Relevant Failure Mechanisms Research Articles

Assessment of flood risk accounting for river system behaviour

In this paper “river system behaviour” is defined as the complex interaction between river flow and the flooding of flood prone areas. A basic aspect of river system behaviour is that a local dike breach may affect hydraulic loads and hence dike failure probabilities at other locations. Important aspects in river system behaviour are discussed as well as the fact that effects of river system behaviour on flood risk may be both beneficial as well as adverse. This paper presents a conceptual approach to quantify effects of river system behaviour on probabilities of dike breach and flood risk. It was successfully applied to two example river configurations. The results of these examples are discussed. It is concluded that for proper flood risk assessment all relevant failure mechanisms, uncertainties as well as all proposed safety improvement measures are to be jointly taken into account. The conceptual approach enable all this. In the authors’ views, there is a need for developing models that account for effects of river system behaviour on flood risk. Such models can serve as a tool for policy makers in evaluating the effects that (regional) safety improvement measures have on the flood risk in the entire river basin.

Analysis and Design of Buried Pipelines for Ice Gouging Hazard: A Probabilistic Approach

In cold environments, marine pipelines may be at risk from ice keels that gouge the seabed. Large quantities of material are displaced and soil deformations beneath a gouge may be substantial. To meet safety criteria, excessive strains are avoided by burying pipelines to a sufficient depth. In this paper, a probabilistic approach for the analysis and design of buried pipelines is outlined. Environmental actions are applied through distributions of gouge width, gouge depth, subgouge soil deformations, and bearing pressure. Three-dimensional pipe/soil interaction problem is modeled using nonlinear soil springs and special beam elements using the finite element method to estimate pipe response for statistically possible ranges of gouge depths, gouge widths, and burial depths. Relevant failure mechanisms have been considered, including local buckling and a variety of strain and stress based criteria. The methodology presented in the paper was developed and successfully used for several pipeline and electrical cable projects in ice gouge environments. Significantly reduced burial depth requirements have been demonstrated through the application of the probabilistic approach and through the use of strain-based design criteria. Because ice actions are applied through displacements of the soil, more ductile pipes are often necessary to meet reliability targets.

LIFETIME EXTRAPOLATION OF PZT CAPACITORS

ABSTRACT We propose a new methodology for lifetime determination of PZT capacitors, based on accelerated tests performed at low voltages and high temperatures. We believe that such a methodology is the only way to characterize the relevant failure mechanism for Time-Dependent Dielectric Breakdown of PZT capacitors. The capacitors lifetime at operating conditions is found to exceed 20 years.

Reliability of thin dielectric for non-volatile applications

With reference to the mainstream flash EEPROM technology, the most relevant failure mechanisms that affect memory reliability are reviewed, showing the primary role played by tunnel oxide defects. The degradation of device performance and single-bit failures induced by program/erase cycling, like the erratic erase phenomenon, are discussed. The impact of stress-induced leakage current on data retention is shown to limit the tunnel oxide scaling.

Flash memory reliability

With reference to the mainstream technology, the most relevant failure mechanisms which affect yield and reliability of Flash memory are reviewed, showing the primary role played by tunnel oxide defects. The effectiveness of a good test methodology combined with a proper product design for screening at wafer sort latent defects of tunnel oxide is highlighted as a key factor for improving Flash memory reliability. The degradation of device performance induced by program/erase cycling is discussed, covering both the behaviour of a typical cell and the evolution of memory array distribution. The erratic erasure phenomenon is illustrated as the most relevant mechanism reported so far to cause single bit failures in endurance tests. Finally, reliability implications of multilevel cell concepts are briefly analysed.

Comprehensive gate-oxide reliability evaluation for dram processes

This paper discusses dielectric breakdown as a major cause for failure of ICs with less than a half-micron linewidth. Focusing on DRAMs the characterization of dielectric reliability using different types of properly designed test structures and appropriate stress methods is reviewed. Considerations concerning screens and upper limits for stress conditions are presented. With a step stress, the sample size can be reduced and efficiently used to cover the specified lifetime. To understand and identify relevant failure mechanisms, physical failure analysis of representative fails is required.

