Early Stage Of Crack Propagation Research Articles

Investigation of quantitative precursory indicators for rock failure using spatio-temporal characteristics of velocity

Stress redistribution and crack development are prominent features in rock failure. However, these processes are complex and challenging to analyze, making early warning of instability in rock engineering difficult. To characterize the rock failure process and obtain reliable precursory indicators, the imaging method that combines active and passive sources was employed to construct the velocity field during the rock failure process in this study. The micro-crack propagation law and its potential connection with the stress environment were investigated by mapping the velocity field, the acoustic emission (AE) density field, and the energy field. Moreover, the Mutation Trend Coefficient (MTC) and Deformation Coefficient (DC) were proposed as precursory indicators of instability based on the spatio-temporal correlation and directional differences of velocity evolution. Results show that the nucleation of AE sources and the release of energy accumulation are key to the formation of the velocity gradient, and progressive rock failure is accompanied by the intensification of velocity anisotropy. Additionally, low-velocity regions usually preferentially extend in the direction parallel to the principal stress, while expansion in the direction vertical to the principal stress is dominant in the plastic stage, indicating that the stress environment controls the spatial heterogeneity of the velocity field. The evolution trend of precursor indicators shows that the increase of MTC from a low value and the continuous decrease of DC can be regarded as the initiation of rock secondary cracks. The proposed indicators and method not only provide beneficial complements to the understanding of the evolution mechanism of rock damage but also have potential in the early warning and identification of the early stage of crack propagation, offering references for the safety prevention and control of rock engineering disasters.

Statistical modeling of microstructurally short crack growth in high cycle fatigue

Capturing the behavior of short cracks under fatigue conditions has become a challenge for researchers. The early stage of crack propagation is influenced by microstructural features, such as grain boundaries and orientations, which leads to significant statistical variations. Models such as the ones developed by Navarro and De Los Rios have been able to describe the process by considering the average properties of the material and estimating the number of cycles related to the short crack stage. However, fatigue should be seen as an extreme value process. In the present work, a more realistic model was developed to consider the statistical nature related to grain orientations to estimate fatigue life dispersion. Eleven crack propagation criteria have been used to compute the number of cycles spent in the short crack stage. Parameters such as the Schmid factor, the potential crack length, and the various angles describing the misorientation between adjacent grains were considered. One thousand random scenarios for short crack growth were studied for each propagation criterion and some statistical analyzes were run to estimate the worst-case scenario out of one million. Results can be well described using the generalized extreme value distribution, capturing the stochastic nature of the fatigue life.

An Attention-Based Detection Method of Displacement Field on Steel Surfaces

Steel structures are prone to fatigue cracks when subjected to cyclic loading, which may lead to catastrophic failure. Generally, the width of fatigue cracks in steel structures is below 0.1 mm at the early stage of crack propagation. Although high-resolution images can be obtained by consumer-grade cameras at low cost, these tiny cracks are difficult to detect by images alone. This paper proposed a crack detection method based on the displacement field on the surface of the structure obtained from images. Video or continuous images of the target structure under loading was first taken and input into an improved Detector-Free Local Feature Matching with Transformers (LoFTR) model, which was capable of densely matching feature points on two pairs of images without distinct visual features. The surface displacement field of the structure was then performed inversely by the coordinate difference of a large number of matched feature points. Eventually, location of the crack was extracted according to discontinuities in the displacement field. A case study was conducted on a cracked steel plate. Results demonstrated a tiny crack with the maximum width of 0.1 mm was detected, which was more effective and accurate in comparison with image-based semantic segmentation methods.

Effect of pre-crack non-uniformity for mini-CT geometry in ductile tearing regime

