Ensuring That Products Will Work

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  By Tom Lecklider, Senior Technical Editor, March 2008

If all products were operated in controlled, benign conditions, there would be little need for environmental testing laboratories. When used in the real world, equipment is exposed to shock, vibration, temperature extremes, dust, humidity, and many more factors that affect performance. As part of a thorough design process, manufacturers subject products to simulated stresses to ensure that specifications will continue to be met in the actual use environments.

The nature of the product determines which tests are most important. For example, an engine air filter intended for rugged military applications would need to withstand severe sand and dust tests in addition to mechanical shock and vibration. If the filter were made of metal, it also might require salt spray and high humidity tests to prove it could endure these conditions for prolonged periods of time.

Because each kind of product is different and is used differently, many types of environmental tests are available. Several of these capabilities are demonstrated in real applications submitted by three accredited testing labs.

Military-Grade Vibration Testing

Standex Electronics, a global manufacturer of custom magnetics, reed switches, and sensors for the medical, military, industrial, and automotive markets, must meet stringent customer and regulatory requirements. Although it operates its own product-line testing equipment, the company also relies on expert, accredited testing services for time-sensitive, large-capacity, and unique scenarios.

To provide rapid, complete military-grade transformer vibration testing for an aerospace application, Standex turned to ESSC Test Laboratory. The lab has 20 years of experience and is ISO/IEC-compliant and accredited by The American Association for Laboratory Accreditation (A2LA).

The transformer test required precise equipment and customized fixturing. Successful testing and useful reporting hinged on how the parts were attached to the vibration table and how well the behavior was analyzed.

Each transformer weighed less than a pound, and 12 were tested at the same time (Figure 1). An LDS 750 System Shaker with a 6,000-lbf rating was used. It was coupled to a slip plate with a 19"x 21" interface plate to which were bolted two hollow aluminum cubes. These fixtures provided convenient mounting locations for six test boards, each with two transformers and related components.

Figure 1
Figure 1. Standex Transformers Similar to Those Tested ??
Courtesy of Standex Electronics and ESSC Test Laboratories

ESSC performed the test in accordance with MIL-STD-202G, Method 204D Vibration, High Frequency. The test entailed 20-minute logarithmic cycling through the 10-Hz to 2,000-Hz frequency range, repeated 12 times over a 12-hour period, with the transformers oriented in each of three directions. These requirements correspond to Method 204D Test Condition E, which applies a peak-to-peak displacement of 0.06" from 10 Hz to about 127 Hz. For frequencies above 127 Hz, the displacement is reduced to maintain a peak 50g acceleration (Figure 2).

Figure 2
Figure 2. Vibration Test Graph
Courtesy of Standex Electronics and ESSC Test Laboratories

Method 204D supports testing with components de-energized or operating under specified load conditions. In this case, Standex elected to test the transformers together with related electrical components in a de-energized state.

For temperature and humidity testing, Method 106G was followed. It requires cycling five times among 25°C @ 95% humidity, 65°C @ 95% humidity, and -5°C, dwelling from three to eight hours at each temperature.

The customer specified three temperatures for thermal shock testing: 130°F, 155°F, and 170°F. The units were soaked at each temperature for 24 hours with inspection every eight hours.

According to Method 107G Thermal Shock, the transformer temperature was reduced to 25°C in less than five minutes after soaking at a high temperature. Similarly, the transformer temperature was increased from room temperature to the required maximum, also in less than five minutes. In contrast to these tests run in air at different temperatures, an almost instantaneous thermal shock can be created under Method 107G by using liquid immersion. The customer chose air in lieu of immersion.

Because ESSC is a division of Cincinnati Sub-Zero, the manufacturer of a long line of temperature-based products and thermal testing chambers, a great deal of practical environmental test experience is available. The lab delivers tests specially suited for design verification, product performance, regulatory compliance, failure analysis, life cycle, and environmental stress. In addition, ESSC uses the latest software to produce follow-up reports that document and analyze testing for customer and regulatory purposes.

Vibration Test Fixturing for Small Parts

Designing and building a product mounting fixture can be one of the most difficult aspects of a vibration testing project. Fixturing for vibration testing is intended to provide good transmissibility. This requirement may be compromised by the needs to secure the unit under test without damaging it and to access it easily for assembly and removal. This is especially important when high-volume testing is required. In addition, the fixturing has to be built quickly and inexpensively.

