Prevent Crack Initiation Research Articles

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong.

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

Preparation and wear properties of high-silicon high-vanadium wear-resistant alloy with nano pearlite matrix and carbides composite structure

Combining high hardness carbide of high vanadium cast iron with nano pearlite, which has high strength and toughness, represents an innovative attempt to explore a new technology for preparing high vanadium alloy. In this study, a high-silicon high-vanadium wear-resistance alloy (HSHVWRA) was designed, incorporating a nano pearlite matrix and carbides composite structure. By increasing the vanadium content, numerous high hardness micrometer vanadium carbides were obtained The CCT curve was shifted to the left by increasing the silicon content in the alloy. Subsequently, the nano pearlite matrix was obtained by air cooling after austenite at 1100 °C. After heat treatment, the inter lamellar spacing of pearlite decreased from 500 to 1500 nm in the as-cast state to 50–200 nm. The heat treatment process significantly improved the wear resistance of HSHVWRA by obtaining a nano pearlite matrix. The weight loss due to abrasive wear in the nano pearlite matrix-based HSHVWRA was reduced by 26.9 % compared to the traditional micro-pearlite matrix-based alloy. In the impact wear test, the wear weight of the nano pearlite material was reduced by 63.7 % compared to the cast sample, resulting in a maximum relative wear resistance of 2.75. Nano pearlite has the ability to prevent crack initiation between the matrix and carbides and inhibit the propagation of fatigue cracks, thereby enhancing the fatigue resistance of the matrix. Additionally, with a higher number of layered sheets, nano pearlite can effectively withstand impact loads. Meanwhile, submicron carbides dispersed in the nano pearlite matrix contribute to a second phase strengthening role and cooperate with nano pearlite to enhance the wear resistance of the materials.

Rapid solidification induced low lattice misfit and associated enhancement of mechanical property in the NiCoCr medium entropy alloy

Medium entropy alloys (MEAs) have a high potential in industrial applications due to their excellent mechanical properties. Here we report a strategy to improve the ductility of the Ni40Co30Cr20Al5Ti5 MEAs by rapid solidification (RS). The as-cast alloy is composed of dendritic structure and L12 nano-precipitates. After RS, the lattice misfit of precipitate and matrix is reduced from 0.251 % to 0.167 %, and the size of the precipitate decreases from 85 nm to 28 nm. Meanwhile, RS also reduces the grain size of matrix. Gliding dislocation can cut through precipitates with low lattice misfit and smaller size, which can avoid strain accumulation and prevent crack initiation at the precipitate-matrix interfaces. Compared with the as-cast alloy, the ductility of the RS alloy is increased by 17 %–42 % without a decrease in strength. Our results can provide a guidance for other experiments using the RS technology (laser cladding, magnetron sputtering, etc.) to achieve the strength-ductility balance in the MEAs.

Effect of microstructural characteristics on the impact fracture behavior of cryogenic 9Ni steel

The percentage of shear fracture largely determines the service performance of 9Ni steel used in low-temperature pressure vessels. Through elemental analysis, microstructural characterization, and mechanical property tests, this study investigates why the percentage of shear fractures is low in 9Ni steel and clarifies the mechanism by which the microstructural characteristics influence the low-temperature impact behavior of 9Ni steel. It was found that cleavage fracture zones, formed when segregation bands appear in the microstructure, decrease the percentage of shear fractures at the impact fracture surface. Specifically, as the segregation area increases from 0.9% to 7.1%, the shear-fracture percentage in 9Ni steel sharply decreases from 100% to 65%, accompanied by a deterioration in low-temperature toughness. The segregation zone is enriched in austenite-forming elements (Ni, C, Mn), leading to a tempered martensite microstructure with a lath shape. The small number of high-angle grain boundaries and low interface bonding strength cannot effectively prevent crack initiation and propagation, resulting in brittle cleavage fracture. In contrast, the non-segregated zone is tempered sorbite with a uniform structure, several high-angle grain boundaries, and a high interface bonding strength. These features hinder crack initiation and propagation. Furthermore, the shear-fracture zone generated in the non-segregated zone exhibits ductile fracture characteristics.

