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Latest News in Testing, Inspection and Certification

NTS News Center - Latest News in Testing, Inspection and Certification

How will MIL-DTL-901E affect your shock qualification?

Photo: Navy.com

For the past 30 years, MIL-S-901D has defined requirements for shock qualification of items installed on U.S. Navy Combatants. For the duration of this period, there have been rumors of a new and improved specification revision. That time has finally come.

MIL-DTL-901E, revision to MIL-S-901D of 1989 has been approved and released as of 20 June 2017. The new “detail specification” or DTL per DOD 4120.24M is an evolutionary document that includes changes from MIL-S-901D, Interim Change #2, the 2012 DSCR-2 initiative clarification letters and cost reduction strategies, and the Total Ownership Cost (TOC) reduction provided by the Deck Simulating Shock Machine (DSSM).

For most Navy shipboard shock practitioners, the most impactful change to the document is the addition of the Medium Weight Deck Simulating Shock Test.  This test is primarily performed on Class II, deck mounted equipment and utilizes the DSSM. The DSSM is typically used in lieu of performing a heavyweight shock test series.

The DSSM shock test machine at NTS Rustburg

In the early 2000s, we recognized that the current model for shock qualification of a vast majority of the proliferating Class II, Deck Mounted, COTS equipment was not sustainable from a costs and schedule perspective.

So, in collaboration with HII-NNS partners, NTS led the pioneering development of a more cost and schedule effective land-based approach to meet this new challenge. These efforts culminated in the fabrication of the DSSM Generation I machine that has been in use at NTS Rustburg, VA since 2004.

The success of the Generation I DSSM led to the development of a scaled-up Generation II machine that is specified in MIL-DTL-901E. All of the certification testing that was necessary prior to inclusion in the new MIL-DTL was conducted at NTS. We have been routinely conducting shock qualification testing on the DSSM as an Alternate Shock Test Vehicle since 2015.

The DSSM development is yet another example of NTS’ focus on customer driven innovation. If you have questions concerning the new MIL-DTL-901E detail specification and how it may affect your program or have a need for Medium Weight Deck Simulating Shock Tests contact our experts today!

Why do I need EMC Compliance?

EMC TestingThere are a variety of reasons why your electrical/electronic products may be required to go through the Electromagnetic Compatibility (EMC) compliance process.

Regulatory Requirements

Most industrialized nations have established agencies or other regulatory bodies responsible for defining and enforcing EMC standards. If EMC regulations exist in a country, equipment manufacturers cannot legally ship their product into that country until compliance with those regulations is met. Professional compliance laboratories like NTS understand the global compliance process and possess the accreditations and capabilities needed to perform testing and certification to meet the relevant standards in all of your target markets.

Customer Requirements

There are many situations in which your customer may dictate EMC requirements. For example, if your customer is an RBOC (Regional Bell Operating Company), an extensive set of tests referred to collectively as Bellcore, are required before the RBOC will purchase your product. If your product or component will be included in a third-party product, your OEM customer will need proof that your product or component will meet appropriate regulatory standards. Your OEM customer will dictate the amount of testing and documentation they require. It is not unusual for an OEM customer to ask for compliance exceeding legal requirements.

Reliability/Performance

In some cases, you may want to self-impose reliability standards for your products; for example, if you only plan to ship your product in the United States, unlike in Europe, you are not required to pass immunity standards. However, you may want to establish your own immunity standards to ensure customer satisfaction by minimizing field failures.

How do I achieve EMC compliance?

Determining Relevant Standards

The first step in the compliance process is to determine the list of target markets where you plan to market and sell your products. Compliance regulations vary from country to country, so an investigation of current standards is required for each market where you intend to operate. Countries such as India have no formal Electromagnetic (EMC) requirements, whereas Taiwan has a very strict submission policy.

