Initial Stage Of Failure Research Articles

Effects of ultrasonic vibration assisted milling with laser ablation pretreatment on fatigue performance and machining efficiency of SiCf/SiC composites

Due to the poor machinability of SiCf/SiC parts in machining, many problems are caused, such as low machining efficiency, poor machining quality and high processing cost, which seriously limit its manufacturing and application. A novel process ultrasonic vibration assisted milling with laser ablation pretreatment (UVAMLAP) was proposed to optimize the fatigue performance and machining efficiency of SiCf/SiC parts. This process was used to fabricate specimens, which were then tested for tensile strength and fatigue performance. The results show that UVAMLAP could enhance surface quality, and increase the tensile strength and residual tensile strength of the sample by 9.4% and 13.5%. This process can avoid damage aggravation in the initial stage of failure, weaken matrix fracture and interface debonding velocity, and reduce fatigue performance degradation caused by machining damage. In addition, comprehensive evaluation based on multi-dimensional indicators such as milling quality, machining efficiency and tool cost for machining strategy was carried out by taking tensile sample machining as an example. The UVAMLAP process can not only improve the machined surface quality, but also reduce the machining time by 31.3% and the tool cost by 75%. Therefore, UVAMLAP provides a feasible process scheme for high-efficiency and low-damage machining of SiCf/SiC parts.

The influence of L12 ordered precipitates on hydrogen embrittlement behavior in CoCrNi-based medium entropy alloys

The recently emerged multicomponent (or medium/high entropy) alloys have generated considerable excitement globally in the last 10 years because of their excellent mechanical and functional properties, particularly in terms of strength-ductility combinations that can surpass most other metallic materials. However, the achieved high strength level (above 1 GPa in many cases) fuels strong concerns about hydrogen embrittlement (HE). Detailed investigation in this field is still scarce, especially pertaining to the face-centered cubic medium entropy alloys (MEA) that are typically strengthened by ordered precipitates. Here, we unravel the effect of γ' (L12) ordered precipitates on H-induced damage behavior and the associated HE resistance in CoCrNi-based MEAs. Compared with the equi-molar CoCrNi MEA, the precipitation-hardened (CoCrNi)94Al3Ti3 MEA shows an enhanced HE resistance even at a higher strength level. Both alloys are fractured due to H-assisted intergranular cracking at the initial failure stage when loaded in the presence of H. The formation of intergranular cracks is primarily attributed to the H-induced decohesion at grain boundaries, where a high stress/strain concentration accompanied by a more intensive dislocation planar slip (or stacking fault formation) caused by H was observed. The presence of γ' precipitates serves to slow down the internal diffusion/migration of H due to the trapping effects. The precipitates with a relatively larger size (∼50 nm) also hinder dislocation planar slip thus decreasing the number of pile-up dislocations at grain boundaries. Both effects collectively reduce the tendency of H-induced intergranular cracking, leading to the improved HE resistance. The work reveals the positive role of ordered precipitates in H tolerance and thus provides some insights in further microstructure design of medium/high entropy alloys for applications in H-abundant environment.

Construction method of ancillary emergency backup service based on battery energy storage system

The power systems with a high proportion of clean energy face both greater uncertainty in power generation and the threat of stability brought by low inertia. After a fault, the rapid drop in power grid frequency, large voltage fluctuations, and insufficient reactive power support can lead to new energy generation units being disconnected from the grid. Fast active and reactive support will play an important role in suppressing fault propagation in low-inertia systems and preventing large-scale power outages. The reserve and frequency regulation market is a necessary means to deal with power grid failure events, but many blackout events occurred due to insufficient reserve capacity and ineffective market transaction mechanisms. As a flexible power regulation resource, BESS (battery energy storage system) has been incorporated into the power ancillary service market planning. In some engineering cases, the frequency regulation ancillary service provided by energy storage has also shown its advantages. However, there is a lack of in-depth research on how to gather BESS to take part in AEBS (ancillary emergency backup services) in the early stage after the failure. Therefore, in view of the dynamic change of power system risks and the different response speed of different backup resources, this paper proposes to establish a segmented combined backup mode to realize the combination of emergency backup at the initial stage of failure and frequency regulation backup during the fault recovery process. A related model of AEBS demand assessment and emergency backup service pricing mechanism is established. And considering the capacity, life loss and opportunity costs of energy storage for AEBS, a LMP (locational marginal price) method is used to calculate the electricity price of AEBS provided by BESS. It has proved the effectiveness by constructing a grid AEBS based on BESS used by the IEEE 30 node system.

