Fracture Transition Research Articles

Multiscale tip asymptotics for a deflating hydraulic fracture with leak-off

This paper deals with the construction of the tip asymptotes for a hydraulic fracture deflating in a permeable elastic medium. Specifically, the paper describes the changing nature of the asymptotic fields during the arrest and recession phases following propagation of the fracture after fluid injection has ended. It shows that as the fracture deflates in the arrest phase, the region of dominance of the linear elastic fracture mechanics tip asymptote $w\sim x^{1/2}$ of the fracture aperture $w$ with distance $x$ from the front shrinks to the benefit of an intermediate asymptote $w\sim x^{3/4}$ . Hence only the velocity-independent $3/4$ asymptote is left at the arrest–recession transition. Furthermore, with increasing receding velocity of the front, a linear asymptote $w\sim x$ develops progressively at the fracture tip, with $w\sim x^{3/4}$ again becoming an intermediate asymptote. These universal multiscale asymptotes for the arrest and recession phases are key to determining, in combination with a computational algorithm that can simulate the evolution of a finite fracture, the decaying stress intensity factor during arrest, the time at which the fracture transitions from arrest to recession, and the negative front velocity during recession.

Hydrogen-enhanced grain boundary vacancy stockpiling causes transgranular to intergranular fracture transition

The attention to hydrogen embrittlement (HE) has been intensified recently in the light of hydrogen as a carbon-free energy carrier. Despite worldwide research, the multifaceted HE mechanism remains a matter of debate. Here we report an atomistic study of the coupled effect of hydrogen and deformation temperature on the pathway to intergranular fracture of nickel. Uniaxial straining is applied to nickel Σ5(210)[001] and Σ9(1-10)[22-1] grain boundaries with or without pre-charged hydrogen at various temperatures. Without hydrogen, vacancy generation at grain boundary is limited and transgranular fracture mode dominates. When charged, hydrogen as a booster can enhance strain-induced vacancy generation by up to ten times. This leads to the superabundant vacancy stockpiling at the grain boundary, which agglomerates and nucleates intergranular nanovoids eventually causing intergranular fracture. While hydrogen tends to persistently enhance vacancy concentration, temperature plays an intriguing dual role as either an enhancer or an inhibitor for vacancy stockpiling. These results show good agreement with recent positron annihilation spectroscopy experiments. An S-shaped quantitative correlation between the proportion of intergranular fracture and vacancy concentration was for the first time derived, highlighting the existence of a critical vacancy concentration, beyond which fracture mode will be completely intergranular.

Mechanical Properties and a Brittle-to-Ductile Fracture Transition in 3D Boron Nitride Foams.

We uncover the fracture characteristics of a boron nitride foam (BNF): a highly promising nanomaterial with a large band gap, superelastic behavior, and high surface area. By applying tension tests to BNF samples and characterizing them using image-processing tools and detailed scanning and transmission electron microscopies, we demonstrate a transition from brittle to a ductile fracture. Complementary mechanical analyses revealed that constraints originating from the synthesis process induce significant prestresses in the BNF and that wall thickness variations explain the fracture transition. We also show that BNF has a nearly zero Poisson's ratio and a high (>200 MPa) shear strength and that it absorbs a significant amount of energy before the fracture occurs. Thus, our findings shed light on the fundamental microscopic-scale mechanics of BNF, paving the way toward its integration into advanced applications, such as wearable electronics and energy absorbers.

