Crack Blunting Research Articles

Ultra-fast high-temperature synthesis and densification of high-entropy diborides and diboride-carbide ceramics

The strong covalent bonds and low self-diffusion coefficients of high-entropy diboride (HEBs) and diboride-carbide (HEB-HEC) ceramics render their densification difficult, which limits their applications. Here, we used an ultra-fast high-temperature sintering technique to synthesize and densify several HEBs and HEB-HEC (containing 5–8 cations) ceramics within an isothermal duration of 1 min and a total sintering period of 6 min. All ceramics formed a single-phase or dual-phase high-entropy solid solution, and showed high density, high hardness and good fracture toughness. The rapid densification mechanisms were determined, and the preferential dissolution of metal cations in HEB and HEC phases was explained using a simplified ideal solution model. A good fracture toughness in the range of 3.6-5.4 MPa·m1/2 resulted from crack deflection or blunting and bridging of the rod-like diboride or boron-rich phase. This paper presents a simple, economical, and efficient method for the synthesis and densification of high-entropy ceramics and other advanced materials.

3D Highly Stretchable Liquid Metal/Elastomer Composites with Strain‐Enhanced Conductivity

AbstractCurrent stretchable conductors, often composed of elastomeric composites infused with rigid conductive fillers, suffer from limited stretchability and durability, and declined conductivity with stretching. These limitations hinder their potential applications as essential components such as interconnects, sensors, and actuators in stretchable electronics and soft machines. In this context, an innovative elastomeric composite that incorporates a 3D network of liquid metal (LM), offering exceptional stretchability, durability, and conductivity, is introduced. The mechanics model elucidates how the interconnected 3DLM architecture imparts softness and stretchability to the composites, allowing them to withstand tensile strains of up to 500% without rupture. The relatively low surface‐to‐volume ratio of the 3DLM network limits the reforming of the oxide layer during cyclic stretch, thereby contributing to low permanent strain and enhanced durability. Additionally, the 3D architecture facilitates crack blunting and stress delocalization, elevating fracture resistance, while simultaneously establishing continuous conductive pathways that result in high conductivity. Notably, the conductivity of the 3DLM composite increases with strain during substantial stretching, highlighting its strain‐enhanced conductivity. In comparison to other LM‐based composites featuring 0D LM droplets, the 3DLM composite stands out with superior properties.

Delayed fracture caused by time-dependent damage in PDMS

An experimental and theoretical study of delayed fracture of polydimethlsiloxane (PDMS) is presented. Previous works have demonstrated that delayed fracture in single edge notch specimens is caused by time dependent damage due to chain scission. Here we study the interactions between damage and the elastic field using different specimens and crack geometries with blunt and sharp cracks. Our experiments show that initial toughness is not well defined, as stable slow crack growth can occur over a range of applied loads. Our experiments demonstrate that there is a universal relation between crack growth rate and applied energy release rate. A model coupling the nonlinear elastic deformation and rate dependent bond scission is proposed and is in good agreement with experimental data.

Descriptor for slip-induced crack blunting in refractory ceramics

Descriptor for slip-induced crack blunting in refractory ceramics

Influence of reverted austenite on fatigue life of 18 % nickel maraging steel

AbstractMaraging steels containing 18 % nickel are based on a low‐carbon iron‐nickel‐cobalt‐molybdenum system. These steels demonstrate a remarkable combination of high strength and high toughness. To investigate the effect of overaging and resulting austenite reversion on the fatigue behavior of C250 maraging steel, specimens in the solution annealed state were overaged at 510 °C for various intervals, resulting in volume fractions of reverted austenite ranging from 2.6 % to 11.4 %. The staircase method was used to calculate the fatigue limit. S−N curves were generated by testing a minimum of four stress levels and at least three samples at each stress level. Basquin's equation was used to fit the experimental stress‐life data, and the Basquin exponent and fatigue strength coefficient were calculated. The presence of 2.6 % reverted austenite in the microstructure improved the fatigue limit and fatigue ratio, without significantly reducing tensile strength. The results can be interpreted in terms of reverted austenite having a crack blunting effect on the propagating cracks. A small amount of reverted austenite in the range of 2 %–3 % vol has a beneficial effect on the fatigue life.

Verification of stress transfer mechanism in fatigue crack growth under overload

Cyclic plastic finite element modeling was carried out to investigate effect of overload on fatigue crack growth under blunting and no-blunting mechanisms at different overload ratios. The stress transfer mechanism was verified and was found more obvious in the case of crack blunting though its effect was time-limited. The level of crack closure varied periodically due to gradual relaxation of compressive residual stress during crack propagation. Both the overload plastic zone and the overload affected zone were linearly correlated with overload ratio, and an overload ratio value of 1.1 was the best choice for studying overload effect.

