Transformer Insulation Film Edge Tear Resistance Testing: Standards, Methods, and Applications

Edge tear resistance testing of transformer insulation films is a critical quality control procedure that evaluates how well polyester, polyimide, and composite insulation films withstand tear initiation and propagation along slit or cut edges during winding, assembly, and long-term service. This technical article explains the failure mechanisms behind edge tearing, the applicable international standards such as ASTM D1922, ISO 6383, and GB/T 11999, and the laboratory methods used by accredited third-party laboratories to measure this property. Understanding these testing procedures helps film suppliers, transformer manufacturers, and insulation system designers ensure dielectric reliability and regulatory compliance.

Technical Overview

Insulation films such as biaxially oriented polyester (PET), polyimide (PI), and polyethylene naphthalate (PEN) serve as interturn, interlayer, and barrier insulation in distribution transformers, dry-type transformers, instrument transformers, and traction equipment. Because these materials are supplied in wide rolls and subsequently slit, cut, punched, or wound into place, the finished insulation almost always contains machined edges. Micro-cracks, burrs, and slitter marks along these edges act as stress concentrators. When the winding is tensioned, thermally cycled, vibrated, or exposed to electromagnetic forces during inrush and short-circuit events, an edge defect can initiate a tear that propagates across the film and compromises the dielectric barrier.

Edge tear resistance testing quantifies the force or energy required to initiate and propagate a tear from the edge of an insulation film. The measured property is closely related to molecular orientation, film thickness, crystallinity, and the quality of the slitting process. Since most transformer-grade films are biaxially oriented, tear behavior is strongly anisotropic, and testing is routinely performed in both the machine direction (MD) and the transverse direction (TD).

Testing is performed by accredited third-party laboratories and in-house quality control facilities using internationally recognized methods, including ASTM D1922, ASTM D1938, ASTM D1004, ISO 6383-1, ISO 6383-2, GB/T 11999, GB/T 16578, and TAPPI T470 for cellulosic materials. The resulting data support incoming inspection, slitting process validation, insulation system qualification, and failure analysis.

Testing Samples

Polyester (PET) Insulation Films

Biaxially oriented PET films in typical thicknesses from 25 µm to 350 µm are the most widely tested sample type. They are used for layer insulation, core wrap, and lead insulation in oil-immersed and dry-type transformers with thermal classes up to 130 °C. Both MD and TD specimens must be submitted because tear resistance differs significantly between directions.

Polyimide (PI) Films

Polyimide films rated for continuous operation at 180 °C and above are tested for high-temperature applications including traction transformers, sealed dry-type units, and aerospace power systems. These films exhibit high tear initiation resistance, so instrument capacity and specimen preparation require special attention.

Composite and Laminated Structures

Film-paper laminates, film-adhesive-film sandwiches, and corona-resistant coated films are tested as finished structures. Testing composite materials reveals whether the laminate construction improves or degrades edge tear performance compared with the individual component films.

Sample Submission Requirements

• Minimum sample quantity of 10 specimens per direction per condition for statistical validity
• Film pieces large enough for die-cutting of full-size test specimens with undamaged edges
• Identification of machine direction, manufacturer lot number, nominal thickness, and thermal class
• Additional material if conditioned, thermally aged, or oil-immersed test conditions are requested

Testing Items

Typical test programs for transformer insulation film edge tear evaluation include the following measurable items:

• Edge tear initiation resistance, the force required to start a tear from a machined or slit edge
• Tear propagation resistance by the Elmendorf pendulum method, reported in millinewtons or gram-force
• Trouser tear strength, the average force required to sustain a propagating tear at a constant separation speed
• Initial tear resistance at low crosshead speed, characterizing resistance to slow, slit-like tearing
• Directional tear strength comparison, MD versus TD, to quantify anisotropy
• Retained tear resistance after thermal aging, evaluated alongside long-term thermal endurance programs such as IEC 60216
• Tear resistance after immersion in transformer oil or exposure to impregnating resins for oil-immersed and cast-resin applications

Testing Methods

Specimen Preparation and Conditioning

Specimens are die-cut with sharp, defect-free cutting tools, since nicked or rough-cut edges bias the result downward. Thickness is verified at multiple points using methods consistent with ISO 4593 or ASTM D6988. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618, because moisture content measurably affects tear behavior in polymer films and cellulosic materials.

Elmendorf Pendulum Method

The pendulum method per ASTM D1922, ISO 6383-2, and GB/T 11999 is the most common procedure for tear propagation resistance. A rectangular specimen is slit along one edge to create a defined tear start. The pendulum is released, and the energy absorbed in tearing the specimen over a fixed distance is converted to tear resistance. The pendulum capacity is selected so that the measured tear energy falls between 20 % and 80 % of the instrument range. A minimum of five valid specimens per direction is tested, and results are reported as the mean tear resistance with standard deviation.

