Dependent Failure Research Articles

Optimal switching and condition-based maintenance of cold standby systems subject to competitive failure and imperfect switchover

Cold standby redundant structures can improve system reliability and availability but are subjected to internal degradation and external shocks during operation. Thus, the switching process in a standby system is prone to failures. This study investigates the optimal switching and condition-based maintenance (CBM) decision of a two-unit cold standby system subject to mutually dependent competitive failure processes with an imperfect switchover. First, periodic switching and offline CBM policies under imperfect switchover are proposed; second, the reliability models of competitive failure—between soft failure caused by system degradation subject to nonfatal shock and hard failure caused by fatal shock—are established. An iterative model of the system state probability distribution is subsequently derived by analysing the characteristics of state transitions in alternate use and maintenance of the active and standby units. An analytical decision model is also established to minimise the average cost rate in a finite horizon to obtain the optimal switching period and maintenance threshold. Finally, a case study of a coal mine main ventilator system validates the proposed model.

Evolution of strain-rate dependent compressive failure behavior of ceramifiable FRP composites at high temperature conditions

Fiber-reinforced polymer (FRP) composites modified with ceramic fillers have excellent thermal stability at high temperatures and are applied in aeronautical and aerospace fields. The effect of strain rate on their mechanical behavior at different temperatures (corresponding to different states of FRP composites) is of interest. In this study, dynamic and quasi-static compressive tests of a kind of ceramifiable polymer composites, B4C-talc modified high-silica/boron-phenolic (B4C-talc_HS/BPh) composites, were examined at 25–1000 °C. A high-temperature split Hopkinson pressure bar with a double-synchronous assembled system was employed to carry out dynamic tests. During the tests, high-speed camera was employed to monitor the progressive failure process, and the failure morphology was examined using an optical microscope. The tests show that the compressive strength decreases with increasing temperature to 600 °C at both low and high strain rates. However, it is found for the first time that the compressive strength increases obviously at 800–1000 °C due to the ceramifiable transition at a high strain rate. Moreover, the influence of temperature and loading rate on the compressive failure behavior of B4C-talc_HS/BPh composites was systematically investigated. Shear force dominates the failure under dynamic load at 25–1000 °C, while it is 25–450 °C under quasi-static load. Piece-wise models are developed to describe the temperature and strain-rate dependent evolution behavior of the compressive strength. This research provides basic experimental data on the mechanical properties of B4C-talc_HS/BPh composites, which can inform the design, evaluation of integrity, and reliability of thermal protection structures.

Time-domain Reliability Estimation of Gear Train considering Failure-dependence

A gear tooth or a gear is usually considered as an independent component in the traditional reliability research. However, gear trains are significantly different from the traditional series systems. Time-dependent configuration of the traditional series systems is fixed. But the meshing tooth-pairs of gear trains are changing all the time during operation. The failures of meshing tooth-pairs are dependent statistically under the common load action. In the paper, the reliability analysis considering time-domain meshing relation of tooth-pairs is studied and meshing relation equations are presented. Then, reliability analysis method of gear trains is developed based on the Monte Carlo simulation with dependent failure and time domain meshing relation. The differences of reliability results between time domain reliability analysis method and traditional series system reliability methods are illustrated by several examples.

Extension of Process Limits in High‐Strength Aluminum Forming by Local Contact Heating

The aluminum alloy EN AW‐7075 T6 is used in the automotive sector for its favorable strength‐to‐weight ratio. However, the limited cold formability is currently addressed by energy‐ and time‐consuming temperature‐assisted processes. In order to limit the effort to critical forming areas only, the state‐of‐the‐art shows promising results for increasing the blank temperature in the range of warm forming. The design of new processes in an industrial context requires appropriate numerical simulation with inherent complexity due to time‐ and temperature‐dependent effects. Herein, the potential of a newly developed tool setup and process chain with integrated local contact heating of the EN AW‐7075 T6 blank is investigated on the basis of a curved hat profile. A thermomechanically coupled FE model of the process is developed and validated. The influence of the local heating layout is analyzed in experimental forming tests and a corresponding process window is derived. The influence of local heating on the occurring failure mechanisms is discussed based on simulation results. The equivalent plastic strain evolution is successfully used to evaluate the local heating dependent failure behavior. A significant increase in the overall formability of the part is achieved by the proposed process chain.

