Mechanism Of Crack Formation Research Articles

Investigation of dynamic crack formation mechanism based on a new crack dynamic driving model

To reveal the dynamic cracking process of deep rock caused by explosive initiation, the mechanical process of rock cracking is theoretically analyzed firstly under the combined action of explosion stress wave and geostress stress. A new crack dynamic driving model (CDD Model) based on the combined coupling effect is established. The feasibility and rationality of it in characterizing the dynamic cracking process are verified with numerical simulation and laboratory test. And the influence of ground stress on dynamic cracking process of presplitting blasting is revealed. The findings show that the dynamic cracking process of presplitting blasting is affected by explosion stress wave, explosive gas pressure, and ground stress. The opening of explosion crack shows a dynamic changing process of first increasing, then decreasing, and finally increasing. For conventional presplitting blasting, when the initial geostress level exceeds 10 ∼ 20 MPa, the stress transient unloading on fracture surface easily leads to the reduction of the explosive fracture opening. And the possibility of secondary opening and expansion of the explosive fracture is reduced. It will limit the driving effect of the subsequent explosion energy on the fracture expansion.

Microstructure, Properties and Crack Suppression Mechanism of High-speed Steel Fabricated by Selective Laser Melting at Different Process Parameters

To enrich material types applied to additive manufacturing and enlarge application scope of additive manufacturing in conformal cooling tools, M2 high-speed steel specimens were fabricated by selective laser melting (SLM). Effects of SLM parameters on the microstructure and mechanical properties of M2 high-speed steel were investigated. The results showed that substrate temperature and energy density had significant influence on the densification process of materials and defects control. Models to evaluate the effect of substrate temperature and energy density on hardness were studied. The optimized process parameters, laser power, scan speed, scan distance, and substrate temperature, for fabricated M2 are 220 W, 960 mm/s, 0.06 mm, and 200 ℃, respectively. Based on this, the hardness and tensile strength reached 60 HRC and 1000 MPa, respectively. Interlaminar crack formation and suppression mechanism and the relationship between temperature gradient and thermal stress were illustrated. The inhibition effect of substrate temperature on the cracks generated by residual stresses was also explained. AM showed great application potential in the field of special conformal cooling cutting tool preparation.

Microstructure evolution and crack formation mechanism of high-nitrogen bearing steel subjected to warm deformation

In this work, the microstructure evolution and crack formation mechanism of the X30CrMoN15-1 high-nitrogen steel under warm deformation (700 ℃ and 800 ℃) have been investigated. The microstructure observation indicates that the warm deformation (both at 700 °C and 800 ℃) facilitates the ferrite transition and the carbonitrides precipitation, which leads to the reduction of retained austenite by 40% in the specimen deformed 700 °C and 18% for the specimen deformed 800 °C. In addition, cracks are likely to generate under the low strain rates of 0.005 s−1. In the specimen deformed at 700 °C, the lamellar carbonitrides are precipitated along the interface between ferrite and M/A islands, while the spherical carbonitrides are precipitated inside the ferrite. The volume expansion generated by ferrite transformation and the growth of recrystallized grain are hindered by carbonitrides, resulting in the significant stress concentration and even crack initiation. For the specimen deformed at 800 ℃, the spherical carbonitrides are mostly precipitated from the recrystallized ferrite grain boundary. The significant dislocation aggregation around the austenite /ferrite interface and strength mismatch between ferrite and austenite together result in the interfacial debonding and even cracks propagation at phase boundary.

The Phenomenon of Cracking in Cement Concretes and Reinforced Concrete Structures: The Mechanism of Cracks Formation, Causes of Their Initiation, Types and Places of Occurrence, and Methods of Detection—A Review

Cracks and cavities belong to two basic forms of damage to the concrete structure, which may reduce the load-bearing capacity and tightness of the structure and lead to failures and catastrophes in construction structures. Excessive and uncontrolled cracking of the structural element may cause both corrosion and weakening of the adhesion of the reinforcement present in it. Moreover, cracking in the structure negatively affects its aesthetics and in extreme cases may cause discomfort to people staying in such a building. Therefore, the following article provides an in-depth review of issues related to the formation and development of damage and cracking in the structure of concrete composites. It focuses on the causes of crack initiation and characterizes their basic types. An overview of the most commonly used methods for detecting and analyzing the shape of microcracks and diagnosing the trajectory of their propagation is also presented. The types of cracks occurring in concrete composites can be divided according to eight specific criteria. In reinforced concrete elements, macrocracks depend on the type of prevailing loads, whereas microcracks are correlated with their specific case. The analyses conducted show that microcracks are usually rectilinear in shape in tensioned elements; in shear elements there are wing microcracks with straight wings; and torsional stresses cause changes in wing microcrack morphology in that the tips of the wings are twisted. It should be noted that the subject matter of microcracks and cracks in concrete and structures made of this material is important in many respects as it concerns, in a holistic approach, the durability of buildings, the safety of people staying in the buildings, and costs related to possible repairs to damaged structural elements. Therefore, this problem should be further investigated in the field of evaluation of the cracking and fracture processes, both in concrete composites and reinforced concrete structures.

