Failure Phenomena Research Articles

Investigation of extremely low cycle fatigue behavior of low yield strength steel LY225 under different stress states

• Monotonic tensile and ELCF tests were conducted for LY225 low yield point steel. • ELCF fracture behavior between LY225 steel and Q235 steel was compared. • The LCVGM parameters of LY225 steel were calibrated based on the experimental results. • Buckling and ELCF behavior of LY225 steel dog bone specimen were experimental studied. • The LCVGM can well simulate the post buckling ELCF fracture of dog bone specimen. Low yield point steel is frequently utilized in the energy dissipation components of structures because of its good seismic performance. A low yield point steel LY225 subjected to different stress states is experimental investigated in the study by the monotonic tensile tests and extremely low cycle fatigue tests. The test results demonstrate that LY225 steel shows cyclic hardening characteristic as well as dimple fracture failure mode. In addition, LY225 steel exhibits higher deformability and energy dissipation capacity than Q235 low carbon steel, especially in pure shear stress state, the deformation capacity and extremely low cycle fatigue life of LY225 steel are more than twice that of Q235 steel. The Lode parameter enhanced cyclic void growth model (LCVGM) comprehensively considering the influence of stress triaxiality and Lode parameter can be employed to predict the extremely low cycle fatigue fracture. The LCVGM parameters of LY225 steel were determined based on the experimental results. It indicates that the fracture failure of LY225 steel is free from the influence of Lode parameter due to its good shear performance. The monotonic tensile test and extremely low cycle fatigue test were carried out on the LY225 dog bone specimens, and the fracture failure phenomenon after buckling occurred in the test. The post buckling crack initiation and growth process were numerically simulated by the LCVGM of LY225 steel. The prediction accuracy of the LCVGM was verified by comparing the test results and simulations. This study may provide valuable information about the fracture behavior of LY225 steel under various stress conditions, as well as an effective approach to analyze the post buckling fracture process on the member level.

Unclamped-Inductive-Switching Behaviors of p-GaN HEMTs at Cryogenic Temperature

In this letter, the unclamped-inductive-switching (UIS) behaviors of GaN-based high-electron-mobility transistors with p-type GaN gate (p-GaN HEMTs) at cryogenic temperature (CT) are first revealed. Unlike the temperature-dependent avalanche-induced failure for SiC devices during UIS process, the withstanding behaviors and final critical breakdown voltage of p-GaN HEMT during UIS process at CT are completely consistent with the results at room temperature. As proved, the resonant circuit formed by device output capacitance and load inductor is the key factor influencing UIS waveforms, and such state is temperature insensitive. Furthermore, the inverse-piezoelectric effect induced by high electric field during UIS process is demonstrated to be the original cause that makes this temperature-independent failure phenomenon. In conclusion, the UIS withstanding capability of p-GaN HEMT will not be degraded in such an extremely low temperature scene, indicating a superiority of p-GaN HEMT for special applications compared with traditional devices.

A concurrent irradiation-mechanics multiscale coupling model

• A novel concurrent irradiation-mechanical multiscale coupling model is developed. • Models of irradiation defect kinetics and irradiation hardening are established. • The difference between post irradiation and concurrent irradiation-deformation tests are disclosed. • Dislocation channel formation is not always correlated with yield drop. The vast majority of our current knowledge regarding the basic mechanisms controlling irradiation effect on mechanical properties is based almost entirely on the results of post-irradiation experiments or theoretical models. However, the concurrent effects of irradiation, mechanical stress, and thermal damage on the failure phenomena of materials and components remain largely unexplored due to its internal multiscale-multiphysics coupling nature. We present here a concurrent irradiation-mechanics multiscale coupling model. The concurrent evolutions of nanoscale irradiation defect clusters, microscale dislocation configurations, and mechanical responses are well captured based on coupling cluster dynamics, discrete dislocation dynamics, and the finite element methods using an effective time marching scheme. Model predictions of defect densities and size are in general agreement with experimental observations. Irradiation hardening is shown to take place also in samples undergoing concurrent irradiation-mechanical loading, similar to samples tested post-irradiation. However, the occurrence of plastic flow localization and dislocation channel formation is not accompanied with apparent yield drop (softening) under concurrent irradiation-mechanical loading conditions, which is different from the post-irradiation case.

