Brittle Crack Research Articles

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.

A generalized machine learning framework for brittle crack problems using transfer learning and graph neural networks

Despite their recent success, machine learning (ML) models such as graph neural networks (GNNs), suffer from drawbacks such as the need for large training datasets and poor performance for unseen cases. In this work, we use transfer learning (TL) approaches to circumvent the need for retraining with large datasets. We apply TL to an existing ML framework, trained to predict multiple crack propagation and stress evolution in brittle materials under Mode I loading. The new framework, ACCelerated Universal fRAcTure Emulator (ACCURATE), is generalized to a variety of crack problems by using a sequence of TL update steps including (i) arbitrary crack lengths, (ii) arbitrary crack orientations, (iii) square domains, (iv) horizontal domains, and (v) shear loadings. We show that using small training datasets of 20 simulations for each TL update step, ACCURATE achieved high prediction accuracy in Mode-I and Mode-II stress intensity factors, and crack paths for these problems. We demonstrate ACCURATE’s ability to predict crack growth and stress evolution with high accuracy for unseen cases involving the combination of new boundary dimensions with arbitrary crack lengths and crack orientations in both tensile and shear loading. We also demonstrate significantly accelerated simulation times of up to 2 orders of magnitude faster (200x) compared to an XFEM-based fracture model. The ACCURATE framework provides a universal computational fracture mechanics model that can be easily modified or extended in future work.

Experimental study and performance analysis of the flexural strength of reinforced ice slabs

In recent years, interest in the prefabricated ice rink for international competition has increased. The ice sheet, whose support is extruded polystyrene foam board, is prone to bending failure. To maintain the advantage of recycling prefabricated ice rinks, the method of reinforcing ice slabs with welded steel mesh (SRI) was proposed. However, the flexural properties of SRI remain unclear. Therefore, three-point bending tests, theoretical derivation, and finite element analysis were conducted in this study to better understand SRI. First, the three-point bending tests with 17 specimens were performed considering the effects of reinforcement ratio (0%∼0.812%). The results showed that the SRI's cracking load capacity and ultimate load capacity improved significantly with 1.84 and 4.49 times higher than the plain ice slab at most. Additionally, reinforcement can reduce the stiffness drop caused by cracking, which can be increased to 0.913 at most. Then, the calculation formula of the cracking load, which is a segmentation function, is derived based plane section assumption and gets good agreement with experimental results. At the same time, the calculation formula of the ultimate load is derived, although the calculation result is conservative, it can reflect the trend of the ultimate load with the variation of reinforcement ratio. Finally, finite element simulation analysis of three-point bending tests of SRI were conducted, using the brittle cracking model and the Coulomb-Moore plasticity model, respectively. Simulation results show that using both models alone have the limitation of use range. As a result, a B + C combined numerical model (the brittle cracking model is used for the tension zone and the Coulomb-Mohr plasticity model is used for the compressive zone) was proposed, and the simulation results agree well with the cracking load of both experiment and formula. Taken together, the welded steel mesh effectively improves the flexural performance of ice slabs and the findings of this study are beneficial for the design and finite element analysis of SRI in the future.

Effect of Post Weld Heat Treatment on Tensile Strength of ASTM A36 Welded Joints: Application on Hull Vessel Material

In fact, in the welding process, there are many problems that arise. Such as cracking caused by excessive stress Residual stress can result in a decrease in the mechanical properties of a material such as brittle fracture, fatigue, and cracking. The purpose of this study was to determine the comparison of tensile strength in ASTM plate welded joints A36 with variations of single vee butt and single bevel butt and knowing the comparison of the effects ofpost-weldd heat treatment on welded joints with variations of single v butt and single bevel butt joints. The method used in this research is experimental. The results of the data obtained in this study The value of the tensile strength of the Single Bevel Butt joint without the influence of Heat Treatment is 153.87 MPa, the tensile strength of the Single Vee Butt joint without the influence of Heat Treatment is 161.75 MPa and the tensile strength of the seam Single Bevel Butt with the effect of Heat Treatment is 196,65 MPa. The tensile strength of the Single Vee Butt seam with the effect of Heat Treatment is 173,36 MPa.