The mini-CT specimen attracts the attention from all over the world for application in fracture toughness measurement. However, one of the shortcomings of this geometry is related to the required tight accuracy of the specimen dimensions, in particular the fatigue pre-crack curvature which often violates the requirements of the ASTM standards. Given the limited thickness of mini-CT geometry, non-uniform pre-crack tends to develop during fatigue pre-cracking, resulting in a large proportion of mini-CT specimens being considered invalid.Previous investigations have demonstrated that mini-CT specimens with excessive crack front curvature can still provide meaningful fracture toughness results. In this paper, the effect of pre-crack front non-uniformity on ductile fracture is studied: first, the difference of macro parameters such as the applied load, J-integral and crack tip stress–strain field are investigated to illustrate the varying fracture behavior related to non-uniform pre-crack. Next, two micro-mechanics-based approaches, the Rice-Tracey void growth model and Thomason void coalescence model, are integrated to compare the ductile fracture initiation conditions associated with uniform and non-uniform fatigue pre-crack. Finally, the experimental verification of the ductile fracture simulations is performed for mini-CT specimens with uniform and 30° tilted initial cracks. The results indicate that the pre-crack non-uniformity plays a major role in the redistribution of local J-integral and stress–strain state, further affects the position of crack initiation and the early stages of crack propagation. Nevertheless, the pre-crack non-uniformity has limited effect on the global properties that are usually expected from fracture toughness tests, such as applied load, J-R curve and critical fracture toughness. The requirements in the ASTM E1820 regarding pre-crack front curvature need thus to be relaxed.

A Finite Element Model for Predicting the Static Strength of a Composite Hybrid Joint with Reinforcement Pins.

This paper presents a finite element model for predicting the performance and failure behaviour of a hybrid joint assembling fibrous composites to a metal part with reinforcement micro pins for enhancing the damage tolerance performance. A unit-strip model using the cohesive elements at the bond interface is employed to simulate the onset and propagation of debonding cracks. Two different traction-separation laws for the interface cohesive elements are employed, representing the fracture toughness properties of the plain adhesive bond and a pin-reinforced interface, respectively. This approach can account for the large-scale crack-bridging effect of the pins. It avoids using concentrated pin forces in the numerical model, thus removing mesh-size dependency, and permitting more accurate and robust computational analysis. Lap joints reinforced with various pin arrays were tested under quasi-static load. Predicted load versus applied displacement relations are in good agreement with the test results, especially for the debonding onset and early stage of crack propagation.

Mechanical properties of high strength concrete incorporating chopped basalt fibers: experimental and analytical study

Many civil engineering structures are constructed in terms of aspect like economy, strength and serviceability requirement. The fibres play a prominent role in bridging the micro cracks at the early stage of crack propagation and makes the structure ductile. The impact of chopped basalt fibre on high-strength reinforced concrete composites is the focus of this research. The chopped basalt fibre hardened property of concrete was investigated and compared it with controlled concrete. Moreover, physical property of concrete i.e. slump cone and compaction factor incorporating chopped basalt fibre volume fraction in variation of 0%,0.75%,1.5%,2.25%,3% to the total volume of concrete mix was also investigated. To analyse the hardened properties of chopped basalt fibre high strength concrete, 135 test samples were made and cured for 7, 14, and 28 days. The test findings show that increasing the volume percentage of fibres reduces slump from 135 mm to 132 mm for BFHS0.75 but does not result in a significant increase in concrete compressive strength of 77.1 MPa for BFHS0.75 However, on addition of chopped basalt fibres at 2.25% (BFHS2.25) the percentage increment of flexural strength increases by 72.8% (10.3 MPa) with control mix (5.96 MPa) and after that it decreases when 3% fibres were incorporated. Similarly, split-tensile strength increases at all fibre dosage and fibre addition of 3% (BFHS3) increases by 12.28% (5.65 MPa) with controlled concrete (5.04 MPa). The analytical results and suggested regression model could be used in real-world situations with fiber reinforced high strength concrete related issues. There is a significant relationship between mechanical property and independent variable through ANOVA in SPSS.

Investigation of compact tensile and fracture mechanical properties of a duplex stainless steel bimetal composite with the interfacial zone

The interfacial zone formed in the manufacturing process of bimetal or laminated metal composites owes complicated microstructures and properties. This special feature greatly affects the fracture toughness and fatigue strength of metal composites. The crack growth behaviours, from a weak layer (AH36 carbon steel) to a strong layer (2205 duplex stainless steel), at the interfacial zone of a hot-rolled bimetal composite (2205/AH36 BC) were investigated. To clarify the potential factors altering the crack growth behaviour near the interface, the variations in fracture force under different loading speeds and directions were obtained from a well-designed micro tensile machine. The results indicate that the influence of crack propagation direction on the tensile fracture force when passing through the interfacial zone is greater than the force at the initial notch of crack. The tension loading speed determines the shielding or amplification effect on the crack tip, verified by the microstructural characteristics along the crack path. The crack tip displacement and yield situations were studied by the mechanism analysis, numerical simulation and microstructure observation. It is found that the influence of interface and strong steel layer on the crack tip opening displacement (CTOD) and yield zone size is great in the early stage of crack propagation. Loading speeds have a greater impact on the microstructure of 2205 steel adjacent to the interface, but a small influence on the CTOD and yield zone size of composite. These results could be used as a reference for guiding the design of new bimetal composite parts.

