I generally write articles about topics I personally struggled to understand from the sources available to us, such as books, online resources, and so on. I believe most technical concepts are fairly straightforward at their core, but the way we express ideas and translate our understanding into writing often makes them harder for others to grasp. That is an area where we can all continue to improve.
As part of that journey, my goal with the Breaking Bad for Reliability newsletter is to be a communicator of Reliability Engineering principles, and I am doing this mainly for two categories of people:
- People who want to become reliability engineers but have minimal information about their responsibilities.
- People or companies who want to hire reliability engineers but do not have a clear understanding of what they actually need, or what skills to focus on in their hiring process.
The paragraph above should probably stay at the beginning of all my articles because that is my motivation: to give back to the community, help young engineers in this journey, and guide companies in their search for experts specific to their needs.
Here is another topic that I personally struggled with early in my career: Reliability Testing. When I first learned about life testing and modeling, such as Weibull, I thought that was the answer, so I tried to apply it to almost every problem I encountered. It was very similar to the saying, “If the only tool you have in hand is a hammer, everything looks like a nail.”
If the only tool you have in hand is a hammer, everything looks like a nail.
Later, I learned about reliability growth, burn-in, HALT, HASS, and so on. But as you learn more, the more confusing it gets. You need context to understand why we have so many different tests and how to decide when and why to apply them. What am I going to get out of these tests to inform design decisions? This is where experience surfaces.
If you are in the same boat, don’t worry, we will get that sorted out in this article.
What you will get out of this article?
My goal here is to explain the big picture of Reliability Testing — the WHAT and the WHY — rather than dive deep into individual techniques (the HOW). I will walk you through the most popular ones, their main purpose, and when to apply them, creating a practical reference for reliability and design engineers.
Reliability Testing
As I discussed in previous articles, Reliability Engineering ensures systems meet performance expectations consistently throughout their lifespan, incorporating time and failure probabilities beyond deterministic design principles, which usually focus on immediate functionality. It examines how systems deteriorate over time using probabilistic models. To learn more about various reliability engineering roles, you can read my Reliability Engineering 101 article.
To develop and verify these models, reliability testing is conducted alongside many other tests such as functional, environmental, and qualification tests throughout the product design phases. Various technical and program constraints necessitate employing a broad spectrum of testing techniques, as illustrated in Figure 1 from Adam Bahret’s book [1]. These techniques fall into three primary categories::
- Qualitative Tests (Improvement): Tests such as Highly Accelerated Limit Testing (HALT) identify unknown failure mechanisms and design margins, contributing to enhanced product robustness without offering specific reliability measurement. Another good example in this category is Highly Accelerated Stress Screening (HASS), which is used during the production phase.
- Quantitative Tests (Measurement): Focused on measuring product reliability using failure data, tests like Life and Accelerated Life Tests (ALT) provide statistical insights into current life expectancy and also support the development of effective maintenance programs.
- Hybrid Tests (Improvement and Measurement): By integrating improvement and measurement elements, methods such as Reliability Growth (RG) tests facilitate tracking reliability growth throughout design phases.
Choosing the right technique depends on its ability to inform specific design decisions and deliver valuable insights within the program’s administrative and technical constraints. Generally, 5 primary factors influence the selection of the most appropriate technique:
- Design Phase
- Physical Hierarchy Indenture
- Decisions Being Informed
- Risk Tolerance
- Resources and Testing Capabilities
I am going to explore each one of these factors in depth and provide several practical examples so that you can build a mental picture of how they might be implemented. After reading this section, I am confident that you will understand why the answer might be reliability growth and not a life test, for example.
Design Phase
Testing objectives evolve throughout the design process to address distinct needs. For example:
- In the early design phase, HALT is invaluable for uncovering design weaknesses, unknown failure mechanisms, and enhancing robustness, preparing the product for the next phases. It is also a good foundation for further life testing by surfacing primary failure mechanisms and margins.
- During the development stage, RG tests are crucial for identifying failures and tracking reliability trends, whether improving or deteriorating, allowing for the early correction of issues. Once the design stabilizes, failure tests such as Life Tests (including ALT) provide valuable data on life expectancy and guide further design refinements. They are also useful for developing effective maintenance programs and warranty periods.