Multi-aspect size effect transition from micro to macroscale: Modelling and experiment

Size effects (SEs) impede the mass production of high-performance miniaturised components via micro-forming. Although many studies have examined SEs from multiple aspects, such as flow stress, deformation, and ductile fracture, the SE transitions characterised by the significant changes of these phenomena across the micro- and macroscale remain ambiguous. These SE transitions must be fully and deeply explored to enable the transformation of microscale deformations to macroscale ones by the design of products with appropriate dimensions, grain sizes and loading boundaries. This study involved experimental and numerical studies of multi-aspect SE transitions in flow stress, heterogeneous deformation (surface roughening and strain localisation) and ductile fracture at the micro- and macroscale in copper (Cu) sheets, which are widely used in electronic industries. The Cu sheets for tensile tests were designed and fabricated to be with a thickness (t) of 0.05–1.50 mm, a grain size (d) of 3–260 μm and t/d of 0.63–65.30. The crystal plasticity finite element model (CPFEM) and the Voronoi-based polycrystalline geometric model (VPGM) with t/d of 1–30 were established. Compared with the phenomenological work hardening model (PM), the dislocation density-based model (DDBM) can better predict the multi-aspect behaviours within the scope of above-mentioned scales. An obvious SE transition point (λ) was observed: when t/d < λ, there is a sharp decrease in materials strength and necking strain, an increase in surface roughness and a transformation of fracture mode, as well as a remarkable scattering of the aforementioned responses. The SE transition points vary from t/d = 3 to 11 for different responses, and generally the stress-related λ is smaller than the strain-related one. The larger the t/d, the closer the stress and strain distributions are to the normal distribution. The distribution irregularity of grain-scale stress to the change of t/d is more sensitive than that of strain. Different distributions in grain orientations are the primary inducers of this scattering of responses when t/d < λ. Grain-scale deformation heterogeneity and scattering could be decreased through controlling scale factor t/d > λ, thereby entering the macroscale deformation domain. Case studies of micro-pin extrusion and thin-walled tube-drawing confirmed that setting t/d > λ or using a boundary constraint could alleviate the negative influences of SEs, thus enabling a more uniform deformation.

Dramatically enhanced creep rupture performance and failure analysis of 9Cr–CrMoV dissimilar welded joint

The relationship between the microstructure evolution and the corresponding creep rupture performance are systematically investigated. Dramatically enhanced creep rupture performance is achieved for the 9Cr–CrMoV dissimilar welded joint by adjusting the heat treatment process (HTP) of CrMoV steels before welding. The crystallographic structure illustrates that the longer normalizing process could lead to the larger grain size, and the shorter tempering time devotes to the residual lath structure and finer second phase particles. The creep rupture test results show that the welded joint using CrMoV steel with original HTP ruptures in the soft zone with an obvious necking. After adjusting the HTP of CrMoV steel, the creep rupture life increases from 9634 h to 16,108 h accompanied with the rupture location transferred to the weld metal (WM), together with the fracture morphology from dimples to cleavage planes under the same creep rupture parameters. The fracture transition and enhancement of rupture life is resulted from the simultaneous strengthening effect of the lath structure and the precipitated fine particles in the matrix.

Numerical analysis of oblique hypervelocity impact damage to space structural materials by ice particles in cryogenic environment

A phase-aware constitutive model is developed and integrated with the meshfree simulations to study the failure mechanisms of space structural materials under the oblique hypervelocity impact of ice. The proposed constitutive model covers material’s dynamic behavior in a wide range of temperature, pressure, and strain rates. The numerical analysis combines the Hot Optimal Transportation Meshfree method with the EigenErosion algorithm to explicitly account for large deformation, brittle and ductile failure, phase transition, and debris cloud formation in the target materials and ice projectile. Specifically, the surface erosion and crater formation in the target under different impact speeds and angles are characterized by the damage’s shapes, depth, and length. The predicted crater depths in the Al6061 targets are validated by the Cour-Palais empirical model. It is evident that the damage to the space structural material by ice particles results from the competition between fracture, plasticity, and phase transition in the materials. The impact angle and velocity determine the crater deformation and the potential thread of the ejected debris.

High performance poly(methyl methacrylate) via hindered urea bond crosslinking

Incorporation of HUB crosslinks into PMMA networks not only simultaneously increased mechanical stiffness, strength, and toughness, but also achieved brittle-to-ductile fracture transition, while maintaining recyclability and healability.

