Selection of a Vibrational/Shock system, for electronic packages used in Air and Ground transport devices, must be carefully considered for its capabilities to provide the necessary data and information pertinent to the specifications set forth by MIL-STD-810F. The advent of combining environmental conditions such as vibration, shock and heat has presented test engineers both a challenge and opportunity to observe the behavior of electronic assemblies, while simultaneously maintaining the required thermal and shock conditions. Knowing the limitations, performance capability, and physical dimensions of the AGREE (Advisory Group on Reliability of Electronic Conditions) system, which includes both the vibration and thermal units, is necessary in defining the specific equipment suited to meet the required needs. Not to be overlooked is the importance of having the right facilities in place to accommodate the size and weight requirements of the test equipment.

Garnering information for a thorough technical assessment of a functional shock system, can be generally achieved by leveraging the parameters found in the specifications set forth by MIL-STD-810F - Test Method (TM) 516.5. This standard defines mechanical shock environments as frequencies not exceeding 10,000Hz, at time durations of not greater then 1.0 seconds. The major share of applications for shock require frequencies < 2,000Hz and durations shorter than 0.1 seconds, a condition most vibration/shock systems are designed to simulate.

Sorting out the requirement as per TM 516.5 for a specific application can be a rather circuitous process and is not always so well defined. There are eight procedures listed in the TM to provide the basis and assistance in collecting and interpreting the information for the item under shock. The AN/ARS-6A, a multiple service Combat Search and Rescue (CSAR) radio being redesigned for sustainment at the EMPF, was used as a specific example for this investigation to determine the proper AGREE system to meet the conditions and requirements, as outlined by the Certification Test Plan. Any Vibration/Shock equipment considered for the ARS-6A would have to meet the Functional Shock and Crash Hazard criteria, as prescribed by Procedure I and V of TM 516.5.

The operational requirements for any vibration/shock system that is designed to meet TM 516.5 criteria can be generally narrowed down into two areas, Input and Output. The Input is the shock response parameters chosen to simulate the test conditions, for example acceleration and duration, while the output considers what form the material characterization response takes, i.e. the generated spectra.

MIL-STD-810F differentiates its test conditions between having adequate historical measured data, and insufficient data as a basis of Shock Response Spectrum (SRS) selection. Table 2-1 illustrates the requirement for applications where a lack of statistically significant measurements and insufficient data cannot justify the criteria for employing a historically accurate test spectrum. Essentially, the standard does allow for selection of parameters outside of the required conditions listed below, if the historical time measurements are available, and there is ample justification. In new applications, the SRS response is not known and follows the recommended parameters in Table 2-1.

Once the test parameters have been implemented, the item responses need to be analyzed, recorded, and tailored appropriately to reveal the relevant information. TM 516.5 considers four descriptive estimates to characterize the measurement responses of duration, amplitude, and frequency. The vibration/shock system that is selected should have the capacity to characterize the following responses:

Effective Shock Duration (Te) - Describes the minimum length of time that contains > 90% of the RMS time history amplitudes that exceed 10 % of the peak RMS magnitude for a specific shock event. The response is a measure of time and amplitude (g force), and exposes events along the time axis that can be characterized as a function of frequency.

Shock Response Spectrum (SRS) - Describes the natural undamped oscillation frequency over a given range of time as determined by the Te and measures the shock response of the material over the frequency range of interest. The response from the SDOF (Single Degree of Freedom) is damped and expressed as the Q factor, with a Q of 5 being recommended where the characteristic spectrum response is not known. Essentially, for multiple random breakpoints, it is desirable to suppress the natural frequency oscillations of the material from confounding the SRS.

Energy Spectral Density (ESD) - The Fast Fourier Transform (FFT) of the total shock is computed and graphed as function of amplitude and frequency (g2-sec/Hz). This shows the distribution of the spectral energy along the frequency band, resultant from the various shock breakpoints.

Fourier Spectra (FS) - Where ESD measures the distribution of energy, the FS is used for isolating outstanding frequency components within the band.

Another aspect of considering shock systems is the ability of the equipment to accommodate shocks in both directions along each of the three orthogonal axes.