Dual composite architectonics: Fracture toughness and self-healing of ZrB2–SiC–TaB2 based UHTC

This study explores the effect of structural architecture of ZrB2–SiC–TaB2 composites on their fracture toughness and ability to self-heal operational surface defects. It is shown that the dual composite architecture provides a synergistic combination of several effects enhancing the fracture toughness of the material: crack arrest in the developed system of polymodal interphase boundaries, and formation of elastic microstress fields. In addition, SiC that is rapidly oxidized and forms an easily fusible borosilicate layer endows MeBx-based dual composites with the ability to self-heal surface defects when operating at temperatures above 1000°С in an oxygen-containing environment, preventing crack initiation from surface stress concentrators. The analysis of the residual strength after self-healing in comparison with the initial data of the studied materials allowed the optimal matrix-inclusion ratio, minimizing the loss of reliability.

Technological Processes for Increasing the Cavitation Erosion Resistance of Nimonic 80A Superalloys.

Nickel-based superalloys are frequently used to manufacture the components that operate under cavitation erosion conditions, such as aircraft gas turbine construction, nuclear power systems, steam turbine power plants, and chemical and petrochemical industries. Their poor performance in terms of cavitation erosion leads to a significant reduction in service life. This paper compares four technological treatment methods to improve cavitation erosion resistance. The cavitation erosion experiments were carried out on a vibrating device with piezoceramic crystals in accordance with the prescriptions of the ASTM G32-2016 standard. The maximum depth of surface damage, the erosion rate, and the morphologies of the eroded surfaces during the cavitation erosion tests were characterized. The results indicate that the thermochemical plasma nitriding treatment can reduce mass losses and the erosion rate. The cavitation erosion resistance of the nitrided samples is approximately 2 times higher than that of the remelted TIG surfaces, approximately 2.4 times higher than that of the artificially aged hardened substrate, and 10.6 times higher than that of the solution heat-treated substrate. The improvement in cavitation erosion resistance for Nimonic 80A superalloy is attributed to the finishing of the surface microstructure, graining, and the presence of residual compressive stresses, factors that prevent crack initiation and propagation, thus blocking material removal during cavitation stresses.

Improved impact toughness of laminate heterogeneous AZ31/GW103K alloys by interface delamination

An AZ31/GW103K (AG) laminate heterostructured material was fabricated by diffusion bonding. The impact resistance of AG was evaluated by Charpy notched impact testing and the fracture mechanism was systematically studied. The impact toughness of AG is nearly twice of the homogeneous structured AZ31 or GW103K. The delamination can effectively prevent crack initiation and propagation under the high-speed impact, which significantly improves the overall impact toughness by consuming high impact energy. Simultaneously, the cracks crossing over the Al2(Gd, Y) zone at the interface consume considerable impact energy. Besides, the conventional thermal strain, composition diffusion, and the mechanical incompatibility of AG can also consume energy by coordinating plastic deformation. During impact, the primary deformation mechanism of the GW103K layer is {0001} basal plane slip. While the AZ31 layer is relatively complicated, primarily of twinning, multiple dislocation slip, and stacking faults.

Investigation of Strain Fatigue Behavior for Inconel 625 with Laser Shock Peening.

