Main Crack Research Articles

Resistance of fatigue crack propagation induced by large α colony in a Ti‐Al‐V‐Mo‐Zr alloy

AbstractFor Ti alloys used in oil drilling pipe, the fatigue crack propagation (FCP) resistance is one of the key properties for petroleum industrial applications. In this work, the fatigue crack propagation behaviors of bimodal, equiaxed, lamellar, and Widmanstatten microstructures have been systematically investigated and characterized by SEM and EBSD. The results show that the Widmanstatten microstructure containing α colonies has higher yield strength (877 MPa), moderate elongation (9.3%), and better FCP resistance compared to other specimens. During the process of crack propagation, the large α colonies can effectively deflect the cracks and induce secondary cracks, which significantly impedes the main crack propagation. Additionally, tensile twinning can occasionally occur within the large α colony along the crack, improving the plasticity of the crack tip and resulting in a decrease in fatigue crack propagation rate (FCPR).

Mechanism of mean stress sensitivity in Ti-6Al-4V under different stress ratios in high cycle fatigue

Initially, this article precisely determines that the “facet” on the fracture surface of the Ti-6Al-4V alloy during high cycle fatigue (HCF) loading under positive stress ratios, as determined by white light interferometry and transmission Kikuchi diffraction (TKD), originates from cracking along the slip planes that exhibit a grain orientated α grain in the loading direction, predominantly involving shear deformation along the crystallographic planes. The article emphasizes the crack initiation mechanisms in Ti-6Al-4V alloy related to internal microstructures under positive stress ratio HCF loading, elucidating relationship between texture-induced crack initiation and growth, and anomalous mean stress sensitivity. Under positive stress ratio fatigue loading, preferentially orientated texture regions tend to generate irreversible dislocations parallel to the basal plane, gradually accumulating within multiple locations to form persistent slip bands (PSBs), which, under the influence of internal stresses, sequentially collapse and rupture, leading to nucleation of microcracks. Upon encountering unfavorably oriented texture regions or grains, PSBs or existing microcracks experience dislocation accumulation, which in turn induce grain refinement, thereby causing further crack nucleation. Consequently, microcracks, through aggregation and coalescence during the fatigue loading sequence, form the main crack, ultimately precipitating fatigue failure and engendering the anomalous mean stress sensitivity observed in the Ti-6Al-4V alloy.

Fracture behavior in blasting under different static stresses and high stress unloading

AbstractIn this paper, the effects of static stresses on the blast‐induced crack in polymethyl methacrylate specimens with an empty hole were investigated by caustic system. The propagation behavior and re‐initiation process of blast‐induced crack under initial static stress and high stress unloading were investigated. The results show that when the direction of the empty hole is perpendicular to the static stress, the static stress reduces the length of the main crack. However, when the static stress is very high, the length of main cracks does not decrease. In addition, the static stress reduces the crack velocity and dynamic stress intensity factor (DSIF) and suppresses the increase of DSIFs and crack velocity due to reflected wave. The crack arrested under high stress unloading propagates again in the later stage. Moreover, the duration of high stress unloading lasts approximately 1 ms.

Hot processing performance and fracture toughness control of AlxCrFeNiCu1.5 (x = 0, 0.2, 0.4, 0.6, 0.8) high entropy alloys