Self-Certification

Over the past several years, regulatory agencies around the world have been moving away from agency submittal applications because they are slow and inefficient. Most government agencies now allow for self-testing and self-certification. This means that you can simply affix a CE or FCC mark to your product and begin shipping without gaining government regulatory approval first.

However, self-testing and self-certification does not relieve your company from meeting the standards implied by the mark you place on your product. In fact, if you place a mark on your product when it doesn’t comply, it could result in serious consequences.

NTS has the formal accreditations to provide both the testing and the reports you need to meet a wide range of regulatory self-declaration requirements. We also create the reports that you must keep on file should an issue arise requiring proof of compliance.

Third party Verification

This is the process of having an independent party validate a product’s compliance. Third party verification adds significant credibility to a product’s test/compliance program, either for marketing or regulatory purposes.

Formal Certification

With certain types of products and in certain countries, a formal certification process may be required. For example, in Taiwan, a formal submission and approval is required, whereas the European Union might allow self-certification. In the USA a formal FCC submission, called a Certification, is required for any products designed to transmit radio signals. In addition to the testing costs to meet the relevant standards, you should also expect to pay agency submittal fees ranging from a few hundred to a few thousand dollars depending on the country and the type of product.

MS Project: A Simple Way to Streamline the Testing Process

At our Rustburg, VA EMI/EMC testing facility, customers have the opportunity to work closely with our tight-knit team; managers and engineers to technicians and administrative staff. To ensure every program runs smoothly from start to finish we utilize Microsoft Project to build and post schedules, providing customers with a detailed timeline of each testing phase. This is especially helpful for complicated projects requiring full EMI testing with multiple pieces of equipment over a long period of time.

Our technical writer in Rustburg has offered this short guide below to get you started with the basics of MS Project. You may find it useful to manage the pre and post-testing phases of development!


Microsoft Project is a powerful project management tool. It has many advantages over Excel, primarily because it is more dynamic and scheduling changes can be easily made following a change in project timeline.

You can track each task of a project, as well as its duration, hierarchy, and resource requirements. This information can then be viewed in numerous different ways, most commonly in a Gantt chart or Timeline. These easy to view, but still very detailed formats, allow the NTS Rustburg team to effectively communicate and exchange project information within our group and with our customers.

Starting a New Project

  • Open Microsoft Project 2013 program
  • Select Blank Project
  • Go to Project Tab, select Project Information, enter Project Start date

1

Entering Tasks

  • Double click on cell under Task Name heading in Gantt chart
  • Enter Task Information into dialogue box
  • Name task, enter duration, select Auto Scheduled for Schedule Mode

2

  • Tasks can also be added from Task tab, Insert Task
  • Summary Tasks and Milestone Tasks can also be added from Task tab
  • A Summary Task will include tasks below it in the Gantt chart. Summary tasks will be outdented to the left and displayed in bold. The Duration (and Start and Finish dates) of Summary Task will include all of the tasks included in Summary Task.
  • Milestone Tasks mark an event in a project and have a zero day duration.

3

Change Working Time

  • Project tab, Change Working Time
  • Project start day was set to 5/25/2015 which is a holiday.  After entering the holiday in the Change Working Time dialogue box, the project Start Date will automatically change (to next working day 5/26/2015).

6

Assigning Task Precedence

  • Many tasks during a project must be completed in a certain order, e.g. testing must be completed prior to writing of test report.
  • In Predecessors column of Gantt chart, assign a row number to any task which must be completed before the next. Because tasks are Auto Scheduled, Start and Finish dates will automatically adjust on Gantt chart. Highlighted cells show ant changes in response to assigning Predecessors. Note: Project can’t link a summary task to one of its subtasks.
  • When the inevitable change to your schedule occurs, Project will automatically adjust the schedule and update the dates of tasks that come following the change, e.g. Technical Writer takes 2 days instead of 1 day to write the report.
  • Once you become comfortable entering tasks, there are many other features of Microsoft Project that can be utilized to create a very detailed and powerful project schedule. You can assign resources to a task, such as a Technician, Engineer, or Technical Writer.

