Individual Failure Modes Research Articles

Effect of spatial variability of soft clays on geotechnical design of braced excavations: A case study of Formosa excavation

This case study presents the effect of spatial variability of soft clays on the geotechnical serviceability assessment of braced excavations. Finite element modeling is performed to predict the excavation-induced wall and ground movement. The spatial variations in two key soil parameters are modeled as two correlated lognormal random fields. The system reliability-based serviceability assessment conducted in this case study shows that the system probability of serviceability failure is greater than or equal to that for each single failure mode, indicating that serviceability assessment considering an individual failure mode can result in unsafe predictions.

(Invited) Durability of Polymer Electrolyte Membranes for Automotive Applications

There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical, mechanical and thermal degradation. Chemical and mechanical degradation can occur during normal fuel cell operation whereas thermal degradation should only occur at extreme temperatures such as those that can be induced by membrane shorting. This talk will focus on the individual failure modes; with attention dedicated to describing the fundamental model-based mechanistic understandings, appropriate accelerated stress tests that enable rapid screening and relevant in-situ diagnostics to track membrane health during fuel cell operation, as well as discussions of effective mitigation strategies to prevent or minimize the risk of failure caused by the specific modes of membrane degradation with focus on the effect of simultaneous chemical and mechanical degradation. Both chemical and mechanical degradation of perfluorosulfonic acid (PFSA) membranes have been extensively studied, and effective mitigation strategies have been developed that can significantly extend PFSA proton exchange membrane (PEM) lifetimes. For example, porous polymer supports such as (expanded polytetrafluoroethlene) ePTFE can be used to make PFSA membrane which exceed US Department of Energy mechanical durability requirements and hydroxyl radical scavenging additives such as Ce3+ and Mn2+ can enable PFSA membranes to meet chemical stability requirements. Accelerated stress tests (ASTs) have been developed that can be used to screen various membrane types and mitigation strategies, and effective in-situ diagnostics have been developed to track degradation of PFSA membranes during fuel cell operation. The failures observed in these accelerated tests as seen by postmortem analysis mimic the failures of PFSA membranes in field tests. A new highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. The HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the gas outlets. HASTs were conducted using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a state-of-the-art robust membrane containing both an ePTFE mechanical support and a Ce-based chemical stabilizing additive. Result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. Further results will be presented that decouple the cerium migration effect from the mechanical/chemical synergistic degradation effect.

Seismic intensity measures for above‐ground liquid storage tanks

SummaryA series of scalar and vector intensity measures is examined to determine their suitability within the seismic risk assessment of liquid storage tanks. Using a surrogate modelling approach on a squat tank that is examined under both anchored and unanchored support conditions, incremental dynamic analysis is adopted to generate the distributions of response parameters conditioned on each of the candidate intensity measures. Efficiency and sufficiency metrics are used in order to perform the intensity measure evaluation for individual failure modes, while a comparison in terms of mean annual frequency of exceedance is performed with respect to a damage state that is mutually governed by the impulsive and convective modes of the tank. The results reveal combinations of spectral acceleration ordinates as adequate predictors, among which the average spectral acceleration is singled out as the optimal solution. The sole exception is found for the sloshing‐controlled modes of failure, where mainly the convective period spectral acceleration is deemed adequate to represent the associated response due to their underlying linear relationship. A computationally efficient method in terms of site hazard analysis is finally proposed to serve in place of the vector‐valued intensity measures, providing a good match for the unanchored tank considered and a more conservative one for the corresponding anchored system.

Reliability of tunnel lining design using the Hyperstatic Reaction Method

The reliability analysis of tunnel linings is a challenging task, due to the complex nature of soil-structure interactions, and due to the large uncertainty in soil properties and soil-structure interaction parameters. Usually, numerical models are employed to properly describe structural geometry and soil-structure interactions, rendering reliability solutions computationally expensive. In the past, tunnel reliability was addressed by local point estimate methods, which are very economical in the number of points where the numerical solutions are computed, but which can be quite inaccurate. More recently, surrogate modelling techniques have been employed to alleviate the computational burden, producing global response approximations. In this paper, an alternative procedure is proposed, which consists in the direct coupling between mechanical and reliability solution algorithms. Such direct coupling is viable because the very efficient Hyperstatic Reaction Method is employed to model the soil-structure interactions. Typical concrete tunnel lining is addressed. Solutions are computed for several failure modes of the tunnel lining, also considering that failures can occur at any point along the tunnel perimeter. Solutions for individual failure modes are computed by FORM, and system reliability is computed by different Monte Carlo simulation techniques.

