Chance Failure Research Articles

The effect of a certain class of potential common mode failures on the reliability of redundant systems

This is a theoretical investigation of the importance of common mode failures (cmfs) on the reliability of redundant systems. These failures are assumed to be the result of fatal shocks, e.g. from earthquakes and explosions, which occur at a constant rate. This formulation makes it possible to predict analytically results obtained in the past which showed that the probability of a cmf of the redundant channels of the protection system of a typical nuclear power plant was orders of magnitude larger than the probability of failure from chance failures alone. Furthermore, since most reliability analyses of redundant systems do not include potential cmfs in the probabilistic calculations, criteria are established which can be used to decide either that the cmf effects are indeed insignificant or that such calculations are meaningless, and more sophisticated methods of analysis are required, because cmfs cannot be ignored.

Ion Propulsion Flight Experience, Life Tests, and Reliability Estimates

The application of low-thrust ion propulsion systems to space missions requires long duration (~ 5,000-20,000 hr) component operation. Thus, components must be developed with wearout mean times to failure in excess of these required mission times. Furthermore, chance failures which occur during the useful life of components must be minimized. This paper makes an assessment of both early and wearout failure modes of ion propulsion systems by examining the results of existing developmental and long duration testing. Estimates of chance failure rates of system components are also presented along with design concepts which maximize total propulsion system reliability. I. Introduction B ECAUSE of the long time operation required for most space applications, ion propulsion system reliability is a major concern. For this reason, the long duration and space testing of critical components, subsystems, and systems associated with primary and auxiliary ion propulsion systems are of great importance. It should be recognized, however, that although a large number of long duration tests have been conducted to date on ion propulsion systems, subsystems, and components, these tests have been part of a technology development phase and are not to be considered statistical reliability testing of fully developed systems. Thus, even though space-qualified ion propulsion systems are becoming available, system reliability can only be inferred from the results of past and present developmental testing and estimated by analytical techniques.

The Reliability of Complex Systems with Dependent Subsystem Failures: An Absorbing Markov Chain Model

Complex systems usually fail through a gradual deterioration of component subsystems until a critical subsystem failure renders the unit inoperative. Given exponential life distributions of chance failures for individual subsystems, absorbing Markov chains capture the statistics of this deterioration. Dependent subsystem failures are readily accomodated. Matrix formulations yield estimates of the system mean time to failure for all states of deterioration. Examples illustrate several distinct forms of subsystem interdependencies; and their implications for systems design, maintenance and repairs are examined.

The Reliability of Complex Systems With Dependent Subsystem Failures: An Absorbing Markov Chain Model

Complex systems usually fail through a gradual deterioration of component subsystems until a critical subsystem failure renders the unit inoperative. Given exponential life distributions of chance failures for individual subsystems, absorbing Markov chains capture the statistics of this deterioration. Dependent subsystem failures are readily accomodated. Matrix formulations yield estimates of the system mean time to failure for all states of deterioration. Examples illustrate several distinct forms of subsystem interdependencies; and their implications for systems design, maintenance and repairs are examined.

Effects of gamma irradiation on radiation sensitivity in the diploid potato species Solanum chacoense

The effect of ancestral exposure to radiation on the radiation sensitivity of progeny was tested the diploid potato species Solanum chacoense f. gibberulosum (Juz. and Buk.) Corr. Approximately 83 per cent of seedlings of a parental population that received 0 and 3 kR of gamma radiation survived and grew normally while only 26 per cent of those receiving 6 and 9 kR survived and appeared to grow normally. The survival and vigor of seedlings derived from plants that had survived these doses were compared following exposure to the same doses. The results indicated that radiation damage measured in terms of survival and vigor was increased following irradiation among most of those plants whose ancestors had received doses of 6 and 9 kR of radiation. This increase was attributed to the hereditary transmission of radiation-induced genetic damage in the form of recessive lethals and detrimentals. Certain lots failed to show increased radiation sensitivity as measured by decreased survival. This result was interpreted as resulting either from chance failure of mutation in the parental plants involved, or to somatic elimination of radiation damage induced in young seedlings.