5

Changing the View

  • Finally, on the Task tab under Gantt Chart, View can be changed to Resource Usage to see how many hours have been allocated to a specific person

6

We hope this brief tutorial might encourage you to explore MS Project for your scheduling process!

A Quick Guide to Expediting Your Equipment Under Test from NTS Rustburg

There is only one commercial site in the United States that can provide global solutions capabilities for all Navy equipment qualification testing, as well as extensive test and qualification capabilities for ground, avionics, and space applications. This test site is the Rustburg Division of National Technical Systems, strategically located in Central Virginia, within driving distance of the Eastern seaports, various military bases and Government facilities, major prime contractors, and Washington D.C.

Besides geographical and environmental factors, what makes this test and engineering support site appealing is its unique blend of capabilities and proven expertise spanning from engineering analysis and simulation to various levels of physical testing for virtually any size EUT.

The main purpose of qualification testing is to ensure that the equipment will perform reliably and according to the operational requirements once it is integrated in the end-user platform. To this end, emulation tests are usually divided into two categories: those tests that subject the EUT to the environment in which the equipment will have to operate (susceptibility tests), and those tests that evaluate the effects that the EUT will have on other systems which are part of the overall operational environment (compatibility tests). Along these lines tests can be environmental in nature (e.g., shock and vibration, temperature and humidity, drip and immersion, airborne and structure borne noise, etc.) or electromagnetic in nature (e.g., EMI, EMC, ESD, magnetic, power quality, etc.).

The hardest tests, from the EUT point of view, are those that apply very high energy levels to the equipment during very short periods of time to emulate, for example, the near-hull explosion of a submerged mine or a torpedo, or the electromagnetic pulse caused by the detonation of a nuclear weapon in the stratosphere.

In the first case, Heavyweight Shock tests are performed according to MIL-S-901D using Floating Shock Platforms (FSP, up to 70,000 pounds) and Extended Floating Shock Platforms (EFSP, up to 125,000 pounds). To fully emulate the reverberation modes caused by the shock waves generated by the explosion of strategically placed underwater explosive charges, equipment carrying Deck Simulator Fixtures (DSFs) are installed on the floating platforms (barges) in order to provide the 8, 14, 25, and 40 Hz damped oscillatory stimuli to the installed EUTs.

Figure 1, Heavyweight Shock Testing

In this emulation scenario the main shock wave and secondary shock wave caused by the explosive bubble hit the barge (left) causing the DSFs to produce the reverberation modes as well as transmitting rotational and athwartship forces which also stress the equipment and any installed shock absorbers.

Figure 2, Deck Simulating Shock Testing (DSSM)

For smaller EUTs (up to 1,500 pounds) a Deck Simulating Shock Machine (DSSM, right) can also be used to optimize test time and associated cost. This machine (DSSM) was very recently qualified by the Navy and it is the only one available at a commercial test site (the Navy has the only other existing DSSM).

Figure 3,  Lightweight Shock Machine  (LWSM)

For yet smaller EUTs, the Lightweight Shock Machine (LWSM, left) is routinely used. All these platforms and machines (and associated tests) are Naval Sea Systems Command (NAVSEA) certified.

Other environmental tests performed at NTS Rustburg include Vibration (MIL-STD-167), Temperature, Humidity, Drip and Immersion (MIL-STD-810), and Airborne and Structure Borne Noise (MIL-STD-740-1/MIL-STD-1474D and MIL-STD-740-2).

Rustburg Figure 4Electromagnetic testing capabilities were added fairly recently to NTS Rustburg (May 2012). The new Electromagnetic Interference (EMI)/Electromagnetic Compatibility (EMC) facility is A2LA certified for MIL-STD-464 (system level environmental EM), MIL-STD-461 (EMI/EMC), MIL-STD-1399 Sections 300A and 300B (shipboard power quality), MIL-STD-704 (aircraft power quality), MIL-STD-1686C and DO-160 Section 25 (Electrostatic Discharge), DOD-STD-1399 Section 070 (DC Magnetic Field), and RTCA/DO-160F-H (Aerospace). The semi-anechoic test chambers (above) are also used for Airborne and Structure Borne Noise testing, taking advantage of the very quiet background environment. These tests are A2LA certified as well.