A Zone-Based Model for Analysis of Dependent Failures in Requirements Inspection

In the software development life cycle, the quality of the requirements specification affects the overall quality of the subsequent phases and hence, the software product. The requirements specification is usually inspected by an inspection team to detect defects. To enhance the quality of the requirements specification, one conventional strategy usually used is adding redundancies to the inspection team. However, this strategy suffers from the problem of dependent failures of the redundant inspectors which was not studied systematically in previous research. To analyze the dependent failures and independent failures in an inspection team, this paper first defines the independent failures and dependent failures in an inspection team from the perspective of human errors. Then a quantification model, i.e., the Zone-based Model, is proposed to analyze the dependent and independent failures. The Zone-based Model considers the following situations: 1) the probability of failures of an inspector may be high; 2) the probability of failures of the inspectors may be different; 3) the failures in an inspection team can be a combination of dependent failures and independent failures. By considering all those situations, the Z model has a meaningful interpretation and a convincing assessment of the failures of an inspection team. To verify the effectiveness of the new model, the Zone-based model is compared to conventional models using simulation data. The results show that the Zone-based model is significantly better than the traditional models in analyzing the independent and dependent failures.

Multi-unit seismic probabilistic risk assessment: A Bayesian network perspective

The Fukushima Dai-ichi highlighted multi-unit accidents in nuclear power plant (NPP) probabilistic risk assessment (PRA). This paper focuses on multi-unit seismic PRA because seismic events are contributors to NPP site risk. This paper integrates works on modeling ground motion spatial variability at a NPP site and dependent seismic failures into a coherent and rigorous framework using Bayesian networks, and explores the relative importance of different types of correlation on NPP site-based risk metrics. A study of a hypothetical dual unit NPP site considered 15 cases with varying degrees of correlation in spatial ground motion (GM) and seismic failures. Based on the results, the current assumption of perfect GM correlation across a NPP site underestimates risk metrics that consider either unit experiencing core damage (CD) (site CD), which is not conservative. The risk metric that considers both units experiencing CD (concurrent CD) shows differences between the partial and perfect GM correlation assumptions when multiple component failures contribute to the CD in each unit. Within a GM correlation assumption, the site CD increases as the fragility correlation increases, a counterintuitive trend. The risk assessment case outlined in this paper provides more realistic results than the cases reflecting the current state of practice.

A Universal Scaling Formulation of the Generalized Peck’s Equation for Corrosion Limited Lifetime in Self-Heated ICs

Since the early 1980s, the highly accelerated stress test in conjunction with conventional Temperature-Humidity-Bias testing has been the industry-standard method for interpreting relative humidity (RH) and temperature (T) dependent failure modes in non-hermetically packaged ICs, solar modules, and other electronic components. In recent years, however, testing requirements based on JEDEC standards and the corresponding lifetime extrapolation to end-use conditions using Peck’s equation have been criticized as overly conservative for modern self-heated logic/power ICs. In practice, the self-heated ICs correlate RH, and T in a way that is not explicitly accounted for by Peck’s equation. In this study, we investigate the IC lifetime due to electrochemical failures – Cu-Al bond wire corrosion – by considering the effects of ambient RH, T, duty cycle, and self-heating temperature rise using an integrated self-consistent finite element-based Multiphysics model and propose a generalized Peck’s equation for self-heated ICs. Our study reveals how the asymmetry between On and Off-state heating leads to dramatic suppression of the effective RH experienced by an integrated circuit and, most importantly, identifies that the corrosion of Cu-Al bond wire occurs in spikes during the on-off transition when both RH and T are high. The generalized Peck’s equation also predicts an elegant scaling relationship among the fundamental variables associated with a self-heated IC lifetime. The prediction is validated by experimental lifetime data from self-heated IC published in the literature. The experimental validation of the physics-informed generalized Peck’s equation suggests the possibility of more accurate (and optimistic) lifetime predictions of modern ICs and solar modules.