Additive manufacturing of Ni-based superalloys: Residual stress, mechanisms of crack formation and strategies for crack inhibition

The additive manufacturing (AM) of Ni-based superalloys has attracted extensive interest from both academia and industry due to its unique capabilities to fabricate complex and high-performance components for use in high-end industrial systems. However, the intense temperature gradient induced by the rapid heating and cooling processes of AM can generate high levels of residual stress and metastable chemical and structural states, inevitably leading to severe metallurgical defects in Ni-based superalloys. Cracks are the greatest threat to these materials’ integrity as they can rapidly propagate and thereby cause sudden and non-predictable failure. Consequently, there is a need for a deeper understanding of residual stress and cracking mechanisms in additively manufactured Ni-based superalloys and ways to potentially prevent cracking, as this knowledge will enable the wider application of these unique materials. To this end, this paper comprehensively reviews the residual stress and the various mechanisms of crack formation in Ni-based superalloys during AM. In addition, several common methods for inhibiting crack formation are presented to assist the research community to develop methods for the fabrication of crack-free additively manufactured components.

Uncovering the white etching area and crack formation mechanism in bearing steel

The microstructure of a damaged bearing from the field was characterized in this work with the intention to better understand microstructural features behind formation of White Etching Cracks (WEC) in bearings. Microstructural characterization of the altered white etching area (WEA) involved conventional electron backscattered diffraction (EBSD), followed by transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). In addition, automated crystallographic orientation mapping in TEM was performed on lamellae from selected regions of the WEA extracted via focus ion beam milling. The results revealed that the orientation of detectable grains within WEA is similar to that of the vicinal bulk material. WEA consists of small spherical grains (average 30 nm) and the orientation of the grains varied significantly in the deformed zone, suggesting that recrystallization had occurred. The interface between bulk material and the deformed zone is very sharp. Furthermore, needle-like grains, most likely originating from the zone undergoing only modest levels of severe plastic deformation, occurred in WEA. The occurrence of different grain sizes in WEA and incomplete plastic deformation strongly support the hypothesis of WEC formation via severe plastic deformation followed by recrystallization.

Study on optimization of surface processing technology of silicon nitride bearing ring

Based on the difficult machining characteristics of silicon nitride materials, this manuscript focuses on optimizing the precision machining process of silicon nitride bearing components and improving the machining efficiency and quality of silicon nitride bearing components. Firstly, the mechanism of crack formation and propagation in hard and brittle materials under the action of abrasive particles is discussed in this paper, and based on this, a kinematic model of single abrasive grain cutting on hard and brittle materials is established. The effect of workpiece linear velocity, grinding wheel linear velocity, grinding wheel oscillation velocity and feed velocity on inner surface roughness of Si3N4 ring was discussed through grinding test. The experimental results show that the surface roughness can reach about Ra0.20 ∼Ra0.33 μm through a large number of grinding experiments by adjusting the combination of process parameters μm. On this basis, use the optimized process parameters calculated by the surface quality prediction model constructed in this manuscript to conduct grinding test again, and the surface roughness value reaches about Ra0.19 ∼Ra0.23 μm. The purpose of optimizing the process parameters is realized. Finally, the surface roughness can be further reduced and maintained at Ra0.05 ∼Ra0.06 μm by further superfinishing the surface after the process optimization with an oilstone. The research work of this manuscript realized the optimization of precision machining process of silicon nitride bearing ring and the rapid optimization of machining process through the established prediction model, which improved the precision machining efficiency of ceramic bearing components. Through the study of this manuscript, a process route of controllable processing of inner surface quality of Si3N4 ring is formed, from preliminary selection of process parameters by optimization model to precision grinding and then to ultra-finishing of whetstone, which provides theoretical reference for efficient and precise manufacturing of practical bearing components.