Damage patterns in float glass plates: Experiments and peridynamics analysis

In this study ring bending tests on plates are performed to analyze the tensile strength of tin and air sides of float glass. Fracture stress values computed from experimental data applying the plate theory are analyzed by means of Weibull distribution. Probability of failure vs. fracture stress plots show much higher scatter for the air side than for the tin side. The linear peridynamic solid constitutive model coupled with damage is applied to simulate the ring bending, up to the critical state of crack initiation and early stage of crack propagation. A convergence study with respect to the number of nodes is performed. The simulated sequence of initial damage patterns, including the bond breakage inside the load ring and the formation of ring damage zone followed by initiation of radial cracks agrees with experimental observations for most specimens with tin side subjected to tensile deformation. A novel procedure to identify the critical bond stretch in the damage model based on experimental data for the critical ring force is proposed. The corresponding characteristic values are presented for both tin and air sides of the float glass.

Simulation of ductile tearing during a full size test using a non local Gurson–Tvergaard–Needleman (GTN) model

Ductile tearing of a full size precracked pipe is experimentally investigated. In order to model and interpret the test, the pipe material is characterized using smooth and notched tensile bars and precracked C(T) specimens. This experimental database is used to fit the parameters of the non local Gurson–Tvergaard–Needleman (GTN) proposed in Zhang et al. (2018) and Chen et al. (2020). The model is used in finite element simulations using specific elements allowing for the control of strain/damage localization as well as volumetric locking. Mesh size independence is checked on notched tensile bars. The model is then able to represent the early stages of crack propagation in the pipe. In particular, experimentally observed crack branching is reproduced, whereas this appeared much more difficult to obtain using a local GTN model.

Initial crack propagation of integral joint in steel truss arch bridges and its fatigue life accession

Integral joints are an important component in steel truss bridges. To investigate their initial crack propagation performance and assess their fatigue life, a finite element model of Bridge B as well as a local model of a steel joist with an integral joint are introduced. Subsequently, the stress–time curves of each component are transformed into equivalent stress amplitudes using the rain flow counting and Miner equivalent stress methods. Next, an ABAQUS automatic crack propagation program is developed to simulate the composite crack propagation at the most dangerous node, SE6. The results show that the crack propagation rate is low in the early stage of crack propagation; however, this process accounts for the main aspect of the remaining life in crack propagation. Finally, the effects of different initial crack sizes, nodal plate thicknesses, and initial angles of cracks on the stress intensity factor are investigated.

Effects of γ/γ lamellar interfaces on interlamellar crack propagation behaviors of TiAl alloys

Interlamellar fracture is one of the failure modes of fully lamellar (FL) TiAl alloys. Here, the effects of three types of γ/γ lamellar interfaces, which are termed true twin (TT), 120° rotational order fault (ROF), and pseudo-twin (PT), subjected to uniaxial tensile loading on the interlamellar crack propagation behaviors of an FL γ-TiAl alloy have been investigated by molecular dynamics method. The results show that the introduction of the PT and ROF interface weakens the tensile strength of single crystal TiAl, while the introduction of the TT interface can effectively improve the tensile strength of the material. Moreover, the cracks in the three interfaces exhibit different propagation behaviors and propagation rates. Specifically, the ROF and PT interfaces exhibit the most desirable and least desirable crack resistances, respectively, which are related to the geometric compatibilities of the interfaces and plastic deformation of crack tips. We also found that the sequence of fracture toughness (ROF > TT > PT) is consistent with that of the flow stresses (ROF > TT > PT). In addition, in the early stage of crack propagation, deformation twins serve as a toughening mechanism that reduces crack tip stress and retards crack extension.

