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TEST & MEASUREMENT
mismatches within the product. The more extreme the temperature cycle, the higher the acceleration factor.
Vibration promotes mechanical failures due to cyclic stressing. The deterioration of material strength due to cyclic stressing is known as fatigue. If a product's operational vibration environment is known, then it may be accelerated too. Power cycling is the act of repeatedly turning a piece of equipment off and then on again to check that an electronic device reinitialises its configuration and continues operating normally. The benefit of accelerated life testing is that it helps detect the design flaws which are most likely to give rise to a product's ‘infant mortalities'. The disadvantage is that this method may precipitate some unrepresentative failures and Highly Accelerated Life Testing may instead provide the answer here.
HIGHLY ACCELERATED LIFE TESTING A key difference between highly accelerated life testing (HALT) and traditional accelerated life testing is that stress factors, such as high temperatures, are applied directly to the component or sub-assembly under test and not to the system as a whole. Thermal and mechanical stimuli are also applied separately, and then together, in order to determine the operating and destruct limits of the item under test. Defect analysis is a key stage in the HALT process and is conducted when the operation and destruct limits are known. The operating limit is defined as the point at which the unit remains operational but any further increase in stress
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causes a recoverable failure. At this stage, all major flaws in the design should be exposed. Most may require a simple fix, some may require major modification, and it may also be considered that the design is sufficiently rugged and that no further action is required. There is a common misunderstanding by engineers that HALT tends to lead to ‘over engineered’ products. In fact, a HALT appraisal allows designers to establish the limitations of their product designs and ensure that a product will have the level of reliability expected by the end user. Whatever testing method is chosen, to remain competitive, companies must select the appropriate techniques for their product that are fast, cost- effective, and produce worthwhile results.
HISTORICAL DATA
The reliability history of a product that is actively sold can sometimes be relied on too much when developing the next generation. What can be perceived as the slightest design alteration, such as the use of different components, can actually have a significant impact on the product’s reliability.
In such a situation, a gap analysis must therefore be performed between the existing product and the new version under development to gain a clearer understanding of the upgraded product’s reliability.
PRODUCT MISUSE
Maintenance and human factors, such as product misuse, can also be factored into the design process and demonstrated by testing, but they are
often overlooked as part of the design verification process. Other than compromising performance, problems arising from maintenance can affect safety, reliability and ultimately undermine the value end-users see in a product.
Even if the most robust tests are performed however, it is still very difficult to fully anticipate end-user behaviour. It is not easy to predict what people might do with the product, as often end-users ignore instructions, or use products in ways that designers would never have envisaged. However, more can be done with the design or to support the design to overcome some of these issues, and we are increasingly seeing standards that try to compensate for this element of ‘abnormal use’.
MAKING THE RIGHT CHOICE As time to market constraints require accelerated testing that cannot guarantee 100 per cent reliability and end-user behaviour cannot be predicted, reliability is difficult to guarantee. To remain competitive, companies must select the appropriate techniques to develop a product’s reliability that are fast, cost-effective, and produce worthwhile results. Without the ability to gauge reliability throughout the design lifecycle there is no assurance that the final product will meet market expectations. And, on an increasingly technologically level playing field, brand reputation of reliability and longevity is a vital element to differentiate a product from that of the competition.
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