The illustration in Figure 2-1 shows a range of motion occurring in both directions simultaneously. The force being applied in the three orthogonal axes stipulates that the item be rotated as such to accommodate the applied force and maintain center of gravity for the accurate application of the test conditions. In the case of the AN/ARS-6A, for example, rotation of the item would require specifically designed head plates, and in the case of simultaneous thermal testing, head expanders. Each application may require specific tooling, and in some instances, where center of gravity may be difficult to maintain, a Trunnion reaction Base may be needed to pivot the shaker- not a minor item both in cost, weight, and installation.

The other part of the AGREE equation is the Thermal environment in which shock occurs. This is where familiarity with the requirements of a specific application is essential to prevent unnecessary costs or unacceptable test conditions. MIL-STD-810F does allow and encourage the use of concurrent conditions to simulate real life environments that would be encountered during operational and stationary phases. The specification does not always define the environmental parameters and conditions required for a particular combination of tests. This is often left to the discretion and judgment of the engineers in simulating the appropriate environment. The stipulation, however seems to imply that any combination used, has the necessary elements of both (i.e. for shock and high temp) conditions in order to meet the level of acceptability.

The air transport, handling, and deployment requirements (MIL-STD-810F, Part I- Pg 13-14) which includes rotary and fixed propeller, call for the following TMs where temperature simulation is required:

501- High temperature (Dry/Humid)
502- Low temperature (Freezing)
503 - Thermal Shock
520 - Temperature, Humidity, Vibration, Altitude

Whatever thermal system is selected, discretion must be used to select a chamber capable of providing conditions appropriate for the assembly that is being tested. The "one size fits all" chamber may not be necessarily the best.

Concurrent environmental testing has been occurring for quite some time, but has gathered more momentum in the past several years as more organizations other than the DoD began to contribute some definition around test methods. The most common AGREE chamber is the thermal cycling unit in combination with a shock/vibration system. One unit under consideration for the AN/ARS-6A system was capable of achieving a temperature range from -77°C to 173°C, with a maximum ramp rate of 6°C/min over the entire range, to simulate both hot and cold environments.

The vibration systems should be capable of being placed underneath the cycling oven and modified to fit into the chamber. A thermal kit comprising of a neoprene gasket is used to insulate, seal, and isolate the vibration unit from the temperature chamber. The head expanders and fixtures attach to the head plate, and are the only parts of the shock unit exposed to the oven chamber. A chamber with dimensions of 30x30x30 inches has enough flexibility to fit various test size assemblies and accompanying fixtures.

Based on the specifications outlined in TM 516.5, some important performance metrics must be taken into account when choosing a shaker system. Velocity, displacement, and force are physical attributes of a shock vibration system which can produce the required g force and Transient Duration Time needed for the SRS. Estimating the amount of force needed is not always as simple as F=MA, which assumes a flat response over the frequency spectrum. In choosing a system one should observe the sine, random, and shock force ratings and be aware that often times, unless specified, the ratings may only include the weight of the shaker armature, not the test fixtures or the test specimen itself.

The Crash Hazard shock test parameters as defined by TM 516.5 can effectively require a 75g force with duration of 15-23 milliseconds. In the case of the AN/ARS-6A System 4, an estimated 7500 lbs. of force is needed assuming a total of 100 lbs. of mass. These are estimates, which depend also on pulse shape as well as other factors, even though the biggest contributor to achieving maximum force is the total load. For shock test environments, a variety of factors may impact performance such as pulse and peak amplitude, pulse duration, and test load.

A typical frequency range for sinusoidal and random tests is generally specified between 20 and 20,000Hz. The maximum displacement is critical at lower frequencies, while limited by acceleration at the higher frequencies. Random vibration depends on a power spectral density with a fairly flat response over the entire frequency range. Resonant damping of the mechanical armatures and other moving parts is critical to preventing competing or offsetting frequencies, which may interfere with the natural characteristic frequencies of the test part. A large number of programmable breakpoints and bandwidths are desirable in providing random harmonics.

Finally, and certainly not the least important aspect of selecting an AGREE System, consierations of the available facilities to house such an immense system is critical. The weight of the shaker system alone may exceed 6000 lbs. over a 3 x 3 foot area. Adding the additional thermal chamber can well top the typical floor load rating of 200-300 lbs./sqft. Additionally, an isolated room would be necessary to dampen the noise and vibratory effects of the running unit. Given all of the factors, it may still be worth the test flexibility and long term cost savings of installing the needed facilities to perform the AGREE test on a critical system such as ARS-6A.