Numerical Investigation on Mixed Mode (I-II) Fracture Propagation of CCBD Specimens Under Confining Pressure

A new numerical method, verified by the analytical solution of the weight functions and experimental paths, is developed to evaluate the crack initiation and propagation generally in mixed mode (I-II). This numerical method combining the interaction integral method and the maximum tangential stress (MTS) criterion is based on the finite element method of secondary development. The influence of combined confining pressure and diametric forces on crack propagation trajectories for CCBD specimens are studied. It is indicated that the crack propagation direction independent of the confining pressure keeps the same with the line of original crack as the loading angle is equal to [Formula: see text]. But when the loading angle is greater than [Formula: see text], the curvature of the curve trajectory in the early stage of crack propagation increases with a larger confining pressure. Further, it is found that larger values of the loading angle and relative length will make the effect of confining pressure more significant at the early stage of crack growth.

Influence of shape of weld toe and hardness of weld metal on fatigue properties in GMA welded fillet lap joint of UHSS sheet

ABSTRACT This study aims to clarify the influence of shape around weld toe and hardness of weld metal on fatigue crack life in fillet lap joints for automotive chassis elements. Gas metal arc (GMA) welding was performed using 980 MPa class hot rolled steel sheet as the test material. The shape around weld toe and hardness of weld metal were changed by the combination of the shield gas composition and the welding wire. The smoothing of the weld toe significantly improved the fatigue life compared to the hardened weld metal. The total fatigue life was separated into crack initiation life and crack propagation life by measuring the decreasing ratio in the stiffness of the specimen during the fatigue test. Then, the effect of smoothing the weld toe and hardening the weld metal on each life was examined. The changes in the propagating crack front shape and the propagation rate during fatigue test were examined by the beach mark technique. As a result, it was suggested that the smoothing of weld toe improves not only the crack initiation life but also the propagation life by delaying the crack propagation rate in the early stage of crack propagation. The mechanism of retardation of crack propagation rate due to smoothing of weld toe was discussed from the viewpoint of crack shape and aspect ratio.

Torsional fatigue mechanisms of an A357-T6 cast aluminium alloy

The mechanisms controlling the fatigue response of an A357-T6 cast aluminium alloy under cyclic torsional loading are investigated. Surface crack monitoring coupled with Electron BackScattering Diffraction (EBSD) analysis is used to study crack initiation. Determination of S-N curve combined with interrupted in situ fatigue testing using synchrotron tomography allows the study of the propagation behaviour. It is observed that fractographic morphologies depend on the stress level. At intermediate-low stress levels (τmax < 100 MPa), the grain structure controls the crack initiation and propagation periods. Cracks are usually nucleated in mode II from slip planes close to the specimen axis or perpendicular to it. Mode II crack growth dominates the early stages of crack propagation as mode III inward crack growth is rapidly decelerated. This behaviour leads to the formation of characteristic shallow surface cracks. Once the crack is long enough for the mode II driving force to be overtaken by mode III, propagation branches into ~ 45∘ mode I.

Tooth Crack Severity Assessment in the Early Stage of Crack Propagation Using Gearbox Dynamic Model

Localized tooth crack in gearboxes may be reflected in impulse components of gearbox vibration signals. Crack induced impulses have been used for crack detection and fault diagnosis. In reported studies, researchers have used statistical indicators of the identified impulses, such as root mean square (RMS) and kurtosis, to track the growth of crack. These reported statistical indicators are only effective when crack levels are high and they are unable to detect tooth crack and assess crack severity in the early stage of crack propagation. In addition, no reported studies have focused on studying how tooth crack level affects crack induced impulses. Specifically, what the dominant segments of crack induced impulses are and which segment is affected more by crack growth within a certain crack level range. This paper uses dynamic modeling to study how crack level affects crack induced impulses. First, impulses are generated with a spur gearbox dynamic model under constant working conditions. Second, an exponentially damped sinusoidal model is utilized to fit the impulses and the Matrix Pencil Method is used for model parameter estimation. Finally, relationships between crack level and impulses are studied based on the obtained model parameters. The results have shown that the segments in the fifth and the sixth frequency bands of impulses are two dominant segments, while other segments have little contribution, for the gearbox system under investigation. Within a certain crack level range, there exists an impulse segment which is more affected by the crack level. In terms of the early stage of crack propagation, the segment in the sixth frequency band of the impulse is more affected by crack growth. On this basis, three new statistical indicators have been developed with the segment in sixth frequency band of the impulse and have shown their effectiveness for tooth crack severity level assessment in the early stage of crack propagation. These results have good potential for detection and severity assessment of early tooth cracks in gearboxes.