Modelling mesh independent failure loads of a soft strain-softening clay using a rate dependent model

If no regularisation method is used during finite element modelling of strain-softening (brittle) materials, results become mesh dependent: manifesting as very thin shear bands whose thickness and orientation depends on the mesh discretisation. The objectives of this paper were to determine if mesh independent results were achieved, and to track the evolving width of the shear band when considering naturally occurring properties of soft clays, including a rate dependent stress-strain-strength relationship. The constitutive model included a destructuration process and strain-rate dependency, and the analyses allowed for local pore water flow. A bearing capacity problem was loaded to failure. Analyses were conducted with different mesh discretisation, loading rates and permeabilities. The results demonstrated mesh independent failure loads, and the strain-softening occurred in zones with thickness independent of mesh size. Local pore water flow delayed the initiation of strain softening and resulted in higher failure loads. The softening zones grew in thickness to over one meter, reinforcing that the finite element method is suitable for the determination of the bearing capacity of strain-softening sensitive clays. In conclusion, the rate dependent constitutive model is a promising method for regularising strain localisation in soft brittle clays under quasi-static loading conditions.

System reliability analysis for independent and nonidentical components based on survival signature

Survival signatures have been widely used for analyzing arbitrary system structures having independent failure events. It is challenging to study system reliability when diverse components are present since the fundamental premise of the survival signatures is that components should be interchangeable within the same subsystem. In this research, two methods based on the fundamental survival signature idea are suggested to examine the reliability of complex systems with independently but not necessarily identically distributed (INID) components. The first algorithm is based on the weighted random sampling (WRS) method to calculate the survival signatures. The second algorithm adopts the idea of divide-and-conquer in computing the probability structure. The application of these two algorithms and the exhaustive algorithm in analyzing the example systems are presented. The results show that the two algorithms can significantly reduce the computation time compared to the traditional methods. Finally, the developed algorithms are used in the reliability analysis of a real practical problem, the Tsinghua campus water supply pipeline system. The results regarding the predictions of system reliability as well as the component importance index are discussed.

Resilience of wave energy farms using metocean dependent failure rates and repair operations

Emerging offshore renewable energy technologies are expected to become an important part of the future energy system, and reliability for these new technologies in different metocean scenarios must be guaranteed. This poses a challenge in extreme weather scenarios like storms, in particular for less mature technologies such as wave energy. Not only the offshore survivability must be controlled; the restoration after disruptive events and failures should be addressed and optimized. Offshore operations are costly and cannot be carried out if the weather is too harsh, and the resulting downtime after failures may be financially devastating for projects. In this paper, the resilience of large wave energy systems is studied with respect to wave conditions, metocean dependent failure rates, and weather windows available for offshore repair operations. A metocean- and time-dependent failure rate is derived based on a Weibull distribution, which is a novelty of the paper. The performance of the farm is assessed using the varying failure rates and metocean data at different offshore sites. Critical metocean thresholds for different offshore vessels are considered, and the resilience is quantified using relevant measures such as unavailability and expected energy not supplied. The resilience analysis is coupled to an economic assessment of the wave farm and different repair strategies. Our results show that the commonly used assumption of constant failure rates is seen to overestimate the annual energy production than when a more realistic varying failure rate is used. Two offshore sites are compared, and the availability is found to be higher at the calmer site. Most of the evaluated repair strategies cannot be considered to be economically justified, when compared to the cost of the energy not supplied.

