Can Reliability Be Tested and Measured?
August 1st, 2017
At Elite, we’re often asked this reliability question and the answer, of course, is “yes… but it depends”. It depends on the complexity of the product, the design and manufactured robustness of prototypes, the availability of samples, and many other related factors.
Reliability metrics are widely used in the electric power industry, railroads, and airlines where operators collect information on their equipment for usage, uptime, and performance. They use this information to develop statistics that can help them anticipate product lifetimes and plan for maintenance and replacement of parts and critical systems. The reliability statistics are used for systems already operating in the field and whose conditions can be tracked. But it’s a bit more challenging to generate reliability metrics when the information is based solely on testing prototypes in a lab and especially on complicated electronic products.
At Elite, the more common reliability programs are success-run tests on electronic modules, test-to-failure for simple components, and accelerated test-to-failure for mechanical parts. These test programs are performed to determine life-on-test. Then, based on this information, metrics are calculated that express expected reliability over time. The reliability performance is generally stated in terms of parameters such as "parts per million," FITs, or MTTF, or R(t) connected to a relevant life distribution.
In the automotive industry, the design validation phase for electronic modules includes over-stress testing, time-based endurance tests, and fatigue tests. The overstress tests are mechanical shock, water spray, ESD, along with others. These tests are applied to simulate field conditions or high, short duration stresses which confirm that the products are capable of handling occasional extreme environments. Cumulative damage tests such as those associated with temperature and vibration stresses are run at levels below the over-stress amplitudes but above endurance limits. These temp and vibe tests accelerate failure mechanisms that degrade product performance over time. When an electronic module completes an automotive success-run program it will have demonstrated reliability metric “R” with an associated confidence interval (C) - for example, reliability 97% (R97) with a confidence interval of 50% (C50).
Mechanical components such as motor-driven actuators, levers, doors, knobs, or push buttons can accumulate years of operation and many cycles of repetitive stress. These components may be exposed to rotational motions or linear displacements that apply wear, fatigue, or other time-based degradation conditions. Accelerated testing for mechanical products involves applying stresses at one or more high levels until failure occurs. Based on failures from accelerated testing, a stress-life curve can be drawn and analytical tools such as Weibull analysis can be used to extrapolate the life of the product under test to live at normal conditions.
What can Elite do to support clients with reliability testing?
We work with clients from start-to-finish to quantify reliability using accelerated testing and we help clients generate a metric that will predict life with statistical confidence.
For mechanical reliability evaluations the Elite process begins with analyzing the product, its environment, and the anticipated life stresses including those applied in shipping, handling, and operational use. From there a Failure Modes and Effects Analysis (FMEA) is prepared to define the failure conditions, failure mechanism, and relevant stresses to be applied. Next a reliability test plan is created to identify the sample size, applied stresses, and test duration. The reliability test plan also provides instructions for dealing with minor faults and end-of-life failures.
Next, Elite’s Machining, Automation, and Controls team creates the mechanical or electrical cycling rig that will produce the motions and stresses that the product will experience throughout its life. Elite test engineers work alongside our Machining, Automation, and Controls team to ensure the life stresses are applied properly and that the automation equipment operates reliably throughout the entire duration of the program.
The results from our life testing are captured by Elite test engineers and the test items are evaluated by our failure analysis partners. The analytical results confirm the failure modes and mechanisms. These data, along with test duration for each part, are entered into calculations that establish the life distribution.
At the conclusion of testing, the output from the reliability tools (i.e. Weibull analysis) along with the failure analysis findings provide manufacturers a comprehensive assessment of reliability and a meaningful metric to support contractual requirements for reliability demonstration. The information can be used to validate designs, or to target design modifications that will improve life in the field and ultimately reduce warranty returns and manufacturing costs.
If you are an equipment manufacturer and your contractual requirements or corporate quality system specifies a quantifiable reliability metric, then contact Elite today. Our engineers can provide turn-key solutions for test planning, automation, test execution, and analysis.