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TEST & MEASUREMENT R


eliability testing is an essential element of the design process as end-users demand products that last longer and deliver value for money. Designers must therefore adopt a robust approach to ensuring product reliability, while ensuring that they remain competitively priced. Designing-in reliability goes beyond regulatory, technical or performance requirements, defining other parameters such as fragility or fault conditions, which identify the limits of the design. Just meeting the regulatory or performance requirements for a product does not necessarily mean it is reliable. However, a common difficulty facing designers is establishing exactly what is meant by reliability. The term ‘reliability’ is defined as “the ability of an item to perform a required function under stated conditions for a stated period of time”. The ‘required function’ includes the specification of satisfactory operation as well as unsatisfactory operation, while for a complex system, unsatisfactory operation may not be the same as failure. The ‘stated conditions’ are the total physical environment including mechanical, thermal and electrical conditions. The ‘stated period’ of time is the time during which satisfactory operation is desired and is often called the service life of a product. There are also different measures of reliability, depending on the application of the end product:


Survivability - is the probability that an item will perform a required function under stated conditions for a specified period of time, but without failure. Survivability applies only to applications in which failures will not be routinely repaired, whereas the generic definition of reliability does include the possibility of repair.


Availability – this applies where there is the possibility of both repair and failure, and it is a measure of the degree to which an item is in an operable state when called upon to perform.


Maintainability - refers to the maintenance process associated with system reliability and is the degree to which an item can be retained in, or restored to, a specified operating condition.


BEYOND LIFE CYCLE TESTING The traditional approach to reliability evaluation has been life cycle testing, which involves tests being carried out within the product's ‘expected environment' or using actual operational conditions. However, this is an unrealistic approach, as it would not only be costly, but also lengthen a product's time to market. For example, if a five- year life cycle for a product is expected, a traditional reliability evaluation programme would require testing to encompass 43,800 hours of usage. This would not only be costly, but also delay the completion of the final design


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PROLONGING PRODUCT RELIABILITY


and product's entry into the marketplace. Also, if this kind of testing is performed at normal operational conditions, it is not likely to yield a statistically significant number of failures unless tens of thousands of products are tested. Pressures to get a product to market quickly mean that in reality the time available for reliability evaluation is very short. So, what are the alternatives when designing in reliability to products?


ACCELERATED LIFE TESTING & STRESS SCREENING


Not only do accelerated life testing and environmental stress screening give a level of confidence that a product will not develop faults after delivery, they also provide a process to identify any design defects, component problems or production related issues.


Accelerated life testing is based on using real-life operational data, trying to accelerate fault conditions by applying key operational failure-causing stresses at levels above those that the product would experience in its application environment.


The resulting data is a distribution of failure times, albeit at more stressful conditions than ordinary operating conditions, that must then be related to the distribution of failure times that would be anticipated under normal operational conditions. This would call for an accelerated life model to be created which is typically


characterised by a linear relationship between failure times at different sets of conditions.


The most common


failure-causing stresses that contribute to the


impairment of a product's reliability are temperature cycling, vibration and fatigue, and power cycling. For example, extremes of temperature can cause unanticipated product defects such as parts binding, overheating, distortion, malfunction, fire hazards and reduced product lifetime.


Spring 2024 UKManufacturing


By Rob Greenwood, senior manager at TÜV SÜD, a global product testing and certification organisation.


Temperature cycling induces stresses within a product due to differential expansion of components and materials. Extending the temperatures (both high and low) to which a product is exposed accelerates creep due to coefficient of thermal expansion (CTE)


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