Brittle Crack Research Articles

The dry-sliding and micro-scratching behaviors of a Mg65Zn30Ca5 bulk glassy alloy

The macro-sliding and micro-scratching behaviors of a Mg65Zn30Ca5 bulk glassy alloy (BGA) were studied in the present work. The dry sliding results show that the wear mechanism of the Mg-based BGA includes the abrasion and oxidation wear. Increasing the normal load does not change the wear mechanism of macro-sliding. The cross-sectional morphology of the subsurface under the worn scar shows obvious brittle cracks across shear bands. The micro-scratching results illustrates that as the normal load increases, plastic deformation occurs and accumulates, then micro-cracks form and propagate, and cracks generate and expand in the scratch when the load excesses 5 N. The cross-sectional morphology below the scratch scar after 1 min repeating scratching shows river pattern fracture surface with nanoscale swirling periodic corrugations, which indicates the brittle fracture characteristic of the Mg-based BGA.

Effect of temperature on the mixed-mode (I&II) fracture response of a rolled magnesium alloy

This work investigates the effect of temperature on the mixed-mode (I and II) fracture response of a rolled AZ31 Mg alloy by conducting experiments using asymmetric pre-cracked four-point bend specimens at room temperature and 100 °C. Digital image correlation analysis of in-situ optical images of the specimen is undertaken to map the strains and deformation on the surface. The deformation response has been further examined through microstructural characterization by optical microscopy, electron backscatter diffraction, and fractographic studies. The results show that the fracture toughness Jc enhances by a factor of about 3.5, irrespective of mode mixity over the above temperature range. A change in fracture mechanism from brittle cracking attributed to tensile twinning near the tip to ductile void growth and coalescence occurs over the above temperature range. Also, Jc drops by about 2.7 as fracture mode changes from mode I to II at both temperatures, notwithstanding the vast difference in fracture mechanism. The above trends are rationalized from the role of tensile twinning, which while critically influencing the fracture processes near the tip, can also contribute to Jc through substantial background plastic dissipation.

Atomic Cluster Expansion for Quantum-Accurate Large-Scale Simulations of Carbon.

We present an atomic cluster expansion (ACE) for carbon that improves over available classical and machine learning potentials. The ACE is parametrized from an exhaustive set of important carbon structures over extended volume and energy ranges, computed using density functional theory (DFT). Rigorous validation reveals that ACE accurately predicts a broad range of properties of both crystalline and amorphous carbon phases while being several orders of magnitude more computationally efficient than available machine learning models. We demonstrate the predictive power of ACE on three distinct applications: brittle crack propagation in diamond, the evolution of amorphous carbon structures at different densities and quench rates, and the nucleation and growth of fullerene clusters under high-pressure and high-temperature conditions.

The role of nickel in forming a structure providing increased service properties of reactor structural materials

Nickel is an essential alloying element in steels used as structural materials in the most common nuclear power reactors of the VVER type. The paper considers reviews the results of structural studies of traditional and advanced materials of the vessels and internals of VVER-type reactors with high nickel contents in their compositions. It is shown that an increased nickel content (up to 5 wt.%) in the steels of VVER pressure vessels contributes to the formation of a more dispersed structure with a smaller size of substructural elements and an increased density of dislocations, as well as a higher volume density of carbide phases. The revealed features of the structure of the reactor pressure vessel steel with high nickel content have the prerequisites for improving the strength and viscoplastic properties due to the increased number of barriers both for the dislocation motion and brittle crack propagation. Using the example of materials for VVER internals, it is shown that the nickel content increased in them up to 25 wt.% contributes to an increase in the volume density of radiation defects (dislocation loops of various types) and radiation-induced phase precipitates (G-phase). As nickel increases from 10 to 25 wt.%, there is a tendency to reduce swelling, which contributes to less shape change of the components of the reactor vessel internals. At the same time, in the steel with the highest nickel content, the highest nickel content was found in the near-boundary regions of the matrix, which contributes to greater austenite stability and a lower probability of the formation of an embrittling α-phase. The data obtained in the work on the effect of nickel alloying on the steel structural phase state and service characteristics were used in the development of new materials for the vessels and internals of advanced reactors.