Designing a Computer Simulation Model to Study the Failure Mechanism in Rotating Shaft

The phenomenon of the failure of rotating mechanical structures is considered one of the dangerous phenomena that researchers are trying to study to find effective ways to avoid it. Many mathematical models describing the mechanism of crack formation in structures and the occurrence of failure in them have been adopted. In this paper, a computer simulation model of the failure mechanism in the intermediate conduction shaft of the ship's propeller (oil shuttle tanker) was designed based on the mathematical model that describes the stages of failure according to the number of revolutions that the shaft bears and the dimensions of fatigue cracks in it. In the beginning, a detailed mathematical model of the crack propagation mechanism was deduced as an initial stage, through which the differential equations that express the failure of the shaft were described. In the second stage, numerical simulations were implemented using the Euler and Rung-Kutta numerical methods used in solving ordinary differential equations. Also, for both methods using the Matlab language to reach the results and accurate graphs and analyze them. In this way it is possible to predict the number of cycles that the column will bear before the collapse, and this will reduce the occurrence of sudden collapses and save time, effort, and costs.

Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.

Study on the Development Law of Mining-Induced Ground Cracks under Gully Terrain

Coal seam mining in the gully area easily causes ground cracks and even induces landslides, which endanger the safety of mining areas. In this paper, combined with the mining conditions of a mining area in southern Shanxi Province, China, ground crack mapping, crack width dynamic monitoring, and the numerical simulation method are used to study the static and dynamic evolution law and the formation mechanism of ground cracks in the gully area. The research shows that ground cracks mainly include dynamic in-plane cracks and boundary cracks. The dynamic in-plane cracks show the characteristics of “opening first and closing later”. The boundary cracks show the characteristics of “only opening and not closing”. It is found that the closure of the dynamic in-plane cracks will decrease (compared with plain areas). The development of ground cracks experiences three stages: the initial formation stage, the dynamic development stage, and the gradually stable stage. The “goaf–surface” structure model and force chain arch structure model are established to more intuitively analyze the formation mechanism of ground cracks. The research results have a specific reference value for preventing ground disasters caused by underground coal mining and land ecological restoration.

Prediction of magnesium alloy edge crack in edge-constraint rolling process by using a modified GTN model

Edge cracks are a type of damage that critically affect the quality and properties of rolled Mg alloys sheet. Accurate damage prediction plays a key role in root cause analysis and process optimization. Although the phenomenological prediction of edge damage in Mg alloy rolling has made some progress so far, the prediction of edge crack behavior and analysis of the mechanisms of crack formation and evolution during rolling have not yet been well reported. In this research, a modified Gurson-Tvergarrd-Needleman (GTN) model was proposed by modifying the growth mode of shear damage. The three-dimensional single-unit finite element (FE) model test results indicated that the proposed model improved the accuracy of the fracture response under negative stress triaxiality. Using AZ31B Mg alloy as the case study material, physical experiments and FE analysis of the rolling process were conducted. The simulations showed that the proposed model can accurately reflect the morphology and location of cracks during the rolling process, and revealed that the high-stress triaxiality at the Mg alloy edge was a significant cause of edge cracking. On this basis, an edge-constraint rolling process was proposed to inhibit edge cracking by embedding the billet into a U-shaped sheet. The numerical results showed that the minimum value of edge stress triaxiality could be reduced from -0.8 to -2.05 during edge-constraint rolling, and the accumulation of damage could be limited. The sheet forming quality was verified with different reduction rates of 30, 35, and 40%, and the results showed that the edge cracks could be effectively avoided during edge-constraint rolling, which provides an important basis for optimizing the rolling deformation process of Mg alloys.

Numerical investigation on origin and evolution of polygonal cracks on rock surfaces

We studied the formation and evolution mechanism of polygonal cracks on rock surfaces under cooling by modelling meso-damage mechanics, continuum mechanics and thermodynamics. Factors that affect rock surface damage include ambient temperature, lithology difference and boundary restrictions. We established and simulated a heterogeneous model with a surface weak layer for three types of boundaries. These were biaxial constraint, uniaxial constraint and free boundary. The initiation and propagation of polygonal cracks were reproduced for varying thickness and homogeneity of the weak layer. The results show that the boundary constraints strongly influence the polygonal cracking. Many polygonal or parallel cracks are generated on the rock surface under biaxial or uniaxial constraint. The unconstrained rock surface displays polygonal cracks at the center and parallel cracks in the surrounding areas. The thicker the surface weak layer, the larger the average area of formed blocks. Small blocks and short cracks are more numerous than large blocks and long cracks. As the heterogeneity index increases, the rock layer is more likely to produce blocks with relatively regular shapes. Quadrilateral, pentagonal and hexagonal blocks dominate regardless of changes in layer thickness and heterogeneity. However, the number of edges of the polygonal blocks is sensitive to rock heterogeneity. The polygons tend to become more complex with increasing inhomogeneity. This study contributes to understanding the complex formation mechanisms of polygonal cracks on rock surfaces in nature. Additionally, the simulations of three-dimensional fracture geometry provide a basis for developing algorithms to generate discrete fractures and blocks in discrete fracture network (DFN) analyses.