Nanoscale ductile fracture and associated atomistic mechanisms in a body-centered cubic refractory metal

Understanding the competing modes of brittle versus ductile fracture is critical for preventing the failure of body-centered cubic (BCC) refractory metals. Despite decades of intensive investigations, the nanoscale fracture processes and associated atomistic mechanisms in BCC metals remain elusive due to insufficient atomic-scale experimental evidence. Here, we perform in situ atomic-resolution observations of nanoscale fracture in single crystals of BCC Mo. The crack growth process involves the nucleation, motion, and interaction of dislocations on multiple 1/2 < 111 > {110} slip systems at the crack tip. These dislocation activities give rise to an alternating sequence of crack-tip plastic shearing, resulting in crack blunting, and local separation normal to the crack plane, leading to crack extension and sharpening. Atomistic simulations reveal the effects of temperature and strain rate on these alternating processes of crack growth, providing insights into the dislocation-mediated mechanisms of the ductile to brittle transition in BCC refractory metals.

Role of gradient heterogeneous interfaces in synergetic strengthening and toughening of layered CuCrZr/Cu composites

Layered CuCrZr/Cu composites with different densities of gradient heterogeneous interfaces were designed and prepared by powder metallurgy combined with rolling and aging. The prepared layered CuCrZr/Cu composites achieved better comprehensive properties than the homogeneous CuCrZr alloy. The evolution of local strain was analyzed by high resolution digital image correlation (DIC). A strain-transfer from CuCrZr layer to adjacent Cu layers appears at the gradient heterogeneous interface, which relieves the strain localization of CuCrZr layers in layered CuCrZr/Cu composites. The incompatible deformation between the gradient heterogeneous microstructures across the layer interface leads to the generation and accumulation of geometrically necessary dislocations (GNDs), which results in extra work hardening and hetero-deformation induced (HDI) strengthening. The increase in the density of gradient heterogeneous interfaces can enhance the interface constraint behavior and produce a greater HDI strengthening effect, which tends to improve the work hardening ability and HDI stress of layered CuCrZr/Cu composites. Furthermore, various crack propagation modes of crack deflection, multiple cracks and crack blunting caused by uneven stress/strain partition at the gradient heterogeneous interface of layered CuCrZr/Cu composites were characterized by in-situ tensile tests.

A coupled diffusional-mechanical model accounting for hydrogen enhancements of strain-induced dislocations and vacancies

Informed by the current understanding of hydrogen-dislocations/vacancies interactions, a coupled diffusional-mechanical model accounting for hydrogen-enhanced strain-induced dislocations and vacancies is developed. This model is applied to capture hydrogen diffusion and trapping in the uniaxially tensioned column and type-I loaded crack specimens of nickel alloy, with special attentions paid to the influence of hydrogen-vacancies interaction on them, which is seldom discussed in previous studies. The results show that the hydrogen-enhanced hardening rate and the hydrogen-promoted vacancy enrichment for the nickel alloys observed in experiments can be addressed well by the present model. The strain-induced vacancies rather than dislocations dominate the hydrogen trapping behavior, due to their higher hydrogen binding energies for the nickel alloy. The hydrogen-saturated state of vacancies can heavily impact hydrogen diffusion/trapping, which could be characterized by the effective trap concentration. The supersaturated vacancies caused by hydrogen-enhanced strain-induced vacancies can promote void growth through vacancy condensation, thus accelerating plastic localization or interface rupture. For the type-I loaded blunt crack specimen, the hydrogen-enhanced dislocation multiplication contributes to intergranular cracking in the pressure valley, while hydrogen-enhanced vacancy generation facilitates void growth in the plastic zone, both of which can exacerbate plastic localization ahead of crack tip. A quantitative model for plasticity localization that accounts for the influences of hydrogen-induced IG cracking and void growth would be helpful in physically based modeling of hydrogen embrittlement.

