Induced Crack Initiation Research Articles

Stress wave superposition effect and crack initiation mechanism between two adjacent boreholes

In rock blasting, the effect that stress wave superposition between boreholes has on rock fragmentation remains controversial. Here, we experimentally study the stress wave superposition effect between two adjacent boreholes by combining the model experiment method and the high-speed digital image correlation experimental system. Numerical simulations based on the continuous-discontinuous element method are also used to analyze the crack initiation mechanism between two adjacent boreholes. The model experiment results show that, in the stress wave superposition area between boreholes, the stress component in the direction of the connection line of the two boreholes is mainly compressive, while that in the direction perpendicular to the connection line is mainly tensile. Superposition of the blasting stress waves between the boreholes strengthens both stress components, but the strengthening effect on the stress component in the direction perpendicular to the connection line (mainly tensile) is significantly less than that in the direction of the connection line (mainly compressive). Moreover, these stress enhancements are not sufficient to induce crack initiation in the stress wave superposition area between boreholes. Numerical simulation analysis further shows that crack initiation in the stress wave superposition area has relatively stringent requirements on the explosive parameters, rock properties and borehole spacing, which are difficult to observe in engineering practice and model experiments.

Big Data Fast Analysis Method of Random Stress Spectrum for Crane Equipment

With the increase of service time, the random stress spectrum of crane equipment causes cumulative damage. Cumulative damage induced crack initiation is one of the important failure modes of crane equipment. Through the rapid analysis of the random stress spectrum obtained in real time by the structural health monitoring system of lifting equipment, the cumulative damage of the structure can be analyzed and the remaining life of the structure can be predicted. In this paper, based on the traditional rain flow counting method, the big data fast analysis method of random stress spectrum is studied by adopting stack data structure. The program of fast rain flow counting method based on stack structure with two parameters is compiled. The fast rain flow counting method based on stack structure is used to analyze and calculate the big data of stress monitoring collected by the structural health monitoring system of metallurgical crane, and the stress amplitude-mean-frequency matrix of the hot spot area of fatigue damage of the main girder is obtained, which lays an important foundation for the health status diagnosis of metallurgical crane equipment.

Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (Ni/Nf) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.

Microstructure evolution and mechanical properties of diffusion bonding Al5(TiZrHfNb)95 refractory high entropy alloy to Ti2AlNb alloy

In this study, the refractory high entropy alloy Al5(TiZrHfNb)95 was diffusion bonded (DB) with the intermetallic compound (IMC) Ti2AlNb under 5 MPa at 970–1100 °C in vacuum. The microstructure and element distribution were analyzed using a scanning electron microscope (SEM) and mechanical properties were evaluated by nanoindentation and shear tests. The typical microstructure of DB joint was Ti2AlNb substrate/solid solution/scattered Al3Zr5/solid solution/Al5(TiZrHfNb)95 substrate, which the discrete brittle Al3Zr5 phase was arranged along a straight line in the bonding surface, and gradually elongated and dissolved as the bonding temperature increased from 970 °C to 1100 °C. The formation of Al3Zr5 was attributed to the lowest Gibbs energy of all the possible IMCs in the range of the welding temperature. In the joining region, the IMC Al3Zr5 had the highest hardness (9.01 GPa) and modulus (143.37 GPa) but the lowest elastic recovery (20.37%), thus, it tended to cause stress concentration at the phase interface and induced crack initiation, which had a detrimental effect on the shear strength of joints. The shear strength of DB joints improved with the increase of bonding temperature and reached the highest (427 MPa) at 1100 °C owing to is lowest Al3Zr5 content. As the bonding temperature increased, the fracture of the joint transformed from intergranular brittle to ductile.