Spatial-temporal reliability analysis of corroding cast iron water pipes

Destructive effects of pipeline structural failure on the worldwide social, environmental and financial aspects reveal the importance of accurate reliability assessment of these infrastructures. In this research, the structural integrity of water cast iron pipes is assessed by a spatial-temporal reliability analysis. Random field representation is used for taking the spatial variability of corrosion depth into account. A probabilistic model for correlation length of corroded cast iron surfaces is developed using the data collected from in-service pipe samples in the West of Melbourne, Australia. To this aim, the randomness associated with involved parameters in the deterioration process is acknowledged by Monte Carlo simulation, and limit states criteria are checked for individual failure mode and also for combination of them in order to execute a multi-failure mode reliability assessment for finding the likelihood of the time of pipeline failure.

Crashworthy response of fibre metal laminate top hat structures

The axial crushing response of fibre metal laminates (FML), in particular GLARE top-hat structures, has been investigated using experimental and numerical techniques, which are reported in this paper. Crushing performance (crush force, energy absorption) was evaluated for formed GLARE top-hat structures as an analogue for an energy absorbing aircraft sub-floor structure. A numerical simulation methodology was established which could accurately predict the crushing performance of GLARE top hat structures using commercial explicit finite element (FE) analysis tool LS-DYNA. Material characterisation at coupon level was conducted to measure the appropriate material properties required for the model to achieve good predictability. The successful demonstration of GLARE energy absorbing structures widens options to enhance crashworthiness; numerous applications of such GLARE structures could be envisaged, including reinforcement of aircraft sub-floors.The crushing modes of GLARE top-hat structures were found to be complex, exhibiting mixed-mode failure which was a combination of the individual constituent failure modes. The metallic layers plastically deformed by folding and tearing while the composite layer failed with a wide range of failure patterns i.e. splaying, delamination and cracking. The top hat structures, most importantly, crushed in a stable progressive manner making them suitable for energy absorbing (EA) applications. The GLARE top hat structure’s crushing response is superior to its bare metal equivalent.Given the complexity of the FML crushing process, there is tremendous scope for optimisation of the laminate parameters to maximise energy absorption. The good agreement obtained between the numerical and experimental results highlights the benefits of using simulation to predict the overall crashworthiness of FMLs.

Assessing Relative Risk for the Failure Modes of a Bridge Scour Countermeasure

AbstractWidely used failure modes and effects analysis (FMEA) can be very effective in assessing the relative risks of the failure modes of a bridge scour countermeasure. In this study, an approach is developed for assessing the relative risks of failure modes of a bridge scour countermeasure overcoming the limitations of risk priority numbers (RPNs) associated with individual failure modes. Appropriate application of the RPNs is established by ensuring that high values of RPNs, indicating a relatively higher risk of failure, have higher severity and occurrence ratings compared with the low RPNs, indicating low risk. Replacement of the use of RPNs with risk priority codes (RPCs) enables engineers to prioritize the risks for any severity-occurrence pair using the same scale as the ratings, unlike the RPNs. Given the simplicity and effectiveness of this technique, RPCs are shown to be preferable for assessing and prioritizing the relative risks of the failure modes of a bridge scour countermeasure.

Failure mode and effects analysis of skin electronic brachytherapy using Esteya® unit.

PurposeEsteya® (Nucletron, an Elekta company, Elekta AB, Stockholm, Sweden) is an electronic brachytherapy device used for skin cancer lesion treatment. In order to establish an adequate level of quality of treatment, a risk analysis of the Esteya treatment process has been done, following the methodology proposed by the TG-100 guidelines of the American Association of Physicists in Medicine (AAPM).Material and methodsA multidisciplinary team familiar with the treatment process was formed. This team developed a process map (PM) outlining the stages, through which a patient passed when subjected to the Esteya treatment. They identified potential failure modes (FM) and each individual FM was assessed for the severity (S), frequency of occurrence (O), and lack of detection (D). A list of existing quality management tools was developed and the FMs were consensually reevaluated. Finally, the FMs were ranked according to their risk priority number (RPN) and their S.Results146 FMs were identified, 106 of which had RPN ≥ 50 and 30 had S ≥ 7. After introducing the quality management tools, only 21 FMs had RPN ≥ 50. The importance of ensuring contact between the applicator and the surface of the patient’s skin was emphasized, so the setup was reviewed by a second individual before each treatment session with periodic quality control to ensure stability of the applicator pressure. Some of the essential quality management tools are already being implemented in the installation are the simple templates for reproducible positioning of skin applicators, that help marking the treatment area and positioning of X-ray tube.ConclusionsNew quality management tools have been established as a result of the application of the failure modes and effects analysis (FMEA) treatment. However, periodic update of the FMEA process is necessary, since clinical experience has suggested occurring of further new possible potential failure modes.