Trouser Tear Method

The trouser tear method per ISO 6383-1, ASTM D1938, and GB/T 16578 uses a rectangular specimen cut along its centerline into two legs. The legs are gripped separately and pulled apart at a constant crosshead speed, typically 200 mm/min for the ISO procedure or 250 mm/min for the ASTM procedure. The average force plateau during steady propagation is recorded and normalized against specimen thickness. This method provides a fundamental tear strength value well suited to comparing film formulations and orientations.

Edge Tear and Initial Tear Methods

Edge tearing resistance of paper-based and cellulosic insulation components is measured with a finch-type edge tear apparatus in the manner described in TAPPI T470, in which load is applied directly to the specimen edge. Initial tear resistance of plastic films is determined with ASTM D1004 using a notched specimen pulled at a low crosshead speed, which reflects resistance to slow, slit-like tear growth that can occur at stressed edges during service.

Data Evaluation and Reporting

• Individual and mean tear values with standard deviation and coefficient of variation
• Separate reporting of MD and TD results, together with the anisotropy ratio
• Instrument identification, pendulum capacity, crosshead speed, and specimen dimensions
• Conditioning atmosphere and duration, and any deviation from standard conditions
• Exclusion of invalid specimens, such as those tearing off-axis or failing at the grip

Applications

Edge tear resistance data are used throughout the transformer value chain. Film producers use the test for release qualification and statistical process control of casting, orientation, and slitting lines. Transformer manufacturers apply it during incoming inspection to verify that supplier material meets purchase specifications, and during winding process development to confirm that tension settings and guide geometry do not degrade film edges. Insulation system qualification programs, including those structured around UL 1446 and related recognized frameworks, incorporate tear data as supporting mechanical evidence. Failure analysts use edge tear testing together with microscopic examination of torn edges to determine whether an insulation breach originated from a slitting defect, mechanical damage during assembly, or aging-induced embrittlement.

• Incoming quality control of PET, PEN, and polyimide insulation films
• Validation of slitting, punching, and cutting processes for edge quality
• Design verification for dry-type, oil-immersed, cast-resin, and traction transformers
• Thermal aging studies and retained-property evaluation for insulation life assessment
• Root-cause investigation of dielectric failures attributed to torn insulation layers
• Independent verification and arbitration testing by accredited third-party laboratories

FAQ

What is the difference between edge tear resistance and tear propagation resistance?

Edge tear resistance describes the resistance of a material edge to the initiation of a tear when stress is concentrated at that edge, which is the dominant failure mode for slit insulation in windings. Tear propagation resistance describes the energy or force needed to continue an existing tear through the material. Both properties matter, because a high-initiation, low-propagation film can still fail rapidly once a tear starts.

Which standards are most commonly specified for this testing?

ASTM D1922 and ISO 6383-2 cover the Elmendorf pendulum method, ISO 6383-1 and ASTM D1938 cover the trouser tear method, ASTM D1004 covers initial tear resistance, and GB/T 11999 and GB/T 16578 are the corresponding national methods. Cellulosic paper and pressboard edge tearing is addressed by TAPPI T470. Purchasing specifications should name the specific method, direction, and acceptance limit.

Why do MD and TD tear results differ for the same film?

Biaxially oriented films have molecular chains aligned preferentially in the plane of the film, and the orientation distribution differs between the machine and transverse directions. A tear propagates more easily when it runs parallel to a weak orientation axis, so MD and TD tear values can differ by a large factor. Both directions must be measured to fully characterize winding suitability.

How does humidity and temperature affect the results?

Polymer films absorb small amounts of moisture that can plasticize the material and raise tear resistance slightly, while cellulosic insulation is far more sensitive to humidity. Standard conditioning at 23 °C and 50 % relative humidity is therefore mandatory for comparable results. For high-temperature service, testing after thermal aging gives a more realistic picture of long-term performance.

How many specimens are needed for a valid test?

A minimum of five valid specimens per direction is typical for the pendulum method, and ten or more are recommended when characterizing anisotropic films or resolving borderline acceptance values. Specimens that tear off-axis, fail at the grips, or show preparation defects are excluded and replaced.

What acceptance criteria are typically applied?

Acceptance limits are set by the material specification between purchaser and supplier, often referencing the minimum tear values published in relevant IEC, ASTM, or GB product standards for the insulation class. Generic universal limits are not appropriate because tear resistance scales strongly with thickness, polymer type, and orientation, so each film grade must be evaluated against its own specification.

Can testing be performed on material taken from a failed transformer?

Yes. Laboratory teams can die-cut specimens from recovered film sections, provided enough undamaged area remains away from the failure site. Comparing the tear resistance of service-aged material with that of retained new material is a well-established technique for diagnosing aging embrittlement and confirming root cause.