Porosity characterization and respective influence on short-beam strength of advanced composite processed by resin transfer molding and compression molding

This work has been developed for a comparative purpose concerning the processing and respective mechanical performance of CFRP composites processed by resin transfer molding (RTM) and compression molding (CM) techniques. Thermal and viscosimetric tests before processing certified the optimal parameter procedure. Both composites were submitted to short-beam shear tests and through microscopy to determine failure mechanisms. CM specimens presented a decrease of 27% in shear strength caused by the presence of macro porosity that induced crack initiation and connection of different delamination plies, causing the speeding up of crack propagation and jump of the interlaminar layer. The low capillary effect and higher viscous force were responsible for macro porosity, inducing heterogeneous impregnation in CM and to the direction reduce in mechanical behavior. On the other hand, more homogeneous impregnation in RTM specimens was responsible for the absence of macro porosity, ensuring higher values of shear strength and lower void volume fraction.

Hydrogen embrittlement behavior of 13Cr-5Ni-2Mo supermartensitic stainless steel

This work presents a comprehensive study of the hydrogen embrittlement (HE) behavior of a 13Cr-5Ni-2Mo supermartensitic stainless steel through two aspects: hydrogen diffusion and hydrogen concentration. Precipitated austenite reduced the effective hydrogen diffusion coefficient and hydrogen permeation flux, and thus increased the HE resistance. Further increasing the austenite content could not provide additional HE resistance as a martensitic transformation is prone to occur when a high hydrogen concentration and stress are coupled, leading to hydrogen induced crack initiation. The HE susceptibility of 13Cr-5Ni-2Mo steel is much higher than that of 960QC steel because of the higher adsorbing hydrogen concentration.

Initiation of hydrogen induced cracks at secondary phase particles

The goal of this work is to propose a general mechanism for hydrogen induced crack initiation in steels based on a microstructural study of multiple steel grades. Four types of steels with strongly varying microstructures are studied for this purpose, i.e. ultra low carbon (ULC) steel, TRIP (transformation induced plasticity) steel, Fe-C-Ti generic alloy, and pressure vessel steel. A strong dependency of the initiation of hydrogen induced cracks on the microstructural features in the materials is observed. By use of SEM-EBSD characterization, initiation is found to always occur at the hard secondary phase particles in the materials.

EBSD characterization of hydrogen induced blisters and internal cracks in TRIP-assisted steel

The hydrogen induced blistering and internal cracking behavior of a TRIP-assisted (Transformation Induced Plasticity) high strength steel is investigated. TRIP-assisted steels are multiphase steels with a microstructure consisting out of ferrite, bainite, retained austenite, and some martensite. Each microstructural constituent demonstrates a different behavior in the presence of hydrogen. Additionally, such steels exhibit an austenite to martensite transformation upon deformation, which influences the hydrogen induced crack formation of the material. In order to assess this influence and to better understand the interaction of such a multiphase material with hydrogen, the hydrogen induced cracking effect is evaluated for a TRIP-assisted steel in the undeformed and cold deformed, i.e. 10% by tensile deformation, condition. The austenite/martensite phase ratio differs for both conditions, allowing one to evaluate the impact of the TRIP effect on the hydrogen sensitivity of the material. The materials are cathodically charged with hydrogen at various charging conditions, i.e. charging time and current density, in order to assess their sensitivity to hydrogen induced cracking. Subsequently, crack initiation and propagation is investigated by microstructural analysis using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Hydrogen induced crack initiation occurs preferably at martensitic islands, while crack propagation takes place along segregation lines, which also consist of martensite networks. Increased deformation leads to less internal crack initiation, but to a more outspoken crack propagation. On the one hand, deformation leads to strain hardening of the material, which leads to an increase of the critical hydrogen pressure necessary for internal crack initiation. On the other hand, deformation of TRIP-assisted steel results in the formation of martensite and provides a more favorable crack path for continuous crack propagation. Moreover, the synergetic effect of hydrogen induced and assisted cracking mechanisms is illustrated.

