Medical Device Highly Accelerated Life Testing: Everything You Need to Know
Highly Accelerated Life Testing HALT
HALT, or Highly Accelerated Life Testing was a term coined by Gregg K. Hobbs in 1988, describing an aggressive methodology of stress testing. It was intended to rapidly discover flaws in a device or part by subjecting it to stress levels well beyond expected limits, using extreme and rapid changes in vibration and temperature. The objective was to test until failure, repair the failure, then continue testing until destruction. By identifying these successive flaws, adjustments to the design could be made, making the device far more robust. While other methods can be used to identify such flaws, the benefit of HALT testing was the much greater speed of the process.
Medical device HALT testing
Hobbs originally developed and refined HALT methodology for military and aerospace parts, so how does one apply that method to a medical device? After all, a heart monitor is unlikely to withstand the same forces that a satellite or jet engine undergo.
“There is no single stress regimen that is good for all products,”
In an editorial published in the Sound and Vibration journal, Hobbs stated “There is no single stress regimen that is good for all products,” which brings us to another important point in HALT methodology: the method must be tailored to the device under test. In addition to this, while the best results are obtained by subjecting a device to all possible stressors at once, this often isn’t possible due to physical limitations of test equipment, of the device itself, or simply due to budget restraints. By reducing the number of stressors, we can further tailor the test to meet the needs of each client.
Although some would argue this veers into the territory of ALT, an Accelerated Life Test, tailoring the test like this can give equally valuable results.
When is a HALT useful?
Hobbs argued that a HALT is at its most valuable during the prototype phase, and the logic is sound: when weaknesses are discovered early on, they can be eliminated before the point where alternations to the design become difficult to implement. However, it is valuable at any point before shipping the actual product, as the costliest changes one can make are generally when devices are already on the market.
HALT is also valuable when designing or testing replacement parts, or when undergoing a change in supplier, allowing manufacturers to verify that the same stress levels are achievable after a change in parts.
HALT vs Service Life Testing
HALT and service life testing are often used interchangeably, but there are key differences between the two approaches. While both are accelerated tests, a HALT, as discussed, is a test to the point of destruction, whereas a service life test is intended to replicate usage over a specific length of time.
Service life tests tend to be performed with stresses within or just above tolerance, and often involve operating the device in a way that is as close to normal usage as possible, e.g.: a button is likely to be pressed once per use, and the device is expected to be used once a day, therefore it needs to be pressed 3,650 times to replicate ten years of use.
Used in concert, a service life test can be used to verify improvements made as a result of a HALT process. If the goal was to develop a device that is intended to last for ten years in the field, the service life test will rapidly replicate the stresses expected during that period: if it survives, functional and intact, then the improvements were a success.
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