Chris Finch, technical sales manager at Trace Laboratories-Central, explained that traditional vibration fixtures are made by drilling and tapping aluminum cubes or plates and bolting down the product via its built-in flanges. If the unit doesn't have built-in mounting holes, more creative methods are necessary.

One method commonly used to secure test samples clamps the unit to an aluminum plate. Typically, the clamping fixture is built by drilling and taping holes in the plate and inserting threaded rods. An aluminum bar then is used to create the clamp, with protective foam between the samples and the fixture. Another method is to double-stick tape the product to the shaker table. Each method is application specific and has cost and efficiency advantages.

There are instances when traditional vibration fixtures are not an effective means of mounting a test unit. Fixturing issues can occur when conducting vibration testing on oscillators, transceivers, Ethernet adapters, flash drives, and similar components.

Often, these units are small, and many are tested at a time. They are round or have nonparallel sides that can be crushed if not handled properly. These components typically do not have mounting holes and cannot easily be attached to an aluminum plate. Using the sandwiching method will not work because the components can be damaged or slip out due to their nonstandard shape.

Instead, Trace Laboratories uses paraffin wax to rigidly mount these multiple, small, hermetically sealed units to the vibration equipment. Paraffin wax is pliable, inexpensive, and easy to use. It also is noncorrosive and an extremely good electrical insulator with an electrical resistivity of at least 1013 Ω-m.

Multiple test units can be placed into a block of paraffin wax, and the wax is placed onto the vibration adapter plate. The vibration inputs of the shaker system are directly transferred to the test units. Paraffin wax allows the vibration testing to be conducted without the significant time and expense of complicated mounting fixtures or fear of damaging fragile components.

Although some laboratories still are using double-stick tape, paraffin wax is a better alternative. Double-stick tape can provide quick, inexpensive mounting of lightweight components; however, it cannot be used for all product geometries and is difficult to remove. And, while double-stick tape holds a unit by one surface, a unit can be embedded into the wax, allowing for better support and vibration transmission.

Upon completion of the vibration, the units can be manually removed and easily cleaned. The paraffin wax is removed by melting it with a heat gun.

Removal and cleaning can be conducted quickly, and additional samples can be under test in a few minutes. As long as it remains clean, the wax can be used repeatedly.

There are some considerations when working with paraffin wax. The testing should only be conducted at room temperatures because paraffin wax can begin melting at temperatures as low as 47°C. If the units are powered and monitored during the testing, be aware of their heat generation. In addition, the samples must be able to withstand 47°C during removal of the wax.

Paraffin wax should only be used on hermetically sealed units since the wax may be difficult to remove from part openings or crevices. Care must be taken when performing shock test fixturing because the wax may not properly hold the units during high impact.

Always ensure proper transmissability by mounting a response accelerometer to the test units. As a rule of thumb, only consider paraffin wax mounting for components that are too small or delicate to be mounted conventionally. Much of the experience at Trace Labs has been with very lightweight ICs, as shown mounted in paraffin wax in Figure 3.

Figure 3
Figure 3. ICs Mounted in Paraffin Wax for Vibration Testing
Courtesy of Trace Laboratories                                                     

Explosive Pressure Pulse Simulation

A customer recently asked Aero Nav Laboratories whether simulation tests could be performed to determine the survivability of a piece of equipment when subjected to explosive pressure pulse blasts. The equipment was designed to be robust and expected to survive moderate levels of pulse blasts like what would be experienced in the survivability zone of an explosion. This zone is defined as the area where the explosive effects are less severe than would be seen at the point of inception of a blast.

Sheldon Levine, Aero Nav Laboratories vice president of marketing and business development, explained that the lab is not equipped to perform testing using actual explosives on-site and, furthermore, is limited by municipal regulations. Another means of achieving the test objectives was required.

A blast tube was constructed using a heavy-walled steel pipe open at one end and equipped with a pressurized air chamber at the other (Figure 4). A valve was provided to rapidly release the entrapped pressure within the chamber. The equipment under test was fixtured to simulate its actual mounting and installed in the pipe. The open end of the pipe was set up with a converging section to maintain the pressure and velocity of the air as it traversed the length of the pipe.

