Reliability Engineering - Design For Reliability

Design For Reliability

Reliability design begins with the development of a (system) model. Reliability and Availability models use block diagrams and fault trees to provide a graphical means of evaluating the relationships between different parts of the system. These models may incorporate predictions based on failure rates taken from historical data. While the (input data) predictions are often not accurate in an absolute sense, they are valuable to assess relative differences in design alternatives. Maintainability parameters, for example MTTR, are other inputs for these models.

The most important fundamental initiating causes and failure mechanisms are to be identified and analyzed with engineering tools. A diverse set of practical guidance and practical performance and reliability requirements should be provided to designers so they can generate low-stressed designs and products that protect or are protected against damage and excessive wear. Proper Validation of input loads (requirements) may be needed and Verification for Reliability "performance" by testing may be needed.

One of the most important design techniques is redundancy. This means that if one part of the system fails, there is an alternate success path, such as a backup system. The reason why this is the ultimate design choice is related to the fact that high confidence reliability evidence for new parts / items is often not available or extremely expensive to obtain. By creating redundancy, together with a high level of failure monitoring and the avoidance of common cause failures, even a system with relative bad single channel (part) reliability, can be made highly reliable (mission reliability) on system level. No testing of reliability has to be required for this. Furthermore, by using redundancy and the use of dissimilar design and manufacturing processes (different suppliers) for the single independent channels, less sensitivity for quality issues (early childhood failures) is created and very high levels of reliability can be achieved at all moments of the development cycles (early life times and long term). Redundancy can be applied in systems engineering by double checking requirements, data, designs, calculations, software and tests to overcome systematic failures.

Another design technique to prevent failures is called physics of failure. This technique relies on understanding the physical static and dynamic failure mechanisms. It accounts for variation in load, strength and stress leading to failure at high level of detail, possible with use of modern Finite Element Method (FEM) software programs that may handle complex geometries and mechanisms like creep, stress relaxation, fatigue and probabilistic design (Monte Carlo simulations / DOE). The material or component can be re-designed to reduce the probability of failure and to make it more robust against variation. Another common design technique is component derating: Selecting components whose tolerance significantly exceeds the expected stress, as using a heavier gauge wire that exceeds the normal specification for the expected electrical current.

Another effective way to deal with unreliability issues is to perform analysis to be able to predict degradation and being able to prevent unscheduled down events / failures from occurring. RCM (Reliability Centered Maintenance) programs can be used for this.

Many tasks, techniques and analyses are specific to particular industries and applications. Commonly these include:

• Built-in test (BIT) (Testability analysis)
• Failure mode and effects analysis (FMEA)
• Reliability Hazard analysis
• Reliability Block Diagram analysis
• Fault tree analysis
• Root cause analysis
• Sneak circuit analysis
• Accelerated Testing
• Reliability Growth analysis
• Weibull analysis
• Thermal analysis by Finite Element Analysis (FEA) and / or Measurement
• Thermal induced, shock and vibration fatigue analysis by FEA and / or Measurement
• Electromagnetic analysis
• Statistical interference
• Avoidance of Single Point of Failure
• Functional Analysis (like Function FMEA) and functional Failure Analysis (FHA or FFA)
• Predictive and preventive maintenance: Reliability Centered Maintenance (RCM) analysis
• Testability analysis
• Failure diagnostics analysis (normally also incorporated in FMEA)
• Human error analysis
• Operational Hazard analysis /
• Manual screening
• Integrated Logistics Support

Results are presented during the system design reviews and logistics reviews. Reliability is just one requirement among many system requirements. Engineering trade studies are used to determine the optimum balance between reliability and other requirements and constraints.