Your Reliability Engineering Professional Development Site
by Fred Schenkelberg Leave a Comment
Plan and conduct standard design of experiments (DOE) (e.g., full-factorial, fractional factorial, Latin square design). Implement robust-design approaches (e.g., Taguchi design, parametric design, DOE incorporating noise factors) to improve or optimize design.
The steps to accomplish a basic DOE.
by Fred Schenkelberg Leave a Comment
Plan and conduct standard design of experiments (DOE) (e.g., full-factorial, fractional factorial, Latin square design). Implement robust-design approaches (e.g., Taguchi design, parametric design, DOE incorporating noise factors) to improve or optimize design.
There is a bit of statistical magic about use an array.
by Fred Schenkelberg Leave a Comment
Plan and conduct standard design of experiments (DOE) (e.g., full-factorial, fractional factorial, Latin square design). Implement robust-design approaches (e.g., Taguchi design, parametric design, DOE incorporating noise factors) to improve or optimize design.
A run down of the language of DOE.
by Fred Schenkelberg Leave a Comment
Plan and conduct standard design of experiments (DOE) (e.g., full-factorial, fractional factorial, Latin square design). Implement robust-design approaches (e.g., Taguchi design, parametric design, DOE incorporating noise factors) to improve or optimize design.
There are different ways to setup experiments, which do you commonly use?
Design of Experiments (article)
by Fred Schenkelberg Leave a Comment
Plan and conduct standard design of experiments (DOE) (e.g., full-factorial, fractional factorial, Latin square design). Implement robust-design approaches (e.g., Taguchi design, parametric design, DOE incorporating noise factors) to improve or optimize design.
Lets take a look at how we approach experiments.
by Fred Schenkelberg Leave a Comment
Plan and conduct standard design of experiments (DOE) (e.g., full-factorial, fractional factorial, Latin square design). Implement robust-design approaches (e.g., Taguchi design, parametric design, DOE incorporating noise factors) to improve or optimize design.
This tool set allows you to maximize the value of your experiments.
by Fred Schenkelberg Leave a Comment
by Fred Schenkelberg Leave a Comment
Describe how tolerance and worst-case analyses 9e.g., root of sum of shares, extreme value) can be used to characterize variation that affects reliability.
Work with your engineering team to establish meaningful tolerances.
Purpose of Tolerances (article)
Worst Case Tolerance Analysis (article)
Root Sum Squared Tolerance Analysis Method (article)
Reliability and Root Sum Squared Tolerances (article)
Reliability and Monte Carlo Determined Tolerances (article)
Why do Tolerance Analysis (article)
1-50. What would a reliability professional be expected to do in a “worst-case” design analysis?
(A) Analyze the worst rejects.
(B) Analyze only those products failing to meet specification requirements.
(C) Assume that all subassembly tolerances are at their maximum limit.
(D) Determine whether product requirements can be met with subassemblies assumed at their worst combination of tolerances.
(D) Determine whether product requirements can be met with subassemblies assumed at their worst combination of tolerances.
The process to analyze a design by worst cases analysis is defined by setting the individual component values at not the maximum values, rather the values which create the least desirable situation. Worst case tolerance analysis is a conservative method to evaluate the impact of tolerances on product performance.
Worst here does not imply failures or rejects, instead looking at failures or rejects is part of a failure analysis process or potential improvement project.
1-95. An engineer is using an extreme worst-case method of variability analysis and finds that the indicated performance characteristics values fall within specifications. What can be concluded?
(A) The probability of occurrence will be unknown.
(B) The system can withstand high component parameter drift.
(C) A low parameter tolerance limit is needed.
(D) A high parameter tolerance limit is needed.
(B) The system can withstand high component parameter drift.
Component parameter values may change over time, beyond the process related variability. A design the functions correctly even when using the extreme worst set of component parameters indicates a robust design with ample margin for the nominal set of components. In general, the components that have parameter drift may do so over wider ranges than a design unable to demonstrate functionality using this method.
by Fred Schenkelberg Leave a Comment
Apply these techniques to develop models that can be used to evaluate undesirable (FTA) and desirable (STA) events.
A map or visual of the chain of events that lead to a failure or success.
Basic Description of a Fault Tree Analysis (article)
A Brief Introduction to Fault Tree Analysis (article)
Fault Tree Analysis 8 Step Process (article)
Benefits of Fault Tree Analysis (article)
Intro to Fault Tree Analysis (article)
1-85. In comparison to a failure mode, effects, and criticality analysis, fault tree analysis (FTA) suffers from which basic problem?
(A) FTA can only be used to analyze electronic systems.
(B) In FTA it is difficult to assign probabilities to the various events.
(C) FTA takes significantly more time to accomplish.
