Test veteran describes fundamentals of shock and vibration measurement
Steve Brenner, an engineer who has contributed to reliability and design-verification testing on the Lunar Module and the Space Shuttle, discussed the basics of ESS, HALT, and HASS.
Greg Reed, Contributing Editor -- Test & Measurement World, 2/8/2008 10:57:00 AM
Steve Brenner, a consultant who has worked in the field of environmental simulation and reliability testing for more than 35 years, has contributed to reliability and design-verification testing on semiconductor manufacturing equipment as well as on the Lunar Module and the Space Shuttle. His experience spans Grumman Aerospace, the US Air Force, and Lockheed Missiles and Space. Brenner also spent 18 years with Kaiser Electronics in San Jose, CA, where he managed the environmental test lab and was involved with the design of hardware intended for severe environments.
In an exclusive interview, Brenner discussed the basics of environmental stress screening (ESS), highly accelerated life testing (HALT), and highly accelerated stress screening (HASS), and explained the importance of proper fixturing.
Q: What are the fundamentals of ESS, HALT, and HASS?
A: The first thing we need to do even before discussing fundamentals is to define these terms and put them in their proper place. HALT is a process that is implemented during the design phase of a project. It is used to identify areas in the design that may reduce the product's reliability. ESS is a process that is implemented on production hardware. The purpose of ESS is to identify any production units that have failures in them that would not normally appear until after shipment. HASS is one of many ESS methods.
In HALT, the environments of temperature extremes and vibration are used as "tools" to induce failures in a short amount of time that normally would not appear for weeks, months, or even years of real-world usage. The HALT process includes temperature step stress, extremely rapid temperature cycling (up to 60ºC/min), step stress random vibration in all six axes simultaneously, and combined environments. This process does not simulate service environments at all, but it has been shown to be very effective in accelerating the failures that would occur after a long period of normal service use.
ESS also uses temperature and often vibration as tools to make hidden failures appear so that those failures can be dealt with before the product is shipped. There are many different types of ESS being practiced, all with the same purpose. Almost all include temperature cycling, and are sometimes combined with sine or random vibration. The vibration is performed before, during, or after the temperature cycles.
HASS is a type of ESS that is performed using the same type of equipment and the combined environments cycles of HALT. It is usually much faster then more traditional types of ESS, and it has been shown to be very effective at inducing the hidden failures to appear quickly.
Q: What are the primary challenges you face while working with clients in the field?
A: It's sometimes hard to sell the concepts of using temperature and vibration as "tools" instead of simulations in HALT or ESS testing. People think if we are applying environments in excess of expected service conditions, we are overtesting, but we're really not.
Additionally, I often run in to managers who think along the lines, "If we design our equipment properly, we won't need to test." What they need to understand is that testing is an integral part of the design process. We've come a long way with computer simulations such as finite element analysis ( FEA) and modal analysis, and those techniques have discovered problems well before the testing, but there are some things that just won't show up until we apply an environment to the hardware.
Q: Vibration presents a challenge for land vehicles, helicopters, aircraft, rockets and spacecraft. How do test labs utilize shakers to simulate real-life conditions?
A: Actually, they usually don't, but they do their best. Since the advent of electrodynamic shakers, we have designed the shakers and fixtures to induce the vibration in a single axis at a time, which is not the way it actually happens on vehicles. Additionally, we design our fixtures so that the vibration input is the same at all test unit mounting points, which is also not the way it happens on vehicles. Another issue is that our fixtures are very stiff, and often the mounting of test units in vehicles is much more flimsy.
The next version of MIL-STD-810 will include a method for three-axis vibration, which begins to address these discrepancies. However, the vibration of hardware on electrodynamic (and hydraulic) shakers has been extremely successful in verifying designs that are robust enough to handle the dynamic inputs in vehicles. This tells us we must be doing something right, even if we aren't precisely simulating real- world conditions.
Q: How important is proper fixturing for vibration and shock testing as well as for ESS, HALT, and HASS?
A: I don't think there is any part of the vibration and shock test process more important than the fixturing. You can have the best shaker, the best control system and instrumentation, and the best test profile, but an inadequate fixture can cause overtesting, undertesting, or both, and you may not even know it. Hand in hand with good fixturing is correct control accelerometer location, which also is vital to get the right test.
I have seen many labs performing tests with the same equipment and setups for years, not realizing that they were not performing the tests they thought they were. To put it another way, their test units were not being subjected to the vibration parameters that were being measured.
For ESS fixtures, if vibration is being performed as part of ESS on an electrodynamic or hydraulic shaker, then the same requirements exist for each class of shaker. For HALT, things are a bit different. Since the purpose of HALT vibration is to stimulate as many of the test unit's resonances as possible, and since the mechanical vibration in a HALT facility is not very uniform anyway, some type of clamping mechanism is usually adequate. This would not be acceptable for electrodynamic of hydraulic shakers testing over 200 Hz.
HASS fixturing falls somewhere in between. Since HASS (as is all ESS) is part of a production process, throughput and uniformity are issues. HASS fixtures often hold multiple units and are often similar to electrodynamic fixtures.
Q: How are accelerometers used to measure vibration over the road, over the waves, in flight and during rocket launch powered flight?
A: Accelerometers are used to measure vibration response of structures to dynamic inputs. When used to record data in the field, the output of accelerometers is usually immediately digitized and recorded on digital tape, flash memory, or a hard drive. In cases of flight testing, the data is transmitted to ground facilities via telemetry. After being recorded, the data can be analyzed using many different types of equipment.
Q: Any final thoughts?
A: There is training available for the correct ways to perform dynamic and climatic testing, and getting people trained is the most cost-effective method I'm aware of to ensure properly run tests.
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