Quasi-static and dynamic compressive behavior of BN-BAS composite ceramic at room temperature and 1000 °C

Abstract Boron nitride-barium aluminosilicate (BN-BAS) composite ceramic specimens were prepared by using hot-press sintering at 1700 °C and 20 MPa from composite powders prepared by employing the sol–gel method. The dynamic compressive behaviors of these BN-BAS composite ceramic specimens were investigated by using split-Hopkinson-pressure-bar set-ups at room temperature and 1000 °C. The effects of the strain rate on the dynamic compressive strength, fragment size, and microstructure were investigated. The quasi-static mechanical properties of the composite ceramics were also tested. The results indicated that the quasi-static and dynamic mechanical properties of BN-BAS composite ceramics at 1000 °C were lower than those at room temperature, and this was caused by the crack propagation in the ceramic specimens at high temperatures owing to the high thermal expansion coefficient of hexacelsian. The dynamic compressive strength of the composite ceramic increased linearly with an increasing strain rate. Flake-like h-BN particles absorbed a large amount of energy through bridging and pull-out, which prevented macroscopic breakage of specimens at strain rates lower than 850 s −1 at the initial failure stage at room temperature. As the strain rate increased, the cracks changed from long and sparse to small and dense reticularly distributed ones, and the fragment size decreased accordingly.

The Whole-Process Low-Voltage Ride-Through Control Strategy for Photovoltaic Power Generation Based on Adaptive Maximum Power Tracking

The impact of the large-scale disconnection of photovoltaic generators on the grid cannot be ignored, so the low-voltage ride-through problem of photovoltaic generators needs to be solved urgently, especially in the stage after fault recovery, there is a problem that the converter is difficult to switch back to the original control strategy. Therefore, a new control strategy of the whole-process low-voltage ride-through control strategy for photovoltaic power generation based on adaptive maximum power tracking is proposed. In the initial stage of failure, an adaptive maximum power control strategy is proposed. This control strategy transforms maximum power point tracking control into adaptive maximum power control by feedback of voltage drop amplitude. It can control the output of photovoltaic power quickly and effectively and realize the zero-voltage crossing of the photovoltaic grid-connected system. In the stage after fault recovery, the switching function is established by comparing the network voltage with the rated voltage. The operating state of the photovoltaic system can be judged by different output of the switching function. It realizes the fast switching of the photovoltaic side control strategy during the fault recovery stage. Simulation results show that the proposed method can switch the control strategy reliably and realize the whole-process low-voltage crossing of photovoltaic power generation quickly.

Nyexon Rock Avalanches: A Special Intrusion Restraint Mechanism, Tibet, China

The Nyexon Rock Avalanches in the southern Qinghai-Tibet Plateau is a huge scale earthquake-induced slope disaster in the Holocene, the accumulation area has distinct sedimentological characteristics, which is of great significance for studying the intrusion and restraint mechanism during long-distance transportation of large rock avalanches or debris avalanche. This long-distance transportation induced a series of landform types, such as ridges, hills, and ravines; they are widely distributed in all areas and extensively developed shear zones, jigsaw cracks, and other structures within the sedimentary structure. With the analysis of DEM data and geological survey, two main types of basement structures and their transition relationships are distinguished; they play an essential role in the restraining bottom during rock avalanches. In the sedimentary structure, the block facies and mixing facies occupy the main body of the deposition from the center to the distal area. Under the basement restriction, mixing facies are formed between the bottom of the sedimentary layer and the basement sedimentary structure; the shear band is mainly developed along with the mixing facies and basement facies, which is accompanied by basement liquefaction and rheology. A sedimentary facies model is established based on the sedimentary structure sequence of the Nyexon Rock Avalanches transportation. After analyzing the failure mechanism of the rock avalanches, it is believed that in the initial stage of failure, the rock avalanches is transformed into a particle flow that is similar to the debris avalanche, which is restrained by the basement structure and lateral bound; then, an accumulated obstacle highland is formed in the central area after deceleration, making the transportation of the main fluid to deflect quickly.