Study of Static Flexural Strength Anisotropy of Yttrium Calcium Oxyborate Crystals

As a promising optical and piezoelectric crystal, yttrium calcium oxyborate (YCOB) has been criticized for its easy fracture during both the cooling and machining processes. Herein, in order to evaluate the mechanical energy (G) required for crack propagation on one crystal plane, we measured the bending strength of the YCOB samples with different orientations via employing a three-point bending test. The surface topography of the polished sample was measured, and the average surface roughness is limited to 10 nm. Fracture surfaces were also observed and analyzed. The results showed that the bending strengths of Y-cut and Z-cut samples were higher than other cut directions in the XZ principal plane, showing obvious anisotropy. It can be found that the cleavage planes (2¯01) and (101) were always observed in fractured samples in multiple directions in the XZ principal plane. The contribution of the cleavage planes (2¯01) and (101) to the fracture of the specimens was discussed in detail by the theorem of energy minimum. Finally, based on the obtained scanning electron microscopy results, we propose the mechanism for crack advance in YCOB crystals. A perfectly brittle crack in a crystal prefers cleavage planes with low surface energy. It is believed that these findings may provide fresh ideas to overcome the cleavage fracture of larger size YCOB crystals.

Cryogenic liquid nitrogen stimulation-induced cracking in gas-enriched shale reservoirs: A numerical investigation

Cryogenic shock-induced brittle cracking has promising application prospects in reservoir stimulation. In this paper, a fully coupled thermo-hydro-mechanical (T-H-M) model combined with the statistical meso-damage theory is proposed to investigate the trans-scale progressive cracking process of shale gas reservoirs stimulated by liquid nitrogen (LN2). In this theoretical framework, shale meso-heterogeneity in both thermo-mechanical and hydraulic properties is characterized by two independent statistical distribution functions; temperature-dependent rock properties and temperature- and pressure-dependent fluid thermodynamic properties are described using some experimental and theoretical formulas. The numerical model is implemented in a finite element context and subsequently validated against existing classical coupling results. Numerical results demonstrate that LN2 fracturing accompanied by the unique cryogenic shock effect leads to a complex cracking process involving multiple-point fracture initiation, asymmetric macrofracture propagation, and primary fracture bifurcation. Sensitivity analysis indicates that approximately equidistant or equilength radial fractures may be generated at a lower stress difference, lower permeability, or higher elastic modulus; both fracture initiation pressure and induced fracture morphology are more sensitive to injected fluid temperature relative to initial reservoir temperature; a larger thermal expansion coefficient is beneficial for inducing more fractures, but not applicable to the thermal conductivity. Preferable candidates for LN2 stimulation to generate multiple fractures in shale reservoirs are recommended. It is also found that compared with H2O and supercritical CO2, fracturing using LN2 can generate fractures with lower initiation pressure and greater complexity. Preliminary results confirm the potential of using LN2 fracturing to improve the stimulation effect of shale reservoirs.

Development, mechanical characterization and high temperature tribological evaluation of magnetron sputtered novel MoS2-CaF2-Ag coating for aerospace applications

In this research paper, mechanical and tribological characteristics of novel MoS2-CaF2-Ag composite coating are presented. A single layer MoS2-CaF2-Ag composite coating of ∼530 nm thickness was developed on highly polished AMS 5898 bearing steel substrate using direct current (DC) and radio frequency (RF) magnetron sputtering of MoS2, CaF2 and Ag targets in an Argon plasma. The coating was studied for its coefficient of friction (COF) in normal relative humidity (RH) 60 ± 5 % and temperature ranging from room temperature (RT) to 250°C using rotary ball-on-disc tribometer. Field emission scanning electron microscopy (FESEM) with energy dispersive spectroscopy (EDS) was used to study the surface morphology and elemental mapping of the as-deposited coating and also for in-situ examination of the wear tracks. The developed coating demonstrated good hardness, dense microstructure and low COF (0.03–0.06). Minimum wear rate of 1.97 × 10−5 (mm3/Nm) was observed at 150 °C. Blister wear, brittle cracking, spallation and delamination was observed at RT and 50 °C and combination of abrasive, adhesive and oxidative wear at 100 °C to 250 °C. The MoS2-CaF2-Ag composite coating proved efficient in reducing COF and wear under ambient atmospheric conditions, making it highly suitable for solid lubrication of tribo systems in space applications.

Applying state‐of‐the‐art microscopy techniques to understand the degradation of copper for nuclear waste canisters

AbstractThe metallurgy, mechanical properties, and corrosion of Cu, proposed as the corrosion barrier for Canadian used fuel containers (UFCs) for use in a deep geological repository (DGR), have been studied for decades. Bulk properties have been reported, and the knowledge applied to strengthen the performance of Cu produced by electrodeposition and cold spray. Despite this success, many degradation mechanisms are unclear from bulk testing, which cannot capture nanoscale phenomena driving degradation. This study provides examples using state‐of‐the‐art microscopy to understand mechanisms of Cu degradation, specifically for UFCs. Subtle changes in the electrodeposited Cu microstructure due to oxygen segregation are observed using atom probe tomography (APT). In aggressive sulfide‐containing environments, corrosion of the cold‐sprayed Cu occurs along particle–particle interfaces, likely inherent to the manufacturing process. Microscale tensile testing at particle–particle interfaces confirms brittle cracking in the cold‐sprayed Cu. Although experiments are not consistent with DGR conditions, results do warrant further study on the performance of the cold‐sprayed Cu in particular.

