Reliability HotWire: eMagazine for the Reliability Professional
Reliability HotWire

Issue 18, August 2002

Hot Topics

Accelerated Life Testing...Why? 

Today's market is fast and furious and timing is everything. What may have taken years to accomplish in the past can now be done in months, or even weeks. If you can release a reliable product before your competitors can release theirs, then you have gained a great advantage. But how can you gain this advantage in time to market? One factor that can play an important role is accelerated life testing. 

Accelerated life testing is still a relatively new subject in reliability engineering, but it is starting to gain greater acceptance in the industry. Accelerated life testing is the process by which a product is forced to fail more quickly than it would have under normal use conditions. Forcing the product to fail more quickly reduces test time and still allows you to understand the life characteristics of the product. Conceptually, this is a wonderful idea, but there are still many concerns. How do I set up an accelerated life test? What are the benefits? How much will it cost and how much time/money will it save me? The answers to these questions are not simple and straightforward, but when is anything worth having easily accomplished?

Qualitative or Quantitative?
There are two types of accelerated tests that you can choose to conduct: qualitative and quantitative. Qualitative tests have been referred to as elephant tests, torture tests, HALT (Highly Accelerated Life Tests), and shake and bake tests. They are generally performed on small samples with the specimens subjected to a single severe level of stress, to a number of stresses or to a time-varying stress (e.g. stress cycling, cold to hot, etc.). If the specimen survives, it passes the test. Otherwise, appropriate actions are taken to improve the product's design in order to eliminate the cause(s) of failure. Qualitative tests are used primarily to reveal probable failure modes. However, if not designed properly, they may cause the product to fail due to modes that never would have been encountered in real life. A good qualitative test is one that quickly reveals those failure modes that will occur during the life of the product under normal use conditions. In general, qualitative tests do not quantify the life (or reliability) characteristics of the product under normal use conditions; however, they provide valuable information as to the types and levels of stresses one may wish to employ during a subsequent quantitative test.

Quantitative accelerated life testing (QALT), unlike the qualitative testing methods described previously, consists of tests designed to quantify the life characteristics of the product, component or system under normal use conditions, and thereby provide reliability information. Reliability information can include the estimation of the probability of failure of the product under use conditions, mean life under use conditions, projected returns and warranty costs. It can also be used to assist in the performance of risk assessments, design comparisons, etc.

So, which type of accelerated test should you conduct? Well, it depends. Both types of accelerated tests will allow you to collect information to increase the reliability of the product. But if you actually would like to be able to quantify what the reliability is, then you would conduct a quantitative test. In reality, you would probably conduct both types of tests and you can use the information gained from the qualitative test to help in setting up the quantitative test.

Designing an Accelerated Life Test
Designing an accelerated life test is not necessarily an easy proposition, especially the first time. In addition, the initial accelerated life test will probably cost you more money and time than if you had conducted a non-accelerated life test. However, there is an increasing rate of return when conducting accelerated life tests. The first accelerated life test is a learning experience that will more than pay for itself down the road, but you must be willing to pay the toll to cross the bridge. The initial investment can be a hard sell to management. But if you can determine how the product will perform in the field in a few weeks rather than a few months, isn't that worth something pretty valuable? Accelerated life test stresses and stress levels should be chosen so that they accelerate the failure modes under consideration but do not introduce failure modes that would never occur under use conditions. Normally, these stress levels will fall outside the product specification limits but inside the design limits (below the destruct limits), as shown in Figure 1.

Typical stress range for a component, product or system

Figure 1: Typical stress range for a component, product or system.

The choice of stresses and stress levels, along with the process of setting up the experiment, is of the utmost importance. Consult your design engineer(s) and material scientist(s) to determine what stimuli (stress) is appropriate, as well as to identify the appropriate limits (or stress levels). If these stresses or limits are unknown, multiple tests with small sample sizes can be performed in order to ascertain the appropriate stress(es) and stress levels. Proper use of Design of Experiments (DOE) methodology is also crucial at this step. In addition to proper stress selection, the application of the stresses must be accomplished in some logical, controlled and quantifiable fashion. You must maintain accurate data on the stresses applied as well as the observed behavior of the test specimens. Bottom line...you must do your homework upfront before conducting an accelerated life test. Any error that exists in any guess will most certainly be discovered later, which in general, usually means you go back to square one. Spending the time and money ahead of time is definitely an investment, but it is an investment that you cannot afford not to take.

The actual acceleration factor that you can expect from an accelerated life test will depend on the characteristics of the product being tested. If you are expecting acceleration factors on the order of 100s or even 1000s, then unfortunately you will be disappointed. It is possible to design a test with an acceleration factor of, say, 500, but the test is not likely to yield useful information. 

Benefits
The first and most obvious benefit of accelerated life testing is the time savings, which is based on the decrease in test duration due to the increased stress levels. Once you have taken the time to properly set up the test, the process will become easier for each successive test since you will now have the knowledge of the types of stresses to apply and at what levels. Assuming that the product does not undergo any major design changes, the same or similar test configuration can be used in the future. And since the test duration has been decreased, multiple tests can now be conducted over a given period of time. Of course, the most important benefits of this testing methodology are financial. How much money does your company stand to lose if you release the product a few months after a competitor releases its version? Can you afford to release later than your competition? The answer is probably going to be no.

Conclusion
It is apparent that accelerated life testing cannot be applied to every single product. For certain products it can be applied more easily than others. But the possible benefits cannot be overlooked: faster to market, savings of time and money. Of course, this is all assuming that the test has been properly set up in the first place. A well-designed accelerated life test will apply stress(es) at levels that exceed the stress level the product will encounter under normal use conditions in order to accelerate the failure modes that would occur under use conditions.

When properly applied, accelerated life testing can provide valuable information and in today's market, the sooner that information can be gathered the better off you will be. And once you have the information, analyzing the data has become much easier using ReliaSoft's ALTA software. Accelerated life testing is simply one of the available methods that you can use to learn about your product more quickly. Once you have gathered the information, you can make knowledgeable decisions about how to proceed.

 

ReliaSoft Corporation

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