Numerical study of specimen with steel inclusion: Influence of interfacial transition zone

In this paper, the influence of the interfacial transition zone (ITZ) between steel inclusion and fine-grained cement-based composite on fracture behaviour is investigated. Specimens of the nominal dimensions 40 × 40 × 160 mm with the steel inclusion of the prismatic shape with nominal dimensions 8 × 8 × 40 mm were provided with an initial central edge notch with a depth 12 mm.The aim of this paper is to analyse the behaviour of such specimen by means of numerical modelling by finite element method (Ansys software). A simplified 2D model (plane strain) based on the fracture test configuration was created. The crack propagation assessment was based on the criterion of the average value of tangential stress σθθ calculated over certain distance d in dependence on the polar coordinate θ. It is assumed that the crack is initiated when the average stress σθθ(θ0) reaches its critical value σθθ,c(θ0), which depends on the fracture toughness KIC of the material and on the distance d.From the detailed numerical analysis of the described fracture test, we concluded that the crack propagation path depends strongly on the fracture properties of ITZ. The central inclusion works as an obstacle to crack propagation only in the initial stage of failure. The mechanical fracture parameters of the ITZ strongly influence the overall fracture behavior of test specimens.

From diffuse damage to discrete crack: A coupled failure model for multi-stage progressive damage of composites

The multi-stage failure process and the coupled failure mechanisms have posed great challenges to damage analyses of fibre-reinforced composite laminates. In this study, a coupled failure model (CFM) is proposed which incorporates both smeared crack model (SCM) and discrete crack model (DCM) through an energy-consistent thermodynamic framework. While SCM is well suited for describing crack nucleation and diffuse damage at initial failure stage, DCM offers a great tool to model the subsequent crack growth and its interaction with other failure modes. A hybrid SCM/DCM formulation is developed for CFM, so that the progressive failure process from diffuse damage to propagation of major cracks is characterized. Furthermore, inherent limitations of each standalone model, such as convergence difficulty and computational inefficiency, are effectively resolved by the proposed approach. Performance of CFM is demonstrated through a few numerical examples, including element validation, Off-axis tensile and open-hole tension tests of composite laminates.

Premature failures of railway axles after repeated pressing

Operational fatigue failures of press-fitted railway axle seats were investigated to identify the conditions leading to premature crack formation. The typical orientation of the fatigue cracks are described in the particular stage of crack propagation. The complex microstructural analyses of the steel grade A1N served for detection of the microstructural influence in the initial stage of failure. Numerical analyses were conducted for the quantitative evaluation of the influence of the localized drop in contact pressure due to re-pressing during standard maintenance. The crevice corrosion process, emphasized by the selective corrosion of pearlitic steel, was identified; it leads to suppression of the initial stage of the fatigue process driven by shear stress as is typical for fretting fatigue on the surface of press-fit seats. A real-scale experiment of the Minden test type was performed using an axle with operationally initiated cracks as validation of the computational model for predicting the development of non-destructively indicated defects.

