Useful Life is when the system’s Early Life issues have been worked out and the system is trusted for normal operation. During this phase, failures are considered to be “random chance failures,” which typically yield a constant failure rate. Useful Life continues until the product life cycle reaches the Wear Out phase.
During Useful Life, you apply the concepts of reliability, availability, serviceability, and manageability (RASM) engineering. The failure rate “λ” or mean time between failure (MTBF) is considered to be constant, that:
[2]
Have you ever wondered why a hard drive has a MTBF of 1 million hours (over a 100 years) but wears out after three to four years of use? So what does MTBF really mean?
The short answer is that MTBF does NOT account for Early Life or Wear Out failures but only random chance failures that are present during Useful Life. Thus, its sole purpose is to calculate the probability of success (reliability) and the probability of being ready to do its job (availability) during Useful Life. MTBF should not be used to calculate how long the system should last (when the system will enter Wear Out).
Reliability (R(t)) is defined as the probability that a device or a system will function as expected for a given duration in an environment. Duration is usually measured in time (hours), but it can also be measured in cycles, iterations, distance (miles), and so on. The reliability formula used for Useful Life, when the failure rate is constant, is:
[3]
t = Mission Time, Duration
Note: However, if the failure rate is not constant, then the above equation does not apply.
Once you have calculated the reliability of a system in an environment, you can calculate the unreliability (the probability of failure). Since the system has two states, functioning as expected or not, the two states are complementary[4]. The probability of success“R(t)”and the probability of failure“F(t)”are equal to 1. This topic is addressed in detail in the RASM Series white paper “The Mathematics of Reliability.”
[5]
Thus
Example:
A system has an MTBF of 300,000 hours. What is the predicted reliability for a six-month mission?
6 months = 4380 hours
The reliability (the probability of success) is predicted to be 98.55%.
What is the predicted probability of failure at 4380 hours?
The probability of failure is predicted to be 1.45%.
The probability of failure represents the risk of failure and can be used to help plan for the number of spares needed. Along with this information, you need to understand the usage profile and the mean time to repair (MTTR). Sparing and MTTR are addressed in other RASM Series white papers.
A strong understanding of Useful Life characteristics is key to planning a “sparing strategy” for the system.