Qualitative SEM analysis of fracture surfaces for dental ceramics and polymers broken by flexural strength testing and crown compression.

To perform qualitative analysis using scanning electron microscopy (SEM) of fracture surfaces for ceramic and polymeric dental materials broken via standardized flexural and crunch-the-crown (CTC) tests. Zirconia, glass-ceramic, and polymeric (Trilor; TRI, Juvora; JUV, Pekkton; PEK) materials were loaded using crowns for CTC tests, discs (zirconia and glass-ceramics) for piston-on-3 ball tests, bars (polymer) for 3-point bend tests, and bars (zirconia, glass-ceramics) for 4-point bend tests. SEM was used to characterize the fracture surfaces and identify fracture surface features (e.g., origin, mist, hackle, and the direction of crack propagation [DCP]). Electron dispersive spectroscopy was used to identify the local chemistry. Fracture surface features were found to be less visually apparent for glass-ceramics than zirconia. For zirconia bars, fractures originated roughly midway between the corner and center for processing defects related to sintering. Fractures originated at the bottom corners of glass-ceramic bars (void or surface flaw) and PEK bars (surface flaw). TRI bar failures exposed glassy fibers. Fracture features were generally less discernable for discs compared to bars for zirconia and glass-ceramics. Ceramic crowns fractured into 2 to 3 pieces, with fractures originating at the occlusal surface and clear evidence for the DCP. Failures of TRI and JUV specimens (bars and crowns) were less catastrophic than for the ceramics, with exposed fibers (TRI) and surface cracks (JUV). PEK crown and bar fractures presented dimple (ductile) features formed due to microvoid coalescence followed by brittle crack propagation. The critical flaws responsible for failure initiation were a function of material composition and test configuration. Fractographic analysis can reveal problems associated with the manufacturing of materials, their handling, grinding and finishing/polishing procedures, the structural design and choice of material, and the quality of the final laboratory-delivered restoration.

Fractal Analysis on Machined Surface Morphologies of Soft-Brittle KDP Crystals Processed by Micro Ball-End Milling.

The micro-defects on KH2PO4 (KDP) optic surfaces are mainly repaired by the micro-milling technique, while it is very easy to introduce brittle cracks on repaired surfaces, as KDP is soft and brittle. To estimate machined surface morphologies, the conventional method is surface roughness, but it fails to distinguish ductile-regime machining from brittle-regime machining directly. To achieve this objective, it is of great significance to explore new evaluation methods to further characterize machined surface morphologies. In this study, the fractal dimension (FD) was introduced to characterize the surface morphologies of soft-brittle KDP crystals machined by micro bell-end milling. The 3D and 2D fractal dimensions of the machined surfaces and their typical cross-sectional contours have been calculated, respectively, based on Box-counting methods, and were further discussed comprehensively by combining the analysis of surface quality and textures. The 3D FD is identified to have a negative correlation with surface roughness (Sa and Sq), meaning the worse the surface quality the smaller the FD. The circumferential 2D FD could quantitively characterize the anisotropy of micro-milled surfaces, which could not be analyzed by surface roughness. Normally, there is obvious symmetry of 2D FD and anisotropy on the micro ball-end milled surfaces generated by ductile-regime machining. However, once the 2D FD is distributed asymmetrically and the anisotropy becomes weaker, the assessed surface contours would be occupied by brittle cracks and fractures, and corresponding machining processes will be in a brittle regime. This fractal analysis would facilitate the accurate and efficient evaluation of the repaired KDP optics by micro-milling.