In order to develop high-entropy alloys with both good hot processing performance and fracture toughness, AlxCrFeNiCu1.5(x=0, 0.2, 0.4, 0.6, 0.8) alloys were designed and prepared. The alloy was firstly hot rolled by a twin-roll rolling device, and then the hot processing performance of the alloy under different Al contents was explored, whether the fracture toughness was at an ideal level, and if so, why it was at an ideal level. The results showed that the phase composition after hot rolling and annealing were two phases (face-centered cubic FCC and body-centered cubic BCC). With the increase of Al content, the FCC phase gradually transformed into BCC phase, and the increase of BCC phase led to the decrease of plasticity, which led to the decrease of hot processing performance. With the increase of Al content, the average fracture toughness value increased first and then decreased. Among them, the Al0.2CrFeNiCu1.5 and Al0.4CrFeNiCu1.5 alloys had the relatively better fracture toughness, which was because that the alloys had the appropriate proportion of FCC and BCC phases, and the strength and plasticity could achieve an ideal match. Among these alloys, the Al0.2CrFeNiCu1.5 alloy had the relatively better hot processing performance and ideal fracture toughness, and its average fracture toughness was 35.71 MPa·m1/2. And compared with Al0.2CrFeNiCu1.5 alloy, the Al0.4CrFeNiCu1.5 alloy had the worse hot processing performance, but it yielded the best fracture toughness, with an average value of 45.27 MPa·m1/2. In the crack propagation morphology, we found the main crack terminated at the notch upper part for the Al0.2CrFeNiCu1.5 alloy, suggesting that the material has good plasticity. And we also found crack multiple deflections and secondary cracking for the Al0.4CrFeNiCu1.5 alloy, which was helpful to improve the fracture toughness.

Assessment of fracturing and blockiness of the Pshenychne gabbro deposit massif

Proper development of the deposit and the choice and use of effective technologies for block stone extraction are impossible without evaluating the structural indicators of the massif. It consists in studying the propagation of cracks in the array, orientation of the main crack systems, studying the anisotropic properties of stone, and predicting the yield of commercial stone blocks in the deposit. This allows both to increase the efficiency of block extraction and to reduce the loss of stone during its extraction, which increases the economic performance of field development. The assessment of fracturing and blockiness of the Pshenychne gabbro deposit was carried out based on the results of studying the core documentation of 11 exploration wells and the documentation of a pilot production quarry. Rose diagrams, pie charts, and fracturing plans were constructed in isolines, and crack systems common in the field were identified.

Estimation of multiple cracks interaction and its effect on stress intensity factors under mixed load by artificial neural networks

One of the goals of fracture mechanics in the analysis of problems involving cracks is to investigate crack growth and propagation. Each crack grows and propagates in the environment based on its geometry and loading conditions, which can occur under pure mode or a combination of modes. The growth and propagation pattern of the crack in these conditions aligns with the fracture mode. In the presence of multiple cracks in the environment, the growth and propagation paths of these cracks differ from the isolated crack case, where they can either grow independently under certain conditions or merge together. Nowadays, extensive research has been conducted on cracks and their mathematical relationships with respect to the geometry and loading conditions of various components. Since in many cases, a large number of cracks may occur in proximity to each other, the intensification and protective effects of each crack on others and their impact on the overall behaviour of components are of great importance. This study focuses on investigating the behaviour of interaction and the stress intensity factor for an infinite plate with parallel adjacent cracks. Remote stress boundary conditions in the form of tensile and compressive stresses have been applied to this plate, and using the J-integral and the finite element method with the Abaqus finite element analysis software, this subject has been examined. The obtained results indicate that depending on the type of vertical and horizontal distances and the angle of their placement between the adjacent crack and the main crack in the first, second, and mixed modes, the adjacent crack can have intensifying, protective, or neutral effects. Furthermore, a good estimation of the secondary crack's influence on the main crack has been provided using a neural network method (MLP), which creates an appropriate weight function for the upcoming sample.

Experimental study on flexural performance of composite slabs with bent bars and postcast ultrahigh‐performance concrete joints

AbstractThis paper describes the use of joints with bent bars and postcast ultrahigh‐performance concrete (UHPC) joints to enable formwork‐free and support‐free construction of composite slabs. To investigate their flexural performance, static loading tests on three full‐scale simply‐supported slabs and one monolithic slab were carried out. Flexural capacity, stiffness, and crack development were recorded, resulting on flexural failure experienced by all specimens. Main cracks of composite specimens developed along the UHPC–normal concrete interface, and cracks on both sides of the UHPC joint developed evenly, similar with that of the monolithic specimen. Besides, composite slabs and a monolithic slab showed similar flexural capacity. UHPC joint was not cracked when failure occurred, and bottom bars were anchored and connected in the UHPC joint, proving that the flexural anchorage method is feasible. The average strain of the composite specimen across the UHPC joint was still consistent with the assumption of plain section, and there was an approximate linear relationship between the expansion width of the joint at the bottom of the slab and the maximum crack width at bottom bars. Moreover, middle bars contributed to the flexural capacity and stiffness of composite specimens, while horse stool reinforcement may effectively prevent the interface shear failure on the composite interface. Based on test results, the formula to control cracking of UHPC is given in this paper. The design methods on flexural capacity, stiffness, and expansion width of composite slabs are also presented and discussed.

