Page 46 - Root Cause Failure Analysis

Root Cause Failure Analysis Methodology 37
Other

Other testing techniques (e.g., meggering, ultrasonics, acoustic emissions) can be
used to measure the cause-and-effect relationship between variables. Each technique
is appropriate for specific applications and provides a means of quantifying or prov-
ing that specific forcing functions contributed to the event or incident being investi-
gated.

There also are a variety of tests that can be conducted to evaluate the metallurgical
properties of the failed component. These include tensile strength, residual stress, and
other properties of the component as well as the actual failure mechanism (i.e., stress
rupture, compression fracture, etc.).

Unless the plant has a fully functional metallurgical laboratory, these tests may
require the use of an outside lab. The cost of the evaluations will vary, depending on
the type of component and the extent of the analysis. Used selectively, metallurgical
analysis is an effective tool and can be a useful part of a RCFA.

DETERMINING THE ROOT CAUSE
At this point in the investigation, the preceding steps, sequence-of-events diagrams.
and logic trees should have identified the forcing functions that may have contributed
to the problem. These data should have provided enough insight into the failure for
the investigator to develop a list of potential or probable reasons for the failure.

The guidelines for specific machine types, which are provided in Part Two, and the
troubleshooting guides in Part Three should be used as part of this activity. The most
common failure modes of machinery are defined in the tables in Part Two, which pro-
vide the obvious symptoms associated with a machine problem or failure and their
common causes. Coupled with the data developed during the investigation, these
tables should help in reducing the number of potential causes to no more than rwo or
three.

A method that can be used to evaluate potential factors that may have caused or con-
tributed to the event is the cause-and-effect analysis. Graphical representation of the
variables or factors identified by the investigation using a fishbone diagram provides a
means of evaluating potential root cause(s).

The advantage of the fishbone diagram is that it forces the investigator to logically
group each of the factors identified during the investigation. This process may auto-
matically eliminate some factors and uncover other issues that must be addressed.
Once all the identified factors have been graphically displayed, the investigator or
investigating team can systematically evaluate each one.

Analyzing the short list of potential root causes to verify each of the suspect causes is
essential. In almost all cases, a relatively simple, inexpensive test series can be devel-