Analysis of the crack location in notched steel bars with a multiple DC potential drop measurement

The direct current potential drop (DCPD) technique is a frequently used method to measure crack propagation in metallic materials. With a single potential probe attached to the specimen, the size of an initiated crack can be estimated. Therefore, this method is used in crack propagation experiments to determine the crack propagation rate. However, with a single potential probe it is difficult to detect small cracks and impossible to determine the location of the initiated crack.In the present study, three potential probes were attached to cyclic loaded notched steel bars. For the case of an initiated single crack, this experimental set-up delivers different potentials depending on the distance of the probe to the initiated crack. A geometric model to determine the position of the initiated crack from the increase of the individual probe signals is presented.The experimental verification of the model has shown that in case of a single crack, the position can be determined clearly, even in the early stages of crack propagation. Moreover, the sensitivity of the potential drop measurement for the detection of an initiated crack is enhanced by this method. In case of multiple crack initiation sites the scatter of the calculated angle can be used as a criterion for crack detection.

In Situ Microscopic Observation and Crystal Plasticity Simulation of Fatigue Crack Formation in Ti-6Al-4V Alloy

The fatigue behavior of metallic materials is a multi-scale problem (from a time and length-scale perspective) intimately influenced by microstructural features that determine the early stages of crack propagation. Prediction... | Find, read and cite all the research you need on Tech Science Press

Short crack propagation from cracked non-metallic inclusions in a Ni-based polycrystalline superalloy

Fatigue cracks initiating from surface and sub-surface non-metallic inclusions (NMIs) have recently been demonstrated to be a necessary but not sufficient explanation for atypically short low-cycle fatigue life in Inconel 718 alloy at intermediate temperature. Therefore, the early stages of short crack propagation from surface NMIs were investigated in a crystallographic and two-dimensional versus three-dimensional morphological manner after room temperature low cycle fatigue (LCF) testing. In the present investigation, NMIs were purposely pre-cracked using different techniques to suppress the natural crack initiation period and thus the incubation period prior to the early stages of crack propagation. Under such fatigue testing conditions, different mechanisms of crack transmission from pre-cracked NMIs were identified: (i) no propagation, (ii) NMI/adjacent metallic grain interfacial debonding, (iii) transgranular crack propagation within the adjacent metallic grain. Focused-ion-beam cross-section observations of numerous fatigue tested NMIs aimed to define a morphological criterion for non-propagating NMIs. Large cracked NMIs at the surface (2c) with limited extension into the depth (a) did not propagate under such fatigue conditions for 2c/a ratio higher than 3. Furthermore, specific crystallographic relationships between NMIs and the adjacent metallic grain explained different crack propagation configurations from pre-cracked NMIs, i.e. interfacial debonding and transgranular crack propagation involving single or multiple-slip activity.

Study on Random Fracture and Crack Growth of Gear Tooth Waist

The gear tooth fracture usually occurs at the root, but sometimes also occurs at the waist, or even at the top. The random fracture is defined as the rupture at the waist or top of the tooth. The random fracture at the waist is studied in this paper. In order to simulate a manufacturing defect on the tooth surface, a mic-notch (a minute notch) was cut at the waist of the twelve teeth in the two test gears. A gear-running test was carried out under ladder loading till a gear tooth fractured. The fracture appearance illuminates that the failure is fatigue fracture. The initial crack of the notch grew in the five teeth, and no crack propagation was not found in the other seven teeth. The stress intensity factor and the crack propagation length are comparatively studied by three methods such as linear elastic fracture mechanics theory (LEFMT), FRANC3D simulation and the test. In the early stage of crack propagation, the theory values of LEFMT are close to the simulation, but the difference gets larger and larger with the increase in crack length till the gear tooth is broken. However, the difference of crack propagation length between simulation and the test is less, and the error is in the range of 2.4–13.3%. Therefore, the simulation could truly predict the crack growth length.

Isothermal and thermomechanical fatigue behavior of aluminide coated near α titanium alloy

Isothermal and thermomechanical fatigue (TMF) behavior of titanium aluminide coated near α titanium alloy Titan 29A has been studied. TMF behavior was studied in the temperature interval of 300°C↔600°C and isothermal low cycle fatigue test was carried out at 300°C and 600°C. By observation of cross-sectional microstructure and fracture surfaces of failed specimens, the likely sites of crack initiation and early stages of crack propagation have been identified. Fatigue lives exhibited by the alloy under isothermal and TMF loading have been analyzed in light of microstructural observations to gain insight into the operating damage mechanisms.