Test like you fly.(ENVIRONMENTAL TEST)

Although test-like-you-fly directly relates to aerospace projects, the underlying intention is to mitigate risk by running the most appropriate tests. In that sense, the phrase applies more broadly. Skipping or compromising on any part of a test, even one that may seem to be inconsequential, has led to mission failure. So, it is no wonder that test-like-you-fly has gained a special meaning among mil/aero professionals.

It seems obvious that environmental testing should expose equipment to the conditions likely to be experienced in service. However, determining the actual deleterious process occurring in a real environment may not be straightforward. Temperature extremes might cause less damage than a rapid rate of change, and high levels of vibration could be less important than dwelling at a specific resonance.

Many other environmental factors are less well defined than temperature and vibration. Nevertheless, the confidence with which equipment can be put into service directly relates to its environmental test performance, assuming that the tests are truly representative.

In addition to ensuring that the DUT functions properly, testing also must verify its modes of failure. This is especially important when failure results from exposure to fire and has security or safety implications.

Fire

For example, network equipment building system (NEBS) fire tests are designed to ensure the continued operation of a telephone exchange. These tests are distinct from those required to prove operational functionality within the telephone network. Equipment intended for use in an exchange must comply with strict design guidelines, including the use of self-extinguishing materials. A rack of equipment must have sufficient baffles and separate chassis to minimize the spread of flames from any part of the rack to the rest.

There's clearly a security risk associated with telephone exchange fire damage but typically a small safety risk. In contrast, aircraft or shipboard fires endanger the lives of all on board if not quickly brought under control. Extensive fire testing of fuel lines and hydraulic hoses is performed to confirm their capability to continue functioning while exposed to flames.

It's especially important that the rate of degradation allows sufficient time to extinguish the fire before the fuel or hydraulic fluid ignites. To verify hydraulic-hose performance, AERO NAV Laboratories designed and constructed the fire test chamber shown in Figure 1. Several propane gas burners can be used, the number depending on the size of the DUT.

[FIGURE 1 OMITTED]

Sheldon Levine, the lab's vice president of marketing, described typical test requirements, "Fire-resistant hydraulic hoses must be subjected to direct flame impingement while flowing fluid under specified flow and pressure conditions. The hose is subjected to the flames for a period of 30 minutes, a time sufficient to allow the fire to be sensed and extinguished. The hose must withstand the flames while maintaining pressure without leaking. In addition," he concluded, "it must pass a hydrostatic pressure test after the fire test."

Lloyd's of London Flexible Hose Fire Test OSG/1000/499 requires that a hose withstand 705[degrees]C (1,300[degrees]F) for 30 minutes at rated working pressure. Typically, testing to this Lloyd's specification is performed on hydraulic hose used in marine applications such as offshore drilling rigs as well as ships.

High-pressure hydraulic hoses are made with several layers of steel spirals that provide strength while maintaining flexibility. The innermost tube often is synthetic rubber, and adhesive layers may be applied between each steel layer with an overall protective sheath providing abrasion resistance. For this type of hose, a static pressure fire test is the most stringent because heat conducted from the flame to the central tube is not removed by the flowing fluid. Mr. Levine said that the fire chamber also was used to test fluid-line gaskets in a no-flow condition.

In a 1976 article about brazing the titanium alloy tubing used in Concorde's hydraulic system, the author described the direct-flame tests applied to materials and components within the engine bays. This environment is quite different to the rest of the airframe, which was expected to operate at slightly elevated temperatures because of the plane's high speed.

For the engine bays, components "are required to withstand a 'torching flame' test that simulates the conditions that can arise should perforation of an engine combustion chamber occur while the engine is delivering thrust, projecting a searing tongue of flame into the engine bay until engine shutdown is achieved .... [These] tests are carried out on typical assemblies that include both mechanical and brazed joints, pressurized to the working pressure (4,000 lb/in. (2)) with the appropriate hydraulic fluid under no-flow conditions to offer no benefit from cooling flow and to equate to the most severe conditions that can occur in service." (1)

The paper goes on to discuss the unsatisfactory results of initial tests and the subsequent change to a higher melting-point, gold-bearing brazing alloy that could withstand prolonged exposure to such extreme temperature.