Interface debonding by growth of interface defect ahead of main crack approaching with oblique paths in layered systems

Interface debonding by growth of interface defect ahead of main crack approaching with oblique paths in layered systems

Multi-method comparison and multi-index evaluation of cracking characteristics of asphalt mixture

The micro crack inside asphalt pavement are potential and antecedent factors for macroscopic cracking of the pavement. The position and quantity of micro cracks have a significant impact on the service life and failure mode of asphalt pavement. The purpose of this study was to conduct multi-scale analysis and multi-index evaluation on the whole process of generation, expansion, and transformation of micro-crack inside asphalt pavement (asphalt mixture) based on multiple testing methods. Firstly, synchronous tests were conducted on the cracking process of asphalt mixture by using three-point bending loading method, acoustic emission (AE) monitoring method, and digital image correlation (DIC) analysis method. Secondly, a three-dimensional finite element model of the asphalt mixture beam was established for AE source location simulation. Finally, the advantages and limitations of multi-methods were compared and analyzed. The principal strain points with different sizes were randomly distributed in the tensile area of the asphalt mixture beam bottom at the initial loading stage, and the number of the principal strain points increased with the decrease of the loading rate. The macroscopic main crack of the asphalt mixture beam generated and expanded from the maximum principal strain point (P1) at the later loading stage. The AE dominant-frequency distribution ranges of micro-crack expanding process, macro-crack generating process, and failure process of asphalt mixture beam were 78.125–156.25 kHz, 234.375–312.5 kHz, and 0–78.125 kHz, respectively. Four stages of damage evolution and cracking process inside the asphalt mixture beam were obtained by the AE source location simulation. A comprehensive and systematic evaluation of bending cracking characteristics of asphalt mixture beam was realized by obtaining multiple indexes from multiple scales with multiple methods.

On the influence of secondary branches on crack propagation in rolling contact fatigue

Gear and bearing failures are most often caused by rolling contact fatigue (RCF). Understanding the growth of a main surface-initiated crack into the depth of the piece, as well as the growth of subsurface-initiated branches towards the surface is necessary, in order to improve fatigue life prediction and reduce fatigue damage repair or component replacements before catastrophic failure. The purpose of this work is to analyze, based on 3D finite element simulations, the initiation and mode I growth of secondary branches in a case hardened gear, their influence on the main crack growth in mode II, and the competition between mode I and mode II crack growth. The residual stress field issued by the case hardening is taken into account and its role is analyzed. The reasons why branch cracks develop only from the upper main crack face and why only those initiated when the main crack is still shallow can reach the surface, inducing spalling are explained.

Compressive Fracture Behavior and Acoustic Emission Characteristics of Sandstone under Constant Crack Water Pressure

Abstract Engineering rock containing flaws or defects under a large water source is frequently subject to the couple influence of constant crack water pressure and geostress. To investigate the fracture behavior of precracked rock under hydromechanical coupling with constant crack water pressure, compression tests were conducted on red sandstone specimens containing a single crack of different angles using a device to realize the constant crack water pressure during loading, and the failure process of rock specimens was monitored by acoustic emission (AE) technique. The results show that the presence of constant crack water pressure has a significant promotion effect on the development of shear wing cracks, and the promotion effect is influenced by the prefabricated crack angle and water pressure. As the constant crack water pressure increases, the failure mode of the 0° precrack specimen changes from “X”- shear failure to the single oblique shear failure along the shear wing crack direction, the main failure crack of the inclined precracked specimens (precrack angles of 15°, 45°, and 60°) changes from a small acute angle with the prefabricated crack to a direction along the shear wing crack, and irregular cracks occur at the chipped prefabricated crack in the 90° precracked specimen. With an increase in the constant crack water pressure, the average energy for a single hit, cumulative AE energy, and cumulative AE hits decrease, and the proportion of the tensile cracks increases and that of the shear cracks decreases.