Crystal orientation effect on fretting fatigue induced geometrically necessary dislocation distribution in Ni-based single-crystal superalloys

The effect of crystal orientation on fretting fatigue induced crack initiation and dislocation distribution is studied by in-situ SEM observation and electron back-scattered diffraction (EBSD) in this paper. Cracks and slip lines are observed in the fretting contact area of Ni-based single-crystal (NBSX) superalloys. The in-situ SEM observation captures different crack and slip line behaviors under different crystal orientations. The EBSD analysis results show obvious misorientation and orientation deviation in the fretting contact area. For both crystal orientations, the geometrically necessary dislocation (GND) density distributions in the contact area are obtained by using Hough-based EBSD methods. The peak position of grain reference orientation deviation (GROD) and GND density matches with the fretting fatigue crack formation position. EBSD analysis shows that the dislocation density distribution on each slip system is closely related to the crack initiation direction. The direction of slip system with the maximum dislocation density agrees with the crack initiation direction obtained by in-situ observation.

Modeling of an excavation-induced rock fracturing process from continuity to discontinuity

A convenient and efficient approach to simulate the excavation-induced rock fracturing process from continuity to discontinuity is proposed. The excavation boundary is considered to be an internal discontinuity, which is enriched by a Heaviside function. By using partition of unity concept and level set method, the cellular automaton updating rule for elasto-plastic fracturing induced by excavation is developed. The excavation induced rock mass strain localization is described by plasticity. A plastic strain-dependent criterion is proposed to link continuity and discontinuity, by which crack initiation and propagation of rock mass are described. By using the cellular automaton neighborhood information and level set method, the crack propagation path and excavation boundary can be tracked efficiently. The excavation and the induced crack initiation and propagation can be represented and simulated without explicit remeshing. This approach is implemented in a self-developed numerical model, i.e. a rock continuous-discontinuous cellular automaton (CDCA). The reliability and versatility of CDCA in the modeling of excavation-induced crack initiation, propagation, coalescence and block formation in rock mass are well demonstrated. The method helps estimate the onset of stable and unstable failure development, as well as the magnitude of plastic strain before cracking in rocks.

Microstructural evolution and mechanical properties of laser beam welded joints between pure V and 17-4PH stainless steel

The microstructures and mechanical properties of joints between 17-4PH stainless steel and pure V were investigated. During laser welding of these joints, the laser beam was first placed at the middle of the joint interface (LW-0) and then shifted by 0.1 mm (LW-0.1) or 0.2 mm (LW-0.2) towards the 17-4PH side. The effects of post-weld heat treatment on the mechanical properties of the welded joints were examined. The microstructure of the LW-0 joint, which was mainly composed of (FeV)ss due to uniform mixing of Fe and V, changed to an (Fe)ss-rich phase at the 17-4PH//FZ side and (FeV)ss at the FZ//V side upon shifting the laser beam towards the 17-4PH side. A small fraction of σ-phase was formed, which induced crack initiation and propagation, when the laser beam was not shifted. The grain morphologies in the fusion zones were highly dependent on the Fe–V composition. At a V content above 28 at.%, (FeV)ss readily forms and a coarse grain structure was observed; below this concentration Fe-rich phases formed with a columnar grain structure upon solidification. The hardness and tensile properties of the joints were dependent on the V concentration in the melt zone. The hardness increased in the order of LW-0.2 < LW-0.1 < LW-0, whereas the tensile strength decreased in the order of LW-0.2 > LW-0.1 > LW-0. The hardness significantly increased after annealing due to the precipitation of the σ-phase, which was dispersed throughout the fusion zones of LW-0 and LW-0.1, or at the FZ//V interface for LW-0.2.

Enhancement by Metallic Tube Filling of the Mechanical Properties of Electromagnetic Wave Absorbent Polymethacrylimide Foam.

By the addition of a carbon-based electromagnetic absorbing agent during the foaming process, a novel electromagnetic absorbent polymethacrylimide (PMI) foam was obtained. The proposed foam exhibits excellent electromagnetic wave-absorbing properties, with absorptivity exceeding 85% at a large frequency range of 4.9–18 GHz. However, its poor mechanical properties would limit its application in load-carrying structures. In the present study, a novel enhancement approach is proposed by inserting metallic tubes into pre-perforated holes of PMI foam blocks. The mechanical properties of the tube-enhanced PMI foams were studied experimentally under compressive loading conditions. The elastic modulus, compressive strength, energy absorption per unit volume, and energy absorption per unit mass were increased by 127.9%, 133.8%, 54.2%, and 46.4%, respectively, by the metallic tube filling, and the density increased only by 5.3%. The failure mechanism of the foams was also explored. We found that the weaker interfaces between the foam and the electromagnetic absorbing agent induced crack initiation and subsequent collapses, which destroyed the structural integrity. The excellent mechanical and electromagnetic absorbing properties make the novel structure much more competitive in electromagnetic wave stealth applications, while acting simultaneously as load-carrying structures.

