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回复：（bibikey）[求助]加速寿命试验
本帖最后由 wdhd 于 2016-3-15 10:21 编辑
Quantitative Accelerated Life Tests
For all life tests, some time-to-failure information (or time-to-an-event) for the product is required since the failure of the product is the event we want to understand. In other words, if we wish to understand, measure and predict any event, we must observe the event!
Most products, components or systems are expected to perform their functions successfully for long periods of time, such as years. 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.
Two methods of acceleration, usage rate acceleration and overstress acceleration, have been devised to obtain times-to-failure data at an accelerated pace. For products that do not operate continuously, one can accelerate the time it takes to induce failures by continuously testing these products. This is called usage rate acceleration. For products for which usage rate acceleration is impractical, one can apply stress(es) at levels which exceed the levels that a product will encounter under normal use conditions and use the times-to-failure data obtained in this manner to extrapolate to use conditions. This is called overstress acceleration.
Usage Rate Acceleration
For products that 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. For example, a microwave oven operates for small periods of time every day. One can accelerate a test on microwave ovens by operating them more frequently until failure. The same could be said of washers. If we assume an average washer use of 6 hours a week, one could conceivably reduce the testing time 28-fold by testing these washers continuously.
To simplify the example, other stresses or secondary effects that might be encountered, such as turning the unit on and off, or overheating due to continuous usage, are not mentioned. The practitioner should take these effects into account when the test is formulated. For example, to overcome this challenge, the test washers could run their cycles with 30 minute rest periods to take other stresses into account.
Data obtained through usage acceleration can be analyzed with the same methods used to analyze regular times-to-failure data. These typical life data analysis techniques are thoroughly described in ReliaSoft's Life Data Analysis Reference [= 4) BSPSPopupOnMouseOver(event);;" href="javascript:BSSCPopup('reference_31.htm');">31] and facilitated by ReliaSoft's Weibull++ software package (http://Weibull.ReliaSoft.com).
The limitation of usage rate acceleration arises when products, such as computer servers and peripherals, maintain a very high or even continuous usage. In such cases, usage acceleration, even though desirable, is not a feasible alternative. In these cases the practitioner must stimulate, usually through the application of stress(es), the product to fail. This method of accelerated life testing is called overstress acceleration and is described next.
Overstress Acceleration
For products with very high or continuous usage, the accelerated life testing practitioner must stimulate the product to fail in a life test. This is accomplished by applying stress(es) that exceed the stress(es) that a product will encounter under normal use conditions. The times-to-failure data obtained under these conditions is then used to extrapolate to use conditions. Accelerated life tests can be performed at high or low temperature, humidity, voltage, pressure, vibration, etc. in order to accelerate or stimulate the failure mechanisms. They can also be performed at a combination of these stresses.
Stresses and Stress Levels
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 as illustrated next:
Fig. 1: Typical stress range for a component, product or system.
This choice of stresses and stress levels and 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. Accurate data on the stresses applied as well as the observed behavior of the test specimens must be maintained.
It is clear that as the stress used in an accelerated test becomes higher the required test duration decreases. However, as the stress level moves farther away from the use conditions, the uncertainty in the extrapolation increases. Confidence intervals provide a measure of the uncertainty in extrapolation. Confidence intervals are presented in Appendix A: A Brief Statistical Background.
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发表于 2005-9-4 08:00
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