Influence of Geotextile Materials on the Fractal Characteristics of Desiccation Cracking of Soil

In recent years, the irregular cracks formed during the damage evolution of civil engineering materials have been able to be quantitatively described by using fractals. In this study, the fractal characteristics of the desiccation cracking of soil were investigated under different substrate contact and permeability conditions through a natural drying test in the laboratory. Three kinds of base contact conditions of soil, namely, grease, geomembrane, and geotextile, were designed, and two samples for each contact condition, including one parallel sample, were used. The continuous drying experiment was carried out at a constant ambient temperature. The crack morphology under different spacings was analyzed quantitatively using digital image processing technology. The fractal dimensions of three soil substrate contact conditions (grease, geomembranes, and geotextiles) were between 1.238 and 1.93. When the crack network on the soil surface stops developing, the fractal dimensions under the three experimental conditions are 1.88, 1.93 and 1.79, respectively. In the final state of crack development, the crack intensity factor of the sample with grease at the bottom is 2.99% and 4.02% higher than that of the sample with geomembranes and geotextiles at the bottom, respectively. The residual water contents of the samples with bottom contact conditions of grease, geomembrane, and geotextile increase successively, which are 3.12%, 5.76% and 9.71%, respectively. The effects of interface friction and permeability on soil cracking behavior are analyzed, and the evolution characteristics and formation mechanisms of cracks in soil are revealed.

The formation mechanism of microcracks and fracture morphology of wood during drying

The surface checks or internal cracks from drying could significantly affect the processing and utilization of the wood. Therefore, it is of great significance to investigate the formation mechanism of cracks and reduce its occurrence. In this work, the development of cracking was quantified and its relationship with moisture content (MC) gradient and anisotropic shrinkage was analyzed. Results showed that the slight shrinkage occurred at the MC of 40%, and the shrinkage ratio at the end of drying was about 7.38% in the tangential (T) direction and 4.35% in the radial (R) direction. The shrinkage ratio of T/R in the MC range of 30%–15% was about 1.8, which was higher than that (1.65) in the MC range of 15%–5%. Cracks initiation was observed at average MC of 45%. The area and number of cracks reached a maximum at 15% MC. During the late stage of drying, the drying stress of the surface layer was reversed from tensile to compressive, which leads to the narrowing or closure of the cracks. Moreover, the fracture morphology of wood cells was observed at the microscopic level, indicating that the failure firstly occurred in the intercellular layer between the ray tissue and the tracheid in the latewood. The ray cells presented intact, and the lamellar structure between ray tissues and between the ray cells and the tracheids were the weak points of wood during drying. The results would contribute to keeping in minimum drying defects and improving the drying quality of wood.

The fracture mechanical behavior simulation of calcium-deficient hydroxyapatite crystals by molecular dynamics and first-principles calculation

Natural hydroxyapatite provides certain strength and stiffness to biological bones, and most of the studies on the strength of bone tissues have been carried out on hydroxyapatite (HAP). However, the Ca/P ratio of hydroxyapatite in bones is actually about 1.50, and the natural hydroxyapatite belongs to calcium-deficient hydroxyapatite (CDHA) with Ca vacancy defects. Therefore, this work focused on the effect of Ca vacancy defects on CDHA crystals through investigating the generation and expansion of microcracks under uniaxial tensile loading by combining molecular dynamics and first principles method. A series of crystal models with different Ca vacancy ratios are constructed and find that Ca vacancies degrade the mechanical properties of hydroxyapatite. Meanwhile the fracture behavior of crystals is detailed and find that the cracks arise at vacancies and extend along the direction of vacancies. Also, first-principles calculation is performed to reveal the mechanism of crack formation in MD simulations. It is found that the decrease of Ca–O bonding of CDHA causes the decrease of the stability of the crystal structure by analyzing the DOS of HAP and CDHA, and the cracks originate from Ca vacancies. This work performs more realistic simulations of CDHA with calcium vacancy defects in actual bone tissue and directly reveals the development and progression of bone fragility at the nanometer scale.

Crack Formation and Control in an AlCoCrFeNi High Entropy Alloy Fabricated by Selective Laser Melting.