Mathematical Modeling and Experimental Study of Cutting Force for Cutting Hard and Brittle Materials in Fixed Abrasive Trepanning Drill.

Hard and brittle materials have excellent physical and mechanical performance, which are widely applied in the fields of microelectronics and optoelectronics. However, deep-hole machining of hard and brittle materials is very difficult and inefficient due to the high hardness and brittleness of these materials. To improve the quality and efficiency of deep-hole machining of hard and brittle materials, according to the brittle crack fracture removal mechanism of hard and brittle materials and the cutting model of the trepanning cutter, an analytical cutting force prediction model of hard and brittle materials processed using a trepanning cutter is established. This experimental study of K9 optical glass machining shows that as the feeding rate increase, the cutting force increase, and as the spindle speed increase, the cutting force decrease. By comparing and verifying the theoretical and experimental values, the average errors of axial force and torque are 5.0% and 6.7%, respectively, and the maximum error is 14.9%. This paper analyzes the reasons for the errors. The results indicate that the cutting force theoretical model can be used to predict the axial force and torque of machining hard and brittle materials under the same conditions, which provides a theory for optimizing machining process parameters.

A phase-field framework for brittle fracture in quasi-crystals

Most of quasi-crystals are brittle at room temperature due to their specific cluster structures. Phase-field fracture models have demonstrated a powerful ability to predict brittle crack evolution. This paper develops a phase-field framework for modeling macroscopic brittle fracture in quasi-crystal solids. The kind of quasi-crystals is not preset in the phase-field model; therefore, this model is valid for almost all quasi-crystal solids. The phase-field model for the first time introduces a volume fraction parameter into the fracture toughness to reflect the effect of phason wall. Furthermore, a new numerical implementation approach of phase-field fracture models in Comsol Multiphysics is presented. Several examples of 1D hexagonal and 2D decagonal quasi-crystals are performed. The developed model is validated against the reported results of benchmark problems. Further investigations indicate that the phason field peculiar to quasi-crystals has a certain influence on the crack paths in asymmetric fracture problems and on the force–displacement curves in both symmetric and asymmetric fracture problems. When the fracture toughness is fixed, the increase of the volume fraction parameter results in the decrease of the peak force and the increase of the failure displacement. When the influence of the phason field on the fracture toughness is considered, both the peak force and failure displacement would sharply decrease, which indicates that the degradation of fracture toughness due to the phason field plays a more important role than the phason elastic energy in the fracture of quasi-crystals. Furthermore, the relation between the phonon-phason energy transformation and the elastic constants of QCs has been qualitatively analyzed. The developed phase-field modeling framework provides a guidance for understanding the fracture mechanism of quasi-crystals and assessing the safety of quasi-crystal structures in engineering practice.

In situ fragmentation of Al/Al2O3 multilayers on flexible substrates in biaxial tension

A unique deposition approach combining atomic layer deposition (ALD) and magnetron sputtering was used to fabricate a series of thin film multilayer structures of Al (50 nm) and Al2O3 (ALD, 2.4–9.4 nm) on flexible polymer substrates without breaking vacuum. The multilayers together with 50 nm and 150 nm Al reference films were analyzed by cross-sectional TEM analysis and experimentally strained in biaxial tension to investigate their deformation behavior. Al film stresses and peak widths, measured in situ with Synchrotron X-ray diffraction, are in good agreement with post-mortem surface SEM and through-thickness FIB analysis of the multilayers. It was revealed that brittle cracking of the multilayer can be avoided, and that the lateral and through-thickness crack resistance improve as a function of decreasing oxide layer thickness. An attempt to model the full biaxial yield surface of the multilayers, which remains experimentally challenging, appears to be valid up to 2.4 nm oxide thickness. Model predictions are further compared to compression data, obtained from the unloading segments of the tensile tests. Describing the mechanical behaviour under multiaxial stress conditions is of utmost importance for a diverse understanding of these multilayers across a variety of potential carrier systems and loading cases.