Multifunctionality in ultra high molecular weight polyethylene nanocomposites with reduced graphene oxide: Hardness, impact and tribological properties

High performance in multiple properties is still a challenge to address in the case of polymer composites with engineered polymers. Herein, reduced graphene oxide was dispersed in an ultra high molecular weight polyethylene matrix through a novel and optimized process to produce nanocomposites with high performance at low nanofiller contents. The graphene layers assisted the formation of long-range ordered lamellar structures and microfibers between the crystals. We found that there is a fracture transition from fragile to ductile depending on the impact speed. This transition occurs at 0.25 wt% of rGO content, which is extremely useful for several high-performance applications that require a good combination of impact and wear resistance during operation. Graphene enhances the wear resistance performance of UHMWPE matrix, leading to a reduction of 40% of volume loss in abrasive wear. The results indicate an advance in the classification of the UHMWPE from engineering polymers to high performance polymers.

Synergistic effect of residual stress and phosphorus grain-boundary segregation on embrittlement of coarse-grained heat-affected zone in a novel Ni-Cr-Mo reactor pressure vessel steel

SA508Gr.4N steel is a novel Ni-Cr-Mo low-alloy steel for the new-generation reactor pressure vessel. Herein, a coarse-grained heat-affected zone (CGHAZ) in a P-doped SA508Gr.4N steel is simulated using Gleeble thermal cycling simulation with a peak temperature of 1320 ℃ under a heat input of 100 kJ cm−1. For the as-simulated specimen, due to the synergistic effect of high residual stress and P grain-boundary segregation, the fracture appearance changes in the order of ductile fracture to intergranular fracture, and then to cleavage fracture with decreasing impact temperature. With the residual stress relieved by annealing at 620 ℃, the specimen toughens to a certain extent even after ageing at 560 ℃, causing a decrease in ductile-to-brittle transition temperature (DBTT), although the boundary cohesion declines to a certain degree owing to the increase of P segregation. Simultaneously, the intergranular-to-cleavage fracture transition temperature (ICFTT) shifts to a lower temperature below –190 ℃, enabling the specimen to exhibit intergranular fracture even at −190 ℃. With the ageing temperature lowered to 500 ℃, the boundary segregation of P is raised to a high level, leading to a large increase in DBTT and a further decrease in ICFTT. Accordingly, the synergistic effect of residual stress and P boundary segregation affects the fracture nature of CGHAZ, thereby making fracture appearance have different characteristics. A mechanism diagram for the above effect is proposed based on the experimental results.

Structural to interfacial fracture transition in epoxy coated hydrating cement

In this work, the fracture behavior of an epoxy coated hydrating cement paste system is investigated with the aid of a slant shear test. A finite element model based on the phase-field method (PFM) is used to simulate the crack propagation and understand the transitions from structural to interfacial fracture and vice versa. At construction sites, it is a usual practice to coat epoxy only after the concrete is completely cured, i.e. after 28 days. However, during hydration, the surface energy of the cement paste varies with time. Therefore, it is necessary to investigate the temporal dependency of the interfacial strength on the instant (time) of coating. With this in mind, we investigated fracture processes in the cement paste, coated with epoxy at various time instants, from an early age (2 days) to the later age (28 and 50 days) of cement paste. From the experimental and numerical results, the cohesive failure of cement paste (bulk or structural failure) is primarily observed for 2-days hydrated cement paste. In contrast, adhesive failure at the interface between cement paste and epoxy (interface fracture) is observed for 28 and 50-days hydrated cement paste. In our fracture analysis, we have incorporated the surface roughness and the bulk porosity of cement paste which evolves over time during the hydration process. From the results of numerical analysis, it is found that the fracture behavior of the cement-epoxy system can be controlled by tuning the material properties of the cement paste and the interface i.e., elastic modulus, E and fracture energy Gc. Also, it is observed that the surface roughness and the bulk porosity of cement paste significantly influence the crack trajectory as well as the load–displacement response of the cement-epoxy system.