Today's electronic assemblies seldom are subjected to direct flame tests, but dust and salt spray are among the many types of environmental tests often applied, especially when qualifying mil/aero and automotive equipment. Each of these tests has a long history beginning with actual dust and salt spray as the names suggest. However, over the years, several test variants have been created to more precisely address a range of conditions.

Dust

Completely sealed equipment is capable of working well regardless of the dust level in the environment. Of course, this is not the case for equipment that relies on external air and fans for cooling. Filters generally are used, and they eventually become clogged unless regularly cleaned.

Air filters also are used on internal combustion engines, and it was in conjunction with AC Spark Plug engine filter design that dust testing began. Arizona road dust, actual dust from the Salt River Valley, originally was used. (2)

A large percentage of extremely fine, highly abrasive particles was the feature most concerning about Arizona dust. Before a standardized production process was developed, a canvas cloth collected airborne dust from behind or around tractors. A comparison of Salt River Valley and Imperial Valley California dust is shown in Table 1. The large difference in particle size distribution is obvious.

From 1982 onward, Powder Technology (PTI) became progressively more involved in the production of test dust to Society of Automotive Engineers Air Cleaner Test Code Specification J726. In 1992, AC Spark Plug ceased test dust production, hiring PTI to fulfill outstanding orders. And, in 1997, ISO 12103-1 Road Vehicles--Test Dust for Filter Evaluation was published listing four grades: ultrafine, fine, medium, and coarse.

PTI literature describes each of the test dust grades in detail, comparing them to AC Spark Plug dust and listing suitable test applications. For example, ultrafine test dust is nominally smaller than 10 microns in size and used as a test contaminate for fuel-system components and water-filter performance evaluation.

Trace Laboratories recently conducted a test that simulates dust infiltration occurring in the working environment. Chris Finch, the lab's technical sales manager, described the situation: "A customer that makes joystick controllers for earth-moving equipment required that the product be life-tested for 5,000,000 cycles while subjected to a dust-filled environment. During normal dust testing, you place the item in the chamber for a set period of time without cycling the unit under test but while agitating the dust and maintaining a set concentration level. Trace's dust durability test setup includes a Lab VIEW-based program to mechanically and electrically cycle the joysticks under a simulated vehicle load. The program provides continuous monitoring and storage of all test data.

"The dust is constantly cycled for 45 seconds and allowed to settle for four minutes," he explained, "and the software is designed to monitor and control the test rig and stop the testing if there is either a mechanical or electrical product failure. Throughout the test cycle, the units also are observed visually for other anomalies."

Several key points summarize the joystick dust test:

* Simulates dust infiltration over a 23[degrees]C to 40[degree]C temperature range.

* Uses ISO A12103-1 A4 coarse dust.

* Continuously monitors electrical function.

* Simulates in-vehicle electrical load conditions.

* Verifies the operation in real-world conditions found during the normal operation of the system.

* Detects both electrical and mechanical wear or malfunction that would not be detected in separate test sequences.

While researching dust testing, several articles were found relating to the U.S. Army M4 carbine dust test conducted in late 2007. In this case, a loaded carbine was exposed to extreme dust conditions for 30 minutes and then test fired using 120 rounds. This cycle was repeated with the carbine wiped down and lubricated after every 600 rounds and cleaned after every 1,200 rounds until 6,000 rounds had been fired. (3)

Whether or not this test regimen adequately represented the actual conditions in which the weapon would be used can be argued. However, possibly more important in view of the detail with which PTI compared the composition of its test dust product to that of the earlier AC Spark Plug dust is a comment in an October 2004 Desert Research Institute report:

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