Digital Twins to Predict Crack Propagation of Sustainable Engineering Materials under Different Loads

Computer-aided engineering (CAE) is an essential tool in a digital twin not only to verify and validate a virtual twin before it is transformed into a physical twin, but also to monitor the use of the physical twin for enhanced sustainability. This paper aims to develop a CAE model for a digital twin to predict the fatigue life of materials. Fatigue damage is represented by the size of a macro-crack that grows with a cluster of micro-cracks subjected to three different loads. The growth angle is related to the maximum circumferential tensile stress, and the growth rate is determined by the stress intensity factor (SIF) at the crack tip. The prediction model takes into consideration the main factors, including micro-cracks, crack closures, and initial configurations. Simulations are developed for the growth of macro-cracks with radially distributed micro-cracks and randomly distributed micro-cracks, and we find that (1) the macro-crack in the second case grows faster than that in the first case; (2) a pure shear load affects the macro-crack propagation more than a combined shear and tensile load or a tensional load; (3) the external stresses required to propagate are reduced when the inclination angle of the micro-crack is small and within (−25° < β < 25°); (4) micro-cracks affect the propagating path of the macro-crack and generally guide the direction of propagation. The developed model has been verified and validated experimentally for its effectiveness in predicting the fracture or fatigue damage of a structure.

Experimental and Numerical Investigation of Rock Failure Process under Hydromechanical Coupling Action

Abstract In order to study the initiation mechanism of rocks under hydromechanical coupling, hydromechanical coupling triaxial tests and acoustic emission tests were carried out on basalt in the Xiluodu hydropower station dam site area in southwestern China. The test results indicate that the basalt displays typical hard brittle behavior, and its peak strength increases as confining pressure rises. Conversely, the peak strength decreases gradually as the initial water pressure increases, which leads to decreased hardness. Meanwhile, tensile failure is the main crack initiation mode under hydromechanical coupling action. During the stable crack growth stage, tensile failure is predominant, complemented by shear failure, with failures mainly occurring in the rock middle position. Contrary to this, during the unstable stage, the rock failure is mainly due to shear failure. The critical pore water pressure failure criterion of rock crack initiation under hydromechanical coupling conditions is derived based on the test results and introduced into the numerical simulation. The hydromechanical coupling failure process and pore water pressure distribution law of basalt are analyzed, and the rationality of the critical pore water pressure failure criterion is verified. These findings are significant for understanding the rock failure process under hydromechanical coupling action and provide a valuable reference for future research.

Research on ultrasonic bone cutting mechanism based on extended finite element method.

The research on the crack propagation mechanism of bone has important research significance and clinical medical value for the selection of cutting parameters and the development of new surgical tools. In this paper, an extended finite element method (X-FEM) model of ultrasonic bone cutting considering microstructure was developed to further study the ultrasonic bone cutting mechanism and to quantitatively analyze the effects of cutting direction, ultrasonic parameters, and cutting parameters on the mechanism of ultrasonic bone cutting crack propagation. The results show that ultrasonic bone cutting is essentially a controlled crack propagation process, in which brittle crack and fatigue crack are the main crack propagation mechanisms. In order to improve the efficiency of ultrasonic bone cutting, large amplitude and high-frequency ultrasonic vibration are preferred. Compared with the other two cutting directions, the crack propagation deflection angle in the transverse cutting direction is the largest, resulting in the worst cutting surface. Therefore, in the path planning of orthopedic surgical robots, the transverse cutting direction should be avoided as much as possible. Frequency only has a significant effect on the crack propagation rate and has a positive correlation. There is a positive correlation between the deflection angle, propagation length, propagation rate, and amplitude, which provides the possibility to control the direction and length of crack propagation by controlling the amplitude of ultrasonic. The feed speed is much lower than the ultrasonic vibration speed, which makes the influence of ultrasonic vibration speed on the crack propagation characteristics dominant. The X-FEM model of ultrasonic bone cutting provides an effective method for selecting reasonable machining parameters of orthopedic robot and optimize the design of ultrasonic osteotome.

