How to Determine Conditions for Sine Vibration Testing？

Sine vibration testing is a fundamental mechanical environmental test designed to simulate the effects of sinusoidal vibrations that products may encounter during transportation, storage, and actual use within a controlled laboratory setting. These vibrations are often induced by rotating, pulsating, or oscillating forces originating from sources like aircraft, vehicles, ships, airborne equipment, and ground machinery. The severity of a sine vibration test, or its rigor level, is jointly defined by three key parameters: frequency, amplitude, and test duration.

At Dongguan Precision Test Equipment Co., Ltd., we provide advanced vibration test systems to meet a wide range of testing standards. Understanding how to determine these crucial test conditions is essential for accurate and meaningful product evaluation. Let's break down the key aspects.

I. Defining Essential Terms (Based on GB/T 2298-2010)

To ensure clarity, let's define some relevant terms according to the Chinese national standard for mechanical vibration, shock, and condition monitoring:

1. Sinusoidal Vibration: A periodic vibration whose vibration parameter is described by a sine function of time.
2. Frequency (f): The reciprocal of the period. Measured in Hertz (Hz), representing cycles per second.
3. Amplitude:
   • Displacement (Relative Displacement): The time-varying quantity representing the change in position of a point on an object relative to a reference frame.
   • Peak-to-Peak Value (of a Vibration): The difference between the maximum positive and maximum negative values of a vibration within a given time interval. Its magnitude depends on the measurement system's response or rise time.
4. Cross-over Frequency: The frequency at which a specific characteristic of vibration transitions from one relationship to another. For example, the frequency at which the vibration amplitude or RMS value changes from a constant displacement-frequency relationship to a constant acceleration-frequency relationship.

II. Navigating the Standards Landscape

A multitude of vibration testing standards exist, varying across countries, regions, and industries. Some prominent standard organizations and their frameworks include: ASTM, ISTA, MIL-STD, EN, IEC, ETSI, JIS, SAE, JASO, ISO, and AEC.

Commonly referenced sine vibration test standards include:

• GB/T 2423.10-2008: Environmental testing for electric and electronic products - Part 2: Testing methods - Test Fc: Vibration (sinusoidal)
• IEC 60068-2-6-2007: Environmental testing - Part 2-6: Tests - Test Fc: Vibration (sinusoidal) 
   

   
• ISO 8318:2000: Packaging - Complete, filled transport packages and unit loads - Vibration tests using a variable frequency
• GB/T 4857.10-2005: Packaging - Basic tests for transport packages - Part 10: Sinusoidal variable frequency vibration test method

III. Understanding Test Methods: Swept Sine vs. Fixed Frequency

Within sine vibration testing, two primary test methods are employed:

1. Swept Sine Testing:

This method involves maintaining one or two vibration parameters (displacement, velocity, or acceleration) at a constant level while continuously varying the vibration frequency within a defined range. The sweep can be:

• Linear Sweep: The frequency changes linearly with time (e.g., Hz/s or Hz/min). This is often used for identifying resonant frequencies.
• Logarithmic Sweep: The frequency changes logarithmically with time (e.g., oct/min or dec/min). A sweep rate of one octave per minute means the frequency doubles (or halves) every minute. Logarithmic sweeps are often used for endurance testing, as they spend equal time across frequency decades, resulting in slower sweeps at lower frequencies and faster sweeps at higher frequencies.

Swept sine testing is primarily used for:

• Vibration Response Analysis (Resonance Search): Identifying the natural frequencies (resonances) of a product and assessing its stability during operation to pinpoint areas requiring vibration isolation or reinforcement.
• Endurance Swept Sine Testing: When a product exhibits no significant resonances within its operating frequency range, or has multiple minor resonances, an endurance sweep is performed. This typically involves a logarithmic sweep with constant displacement amplitude at lower frequencies and constant acceleration amplitude at higher frequencies, with a cross-over frequency typically between 55-72 Hz and a sweep rate of one octave per minute.
• Post-Endurance Resonance Check: Repeating the initial resonance search after endurance testing to determine if any resonant frequencies have shifted, indicating potential structural changes or degradation.

2. Fixed Frequency Testing:

This method involves subjecting the test specimen to vibration at specific, fixed frequency points with varying levels of other vibration parameters (amplitude). It is primarily used for:

• Resonance Dwell Testing: Applying vibration at a significant resonant frequency identified during the resonance search to evaluate the product's ability to withstand prolonged vibration at its natural frequency.
• Predetermined Frequency Testing: Simulating known vibration frequencies encountered in the product's actual operating environment to assess its performance and durability under those specific conditions.

