environmental stress testing

Shock Transients of the Mechanical Type

Posted on October 27, 2021 by admin - Environmental Stress

*Elite’s Nick D. preparing for a Mechanical Shock test*

While we feature a complete blog series on electrical transients, the technical team at Elite is also starting this month a new companion series on mechanical shock transients. Follow along as we discuss the basics of the Mechanical Shock environment and see how testing is performed. We’ll also point out how “shockingly” analogous the electrical and mechanical transient characteristics are!

Mechanical Shock Testing Basics

Manufacturers want robust products that satisfy customer needs and meet lifetime and warranty objectives, as well as they satisfy requirements for validation and qualification. Mechanical shock survival is key to meeting those goals.

The concept of mechanical shock is intuitively simple: its drop, bang, bump, slam, and crash. From a physics perspective, it’s a rapid transfer of kinetic energy to a mechanical system that can create a variety of product problems.

Shock is an over-stress failure as opposed to a fatigue phenomenon such as vibration stress. Depending on the shock-pulse characteristics and the product’s mechanical resonances, the equipment responses may result in the deformation of structural members, deflection of printed circuit boards, or fractures at microprocessor die-bond wires. Shock transients can cause momentary discontinuities at electrical connectors or chatter at the contact of mechanical relays. High-frequency shock pulses can affect the performance of piezoelectric crystals used for electrical timing and digital clocking circuits.

There are a variety of options for manufacturers to evaluate their products against the shock environments they operate in. They can test the finished product in the end environment, test components and subsystems using field data, use classical shock pulses, or used specification-defined equipment.

Shock Test Approaches

1. Test In the Actual Shock Environment

Products can be instrumented with accelerometers, strain gauges, or other sensors, and then be exposed to the actual shock environment. Automotive original equipment manufacturers (OEMs) routinely test-drive vehicles through potholes and road bumps, or into impact barriers. Cell phone manufacturers drop products at different heights on concrete or wood surfaces to evaluate handling toughness. Shipbuilders intentionally detonate controlled underwater explosives and evaluate how the shock wave interacts with mission-critical systems on board.

This in-situ shock approach is the most realistic test because it evaluates the complete system performance in an environment closest to the actual application. However, at this phase of product development testing requires near-production prototypes or the finished product itself. If the product does not survive in the shock environment, then design changes late in development are costly and delay product delivery dates.

2. Record Field Data and Apply Shock Pulse in the Lab Characterized as the Shock Response Spectrum (SRS)

The shock environment can be measured using accelerometers mounted at the end-use points for the product. The actual field data can be post-processed, replicated, and applied to sub-system components. With field data, testing of small parts, components, and subsystems can occur early in the development process. Field-recorded shock pulses are often a complex waveform of acceleration versus time. The challenge is that not all waveforms can be easily recreated or not produced repeatably. In addition, because of the random nature of how shock interacts with mechanical systems, no two measured shock pulses will be identical, which raises the question as to which field shock pulse should be applied.

As an alternative, shock-field data can be used to create a more repeatable and controlled shock input by creating a Shock Response Spectrum (SRS). This approach creates an impact input to the test item that has the frequency character and amplitude of the measured field-shock environment. Although the time waveform input of the SRS pulse may not look identical to the field waveform, the response of a single-degree-of-freedom system to the SRS pulse is the same as it were the complex field waveform.

3. Apply Simple Classical Shock Pulses

The most effective shock test pulse is one that replicates the actual environment where the product will be used. However, many manufacturers don’t have field shock data or the ability to measure the end use application. In addition, conducting shock testing using the SRS shock pulse approach requires specialized equipment and the process does not lend itself to quick and efficient shock testing.

As a result, manufacturers turn to simple classical shock pulses that take the form of half-sine, saw-tooth, or trapezoidal acceleration versus time waveforms. Classical shock pulses are relatively easy to produce, can be generated on a wide range of equipment, and the pulses can be generated repeatably with tightly controlled waveform specifications. For this reason, classical shock pulses are the most applied shock test.