Using Reliability Modeling and Accelerated Life Testing to Formulate a Cost Model of Photovoltaic Systems with Different Architectures

As the number of photovoltaic (PV) systems grows annually and ages, the importance of long term reliability to the levelized cost of electricity (LCOE) has become increasingly clear. Likewise innovative PV system architectures being considered present opportunities for differentiation and growth into new markets, but still need to have a model for system reliability in order to justify their LCOE. Other benefits of reliability modeling include setting reliability goals for individual failure modes, determining optimum replacement and maintenance strategies, as well as prioritizing which failure modes require the most consideration in terms of research and testing. Physics of failure and materials properties are key inputs to the outcome of these models. This paper presents the results of reliability models based on accelerated testing, field data and theoretical calculations that were undertaken in order to determine the cost model of reliability of PV systems of different system architectures. Three examples for PV modules are presented here (bypass diodes, solder joints and microinverters) and are used to show how understanding of the reliability of systems based upon materials properties can be used to formulate models for lifetime and costs.

Two-dimensional strain-based interactive failure theory for multidirectional composite laminates

A 2-D strain-based interactive failure theory is developed to predict the final failure of composite laminates subjected to multi-axial in-plane loading. The stiffness degradation of a laminate during loading is examined based on the individual failure modes of the maximum strain failure theory, and a piecewise linear incremental approach is employed to describe the nonlinear mechanical behavior of the laminate. In addition, an out-of-plane failure mode normal to the laminate is also investigated to more accurately predict the failure of multidirectional laminates. The theoretical results of the failure model presented are compared with the experimental data provided by the World-Wide Failure Exercise, and the accuracy of the model’s predictive capabilities is investigated.

Low Temperature PEM Degradation: Failure Modes & Mitigation Strategies

There are three critical membrane degradation mechanisms that can lead to failure of polymer electrolyte fuel cell systems: chemical degradation, mechanical degradation, and membrane shorting. This talk will focus on the individual failure modes; with attention dedicated to describing the fundamental model-based mechanistic understandings, appropriate accelerated stress tests that enable rapid screening and relevant in-situ diagnostics to track membrane health during fuel cell operation, as well as discussions of effective mitigation strategies to prevent or minimize the risk of failure caused by the specific modes of membrane degradation. The effect of simultaneous chemical and mechanical degradation will also be discussed.Both chemical and mechanical degradation of perfluorosulfonic acid (PFSA) membranes have been extensively studied, and effective mitigation strategies have been developed that can significantly extend PFSA proton exchange membrane (PEM) lifetimes. Accelerated stress tests (ASTs) have been developed that can be used to screen various membrane types and mitigation strategies, and effective in-situ diagnostics have been developed to track degradation of PFSA membranes during fuel cell operation. The failures observed in these accelerated tests as seen by postmortem analysis mimic the failures of PFSA membranes in field tests. Durability of PFSA membranes will be discussed and an overview of the degradation of alternate PEM chemistries, such as hydrocarbon membranes, will also be provided. Insight into the degradation of these materials is limited because there is hardly any field data which can be used to determine the root causes of failure and validate that the ASTs are accelerating the relevant stressors and, also, because appropriate diagnostics, such as those used for PFSA membrane systems, have not been developed.This talk will also address shorting of PEMs in fuel cell systems. Shorting is a two step process during which soft shorts are created by penetration of conductive materials though the thickness of the membrane. Soft shorts become hard shorts when there is a voltage excursion that leads to high local temperatures and subsequent thermal decay of the PEM. These hard shorts are the severe events that lead to fuel cell stack failure. Results and analysis suggest that the PEM type is not the limiting factor in preventing membrane shorting, and that mitigation is best achieved by a combination of design and operating strategies.

Acoustic emission monitoring of damage progression in CFRP retrofitted RC beams

The increased use of carbon fiber reinforced polymer (CFRP) in retrofitting reinforced concrete (RC) members has led to the need to develop non-destructive techniques that can monitor and characterize the unique damage mechanisms exhibited by such structural systems. This paper presented the damage characterization results of six CFRP retrofitted RC beam specimens tested in the laboratory and monitored using acoustic emission (AE). The focus of this study was to continuously monitor the change in AE parameters and analyze them both qualitatively and quantitatively, when brittle failure modes such as debonding occur in these beams. Although deterioration of structural integrity was traceable and can be quantified by monitoring the AE data, individual failure mode characteristics could not be identified due to the complexity of the system failure modes. In all, AE was an effective non-destructive monitoring tool that can trace the failure progression in RC beams retrofitted with CFRP. It would be advantageous to isolate signals originating from the CFRP and concrete, leading to a more clear understanding of the progression of the brittle damage mechanism involved in such a structural system. For practical applications, future studies should focus on spectral analysis of AE data from broadband sensors and automated pattern recognition tools to classify and better correlate AE parameters to failure modes observed.

