QMT Features: October 2011
Extending product reliability
Product reliability testing has become increasingly important as cash-strapped consumers demand a long product life. By Jean-Louis Evans,
managing director at TÜV SÜD Product Service.


In today’s economic climate consumers demand value for money, a move away from delivery price which was often the sales mantra pre economic bust. This means that manufacturing a reliable product at an attractive price point has become a key competitive differentiator.

Manufacturers therefore need to adopt a more robust approach to ensuring product reliability, which would suggest an associated hike in production costs. How can manufacturers ensure that their products remain competitively priced but are also more reliable?

An early approach to evaluating the reliability of a product was design verification testing (DVT), also known as product assurance testing. This was a series of simple tests devised to show that the product would survive repeated use.
However, the problem with this approach is that it has to fully replicate the total estimated usage time in order to identify potential product failures. For example, if a five-year life cycle for a product is expected, a DVT programme could require 43,800 hours of testing. Not only would this be costly, it would also significantly delay time to market for a new product and lose the manufacturer any competitive advantage. Neither is this approach likely to result in a realistic number of failures unless tens of thousands of products are tested simultaneously.

If such full life cycle testing is not practical because of the high costs and time lag involved, what are the alternatives?

Accelerated Life Testing & Environmental Stress Screening
Accelerated life testing and environmental stress screening (ESS) are now recognised by major multinationals operating in Europe and around the world as legitimate product reliability test methods.

As failures are most likely to occur during the early use of a product, ESS has become an established part of the production process. This testing approach stresses the equipment during the early stages of its life, helping to identify defects that are caused during the manufacturing process.

The alternative approach of accelerated life testing tries to expedite fault conditions by applying key operational failure-causing stresses at levels above those the product would experience during real use. This often results in failures occurring earlier than if the units were tested at normal usage, or in some cases failures might arise that would not usually be seen. It is therefore important that these accelerated effects are taken into account when the test is formulated.

The key operational failure-causing stresses that contribute most commonly to the failure of a product are:

•    Temperature cycling – extending the temperature, (both high and low), to which a product is exposed accelerates stresses due to differential expansion of components and materials. The more extreme the temperature cycle, the higher the acceleration factor.
•    Vibration – this promotes mechanical failures and the deterioration of material strength, due to such cyclic stressing, is known as fatigue. If a product's normal operational vibration environment is known, then it can be accelerated too.
•    Power cycling – this is the act of repeatedly turning a piece of equipment off and then on again to check that an electronic device re-initialises its configuration and continues operating normally.

Putting a product through thermal and vibration stresses in combination with power cycling will accelerate a product's failure. However, it is vital to keep in mind that the test programme has to be devised and implemented in a way that should not damage the product due to extreme stresses.

The main benefit of accelerated life testing is that it helps detect the design flaws which are most likely to lead to a product's ‘infant mortality'. The disadvantage is that it may create some unrepresentative failures. Highly Accelerated Life Testing may provide the answer in such circumstances.

Highly Accelerated Life Testing
Highly accelerated life testing (HALT) was developed in the USA during the 1980s. HALT is actually an extension of accelerated life testing, but takes a more practical approach rather than a predictive one as the test levels used are not based on operational data.

There is a common misunderstanding that HALT has a tendency to lead to over-engineered products. This is not the case as a HALT appraisal allows designers to establish the limitations of their product designs and has exposed design flaws within hours when traditional test methods might have taken days or weeks. This testing methodology is therefore particularly suited to products in the development or prototype stage.

During HALT testing, thermal and mechanical stimuli are applied separately, and then together, in order to determine the operating and destruct limits of the item under test. A key difference between 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 product as a whole.

Once the operating and destruct limits of a product are identified, the next stage of defect analysis can be conducted. The operating limit is the point at which the unit remains operational, but any further increase in stress causes a recoverable failure. The destruct limit is the level at which the product stops functioning and remains inoperable. It is at this stage that all major flaws in the design should be exposed.

Don’t rely on 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 alteration, such as the use of different components, or a change in the manufacturing process, 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.

Survival of the fittest
Even if the most robust tests are performed, it is still very difficult to fully anticipate end-user behaviour. While reliability testing focuses on performance, it is not easy to predict what people might do with the product; often end-users ignore instructions, or use products in ways that designers would never have envisaged.

We are increasingly seeing standards that try to compensate for this element of ‘abnormal use’. In such a situation, tests are taken outside the laboratory to observe how the product might be used and how maintenance will be managed. However, in the rush to release a product onto the market this element is often overlooked if it is not a defined requirement in a standard.

On an increasingly technologically level playing field, it is brand reputation that sets one manufacturer apart from another. A significant part of reputation is a positive user experience, created through reliable products that satisfy the intended use and meet consumer expectation. Product reliability testing has therefore become increasingly important as cash-strapped consumers demand a long product life.

While time to market constraints require accelerated testing that cannot guarantee 100 per cent reliability, and end-user behaviour is unpredictable, such tests remain a fundamental element to successfully differentiate a product from a competitor’s by positively impacting brand reputation. l
email: info@tuvps.co.uk
www.tuvps.co.uk
  
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