Computational modeling and analysis of the interfacial debonding in copper/copper and copper/polyether-ether-ketone coatings deposited by cold spraying

Cold Spraying (CS) is a promptly developing solid-state additive material deposition technique that is frequently used for high-value metallic component repair. The purpose of this research is to investigate the interfacial adhesion strength of cold-sprayed copper (Cu) coatings deposited on Cu and polyether-ether-ketone (PEEK) substrates for repair applications. In light of this, we propose a computational framework that accounts for both of the two main failure mechanisms, i.e., failure at the Cu/Cu interface caused by plasticity and damage initiation and failure at the Cu/PEEK interface due to brittle crack. Towards this end, different damage models are employed to describe the ductile damage of bulk materials, and a large deformation Cohesive Zone Model (CZM) is developed to model Cu/Cu interface debonding, allowing for stable calculations under large interfacial deformation. The Virtual Crack Closure Technique (VCCT) is used to simulate brittle debonding failure of the Cu/PEEK coating. The threshold stresses for coating debonding, found experimentally, are successfully compared to the numerical predictions. Such debonding modeling techniques might be useful for predicting coating behavior and bonding strength in cold spraying under various loading situations and material combinations.

Coupled versus energetic nonlocal failure criteria: A case study on the crack onset from circular holes under biaxial loadings

The phenomenon of brittle crack onset stemming from a circular hole in an infinite plate subjected to remote biaxial loading is herein investigated. A thorough analysis on the influence of the loading biaxiality reveals the existence of a wide casuistry in the sign and trend distributions of the stress field and Stress Intensity Factor, thus rendering it an exhaustive case study for assessing different failure criteria. Subsequently, three different approaches are used to determine the biaxial safety domains, two of which rely on the coupling of stress and energy conditions for failure, namely Finite Fracture Mechanics and Cohesive Zone Model, plus the purely energy-driven Phase Field model of fracture. Noteworthy, Finite Fracture Mechanics predicts the existence of a region in the loading space where failure is exclusively governed by the energy condition. Likewise, it is mathematically proven that the system of equations governing Dugdale's Cohesive Zone Model is equivalent to the first-order minimization condition of the energy balance, the resultant predictions being fairly close to those obtained by Finite Fracture Mechanics. Lastly, the Phase Field model of fracture is numerically implemented in the context of Finite Elements while paying special attention to the choice of the energy decomposition, whereof two are implemented: No-Decomposition and No-Tension decomposition. Specifically, the latter showcases satisfactory agreement with both Finite Fracture Mechanics and Dugdale's Cohesive Zone Model, thus posing a solid contender for studying complex fracture scenarios upon combined tension-compression stress states.

Microstructure-based crack formation in tungsten exposed to cyclic transient heating

The performance of plasma facing material (PFM) in nuclear fusion reactor is dramatically influenced by the synergy between steady heat loading and transient events. The negative and positive thermal stresses induced by a temperature gradient can trigger crack damage and even complete failure. Deep understanding the cracking mechanism is of great significance to optimize structure design and to improve service capability of PFM. Herein we combine the extended finite element method (XFEM) with the cohesive zone method (CZM) in order to unravel the characteristics of inter-granular micro-cracking formation in polycrystalline tungsten (W) by taking account of topological microstructure and external loading condition. From the macro component to the mesoscopic structure, the temperature and stress distributions of W/Cu/CuCrZr monoblock under steady state coupled with cyclic transient event is first numerically investigated and compared with the results from experimental measurement. Then, by using the displacement rate estimated from the heat-energy-variation-induced accumulated plastic strain on the surface of monoblock as a bridge, the inter-granular crack propagation behaviors in the equiaxial grain (EG), gradient grain (GG) and heterogeneous grain (HG) structures of the cross-sectional component and in different surface grain sizes of component are predicted, respectively. It is found that higher heat loading with longer duration time leads to the increment of cumulative plastic strain level so that crack extension rate is accelerated. The GG structure exhibits a remarkable improvement on crack development since the change of inter-granular crack path at the triple point is influenced by the inclined boundary with respect to the loading direction. These results provide valuable insights into the correlation of grain morphology and brittle crack growth, especially focusing on further improvement of PFM.