Fault diagnosis of vibration signal of synchronous condenser based on wavelet model

A large synchronous condenser has a complex structure and many parts, which is very prone to failure. However, the regular electrical quantity generally does not change significantly at the initial stage of failure, and may not reflect the fault characteristics. Some non-electrical quantities, such as vibration characteristics, can be used to diagnose and predict faults in a timely manner. Based on this, a multilayer-perceptron based wavelet model is proposed for fault diagnosis. Vibration signals are taken in real time at different positions on the surface of the condenser, and the energy eigenvalues are calculated by fast Fourier transform of the data, and the wavelet neural network model is input for training to obtain the nonlinear mapping relationship between the vibration signals and the fault types of the condenser, so as to realize the fault diagnosis of the condenser. Experimental results show that this method can effectively detect and diagnose the fault of the synchronous condenser in the early stage.

Experiments and modelling of cantilever failures for cohesive riverbanks

ABSTRACTThis paper presents experiments and modelling of cantilever failures for a cohesive riverbank. Two types of cohesive materials with different bank cohesions were investigated under similar hydraulic conditions to clarify and simulate the failure phenomena. The experimental results showed that fluvial erosion undermined the banks during the initial stage of failure, and tension cracks then developed at the upper surface of the banks. Beam failure occurred thereafter. Additionally, a numerical model was developed using a triple-grid approach to simulate the behaviour of a cantilever within the framework of fluvial erosion, cantilever failure, slump block, and sediment transport. Moreover, cantilever failure was analysed using a limit equilibrium method to compute the stability of the overhanging block. The numerical model exhibited good agreement with the experimental results in terms of the spatial-averaged bank width along the banks and of the dominant failure type, which was beam failure.

Buckling and energy absorption of novel pre-folded tubes under axial impacts

Dynamic response of novel pre-folded tubes subjected to axial impacts is investigated numerically. A numerical model for novel pre-folded tubes with different fold angles is proposed to study its dynamic buckling and energy absorption. Results show that the deformation process of the tube under the impact force can be divided into three phases, i.e., initial peak, stable progressive buckling, and densification stages. It is shown that at the initial failure stage, the initial peak force is much lower than one of the square tube. The buckling modes of the corrugated tubes can be summarized as the progressive buckling, dynamic plastic buckling, and mixed bulking modes. The fold angles can improve the energy absorption capacity of the pre-folded tube efficiently.

Turbine generator status diagnostic system based on petri nets

The objective of the study is to build an automated system to monitor and control the turbine generator (TG) processes, based on acoustic analysis of the stator failures, for the purpose of improving the operating efficiency of the nuclear power plant (NPP) as a whole. This requires the development of an information and measurement system (IMS) which receives information on the status of the TG stator windings from the noise detectors installed on the TG surface. Acoustic analysis of the TG “hot” spots is used to identify the TG status.Petri nets were used to build a stochastic model of the automated TG monitoring and control system. The model in question allows local changes in the status of the TG stator windings to be detected and makes it possible to reduce the problem dimensionality through the selection of a subset of conditions leading to a risk of the stator failure. The model is simple and features a high speed of response. The use of the IMS structure under consideration enables the TG condition to be detected at the initial failure stage and the measurement process rate to be increased tenfold.

Effects of TiN, CrN and TiAlN coatings using reactive sputtering on the fatigue behaviour of AA2024 and medium carbon steel specimens

ABSTRACTThe effects of ceramic coatings (TiN, CrN and TiAlN films) on the fatigue behaviour of 2024 aluminium alloy (AA2024) and medium carbon steel (MCS) specimens under cyclic loading have been investigated in this study. These ceramic thin films of about 2 μm thickness were deposited on the substrates using radio frequency reactive magnetron sputtering, with metallic Ti, Cr and Al targets in an Ar/N2 gas environment. The (111), (200) and (220) and (311) peaks of a face centre cubic (fcc) structure in CrN films, and the (111), (200) and (220) peaks in TiN/TiAlN films were observed using X-ray diffraction method, respectively. Fatigue tests were performed using rotational bending conditions at a speed of 3000 rpm with five stress levels (115, 173, 231, 288 and 346 MPa for AA2024; 288, 346, 404, 462 and 519 MPa for MCS) in air. Scanning electron microscopy was used for the analysis of surface morphologies and initial failure stage. The results show that the fatigue life of coated specimens is significantly increased under lower cyclic loading, for example, from 42% at 288 MPa, to 98% at 230 MPa and up to 434% at 173 MPa, for AA2024 with TiN coatings. The improvements of mean fatigue life for MCS coated with TiN films are 88% at 519 MPa, 197% at 462 MPa and up to 597% at 404 MPa, respectively. The experimental results also depict that the coated specimens with higher surface hardness (TiAlN) tended to increase in fatigue strength and fatigue life over uncoated and TiN/CrN specimens.