Figure 4
Figure 4. Blast Tube
Courtesy of Aero Nav Laboratories

When an air-blast event occurs, two effects will exist: peak pressure in the wave front and drag loading. The peak pressure is the maximum pressure in the wave front as it passes the test item. Drag loading represents the effect of the bulk of the air mass moving past the test item. Drag loading is the same as that encountered by an object moving in a fluid stream.1

In a ground-level explosive event in an open field, the explosive effects radiate outward in a semihemispheric manner. However, when constrained within a long, relatively small-diameter pipe, the effects are directed axially outward toward the open end. Consequently, the equipment under test in the tube experiences the full effect of the sudden release of pressure and the rapid movement of the air past the unit.

An idea of the magnitude of the pressure created by an explosion can be gained by reviewing several documents on the U.S. Department of Energy website. For example, Appendix B of a 1995 Defense Nuclear Facilities Safety Board Memorandum presents an interesting comparison of the pressures that might be developed by burning hydrogen with oxygen vs. exploding a hydrogen-oxygen mixture.2

Burning a confined hydrogen-oxygen mixture was calculated to produce a pressure about 10x higher than atmospheric. A containment vessel must withstand twice that value to account for reflected pressure. However, an actual explosion with reflected shock waves would create a maximum pressure greater than 122 atmospheres or greater than 1,800 psi.

The peak pressure at the site of an explosion can be extremely high. In contrast, the testing carried out by Aero Nav Laboratories simulated a lower peak pressure and drag loading some distance from the blast site in the survivability zone.

For the device being tested, a blast tube was considered to provide a sufficient simulation of the actual explosive event. The pressure buildup and subsequent decay in the blast tube were measured by a pressure transducer placed on the upstream side of the equipment under test. Figure 5 presents a typical trace showing the pressure vs. time.

Figure 5

Figure 5. Simulated Air Blast Pressure vs. Time
Courtesy of Aero Nav Laboratories

The pressures, usually shown as psig, are presented as percentages of peak pressure since the actual values are considered proprietary. Rapid buildup of pressure in an almost linear manner is followed by a slower, more gradual drop-off.

The entire event takes place in slightly more than 400 milliseconds. The rise of pressure to its peak level occurs within 100 milliseconds. The peak pressure would be much higher very close to the actual site of a blast; however, the pressure pulse shown in Figure 5 corresponds to expected conditions in the survivability zone.

Multiple test runs were performed at various peak pressures. The equipment under test was found to have survived the effects of the air blasts with no physical damage or functional degradation.

Summary

If it's done correctly, simulated environmental testing can quickly prove a design's value or lack thereof. A product designed to survive the environment in which it will be used won't be damaged by testing. On the other hand, marginal test results or outright failure should set off warning bells.

Achieving a good correlation between simulated tests and the actual use environment is critical. As the Aero Nav Laboratories example showed, the actual test apparatus doesn't need to resemble the real environment as long as the values of important parameters are similar. In this case, peak air pressure and drag loading had the greatest effect on the product being tested. The tests were meaningful because the simulated quantities sufficiently matched real values expected in the survivability zone.

Sometimes, successful testing relies on a lab's experience and ingenuity. Because they know how a test should be run, seasoned test engineers can easily spot errors. For example, resonance caused by a loose fixture could be confused with resonance in the product itself. This problem and others related to fixturing are compounded when the product being tested is very small, delicate, or an odd shape. Trace Labs' use of paraffin wax seems an obvious solution, but it's a good bet not all test engineers are familiar with it.

Beyond technical test capabilities, sometimes customers expect a test lab to help solve business-related issues such as tight delivery schedules or unanticipated large orders. These kinds of considerations were a factor in Standex's decision to contract ESSC Test Labs for transformer testing.

What kinds of environmental testing do your company's products require? They may not be as extreme as a simulated explosion or as extensive as those called for under MIL-STD-202G. Nevertheless, whatever is needed, it's likely that standardized tests already exist and a number of labs have the necessary equipment and experience to perform them.

References

1. Glasstone, S. and Dolan, P.J., The Effects of Nuclear Weapons, Third Edition, 1977.
2. Cunningham, G. W., "Defense Nuclear Facilities Safety Board Memorandum," 1995, www.hss.energy.gov/deprep/1995-2/tr95m27a.pdf

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