Thermal Shock Testing, often referred to as Temperature Shock Testing, Temperature Cycling, or High-Low Temperature Shock Testing, is a crucial environmental test used to evaluate the ability of materials and products to withstand rapid and extreme temperature changes. At Dongguan Precision, we understand the importance of this testing in ensuring the reliability and durability of your products across various operating environments.

According to standards like GJB 150.5A-2009 3.1 and MIL-STD-810F 503.4 (2001), a rapid change in the surrounding atmospheric temperature exceeding 10 degrees Celsius per minute is defined as temperature shock. However, it's important to note that actual temperature shock tests often employ even more severe rates of change, frequently cited as being greater than 20°C/min, 30°C/min, 50°C/min, or even faster.

What Causes These Rapid Temperature Changes?

Various real-world scenarios can lead to rapid temperature fluctuations, as highlighted in standards like GB/T 2423.22-2012 (Environmental testing - Part 2: Tests - Test N: Temperature change):

• Transferring equipment between drastically different temperature environments (e.g., indoors to outdoors).
• Sudden cooling due to rain or immersion in cold water.
• Conditions experienced by externally mounted airborne equipment.
• Specific transportation and storage conditions.
• Internally generated heat gradients within powered equipment.
• Rapid cooling of components with active cooling systems.
• Manufacturing processes.

The frequency, magnitude, and duration of these temperature changes are all critical factors.

Why is Temperature Shock Testing Important?

As outlined in GJB 150.5A-2009 (Military Equipment Laboratory Environmental Test Methods, Part 5: Temperature Shock Test), this test is applied in several contexts:

• Normal Environment Simulation: To assess equipment intended for use in areas where rapid air temperature changes are likely. This evaluates the impact on external surfaces, externally mounted components, and internal parts near the surface during transitions between hot and cold environments, rapid ascents to high altitudes, or even air drops from aircraft.
• Safety and Environmental Stress Screening (ESS): To identify potential safety issues and latent defects in equipment exposed to temperature change rates below extreme levels (within the design limits). It can also be used as a screening test with more extreme temperatures to reveal potential weaknesses.

The Effects of Temperature Shock:

Rapid temperature changes can have significant and varied effects on equipment, particularly on parts near the outer surfaces. The further away from the surface (depending on material properties), the slower the temperature change and the less pronounced the impact. Protective packaging can also mitigate these effects. Temperature shock can cause temporary or permanent operational impairments. Examples of potential issues include:

A) Physical Effects:

1. Fracturing of glass containers and optical instruments.
2. Seizing or loosening of moving parts.
3. Cracking of solid propellants in explosives.
4. Differential expansion or contraction rates of dissimilar materials, leading to induced strain.
5. Deformation or rupture of components.
6. Cracking of surface coatings.
7. Leakage of sealed enclosures.
8. Failure of insulation.

B) Chemical Effects:

1. Separation of components.
2. Failure of protective chemical agents.

C) Electrical Effects:

1. Changes in electrical and electronic components.
2. Electronic or mechanical failures due to rapid condensation or frost formation.
3. Electrostatic discharge.

The Purpose of Temperature Shock Testing:

• Engineering Development: To identify design and manufacturing flaws early in the product lifecycle.
• Product Qualification and Acceptance: To verify a product's ability to withstand temperature shock environments, providing data for design finalization and mass production approval.
• Environmental Stress Screening (ESS): To eliminate early-life failures in products.

Types of Temperature Change Tests:

According to IEC and national standards, there are three main types of temperature change tests:

1. Test Na: Rapid temperature change with specified transition times; air as the medium.
2. Test Nb: Temperature change with a specified rate of change; air as the medium.
3. Test Nc: Rapid temperature change using two liquid baths; liquid as the medium.

Tests Na and Nb use air as the heat transfer medium and typically have longer transition times compared to Test Nc, which utilizes liquids (water or other fluids) for much faster temperature transitions.

Relevant Standards:

Other relevant standards include MIL-STD-883 (Method 1010), JESD22-A104D, JESD22-A106B, JIS C 60068-2-14:2011, JASO D 001, EIAJ ED-2531A, GB897.4-2008/IEC60086-4:2007, GJB548B-2005 (Method 1011.1), GJB128A-97 (Method 1056), and various internal company standards (e.g., automotive).