Each of the electrical/electromagnetic standards mentioned above are fairly complex in nature and includes many individual tests.  For example, MIL-STD-461F covers all test cases depicted below.

Figure 5, MIL-STD- 461 Test Cases

In the highly competitive test and qualification market it is extremely important to continuously achieve all possible cost savings for the customer while consistently preserving compliance with the standards and maintaining the highest quality of service. Along these lines the main cost reduction drivers can be identified with expeditious, high quality documents preparation and delivery, the capability of offering all required tests at one location (to help the customer manage both travel and shipping and staging cost), the continuous optimization of test sequencing (critical path based schedule, to avoid any non-test periods while on location), and the reduction of individual tests time. In response to these challenges NTS Rustburg has been consistently delivering high quality Test Procedures within approximately two weeks of contract award, and Test Reports within two weeks of test campaign completion. NTS Rustburg has also been consistently adding certified test cases to provide a one-stop shop for all required tests, it has adopted a sophisticated company-wide scheduler (VLAB, connected to the central ERP system) to fully optimize programs test schedules based on resources and assets allocation and availability, and it has developed standards compliant, innovative test equipment implementation and tailored test solutions to reduce test time and associated bid price and execution cost.

Figure 6, Helmholtz Coils

One example of this innovation and ensuing cost reduction is the in-house design and implementation of very large Helmholtz Coils (left). These coils have an area of approximately 2.5 by 2.5 meters and can create the required DC magnetic fields (DOD-STD-1399 Section 070) and AC magnetic fields (MIL-STD-461, RS101 test method) while encompassing entire racks within the generated fields.

Rustburg Figure 7

By comparison, the traditional approach to the RS101 test has been to use a small coil (right) and apply a localized magnetic field to individual 30 by 30 centimeters square surfaces and at each electrical interface connector. For large EUTs, using the small coil for this test can take days. With the large Helmholtz coils, besides creating a more realistic scenario by immersing the entire EUT in a homogeneous magnetic field, complete tests in all three axes (X, Y, and Z) only take a few hours, thus greatly reducing the cost of these tests to the customer.

If you would like to know more about how best to expedite the testing phase of your program by utilizing the experienced technicians and engineers at NTS Rustburg, please contact the author of this article, Dr. Francesco Lupinetti, EMI/EMC Director at 434-608-1234 or Francesco.Lupinetti@nts.com.

Evolution of NTS’ Deck Simulator Fixtures in Accordance with MIL-S-901D

An aerial view of NTS Rustburg

An aerial view of NTS Rustburg

Evolution of NTS’ Deck Simulator Fixtures (DSFs) for Testing Deck Mounted, Shock Isolated (Class II) Equipment in accordance with MIL-S-901D

By Calvin P Milam, Division Manager, NTS Rustburg

NTS Rustburg, located in Central Virginia, was founded as a place to conduct heavyweight shock testing for US Navy shipboard components. The concept of a heavyweight shock test is to install the equipment under test on the Floating Shock Platform (FSP), set off underwater explosions at varying distances from the FSP, and make observations as to the survivability of the items being tested. The controlling document for conducting heavyweight shock testing is MIL-S-901D.

The fundamental philosophy of MIL-S-901D is to assure that essential Navy Shipboard equipment and systems will operate safely and reliably in a combat environment. A significant subset of essential equipment installed aboard naval combatants is deck mounted and is installed on some type of shock isolation system to improve survivability. In MIL-S-901D terms, Class II equipment is defined as equipment installed with resilient mounts between the equipment and the ship structure or more commonly referred to as shock isolated. Common types of shock isolators are wire rope isolators, arch-mounts, and numerous other shock mitigating devices. Deck mounted equipment (for surface ships) is typically mounted on the main deck and above decks, platforms and non-structural members below the main deck.