A New Approach to the Challenge of Machinery Prognostics

Current generation mechanical diagnostic equipment is designed to identify individual events or trends in the output of sensors mounted on a mechanical component, subsystem, or system. Such equipment can provide a useful indication that a failure condition may be developing, but it cannot provide reliable predictions of the remaining safe or operational life. Typically, these diagnostic systems simply compare the output of individual sensors against a priori thresholds to establish a measure of the system’s health. Two problems result from this approach: (1) There is no advantage taken of possible synergy among the sensors, i.e., the determination of health is one dimensional; and (2) the diagnosis provides only a statement regarding the current equipment health, but does not provide a prediction of the time remaining to failure. This often leads to an operational environment in which diagnostic equipment outputs are either ignored because of frequent false alarms or frequent (and costly) time-based preventive maintenance is performed to avoid hazardous failures. This paper describes a new approach to the development of a more robust diagnosis and prognostic capability. It is based on the fusion of sensor-based and model-based information. Sensor-based information is the essence of current diagnostic systems. Model-based information combines dynamic models of individual mechanical components with micromechanical models of relevant failure mechanisms, such as fracture and crack propagation. These micromechanical models must account for initial flaw size distribution and other microstructural parameters describing initial components condition. A specific application of this approach is addressed, the diagnosis of mechanical failure in meshing gears. Four specific issues are considered: (a) how to couple a validated numerical simulation of gear transmission error (due to tooth spacing irregularity, contour irregularity, or material inhomogeneity) with physically and empirically based descriptions of fatigue crack growth to predict a failure precursor signature at the component level; (b) how to predict the manifestation of this signature at the subsystem or system level where sensors are located; (c) how to fuse this model-based information with the corresponding sensor-based information to predict remaining safe or operational life of a gear; and (d) issues associated with extending this methodology to bearings and other rotating machinery components.

Reliability of Electronics in Harsh Environments

This paper reports experiences gained in several research programmes, showing the necessity to develop improved environmental tests that better accelerate the mechanisms that degrade electronics. The tests specified in standards today do not consider relevant failure mechanisms. The paper describes the authors' philosophy for performing reliability testing. Examples of various reliability tests are given.

Study on mechanism of combined action of abrasion and cavitation erosion on some engineering steels

The abrasion and cavitation erosion properties as well as the relevant failure mechanisms of several steels to be used for hydraulic turbine parts were investigated. The tests were carried out using rotating disc equipment under simulated working conditions, instead of in the conventionally used venturi tube or vibration apparatus. Different weight loss curves were obtained. The experimental results obtained from our scanning electron microscopy, transmission electron microscopy, light microscopic observations, X-ray analysis and other test methods proved that it is more reasonable to consider the cavitation erosion as a failure of low cycle (plastic strain) fatigue. Hence, it was derived from the Manson-Coffin relation that the resistance to cavitation erosion depends on the square of the product of the hardness and the true fracture strain ϵ f of the steel. Sand—water abrasion is actually a microcutting of the surface layer of the steels and, here, the hardness of the steels is proved to be the main resistance factor. The failure of steels in muddy and sand-carrying water is due to the combined action of abrasion and cavitation erosion, and the consequent weight loss rate is the weighted sum of the individual rates of abrasion and cavitation erosion. Equations that relate the weight loss rate of abrasion and also of cavitation erosion to the mechanical properties of the steels were developed. The calculated values and the experimental data agree well. Conclusions drawn give useful reference principles for the microstructural design of steels to be selected and also for the improvement of their heat treatment.

Stochastically Dominant Modes of Frames by Mathematical Programming

Most important in calculating the failure probability of structural systems is the search for the stochastically most relevant failure mechanism of ductile structural systems—particularly framed structures. For this reason an automatic method for finding all modes can be employed, but for very large systems these procedures may become intractable. Mesh and nodal methods for frames are formulated with particular attention to simple data preparation. The generalized mesh description leads to the simpler and more efficient formulation of the mechanism compatibility equations. Two nonconvex problems, one with bilinear constraints and the other with a fractional objective function, have been derived. The latter formulation can be solved quite effectively when cast in the form of the maximization of a quadratic convex function over a linear domain. This is followed by mathematical programming techniques that are more appropriate for nonconvex optimization problems. The paper concludes with a discussion of the methodologies for finding other important modes.