Experimental investigation of time dependent dielectric breakdown and failure mechanism for Hf0.5Zr0.5O2 ferroelectric field effect transistors

The experimental investigation of the time dependent dielectric breakdown (TDDB) was performed to study the dielectric breakdown mechanisms for Hf0.5Zr0.5O2 (HZO) ferroelectric field-effect transistors (FeFETs) with the SiO2 interfacial layer (IL). Constant voltage stress method combined with the TCAD device simulation was used to study the breakdown mechanism of the TaN/HZO/SiO2/Si gate stack. Due to the much higher electric field in the IL, the breakdown of the SiO2 IL is the major reason for the breakdown of the gate stack. In addition, it is found that the IL breakdown will speed up the disappearance of the memory window (MW), leading to the malfunction of the FeFET as a storage device. Finally, the Weibull distributions of time-to-breakdown (TBD) and charge-to-breakdown (QBD) were characterized, revealing decent TDDB performance of the FeFETs. Careful design of IL thickness is necessary to balance the tradeoff between the device performance and the reliability lifetime of HZO FeFETs.

Abstract #1408177: Spinal Muscle Atrophy Presenting with Non-Diabetic Ketoacidosis

Spinal muscle atrophy (SMA) is an untreatable neurodegenerative disease related to loss of survival motor neuron (SMN) 1 gene resulting in muscle atrophy leading to significant morbidity and mortality. Patients with SMA may present with metabolic disturbances including impaired glucose tolerance, fatty acid metabolism and hyperlipidemia. We present a case of SMA presenting with recurrent metabolic acidosis. 51-year-old female with hypertension, osteoporosis, and SMA (homozygous deletion of exon 7 in the SMN1 gene) with quadriplegia and chronic ventilator dependent respiratory failure was admitted with diarrhea, poor oral intake, nausea, and abdominal pain. She did not report sick contacts or fever and had no history of diabetes. She had similar past episodes of metabolic acidosis with acute illness. Vital signs were BP 135/74mmHg, HR 110/min, RR 20/min, afebrile, and SpO2 96% on room air. On exam she appeared ill and debilitated with dry oral mucosa, + abdominal distension and tenderness with no rebound. Labs showed anion gap metabolic acidosis, Na 136mEq/L, K 3.9mEq/L, Cl 102 mEq/L, CO2 12 mEq/L, Anion gap 22, pH 7.13, Creatinine <0.01mg/dl, glucose 110 mg/dl, 2+ urine ketones, beta hydroxybutyrate 7.39 mmol/L, WBC 23.97 K/ul and HbA1c 5.2%. She was treated for non-diabetic ketoacidosis with IV fluids and fixed dose IV insulin infusion with dextrose infusion in tandem. She had hypernatremia, hypocalcemia, hypophosphatemia and hypokalemia likely due to poor nutrition and dehydration, and with aggressive electrolyte replacement, she showed significant clinical improvement during hospitalisation. Patients with SMA are prone to metabolic derangements e.g. ketoacidosis. Several factors may contribute to ketoacidosis such as decreased muscle mass, defects in endocrine pancreatic function and abnormal fatty acid metabolism regardless of triggering factors. One study investigated glucose metabolism and pancreas development in the Smn (2B/-) intermediate SMA mouse model and type I SMA patients. In mice it demonstrated a dramatic cell fate imbalance within pancreatic islets, with a predominance of glucagon-producing α cells at the expense of insulin-producing β cells and mice displayed fasting hyperglycemia, hyperglucagonemia, and glucose resistance. Similar abnormalities in pancreatic islet cells were seen in deceased children with the severe infantile form of SMA indicating that defects in glucose metabolism may play an important contributory role in SMA pathogenesis. There are no present guidelines to manage this entity, but case reports indicated a role of volume resuscitation and electrolyte replacement for improvement and future prevention by avoiding fasting.