A new implicit gradient damage model based on energy limiter for brittle fracture: Theory and numerical investigation

We present a general form of the gradient-enhanced damage theory and its numerical implementation using finite element method (FEM) in modeling quasi-static brittle crack growth in one- (1D), two- (2D) and three-dimensional (3D) bodies. Coupled equations of the equilibrium and a new implicit gradient damage formulation are introduced to govern the deformation of the solid and evolution of the damage. The resulting nonlocal damage evolution equation featuring the growth of diffusive crack is integrated with a characteristic length scale to eliminate the common mesh-bias issue in FEM implementation. In contrast to the traditional gradient-enhanced damage approaches, the nonlocal damage field here is defined as the primary variable of the damage evolution equation without interpolation mismatch between the displacement and nonlocal damage fields. For derivation of the material constitutive law and local damage parameter, a novel strain energy density (SED) function based on the energy limiter theory for brittle crack growth problems under small strain regime is introduced. To further improve the performance of the developed model, an initial SED threshold, which is used for determining the critical point when damage starts to initiate in the material, is integrated into the novel energy limiter theory. For preventing nonphysical failure in compression domains, the spectral decomposition technique for the strain tensor is adopted to split the reference SED. With integrating the energy limiter into the developed theory, unlike the conventional nonlocal damage theories where the interpretation of the length scale is still ambiguous, the developed nonlocal damage model defines the length scale parameter as the problem-dependent factor. The performance and ability of the proposed model are demonstrated via a set of representative numerical examples in 1D, 2D and 3D fracture problems.

Fabrication of Fe–30Al alloy using plasma arc welding powered twin-wire directed energy deposition-arc process: Droplet transfer, microstructure, and mechanical property investigation

Intrinsic brittleness and cold cracking sensitivity of Fe–Al intermetallic compounds make their fabrication and processing difficult. In recent years, an innovative additive manufacturing (AM) technology, twin-wire directed energy deposition-arc (TW-DED-arc), has been developed to prepare defect-free Fe–Al alloys via in-situ alloying of Fe and Al elements in a gas tungsten arc welding (GTAW) generated molten pool. However, unconstrained plasma of regular tungsten arc leads to excessive heating area and residual stress during AM buildup. And relatively weak arc pressure of GTAW makes composition inhomogeneity a concern in the in-situ alloying molten pool. To solve the two potential drawbacks, plasma arc welding (PAW) possessing higher energy density and arc pressure is used to power the TW-DED-arc process and fabricate Fe–30Al alloy. In the present research, a special wire-feeding strategy for stable twin-wire feeding of Fe and Al wires is successfully developed to overcome the droplet transfer difficulty induced by constrained arc of PAW and large thermophysical properties difference of heterogeneous filler wires. And according to microscopy characterization results, the innovative PAW powered TW-DED-arc process is capable of fabricating defect-free Fe–30Al bulk sample with homogeneous Al content and epitaxial columnar grain structures. The successful application of PAW-powered TW-DED-arc technique in fabrication of iron aluminide provides stabler and more efficient route for iron aluminide component.

An experimental study on casing perforation erosion during sand fracturing

A sand-fracturing casing perforation erosion experimental device was designed to explore the casing perforation erosion behavior under different working conditions in the study. The influences of different factors on the casing perforation erosion mechanism were investigated based on macro-scale observations and SEM images. The serious perforation erosion occurred in the direction of the flow inlet and led to asymmetric funnel. The influences of various parameters on the perforation expansion rate were ranked according to the following order: sand-passing quantity of perforation > pump displacement > sand concentration > proppant size. Under a low sand concentration, the erosion effect of proppant particles on casing perforations was mainly caused by cutting and ploughing. Under a high sand concentration, shallow craters and brittle cracking were increased. The findings can guide the control of casing perforation erosion and improve the understanding of the perforation target erosion wear mechanism of the solid-liquid two-phase flow.