Historically, military development has been the main driving force in technology development but with the increased investment by the private sector in computers and electronics in the 1980s, military development began to lag commercial technology. As a result, throughout the 1980s, the use of commercial-off-shelf equipment (mostly electronics) on naval combatants gradually gained momentum. To keep up with rapidly changing technologies, the navy could no longer rely on the long cycle of testing and approval typically associated with ruggedized MIL-SPEC equipment. The solution was to use COTS equipment packaged in shock mitigated enclosures designed to improve shock fragility levels.

Prior to the proliferation of COTS equipment, most deck mounted shipboard equipment was tested to Hull levels on medium and lightweight shock machines to Hull input (vice lower G, Deck) levels. Thus, shock isolated, Class II, deck mounted equipment presented a new dilemma for shock qualification. Hull inputs provided a higher G-input but under tested the equipment. During ship shock trials, equipment that was shock isolated and installed on decks had a higher than anticipated failure rates because of the higher displacements associated with decks and closer coupling of the isolator and deck frequencies.

Initially, test labs designed customized shock test fixtures to simulate the deck environment associated with a given piece of equipment. This approach was not only expensive but often lead to compromised tests because the test fixture modal mass was typically insufficient to drive shock isolated test item. Simply stated, the simulated deck was too light to drive the test item.

To address this problem, NTS designed and fabricated a universal Deck Simulator Fixture (DSF) in 1991. The new DSF provided a cost effective and technically sound approach for shock testing deck mounted, COTS (Class II) equipment. The DSF was designed to test numerous items simultaneously at frequencies ranging from 12 to 25 Hz. The design was finite element (modal and static) based and intended to accommodate a wide range of test payloads. A key feature of the design was to provide a means to adjust the deck frequency by adjusting span lengths and or adjusting the payload mass. An isometric view of this DSF design is shown in Figure 1.

Navy Standard DSF

Figure 1. Navy Standard DSF

This design is routinely used and well over a thousand items have been tested on this DSF design. The design shows four longitudinal beams and the beams are pinned via bolted connections to supporting footstools beneath the beams. The footstools are welded to the inner-bottom of a Floating Shock Platform (FSP). Pin locations are adjusted to suit the payload and target DSF frequency. Throughout the 1990s, this was the go-to platform for shock qualifying deck mounted, Class II, shock isolated equipment in the frequency range from 12 to 16 Hz as required by MIL-S-901D. The design has been in use for so long that it is commonly known as the Navy standard DSF.

As COTS equipment proliferated onto navy platforms, the test frequency deck environments continued to evolve. A number of low frequency decks in the range of 8Hz were identified on naval combatants, particularly large deck ships. Thus, the MIL-S-901D specification evolved to include low frequency testing; however, at that time there was not a viable low frequency (~8Hz) test platform in use. Sometimes, expensive and technically deficient (i.e. too light) DSFs were utilized because there were no alternatives. Once again, NTS stepped up to provide a timely, cost effective and technically sound alternative for shock testing Class II, COTS equipment installed on low frequency decks. The basic approach was similar to the development of the initial deck.  Innovative thinking that included a tune-able design coupled with finite element analyses and confidence testing lead the way for the low frequency DSF shown in Figure 2.

NTS Low Frequency DSF

Figure 2. NTS Low Frequency DSF

This new design utilized existing DSF platforms to minimize costs. The design featured bolt-on extensions for the two outboard beams and offered a tune-able DSF platform for testing at both 8 and 14 Hz nominal target frequencies. The outboard beams are pinned to the supporting footstools while the inboard beams are not bolted to the FSP and provide added mass. This approach has been routinely used since the late 1990s to shock qualify Class II, COTS equipment for low frequency applications on US Navy ships.