Multiscale crystal-plasticity phase field and extended finite element methods for fatigue crack initiation and propagation modeling

This paper presents a physics-based prediction of crack initiation at the microstructure level using the phase field (PF) model without finite element discretization, coupled with an efficient and accurate modeling of crack propagation at macro-scale based on extended finite element method (XFEM). Although the macro-scale model assumes linear elastic material behavior, at micro-scale the behavior of plastically deforming heterogeneous polycrystals is taken into account by coupling the PF model and a crystal plasticity model in the fast Fourier transform computational framework. A sequential coupling has been established for the multiscale modeling where the macro-scale finite element (FE) model determines the hot spots at each cyclic loading increment and passes the associated stress/strain values to the unit-cell phase-field model for accurate physics-based microstructure characterization and prediction of plasticity induced crack initiation. The PF model predicts the number of cycles for the crack initiation and the phenomenological crack growth models are employed to propagate the initiated crack by the appropriate length to be inserted in the FE mesh. Finally, the XFEM solution module is activated to perform mesh independent crack propagation from its initial crack size to the final size for the total life prediction. The effectiveness of the proposed multiscale method is demonstrated through numerical examples.

The response of accumulated stress and microstructural refinement on cyclic deformation and associated mechanism in high strength bainite-martenite dual-phase steel

We elucidate here the response of accumulated stress and microstructural refinement on the ultra-high cyclic deformation and associated crack initiation mechanism in high strength bainite-martenite dual-phase steel, where non-inclusion induced crack initiation is the dominated failure mode. The non-inclusion induced crack initiation involved nucleation of microvoids in the coarse bainitic lath when the applied stress accumulated. The density of the microvoids increased with the accumulation of applied stress, and the microvoids that were near the grain boundary connected to form a microcrack. Failure occurred when the microcracks combined to form a macrocrack. The premise of crack propagation is that the total stress intensity factor at the crack tip is larger than granular bright facet (GBF) crack growth threshold value. A mathematical criterion is proposed for non-inclusion induced cracking. The study suggested that the probability of crack initiation is reduced when the microstructural constituent, bainitic lath, is refined.

Evaluation of the hydrogen embrittlement susceptibility in DP steel under static and dynamic tensile conditions

Hydrogen (H) induced mechanical degradation is studied in DP steel by performing tensile tests under static and dynamic conditions. Tensile specimens were electrochemically H charged and tensile tests were done ex-situ after charging. Different H contents were charged into the samples by modifying the current density. The strain rate is increased from static (1.67*10−2 and 1.67 s−1) to dynamic (450 and 900 s−1) conditions to verify the effect of H diffusivity during the tensile tests on the hydrogen embrittlement (HE) susceptibility. Therefore, a reproducible methodology was established by using a standardized tensile machine for static testing and split Hopkinson bar experiments for dynamic conditions. The HE degree increased with current density due to higher amount of H, as confirmed by melt extraction. The HE% also increased with slower strain rates since H was able to diffuse to a crack tip, hence accelerating failure. Even at the highest strain rate (900 s−1), the material lost about 10% of its ductility due to H present in the sample and not because of H diffusion during the test. This was concluded since H induced brittle failure initiated at the edges of the samples at slow strain rates. Though at a strain rate of 1.67 s−1, fracture initiated in a ductile way from the center similarly as for tests performed without charging. Fractographic visualization of the fracture surfaces revealed an embrittled central line when charged with H, which evolved into a major crack. MnS inclusions were found in this central line accounting for the H induced crack initiation.