Performance of box-shaped column connection achieved with local grouting and bolts

To prevent the traditional box-shaped column flange connection from affecting the facade of the overall structure, this study proposed a box-shaped column connection achieved by local grouting and bolts. One integral box-shaped column specimen and two box-shaped column connection specimens were designed. The seismic performance of the connection and the influence of local grouting on the mechanical performance of the connection were studied. The failure phenomenon of the specimen was observed, and the mechanical performance of specimens, such as hysteretic behavior, bearing capacity, stiffness degradation, ductility, and energy dissipation capacity were examined. Results are as follows: the failure mode of the integral box-shaped column specimen and the specimen connected by local grouting and bolts is plastic hinge failure; the effect of local grouting can improve the integrity of the connection and obtain good energy dissipation capacity; the effect of local grouting can increase the bearing capacity of specimen by 24.84% and reduce the bolt tensile force by 34.37%; the steel connector at the column–column joint prevented the buckling deformation of the box-shaped column; the ductility is increased by 22.06%; no slip occurred between the upper and lower steel connector. Therefore, the new joint is safe and reliable. Through numerical simulation analysis, the research results show that the joint can be equivalent to rigid connection. The peak bearing capacity of the box–column joint is checked according to the Code for Seismic Design of Buildings: GB50011–2010. The calculation results are consistent with the experiment results.

Neurodidactic factors in the prediction of academic dropout in Andalusian university students

Persistence and dropout are two sides of the same coin. Together with personal and social factors, associated with the quality of teaching provided by universities determine students' ability to persist in achieving their academic degree or, on the contrary, to drop out of university studies. Our working hypothesis is that the impact on improving the quality of teaching by considering neurodidactic factors, which are currently being researched and experimented with, can improve the overall persistence rate by reducing the dropout rate. In this preliminary study, we intend to make a first approach to the phenomenon of academic failure in Andalusian universities from the prediction and diagnosis of risk groups and the recommendation of preventive measures. Among the measures we proposed for prevention, we highligh those that have an impact on neurodidactic factors. Our work consisted of applying an instrument to diagnose the risk of dropping out of university studies to a sample of first-year university students in Andalusian universities. The instrument was applied at the beginning of the second semester. Out of the 976 students surveyed, we have established a risk group of 34 students. We will propose measures oriented towards the neurodidactics factors that predict dropout and call for the use of ICT in the implementation of preventive measures.

Mechanism Study and Tendency Judgement of Rockburst in Deep-Buried Underground Engineering

Rockburst is a type of dynamic instability failure phenomenon and frequently brings huge losses to underground engineering projects such as mines and tunnels. In order to explore rockburst mechanisms and predict rockbursts better, relying on the background of Wulaofeng deep-buried highway tunnel, in situ stress measurement was performed using new wireless devices, and mechanics tests of surrounding rock samples taken from different burial depths were carried out. The rockburst mechanism was explored from the microscopic perspective based on the analysis of scanning electron microscopy (SEM). Rockburst tendency was judged comprehensively by a tendency analysis, grade prediction and numerical simulation. The result showed that the mechanical parameters of granite rocks in the deep-buried section were larger than those in the entrance section, and the fractured morphology mainly comprised sheet and monolithic block, corresponding to transgranular fracture and intergranular fracture. Rocks with few types of mineral cementation, good crystallization and small particle size differences had better energy storage and release characteristics. There was little difference in the rockburst tendency of rocks with different buried depths, but there were obvious differences in the rockburst grade. In the deep-buried section of the tunnel, the rockburst grade was of a moderate–heavy level and the rockburst risk at the vault and right spandrel of the cross section was more severe, which was basically consistent with the situation at the tunnel site. This study can provide a theoretical basis for the prevention and control of rockbursts in Wulaofeng tunnel and other similar engineering projects.