Method to Identify Dependability Objectives for Use in Multiobjective Optimization Problem

Intelligent mechatronic systems, such as self-optimizing systems, allow an adaptation of the system behavior at runtime based on the current situation. To do so, they generally select among several pre-defined working points. A common method to determine working points for a mechatronic system is to use model-based multiobjective optimization. It allows finding compromises among conflicting objectives, called objective functions, by adapting parameters. To evaluate the system behavior for different parameter sets, a model of the system behavior is included in the objective functions and is evaluated during each function call. Intelligent mechatronic systems also have the ability to adapt their behavior based on their current reliability, thus increasing their availability, or on changed safety requirements; all of which are summed up by the common term dependability. To allow this adaptation, dependability can be considered in multiobjective optimization by including dependability-related objective functions. However, whereas performance-related objective functions are easily found, formulation of dependability-related objective functions is highly system-specific and not intuitive, making it complex and error-prone. Since each mechatronic system is different, individual failure modes have to be taken into account, which need to be found using common methods such as Failure-Modes and Effects Analysis or Fault Tree Analysis. Using component degradation models, which again are specific to the system at hand, the main loading factors can be determined. By including these in the model of the system behavior, the relation between working point and dependability can be formulated as an objective function. In our work, this approach is presented in more detail. It is exemplified using an actively actuated single plate dry clutch system. Results show that this approach is suitable for formulating dependability-related objective functions and that these can be used to extend system lifetime by adapting system behavior.

Probabilistic Framework for Strength Limit and Service Limit Checks of Drilled Shafts considering Soil Spatial Variability

This paper presents a performance-based, probabilistic framework for design of a drilled shaft under axial and lateral loading that can consider spatial variability of soil properties at a project site. The performance criteria of a drilled shaft are stated in relation to the limiting tolerable deformations for strength limit and service limit, respectively. The computational algorithm for calculating the deformation of a drilled shaft is based on the commonly adopted load transfer method and the p-y method. “Geotechnical failure” is defined as an event in which the specified performance criteria are not met. Three failure modes are considered: axial movement, lateral deflection, and angular distortion. The spatial variability of soil properties is considered by using random field modeling techniques in which correlation length is introduced to account for site variability in addition to mean and variance. The method of fitting a sample autocorrelation function to a prescribed correlation function by using the method of ordinary least squares is introduced for determining site-specific correlation length for soil parameters. Geostatistical principles known as kriging are employed to estimate unknown parameters at unsampled locations from neighboring sampled locations. A numerical example is given to illustrate the application of the proposed methodologies. The example demonstrates that correlation length is an important statistical descriptor for characterizing site variability. Performance-based design provides unified consideration for both strength limit and service limit. Finally, the overall probability of failure for a drilled shaft when all three failure modes are considered is greater than the failure probability for any individual failure mode.

Determination of Turbine Blade Life from Engine Field Data

It is probable that no two engine companies determine the life of their engines or their components in the same way or apply the same experience and safety factors to their designs. Knowing the failure mode that is most likely to occur minimizes the amount of uncertainty and simplifies failure and life analysis. Available data regarding failure mode for aircraft engine blades, while favoring low-cycle, thermal mechanical fatigue as the controlling mode of failure, are not definitive. Sixteen high-pressure turbine (HPT) T-1 blade sets were removed from commercial aircraft engines that had been commercially flown by a single airline and inspected for damage. Each set contained 82 blades. The damage was cataloged into three categories related to their mode of failure: (1) Thermal-mechanical fatigue, (2) Oxidation/Erosion, and (3) From these field data, the turbine blade life was determined as well as the lives related to individual blade failure modes using Johnson-Weibull analysis. A simplified formula for calculating turbine blade life and reliability was formulated. The L(sub 10) blade life was calculated to be 2427 cycles (11 077 hr). The resulting blade life attributed to oxidation/erosion equaled that attributed to thermal-mechanical fatigue. The category that contributed most to blade failure was Other. If there were there no blade failures attributed to oxidation/erosion and thermal-mechanical fatigue, the overall blade L(sub 10) life would increase approximately 11 to 17 percent.

