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RELIABILITY ENGINEERING AND RISK
ANALYSIS A PRACTICAL GUIDE 3RD
EDITION MODARRES SOLUTIONS MANUAL
EXAMINATION TEST 2026 COMPLETE
QUESTIONS AND ANSWERS
⩥ Availability. Answer: - used for repairable systems
- the probability that the system is operational at any random time t
- can also be specified as a proportion of time that the system is available for use in a given interval (0,T) ⩥ MTTF (Mean Time To Failure). Answer: average time that elapses until a failure occurs (does not provide information about the distribution of the TTF) ⩥ MTBF (Mean Time Between Failures). Answer: average time between successive failures (used for repairable systems) ⩥ Reliability Engineering. Answer: - use product life data to determine the probability and capability of parts, components, and systems
- use to perform their required functions for desired periods of time without failure, in specified environments
⩥ Life Data. Answer: can be the measure lifetimes of a product in the marketplace such as
- time the product operated successfully
- or the time the product operated before it failed ⩥ Reliability Assessment. Answer: based on the results of testing from in-house (or contracted) labs and data pertaining to the performance results of the product in the field ⩥ Key Reasons for Reliability Engineering. Answer: it is essential that it knows the reliability of its product and is able to control it in order to produce products at an optimum reliability level
- this understanding yields the minimum life-cycle cost for the user and minimizes the manufacturer's cost ⩥ minor nuisances (ex). Answer: failure of a television's remote control ⩥ catastrophic failures (ex). Answer: involving loss of life and property ⩥ Implementing a Reliability Engineering Program. Answer: the typical manufacturer does not really know how satisfactorily its products are functioning
⩥ Availability A=MTBF/(MTBF+MDT). Answer: the probability that a component or system is performing as designed at a given point in time when used under stated operating conditions ⩥ MDT. Answer: mean downtime for repair or downtime ⩥ Reliability theory. Answer: developed apart from the mainstream of probability and statistics ⩥ Reliability Theory. Answer: provides a way to examine a multiple component system calculating its overall reliability, the probability that the system will work (the foundation of reliability engineering) ⩥ First - reliability is a probability Second - reliability is predicated on 'intended function' Third - reliability applies to a specified period of time Fourth - Reliability is restricted to operation under stated conditions. Answer: Reliability Engineering Concerns ⩥ - inherently incapable
- overstressed
- variation
- wearout
- time dependen mechanisms
- sneak
- errors
- others. Answer: Why do engineered items fail ⩥ Failure rate equation. Answer: lambda = (MTBF)^- 1 failure rate = (mean time between failure) ^- 1 ⩥ Non-repairable items. Answer: reliability is the survival probability over the item's expected life, or for a period during it's life, when only one failure can occur ⩥ hazard rate. Answer: items instantaneous probability of the first and only failure MTTF (Mean Time To Failure) ⩥ Repairable items equations. Answer: MTBF (Mean Time Between Failure) availability = MTBF/(MTBF + MTTR) ⩥ reliability principals. Answer: - design for reliability from the initial concept of the idea
- use proven components that have been used on previous products
⩥ Independent Variables. Answer: set up independently before the experiment begins (reading scheme - no reading scheme; baby crawled - not crawled) ⩥ Dependent variables. Answer: dependent on experimenter's manipulation of the independent variable (reading test score; movement ABC score) ⩥ Confounding variables. Answer: change outcome of experiment in some unforeseen way (reading lessons at home; age for MABC test) ⩥ Validity. Answer: how confident we are that our interpretation of the data is valid that our findings actually show what we think they show ⩥ External Validity. Answer: a random sample is necessary to ensure results generalize - would a sample from here on education knowledge accurately describe knowledge in the British population as whole? ⩥ Internal Validity. Answer: random assignment of participants to groups helps ensure that our results mean what we think they mean ⩥ Reliability (occurrence). Answer: how confident we are that a given finding can be reproduced - that it can be replicated that it is not a chance result, a 'freak occurrence'
⩥ Data variable samples and populations. Answer: - univariate (one variable)
- bivariate (two variables)
- multivariate (two or more variables) ⩥ Qualitative or categorical Quantitative
- discrete
- continuous. Answer: Types of Variables ⩥ Graphs for univariate data distributions. Answer: qualitative
- pie charts
- bar charts ⩥ Quantitative data. Answer: - scatter plot
- stem and leaf plots
- relative frequency histograms ⩥ data distributions. Answer: - shapes - symmetric, skewed left, skewed right, unimodal, bimodal
- proportion of measurements in certain intervals
⩥ Median [data set]. Answer: the middle value of the data set, most appropriate for ordinal level data ⩥ Mode [data set]. Answer: most commonly occurring value, most appropriate for nominal data ⩥ Dispersion. Answer: the extent to which the scores vary, all clumped together or spread out (box plot) ⩥ Range [Dispersion]. Answer: the distance from the lowest to the highest score, in our previous example the range was 10 {0, 0, 0, 2, 4, 5, 10} ⩥ Standard Deviation {s or sigma) [Dispersion]. Answer: the average deviation from the mean, in previous example, it is 3.4 MSexcel or SPSS can easily work this out ⩥ Variance (sigma squared). Answer: result in squared units which does not have a direct intuitive interpretation ⩥ Normal Distribution. Answer: a bell curve, a reflection of naturally occurring values where the mean, the median and the mode are the same
⩥ Variation. Answer: you want to understand how the response is dependent on variation in the explanatory variables, but you are also interested in lack of dependence ⩥ Hypothesis testing. Answer: you have an idea you want to test ⩥ control experiment. Answer: one where you don't apply the treatment or don't enable the part of your experiment that is supposed to produce the different outcome ⩥ Replication. Answer: - must be independent
- not part of a time series
- not grouped together in space
- of an appropriate spatial scale
- covers the normal variation in initial conditions ⩥ Strong inference. Answer: - a clear hypothesis
- an acceptable test ⩥ weak inference. Answer: natural experiments ⩥ mechanisms cause
⩥ probability (book). Answer: any attempt to quantify it must involve the use of statistical means ⩥ reliability for non-repairable systems. Answer: survival probability over the item's expected life, only one failure can occur ⩥ reliability for repairable systems. Answer: probability that failure will not occur in the period of interest, when more than one failure can occur ⩥ Process Reliability. Answer: Improve processes that would increase product reliability ⩥ Fatigue. Answer: the most common mechanism of failure, responsible for 90% of all structural and electrical failures ⩥ High-cycle fatigue. Answer: based on stress reversals to determine the number of cycles to fatigue failure ⩥ Low-Cycle fatigue. Answer: based on strain reversals and is used for situations where the material has plastic deformation ⩥ Brittle Fracture. Answer: an overstress failure mechanism that occurs rapidly with little or no warning when the induced stressed in the component exceeds the fraction strength of the material
⩥ creep. Answer: a time-dependent deformation process under load, thermally activated process ⩥ delamination. Answer: the debonding or the separation of adjacent material layers which were bonded before ⩥ plastic deformation. Answer: when the applied mechanical stress exceeds the elastic limit or yield point of a material, is permanent ⩥ electrically induced failures. Answer: failures caused as a result of electrical over stress three types electrostatic discharge gate oxide breakdown electromigration ⩥ Chemically induced failures. Answer: chemical process such as electrochemical reactions that can result in cracking of vias, traces, or interconnects leading to electrical failures two types corrosion intermetallic diffusion