Architecture design and significant toughness enhancement of Ti6Al4V based composites with gradient laminate distributed TiB

Titanium Matrix Composites (TMCs) are well-known for their high specific strength and elastic modulus, however short of toughness. In the present work, in order to achieve better toughness, TiB/Ti6Al4V composites with a novel multiscale gradient-laminate network structure were fabricated via powder metallurgy method combining powder stacking process. The composites exhibit a laminated structure with gradient volume fraction of TiB on the macroscopic scale and a network structure of TiB distribution on the microscopic scale. Three-point bending tests were carried out to reveal the mechanical properties of the composites. The bending strength and fracture strain achieve 1950 MPa and 0.097 in the gradient-laminated composites, which were improved significantly compared to those in the monolithic composites. The bending toughness (148.3 × 103 kJ·m-3, represented by work of fracture during the three-point bending test) of the gradient-laminated composites is about 70% higher than that of the composites without laminated architecture. Fractography suggests that the crack deflection and prolonged crack propagation path contribute to the increase of fracture strain. The crack blunting, network microcracks and interface cracks are also beneficial to improving the bending strength and toughness.

Developing a Single-Specimen Technique for Low-Temperature R-Curve Determination of Asphalt Concrete Using a Modified Unloading Compliance Method

Fracture resistance curves (R-curves) have served as a robust tool in characterizing the entire fracture process of engineering materials. However, obtaining such curves for asphalt concrete (AC) mixtures is cumbersome due to the non-linear inelastic behavior of the mixtures. In this research, a single-specimen technique is developed based on the unloading compliance method which is used for metals. AC mixtures with limestone aggregate and PG58-22 binder were prepared. Beam specimens were fabricated and single-edge notched beam (SE(B)) fracture testing was conducted at low temperatures. A loading-partial unloading regime was used in the experiments and crack growth increments were captured by digital images throughout the tests. Using a multi-variable regression analysis, modified compliance equations were obtained for AC and R-curves of the mixtures could be constructed. It was revealed that the R-curve developed by ASTM E1820 compliance method could potentially overestimate the resistance of the mixtures against low-temperature fracture. The constructed R-curve exhibits a lower semi-vertical region addressing lower resistance of the mixture in the crack blunting phase. Also, the post-peak phase of the fracture shows a significantly lower slope in the constructed R-curve which denotes lower resistance of the mixture against unstable crack propagation.

In-situ observation of fracture behaviors in TiB2p/Al7050 composites with different gradient structure

The study employed the vacuum hot-pressed sintering method to fabricate TiB2p/Al7050 homogeneous composites (HC) and TiB2p/Al7050 functionally gradient materials (FGM), including three-layer conventional FGM (TFGM) and five-layer bionic interleaved FGM (FFGM). The crack-initiation fracture toughness (KIc), crack-propagation fracture toughness (KJc), and crack propagation performance were investigated to examine the impact of gradient structure and TiB2p content. The results highlighted the crucial role of TiB2p content in determining the fracture toughness of the three composites. The fracture toughness of HC requires consideration of the synergistic effects of strength and plasticity, while that of FGM is influenced by the combined effects of strength, plasticity, and interlayer synergistic factors. Additionally, the in-situ observation of the crack propagation path revealed a toughening mechanism that contributes to the improved fracture toughness of these composites. This mechanism includes crack deflection, branching, plastic deformation of the matrix, and crack blunting, which consumes the energy required for crack propagation, reduces the stress concentration at the main crack tip, and delays crack propagation. It is worth mentioning that among these composite materials, FFGM demonstrates the highest fracture toughness, with KIc and KJc increasing up to 22.39% and 22.23% above the average, respectively. The excellent performance can be attributed to its highly effective toughening mechanism. Overall, the study provides valuable insights into the design and development of FGM with superior fracture resistance. Hence, these findings can support the development of advanced materials with improved toughness and potential applications in various fields.

The initiation criterion for open-crack considering fissured water under biaxial compression

Fissured rock is under compression-shear load for a long time in the geotechnical engineering, while the blunt crack tip and fissured water also make the compression-shear fracture mechanism of the open-crack more complicated. So, the profound study on the distribution of the stress at the open-crack tip is made and the open-crack initiation criterion under hydrostatic and confining pressure is also established. First of all, according to Muskhelishvili's tensile stress hypothesis, the stress intensity factor KI under compression is studied and the stress field at the open-crack tip is modified, which involves the curvature radius ρ, high-order and non-singular terms, then the revised maximum tangential stress (MTS) criterion is proposed. Second, the effect of the curvature radius ρ and half-length a on KI and the initiation angle θ are discussed respectively. It is found that KI increases with increasing ρ and decreasing a. It is also found that when the loading angle β = 0 ∼ 45°, the initiation angle θ increases with decreasing ρ and increasing a. In the range of β = 45 ∼ 90°, the initiation angle θ is insensitive to ρ, a and tends to converge. The initiation stress goes down firstly in the range of β = 0 ∼ 50° and then gradually goes up with increasing β. Third, the model for the open-crack filled with fissured water under biaxial compression is discussed. The trend of the wing-crack initiation angle θ is similar to that under uniaxial compression with increasing β and a, but the upper limit of θ decreases with increasing the hydrostatic pressure pw. Finally, the revised criterion is preliminarily verified with the indoor test.