Lay-up effect on the open-hole shear strength of composite laminates

The effect of lay-up on the open-hole shear (OHS) strength of quasi-isotropic carbon fibre/epoxy composite laminates was investigated. It was found that laminates with equivalent specific stiffness but different laminate lay-up, i.e. with different arrangements of plies and ply thicknesses, exhibit significant differences in ultimate OHS strength of up to 16.3% and 27.9% for the prepreg material systems IM7/8552 and T800/epoxy, respectively. Using Digital Image Correlation (DIC), X-ray Computed Tomography (CT), and Finite Element Analysis (FEA), it is shown that the lay-up dependent OHS strengths were associated with distinctly different failure modes. These lay-up dependent failure modes are challenging to predict. It is therefore proposed that the OHS test could be used for material screening, and as a new benchmark case for the development and validation of advanced progressive damage modelling frameworks for laminated composites, complementing the commonly used open-hole tensile (OHT) test.

Scaling Effect of Long Rod Penetration

Long rod penetration tests show a scaling effect that cannot be explained by rate dependent strength. We propose here that such a scaling effect may be explained by a rate dependent failure response. We start by revisiting the well-known result that long rod penetration efficiency depends on strain to failure of both projectile and target materials. We then make the strain to failure depend on strain rate, using our overstress approach concept. With this approach, the effective strain to failure increases with strain rate. As strain rate increases with decreasing scale, we also get that penetration efficiency decreases with decreasing scale, which has been observed in tests. In the paper we show results of hydrocode runs that demonstrate the relation between the strain rate sensitivity of strain to failure and the scaling effect of long rod penetration

Investigation of strain rate dependent microscopic failure mechanisms in short fiber reinforced plastics using finite element simulations

For predicting the strain rate dependent failure of short fiber reinforced plastics (SFRP) a two-phase simulation model is developed using the finite element method and comparing the results to microscopic specimen tests for uniaxial tension under quasi-static (0.007/s) and dynamic loads (250/s). Experimentally the failure behavior of SFRP is observed to be strain rate dependent. The global strain at failure and the absorbed energy increase with strain rate. Moreover, locally an influence of the strain rate on the amount of material involved in the deformation can be observed. The suggested model can represent these effects accurately. Also, the present micro-mechanical effects and their influence on the strain distribution are investigated by unit cell simulations. Thereby the material model of the fibers, the matrix, and the boundary layer are varied respectively. These reveal the important role of strain rate dependent decohesion leading to a correct representation of the plastically deformed volume. Consequently, the distortion energy density is evaluated and is found to be constant at failure for all strain rates.

Middle Meningeal Artery Embolization for Chronic Subdural Hematoma: Predictors of Clinical and Radiographic Failure from 636 Embolizations.

Background Knowledge regarding predictors of clinical and radiographic failures of middle meningeal artery (MMA) embolization (MMAE) treatment for chronic subdural hematoma (CSDH) is limited. Purpose To identify predictors of MMAE treatment failure for CSDH. Materials and Methods In this retrospective study, consecutive patients who underwent MMAE for CSDH from February 2018 to April 2022 at 13 U.S. centers were included. Clinical failure was defined as hematoma reaccumulation and/or neurologic deterioration requiring rescue surgery. Radiographic failure was defined as a maximal hematoma thickness reduction less than 50% at last imaging (minimum 2 weeks of head CT follow-up). Multivariable logistic regression models were constructed to identify independent failure predictors, controlling for age, sex, concurrent surgical evacuation, midline shift, hematoma thickness, and pretreatment baseline antiplatelet and anticoagulation therapy. Results Overall, 530 patients (mean age, 71.9 years ± 12.8 [SD]; 386 men; 106 with bilateral lesions) underwent 636 MMAE procedures. At presentation, the median CSDH thickness was 15 mm and 31.3% (166 of 530) and 21.7% (115 of 530) of patients were receiving antiplatelet and anticoagulation medications, respectively. Clinical failure occurred in 36 of 530 patients (6.8%, over a median follow-up of 4.1 months) and radiographic failure occurred in 26.3% (137 of 522) of procedures. At multivariable analysis, independent predictors of clinical failure were pretreatment anticoagulation therapy (odds ratio [OR], 3.23; P = .007) and an MMA diameter less than 1.5 mm (OR, 2.52; P = .027), while liquid embolic agents were associated with nonfailure (OR, 0.32; P = .011). For radiographic failure, female sex (OR, 0.36; P = .001), concurrent surgical evacuation (OR, 0.43; P = .009), and a longer imaging follow-up time were associated with nonfailure. Conversely, MMA diameter less than 1.5 mm (OR, 1.7; P = .044), midline shift (OR, 1.1; P = .02), and superselective MMA catheterization (without targeting the main MMA trunk) (OR, 2; P = .029) were associated with radiographic failure. Sensitivity analyses retained these associations. Conclusion Multiple independent predictors of failure of MMAE treatment for chronic subdural hematomas were identified, with small diameter (<1.5 mm) being the only factor independently associated with both clinical and radiographic failures. © RSNA, 2023 Supplemental material is available for this article. See also the editorial by Chaudhary and Gemmete in this issue.

