# Manual for Life Cost Based FMEA

MTBF =

T K

=

1 λ

(7)

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

expressed as:

n

^{MTTR = }∑

i =1

i n T

(8)

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

(9)

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:

1.

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

practice

are

liable

for

failures

at

this

stage.

Examples

include

premature

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

15/26

January 2009