Experimental study on out-of-plane flexural behavior of steel-concrete-steel composite walls

In this study, six steel-concrete-steel composite wall (SCSCW) specimens have been designed and manufactured, and their out-of-plane flexural behaviors under combined axial and horizontal loads are experimentally studied. The study parameters include the connector arrangement scheme and applied axial load. The effects of these parameters on the failure modes, experimental load–displacement curves, bearing capacities, ductility coefficients, stiffness, energy dissipation capacities, and strain distributions of the specimens are analyzed. All specimens exhibit sectional flexural failure. The failure phenomena include the out-of-plane local buckling, concrete crushing, and welding fracture of the steel plates. These specimens show excellent ductility with an average ductility coefficient 2.27. The strength degradation coefficients of the specimens are >0.85. Increasing the number of connectors can restrict the local buckling of external steel plates, and the bearing capacities of the specimens are also improved. The connector arrangement scheme and axial load significantly influence the secant stiffness of the specimens during the initial stage of the experiments. When the connector arrangement schemes are identical, the higher the axial load, the better the energy dissipation capacity of the specimens. A simplified calculation model for the flexural capacity of SCSCWs is proposed considering the effects of local buckling and side plates, which has high accuracy and efficiency, and average ratio between the calculated and experimental peak loads is 0.96. A brief design example for the SCSCWs is provided to illustrate the design process considering the axial and out-of-plane loads.

Study on failure assessment and experimental validation of automotive steering knuckle in McPherson front suspension system

The failure phenomenon of automotive components within the service life is one of the most concerning issues in the automotive industry. In this paper, the concept of deformation gradient is introduced into the CAE simulation results to obtain the critical strain of the component under large deformation conditions. Subsequently, a complete quasi-static failure prediction method combined with the Johnson-Cook criterion is proposed. In addition, considering the cyclic hardening effect of the components’ material, the subcritical plane model is improved based on the Fatemi-Socie model to make the fatigue life prediction more accurate. According to the assembly relationship of the McPherson front suspension system, the proposed method and the improved model are used to predict the failure of the steering knuckle. Finally, the predicted results are compared with the rig test results. The comparison results validate the applicability and accuracy of the proposed quasi-static failure prediction method and fatigue failure prediction model.

Finite element analysis of microscopic fracture process in titanium metal matrix composite shafts using a unit cell approach

The aim of this study is to investigate the microscopic fracture process in titanium metal matrix composites (TiMMC) shafts based on finite element (FE) analysis. A unit cell approach is used as a tool to design a representative volume element to explain the complex failure phenomenon on a microscopic level. FE mesh convergence is examined and validated with the equivalent plastic strain and contact pressure. The accuracy of the numerical modelling is verified by comparison with experimental investigations on fractured TiMMC shafts. The results show that FE modelling can successfully analyse the complicated fracture process that occurs on a microscopic scale of TiMMC shafts.

Bank Portfolio Allocation Strategy and Its Probability of Failure: Case of the Russian Banking Sector Purge

This paper aims to discover portfolio allocation strategies that facilitate a bank’s stability. The paper examines the phenomenon of massive failures of Russian banks in the period from 2013 to 2019, in order to identify which of the banks’ strategic decisions regarding assets and liabilities, as well as portfolio structure, lead to higher stability. The dataset contains financial indicators and prudential ratios of 895 commercial banks operating in Russia during that period. 507 banks, or 57% of all banks, lost their license during the considered period. Cases of bank failures were classified depending on whether the Central Bank identified any illegal activities conducted by the failed bank. The high failure rate provides an opportunity to study the differences between failed and non-failed banks in order to determine the factors associated with lowerfailure probability. Following the approach applied in most of the previous studies, we use a logistic regression to model the effect of different asset and liability portfolio structure on the failure probability. The hypothesis that failure probability of a bank is affected by its strategic focus of forming an assets and liabilities portfolio was statistically confirmed. We found that the focus of a bank’s activity on providing loans to individuals and attracting deposits from companies leads to lower failure probability, confirming the results of previous studies. Also, we found that more active cooperation with other banks in terms of both borrowing and lending is associated with lower failure probability. Furthermore, we found that banks are less likely to borrow from or lend money to their fellow banks that later fail with illegal activity accusations.Finally, we found that unlike the EU banks, Russian banks with higher profitability ratios are more stable. The results are relevant for industry practitioners in facilitating the development of a more resilient bank strategy, as well as for regulators for incorporation in early warning models.

