The definition of reliability includes four elements.
One of them is the intended environment where the device or system will experience a range of stresses.
The knowledge of where and how an item will operate enables:
- The proper design to meet customer expectations
- The essential durability during transportation, storage, and use
- The complete accounting of applied stresses and their variations
- The effective application of stress mitigation or reduction techniques
- The efficient installation and monitoring of stress tracking
The operation or function of an item occurs in some local.
The set of environmental stresses in your office differs from the environments found under the hood of a vehicle or the exterior of an orbiting communications satellite. Part of the initial design process, and later the operation of a product includes an intimate knowledge of the types and amounts of applied stresses.
We focus on the externally applied stresses which may impact the performance or the longevity of the item.
Not all stresses are harmful nor degrade an items ability to function.
Each product has a different set of stresses that require detailed understanding.
Types of stresses
The common weather set of stresses include:
- Temperature, high, low, transients, cycling, and shock changes
- Solar Radiation
- Rain/Snow/Sleet – participation
- Salt Fog
- Icing/Freezing Rain
- Driving Rain
- Hail Stone strikes
Beyond weather conditions, other environmental stresses include:
- Sand and Dust
- Shock, Pyroshock, Ballistic Shock, Gunfire Shock
- Acoustic Noise
- Contamination by air or fluids
- Smoke, gasses, particulates
- Explosive or Acidic Atmosphere
- Freeze/Thaw Cycling
- Power quality (line voltage, current, and transients)
- Electrostatic discharge (ESD)
- Electromagnetic radiation immissions (EMI)
- Lighten Strikes
- Radiation: alpha, beta, or gamma
Yet other environmental stresses may apply.
Consider where the product is produced, stored, how transported, installed, used, and disposed of, for the full range of stresses that item may experience.
A subset of those stresses which may impact the ability of the item to function and do so over time is the set to fully characterize.
Types of use factors
The use of an item also places stress on an item.
Let’s consider an inkjet printer for home or small office use.
It may print full-color images once a month or every day. It may only print an occasional school report or a full set of photos from a recent trip. I may experience regular dusting, cleaning, and maintenance, nor none.
The frequency of use, the duration of each use, the type of paper used, and so on, impact the ability of the printer to function and do so over time.
Generate a list of use factors by considering each phase in the existence of a product, plus the different type of use cases or situations. Beyond understanding the weather conditions to expect during storage, how long will the storage last?
The set of stresses experienced by a motor is quite different from continuous 24/7 operation versus sitting on a storeroom shelf as a spare part.
Look at the day in the life of your product
Some product experiences natural variation in use due to loading or demand place on the item.
An expresso machine during the morning rush is in operation nearly full time for a few hours, while late in the afternoon it may only operate once an hour. The cycling of use and maintenance/cleaning applies different sets of stresses on the product.
Desktop and laptop computers often include a sleep function when the system is idle for some amount of time. The transition to sleep mode shuts down many functions within the system thus reducing the load on the electronics, hard drive, and power supply.
The mini-cycles of temperature cycles may be as damaging as the power on/off cycles it experiences each day.
For handheld or transported items, dropping can be a regular part of the expected use or a rare accidental experience. How often and from what height will the drop occur and on to what kind of surface?
Strikes or impacts, sitting or stepping on an item, likewise maybe regular occurrences or rare.
Consider the day in the life of our product, view the world of stresses as experienced by your product. Consider the different types of applications and customers.
What percentage of the customers are light or heavy users, for example?
The list of use factors that impact the functionality or reliability provides a set of factors to characterize and understand.
Characterization of stresses and use factors
Think how the information concerning a stress assists in the design, verification, and validation of an item. The richness of the stress characterization permits:
- The understanding nominal and the range of stress values
- The ability to make tradeoff calculations concerning different stress conditions and functional or reliability impact
- The ability to design to specific safety factors for specific scenarios and situations
- The ability to select materials, components, construction & assembly techniques appropriately
- The ability to conduct simulations with appropriate range of stress conditions
- The ability to model failure mechanisms using physics of failure models
- The ability to perform stress/strength analysis
Listing the stress and use factors with only nominal values does establish the baseline of understanding, yet doesn’t provide the necessary information for most use of the environmental data.
Including the minimum and maximum values for the expected stresses and use factors, provide only the limits.
Using the worst case set of environmental stresses it is not always feasible nor economical to do so. Creating a product able to withstand the maximum temperature exposure, for example, may add significant cost for the thermal management of the item, wherein the vast majority of the use situations the item will not experience harsh high temperatures.
Characterization may include a histogram or distribution describing the variation of stress values. This is particularly useful for stress/strength calculations.
You may find a suitable tabulation of weather data using the NOAA and other weather station reading databases. Another method to gather the appropriate data is to measure the stress values directly over time.
A set of sensors and a data logger may provide the appropriate data quickly for a range of different situations and use profiles.
Some systems include stress and use monitoring functionality. An hour meter on an industrial motor, the odometer for a vehicle, the moisture indicator on an iPhone, provide a means to collect stress data.
Some systems include sensors and algorithms to collect data and to monitor performance. These complex system may provide warning of the need for maintenance.
My car does this in part based on miles driving, yet also considers other stress factors.
One note of caution
A set of test or protocol conditions is not a characterization of environment stresses or use factors.
It is a test that someone created to evaluate an item, technology for its ability to operate under those conditions. 168 hours at 85°C with three samples, may represent a specific situation for your product, or most likely will not.
Passing a standard environmental test may or likely may not have any significant meaning. If your product only experiences 168 hours at 85°C then the test represents the expected temperature stress experience for the product and provides a meaningful result.
Be sure to relate any environmental testing to the associated characterization to better understand how to interpret the test results.
How every you collect the data, report the data in a manner useful for your team to make informed decisions.
What is your favorite environmental stress factor (mine is fire ants)? How do you characterize the environment your product will experience?
Do you know of public data on specific stresses? Please comment and add to the information in this article below.
The Environmental Test Manual (article)