This approach for testing COTS at relatively low frequency worked well; but testing large items on high displacement shock mounts posed a problem. To address this issue in late 2012, NAVSEA issued Letter 9072 Ser 05P1/464 titled “Cost Avoidance and Clarification of MIL-S-901D Shock Test Requirements for Shock Qualification of Equipment on Heavyweight Floating Shock Platform Types with or without Deck Simulating Fixtures.” The letter introduced a number of new DSF parameters and introduced both new requirements and methodologies for defining the appropriate DSF environment. Some of the new parameters included dead mass, DSF live mass, and live mass (Equipment and Foundations). Without getting into to excessive detail, the new parameters set limits for different mass categories based on the mass Shock Response Frequency (SRF) relative to the DSF target frequency.

After the math exercise is completed in accordance with the NAVSEA letter, the bottom line for the majority of class II, deck mounted equipment is as follows:

  • 12 to 16 Hz DSF, the effective modal mass of the DSF is 5 times greater than the shock isolated test item.
  • 7 to 9 Hz DSF, the effective modal mass of the DSF is 10 times greater than the shock isolated test item.

In most cases, the necessary dead mass could be added to the Navy standard DSF shown in Figure 1 for 12 to 16 Hz testing; however, the addition of the mass to achieve a 10:1 ratio as required for testing in the 7 to 9 Hz range would significantly yield the DSF.

NTS was again challenged to provide a new DSF solution for shipboard shock qualification. The same customer driven recipe was applied that lead the way for previous DSF designs. A tune-able design coupled with finite element analyses (see Figure 3) and confidence testing was the prerequisite for the NAVSEA letter compliant DSF shown in Figure 4.

Figure 3. Fundamental Mode for NAVSEA Letter Compliant DSF

Figure 3. Fundamental Mode for NAVSEA Letter Compliant DSF

Figure 4. NAVSEA Ltr 9072 Ser 05P1/464 Compliant DSF

Figure 4. NAVSEA Ltr 9072 Ser 05P1/464 Compliant DSF

Again, the design was a modification to the existing navy standard DSF to minimize cost. The Standard design was modified by welding extensions to all of the longitudinal beams and permanent dead mass was attached to the underside of the DSF. The DSF is tuned by adding or subtracting dead mass to the top of the DSF or by adjusting the span length by changing hold-down bolt locations on all four beams. The goal for the new deck design was to have an effective modal mass in the 30,000 to 40,000 pound range and have a frequency range from roughly 6 Hz to 16 Hz without yielding the DSF. The latest DSF was successfully tested in a confidence shock test trial in May 2014 and subsequently placed in service for shock qualification test trials.

If you have skipped to the bottom of this article because Heavyweight Shock Test, MIL-S-901D, and DSF are not a high priority, there is still something to be gained. The evolution of DSFs at NTS Rustburg is one example of customer driven investment at NTS over a long period of time. As navy requirements for DSF have evolved, NTS has been at the forefront of providing technically sound, cost effective solutions. The next evolutionary step for DSFs at NTS is the land-based Deck Simulating Shock Machine or DSSM (see Figure 5). This new approach is still a work in progress; however, Navy certification is soon anticipated. The evolution of DSFs at NTS is one small example of our commitment to the NTS Mission statement “One company that is the natural first choice for high technology test, inspection and certification solutions.”

For more on heavyweight shock testing, DSFs or how NTS Rustburg’s unique capabilities can help with your MIL-S-901D testing please contact the laboratory at 434.846.0244 or sales@nts.com

Figure 5. DSSM Gen II

Figure 5. DSSM Gen II

References

  1. MIL-S-901D, “Shock Test, H.I. (High Impact) Shipboard Machinery, Equipment, and Systems, Requirements for, ” 17 Mar, 1989
  2. NAVSEA Ltr 9072 Ser 05P1/464, 21 Nov 2012