Manual for Life Cost Based FMEA
Mean Time To Repair (MTTR) MTTR is the statistical mean or average of the distribution time to repair. MTTR
value is calculated by taking the cumulative period and dividing it by the total number
totals of repair of incidents.
time over a specific Mathematically it is
MTTR = ∑
i n T
where, n= number of incidents and T is the time to repair the i th incident.
Availability is the any given time. operating hours. It
probability that a system Availability is the ratio can be expressed as:
will operate to satisfactory conditions at between Uptime and Total Scheduled
Availability (A) =
Uptime Scheduled Operating Hours
MTBF MTBF + MTTR
Field failures do not generally occur at a uniform rate, but follow a distribution in time commonly described as a "bathtub curve." The reliability bathtub curve is a conceptual model that is used to describe reliability at a component level over its life cycle. The bathtub consists of three stages as shown in Figure 3. Not all products follow the bath tub curve model but most do.
Figure 3. Reliability Bathtub Curve
The three phases are:
Infant Mortality Phase: characterized by a rapidly
The early life period of device operation is declining failure rate. It occurs between 0 and
10,000 hours (~1 year) of device operation. Quality control poor workmanship, contamination, and other substandard
defects due to manufacturing
failure due to material, etc.
poor manufacturing or assembly errors, use of poor quality This has led to the development of burn-in procedures to
FMEA MANUAL By S. Rhee and C.M. Spencer