- As the product enters the production phase, Highly Accelerated Stress Screening (HASS) is applied to quantify manufacturing-related issues. Unlike HALT, which identifies design limits, HASS uses the operating limits established by HALT to screen out units with manufacturing-related flaws. This technique should be used carefully for critical products with very limited life, such as reusable rockets, since even these screening tests can slightly stress the product and potentially impact its remaining life.
This phased approach incrementally enhances reliability from conception through production.
Physical Hierarchy Indenture
Incorporating hierarchy levels in the strategy ensures tests are conducted at the most appropriate stage of product integration, optimizing resource use and providing meaningful results:
- Component-Level Testing: HALT, ALT, and stress margin testing uncover weaknesses in materials and design before integration. Since components are generally cheaper than assemblies or subsystems, failure tests are usually more feasible here. However, life tests done at lower hierarchy levels miss interface-related failure mechanisms, which might pose a significant risk. Life tests, or test-to-failure experiments, at this level are particularly valuable for estimating life when higher-level testing is impractical.
- Assembly-Level Testing: RG testing at the assembly level evaluates integrated reliability, revealing failure modes not visible at the component level. Life tests at this stage can yield faster insights due to additional potential failure mechanisms such as interface issues, though at higher cost and facility complexity.
- System-Level Testing: Testing at this level evaluates overall product reliability in its fully integrated form. Life tests at this level can be highly beneficial for understanding total product life; however, they may be impractical due to cost and complexity. System-level testing is typically reserved for final validation under operational conditions, where Reliability Demonstration (RD) tests are particularly useful. RG testing can also be effective at this level to iteratively identify and mitigate failures, balancing insights with logistical and practical constraints.
Decisions Being Informed
Reliability tests are essential for data-driven design decisions at different phases, whether related to modifications, verification, or validation. Grounded in the principles of the bathtub curve, which illustrates changes in population failure rates over time, reliability tests guide design and process decisions through its three distinct phases, as shown in Figure 2.
Specific decisions informed by reliability tests may involve selecting materials and suppliers, diagnosing design and process flaws, determining product life and maintenance strategies, or assessing reliability trends and applying interventions.
- Life Test: Essential for understanding wear-out mechanisms, providing insights into life expectancy and preventive maintenance beyond what HALT can offer.
- HALT: Best for new designs with unknown failure mechanisms and margins, quickly identifying flaws and aiding material and supplier selection, though not offering quantitative reliability data.
- Reliability Growth Test (RG): Tracks whether reliability is improving or deteriorating over design phases. RG analysis generally uses data from development and qualification tests rather than being a standalone test.
- Reliability Demonstration Test: Confirms that products meet specified reliability requirements under controlled conditions, critical for transitions from development to production.
Risk Tolerance
The complexity and cost of reliability tests necessitate a strategic analysis to balance potential returns with the need for these tests. The selection process is primarily guided by two critical risk factors:
- The mission-critical nature of the item
- The level of uncertainty about its reliability
Efforts should focus on eliminating unnecessary tests, as some, like Life Tests, require substantial time and resources. Conducting such extensive testing should only be justified if the item is critical and there is significant uncertainty about its reliability. This uncertainty can arise from changes in one or more of the following areas:
- design changes
- environmental condition changes
- operational load changes
To rationalize the reduction or elimination of costly tests, historical operational and failure data may be leveraged, provided there are no recent changes in design, environmental, or operational contexts that could impact reliability. However, in cases where such changes have occurred or when historical data is insufficient, comprehensive testing becomes more justifiable.
Resources and Testing Capabilities
Project, technical, and facility constraints shape testing plans, influencing scope and frequency. Strategies must adapt within program limits, considering:
- Time and Budget: Schedules and costs limit the number and duration of tests. Stress acceleration such as ALT can help when timeframes are short.
- Technical Expertise: Skilled personnel and strong analytical capabilities enable more complex testing.
- Test Facilities: Access to appropriate equipment is essential, and limitations here can affect feasibility and schedule.
In Summary
There are many reliability testing techniques available within the Design for Reliability Process toolkit. What really matters is not just knowing that these tests exist, but understanding which design decision each one is meant to inform — which, in turn, defines when the test should be conducted. Information that comes later than needed does not add any value. Experienced engineers recognize that the value of a test comes from the clarity it brings to decision-making, not just from running the test itself.
References / Resources:
[1] Bahret, A. (2024). The Perfect HALT Test: Improving Your Product’s Robustness. Apex Ridge Publishing.
Leave a Reply