4. Specialty Shock Tests

Certain shock tests are specified not in terms of the field waveform characteristics, SRS, or by a classical pulse shape, rather they are defined by a particular test apparatus. An example is the Navy test using a High-Impact Shock Machine for light, medium, or heavy-weight equipment as described in MIL-STD-901. The details of the machine are outlined in the MIL-STD and the applied pulse amplitude is selected from the shipboard location and delivered by the machine by raising the impact hammer to a specified height.

Similarly, many drop, bounce, or handling tests are not characterized by the acceleration parameters, instead are defined by the drop height, impacting surface, and test-item orientation. These types of tests are run on the device itself or on the packaging.

So out of these four approaches to shock testing, which one is best for your product? The world is full of transient events, whether mechanical or electrical. In our next blog, we will describe in more detail the SRS and Classical Shock Pulses. Continue to Part 2.

Contact us today to start planning your Mechanical Shock testing.

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Optimizing Vibration Fixtures with Simulation

Posted on May 25, 2018 by admin - Environmental Stress

At Elite, every product tested for vibration or mechanical shock has to be properly secured, or “fixtured,” to a vibration or shock machine. The easiest way to fixture is using bars, threaded rods, clamps, or other tools that can grip the test item and secure it to the table. The preferred way is to create a custom assembly, called a “fixture,” which provides a mounting surface and tapped holes for holding the Device Under Test (DUT). The base of the fixture also has attachment points that connect to the vibe/shock machine surface.

A custom-made fixture is especially important for devices that have complex shapes which do not allow for a simple clamping solution. A fixture is also helpful when the DUT must be held in a particular orientation for all axes of testing and to hold multiple samples for a single test run.

Generally speaking, there are two types of fixtures; those that “simulate” the mounting conditions, and those that “stimulate” vibration into the DUT.

Simulate-style fixtures are intended to replicate the actual DUT mounting bracket or mounting surfaces. They are unique structures with their own mechanical response. A simulated bracket fixture will likely alter the characteristics of the vibration/shock dynamics when passing from the machine through the fixture and into the DUT. In some cases, this is desirable, for example when the goal of the test is to provide fatigue input representative of how the DUT is mounted in the actual end-use environment. The simulated fixture will have its own mechanical resonant characteristic which may amplify the input vibration and create a more severe vibration environment for the DUT.

Stimulate-style fixtures tend to be designed with thicker more robust elements and structural members. They are arranged to provide stiffness while being lightweight. Stimulate-style fixtures are intended to transfer the applied vibration/shock input from the table directly into the DUT. A perfect stimulate-type fixture has input transmissibility that is very flat, meaning that the frequency range, amplitude, and phase of the vibration environment are applied directly to the part with no modification to the input vibration environment.

The design of a fixture should first account for being a simulate vs. stimulate purpose. However, the design should also be lightweight to achieve maximum vibration test levels and should allow access to the DUT mounting points in order to easily attach the test item. The design needs to consider future modifications and re-tooling. It also has to balance performance with cost.

Elite engineers take all of these parameters into consideration when developing fixtures for client testing. They have the benefit of years of expertise through hands-on testing and vibration fixture development but are also able to apply the latest CAD and vibration simulation tools, like those from Solidworks.

Once a test item CAD design file is received from a client, it can be loaded into Elite’s Solidworks tool and the fixture structural design can begin. Engineers create the initial design taking into account all the particular details of the test item shape, mounting configuration, attachment and accessibility, and cost.

The next step is to run a full dynamic simulation using the vibration modeling features of Solidworks. From the simulation, we can visually and quantitatively evaluate areas of the fixture that are resonant and redesign the fixture to reduce the resonance magnitude or even shift the resonant frequency beyond the test range.

It is critical to reduce fixture resonant frequencies that coincide with the vibration input profile. When vibration is applied at the fixture resonance conditions large displacements and accelerations (high G levels) are produced and test items can be overstressed. Similarly, when the input vibration and response align out of phase the applied vibration can result in “null” acceleration conditions and under-test the DUT.

