Delserro Engineering Solutions - Page 8 of 11 - Product Reliability & Vibration Testing Since 1982

Vibration Response of Products

Vibration of a mechanical system can be descried as an oscillatory motion about an equilibrium point. Certain vibrations of mechanical systems can be considered desirable, such as in musical instruments like a tuning fork or guitar.

However, often times vibrations of mechanical systems are undesirable, producing wasted energy, unwanted noise and catastrophic failures. Therefore it is critical during the product design phase that engineers are able to accurately characterize the vibration response of the system in order to ensure a safe and reliable product for their customers. We accomplish this through comprehensive vibration testing services.

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Shock Testing: Long Duration Half Sine Shock

Shock testing with long durations can be a challenging endeavor. DES recently had to perform a 35G peak, half sine shock with a 50 millisecond duration. The video below shows this shock test being performed.

This sounds like an easy shock to carry out because a peak of 35G is low compared to many shocks. However, this is a difficult shock to perform because 50 milliseconds is a long duration. Most typical shock durations are less than 20 milliseconds.

A half sine shock impulse has the shape of a half sine wave. More details can be found elsewhere on our blog, in an article titled “Classical Shock Testing“.

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Altitude Testing: Low Pressure Vacuum Chamber

Altitude (Low Pressure) Testing makes use of a vacuum chamber to simulate the effects of high altitude conditions. The pressure inside the altitude chamber can be reduced to correspond to the air pressure at a specific altitude. Products can be placed inside the altitude chamber and tested to determine if they will still function after exposure to a given duration at a specified altitude.

Components sealed with internal fluid such as batteries or capacitors may fail or leak during altitude testing because an internal pressure results at rising altitudes as the external pressure is reduced. It is also possible to power a product during the test to verify that it remains operational during the altitude test. The lower pressure at higher altitudes can reduce the cooling of components which can lead to possible failures. For this type of testing, it is necessary to have power and signal wires that can be fed into the altitude chamber without causing vacuum leaks. DES can provide a generic feed through that can be used for most testing. A custom feed through can also be fabricated if the component to be tested has specialized power or signal cables. It will be necessary to seal these cables to maintain the low pressure.

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Thermal Shock Testing – Temperature Cycling

Thermal shock testing also called temperature shock testing or temperature cycling exposes products to alternating low and high air temperatures to accelerate failures caused by repeated temperature variations during normal use conditions. The transition between temperature extremes occurs very rapidly during thermal shock testing, greater than 15 °C per minute. Alternatively, temperature cycle testing uses slower rates of change between high and low temperatures. The failure acceleration rate for thermal shock testing is determined by the Coffin-Manson equation as previously discussed in DES’s blog article Temperature Cycling Testing: Coffin-Manson Equation.

Equipment with single or multiple chambers may be used to perform thermal shock testing. When using single chamber thermal shock equipment, the products or samples remain in one chamber and the chamber air temperature is rapidly cooled and heated. This usually results in a slower rate of change in the product response temperature as the entire chamber must be cooled down and heated up. However larger products can be tested in single compartment chambers. Some equipment uses separate hot and cold chambers with an elevator mechanism that transports the products between two or more chambers. This results in a more rapid rate of change in the air temperature. However, there is a limit to the size and weight than can be put in a chamber with an elevator mechanism. DES has both types of chambers for thermal shock testing.

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DES Performs Testing for Orion Rocket Program

Delserro Engineering Solutions (DES) was proud to be a part of the recent Orion rocket that launched on December 5th 2014. DES was contracted by a local manufacturer to help qualify their product designs for use on Lockheed Martin’s Orion rocket. DES’s role was to perform specialized shock testing on their products.

Some of the shock levels were over 3200 G’s to simulate the rugged launch environment. Both the local manufacturer and Lockheed Martin Corporation were pleased with the testing. Their products successfully passed the shock tests at DES. They acknowledged DES’s role and informed DES that their products operated successfully during the launch!

What sets DES apart from other labs is our in depth experience and technical capability to understand and reproduce the most complicated vibration and shock profiles. DES continues to perform the most complex vibration and shock tests on products that are used in outer space, rockets, missiles, automotive & truck environments, military environments, hospitals, etc.

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What Kind Of Failures Occur During HALT?

