Tear Strength Testing of Thermally Conductive Pads for Power Battery Applications

Tear strength testing of thermally conductive pads is a critical mechanical evaluation for power battery packs, quantifying a thermal interface material's resistance to crack initiation and propagation during assembly, compression, and long-term service. Performed in accordance with ASTM D624, ISO 34-1, and GB/T 529, the test helps battery manufacturers verify that thermal pads will survive handling, cell expansion cycles, and vibration without compromising heat transfer. This article provides a comprehensive technical reference covering sample types, test items, test methods, applications, and frequently asked questions.

Technical Overview

Thermally conductive pads are soft elastomeric thermal interface materials (TIMs) installed between battery cells, between modules and liquid cooling plates, and between pouch cells and structural housings in electric vehicle power battery packs. Their function is to eliminate air gaps at these interfaces and to conduct heat efficiently from the cell surfaces into the cooling system. To achieve thermal conductivity levels typically ranging from 1 to 8 W/m·K, these pads are heavily loaded with ceramic fillers such as aluminum oxide, aluminum nitride, or boron nitride, often at 60 to 90 percent by weight.

This high filler loading significantly reduces the continuity of the polymer matrix, which in turn lowers the material's resistance to tearing. During battery pack assembly, thermal pads are die-cut, bent, stretched, and compressed; during service, they must endure repeated cell breathing, swelling and contraction, mechanical vibration, and thermal cycling. A pad that tears during installation or operation develops voids at the interface, which sharply increase thermal resistance, create localized hotspots, accelerate capacity fade, and in extreme cases contribute to thermal runaway risks.

Tear strength is defined as the maximum or median force required to propagate a tear through a precut nick or slit in the specimen, normalized by the specimen thickness, and is expressed in kN/m. The principal test standards applied to thermal pads include ASTM D624, ISO 34-1, and GB/T 529, with ISO 34-2 applicable to small-size test pieces. Tear strength testing is routinely performed by accredited third-party laboratories as part of material qualification, incoming quality control, and design validation for battery thermal management systems.

Testing Samples

Silicone Thermally Conductive Pads

Silicone-based pads are the most widely used category in power battery applications. They consist of a platinum-cured or peroxide-cured silicone elastomer matrix filled with thermally conductive ceramics, with typical thicknesses of 0.5 mm to 10 mm and hardness values from Shore 00 30 to Shore A 60. Their high elongation and softness allow excellent gap filling under low clamping pressure, and typical tear strength values range from approximately 5 to 25 kN/m depending on formulation and filler content.

Non-Silicone Gap Filler Pads

Non-silicone pads based on acrylate, urethane, or olefin chemistries are selected where silicone volatiles must be avoided, such as in certain electrical contact environments or sensitive optical assemblies within battery management systems. These materials generally exhibit lower elongation than silicone pads, making tear characterization particularly important for confirming their robustness during automated placement and compression.

Flame-Retardant Cell-to-Cell Pads

Cell-to-cell pads used between prismatic or pouch cells are commonly required to meet UL 94 V-0 flame ratings while maintaining compressibility across large cross-sections. These thicker, highly filled pads are frequently tear-tested at their full supplied thickness to verify that handling and cell expansion will not initiate tearing at pad edges or cut features such as through-holes and notches.

Specimen Preparation and Conditioning

Test specimens are die-cut directly from production-grade sheet material using sharp, calibrated cutting dies to ensure clean edges free of nicks and burrs that could artificially lower results. Specimen thickness is measured at the tear region with a gauge of 0.01 mm resolution. Prior to testing, samples are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 16 hours in accordance with ISO 23529 or the equivalent conditioning clause of the governing test standard.

Testing Items

The core deliverable of the test is the tear strength of the thermal pad, supported by the underlying force and dimensional data. A typical test report from an accredited third-party laboratory includes the following items:

• Tear strength in kN/m, determined by trouser, angle, or crescent test method as specified
• Maximum tear force in N for angle and crescent specimens, or median tear force in N for trouser specimens
• Specimen thickness measured at the tear path, in mm
• Crosshead speed, grip separation, and test temperature at the time of testing
• Failure mode description, including whether the tear followed the intended path or deviated
• Individual and median results for a minimum of five specimens per condition
• Optional retention percentage after aging, such as tear strength retention following heat aging, damp heat exposure, or thermal cycling

Where a broader mechanical qualification package is required, tear strength is commonly tested alongside complementary properties including tensile strength and elongation per ASTM D412 or ISO 37, hardness per ASTM D2240 or ISO 48, compression set per ASTM D395 or ISO 815, thermal conductivity per ASTM D5470 or ISO 22007-2, and flammability per UL 94.

Testing Methods

Apparatus and Test Setup

Testing is performed on a calibrated universal testing machine equipped with a suitable low-capacity load cell, typically in the range of 100 N to 1 kN depending on the expected tear force, together with pneumatic or mechanical grips designed to prevent specimen slippage without inducing premature failure. A calibrated die, a precision thickness gauge, and a conditioned laboratory environment complete the setup.