The Effect of Controlled Shot Peening on the Microstructure and Fatigue Behavior of Wet Copper-Based Powder Metallurgy Friction Plates

The effects of shot peening treatments on the microstructure and fatigue behavior of the teeth root in wet copper-based powder metallurgy friction plates were investigated and an efficient method for selecting optimal shot peening parameters for friction plates was proposed. Different experimental processes including microscopy observation, microhardness, roughness and X-ray diffraction measurements have been performed to characterize the treated surface of specimens. It is found that fatigue life of friction plates firstly increases and then decreases with the increase of Almen intensity, and the optimal fatigue life has improved over 55%. It is considered that both surface grain refinement and high residual compressive stress are the main ingredients responsible for the improved fatigue life. With further increase of Almen intensity, surface roughness of the teeth root gradually increases to a critical point, where rough peened surface may induce crack initiation due to stress concentration, and finally lead fatigue deteriorate.

Fatigue life prediction of engaging spur gears using power density

The high cycle fatigue performance of an input spur gear pair found in the gearbox of a 4LZ-2 combined harvester with a maximum walking power of 15 kW is investigated. A three-dimensional finite element model of the two engaging spur gears is developed to estimate the fatigue life of the pinion gear subject to bending induced crack initiation and propagation. The critical point of the pinion gear is obtained along with the associated bending stress-time history. The novel concept of power density is applied to the finite element result to correlate fatigue crack initiation and subsequent crack growth with the number of loading cycles undergone. After the location of crack initiation is identified, fatigue crack propagation is modelled using linear elastic fracture mechanics. The estimated fatigue life of the pinion gear is 886 h. A fatigue test rig is employed to physically demonstrate the feasibility of the power density concept for predicting gear fatigue life. It is shown that the power density concept is preferred over the Miner rule for higher accuracy in fatigue life prediction.

Non-inclusion induced crack initiation in multiphase high-strength steel during very high cycle fatigue

During very high cycle fatigue (VHCF) crack can initiate at non-inclusion site in the interior of Mn-Si-Cr bainite/martensite (B/M) multiphase steels. The characteristics of non-inclusion induced crack initiation in B/M multiphase steels and the relationship between the size of granular bright facet (GBF) and VHCF limit were studied through ultrasonic fatigue testing, in terms of fracture surface analysis, crack initiation analysis, and failure mechanism. The current study reveals that GBF area became smaller with the increase of VHCF limit, i.e. the steel with higher fatigue-endurance strength exhibited smaller GBF area in the case of steel characterized by non-inclusion crack initiation in the VHCF regime. The relationship between GBF area, fatigue life, and applied stress was consistent with the following equation areaGBF = 29.41LogNf − 0.2916 σa + 47.51. The study provides an understanding of non-inclusion induced crack initiation in VHCF and a guidance for steel fatigue design.

In-situ observation of hydrogen induced crack initiation in a nickel-based superalloy

Hydrogen induced crack initiation in a nickel-based superalloy was studied via in-situ Scanning Electron Microscopy observation under slow strain rate loading. Direct evidence has been obtained of crack initiation and propagation along dislocation slip bands leading to sample fracture. Post-mortem microstructural analysis showed that hydrogen induced cracking and separation along dislocation slip bands, which are along {111}γ crystallographic planes with the highest Schmid factor, leads to the formation of the typical slip-trace-like features on the quasi-cleavage surface.

Analysis on Crack Fatigue Growth Calculation for Nuclear Power Plant In-service Inspection

Fatigue is the common failure mode for nuclear mechanical components during operation which could induce crack initiation and growth. In order to predict the behaviour of fatigue crack propagating, fatigue calculation is introduced. A new method of crack fatigue growth calculation has been presented in this paper according to French RSE-M Rules. Regarding this method, detail analysis on key formulas selecting, essential calculation factors of occurrences combination, K-value calculation and mechanic modelling, characterization interval option has been performed. One fatigue crack growth calculation example of dissimilar weld of Reactor Pressure Vessel outlet nozzle to the safe end has been taken and the result demonstrates a good expectation.