IV. Determining the Test Conditions (Severity Level)

Selecting the appropriate test conditions (frequency, amplitude, and duration) is crucial for a relevant and effective sine vibration test.

1. Frequency and Frequency Range:

• Some standards (especially military standards) directly specify the test frequency or frequency range based on the product's operational environment.
• Commercial standards often define the frequency range using different approaches.

Considerations for Determining Frequency Range:

• Products may experience very low vibration frequencies during transportation or operation (e.g., vehicle-mounted equipment with fundamental frequencies as low as 1.5-4 Hz). However, achieving accurate low-frequency vibration (below 1 Hz) on standard vibration test equipment can be challenging due to waveform distortion.
• When defining the frequency range, it's essential to consider the capabilities of the available vibration test system. Hydraulic shakers are often suitable for lower frequencies (1-200 Hz), while electrodynamic shakers excel across a broader range (typically 0-3000 Hz, with lower limits around 5-10 Hz for modern systems). For wide frequency ranges extending to very low frequencies, a compromise on the lower frequency limit might be necessary when using electrodynamic shakers.

2. Amplitude and its Selection:

In sine vibration testing, amplitude is defined by either displacement (peak or peak-to-peak) or acceleration (peak). Some standards specify only displacement, while others provide both.

• Displacement Amplitude Only: IEC and national standards often specify only displacement amplitude for tests with an upper frequency limit of 10 Hz. Similarly, some military standards for electronic and electrical components specify a constant displacement amplitude (e.g., 0.75 mm single amplitude) within a specific frequency range (e.g., 10-55 Hz).
• Combined Displacement and Acceleration Amplitude: In real-world vibration scenarios, lower frequencies typically exhibit larger displacement amplitudes, while higher frequencies exhibit larger acceleration amplitudes. Modern sine vibration testing attempts to approximate this by using a constant displacement amplitude at lower frequencies (referred to as "constant displacement") and a constant acceleration amplitude at higher frequencies ("constant acceleration"). The frequency at which the control mode transitions from constant displacement to constant acceleration is called the cross-over frequency. IEC and national standards often define two cross-over frequencies: a lower one around 8-9 Hz (primarily for shipboard equipment) and a higher one around 57-62 Hz (mainly for land-based and airborne equipment).

3. Test Duration and its Selection:

The test duration is a critical parameter for assessing a product's ability to withstand vibration. However, determining the equivalent real-world exposure time for a given laboratory test duration can be challenging.

• For swept sine tests, the duration is typically specified by the number of sweep cycles.
• For fixed frequency tests, the duration is usually given in minutes or hours.
• IEC and national standards provide a range of sweep cycle levels (e.g., 1, 2, 5, 15, 20, 50, 100) and fixed frequency test durations (e.g., 10 min, 30 min, 1 hr, 1.5 hr, 2 hr, 10 hr). Military standards for microelectronic devices and electronic/electrical components often specify 12 cycles (three axes) or 36 cycles (three axes). A common sweep rate for swept sine testing is one octave per minute.

The specific choice of test duration is typically based on the failure mechanisms being investigated:

• Impact on Performance: If the focus is on temporary performance degradation (e.g., malfunction, instability) that recovers after vibration ceases, the test duration can be based on the longest expected continuous operating time combined with practical experience.
• Structural Integrity: For assessing structural damage (e.g., solder joint failure, loose screws, connector detachment, component collision), shorter durations (e.g., 30 minutes to 1 hour) might be sufficient to reveal such issues. For screw loosening, connector detachment, and component collision, the longest continuous operating time can also be considered. In some cases, the product's entire lifespan might be relevant.
• Cumulative Stress (Fatigue): To determine a product's ability to withstand accumulated stress, the test duration should be based on the number of stress cycles expected during its service life or based on an infinite life criterion (e.g., 10^7 stress cycles).

Partnering with Dongguan Precision for Tailored Vibration Testing Solutions:

Determining the appropriate sine vibration test conditions requires a thorough understanding of the product's intended environment, relevant industry standards, and potential failure mechanisms. At Dongguan Precision Test Equipment Co., Ltd., our experienced team can provide expert guidance in selecting the optimal frequency, amplitude, and duration for your specific application, ensuring accurate and meaningful vibration testing results. Contact us today to discuss your requirements and explore our advanced vibration test systems.