Transition of creep damage region in P91-Alloy800-SS316LN dissimilar metals weld joint

The creep behaviour of P91-Alloy800-SS316LN dissimilar metals weld joint (DMWJ) has been assessed at 823 K in the stress range of 180–260 MPa. Creep strain accumulation in the DMWJ was higher under high stress regime than the low stress regime. Transition of creep damage and fracture in the DMWJ occurred from P91 steel base metal at high stress regime to the interface region of P91 CGHAZ-IN182 weld metal at lower applied stress with significant reduction in ductility. Kernel average misorientation indicates that the localized increase in strain at the P91 CGHAZ-IN182 weld metal interface region under creep loading in comparison to that of the pre-creep condition. Enhanced heterogeneity in strain distribution, carbon migration, coarser precipitates and soft zone formation at the P91 CGHAZ-IN182 interface region have been attributed to the premature failure of DMWJ in comparison to P91 steel.

Hydrogen-induced transgranular to intergranular fracture transition in bi-crystalline nickel

It is known that hydrogen can influence the dislocation plasticity and fracture mode transition of metallic materials, however, the nanoscale interaction mechanism between hydrogen and grain boundary largely remains illusive. By uniaxial straining of bi-crystalline Ni with a Σ5(210)[001] grain boundary, a transgranular to intergranular fracture transition facilitated by hydrogen is elucidated by atomistic modeling, and a specific hydrogen-controlled plasticity mechanism is revealed. Hydrogen is found to form a local atmosphere in the vicinity of grain boundary, which induces a local stress concentration and inhibits the subsequent stress relaxation at the grain boundary during deformation. It is this local stress concentration that promotes earlier dislocation emission, twinning evolution, and generation of more vacancies that facilitate nanovoiding. The nucleation and growth of nanovoids finally leads to intergranular fracture at the grain boundary, in contrast to the transgranular fracture of hydrogen-free sample.

Injection Parameters That Promote Branching of Hydraulic Cracks

AbstractFluid injection into rock formations can either produce complex branched hydraulic fractures, create simple planar fractures, or be dominated by porous diffusion. Currently, the optimum injection parameters to create branched fractures are unknown. We conducted repeatable hydraulic fracturing experiments using analog‐rock samples with controlled heterogeneity to quantify the fluid parameters that promote fracture branching. A large range of injection rates and fluid viscosities were used to investigate their effects on induced fracture patterns. Paired with a simple analytical model, our results identify the threshold at which fracture transitions from an isolated planar crack to branched cracks when closed natural fractures exist. These results demonstrate that this transition can be controlled by injection rate and fluid viscosity. In relation to the field practices, the present model predicts slickwater and lower viscosity fluid injections promote fracture branching, with the Marcellus shale used as an example.

Experimental investigation of scatter in fracture toughness data of SA516Gr.70 steel in the ductile-to-brittle transition regime for high rate of loading using split Hopkinson pressure bar test setup

Ferritic grade steels exhibit ductile to brittle transition in the sub-zero temperature regime and the fracture toughness data show scatter and temperature dependence. The fracture toughness variation also depends upon rate of loading in addition to neutron irradiation, specimen geometry and loading configuration. The material SA516Gr.70 steel is widely used in nuclear industry as material of transportation cask, vessels and piping etc. The study of the effect of rate of loading on the fracture toughness transition curve in the ductile-to-brittle transition regime of this material is important for designers and safety analysts. In this work, the split Hopkinson pressure bar test technique has been used to carry out fracture tests on sub-sized single-edged notched bend specimens in the temperature range of −100 to −60 deg. C by loading the specimens at a very high loading rate of 700 m/min. This loading rate is at least 6 orders of magnitude higher than the rates (i.e., fraction of mm/min) employed for quasi-static fracture tests. The strain pulses, during high rate of loading, have been captured through strain gages located in the incident and transmission bars of the test setup. The load–displacement data for the specimens have been evaluated from the strain signal and these have been used to evaluate fracture toughness for onset of cleavage fracture in terms of KJc. The scatter and temperature dependence of the data have also been modelled through Weibull’s statistical distribution and master curve bounds according to ASTM E1921. From the results of fracture experiments, it was observed that the fracture transition temperature T0 increased significantly for the high rate of loading when compared to quasi-static data of similar materials in literature. This observation has been explained in terms of micro-mechanism of the cleavage fracture in the process zone ahead of crack-tip which changes due to rate of loading. This study indicates that the rate of loading is an important parameter in the design and integrity analysis of ferritic grade steel components in addition to other parameters such as specimen geometry and loading conditions.

