Accelerated Life Testing Pdf
They can also be performed at a combination of these stresses. From Wikipedia, the free encyclopedia.
When the stress is time-independent, the stress applied to a sample of units does not vary. Confidence Intervals are presented in Appendix A. In addition to proper stress selection, the application of the stresses must be accomplished in some logical, controlled and quantifiable fashion.
For instance, a test that is measuring lifespan may look only at the mean time to failure of the test objects, or it may try to fit a statistical distribution to the data. This method of accelerated life testing is called overstress acceleration and is described next. Introduction to Accelerated Life Testing.
Two methods of acceleration, usage rate acceleration and overstress acceleration, have been devised to obtain times-to-failure data at an accelerated pace. The first step in performing an accelerated life data analysis is to choose an appropriate life distribution. In other words, if temperature is the thermal stress, each unit is tested under the same accelerated temperature, ejercicios de conversion de unidades pdf e. Quantitative accelerated life testing can take the form of usage rate acceleration or overstress acceleration.
Although it is rarely appropriate, the exponential distribution has in the past been widely used as the underlying life distribution because of its simplicity. The times-to-failure at this stress level can then be easily analyzed using an underlying life distribution. Reliability information can include the probability of failure of the product under use conditions, mean life under use conditions, and projected returns and warranty costs.
If we tested specimens of our product at two different stress levels, we could begin to fit the model to the data. This model or function can be described mathematically and can be as simple as the equation for a line.
Introduction to Accelerated Life Testing - ReliaWiki
The times-to-failure data obtained under these conditions are then used to extrapolate to use conditions. For many reasons, obtaining such life data or times-to-failure data may be very difficult or impossible. This is called usage rate acceleration. Obviously, for a company to remain competitive, the time required to obtain times-to-failure data must be considerably less than the expected life of the product.
To understand the process involved with extrapolating from overstress test data to use level conditions, let's look closely at a simple accelerated life test. Data obtained through usage acceleration can be analyzed with the same methods used to analyze regular times-to-failure data.
Even if two models fit data at high stresses equally well, they may differ by orders of magnitude at lower stresses. Each type of test that has been called an accelerated test provides different information about the product and its failure mechanisms. This is accomplished by applying stress es that exceed the stress es that a product will encounter under normal use conditions. Established models must show agreement between extrapolations from accelerated data and observed data across a range of stress factors. Encyclopedia of Quantitative Risk Analysis and Assessment.
Accelerated life tests can be performed at high or low temperature, humidity, voltage, pressure, vibration, etc. Handbook of Reliability Engineering. All factors thought to influence the test object should be involved and tests should be conducted at various levels of each factor. For products which do not operate continuously under normal conditions, if the test units are operated continuously, failures are encountered earlier than if the units were tested at normal usage. In our case, the model is a combination of the life distribution and the life-stress relationship model.
There are numerous variations of this concept. Accurate data on the stresses applied, as well as the observed behavior of the test specimens, must be maintained.
The next figure illustrates a typical behavior of the pdf at the high stress or overstress level and the pdf at the use stress level. For simplicity we will assume that the product was tested under a single stress at a single constant stress level. Clearly, the more points we have, the better off we are in correctly mapping this particular point or fitting the model to our data. Available methods for estimating the parameters of a model include the graphical method, the least squares method and the maximum likelihood estimation method. In such cases, usage acceleration, even though desirable, is not a feasible alternative.
For instance, a reliability test on circuits that must last years at use conditions high longevity would need to yield results in a much shorter time. If a sudden shock was the primary cause of failure, a Highly Accelerated Life Test may be more appropriate. One can accelerate a test on microwave ovens by operating them more frequently until failure. The Weibull and lognormal distributions, which require more involved calculations, are more appropriate for most uses. This is usually referred to as a life distribution, the probability density function of which represents the proportion of products failing at a given time.
In other words, they have attempted to accelerate their failures. In other words, if we wish to understand, measure and predict any event, we must observe how that event occurs!
An Overview of Basic Concepts
Clearly, as the stress used in an accelerated test becomes higher, the required test duration decreases because failures will occur more quickly. How one factors in the effect of time depends largely on what one is measuring. Both of these loading schemes are described next.
Parameter estimation methods are presented in detail in Appendix B of this reference. These lifetime distributions are presented in greater detail in the Distributions Used in Accelerated Testing chapter of this reference. Increasing the number of tests or the number of test objects in each test generally increases how precisely one can infer the test object's behavior at operating conditions. The next figure illustrates such a simplification where we need to determine a way to project or map this single point from the high stress to the use stress. Eight common life-stress models are presented later in this reference.
When the model is known in advance the test only needs to identify the parameters for the model, however it is necessary to ensure that the model being used has been well verified. Accelerated life tests can be classified as constant stress, step stress, cycling stress, random stress, etc. However, as the stress level moves farther away from the use conditions, the uncertainty in the extrapolation increases. This is the type of stress load that has been discussed so far. The combination of both an underlying life distribution and a life-stress model can be best seen in the next figure where a pdf is plotted against both time and stress.
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