Key Test Parameters:

• Laboratory ambient temperature
• High temperature
• Low temperature
• Exposure duration at each temperature extreme
• Transition time or rate of temperature change
• Number of test cycles

Stabilization Time:

GJB 150.5A-2009 4.3.7 (Temperature Stabilization): The temperature of the test item should be uniform throughout its external parts before the transition begins.

GB/T 2423.22-2012 7.2.1: After placing the test sample, the air temperature should reach the specified tolerance range within 10% of the exposure duration.

Relative Humidity:

GB/T 2423.22-2012: Does not explicitly mention relative humidity control.

GJB 150.5A-2009 4.3.8 (Relative Humidity): Most test procedures do not control relative humidity. However, it can significantly affect porous materials (e.g., fibrous materials) where absorbed moisture can move and expand upon freezing. Unless specifically required, humidity control is generally not considered necessary for temperature shock testing according to these standards.

Transition Time:

GB/T 2423.22-2012 4.5 (Selection of Transition Time): For two-chamber methods, if the transition cannot be completed within 3 minutes due to the sample size, the transition time (t2) can be increased as long as it does not noticeably affect the test results, using the formula: t2 ≤ 0.05 * t3 (where t3 is the temperature stabilization time of the test sample).

GJB 150.5A-2009 4.3.9 (Transition Time): The transition time should reflect the actual temperature shock duration experienced during the product's life cycle. It should be as short as possible, and any transition time exceeding 1 minute should be justified.

Air Velocity:

GB/T 2423.22-2012: Does not explicitly mention air velocity in the current version (older versions might have specified ≤ 2 m/s).

GJB 150.5A-2009 6.2.2 (Air Velocity): The air velocity around the test item in the test chamber should not exceed 1.7 m/s, unless a different velocity is justified by the equipment platform environment and specified in the test conditions.

Test Item Mounting and Setup:

The test item should be mounted to simulate its actual usage conditions as closely as possible, with necessary connections for testing instruments. Key considerations include:

1. Ensuring accessibility of plugs, covers, and test points for evaluating protective device effectiveness.
2. Replacing normal electrical and mechanical connections not used during the test with simulated connectors for test realism.
3. Testing individual functional units separately if the item comprises multiple independent units. When testing multiple units together, maintain a minimum distance of 15 cm between units and the chamber walls to ensure proper air circulation.
4. Protecting the test item from irrelevant environmental contaminants.

GB/T 2423.22-2012 7.2.2 (Mounting or Support of Test Samples): Unless otherwise specified, mounting or support structures should have low thermal conductivity to ensure the test sample is effectively insulated. When testing multiple samples, they should be placed to allow free air circulation between them and the chamber surfaces.

Determining the Number of Test Cycles:

Temperature cycling induces mechanical stress in the test item, with internal strain increasing with the number of cycles. A common empirical relationship in reliability engineering is:

$N(\Delta T{)}^k=Constant$

Where:

• N = Number of temperature cycles
• ΔT = Temperature change (difference between high and low temperatures)
• k = Exponent (dependent on the failure mechanism)

This is sometimes referred to as the Coffin-Manson formula and can be rewritten to estimate the number of test cycles (Nf2) needed to simulate a desired service life (Nf1):

$N{f}^2=N{f}^1{\left(\genfrac{}{}{0ex}{}{\Delta T^1}{\Delta T^{2/}}\right)}^k$

Where:

• Nf1 = Number of cycles to failure (actual service life)
• Nf2 = Number of cycles to failure (test)
• ΔT1 = Temperature change (actual service environment)
• ΔT2 = Temperature change (test conditions)
• k = 2 for metals experiencing plastic deformation under cyclic loading, 4 for predominantly plastic parts.

Example Calculation:

For an oil pump bracket assembly with a desired service life of 10 years (2 cold starts per day):

• Nf1 = 10 years * 365 days/year * 2 cycles/day = 7300 cycles
• ΔT1 = 50°C - 0°C = 50°C (actual operating temperature range)
• ΔT2 = 80°C - (-40°C) = 120°C (test temperature range)
• k = 4 (assuming predominantly plastic components)

$N{f}^2=7300(50/$120​)4≈220 cycles

Therefore, approximately 220 temperature shock cycles under the given test conditions can simulate 10 years of actual service life.

Understanding these principles and parameters is crucial for designing and interpreting temperature shock tests effectively. At Dongguan Precision, we provide a range of temperature shock chambers and expert guidance to help you ensure the reliability of your products under extreme thermal conditions. Contact us today to discuss your specific testing needs.