Mission profiles are fundamental to any reliability prediction being valid. Without clearly defined environmental and use profiles there will be a long chain of inaccuracy that accumulates into significant errors within a product.
This is what occurs at the following stages, or tools, if the mission profile or environment profile changes after its completion.
- DFMEA: Failure modes may be excluded. Critical failure modes may be ranked low for occurrence and not be addressed
- DOE: Stresses may not be included in the matrix. Resulting singular and interaction failure modes will not be discovered in advance.
- Design validation: Design will be approved for a base use case when in fact it can not handle nominal stress and requires significant redesign in late stages
- Accelerated Life Testing: Life projections are in accurate by orders of magnitude
- Stress Margin Testing: Actual operational margins are less than projected. Variability in actual use margins pass the threshold and failure rate increases
- Warranty Projection: It’s not possible to have an accurate warranty projection without being able to trust all of the above.
What is the process for creating a correct mission profile?
- Define all stresses ( forces, loads) in a nominal use case.
- Identify which will be highly variable and which are expected to stay static regardless of use.
- How many “levels” can the customer base uses profiles be divided into? Two? Three? Five?
- Test the product in field usage and/or simulations
- Characterize these results in the use case levels
What is the process for creating a correct environmental profile?
- Define what are important environmental factors for the product (UV, heat, humidity, water submersion?)
- Categorize anticipated environments by level (Mild, Nominal, Harsh)
- List the stress to failure mode relationships for the environmental based stresses?
With this level of resolution on environmental and use stress any of the listed exercises listed above (DFMEA, ALT, Warranty Projection) will have an established foundation to be created upon. This assists with ensuring that the outputs of these activities stay relevant to the program throughout all phases.
Take the time to define “Use Case” and “Environmental Conditions” with the development team early in the program. It is part of your program foundation, ensuring planned activities have maximum influence on creating a reliable design.
Bo Eriksson says
Very good summary of the subject. Mission profile is so obviously needed that it sometimes gets difficult to explain why it is needed but I think you made a good job on that.
Fred Schenkelberg says
Thanks Bo, much appreciate he kind words. cheers, Fred
Adam Bahret says
It always surprises me how teams do not invest in this. The gains in having this established vs the small investmetn to create it is so clear.
we are currently working on an international standard for a mission profile data format.
Please feel free to check http://www.mpfo.org.