A map of competing buckling-driven failure modes of substrate-supported thin brittle films

Our in situ experiments of polyimide-supported thin indium tin oxide (ITO) films reveal buckling-driven film cracking in some samples and buckling-driven interfacial delamination in other samples. Although studies of individual buckling-driven failure mode exist, it still remains unclear what governs the competition between these two different failure modes in a given film/substrate structure. Through theoretical analysis and numerical simulations, we delineate a map of competing buckling-driven failure modes of substrate-supported thin brittle films in the parameter space of interfacial adhesion and interfacial imperfection size. Such a map can offer insight on the mechanical durability of functional thin films. For example, interestingly, we show that strongly bonded thin brittle films are more prone to buckling-driven cracking, a more detrimental failure mode for thin brittle ITO transparent conductors widely used in displays and flexible electronics.

Risk analysis of analytical validations by probabilistic modification of FMEA

Risk analysis is a valuable addition to validation of an analytical chemistry process, enabling not only detecting technical risks, but also risks related to human failures. Failure Mode and Effect Analysis (FMEA) can be applied, using a categorical risk scoring of the occurrence, detection and severity of failure modes, and calculating the Risk Priority Number (RPN) to select failure modes for correction. We propose a probabilistic modification of FMEA, replacing the categorical scoring of occurrence and detection by their estimated relative frequency and maintaining the categorical scoring of severity. In an example, the results of traditional FMEA of a Near Infrared (NIR) analytical procedure used for the screening of suspected counterfeited tablets are re-interpretated by this probabilistic modification of FMEA. Using this probabilistic modification of FMEA, the frequency of occurrence of undetected failure mode(s) can be estimated quantitatively, for each individual failure mode, for a set of failure modes, and the full analytical procedure.

Analysis of Accelerated Life Test Data Involving Two Failure Modes

This paper presents an approach to analyze accelerated life testing (ALT) data involving two failure modes. The approach first transforms the ALT dataset into two new datasets that correspond to individual failure modes. Each transformed dataset is modeled using a two-step procedure, and the resulted models associated with individual failure modes are combined into a competing risk model. The approach is illustrated using the ALT data of industrial heaters from the literature. The analysis shows that the shape parameter of the life distribution can change with stress level.

Optimization and Postbuckling Analysis of Curvilinear-Stiffened Panels Under Multiple-Load Cases

Recent studies have indicated that panels with curvilinear stiffeners offer a strong potential for structural tailoring. However, the design complexity requires use of numerical analysis and optimization techniques. This paper considers the problem of optimal design of a stiffened panel with cutouts and curvilinear stiffeners, under multiple-load cases. Multiple failure modes are considered: buckling, damage tolerance, stress, and crippling. An optimization framework is presented to minimize mass of the stiffened panel by a combined shape and sizing optimization subject to constraints on the individual failure modes. Different panel thicknesses are allowed for each region bounded by stiffeners or panel boundaries. The framework uses Python scripts to couple ABAQUS-based analysis with VisualDoc optimization package. A design example is presented to illustrate the capability, and the optimized design is compared with that obtained using industry-standard practices. A postbuckling analysis of the optimal design with four curvilinear stiffeners is carried out and discussed in detail.

Modeling Failure Modes for Residual Life Prediction Using Stochastic Filtering Theory

This paper reports on a theoretical Bayesian modeling development for residual life prediction in the context of condition-based maintenance. At each monitoring point during a components lifetime, the stochastic filter is used to establish a posterior conditional probability density function (PDF) for the residual life. The PDF can then be used in the evaluation of maintenance and replacement decisions. The research documented in this paper extends the modeling principles in accordance with a practical consideration recognized in a number of previous case applications. Many monitoring scenarios provide evidence that the operational components involved may potentially be subject to a number of individual distinct failure modes, rather than a single dominant failure mode as modeled previously. The modeling procedure proposed to handle this scenario is based on the assumption that an individual monitored component will fail according to one of a number of predefined failure modes. Individual stochastic filters are constructed to facilitate the residual life prediction under the influence of each potential failure mode, and the output from each filter is weighted according to the probability that the particular failure mode is the true underlying (unknown) failure mode. The probabilities associated with each failure mode are recursively derived using a Bayesian model, and the condition monitoring information obtained to date. The modeling process is applied to a set of simulated condition monitoring histories incorporating two potential failure modes, and the results are compared with those obtained using a general model with no failure mode assumptions. The results indicate that, when individual failure modes are identifiable using historical data, the modeling process described in the paper could greatly improve residual life prediction capabilities, and prevent the occurrence of costly component failures.