The equiatomic AlCoCrFeNi high entropy alloy (HEA) is prone to cracking during the additive manufacturing process due to the high cooling rates observed, which limits its application to a large extent. In this study, the selective laser melting (SLM) technique was adopted to fabricate the alloy and the mechanism of crack formation was revealed. Most importantly, a new design strategy was proposed to suppress the generation of cracks, and the optimization of the preparation process was also studied in detail. It is found that the interlaminar crack is related to the heat input at the edge of the specimen, and the internal cracks are formed by solidification cracks. Alloys without interlaminar crack can be prepared by means of combination of the side inclination angle and the process parameters. Side inclination angle optimization provides a possibility for the preparation of crack-free AlCoCrFeNi HEA by SLM.

Cause analysis of the cooler leakage in a stirling engine

After a stirling engine operated for 1750 h, the cooler in the engine was found leaking. The leakage led to a rapid decrease of the cycle efficiency. After dismantling, a cooling tube was found leaked by air pressure test. Through macro- and microscopic fracture morphologies, the cause of the leakage was determined as fatigue under alternating stress during operation, but the fatigue source region shows abnormal brittle fracture morphologies. By XRT and inner wall inspections, more cracks initiated from the extrusion bands on the inner wall, and F-containing lubricant was also detected there. Considering the fabrication process of the tube, the formation mechanism of the inner cracks is determined as hydrogen environment embrittlement (HEE) under the conditions of the local surface damage, deformed microstructure and pure hydrogen circumstance during bright annealing. Remedial methods are also proposed to increase the service life and service safety.

Formation mechanism of surface cracks and subsurface cracks in SAE1548: Medium carbon sulfur-containing V-N micro-alloyed bloom

The three-dimensional (3D) morphology, spatial distribution and two-dimensional (2D) distribution of each cross-section of the surface cracks were investigated in detail with the help of industrial computed tomography (CT) equipment. Micro-structure and inclusions analysis around the cracks shows that the surface cracks originated from the mold, are induced by large-size inclusions, and propagates along the austenite grain boundary. The third brittle temperature range of SAE1548 is 650–900 °C due to the precipitation of the precipitates (AlN, V(C,N)) along the austenite grain boundary. However, the surface temperature of the bloom in the pulling and straightening section is just in this range, which makes the surface crack easy to expand and the subsurface crack easy to initiate. The macrostructure study suggested that both the mold cooling and secondary cooling processes are not optimal. As expected, the formation mechanism of cracks in SAE1548 is clarified.

Effect of glass transition on drying crack formation of maize kernels

The change of temperature and moisture content during maize ( Zea mays ) kernel drying can cause glass transition, which can lead to crack formation. However, the influence mechanism of glass transition on crack formation is not clear. This work aimed to study the effect of form and process of glass transition on the crack formation. It was found that the form and process of glass transition of maize kernels were determined by the state diagram, moisture content, temperature and state distribution. Glass transition began at the corner of the kernel and extended from the surface to the centre, which was consistent with the development of cracks. The transition form, transition rate and duration of the transition state depended on drying stage and temperature, which could effectively explain the difference in the degree of cracking. This could be helpful for understanding the crack formation mechanism and optimising the operating conditions in maize drying. • Crack formation depended on the form and process of glass transition. • Moisture content and temperature of corn kernel affected the glass transition process. • The difference of crack degree could be explained effectively by the glass transition. • Occurrence and propagation of cracks were consistent with transition of glassy state.

Research Mechanism of Formation on Transverse Corner Cracks in the Continuous Casting Slab of Peritectic Steel

To figure out the formation mechanism of transverse corner cracks at the inner and outer arc of peritectic steel slab, the metallurgical microscope, scanning electron microscope with energy‐dispersive X‐ray spectroscopy, and electron probe‐X‐ray microanalysis, are used to study cracks. High‐temperature mechanical properties and the stress distribution of the outer arc of the bending section under different roll gap schemes are studied and simulated. The outer arc cracks expand along the grain boundary, while the inner arc cracks not only extend along the grain boundary but also traverse along the grains. The outer arc cracks are mainly generated at the bending section and are formed and expanded along the ferrite film. The inner arc cracks' cracking temperature is higher than the temperature of austenite transformation to ferrite. The third brittle temperature ranges are 650–850 °C, and the transverse corner crack sensitivity zone is 680–837 °C. The inner arc cracks are generated because of the element segregation and low temperature of the corner and then extended at the straightening section, and new cracks exist. The weakened cooling intensity and bending earlier at the bending section help to reduce the equivalent stress on the slab. The optimized experiments are conducted, and the transverse corner cracks are reduced significantly.