With excellent creep resistance, high-temperature thermal strength and high-temperature fatigue strength, Inconel 625 is widely applied to fabricate structural components in the aerospace field, where fatigue life is a key point. Laser shock peening (LSP) is considered to improve the fatigue strength and fatigue crack growth resistance of metal materials. The present work was conducted to investigate the influence of LSP on strain-controlled fatigue behavior of Inconel 625. The surface microstructures of specimens before and after LSP were observed by transmission electron microscope (TEM). The strain-controlled fatigue loading tests with different strain amplitudes ranging from 0.4% to 1.2% were carried out on the specimens, and the topography of fracture appearance was examined by scanning electron microscope (SEM). The investigations showed that the specimens with LSP presented fewer crack initiations, shorter fatigue striations space and smaller dimples or micropores, which account for the enhancement of the fatigue life for the LSP specimens. Furthermore, the plastic deformation, ultra-fine grains, twins and dislocations caused by LSP could prevent crack initiation, crack propagation and ultimate fracture, hence prolonging the fatigue life of the Inconel 625. In addition, it was revealed that the cyclic strain hardening as well as cyclic strain softening remains almost the same to Inconel 625 with or without LSP.

Effect of high-pressure torsion on high cycle fatigue of commercially pure Cu: Some insights from formation of surface micro-cracks

In most technical applications, fatigue is related to highly localized load distributions. While high-pressure torsion (HPT) materials cannot prevent crack initiation, they promise to improve the resistance to crack nucleation and propagation. Herein, commercially pure Cu was processed through HPT at a pressure of 6 GPa up to 50 turns. Small “cantilever” type samples were fabricated from annealed and HPT samples. The cantilever samples were subjected to fully-reversed cyclic bending. The maximum stress amplitude was chosen to reach the high cycle fatigue regime, and the experiments were stopped when the resonant frequency decayed by 20%. Compared to the annealed samples, the HPT samples showed higher lifetimes. The grain size in the HPT samples remained stable during fatigue, and dislocation substructures, a stacking of parallel dislocations, could be observed in all samples. In HPT samples, the area fraction of surface micro-cracks increased with the local stress amplitude. This can be attributed to the inhibition of crack nucleation at low stresses due to the high strength of HPT samples and the crack arrest at the boundaries of their ultra-fine grains. The obtained insights into the microstructure-fatigue response relationship are vital for understanding the initial stages of fatigue failure in ultra-fine-grained materials and their technological adoption for applications in extreme environments.

Influence of film structure on the microstructure and properties of TiAlN coatings on Al-Si alloys

Due to the disadvantages of poor friction and low wear resistance, the application of Al-Si alloys with unique properties in pistons is limited. In this work, TiAlN ceramic coatings were prepared by filtered cathode vacuum arc technique to improve the surface properties of Al-Si alloys. To overcome the high residual stress of hard coatings growth on soft substrate surfaces due to interfacial mismatch and defect accumulation, gradient and multilayer film structures were adopted to improve adhesion strength. The effects of film structure on the microstructure, mechanical and tribological properties of coatings were systematically investigated. The results showed that the coatings with different film structures showed a multiphase microstructure with the dominated phase of TiAlN and a small amount of AlTi and TiN phases. Monolayer and multilayer coatings exhibited TiAlN (111) orientation, while the gradient coating showed (220) orientation due to the growth of columnar to equiaxed crystals. Compared with the monolayer coating, TiAlN coatings with gradient and multilayer structures had significantly reduced compressive stress and improved adhesion and scratch propagation resistance. Among them, the multilayer coating showed the lowest friction coefficient (~0.44) and wear rate (~5.14 × 10 −5 mm 3 /Nm), which were attributed to the high hardness (~31.01 GPa), toughness (H/E = 0.135, H 3 /E 2 = 0.563 GPa), critical load L C3 (~26.58 N). Interfacial effects in the multilayer structure could enhance synergy of the coating, thereby preventing crack initiation and propagation and adhesion failure. • TiAlN coatings with different structures were prepared on Al-Si alloys by FCVA. • Coatings were dominated by fcc TiAlN phase with less AlTi and TiN phases. • Adhesion of gradient and multilayer coatings were effectively improved. • Multilayer coatings showed the best mechanical and tribological properties.

