Extending product reliability
03 August 2016
Product reliability testing has become an important part of the design process as end-users are increasingly demanding products that are more reliable, have a longer life and deliver value for money.
A good example is how the car industry has increased reliability through design that delivers drivers the benefits of longer service intervals and extended warranties.
Designers must therefore adopt a more robust approach to ensuring product reliability, which would suggest an associated hike in production costs. How can you ensure that products remain competitively priced but are more reliable?
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.
The traditional approach to reliability evaluation has been life cycle testing, which involves tests carried out within the product's ‘expected environment' or using actual operational conditions. However, this is an unrealistic approach. For example, if a five-year life cycle for a product is expected, a traditional reliability evaluation program 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 and product's entry into the marketplace.
Accelerated life testing and environmental stress screening have become increasingly accepted as methods of assessing product reliability. Not only do they give a level of confidence that a product will not develop faults after delivery, they also provide a process to identify any design defects or component problems.
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. It also requires the distribution of failure times to be related to those anticipated under normal operational conditions.
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 once the operation and destruct limits have been identified. This test method has been proven to expose design flaws within hours when traditionally this might have taken days or weeks.
As time to market constraints require accelerated testing that cannot guarantee 100 percent reliability and end-user behaviour cannot be predicted, reliability is difficult to guarantee. However, it is often brand reputation that sets one product apart from another, of which product reliability is a key element. 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.
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