Evaluation of using oyster shell as a complete replacement for aggregate to make eco-friendly concrete

The objective of this study is to investigate the feasibility of using oyster shells (OS) as a complete replacement for concrete aggregate, which provides a way to make eco-friendly concrete for the recycling of waste OS and the conservation of natural resources. In this study, the mineral composition of OS was analysed, and a series of experiments were carried out to investigate the effects of curing water, mineral admixtures, and steel fiber (STF) incorporation on the strength of all-OS aggregate concrete (AOSAC), and scanning electron microscopy (SEM) and the finite element method (FEM) were used to investigate the microstructure and meso-mechanical properties of AOSAC. In the FEM simulation, it was found that the main cracks of AOSAC under compression damage develop obliquely from the top and bottom corner to the interior, forming a double inverted triangle region on the surface, and the concrete outside the region quits working. The sequence of damage was observed in the internal slices of the model as interface transition zone (ITZ), OS aggregate, and mortar. In the tests, it was found that seawater curing introduces additional hydration reactions that leads to cracking within the concrete due to the generation of expansive hydrates resulting in reduced strength. The incorporation of fly ash (FA) can alleviate this problem and inhibit the generation of expansion hydrates. The incorporation of STF equivalent to 0.5% by volume results in the maximum increase in the strength of AOSAC, and the AOSAC developed was found to be fully strong enough for structural engineering purposes.

Pioneering research on the role of strain burst in the early-stage stress corrosion crack propagation

This work pioneered the study on the effect of cyclic-loading-induced strain burst on Stage-1b stress corrosion crack (SCC) growth under alternating static and cyclic loading. Results show that Stage-1b growth occurred by microcrack initiation and its linking to the main crack's tip. Increasing static hold duration caused crack growth rate per cycle to increase after an incubation period. This was because the occurrence of strain burst induced a high strain rate, leading to film rupture, and thus microcrack initiation. The effects of different stress levels and durations of static hold, and their interactions with cyclic loading, on crack growth were systematically studied.

Embrittlement Fracture Behavior and Mechanical Properties in Heat-Affected Zone of Welded Maraging Steel.

In welded maraging steels, mechanical properties, particularly ductility and toughness, are often compromised in the heat-affected zone (HAZ). This study focuses on 300-grade maraging steel bars, solution annealed at 1123 K for 1.5 h (5.4 ks) and welded using gas tungsten arc welding, followed by a post-weld heat treatment at 753 K for 13.33 h (48 ks). In situ observations during three-point bending tests on HAZ samples featuring coarsened prior austenite grain sizes were conducted to examine damage behavior and the crack path near the crack tip. The main crack initiated at the peak applied load during the bending test and, upon further loading, exhibited significant deflection and extension accompanied by numerous microcracks and localized crack branching. Distinctive damage features, such as transgranular cracking across block regions, intense intergranular cracking along packet boundaries with a pronounced shear component, and crowding of microcracks ahead of the crack tip, were observed in the HAZ sample during the in situ test. The interaction between the main crack tip and microcracks and its influence on the local crack propagation driving force was discussed using fracture mechanics. Experimental results, including tensile fracture surface observations and in situ images, along with analysis of the stress anti-shielding effect by microcracks, suggest that the HAZ sample exhibits embrittlement fracture behavior with lower ductility and toughness compared to the base metal sample.