Recent Advances in the Analytical Stress Field Solutions for Radiused Notches in Orthotropic Solids.

The main aim of this work is to provide a brief overview of the analytical solutions available to describe the in-plane and out-of-plane stress fields in orthotropic solids with radiused notches. To this end, initially, a brief summary on the bases of complex potentials for orthotropic elasticity is presented, with reference to plane stress or strain and antiplane shear problems. Subsequently, the attention is moved to the relevant expressions for the notch stress fields, considering elliptical holes, symmetric hyperbolic notches, parabolic notches (blunt cracks), and radiused V-notches. Eventually, examples of applications are presented, comparing the presented analytical solutions with the results from numerical analyses carried out on relevant cases.

Dependence of tungsten crack behaviors on heat source parameters under transient heat flow

As a plasma facing material in TOKAMAK, tungsten may suffer plastic deformations, cracking, melting, even ablation induced by transient heat loads, such as Edge Localized Modes (ELMs). Focused on investigation of cracking behaviors of tungsten under transient heat flux and its dependence on pulse parameters, powder metallurgical pure tungsten samples have been tested with 1–100 pulses of intense pulse electron beams (IPEB) at 5 ms and 1 ms pulse duration. Three types of cracks were observed, including primary crack, secondary crack and blunt crack. The primary cracks can be assorted as branch, annular and network cracks according to their patterns. Branch cracks were formed in the irradiation area center, and annular cracks appeared surrounded the irradiation area. The width of annular cracks increases as the pulse number increasing. The initiation and evolution of annular cracks are mainly studies in this paper. Considering the ductile-to-brittle transition property, an ideal elasto-plastic mechanics model is adopted in the thermodynamic calculation model of tungsten under the IPEB irradiation. Irreversible plastic deformation plays an important role, which determined the tensile stress distribution during the temperature decreasing. The formation mechanism of annular crack and its dependence on the heat source parameters are discussed, based on the calculation results of temperature, stress distribution and evolution of tungsten under different pulse parameters with this model. It is found that the formation mechanism of annular crack depends on the high radial gradient of energy density E, and the radius of annular crack obviously depends on the energy density deposited per pulse and the heat source boundary gradient for large volume sample. The influence of pulse number on branch crack and annular crack is different. The widening of the annular crack with the increasing of pulse number is caused by the axial residual plastic strain after pulse irradiation.

Investigating inter/intralayer interface-triggered toughening mechanisms of three-dimensional printed polylactic acid using double-notch four-point-bending method

Three-dimensional (3D) printing technology is widely adopted in smart manufacturing, automotive, and aerospace fields for manufacturing complex geometric structures. Understanding the inter/intralayer interface-triggered toughening mechanisms of the 3D printed polylactic acid (PLA) materials produced by material extrusion is crucial. The fracture performance of the printed specimens in different fracture directions is evaluated with the single-edge-notch three-point-bending (SEN-3PB) test. Based on the double-notch four-point-bending (DN-4PB) method, the deformation mechanism around the crack tip and the crack propagation in the damage zone are systematically analyzed. It is clarified that the geometrical arrangement between the pre-crack and inter/intralayer and the interface bonding strength play decisive roles during the fracture process. The crack deflection, crack branching, and plastic deformation of the filaments are the main toughening mechanisms which endow 3D printed materials with better fracture performance than injection molded ones. However, the crack deflection and crack blunting could be inhibited if the intralayer interfacial bonding strength is excessively high. The results provide a good insight into the toughening mechanisms provoked by the inter/interlayer interface and could guide the design of the 3D printed specimens with optimized fracture performance.