Some Aspects of Reactor Safety Systems

Ostensibly, reactor safety systems compare nuclear and process measurements with predetermined operating limits and initiate automatic warning and protective functions. The importance of system engineering and establishing the levels of safety and availability required for reactor installations are discussed. Safety system logic schemes, operator confidence in the equipment, and protection against the effects of chance failures in safety circuits are considered. The authors suggest that existing reactor safety systems should be examined in light of the philosophical, logical and circuit considerations. Possible component failures should be evaluated on a system effects basis to determine the need for the automatic annunciation of these failures and to provide a measure of the system's capability of providing the required automatic corrective actions and protection against unnecessary plant shutdowns. Since reactor safety systems are a combination of different circuits in which the analog to digital interface is not always displayed, failures are not necessarily annunciated and a cascading of failures is possible. The development of the basic philosophy for these systems is a complex process employing broad and complex technical considerations.

Optimal Replacement under Risk

Large investments in capital goods create concern about economic life of specific items of capital such as farm buildings, machinery, and livestock. Many of these assets are subject to involuntary replacement due to random phenomena. This paper develops a model for economic analysis of asset life under conditions of chance failure or loss. Analytical results are derived which imply routine decision rules and provide insight to the economics of replacement.

An Inventory of Intrinsic Sources of Chance Failures in Electronic Parts

Electronic part failures in operating systems are eventually traced to either (1) an assigned physical (time dependent) defect or (2) an inadvertent use of the part beyond its margin of safety as defined by conservative practices. Part failure rates are valid numerics only after the assurance of conservative application practices of a mandatory nature. The residual part application uncertainties contribute to part failure rates in a manner undistinguishable from the effects of physical defects. Thus, the underlying cause of part failures may be physical or communicative. Only the first of these factors lies in the technical area commonly referred to as "physics of failure". Both factors contribute to the experienced frequency of part failures and combined to produce distinct failure modes as expressed in meaningful statistical summaries. It has been determined that eight basic failure modes can account for the bulk of the physical risk factors reflected into (statistically expressed) electronic part failure rates. These essential categories correspond to the loss of the following operating functions, (1) connection (2) dissipation (3) insulation (4) dielectric (5) magnetic (6)rectification and gain (7) structural and (8) kinematic. Each of these functional demands can be identified in relation to primary, secondary and tertiary defects and used to construct part failure rate models. Example models are cited for the generic part classes called "dielectric and dissipative devices". Each of the eight basic part-failure modes given can be traced to an array of distinct industrial practices which foster the inherent risks developed. This risk traceability is provided by means of an inventory of reliability synthesis factors for using failure rates as elements of chance failure summations. While it is not claimed that the synthesis of electronic part failure rates is a simple matter of arithmetic risk summation, it is considered a key prerequisite to have an organized inventory of risk terms and factors to which meaningful numerical assignments can be made from an accumulated experience with material failure phenomena. Presented here are two basic interrelated inventories consisting of (1) a list of eight basic failure modes and (2) a "genealogy" of part failure risks for the life cycle of electronic parts from design conception to in-system utilization.

Letter to the Editor—Comment on the “Reliability of Aircraft Structures in Resisting Chance Failure”

The purpose of this brief paper is to point out an objection to the assumptions made in a recently published formula on the reliability of an aircraft in resisting chance failure, and to indicate how, with first passage time probabilities, it is possible to derive a more realistic expression for this formula.

Reliability of Aircraft Structures in Resisting Chance Failure

In this analysis, a measure is developed for the capability of aircraft structures to withstand chance failure, as contrasted to fatigue failure, over the design life of any aircraft system. The measure developed is similar to that for reliability, except the failure rate is specified completely in terms of the power spectral properties of the structural response to the environment. This relation of the reliability to the power spectral parameters enables the reliability of a structure to be predicted. For a given environment and reliability requirement, the analysis could also be used as a basis for determining optimal structural designs.