|
Reliability Testing
Reliability testing is the cornerstone of a
reliability engineering program. It provides the most detailed form of
reliability data because the conditions under which the data are collected
can be carefully controlled and monitored. Furthermore, reliability tests
can be designed to uncover particular suspected failure modes and other
problems. The type of reliability testing a product undergoes will change
along different points of its life cycle, but the overriding goal is to
insure that data from all or most of the tests were generated under similar
enough conditions so that an "apples to apples" comparison can be made of
the product's reliability characteristics at different points in the
product's life. It is for this reason that consistent and thorough
reliability specifications and a standard definition of failure are up-front
requirements to implementing reliability testing.
A properly designed series of tests,
particularly during the product's earlier design stages, can generate data
that would be useful in the implementation of a reliability growth tracking
program. This will provide information that will be helpful in making
management decisions regarding scheduling, development cost projections and
so forth. This information will also be useful in planning the development
cycle of future products.
There are several different kinds of tests,
including: Customer Usage Profiling,
Development Testing and
Manufacturing Testing.
Customer Usage Profiling
An important
requirement for designing useful reliability tests is to have a good idea of
how the product is actually going to be used in the field. The tests should
be based on a realistic expectation of the customer usage, rather than
estimates or "gut feelings" about the way the customer will use the product.
Tests based on mere speculation may result in a product that has not been
rigorously tested and consequently may run into operational difficulties due
to use stress levels being higher than anticipated. On the other hand, tests
that are designed with a strong basis of information on how the product will
be used will be more realistic and result in an optimized design that will
exhibit fewer failures in the field.
Customer usage profiles can be designed to
actively gather information on how the customers are actually using an
organization's product. This design can range from a simple questionnaire to
sophisticated instrumentation within the product that feeds back detailed
information about its operation. An incentive is often useful to get
customers to sign on for a usage measurement program, particularly if it is
an intrusive process that involves the installation of data collection
equipment. Additionally, customers are often eager to participate in these
programs in the knowledge that the information that they provide will
ultimately result in a more reliable and user-friendly product.
Developmental Testing
Developmental testing occurs during the early phases of the product's life
cycle, usually from project inception to product design release. It is vital
to be able to characterize the reliability of the product as it progresses
through its initial design stages so that the reliability specifications
will be met by the time the product is ready for release. With a multitude
of design stages and changes that could affect the product's reliability, it
is necessary to closely monitor how the product's reliability grows and
changes as the product design matures. There are a number of different test
types that can be run during this phase of a product's life cycle to provide
useful reliability information:
- Component-level
Testing: Although component-level testing can continue
throughout the development phase of a product, it is most likely to
occur very early in the process. This may be due to the unavailability
of parts in the early stages of the development program. There may also
be special interest in the performance of a specific component if it has
been radically redesigned or if there is a separate or individual
reliability specification for that component. In many cases,
component-level testing is undertaken to begin characterizing a
product's reliability even though full system-level test units are
unavailable or prohibitively expensive. However, system-level
reliability characterization can be achieved through component-level
testing. This is possible if sufficient understanding exists to
characterize the interaction of the components. If this is the case, the
system-level reliability can be modeled based on the configuration of
components and the result of component reliability testing.
- System-level
Testing: Although the results of component-level tests can be
used to characterize the reliability of the entire system, the ideal
approach is to test the entire system, particularly if that is how the
reliability is specified. That is, if the technical specifications call
out a reliability goal for a specific system or configuration of
components, that entire system or configuration should be tested to
compare the actual performance with the stated goal. Although early
system-level test units may be difficult to obtain, it is advisable to
perform reliability tests at the system level as early in the
development process as possible. At the very least, comprehensive
system-level testing should be performed immediately prior to the
product's release for manufacturing in order to verify design
reliability. During such system-level reliability testing, the units
under test should be from a homogeneous population and should be devoted
solely to the specific reliability test. The results of the reliability
test could be skewed or confounded by "piggybacking" other tests along
with it and this practice should be avoided. A properly conducted
system-level reliability test will be able to provide valuable
engineering information above and beyond the raw reliability data.
- Environmental
and Accelerated Testing: It may be necessary in some cases to
institute a series of tests in which the system is tested at extreme
environmental conditions or with other stress factors accelerated above
the normal levels of use. It may be that the product would not normally
fail within the time constraints of the test and, in order to get
meaningful data within a reasonable time, the stress factors must be
accelerated. In other cases, it may be necessary to simulate different
operating environments based on where the product will be sold or
operated. Regardless of the cause, tests like these should be designed,
implemented and analyzed with care. Depending on the nature of the
accelerating stress factors, it is easy to draw incorrect conclusions
from the results of these tests. A good understanding of the proper
accelerating stresses and the design limits of the product are necessary
to be able to implement a meaningful accelerated reliability test. For
example, one would not want to design an accelerated test that would
overstress the product and introduce failure modes that would not
normally be encountered in the field. Given that there have been a lot
of incredible claims about the capability of accelerated testing and the
improbably high acceleration factors that can supposedly be produced,
care needs to be taken when setting up this type of reliability testing
program.
- Shipping Tests:
Although shipping tests do not necessarily qualify as reliability tests
per se, shipping tests or simulations designed to test the impact on the
product of shipping and handling should be a part of the reliability
testing program. This is because the effects of shipping will often have
an impact on the reliability of the product as experienced by the
customer. As such, it may be useful to incorporate shipping tests
alongside the normal reliability testing. For example, it may be a good
idea to put the units of a final design release reliability test through
a non-destructive shipping test prior to the actual reliability testing
in order to better simulate actual use conditions.
Manufacturing Testing
The testing that
takes place after a product design has been released for production
generally tends to measure the manufacturing process rather than the
product, under the assumption that the released product design is final and
good. However, this is not necessarily the case, as post-release design
changes or feature additions are not uncommon. It is still possible to
obtain useful reliability information from manufacturing testing without
diluting any of the process-oriented information that these tests are
designed to produce.
- Functionality
Testing and Burn-In: This type of testing usually falls under
the category of operation verification. In these tests, a large
proportion, if not all, of the products coming off of the assembly line
are put on a very short test in order to verify that they are
functioning. In some situations, they may be run for a predetermined
"burn-in" time in order to weed out those units that would have early
infantile failures in the field. Although it may not be possible to
collect detailed reliability information from this type of testing, what
is lost in quality is made up for in quantity. With the proper
structuring, these tests can provide a fairly good picture of early-life
reliability behavior of the product.
- Extended
Post-Production Testing: This type of testing usually gets
implemented near the end or shortly after the product design is released
to production. It is useful to structure these types of tests to be
identical to the final reliability verification tests conducted at the
end of the design phase. The purpose of these tests is to assess the
effects of the production process on the reliability of the product. In
many cases, the test units that undergo reliability testing prior to the
onset of actual production are hand-built or carefully adjusted prior to
the beginning of the reliability tests. By replicating these tests with
actual production units, potential problems in the manufacturing process
can be identified before many units are shipped.
- Design/Process
Change Verification: This type of testing is similar to the
extended post-production testing in that it should closely emulate the
reliability verification testing that takes place at the end of the
design phase. This type of testing should occur at regular intervals
during production or immediately following a post-release design change
or a change in the manufacturing process. These changes can have a
potentially large effect on the reliability of the product and these
tests should be adequate, in terms of duration and sample size, to
detect such changes.
|