(D) A different FTA is needed for each defined top event.
(D) A different FTA is needed for each defined top event.
The structure of an FTA starts with a unique top event, whereas an FMEA/FMECA start by listing potential failure modes within one study.
by Fred Schenkelberg Leave a Comment
Describe this type of failure (also known as common cause mode failure) and how it affects design for reliability.
Sometimes multiple elements fail due the same initiating cause, how will your system respond?
Common Mode Failures (article)
Common Cause Failures (article)
by Fred Schenkelberg Leave a Comment
Define and distinguish between failure mode and effects analysis and failure mode, effects, and criticality analysis and apply these techniques in product, processes, and designs.
It’s time to consider what potentially could go wrong and take action.
10 steps of FMEA (article)
SOR 083 The Most Common FMEA Mistakes and How to Avoid Them – Part I (podcast)
SOR 084 The Most Common FMEA Mistakes and How to Avoid Them – Part II (podcast)
When to do FMEA (article)
Finding Value in FMEA (article)
1-83. Identify all the considerations that should be taken into account in a criticality analysis.
I. consequences of a failure
II. damage to the environment
III. consequential damage of a failure
IV. potential increase in efficiencies
(A) I and II only
(B) II and III only
(C) I, II, and III only
(D) I, II, III, and IV
(C) I, II, and III only
Criticality is a relative measure of failure mode consequence and its frequency. Thus all the options apply except efficiency. If “efficiencies” here apply to how well the failure manifests or to the manufacturing process, neither concept applies to the concept of FMECA or specifically the criticality element.
1-91. What is the very first step in a failure mode, effects and criticality analysis?
(A) Define the system requirements.
(B) Identify all the failure modes.
(C) Compile a critical items list.
(D) Determine the causes of the failures.
(A) Define the system requirements.
According to The Basics of FMEA by Robin E. McDermott, et. al. there are ten steps to FMEA:
Other references place the scoring after listing causes and detection elements related to each failure mode. All start with understanding the system or component or process under consideration.
1-101. Identify the principal measures of failure during hazard analysis among the following.
I. failure mode
II. failure severity
III. failure probability
IV. failure mechanism
(A) I and II only
(B) I and III only
(C) II and III only
(D) III and IV only
(C) II and III only
The quantification of a hazard does not consider detection as is done in FMEA. How often and the consequence of a failure to humans or environment are the primary factors in hazard analysis.
The failure mode is a symptoms presented when a failure occurs. The failure mechanism is the underlying event or series of events that manifest as a failure mode.
1-102. What does one analyze in a process FMEA?
(A) system design and the impact of failure modes
(B) the specific hardware details of a device
(C) the functional output of a device
(D) how manufacturing failures affect the product operation
(D) how manufacturing failures affect the product operation
Key word here is “process”. FMEA or FMECA are tools to analyze designs or processes from system to component level.
1-105. Conducting a hardware FMECA on a system requires knowledge of which of the following?
(A) the details of the system
(B) the details of the subsystems
(C) the black box functions
(D) the software to be used
(A) the details of the system
FMEAs/FMECAs are versatile tools to determine risks and prioritize improvement efforts. A system study focuses on the system. A common practice is to use the results to the system study to identify subsystems, components, or processes for a detailed FMEA/FMECA study.
1-142. In a failure modes and effects analysis, which of following does not have a risk priority number assigned to it?
(A) detection
(B) function
(C) probability
(D) severity
(B) function
An FMEA does include a listing and discussion of the function of the item or system. It is not scored or ranked. The severity of a failure mode, the probability of a cause, and the ability to detect a cause before the failure occurs are all scored and comprise elements to calculate the risk priority number (RPN).
by Fred Schenkelberg 2 Comments
Apply stress-strength analysis method of computing probability of failure, and interpret the results.
When the stress is larger than the specific units strength; that is a problem.
Stress Strength Normal Assumption (article)
The Stress – Strength Concept in Practice (article)
SOR 059 How to Set Environmental Specifications for Testing (podcast)
1-81. Normal stress and strength distributions are constructed for four different parts as described in the following:
For part 1, the average and standard deviation of the stress and strength distributions are the same.
For part 2, the average and standard deviation of the stress distribution are both higher than for the strength curve.
For part 3, both distributions have the same standard deviation but the strength average is higher than the stress average.
For part 4, Both distributions have the same variance but the stress average is slightly higher than the strength average.
Identify the part with the highest reliability.
(A) part 1
(B) part 2
(C) part 3
(D) part 4
(C) part 3
This takes some careful reading and knowledge that when the strength curve is higher (stronger) than the stress curve for most scales of stress/strength application, then the part is more reliable. Parts 2 and 4 have the mean stress higher than the mean strength suggesting a lower reliability then with part 1, which still isn’t a great situation. Part 3 has a higher mean strength than stress, which will result in a higher reliability than other the other parts.
