Intro To Reliability Engineering

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Reliability Engineering - Introduction

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Topics

Topics that will be covered 1. Need for Reliability 2.Terminology 3. Reliability Metrics 4. Bath-Tub Curve 5. Reliability Testing 6. Accelerated Testing

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Need for Reliability

1. Need for Reliability

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Need for Reliability

What Customer Care about : 1. Product life.........i.e, useful life before wear-out 2. Minimum Down Time...........i.e., Maximum MTBF 3. Endurance...........i.e., # operations, robust to environmental changes 4. Stable Performance.........i.e., no degradation in CTQ's 5. ON time start-up...........i.e., ease of system startup

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Need for Reliability

•We depend on, demand and expect reliable products •Shipping unreliable products can destroy a company’s reputation •Reliability is “Quality changing over time.” •Quality is snapshot at the start of life and reliability is a motion picture of the day –by-day operation. •Reliability is a major economic factor in determining a product’s success

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Correction Vs Prevention

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Need for Reliability

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Terminology

2. Terminology

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Terminology

What do we mean by

1. Reliability 2. Failure 3. Failure Rate 4. Hazard Rate 5. MTBF 6. MTTF

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Terminology

1. Reliability [R(t)]: The probability that an item will perform its intended function without failure under stated conditions for specified period of time. 2. Failure: The termination of the ability of the product to perform its intended function 3. Failure Rate [λ(t)]: The ratio of no. of failures within a sample to the cumulative operating time. 4. Hazard Rate [h(t)]: The instantaneous probability of failure of an item given that it has survived until that time. This is also called as “Instantaneous Failure Rate”. 5. MTBF : Mean Time Between Failures, for a repairable system, the ratio of the cumulative operating time to the number of failures for that item. 6. MTTF : Mean Time To Failure, for non-repairable items. The total number of life-units of an item population divided by the number of failures within that population, during a particular measurement interval under-stated conditions. Reliability Engineering

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Reliability Metrics

3. Reliability Metrics

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Failure Rate

EXAMPLE: A sample of 1000 numbers are tested for a week, and two of them fail. (assume they fail at the end of the week). What is the Failure Rate?

2 failures Failure Rate = 1000 * 24 * 7 hours

= 1.19E-5 failures/hr

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Reliability

Suppose we start the test at t0 with N0 devices. After some time Nf devices of

the total have failed and Ns will have survived ( N0 = Nf + Ns). The Reliability at any time t is given by

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MTBF

EXAMPLE: A motor is repaired and returned to service six times during its life and provides 45,000 hours of service. Calculate MTBF.

Total operating time 45 , 000 MTBF = = = 7, 500 hours No of failures 6

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R(t)

EXAMPLE: If MTBF for a motor is 7,500 hours, the probability of operating for 30 days without failure is ...

 R=e

−

30 ∗ 24 hours 7500 hours

 = 0 .908 = 90 . 8

A mathematical model for reliability during Useful Life

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Bath-tub Curve

4. Bath - tub Curve

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Hazard Function

Hazard Function As shown the hazard rate is a function of time. What type of function does hazard rate exhibit with time? The general answer is the bathtub-shaped function. The sample will experience a high failure rate at the beginning of the operation time due to weak or substandard components, manufacturing imperfections, design errors and installation defects. This period of decreasing failure rate is referred to as the “infant mortality region” This is an undesirable region for both the manufacturer and consumer viewpoints as it causes an unnecessary repair cost for the manufacturer and an interruption of product usage for the consumer. The early failures can be minimized by improving the burn-in period of systems or components before shipments are made, by improving the manufacturing process and by improving the quality control of the products. Reliability Engineering

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Bath-tub Curve

At the end of the early failure-rate region, the failure rate will eventually reach a constant value. During this constant failure-rate region the failures do not follow a predictable pattern but occur at random due to the changes in the applied load. The randomness of material flaws or manufacturing flaws will also lead to failures during the constant failure rate region. The third and final region of the failure-rate curve is the wear-out region. The beginning of the wear out region is noticed when the failure rate starts to increase significantly more than the constant failure rate value and the failures are no longer attributed to randomness but are due to the age and wear of the components. To minimize the effect of the wear-out region, one must use periodic preventive maintenance or consider replacement of the product.

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Bath-tub Curve

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Reliability Testing

4. Reliability Testing

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Reliability Testing

Reliability Testing allows us to 1. Determine if a product's design is capable of performing its intended function for the desired period. 2. Have confidence that our sample-based prediction will accurately reflect the performance of the entire population. 3. Provide a path to “grow” a product's reliability by identifying weak points in the design 4. Confirm the product's performance in the field. 5. Identify failure caused by severe applications that exceeds the ratings, and recognize opportunities for the product to safely perform under more diverse applications.

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Reliability Testing

Reliability Testing answers questions like … ●

What is my product’s Failure Rate?

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What is the expected life?

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Which distribution does my data follow?

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What does my hazard function look like?

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What failure modes are present?

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How “mature” is my product’s reliability?

. . .. ..

These metrics and more can be obtained with the right reliability test

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Reliability Testing

Four Major Categories of Reliability Testing ●

Reliability Growth Tests (RGT) - Normal Testing - Accelerated Testing

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Reliability Demonstration Tests (RDT)

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Production Reliability Acceptance Tests (PRAT)

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Reliability Validation (RV)

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Reliability Testing

Reliability Growth Testing To determine a product’s physical limitations, functional capabilities and inherent failure mechanisms. Used early & throughout the design process Reliability Demonstration Testing To demonstrate the product’s ability to fulfill reliability, availability & design requirements under realistic conditions. Used at end of design stages to demonstrate compliance to specification Production Reliability Acceptance Tests To ensure that variation in materials, parts, & processes related to move from prototypes to full production does not affect product reliability Screens and Audits precipitate and detect hidden defects Reliability Engineering

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Reliability Testing

Reliability Validation To ensure that the product is performing reliably in the actual customer environment / application. Reliability Validation tracks field data on Customer Dashboards

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Accelerated Testing

4. Accelerated Testing

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Accelerated Testing

Scope : Accelerated testing allows designers to make predictions about the life of a product by developing a model that correlates reliability under accelerated conditions to reliability under normal conditions. Model: The model is how we extrapolate back to normal stress levels.

Results @ high stress + stress-life relationship = Results @ normal stress

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Accelerated Testing

Failure data is needed to accurately assess and improve reliability - this poses problems when testing highly reliable parts Since the number of failures r is critical, and not the sample size n on test, it becomes increasingly difficult to assess the failure rates of highly reliable components. Testing at much higher than typical stresses can yield failures but models are then needed to relate these back to use stress. The models that relate high stress reliability to normal use reliability are called acceleration models. When changing stress is equivalent to multiplying time to fail by a constant, we have true (physical) acceleration.

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Accelerated Testing

Physical Acceleration (sometimes called True Acceleration or just Acceleration) means that operating a unit at high stress (i.e., higher temperature or voltage or humidity or duty cycle, etc.) produces the same failures that would occur at typical-use stresses, except that they happen much quicker.

Failure may be due to mechanical fatigue, corrosion, chemical reaction, diffusion, migration, etc. These are the same causes of failure under normal stress; the time scale is simply different.

An Acceleration Factor is the constant multiplier between the two stress levels

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