Mechanical and wear behaviour of graphite nano filler reinforced Al 6061 composites

In this present work Aluminium 6061-Graphite Nano filler composites are developed with varying percentages of Graphite Nano filler. The Aluminium 6061-Nano Graphite composites are prepared by hot extrusion method. The mechanical properties such as Tensile strength, Compressive strength and Hardness is studied experimentally. The wear behavior of the developed composites is studied using a Pin-On-Disc tester. Experimental results reveal there is considerable growth in Tensile strength as well as Hardness with the increase in Nano-Graphite filler content. Also, it is observed that the strength of the nano-graphite/Al 6061 composite is very good compared unreinforced Aluminium 6061. There is increase in wear resistance of the developed composites with increase in Nano-Graphite filler content. The improvements in properties are attributed to uniform dispersion of Nano-Graphite0filler and good interfacial bonding between Nano-Graphite filler and Aluminium 6061 matrix. The fracture study shows the transition of ductile fracture in case of pure Alumunium 6061 to Brittle fracture in Aluminium 6061-nano Graphite composites.

Large-scale asymmetric pulverisation of fault zone: Insights from rock axial strain in static and dynamic loading conditions

The open question of asymmetric large-scale pulverisation of fault zone was experimentally investigated in this paper. It is found that the transition of rock fracture from local splitting to pervasive pulverisation is controlled by strain both in quasi-static and dynamic conditions. This can explain the asymmetric damage in the fault zone as the strain threshold to pulverisation for stiffer rock is lower. In terms of the cyclic dynamic loadings, four types of fracture patterns (lateral tension, axial split, spatial fragmentation, and pervasive pulverisation) were developed with the increase of strain. Subject to the long-term quasi-static and dynamic load, the cumulative damage to the fault zone broadens the scale of pulverisation. In light of the linear correlation of energy absorption to strain, the damage relationships between strain and loading cycles were established and found to offer a good prediction of experimental data.

Effect of hydrogen on the embrittlement susceptibility of Fe–22Mn-0.6C TWIP steel revealed by in-situ tensile tests

The hydrogen embrittlement (HE) behavior on a Fe–22Mn-0.6C twinning-induced plasticity (TWIP) steel was investigated by tensile tests with in-situ scanning electron microscope observation combined with electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques. The tensile test specimens were cathodically pre-charged with hydrogen for 0, 50, 150, and 300 h, which accumulatively reduced the mechanical properties and induced a ductile-to-brittle fracture transition. The threshold of hydrogen content to trigger this ductile-to-brittle transition was further determined by combined thermal desorption spectroscopy (TDS) analysis and theoretical hydrogen diffusion calculation. During the tensile tests, intergranular secondary cracks were observed on the gauge surfaces of the specimens with pre-charged hydrogen. The low angle grain boundaries (LAGBs) exhibited better resistance to both crack initiation and propagation compared with high angle grain boundaries (HAGBs). In addition, the stress concentration together with the hydrogen effect on grain boundaries intersected with deformation twins are proposed as the reasons for the crack initiation and propagation.

Skeletal Stem/Progenitor Cells in Periosteum and Skeletal Muscle Share a Common Molecular Response to Bone Injury.