Static pushover test of spring-underground structure system for seismic performance analysis of underground structure

The static test is appropriate for studying the seismic performance of underground structures due to the large size of the model structure and the controllable stress level. By setting elastic elements around the model structure, the static pushover test of the spring-underground structure system could simulate the initial stress state of the underground structure and soil-structure interactions during the pushover process. Taking the Daikai subway station damaged in the 1995 Hanshin earthquake in Japan as the model structure, a series of static pushover tests of underground structures with a geometric scale of 1/5 were performed. The impact of vertical ground motion on the seismic performance of underground structures was studied by conducting two static pushover tests of spring-underground structure system under different vertical loads; the impact of surrounding soil on the seismic performance and failure process of underground structures was studied by conducting static pushover tests with and without springs under the same vertical load. The relationship between the horizontal pushover force and the structural interstory drift, the concrete strain change process of structural weak vertical member, the structural failure phenomenon, and the seismic performance of the test object can be obtained from the tests, which can provide valuable information for experimental research on the seismic performance of underground structures. The test results demonstrate that the vertical ground motion decreases the ductility of underground structure, and the surrounding soil improves the lateral collapse resistance of underground structure.

Investigation on Intermittent Failure Phenomenon of Electrical Connectors Under Repetitive Mechanical Insertion and Withdrawal Operations

Electrical connectors play an indispensable role in electrical and electronic systems. The frequently occurred intermittent failure events seriously affect the reliability of application systems. In this article, the repetitive mechanical insertion and withdrawal experiment of electrical connector is carried out by using our designed test rig and the intermittent failure phenomena recurs. The typical waveforms of contact resistance and force during the mating operation are presented. The variation laws of static contact resistance and dynamic contact resistance under intermittent failure condition are counted and analyzed. And the dynamic contact resistance is proven to be a suitable sensitive parameter to characterize the intermittent failure event. Based on the mathematical model, variation of surface morphology, and element analysis, the failure mechanism of electrical connectors under repetitive operation is proposed.

Deciphering the Effect of Corrugated p-Base on Reverse Blocking IGCT

The corrugated p-base (CB) structure has been acknowledged as an effective method to improve the maximum controllable current (MCC) of asymmetric integrated gate-commutated thyristor (IGCT), but never clarified in symmetrical devices such as reverse blocking IGCT (RB-IGCT). This study focuses on the influence of the CB structure on the MCC and the failure mechanism of RB-IGCT. First of all, by employing the CB structure in the RB-IGCT samples, it is verified that the CB structure can improve MCC of the symmetrical devices by more than 50%, no matter under high or low dc-link voltages. Moreover, regarding the scenario under the high dc-link voltage, the effect of the CB structure on the dynamic avalanche is analyzed via simulation, and the improvement of MCC and the positive trend of MCC upon temperature are further clarified. As to the scenario under the low dc-link voltage, distinct failure phenomenon and negative temperature dependence of MCC are observed evidentially on the symmetric devices with CB structure. To explain these failure characteristics that are conflicting with the well-acknowledged dynamic avalanche failure, a novel failure mode named cathode retrigger failure is proposed in this study. It is believed that this study not only deciphers the fundamental mechanisms of the CB structure in RB-IGCT, but also provides practical strategy to optimize the symmetric IGCT devices.

Study on bond behaviour of stainless steel reinforced concrete

The stainless steel rebar (SSR) and concrete are mainly bonded to transfer force and coordinate deformation. Therefore, it is necessary to study the bond behavior of stainless steel reinforced concrete. In this paper, the reinforced concrete specimens of carbon steel rebar (CSR), austenitic stainless steel rebar(ASSR) and duplex stainless steel rebar (DSSR) are made by taking concrete strength, steel rebar diameter, anchorage length, thickness of concrete cover and stirrup ratio as test parameters. Through the pull-out test, the failure phenomenon, bond stress-slip curve, ultimate load and ultimate bond strength of three kinds of reinforced concrete specimens are compared. The influence regularity of different factors on the bond behavior of SSR concrete is studied.. The bond strength formulas of ASSR and DSSR concrete are obtained by data fitting. The test results show that: (1) Both CSR concrete specimens and ASSR concrete specimens are mainly damaged by steel pulling out; (2) The failure mode of DSSR concrete specimens is mainly concrete splitting failure; (3) The change of concrete strength has the greatest influence on bond slip of DSSR concrete, followed by CSR concrete, and finally ASSR concrete; (4) CSR concrete has the best bond slip performance, followed by DSSR concrete, and finally ASSR concrete; (5) The recommended design value and calculation formula of anchorage length of ASSR and DSSR are given.