Excellent combination of strength and elongation of TZM alloys achieved by c-ZrO2 and single cross rolling

In this study, new TZM-xZrO2 (x = 0, 0.5, 1, 1.5) (wt%) alloys were designed by powder metallurgy solid-solid mixing method combined with single cross rolling. The results show that c-ZrO2 in TZM alloys was mainly spherical, and the average size of c-ZrO2 particle increases gradually from 1.28 μm to 1.71 μm with the increase of its content. c-ZrO2 can effectively refine the molybdenum grain and adsorb the free brittle element O in grain boundaries in the form of TiO2 layer. The interface was composed of three phase: c-ZrO2, Y2O3 and TiO2, with the presence of [0−11] c-ZrO2 ǁ [−11−2] Y2O3 ǁ [−10−3] TiO2 crystal relationship. The rolled plates mainly exist 〈100〉//ND and 〈111〉//ND two kinds of grain orientation, the addition of c-ZrO2 particles promote <100>//ND orientation ratio, thus weakening the texture strength (maximum pole density value from 8.34 to 5.39), conducive to the improvement of plasticity. When the c-ZrO2 content was 0.5 wt%, the best performance was achieved, with good strength and elongation (ultimate tensile strength: 958 MPa; Elongation: 17.3%), showing excellent strength and elongation product (16.6 GPa·%), 31% higher than TZM alloy, better than most reported molybdenum alloys. With the further increase of c-ZrO2 content, the particles gradually coarsened, the hindering effect on dislocations was weakened, and it was easy to become the nucleation site of main cracks, resulting in the decrease of strength and plasticity. Fine grain strengthening and dislocation strengthening were important factors to improve the strength. The excellent ductility was mainly due to the deflection mechanism of microcracks and the blocking effect of grain boundary enrichment on crack propagation.

Experimental study on mechanical properties of sprayed ECC under quasi-static and cyclic loading

The tunnel lining will be subjected to the cyclic action of high ground stress during service. However, there is a lack of research on the damage evolution mechanism of sprayed engineered cementitious composites (ECC) under cyclic loading. Herein, we studied the strain rate effect and fatigue damage evolution mechanism of sprayed ECC by conducting compression and bending tests at different quasi-static strain rates and high-stress cyclic loading-unloading compression and bending tests. The results show that with the increase of strain rate, the compressive peak stress and bending strength of sprayed ECC show an upward trend, as well as the increase of the change rate. At 8.3 × 10−3 s−1, the compressive SGF is 1.52, with an energy storage limit density of 0.501MJ/m3. At 2.1 × 10−2 s−1, the bending SGF is 1.52, with a peak fracture energy of 49.29 J. In addition, under the compression test, the tensile shear failure of specimens develops from finer double shear planes to coarser multiple shear planes. Under the bending test, the position of the main crack of specimen is gradually transferred from the mid-span to both sides. Under high stress cycle test, the input energy and dissipation energy of the two tests have the similar trend, showing “U” type. The variation of elastic energy and dissipation energy conversion rate exhibits a “U” type with different directions. At the last loading, the dissipation energy conversion rate of compression test is 46.8%, while that of bending test is 70.4%. The damage variable of specimen increases significantly with the increase of the number of cycles, accompanied by an increase in the degree of breakage. In addition, the damage variables based on deformation characteristics are higher than those based on energy characteristics.

Investigation of fracture characteristics of cracked granite suffered from different thermal treatments and water-cooling time

Rock mass may be exposed to high temperature and cooled by water in fire, geothermal resource exploitation and deep underground engineering, which affects the safety of these engineering. In this study, cracked straight through Brazilian disc (CSTBD) specimen was used to study the fracture properties of granite treated with different water-cooling time from 25 °C to 800 °C. A camera was used to record the fracture process of the specimen, the evolution of strain at the specimen surface was obtained by using digital image correlation technology, and the morphological characteristics of fracture surface were observed by using scanning electron microscopy. The results show that water-cooling time has some effects on fracture properties of granite, but its effect is less than the temperature. With the increase of water-cooling time, the mode I fracture toughness decreases first, then increases and finally decreases at the same temperature, and the fracture toughness ranges from 0.419 MPa m1/2 to 0.567 MPa m1/2 at 400 °C. The crack tip opening displacement (CTOD), crack propagation path and fracture surface are related to the energy released during the specimen failure. When the temperature is 25 °C or 200 °C, the energy released is large and the fracture surface is relatively flat with irregular sharp edges, the main cracks are straight lines. At 600 °C, the main crack is zigzag, the CTOD and the maximum principal strain in order can reach 0.143 mm and 0.731 %. This study can help to understand the effect of water-cooling time on engineering rock mass.