ABSTRACTBone regeneration involves skeletal stem/progenitor cells (SSPCs) recruited from bone marrow, periosteum, and adjacent skeletal muscle. To achieve bone reconstitution after injury, a coordinated cellular and molecular response is required from these cell populations. Here, we show that SSPCs from periosteum and skeletal muscle are enriched in osteochondral progenitors, and more efficiently contribute to endochondral ossification during fracture repair as compared to bone‐marrow stromal cells. Single‐cell RNA sequencing (RNAseq) analyses of periosteal cells reveal the cellular heterogeneity of periosteum at steady state and in response to bone fracture. Upon fracture, both periosteal and skeletal muscle SSPCs transition from a stem/progenitor to a fibrogenic state prior to chondrogenesis. This common activation pattern in periosteum and skeletal muscle SSPCs is mediated by bone morphogenetic protein (BMP) signaling. Functionally, Bmpr1a gene inactivation in platelet‐derived growth factor receptor alpha (Pdgfra)‐derived SSPCs impairs bone healing and decreases SSPC proliferation, migration, and osteochondral differentiation. These results uncover a coordinated molecular program driving SSPC activation in periosteum and skeletal muscle toward endochondral ossification during bone regeneration. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Critical behavior of interfacial t-ZrO2 and other oxide features of zirconium alloy reaching critical transition condition

Transition of zirconium alloys during uniform corrosion is not expected but inevitable, since it accelerates the degradation of corrosion property. Up to now, proposed transition mechanisms have not clearly illustrated the critical behavior of interfacial t-ZrO2 and other oxide features under the critical transition condition. Therefore, study on the under-transition sample was carried out. Microscopically, the monoclinic stress increases after transition, strongly supporting the occurrence of transformation from t-ZrO2 to m-ZrO2. A sudden stress decrease in t-ZrO2 occurs in pre-transition region of the under-transition sample. This decrease might be attributed to formation of permeable paths which induce consumption of interfacial oxygen vacancies. This critical behavior possibly destabilizes the grown t-ZrO2 grains. And the post-transition regions will propagate after t-ZrO2 transformation. The transition region in the oxide consists of lateral cracks propagated transgranularly and large numbers of interconnected pores, both of which may originate from t-ZrO2 transformation. Current works suggest that three critical behaviors of t-ZrO2 induce oxide fracture and corrosion transition. The growth nature and evolution mechanism of t-ZrO2 are further illustrated. Even though t-ZrO2 layer similarly evolve as the ZrO suboxide layer, the unstable nature of t-ZrO2 indicates that it is severely affected by grain size, stress value and chemical composition.

Fracture behavior and acoustic emission characteristics of sandstone samples with inclined precracks

Sandstone samples with precracks of different dip angles were collected from a coal mine roof and subjected to uniaxial compression tests, and acoustic emission (AE) and scanning electron microscopy (SEM) were used to study how the crack dip angle affected the fracture mechanism. In the precracked sandstone samples, as the dip angle between the crack line and loading direction decreased, so did the peak stress and its completion time. The SEM observations revealed a fracture transition from tensile cleavage to shear slip, which was manifested by a microstructure change from aggregate to staggered. According to energy conversion, a decreased crack dip angle results in gradually decreasing total and dissipative peak energies, whose variation amplitudes at different stages are consistent with those of the peak stress of the samples. The decreased crack dip angle lowered the stress required to trigger the first appearance of AE energy peaks and ring-down counts, as well as shortening the period before the occurrence of the first AE peak signal. However, the AE energy and ring-down count during the failure stage after the stress peak increased gradually. A stepped increase was observed in the AE ring-down count curves, with each step corresponding to a jump in the stress-strain curve. From the characteristics of the AE signal of the fracture of a precracked rock sample, the occurrence of joints or faults in the rock mass can be reasonably inferred. This is expected to provide a new method and approach for predicting coal and rock dynamic disasters.