Trouser Test Method

The trouser method, described in ISO 34-1 Method A and ASTM D624 Type T, uses a rectangular specimen with a central slit that forms two legs. Each leg is secured in a separate grip, and the legs are pulled apart so that the tear propagates along the slit at a controlled rate. The force trace produced is typically jagged, so the median force rather than the peak force is used in the calculation. The trouser method is well suited to highly extensible silicone pads because it produces steady tear propagation and gives a good representation of tear resistance during tearing of soft materials.

Angle Test Method

The angle method, specified in ISO 34-1 Method B and ASTM D624 Type C, uses a right-angle specimen with a small nick or cut at the internal corner. When the specimen is stretched, the geometry concentrates stress at the nick tip, and the tear initiates from this point. The maximum force recorded during the test is used for the calculation. This method is widely referenced in electric vehicle supply specifications and is favored for its reproducibility and simple specimen geometry.

Crescent Test Method

The crescent method, covered in ISO 34-1 Method C and available within the ASTM D624 family of specimen dies, uses a specimen with a curved profile and a precut nick at the center of the concave face. Similar to the angle method, tear initiation occurs at the stress-concentrated nick and the maximum force is used. The crescent geometry can be advantageous for materials where a controlled initiation point is needed to compare crack growth resistance between formulations.

Calculation and Data Evaluation

Tear strength is calculated by dividing the relevant tear force by the specimen thickness measured at the tear path. For trouser specimens, the median force of the trace is used; for angle and crescent specimens, the maximum force is used. The result is reported in kN/m or N/mm, and the median of at least five valid specimens is taken as the representative value. Test speed is 500 mm/min for angle and crescent specimens in both ASTM D624 and ISO 34-1, while the trouser method in ISO 34-1 is conducted at 100 mm/min. Invalid results arising from tear deviation, grip slip, or specimen defects are discarded and replaced with additional specimens.

Influence Factors and Quality Controls

Key factors affecting tear strength results include filler dispersion quality, degree of cure, die sharpness, nick geometry, test speed, ambient temperature, and specimen thickness. To ensure comparability, laboratories control these variables through calibrated dies, verified conditioning, and standardized test parameters, and they document any deviations in the test report so that results can be properly interpreted by battery design and quality engineers.

Applications

Tear strength testing of thermal pads supports the entire lifecycle of power battery thermal management materials, from development to production surveillance. Typical applications include:

• Incoming quality control of thermal pads at cell, module, and pack manufacturing plants
• Material qualification and supplier change validation for new thermal interface material platforms
• Design validation and production validation (DV/PV) test programs for battery pack development
• Durability assessment through tear strength retention measurement after heat aging, damp heat exposure at 85 °C and 85 % relative humidity, and thermal cycling between -40 °C and elevated temperatures
• Failure analysis when pad tearing is suspected as a root cause of elevated interface thermal resistance or localized overheating
• Benchmarking of competing silicone and non-silicone thermal pad formulations during material selection
• Generation of objective evidence supporting conformance with battery safety frameworks such as UL 2580, GB 38031, and IEC 62660 series requirements
• Independent verification through accredited third-party laboratories for supplier approval and customer documentation packages

FAQ

Why is tear strength so important for power battery thermal pads?

Thermal pads are stretched, folded, and compressed during automated and manual installation, and they experience continuous mechanical stress from cell swelling, contraction, and vibration during battery operation. If the pad tears, an air void forms at the interface, dramatically increasing thermal resistance. This creates localized hotspots that accelerate cell aging and can compromise pack safety, so tear strength is a direct indicator of both assembly robustness and long-term reliability.

Which test method should be selected: trouser, angle, or crescent?

The choice is usually dictated by the applicable material specification. The trouser method is preferred for very soft and highly extensible silicone pads because it provides a stable median-force measurement. The angle method is the most commonly referenced in electric vehicle specifications and offers good reproducibility with a simple specimen. The crescent method is useful when controlled tear initiation is desired. When in doubt, testing to both trouser and angle geometries provides the most complete characterization.

What tear strength values are typically considered acceptable?

Commercial silicone thermal pads generally exhibit tear strengths between approximately 5 and 25 kN/m. Many power battery specifications set a minimum requirement in the range of 5 to 10 kN/m for as-received material, together with a retention requirement, commonly 50 percent or greater, after thermal or environmental aging. The final acceptance criterion should always be agreed between the battery manufacturer and the material supplier based on the specific mechanical loading conditions of the application.

Can thin thermal pads below 1 mm be tested?

Yes. Tear strength results are thickness-normalized, but very thin specimens produce low absolute forces and are more sensitive to die quality and edge defects. A low-capacity, high-resolution load cell and freshly sharpened dies are recommended. Testing should always be performed at the nominal supplied thickness rather than on stacked layers, since stacking alters the tear geometry and invalidates the result.

Is it possible to test tear strength after environmental conditioning?

Yes, and it is a common practice in battery material qualification. Specimens are first exposed to defined conditions such as high-temperature storage, damp heat, thermal cycling, or compression set aging, and then tear-tested under the same standard conditions as unaged reference specimens. The reported tear strength retention percentage quantifies how the material's mechanical integrity survives representative service environments, which is essential information for long-life electric vehicle battery packs.