boundary strengthening of the fine α-Mo. When the volume fraction ratio of the fine and coarse α-Mo grains was approximately 6:1, the fracture toughness value reached 13.1 MPa·m1/2 in the bimodal alloy, showing a 18% increase compared to an ultrafine-grained alloy. Simultaneously, a high compressive strength with 2998 MPa was also maintained.