A Bayes Analysis of a Dependent Competing Risk Model Based on Marshall–Olkin Bivariate Weibull Distribution

This paper considers a competing risk model defined on the basis of minimum of two dependent failures where the two failures are assumed to jointly follow Marshall–Olkin bivariate Weibull distribution. This paper explores some important features of corresponding likelihood functions and performs a full Bayesian analysis of the model for data resulting from normal as well as accelerated life tests. The accelerated model is described by regressing the scale parameters of the model through inverse power-law relationship. Posterior-based inferences are drawn using the Gibbs sampler algorithm after specifying proper but vague priors for the model parameters. The numerical illustration is provided using real datasets. The performance of the model is assured by Bayesian tools of model compatibility and then the entertained model is compared with the competing risk model based on Marshall–Olkin bivariate exponential assumption.

Disseminated tuberculosis presenting as a crescentic glomerulonephritis

A 48-year-old male patient presented with dialysis dependent renal failure with biopsy showing crescentic glomerulonephritis and Positron emission tomography–computed tomography (PET-CT scan) revealing features of pericardial, pulmonary tuberculosis with positive urinary GeneXpert test for tuberculosis bacilli. Clinicians should keep in mind the atypical presentations of tuberculosis while managing rapidly progressive renal failure especially in tropical countries with high prevalence of tuberculosis.

Reliability analysis of uncertain random systems based on uncertain differential equation

Typical failure modes of degradation and shock processes have been applied which are named as soft failures and hard failures, respectively. This paper discusses the reliability of uncertain random systems where randomness is regarded as objective indeterminacy with enough sample data, and uncertainty is referred to epistemic indeterminacy with insufficient sample data. Considering that the combination of internal and external factors, the degradation process is modelled by an uncertain differential equation, and external shocks are driven by an uncertain random renewal process where state variables are uncertain random variables. Chance measure is applied to define the reliability index which is distinguished from traditional reliability assessment methods that utilize probability measure. Three types of reliability models possessed with independent failures are presented, where the general reliability index formulas are provided. The analysis results of Caohai Nature Reserve show that uncertain random modelling method provides an effective approach to system reliability assessment.

A strain rate dependent and anisotropic failure material model for short fiber reinforced plastics based on energy density accounting for local fiber orientation

A failure model for SFRP for FEM simulations is developed to describe the strain rate dependency, the influence of the local fiber orientation and of the stress state on the failure behavior. The material is considered as a continuum while internally calculating the micro-mechanics analytically. The described micro-mechanics are based on experimental observations and on analyzation with numerical studies. In particular the strain rate dependent delamination of fibers and matrix is incorporated in the model. The distortion energy density is defined as the driving value for failure and estimated by the model. This is achieved with the analytic solution by Eshelby for the stress field in the matrix and by introducing an additional phase for the plasticly deformed volume. The validation on characterization specimens as well as component test demonstrates that the influence of strain rate, fiber orientation, and stress state on the failure behavior can be described with only one material parameter, the critical distortion energy density.