Damage Accumulation in Aluminum Alloys under Plastic Deformation and Creep

The accumulation of local and bulk damage in D16 AT and 1201 T1 aluminum alloys used in aircraft engineering is studied. The local damage level is calculated from data of thermoactivation analysis of the residual life of D16 AT alloy specimens after preliminary plastic deformation. The bulk damage level is determined from the elastic-modulus defect by measuring the natural frequency of 1201 T1 alloy specimens. Life tests of the specimens were performed at constant tensile loads and elevated temperatures. The dependence of the local damage on preliminary plastic strain at room temperature is obtained. The residual life of the specimens is calculated with allowance for the damage to the material in the initial stage of failure and compared with experimental results. Data are given on the kinetics of bulk-damage accumulation in various test regimes.

On the similarity of the initial stage of failure of solids and liquids under impulse loading

On the similarity of the initial stage of failure of solids and liquids under impulse loading

Effect of microinhomogeneous deformation on the microfracture characteristics of metals

1. As the process of microfailure of a structure begins, the mechanism of microinhomogeneous deformation changes, which can be seen as a redistribution of the high and low deformation sites [15]. In this context, the proposed model of a polycrystalline material represents the basic process by which the linear nature of the stress dependency and elastic strain on the length of the rods is only observed in the initial stages of failure. When the system is further stressed, as a result of redistribution of the load from failed rods, the linear dependency no longer holds. 2. The quantitative results obtained concerning the effect of initial inhomogeneity on the failure load show that loss of strength in crystalline material is not only a result of the movement of dislocations, but also as a result of the fact that deformation and failure occurs in an inhomogeneous manner. And the higher the initial level of inhomogeneity, the earlier the load curve will deviate from linearity.

The initial stage of failure of chemically deposited coatings

The initial stage of failure of chemically deposited coatings

Initial stages of failure of an orthogonally reinforced composite material

1. ML is the result of crack formation in the hybrid composite material. 2. The loss of continuity in uniaxial loading can be recorded both by the ML method and (in individual cases) by strain gauging. 3. For the hybrid composite in combined loading in the plane stress state, ML indicates (in contrast to strain gauging) the instant of the start of cracking.

The effect of moisture on the physical and mechanical integrity of epoxies

The effect of specific combinations of moisture, heat, and stress on the physical structure, failure modes, and tensile mechanical properties of diaminodiphenyl sulphone (DDS)-cured tetraglycidyl 4,4′diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (27 wt% DDS)] are reported. Sorbed moisture plasticizes TGDDM-DDS epoxies and deteriorates their mechanical properties in the range 23 to 150° C. Studies of the initiation cavity and mirror regions of the fracture topographies of these epoxies indicate that sorbed moisture enhances the craze initiation and propagation processes. The effect of tensile stress-level, applied for 1 h on dry epoxies, on the subsequent moisture sorption characteristics of the epoxies was also investigated. Such studies indicate that the initial stages of failure that involve both dilatational craze propagation and subsequent crack propagation enhance the accessibility of moisture to sorption sites within the epoxy to a greater extent than in the latter stages of failure which involve crack propagation alone. The amount of moisture sorbed by TGDDM-DDS epoxies is enhanced by ∼ 1.6 wt% after exposure to a 150° C thermal spike, as a result of moisture-induced free volume increases in the epoxies that involve rotational—isomeric population changes.