On the Chemical Composition of Non-Metallic Inclusions in Ultra-Clean Gear Steels

Abstract Fisheye failures due to non-metallic inclusions can lead to the reduced endurance fatigue limit of case-hardened, shot-peened gears. Various approaches for preventing crack initiation at non-metallic inclusions are conceivable. As one approach, steel manufacturers have invested much effort in the steelmaking process to produce ultra-clean gear steels. Such ultra-clean gear steels were investigated in a gear research project. For each steel batch, different measures were taken during steel production to avoid or bind off non-metallic inclusions in order to reduce or even suppress the harmful effect of the non-metallic inclusions. However, the influence of the measures taken on the chemical composition of non-metallic inclusions in such ultra-clean gear steels is not investigated in the gear industry in detail so far. Therefore, in the framework of this publication, the chemical composition of non-metallic inclusions in ultra-clean gear steels is investigated. Furthermore, a comparison of the chemical analysis with the crack-initiating inclusions from the experimental investigations is performed.

Effects of graphene and CNTs reinforcement on the friction mechanism of nitrile butadiene rubber under water lubrication conditions

Nitrile butadiene rubber (NBR) is a water-lubricated bearing material that demonstrates excellent vibrational absorption and impact resistance; however, rapid wear under abnormal conditions is still a serious problem. Graphene (Gra) and carbon nanotubes (CNTs) are two types of carbon nanomaterials with different dimensions and excellent mechanical and friction properties. In this work, Gra and CNTs were used to fill the NBR, and a multifunctional friction and wear testing machine was used to study their impacts on the friction between NBR and ZCuSn10Zn2 under water lubrication conditions. The results indicate that both Gra and CNTs can effectively reduce the adhesion and the friction coefficient of NBR during the friction process. The two-dimensional lamellar structure of Gra is more likely to precipitate from the NBR matrix to generate a surface lubrication film, which can effectively reduce the friction coefficient. Both Gra and CNTs can prevent crack initiation and propagation in NBR during the wear process, thereby effectively reducing the wear of NBR. Compared with the two-dimensional structure of Gra, the one-dimensional linear structure of carbon nanotubes can “trap” the NBR molecular chains more effectively during the friction process, thus reducing the initiation and propagation of wear cracks and further improving the wear performance of NBR.

Initiation Mechanism of Transverse Cracks in Wind Turbine Blade Trailing Edge

Transverse crack often occurs in the trailing edge region of the blade when subjected to the excessive edgewise fatigue load. In this paper a refined model was established through local mesh refinement methods in order to investigate the initiation mechanism of crack and its extension in blade trailing edge. The material stress around the crack in trailing edge region under different thicknesses is calculated based on the fracture mechanics theory. The factors affecting the fatigue robustness of blade trailing edge are concluded by investigating the results of finite element analysis and coupons test. Compared with the laminate, the lower fatigue strength of the adhesive is the cause of the transverse crack of the adhesive joint at the trailing edge. The increase of the adhesive thickness at the adhesive joint will significantly increase the stress concentration factor at the crack region and accelerate the crack extension of the laminate. In final, a practical design scheme to prevent crack initiation is given for the manufacture of the wind turbine blade.

Development of a thermo-mechanical behaviour model adapted to the ITER vacuum vessel material