Elite’s Steps to Vibration Fixture Optimization

1. Develop an initial fixture design considering the following:

Fixture purpose: Stimulate or Simulate style fixture.
Single or multiple samples
DUT orientation
Accessibility for DUT mounting and table mounting
Weight, length, width height considerations
Single-use fixtures or future adaptations

2. Run the simulation and identify the following:

Identify natural resonant frequencies, mode shapes, and relative resonance magnitude.
Consider options for reduction of resonance, weight, stiffening elements, and moment of inertia (length, width, height).
Repeat simulation and optimize results for performance, machinability, and least cost.

When you are ready to experience the Elite advantage and put our comprehensive capabilities and proven experts to work for you, please contact us with your questions or request a quote.

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What is HALT HASS Testing?

Posted on February 1, 2018 by admin - Environmental Stress

Watch our webinar to learn about HALT and HASS Testing and when to consider it for your products.

What are HALT and HASS?

Highly Accelerated Life Testing (HALT) is a process that utilizes a stepped stress approach in exposing your product to diverse accelerated stresses to discover the physical limitations of a design and product reliability. Manufacturers can discover their products’ failure modes and determine their failure mechanisms.

Highly Accelerated Stress Screening (HASS) is a production quality assessment to quickly and efficiently identify any weaknesses that the product may have inadvertently developed during the manufacturing stage.

Both are “Test, Analyze, Verify, and Fix” approaches – with Root Cause Analysis along the way!

Why should you perform HALT/HASS?

Highly accelerated life tests find weaknesses and flaws early in the design phase by testing to failure, while highly accelerated stress screening (HASS) catches manufacturing defects on production parts prior to installation without reducing the part’s life. HALT also provides valuable data for reliability metrics at the component level. The test results benefit customers, protect the manufacturer’s reputation, and prevent costly re-design later in the product development cycle.

What is unique about a test chamber?

Unlike other environmental simulation chambers, HALT and HASS chambers offer fast temperature ramp rates (up to 60C per minute) and combine thermal, vibration, and shock simulation in a single apparatus. As with other types of Vibration and Shock Testing, the test items require mounting fixtures to simulate the intended orientation and transmit vibratory energy without interference.

Vibration levels up to 50 Grms can be applied simultaneously in three linear axes (X, Y, and Z) and three rotational axes (pitch, roll, yaw).

How do you specify a HALT/HASS test?

HALT and HASS profiles are composed of several segments defined by the product’s intended end-use environment:

Cold Step Stress and Hot Step Stress: Incrementally decreasing or increasing the temperature to identify product limitations. Select start and end points based on the end-use environment for the product reliability and physical limitations of the components.
Vibration Step Stress: Incrementally increase the vibration levels while pausing on the way up to see how your product responds. Begin at a set Grms level, dwell for a specified duration, then increase to a higher amplitude and repeat the cycle to initiate failures.
Rapid Thermal Transitions (or Thermal Shock): Subjecting your product to pre-defined maximum and minimum temperatures and rapidly cycling between them.
Combined Environment: Simulating real-world conditions where your product will be exposed to multiple random environments simultaneously.

Here are some common HALT/HASS acronyms used to specify test profiles:

“Grms” – Vibrational G’s in the root mean square, where “G” is the acceleration due to gravity.
“PSD” – Power Spectral Density – In a random vibration spectrum, it is the measurement of the amplitude and frequency.
“LOL” and “LDL” – In the cold temp step stress stage, they are the “Lower Operating Limit” and “Lower Destructive Limit”
“UOL” and “UDL” – The “Upper Operating Limit” and “Upper Destruct Limit ” occur in the hot temp step stress stage.

Why choose Elite for environmental testing?

Our team of experts can support your test needs from planning stages to testing, and failure analysis.

Elite’s in-house machine shop and design experts allow us to design and fabricate any custom fixture or automation solutions to meet your specific product requirements.

Request a quote now to start your HALT/HASS plan.

Visit our HALT/HASS Testing page or watch our webinar for more information.