1-87. An oxygen cylinder is rated at 2200 psi, with a standard deviation of 190 psi. If the expected stress will be 1750 psi with a standard deviation of 300 psi, what is the probability of failure of the cylinder?
(A) 0.0345
(B) 0.1025
(C) 0.1817
(D) 0.8975
(B) 0.1025
This is a normal based stress-strength calculation assuming both distributions are normal and independent. Use the following formula to determine the z-value representing the probability of failure.
$$ Z=\frac{{{\mu }_{x}}-{{\mu }_{y}}}{\sqrt{\sigma _{x}^{2}+\sigma _{y}^{2}}}$$
where the x subscripts are for the strength parameters and y is for the stress parameters. Not it uses variance, not the standard deviations. Inserting the values and calculation we fine z is
$$ Z=\frac{2200-1750}{\sqrt{190_{{}}^{2}+300_{{}}^{2}}}=0.1025$$
Then it is off to the z-table, to determine the probability of failure. In this case you should find the probability value, in the body of the table is between z-values of 1.26 and 1.27, with probabilities of 0.1038 and 0.1020, respectively. Use intropolation to determine the value for a z-value of 1.27.
Interpolation uses the idea equivalent ratios
$$ \frac{{{z}_{between}}-{{z}_{lower}}}{{{z}_{higher}}-{{z}_{lower}}}=\frac{{{P}_{between}}-{{P}_{lower}}}{{{P}_{higher}}-{{P}_{lower}}}$$
In this case, insert all the known values and solve for the unknown probability corresponding to a z-value of 1.27.
$$ \begin{array}{l}\frac{1.267-1.26}{1.27-1.26}=\frac{{{P}_{between}}-0.1038}{0.1020-0.1038}\\{{P}_{between}}=\left( \frac{1.267-1.26}{1.27-1.26} \right)\left( 0.1020-0.1038 \right)+0.1038\\{{P}_{between}}=0.1025\end{array}$$
1-103. A metal component has a strength of 7,000 psi and a standard deviation of 800 psi. The component must withstand a load with a mean value of 4,500 psi and a standard deviation of 400 psi. Assuming that both strength and load are normally distributed, calculate the probability of failure for the component.
(A) 0.0026
(B) 0.0186
(C) 0.9814
(D) 0.9974
(A) 0.0026
This is a normal based stress-strength calculation assuming both distributions are normal and independent. Use the following formula to determine the z-value representing the probability of failure.
$$ Z=\frac{{{\mu }_{x}}-{{\mu }_{y}}}{\sqrt{\sigma _{x}^{2}+\sigma _{y}^{2}}}$$
where the x subscripts are for the strength parameters and y is for the stress parameters. Not it uses variance, not the standard deviations. Inserting the values and calculation we fine z is
$$ Z=\frac{7000-4500}{\sqrt{800_{{}}^{2}+400_{{}}^{2}}}=2.795 $$
Then it is off to the z-table, to determine the probability of failure. In this case you should find the probability value, in the body of the table is between z-values of 2.79 and 2.80, with probabilities of 0.0026 and 0.0026, respectively. There is no need to use interpolation to determine the value for a z-value of 2.795, as it is the same for the bounding values thus the resulting probability of failure is 0.0026.
by Fred Schenkelberg Leave a Comment
Identify environmental and use factors (e.g., temperature, humidity, vibration) and stresses (e.g., severity of service, electrostatic discharge (ESD), throughput) to which a product may be subjected.
The Environmental Test Manual (article)
SOR 043 Environmental Testing and Reliability (podcast)
1-10. How would you describes the events and operating conditions an item experiences from mission initiation to completion (e.g., research and development phase, product manufacturing phase, …, to mission completion)?
(A) operational profile
(B) mission profile
(C) design reliability
(D) mission reliability
(B) mission profile
The terms “operating” and “mission” in the question basically give this one away. The mission profile includes the activities, events, stresses, decisions, usage, capability, and environmental factors that fully describe what the system may experience over a use cycle as it accomplishes a specific goal. Think a commercial airliner, all the elements that make up a flight between cities, from fueling, boarding, take-off, to landing, disembarking, and cabin cleaning, as a mission profile associated with safely transporting passengers and cargo to the desired destination.
The operational profile is a subset of the mission profile as it does not include the events associated with accomplishing the goal. Generally the operational profiles focuses on the stresses and loads experienced during the mission or period of use.
The design and mission reliability relate to the probability of the system successfully completing the mission.
by Fred Schenkelberg Leave a Comment
by Fred Schenkelberg Leave a Comment
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