Impact of Nano-CaCO3 and PVA Fiber on Properties of Fresh and Hardened Geopolymer Mortar

Geopolymer is a green substitute for Portland cement but has low tensile strength, high brittleness and easy cracking. Therefore, fibers and nanomaterials are used to strengthen and toughen geopolymer composites. The influence of nano-calcium carbonate and PVA fiber on the properties of fresh and hardened geopolymer mortar were studied herein. The hybrid of long and short fibers with small content (0.8 vol.%) is conducive to flowability, while the hybrid with large content (1.6 vol.%) and nano-calcium carbonate is conducive to flowability. The slump flow and flow rate of geopolymer mortars with low fiber factor (product of fiber volume fraction and length-diameter ratio) decrease with the growth in nano-calcium carbonate content. As the PVA fiber factor reaches 464.8%, the slump flow and flow rate values of mortars with 0, 1 wt.% and 2 wt.% nano-calcium carbonate are close to each other. About 450% is the density packing threshold of PVA fiber in geopolymer composites. The combination of 0.8 vol.% 12 mm + 0.4 vol.% 6 mm fiber + 1 wt.% nano-calcium carbonate presents the highest flexural strength and flexural to compressive strength ratio, with a compressive strength of about 36 MPa. The optimal fiber factor range of PVA fiber in cement and geopolymer mortar is about 400% and higher than 600%, respectively. PVA fibers show more effective enhancement of flexural strength and toughness in geopolymer than cement mortar. The ultrasonic wave velocity and apparent density of geopolymer mortar show a downward trend as a whole with the increase in fiber factor. The intensity rise of the hump between 17° and 38° (2 θ) in the XRD pattern is observed. The SEM indicates that the surface of PVA fiber in geopolymer mortar with nano-calcium carbonate is heavily scratched, and the fiber filaments are rolled up, demonstrating improved bonding between PVA fiber and geopolymer mortar.

Modeling Plastic Damage of Reinforced Concrete in Finite-Element Applications

Testing full-scale structures in the laboratory is not always doable nor financially feasible, especially for large structures. Therefore, these structures are often modeled with Finite Element Analysis (FEA) software and packages, such as Abaqus, to understand their actual behaviors under special simulated loading and material-behavioral considerations. This study developed a material model that simulates load-induced cracking in reinforced concrete (RC) elements in the FEA of structures. The simulation of complete stress–strain concrete behavior under tension and compression (including damage characteristics) requires the application of numerical material models, potentially producing the stress–strain curves. These models include strain-softening regimes that presuppose the use of ultimate compressive concrete strength and that can be practically implemented for existing or future RC structures. The method presented in this study is valuable because it assesses existing RC structures when detailed test results are not available. However, the implemented numerical models were slightly altered in order to compare them with the damaged plasticity models available in the research literature. These are mainly the smeared crack concrete model, the brittle crack concrete model, and the concrete damaged plasticity model, which are all typically suitable for RC elements. The methodologies proposed here provide accurate and realistic modeling of the plastic damage of RC members. Although numerically extensive and very time-consuming, it is demonstrated that the material and damage modeling presented in this paper can be used to accurately represent the behavior of actual structural members tested in research laboratories. This suggests that it is applicable to structures for which experimental tests or monitoring data are not available.

The fracture characteristics and enhancement mechanism of crushing strength for the coke with hot pressing

High-strength coke is essential to the ironmaking and steelmaking industries. The crushing strength of coke could be improved effectively when the hot pressing technique is carried out with proper parameters. The fracture characteristics and enhancement mechanism of crushing strength for the coke with hot pressing were investigated by experiment and finite element method with the brittle cracking model. Results show that the crushing strength of coke is improved greatly when the hot pressing is conducted at 420 ℃∼440 ℃ for the 1/3 coking coal. Coke's crushing strength is mainly affected by porosity and pores' spatial distribution. For the high porosity, the coke has thin pore walls and the cracks could easily penetrate the entire coke. Therefore, the high porosity contributes to the decrease of crushing strength. If the pore distribution is uneven, the cracks expand rapidly in the pores aggregation region and the coke has lower crushing strength. In the initial and middle thermoplastic stages, conducting the hot pressing technique with low stress could enhance the crushing strength of coke. The coke porosity is high when the coal is pressed with high stress at the early thermoplastic stage. The high pressing stress may result in uneven pores distribution if the hot pressing technique is conducted at the middle thermoplastic stage. Therefore, the high pressing stress would impede the enhancement of crushing strength when the coal is pressed at the early and middle thermoplastic stages. When the hot pressing technique is conducted with high stress at the later thermoplastic stage, the coke would have low porosity, even pore distribution, low reactivity and high crushing strength.

