Reliability Engineering and Quality Assurance

Question 1

Reliability definition

Answer 1

Defined as the ability of a system to perform its intended mission when operating for a designated period of time, or through a planned mission scenario (or series of scenarios), in a realistic operational environment.
– A system with a 90% reliability has a 90% probability that the system will operate the mission duration without a critical failure

Inherent within this definition are the elements of probability, satisfactory performance, time or mission-related cycle, and specified operating conditions.
– The tools of reliability engineers include heavy doses of probability and statistics and specialized tools like fault trees and reliability block diagrams, as well as traditional engineering tools of modeling and simulation

Question 2

Reliability, Availability, Maintainability, & Supportability (RAMS)

Question 3

Reliability Block Diagram (BRD) as a concept

Answer 3

Represents the system’s functioning state (success or failure) in terms of the function states of it’s components

The RBD demonstrates the effect of a failure of a component in the success or failure of the system

Question 4

RBDs for a system

Answer 4

All real systems are built up of some system elements that are arranged reliability-wise in series and some system elements that are arranged relaibility-wise in parallel

Question 5

“Reliability-wise in series”

Answer 5

If even one component fails, the whole system fails (like the links of a chain… none of them can fail or the whole thing fails)

Indicates an ‘product’ relationship in terms of Boolean algebra
- That same equation also applied when calculating the probability of system failure (for each factor, you just calculate the probability that that component will fail in the given time period and

The least reliable component

Question 6

“Reliability-wise in parallel”

Answer 6

Indicates Redundancy, extremely common in automotive and aerospace

Quantifying this is a little trickier than it seems:
The reliability of a system build up from components reliability-wise in parallel is 1-ΠF
ΠF is the probability that ALL the components will fail within the mission duration (product of CDF for each component failure)

Question 7

Quantifying Reliability

Answer 7

The exact number depends on the time period

A component’s reliability is represented by the probability that it will fail in the given time period (solved using an integral)
System reliability is represented by the probability that the entire system will fail in the given time period (mission duration)
- For components ‘reliability wise in series’: It is the product of those integrals

Question 8

F-function of a component

Answer 8

Represents the probability that the component will fail.

It is equivalent to one minus the reliability function F=1-R

Question 9

Reliability function of a component

Answer 9

Represents the probability that the component will NOT fail.

It is equivalent to one minus the reliability function R=1-F

Question 10

Mean time to failure (MTTF)

Answer 10

Represents how long the component or system will be expected to last, on average

This is particularly important is exponential failure distributions

Question 11

Failure distributions

Question 12

Calculating MTTF for a system

Question 13

Reduction of complex RBDs

Answer 13

Question 14

“Fault” Definition

Answer 14

A latent defect condition-incompatibility, degradation, or deterioration; OR a hazard that has the potential to materialize into a failure

Question 15

Fault Tree Analysis

Answer 15

Represents an option when it comes to understanding how a system may fail

• Uses Boolean algebra symbols to represent sequencing and dependencies

Question 16

Cut Set in Fault Tree Analysis

Answer 16

Set of basic events whose simultaneous occurrence would ensure system failure if they occur

Described using set theory

MINIMAL cut sets represent the smallest set of events that could cause a system failure (meaning that each of the events in the cut set actually contribute to the root cause and there are no extraneous events in the set)

Each cut set has a probability

Question 17

Root Cause

Answer 17

The actual

Question 18

Basic Assumption of a Cut Set

Answer 18

You are assuming that the basic events are independent

NOT a Markov Chain

Question 19

Probabilistic Risk Assessment

Question 20

ISO 9001

Question 21

Failure Rate

Question 22

Failure Modes and Effects Analysis (FMEA)

Question 23

Common Continuous Reliability Distributions

Answer 23

The two most commonly used reliability distributions are the exponential and the Weibull. Both are among the most mathematically tractable to work with; the exponential is a constant failure rate model, and the Weibull is adaptable to a wide variety of failure rate scenarios.

1) Exponential
2) Weibull
3) Normal
4) Lognormal
5) Beta
6) Gamma
7) Rayleigh
8) Uniform
9) Extreme Value
10) Logistic
11) Log logistic
12) Pareto
13) Inverse Gaussain
14) Makeham
15) Hyperexponential
16) Muth

Question 24

Exponential Failure Model

Question 25

Weibull Failure Model

Question 26

Part Stress Analysis (PSA)

Answer 26

– assessment of a part’s reliability due to construction and application
– utilizes specific attribute data such as component technology, package type, complexity and quality, as well as application-specific data such as electrical and environmental stresses
– applicable when most of the design is completed and a detailed parts list including part stresses is available

Question 27

Part Count Analysis (PCA)

Answer 27

– a less-refined estimator relying on default values of most of the part and application-specific parameters
– applicable during bid proposal and early design phases when insufficient information is available to use the part stress analysis models

Question 28

MIL HDBK 217

Question 29

Bellcore (Telecommunications Industry standard)

Question 30

Accelerated Life Testing

Question 31

Highly Accelerated Life Test v. Highly Accelerated Stress Screen

Question 32

Bathtub Curve