# Presentation: 707503

## Reliability: Its Purpose, Roots, And Activities

- Tim C. Adams
- NASA Kennedy Space Center
- 321-867-2267
- June 17, 2004

## Two Schools of Thought

**Determinism:**
- The doctrine that every event, act, and decision is the inevitable consequence of antecedents (past events) that are independent of the human will.
**Probabilism:**
- The doctrine that probability is adequate basis for belief and action, since certainty in knowledge cannot be attained.
- Many of the tools and techniques in used in Reliability are probabilistic.

## Comparing Concepts

**X: ****Scenario**

**Y: ****Likelihood**

**Z: ****Consequence**

**Risk**

**Reliability**

**Safety**

- What can go wrong?

- How likely is this to happen?

- If it does happen, what are the consequences?

- Uses historical failure data

- Historical failure data is mathematically modeled to predict failures

- The element under study either does or does not meet the failure definition

- Relies heavily on the identification of hazards

- Likelihood is based on judgment and a qualitative scale

- Evaluates hazardous states that could occur from both correct and incorrect
- element behavior

**Comparing Concepts**

## What is Reliability?

**Reliability **is defined as
-  *the *_*probability*_* an item will perform its intended *_*function*_ *for a given *_*time period*_* under a given set of *_*operating conditions*_.
- Another name for reliability (R) is the “**probability of success**” (ps).
- Many organizations speak in terms of the “**probability of failure**” (pf) being unreliability (U).
- Fundamental relationship:   ps + pf  = 1  or  R + U = 1 .

## Other Measures Related to Reliability

**Reliability** deals with reducing the frequency of breakdowns.
**Maintainability** deals with reducing the duration of breakdowns or downtime.
**Availability** deals with mission readiness which is a function of Reliability and Maintainability, that is...
-  A = uptime/(uptime + downtime) = MTBF/(MTBF + MTTR),
- Where MTBF is mean time between failure and MTTR is mean time to repair.

## Where does Reliability originate?

- It is _design engineering_ where most of the true reliability work is done.
- True reliability is built into the design and is called _inherent reliability_.
- Reliability requirements are an integral part of engineering specifications.  As the formal definition implies, at least four items must be contained in a _reliability requirement_** **or specification, namely:
- The intended function (mission) to be performed.
- The desired mission time.
- The operating environment.
- The probability of success (ps) that the product will perform its intended function.

## Design Phase Reliability Tools & Techniques

- _To specify reliability_: Use all four parts of the reliability definition.
- _To prevent failures_: Use the *design strategies* in the following order.
- Improve the design to eliminate the failure mode.
- Design for fault tolerance (redundancy).
- Design to be fail-safe (i.e., failure affects function but no injury or additional damage will occur).
- Provide early warnings of failure through fault diagnosis.
- Note: If these strategies are not viable, the designer may choose to issue special maintenance instructions and/or use Reliability-centered Maintenance (RCM).
- _To improve reliability (part 1_): Use the applicable *design strategies*.
**Zero Failure Design: **Critical failures are entirely eliminated by design.
**Fault Tolerance: **Redundant elements are used to switch over to a backup or alternative mode.
**Derating: **A component is used much below its capability rating.
**Durability: **A component is designed a have a longer “useful life” or is designed for damage tolerance.
**Safety Margins: **Design for all applicable worst-case stresses and environments.