The ITER machine has been classified as a Basic Nuclear Installation French nuclear regulator (INB n°174), which implies that it will be the first fusion reactor to go through complete French nuclear licencing. The combination of mechanic and electromagnetic phenomena with the heat loads caused by neutron streaming requires a multi-physics approach to the damage assessment, which has not yet been implemented in the common nuclear codes and standards.The general damage prevention methodology consists in guaranteeing the structural integrity of a component. The development of design rules has mainly two origins: prevention of damage from monotonic mechanical loads and prevention of damage from repeated application of loads. In most cases, structural integrity is justified within a linear elastic behaviour but when this route is not enough to respect the design criteria, several non-linear approaches to the material's mechanical behaviour can be considered, requiring more elaborated demonstration of the design compliance. Nevertheless, the models proposed in the nuclear model database are sometimes not sufficient to properly describe the experimentally observed cyclic plasticity behaviour and, in particular, the ratcheting and shakedown phenomena.According to ITER community experts in materials and analyses, a thermo-mechanical behaviour model fitting the ITER Tokamak materials data will guarantee the best prediction of the damage considering a nuclear and a multi-physic loading condition.This paper describes the assessment of the non-linear behaviour of Vacuum Vessel (VV) material with a strong thermo-mechanical coupling and a damage parameter to prevent crack initiation. More precisely, Chaboche's models available in the literature (elasto-(visco)-plasticity models, with various types of hardening) have been enriched in order to explicitly take into account the influence of the temperature on the mechanical behaviour and, reciprocally, the influence of the mechanical behaviour on the temperature. Mechanical cycling tests have been performed on the VV constitutive material, emphasizing on the progressive deformation state up to failure mode, i.e., ratcheting. The proposed models have been tested on a homogeneous problem and the results compared with uniaxial test results; this type of simulation is commonly called “0D”analysis. The last part of this document describes the finite element implementation of the constitutive material model and its application to the ITER VV welded support.

Cell structures, phase morphologies and impact toughness of phenolphthalein poly(ether ether ketone)/epoxy composite foams

AbstractThermoplastic phenolphthalein poly(ether ether ketone) (PEK‐C) /epoxy composite foams were prepared by free foaming process (Fre‐foams) and limited foaming process (Lim‐foams), respectively, and the effects of PEK‐C content and foaming methods on the cell structures, phase morphologies, and impact strength of epoxy foams were investigated. The results showed that the gas pressure in the bubbles promoted the resin curing process and polymerization‐induced phase separation process (PIPS) during the free foaming process, while the resin curing process and PIPS were highly constrained in Lim‐foams. The incorporation of PEK‐C in the Fre‐foams showed superior improvement in impact strength compared with the PEK‐C/epoxy Lim‐foams. By adding 12 wt% PEK‐C, the specific impact strength of Fre‐foams could be enhanced by 72%, and the improved specific impact strength was because the phase‐inverted structures could prevent crack initiation and propagation, and moreover the presence of PEK‐C led to significantly refine cell structure and reduce foam density. Interestingly, the impact strength of Lim‐foams was nearly independent on the PEK‐C content, it was attributed to the obviously increased cell size with increasing PEK‐C content, and also that the phase separation process was not carried out.

Study on Thermally Induced Crack Propagation Behavior of Functionally Graded Materials Using a Modified Peridynamic Model

Peridynamic (PD) theory is used to study the thermally induced cracking behavior of functionally graded materials (FGMs). A modified thermomechanical peridynamic model is developed. The thermal crack propagation of a ceramic slab in quenching is calculated to validate the modified PD model. The results predicted by the modified PD model agree with previously published numerical and experimental ones. Compared with the original PD model, the calculation accuracy of the modified PD model for thermal cracking is improved. The thermal cracking in FGMs is also simulated. The effects of material shape, initial temperature, and ceramic fracture toughness on thermal crack propagation behaviors are studied. It can be found that the thermal cracks in FGMs are still in periodical and hierarchical forms. The metal materials in FGMs can prevent crack initiation and arrest the long cracks. The crack number tends to be increased with the increasing initial temperature, while the strengthened ceramic fracture toughness can decrease it.