Improving the ductility and toughness of nano-TiC/AZ61 composite by optimizing bimodal grain microstructure via extrusion speed

In this study, the nano-TiC/AZ61 composites with different heterogeneous bimodal grain (HBG) structures and uniform structure are obtained by regulating the extrusion speed. The effect of HBG structure on the mechanical properties of the composites is investigated. The increasing ductility and toughening mechanism of HBG magnesium matrix composites are carefully discussed. When the extrusion speed increases from 0.75 mm/s to 2.5 mm/s or 3.5 mm/s, the microstructure transforms from uniform to HBG structure. Compared with Uniform-0.75 mm/s composite, Heterogeneous-3.5 mm/s composite achieves a 116.7% increase in ductility in the plastic deformation stage and almost no reduction in ultimate tensile strength. This is mainly because the lower plastic deformation inhomogeneity and higher strain hardening due to hetero-deformation induced (HDI) hardening. Moreover, Heterogeneous-3.5 mm/s composite achieves a 108.3% increase in toughness compared with the Uniform-0.75 mm/s composite. It is mainly because coarse grain (CG) bands can capture and blunt cracks, thereby increasing the energy dissipation for crack propagation and improving toughness. In addition, the CG band of the Heterogeneous-3.5 mm/s composite with larger grain size and lower dislocation density is more conducive to obtaining higher strain hardening and superior blunting crack capability. Thus, the increased ductility and toughness of the Heterogeneous-3.5 mm/s composite is more significant than that Heterogeneous-2.5 mm/s composite.

Modeling the role of interfacial adhesion in the rolling resistance of a hyperelastic wheel

Adhesion theory has achieved remarkable success in the interpretation of sliding friction, while the adhesion mechanism in rolling friction is not fully understood. In this work, an adhesion model was proposed to investigate the rolling contact. With the model, the contact stress and the rolling resistance to free-rolling were calculated. It is found that the tensile stress at the contact trailing edge can result in crack blunting, such that the adhesive rolling of the hyperelastic wheel is a discontinuous crack propagation along the interface. Besides, stick-peel feature in the rolling resistance curves was identified, which is the analogy to the stick-slip in the sliding friction. The proposed model can be used to investigate the adhesion issue in the rolling friction.

Hierarchical precipitates facilitate the excellent strength-ductility synergy in a CoCrNi-based medium-entropy alloy

An interstitial carbon-doped CoCrNiSi0.3 medium-entropy alloy (MEA) has been developed, which exhibits excellent strength-ductility synergy. This has been achieved through the implementation of a three-level hierarchical-precipitate-microstructure design. The CoCrNiSi0.3C0.048 MEA displays an ultra-high yield strength of 1.34 GPa, accompanied by a uniform tensile elongation of 8.62%, surpassing the performance of most carbon/silicon-doped MEAs. The three-level hierarchical precipitate structure comprises the primary Cr23C6 carbides (2–10 μm), the secondary SiC precipitates (200–500 nm) surrounding the grain boundaries, and the tertiary SiC precipitates (∼50 nm) within the grains. The strengthening and toughening behavior of the current MEAs are attributed to multiple mechanisms, including crack branching and blunting in Cr23C6 carbides, dislocation-bypass mechanism around the non-shearable SiC precipitates, hetero-deformation-induced strengthening caused by the interface between matrix/three-level hierarchical precipitates, as well as stacking-fault networks and dense nanotwins activated by precipitates in the matrix. These findings provide valuable insights into the development of high-performance alloys for engineering applications in the future.

The interplay between constituent material and architectural disorder in bioinspired honeycomb structures

The advent of additive manufacturing has enabled the precise manufacturing of lattice materials at different length scales. While honeycombs are usually constructed from a tessellated array of identical unit cells, most structures in nature tend towards stochastic or disordered tessellation. In this work, we explore the effect of disorder properties of honeycombs, in particular, stiffness, strength, fracture toughness, and crack propagation paths using finite element models. The honeycombs are created using Voronoi tessellations with different levels of disorder measured by a regularity parameter δ, where δ=1 corresponds to a regular hexagonal honeycomb and decreasing values of δ correspond to increasing levels of disorder. We consider three constituent materials: very brittle, quasi-brittle, and very ductile (0.0007–0.25 in failure strain). The results show that pseudo-order realizations with 0.5≤δ≤0.8show the best strength-toughness trade-off. With increased plasticity, the improvement in fracture toughness compared to regular honeycombs can be as high as 50% at the expense of a 20% decrease in strength. This increase in fracture toughness is achieved both at crack initiation with crack blunting intrinsically and extrinsically during crack growth by a combination of crack deflection and crack bridging. Also, a single mathematical variable δ usually referred to as a “regularity parameter” to control cell stochasticity cannot account for the variability in tensile strength and fracture toughness parameters for brittle or quasi-brittle constituent materials and significantly more for a ductile constituent material. Thus, a local parameter for disorder is required to explain the large statistical variance seen in the mechanical properties of disordered honeycomb structures across the same irregularity value.