- Process

## Design Phase Reliability Tools & Techniques (cont.)

- _To verify reliability_:  Use the applicable *analytical tools.*
**Design Reviews:** Challenges the design from different viewpoints and identifies and assesses risk (technical, schedule, and cost).
**Reliability Allocation, Modeling, And Prediction: **Provides a hierarchy of design requirements along with the distributed reliability goal, a model for system configuration, and estimated (predicated) reliability of the configuration.
**Design Failure Mode, Effects, And Criticality Analysis (Design FMECA or FMEA): **Starts at the component level.  Asks what can go wrong and how does it affect the system. Is an inductive (bottom up) and systematic method and is mostly qualitative.
**Fault Tree Analysis (FTA): **Starts at the system major failure or undesired event and decomposes to it contributing fault occurrences.  Is a deductive (top down) and unstructured method; uses symbolic logic and is always qualitative (e.g., identifies cut sets) with the option of being quantitative.
**Sneak Circuit Analysis: **Identifies failures not caused by part failures but are caused by logic flaws.
**Worst-Case Analysis:** Typically, used on circuits to evaluates performance when components are at their high and low values.
**Statistical Analysis: **Uses time-to-failure distributions, pass-fail distributions, and stress-strength distributions to measure predicated or demonstrated reliability.
**Quality Function Deployment (QFD): **A method for converting customer needs into engineering requirements.
**Robust Design (Design of Experiments, DOE):** Parameters and tolerance ranges are scientifically established to optimize performance so that the item is robust in a variety of conditions.

- Process

## Design Phase Reliability Tools & Techniques (cont.)

- _To improve reliability (part 2_): Use the applicable *engineering tests.*
**Reliability Growth Tests: **A test that identifies problems and solves them as the design progresses.  Thus, is essentially, a “test, analyze, and fix” method that is used in a closed-loop corrective action manner.
**Durability Tests** – Typically, **Accelerated Tests** that determine the failure rate for the entire expected life.  Duplicates field failures by providing a harsher but representative environment.  Performed instead of testing under normal conditions in order to eliminate testing that would otherwise take months or years.
**Qualification Tests** – Consist of stressing the product for all expected failure mechanisms.  The test can be stopped if there are no failures during the expected life—thus, are performed to measure the achievement of the reliability requirement.  _Note_: **Demonstration Tests** or **Design Approval Tests** are similar and usually require stressing during only a portion of the useful life.  See the tests used in the manufacturing phase.

- Process

## Manufacturing Phase Reliability Tools & Techniques

- _To prevent or reduce failures_: Use the following *analytical tools*.
**Process Failure Mode, Effects, And Criticality Analysis:** Used on the manufacturing process before it is installed.  Similar to Design FMECA.
**Statistical Process Control: **Designed to ensure that the manufacturing process continues to produce products with no more than expected variation in the critical parameters. Often considered a test for determining the control of quality instead of reliability.
- _To prove reliability_: Use the applicable *accounting tests*.
**Environmental Stress Screening Tests:** Also, known as **Burn-in and Screening Tests**.  Tests to catch “infant mortality” failures.  If the product is manufactured properly, these tests are not required.  _Note_: These tests are also performed in the Design Phase such that early failures do not mask the true reliability.  Unfortunately, these tests are sometimes used as the “final word.”  As a result, the screening may not be long enough and weak products may be provided to the customer.
**Production Reliability Acceptance Tests: **Also, known as **Failure Rate (MTBF) Tests.  **Used to detect any degradation in the inherent reliability of a product over the course of production and to assure products being delivered meet the customer’s reliability requirements and/or expectations (by testing a production lot and accepting or not accepting based on a sampling plan).  Also, used to qualify new products.

- Process

## User’s Phase Reliability Tools & Techniques

- Use the following *strategies* in the User’s Phase:
**Failure Reporting, Analysis, Corrective Action System: ** Provides the data needed to identify deficiencies for correction to ensure that inherent reliability is not degraded.  This system is typically used to record data for product failures that occurred during all phases of testing as well as in the field.  Also, the data from this system is typically used to detect trends as early as possible and to respond accordingly in a timely and preventive manner.
- _Note_: **Weibull Analysis**, a type of statistical analysis, is a good *tool* for identifying trends in non-repairable systems.  For repairable systems that are not repaired “good-as-new,” start with the **Laplace Test**.
**Warranties: **An attribute where reliability easily affects the manufacturer’s current and future revenues.  One of the biggest challenges facing manufacturers is competition due to longer warranties.

- Process