Contribution to the quality improvement of manganese steel Z120 MC12

Manganese steel, known by the name of its inventor, steel Hedfield is deemed by its high mechanical shock resistance, it is used in the field of crushing and grinding. This type of steel is obtained by adding manganese (Mn) between (12 & 18%) to get a metallographic structure austenitic. For a very good quality of the manufactured parts, and to prevent crack initiation intergranular generated by the presence of pre-cipitates or clusters of carbides in the areas near the grain boundary, it is necessary to conduct a treatment hyper thermal quenching at 1050°C to dissolve these clusters. On all treated samples, we find the presence of an austenitic structure with a homogeneous distribution of grains having a size to be correct [1]. However, it appears as a network, the presence of a network of black dots (as precipitates) we find abnormal, and which we know neither the nature nor its impact on the metallurgical quality of the material. To solve this problem, we con-ducted a specific analysis by scanning electron microscopy (SEM) to explain the nature of these blackheads and subsequently, tried to judge when its influence on the structure of austenitic manganese steel

Laser 3D Printing of Fe-Based Bulk Metallic Glass: Microstructure Evolution and Crack Propagation

This work presents a comprehensive study of microstructure evolution, crack development and hardness performance of Fe-based bulk metallic glass parts processed by laser 3D printing. The combination of a low scan speed and high energy density generates a low temperature gradient, leading to supercooling and the formation of coarse dendrite and cellular crystal microstructure. Columnar dendrites grow in a single direction with large stress, and cracks are easily formed between the dendrites. The stress intensity factor (SIF) caused by the crack surface tension is small, the crack healing effect is weak, and it is difficult to prevent crack propagation. Cellular grain growth is uniform and more easily accommodates the strain, preventing crack initiation and propagation. In contrast, use of a high scan speed with low energy input produces a fine and uniform dendrite or even nanocrystalline, microstructure with microcracks. The SIF caused by the crack surface tension is large, and the crack healing effect is strong and prevents crack initiation and propagation. For low energy density, the hardness of the sample is similar to the hardness of the cast amorphous. This result indicates that the printing sample retained some amorphous characteristics.

3D printing of biomimetic composites with improved fracture toughness

Recent researches show that material microstructural designs mimicking biomaterials offer an effective way to produce tougher composites. To reveal the underlying microstructure-toughness relationship, five bioinspired material microstructures are investigated experimentally, including the brick-and-mortar, cross-lamellar, concentric hexagonal, and rotating plywood microstructures. The feature sizes of these microstructures are controlled to be one order smaller than the specimen size, providing better pictures of how crack resistance interacts with heterogeneity. Fracture theories are further used to analyze the toughening mechanisms and find the design criteria. Results show that the rotating plywood structure presents a “J” shaped R-curve, while other structures show “Γ” shaped R-curves. The “J” shaped R-curve gives a larger critical energy release rate and tolerates a longer crack, thus preferable for crack arresting. By contrast, the “Γ” shaped R-curve provides a larger critical failure stress and is beneficial for preventing crack initiation. Moreover, combined experimental results and theoretical analysis suggest that heterogeneity improves toughness by 1) creating stiffness variations to slow down crack propagation and prevent crack penetration and 2) guiding cracks along weak interfaces to promote progressive damage. Our results shed new light on the structure-property relationship that will facilitate the design of tougher and better crack resistant composites.

Retrofit against fatigue cracking at web penetration details with a slot in steel girder

Fatigue cracking in steel girder web penetration details is so dangerous that it can break steel girders. The purpose of this study is to clarify fatigue behavior of web penetration details with cross beam flange welded to upper or lower surface of a slot by fillet welding, and verify the effect of retrofitting methods against fatigue cracking in web penetration details. As a result, fatigue cracks were initiated at the web-side toe of the fillet weld along cross beam lower flange edges both in upper and lower welded specimen. In the upper welded specimen, fatigue cracks were propagated into the web plate diagonally upward almost perpendicularly to the maximum principal stress direction. Whereas in the lower welded specimen, fatigue cracks were propagated into the web plate vertically downward faster than in the case of the upper welded specimen, and broke the lower flange. The fatigue life of the lower welded specimen was shorter than that of the upper welded specimen. As a preventive measure, weld toe grinding can extend fatigue life a little, but cannot prevent crack initiation perfectly. As a post cracking measure, a stop hole can stop crack propagation of long cracks, whereas crack removal and shaping up method by grinding can prevent crack propagation of short cracks.