Analysis of impact ionization on RF power LDMOS reliability under RF life test in radar system

This paper focuses on correlation between the electrical parameter shifts and the failure phenomena that appeared after pulsed RF life tests applied to RF power LDMOS transistor (Radio Frequency Laterally Diffused-Metal–Oxide–Semiconductor). The results of the test bench (radar circuit) are measured in real time. The failure rate is more significant at low temperatures after the aging life test. The electrical performances are shifted, notably, IGS, VTH, GM, RDSON, CRSS, gain, and S-parameter S21. A 3D physical simulation (SILVACO-TCAD) is used to highlight the phenomena linked to the characteristic's degradation. The impact ionization induces physical degradations in the study of transistors, which are manifested by a variation in their electrical performance. It showed that the located failure zone is due to the device's shifted characteristics, which are deeply correlated with the structure area. The goal of the paper is to prove the failure mechanism through the characteristics evaluation origin, that is started by the carrier generation, because of the impact ionization phenomenon. The demonstration is based on the physical simulations are conducted at RF power NLDMOS. The outcomes show that the mechanism is due to the augmented carrier injection into the advanced silicon dioxide layer (SiO2) or/and in the Si/SiO2 interface state. The consistency between the physical simulations and experimental results emphasizes the theoretical analysis performed in this work.

Anti-collapse performance assessment of steel beam-column substructures with all-welded connections after exposure to fire

This paper presents both the experimental and numerical studies on the collapse resistance of beam-column substructures with all-welded connections under a middle-column removal scenario induced by fire scenarios. Extensive structural responses and key failure phenomena of specimens were monitored and recorded during push-down tests. The collapse resistance of specimens after exposure to fire deteriorated by comparison with those without fire exposure. In general, as the fire temperatures rose, the potentials of bending mechanisms against the progressive collapse were gradually degenerated, and the corresponding potentials of catenary mechanisms were strengthened instead. Nevertheless, the effect of temperature durations on the development history of resistance mechanisms was almost negligible. Additionally, an abundance of elaborated numerical models that considers the damage and fracture of ductile metals was developed to understand the structural responses of post-fire specimens. The simulated results showed that the vertical capacity and deformability of post-fire specimens were greatly enhanced with increasing beam heights. The span-to-depth ratios intrinsically influenced the contribution proportions of resistance mechanisms in beam-column substructures. With increased span-to-depth ratios, the vertical capacity provided by the bending mechanisms decreased obviously and deactivated works in the early stage, whereas that provided by the catenary mechanisms entered the stages of rapid development. Furthermore, two typical strengthening solutions after the fire events for post-fire substructures were comprehensively compared in terms of the structural behaviors against progressive collapse. From a practical viewpoint, strengthening with welded ribbed plates was considered a superior solution for the post-fire all-welded connections.

Failure study of compressed thin-walled composite columns with top-hat cross-section

This paper presents considerations on the failure of axially compressed thin-walled composite structures with open sections (using an top-hat section structure). The analyzed columns were made of composite material — carbon fiber reinforced polymer (CFRP) using the autoclave technique. The study was conducted in a comprehensive approach using both experimental methods and advanced numerical simulations based on the finite element method (in ABAQUS® program). Investigations were performed up to the total failure of the structure, while considering the phenomena directly contributing to the failure (damage initiation, damage evolution, delamination). During the conducted experimental studies, both the postbuckling equilibrium paths obtained from the universal testing machine and the acoustic emission signals were registered. Regarding the simultaneously conducted numerical simulations, two independent advanced damage models (progressive failure analysis — PFA and cohesive zone model — CZM) were used, allowing for a thorough analysis of the complex mechanism of the composite material failure phenomenon. Both numerical and experimental results show high agreement. This paper provides a more comprehensive understanding how failure occurs such that the design of the analyzed composite sections. The most important aspect of this paper is the potential to enable to understand the failure of the top-hat stiffeners during post buckling such that made it possible to utilize the stable, undamaged region of the post-buckling region. In addition, the paper conducts an advanced study based on the simultaneous use of two independent damage models, allowing in particular for a thorough analysis of the damage with delamination phenomenon detection, on structures with a complex shape of the end cross-sections of the structure.