Evaluation of Absorbed Energy in High-Velocity Particle Impact for Aero-Engine Materials

A single particle impact testing system was developed to estimate an absorbed energy in particle perforation process of aero-engine materials. Thin target plates of 0.1-1 mm thickness were impacted by a 3 mm diameter spherical particle. A wide range of impact velocities from 50-375 m/s was covered using a compressed gas gun. First, particle impact tests were conducted on aluminum and copper plates to evaluate the validity of the experimental system. The experimental results show that there is no significant difference in the effect of the particle impact velocity on the absorbed energy of aluminum and copper, and the ballistic limit can be reasonably estimated based on a simple energy balance equation. Second, impact tests were conducted on TiAl alloys and Ni-based alloys. There was no clear ballistic limit for the TiAl alloys, and the absorbed energy tended to increase as the particle impact velocity increased. The absorbed energy of the TiAl alloy was half that of the Ni-based alloy for specimens of the same thickness. Radial and petal-shaped cracks were observed in the Ni-based alloys after impact tests, while brittle cracks, fractures, and scattering of debris were observed at the periphery of the penetration hole in the TiAl alloys.

Significantly improved strength and plasticity of a refractory high-entropy alloy at small length scale

Refractory high-entropy alloy (RHEA) VNbMoTaW exhibits remarkably high strength in a much wide temperature range, whereas the brittle fracture behavior limits the applications. Here we show a significant size effect on the strength, plasticity and fracture behaviour of the VNbMoTaW RHEA. As sample size goes from millimeter to sub-micrometer scale, the brittle cracking behavior of the RHEA is greatly suppressed, along with a simultaneous enhancement of strength and plasticity. The improved mechanical properties can be attributed to the disappearance of crack initiation sites, the size-dependent dislocation plasticity at small length scale and the size-dependent competition in thermodynamics driving-force for cleavage cracking and plastic yielding. The results imply a promising potential of the RHEA for making microscale devices with high temperature environment.

Study of mechanical properties of grouting defective sleeve

When sleeves are grouted at the construction site, grouting defects occur inside the sleeves owing to the structure of the grout sleeve, plugging time of the grout outlet hole, and connection form of the sleeve. Grouting defects reduce the connection strength of grout sleeves and further affect the safety performance of assembled building structures. In this study, the failure modes of grout defects and their influencing factors were fully investigated, which provides guidelines for the standardization of sleeve grouting construction. First, a uniaxial tensile test was performed on 33 grouting defect specimens to study the mechanical properties of the grouting defect. Second, the Concrete damaged plasticity (CDP) and Brittle Cracking (BC) constitutive models of grout were considered and optimized, and the corresponding numerical models were established to further analyze the failure modes of sleeve grouting. The numerical analysis results were compared with the test results. The comparison results show that adopting the optimized CDP constitutive model in the numerical simulation is more consistent with the test results. Finally, the stress mechanism of the grout sleeve was theoretically analyzed, and the calculation equation for the anchorage length was improved. The experimental study revealed that the ultimate bearing capacity and displacement of the specimen were affected by the specimen size, and the large-size specimen was more prone to pull-out failure. In addition, theoretical analysis showed that the specimen was in a safe state when the anchorage length of the rebar was greater than 6d. Based on the investigation results, the improved anchorage length equation can be used to calculate the anchorage length of the grout sleeve in sleeve grouting construction. Moreover, the middle and lateral grout defects of the grout sleeve should be focused on in sleeve grouting construction to ensure the quality of sleeve grouting.

Edge cracking of WEST tungsten actively cooled plasma facing components after plasma operation

The WEST tokamak operates with actively-cooled tungsten ITER-like divertor plasma facing units (PFUs) since 2018. Poloidally-distributed cracks along the leading edges of PFUs were observed after WEST plasma operations. Because cracks may have an impact on the PFUs lifetime, mechanisms related to the cracks initiation and propagation are still under investigation. Previous numerical simulations lead to assess the location of the privileged initiation of the first crack on the PFU and the rationale for its initiation (disruption events causing brittle crack). In this paper, the goal is to identify, the most likely mechanism involved in the PFU damage process during transient. For this, in the first part of this paper, the observed crack topologies (depth and width of cracks) are described based on the recent post-mortem analysis in WEST. Then numerical simulations assuming initial crack(s) at the leading edge lead to assess the evolution of the stress mechanical field and explains the cracking mechanisms. Results of the simulations assuming several initials cracks (primary, secondary) highlight the leading edge periodic cracking under successive disruptions up to a threshold which corresponds to a minimum distance between two cracks and no further cracks are generated. The cracking threshold length has been assessed between 0.4 mm and 0.8 mm which is fully consistent with the WEST post-mortem observations. Finally, the leading edge crack initiation domain is specified as function of the thermal quench disruption impact factor and the component misalignment.

