Reliability Engineering

Bath Tub curve Infant Mortality Useful Life Wear Out Failure rate Time

LIFE CHARACTERISTIC CURVE Infant Mortality Due to gross built in flaws caused by faulty workmanship Transportation damage or installation errors, etc. Examples: Poor seals or welds or solder joints Poor connections Dirt or contamination on surfaces or in materials Chemical impurities in metals or insulation Voids, cracks, thin spots insulation or protective coatings Incorrect positioning of parts

Useful Life Period Characterised mainly by occurrence of stress related failures Exponential distribution, constant failure rate Wear out Due to deterioration of the design strength of the device as a consequence of operation and exposure to environmental fluctuations, corrosion / oxidation / ionic migration, etc.

Four Types of Part Failures Dependent upon Eliminated by Quality Unrelated to stress or Time Inspection Reliability Stress Screening Wear-out Time Qualification (life) testing or periodic replacement Design May be stress and/or time dependent Proper application and derating

Reliability Reliability is the probability that an item will perform its intended function for a specified time interval under stated conditions

R(t) = e-t R(t) is the probability that the item will operate without failure for the time period t, under specified operating conditions e - base of natural logarithm = 2.71828 λ - failure rate in Fits 1 Fit = 1 Failure / 10-9 hours Fit - Failure Unit , Failure in time

MTBF and MTTR Up Dt Ut Down Time

Availability The probability that the system is operating at time t. MTBF. Availability = ---------------------. MTBF + MTTR. MTBF … Mean time between failures. MTBF = 1/ λ hours. MTTR … Mean time to repair (expressed in hours).

Maintainability The measure of the ability of an item to be retained in or restored to a specified condition when maintenance is performed by personnel having specified skill levels, who use prescribed procedures and resources at each level of maintenance and repair.

Failure Distribution Versus Environment VIBRATION 27% TEMPERATURE 40% MOISTURE 19% SAND & DUST 6% SALT 4% ALTITUDE 2% SHOCK 2%