Impact Fracture Surfaces as the Indicators of Structural Steel Post-Fire Susceptibility to Brittle Cracking.

The results of experimental research on forecasting post-fire resistance to brittle failure of selected steel grades used in construction are presented and discussed in this paper. The conclusions are based on detailed analysis of fracture surfaces obtained in instrumented Charpy tests. It has been shown that the relationships formulated based on these tests agree well with conclusions drawn based on precise analysis of appropriate F-s curves. Furthermore, other relationships between lateral expansion LE and energy Wt required to break the sample constitute an additional verification in both qualitative and quantitative terms. These relationships are accompanied here by values of the SFA(n) parameter, which are different, depending on the character of the fracture. Steel grades differing in microstructure have been selected for the detailed analysis, including: S355J2+N-representative for materials of ferritic-pearlitic structure, and also stainless steels such as X20Cr13-of martensitic structure, X6CrNiTi18-10-of austenitic structure and X2CrNiMoN22-5-3 duplex steel-of austenitic-ferritic structure.

A strategy to promote formability, production efficiency and mechanical properties of Al–Mg–Si alloy

To enhance the formability, production efficiency and mechanical properties of Al–Mg–Si alloy, the 6063 Al alloy was extruded from as-cast, furnace-cooling and air-cooling homogenized billets and followed by online quenching, respectively. The effects of initial billet states, extrusion parameters and online quenching process on the formability, microstructure and mechanical properties of the profile were investigated. It was found that the second phases in as-cast and furnace-cooling homogenized billets not only brought no brittle crack during extrusion, but also refined the grain size. The online quenching of the 6063 Al alloy profiles with complex sections prevented the abnormal grain growth occurring in offline solid solution treatment, so as to improve production efficiency. The strengths of the profiles extruded from as-cast and air-cooling homogenized billets were equivalent. Therefore, most of 6063 Al alloy profiles could be directly extruded from as-cast billet without homogenization treatment, which improved the production efficiency. The furnace-cooling homogenized billet had the lowest flow stress, indicating lower break-through force to activate extrusion. The strength of the profile extruded from furnace-cooling homogenized billet was higher than that of air-cooling homogenized billet. Therefore, the furnace-cooling homogenized billet was recommended for the profile with complex section or higher performance requirement.

Analysis of bulge failure of steel wire-reinforced thermoplastic composite pipe

A steel wire-reinforced thermoplastic composite pipe (SRTP) was used for water injection connecting line pipe had caused bulge failure of the external surface after 6 years of service. Investigations were performed to identify the most probable causes of the pipe failures. The study was conducted by reviewing the background of the failure, visual physical inspection, test production process analysis, and thermal analysis. The results revealed that the failure of this pipe was mainly caused by scratch defects on the inner surface and the poor resin stability near the cracking site compared to its basal resin. It has scratch defects forming a crack source under the combined action of internal pressure, the resin matrix on the inner surface appears brittle cracking. Subsequently, the medium to be corroded after contacting with the metal wire, losing the pressure bearing effect, and finally forming a bulge on the outer surface under the action of low internal pressure.

Strengthening and Toughening Technology of Epoxy Resin

Epoxy resin materials have excellent processability, adhesion, corrosion resistance, electrical insulation, mechanical properties, heat resistance and other properties. At present, they are widely used in weapons, aviation, aerospace, high-temperature adhesives and many other fields. However, epoxy resin has the defects of brittleness, low flexibility, weak impact resistance, poor peel resistance and low heat resistance in the process of use, especially in high-end fields such as weapons, the upper limit temperature is not high, The problems of high brittleness and easy premature cracking exposed in low temperature environment limit the deeper use of epoxy resin materials. This paper analyzes and discusses the advantages and disadvantages of some mainstream epoxy resin modification technologies, and prospects the next research trend, so as to provide reference for further research on strengthening and toughening epoxy resin in the future.