Effective grain size dependence of crack propagation resistance in low carbon steel

Unlike numerous studies focused on the crack initiation occurred in steel materials, the influence of effective grain size on crack propagation resistance does not receive enough attention and the intrinsic mechanism is not elucidated. In this work, comparable matrix microstructures with varying effective grain sizes were designed using different weld thermal simulations. Instrumented Charpy impact tests were conducted on these specimens to document the variation of fracture behaviors. The results showed that the impact toughness changes notably regardless of their similar matrix microstructure. By sub-dividing the total impact absorbed energy into two parts based on the maximum load, we find a new inverse proportional relation between the effective grain size and energy dissipation at the crack propagation stage. This new function, which is akin in form to the classical RKR model, may have board applications for optimizing the fracture behavior because it explicitly displays the effect of metallurgical factors on crack propagation resistance. This relation further implies that the crack propagation resistance is more strongly dependent on the grain size compared to the crack initiation resistance. With focus on the brittle crack propagation stage, a sharp decrease in brittle cracking speed for fine-grained specimens is attributed to the mechanism that numerous tear-ridges and side ligaments formed by local plastic deformation exert a closure stress at the wake of crack tip.

A Comparison of Low-Temperature Deformation Behavior and Fracture in Low-Carbon Steel Specimens Obtained by Electron Beam Additive Manufacturing and Conventional Casting and Normalization

In the present work, the microstructure, phase composition, and temperature dependence of the mechanical properties and fracture micromechanisms of low-carbon steel produced by conventional casting and electron beam additive manufacturing have been studied. Regardless of the manufacturing method, the phase composition of steel consists of ferrite with an insignificant fraction of carbides (pearlite grains in both types of steel and single coarse precipitates in the additively fabricated one). It was shown that the studied steels are characterized by a strong temperature dependence on yield strength and ultimate tensile strength. At T = 77 K, both types of steel are characterized by high strength properties, which decrease with increasing test temperatures up to 300 K. In addition, all deformation curves are characterized by the presence of a yield drop and yield plateau over the entire temperature range under study (77 K-300 K). A decrease in test temperature from 300 K to 77 K leads to a change in the fracture micromechanism of the steels from a dimple fracture to a cleavage one. Despite the similar deformation behavior and strength properties, the additively fabricated steel possesses lower elongation to failure at 77 K due to an insignificant fraction of coarse precipitates, which assists the nucleation of brittle cracks.

Some Features of Cleavage Cracks in Rocks and Metals

The cracking of some rocks, namely granite, serpentinite and sandstone under tensile stress is examined in details. Brazilian testing or diametral compression, three points bending and explosion testing are used as the loading schemes in air at room temperature. Morphology of cracks in the model rocks are compared with cracks in silicon crystals, as the standard of a brittle crack, with cleavage cracks in iridium single crystals and with cracks in gallium-covered aluminum single crystals. The comparison of cracks between themselves has shown that there is additional channel for stress accommodation in the model rocks. This channel does not lead to transformation of a rock into a macroscopically ductile material, but it causes the arrest of the dangerous crack in it under tensile stress. Its influence causes transition from the brittle crack to the pore-like crack on the microscopic scale. The most probable mechanism of this transition is the dislocation emission from crack, which becomes possible in such a natural covalent solid as a rock due to Rehbinder's effect.

The effect of microstructure and mechanical properties on sliding wear and cavitation erosion of plasma coatings sprayed from Al2O3 + 40 wt% TiO2 agglomerated powders

This paper discusses the influence of 40 wt% TiO2 addition to Al2O3 and the selection of plasma spray parameters on the morphology, microstructure, microhardness and adhesion of coatings. Mainly, the tribological behavior and cavitation erosion performance of produced coatings were evaluated. First, the two-factorial design of the experiment, with the stand-off distance and torch scan speed as variables, was used to plan coating deposition by means of atmospheric plasma spraying (APS). Then, a process response using a multiple regression method was used to investigate the influence of factors variation on coatings functional properties. Although these properties were already investigated in alumina-titania coatings, there are still some knowledge gaps. Particular attention was given to the composition of Al2O3 + 40 wt% TiO2, because, so far, it is the most rarely discussed in the literature in terms of, e.g., cavitation or wear resistance. Increasing the torch scan speed and decreasing the spraying distance densifies the microstructure and increases coating's mechanical properties. A dense and low porosity structure facilitates cavitation erosion resistance and minimizes friction coefficient. Cavitation erosion damage relies on brittle cracking initiated at coatings porosity. Therefore, coatings with decreasing porosity show layer-like removal contrary to uniform material removal stated for porous coatings. The sliding wear mechanism has been dominated by abrasive wear and fatigue cracking. Accumulated wear debris undergoes adhesive smearing and removal.