Product failures in HALT testing are defined as either the cessation of any functions or an out-of-specification condition for any performance characteristic. When a failure occurs, it is documented in DES’s HALT log. The exact time and test condition when each failure occurred is noted.

If the product fails to operate, the temperature or vibration will be changed toward ambient room conditions followed by a short dwell period to see if the product recovers. If the product is non-operational after dwelling at ambient conditions, trouble shooting will take place to find the failed component. The failed component will then be removed, repaired or replaced with a new component (as is practical) in an effort to expand the test stresses.

Relevant Failures

Figure 1 - Failure of Improperly Designed Mounting Tabs — Figure 1 – Failure of Improperly Designed Mounting Tabs

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What Equipment Is Used For HALT?

HALT Chambers

Specialized test chambers are needed to perform a HALT. Typical HALT chambers are shown in Figure 1. The specification for HALT chambers is typically the following:

Liquid nitrogen (LN2) is used to cool the air temperature in HALT chambers. This allows for very rapid temperature changes of 60°C per minute and a cold temperature extreme of -100°C.

HALT chamber heating is provided by high power resistive heating elements that can produce changes of 60°C per minute and a hot temperature limit of +200°C.

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What Is A Typical HALT Procedure?

HALT procedures vary from lab to lab but are typically performed similar to DES’s procedure which is summarized below. DES’s HALT procedure is divided into 5 Stages: Stage 1 – Temperature Step Stresses, Stage 2 – Temperature Ramps, Stage 3 – Vibration Step Stresses, Stage 4 – Combined Temperature &Vibration Stresses, and Stage 5 – Temperature Destruct Limits.

Stage 1 is used to determine the HALT Operational Limits for temperature. The goal is not to cause destruction in Stage 1, but sometimes the operational and destruct limits occur simultaneously. The HALT Destruct Limits for temperature and vibration are typically found in Stages 3 to 5.

Temperature Step Stresses – Stage 1 (Figure 1)

Figure 1. Stage 1 Temperature Steps — Figure 1 – Stage 1 Temperature Steps

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What Is HALT (Highly Accelerated Life Testing) And Why Perform HALT?

Highly Accelerated Life Testing (HALT) is a rigorous reliability test method that is used to expose product weaknesses. The goal of HALT is to proactively find weaknesses and fix them, thereby increasing product reliability. Because of its accelerated nature, HALT is typically faster and less expensive than traditional testing techniques.

HALT of circuit boards in one of DES’s HALT chambers

HALT can be effectively used multiple times over a product’s life time. During product development, it can find design weakness when changes are much less costly to make. By finding weaknesses and making changes early, HALT can lower product development costs and compress time to market. When HALT is used at the time a product is being introduced into the market, it can expose problems caused by new manufacturing processes. When used after a product has been introduced into the market, HALT can be used to audit product reliability caused by changes in components, manufacturing or suppliers etc. The bottom line is that HALT can reduce product development time and cost, reduce warranty costs, improve customer satisfaction, gain market share, and increase profits.

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Temperature Cycling Testing: Coffin-Manson Equation

Temperature cycling testing is another method of accelerated life testing for products that are exposed to temperature variations during use in normal operation. The temperature variations can be a result of self heating for products that are repeatedly turned on and off, or can be the result of cyclic environmental changes — such as temperature variations from day to night — or other causes.

These repeated temperature changes can result in thermal fatigue and lead to eventual failure after many thermal cycles. Accelerated life testing can be performed by cycling the product to high and low temperatures that exceed its normal use temperatures.

It should be noted that temperature cycling may also be referred to as thermal cycling or thermal shock testing. However, some test standards, such as MIL-STD-883, make the distinction between temperature cycling being performed as air to air testing and thermal shock being performed with the samples transferred between liquids. This article deals with testing performed using an air to air thermal cycle chamber.

Typical temperature cycling equipment consists of at least one hot chamber and one cold chamber. The test samples are automatically transferred between the two chambers by an elevator-type mechanism. It is also possible to perform temperature cycling in a single compartment chamber where the temperature is ramped between hot and cold. This generally produces a slower rate of temperature change compared to the two chamber method.

Continue reading Temperature Cycling Testing: Coffin-Manson Equation →

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