Feasibility Analysis on the Support of Rock Slopes Against Flexural Toppling Failure Using the DDA Method-A Case Study

As a typical and complex failure type of rock slopes, flexural toppling failure has increasingly received attention, mainly focused on its failure mechanism and introducing a proper numerical method to reproduce the failure phenomena. However, there are few studies related to the supporting mechanism against flexural toppling failure. This study attempted to make a preliminary analysis on the research gap. For this purpose, an anti-dip layered rock slope supported with anchor cables that failed owning to flexural toppling was taken as an example, and the numerical method discontinuous deformation analysis (DDA) was taken as the major analytical tool. Firstly, the rationality of DDA was verified by correctly simulating the failure state of the anti-layered rock slope and the underlying failure mechanism was studied. The results indicated that the lack of the support from slope toe is the crucial factor inducing the flexural toppling failure, which posed a great obstacle to the further excavation of the open-pit mine. Secondly, by introducing the anchor cable support to the slope toe in the DDA model, the interaction mechanism between the anchor cables and the rock slope was investigated. the results indicated that the supporting system of the ordinary anchor cables is very sensitive to the inhomogeneous flexural toppling deformation. The tension values of the anchor cables are very inhomogeneous, and once some of anchor cables beyond their tension limit, the anchor cables failed, which will greatly threaten the safety of the adjacent anchor cables and even the stability of the whole slope. Meanwhile, a novel anchor cable called constant-resistance-large-deformation (CRLD) anchor cable was introduced for a comparative study using DDA. just replacing the anchor cables of the original slope with the CRLD anchor cables, the slope can reach stable state eventually even without the slope toe being enhanced. In addition, the CRLD anchor cables were able to support the slope well against the inhomogeneous deformation, because they can provide high constant resistances and withstand large deformations. This study can provide a deeper insight into the support mechanism for the snit-layered rock slope against flexural toppling failure and introduce a novel supporting method for such slopes.

Immediate reinfection with Omicron variant after clearance of a previous SARS-CoV-2 infection

SARS-CoV-2 Omicron variant is spreading worldwide, causing unprecedented epidemic peaks due to its transmissibility and immune evasion. We searched in the archive of the Regional Microbiology Laboratory (Umbria, Italy) for immediate reinfection (i.e. infection occurring 25–60 days from primary infection) among 454,764 RT-PCR tests from 261,217 individuals. Lineage heterogeneity was assessed by S gene target failure phenomenon or whole genome sequencing. We found that BA.1 Omicron variant may cause immediate reinfection of patients just recovered from Delta infection. Immediate reinfection was not observed for any other combination of variants, including Delta over Alpha variant and BA.2 over BA.1 Omicron lineage.

Failure analysis and countermeasures of a tunnel constructed in loose granular stratum by shallow tunnelling method

To solve the failure problems encountered in the tunnels constructed by shallow tunnelling method in loose granular stratum, taking a tunnel construction project on the Sichuan-Tibet 318 Highway Line in China as an example. Failure phenomena, characteristics, and modes were systematically analyzed during tunnel construction. Then, the evolution at different failure modes was analyzed through DEM calculation and the prevention mechanism was discussed. Finally, corresponding construction countermeasures were put forward from the aspects of pre-support, excavation, bolt driving, and collapse prevention. The results show that: (1) The conventionally strong support system cannot play an active role in the tunnel. The core problems in construction are how to reduce the disturbance of excavation to surrounding rock and realize the restraint of the support system to the surrounding rock in the arch-foot area. (2) According to the loss of the surrounding rock, the failure modes can be divided into two types: Ⅰ-the rapid development failure from vault collapse and Ⅱ-the lag failure of ground cavity moving up. Construction optimization aims at avoiding the mode-Ⅰ and alleviating the mode-Ⅱ. (3) For the mode-Ⅰ, maintaining the stability of the initial ground-arch is the practical measure; For the mode-Ⅱ, when the cavity area is more significant than 5 m2, it must be filled in time. (4) The field deformation data of surrounding rock after treatment shows that the effect of the proposed countermeasures is remarkable, which can provide valuable references for the tunnel design, construction, and disaster control in a similar area.