Tensile strength testing of transformer insulation films is a critical quality control and design verification procedure that evaluates the mechanical integrity of polymer dielectric films used for interturn, interlayer and barrier insulation in transformers and rotating electrical machines. Performed in accredited third-party laboratories in accordance with internationally recognized standards such as IEC 60674-2, ASTM D882 and ISO 527-3, the test quantifies tensile strength, elongation at break and tensile modulus to ensure that films withstand winding stresses, thermal aging and long-term mechanical loading. This article presents a comprehensive technical overview covering sample types, testing items, test methods, applications and frequently asked questions.
Technical Overview
Insulation films are thin polymeric materials, typically ranging from 10 to 350 micrometers in thickness, that serve as essential dielectric barriers in transformers. During coil winding, core assembly, clamping and operation, these films are subjected to significant mechanical stresses in the form of tension, bending, abrasion and compression. A film with insufficient tensile strength may tear during winding or crack under thermal cycling, creating partial discharge sites and ultimately leading to dielectric failure.
Tensile strength testing measures the maximum tensile stress a film can sustain before fracture, together with its elongation at break and elastic modulus. The test is conducted on a universal testing machine operating at a constant rate of extension, using calibrated load cells and precision grips suitable for thin, flexible specimens. Because most electrical insulation films are biaxially oriented during manufacturing, testing is performed separately in the machine direction (MD) and transverse direction (TD) to fully characterize the anisotropic mechanical behavior of the material.
The test is typically governed by IEC 60674-2, ASTM D882, ISO 527-1 and ISO 527-3, GB/T 13541, or JIS K 7127, depending on the market region, customer specification and material type. Results are used for incoming material inspection, supplier qualification, design verification and failure analysis in transformer manufacturing.
Testing Samples
Polyester (PET) Insulation Films
Biaxially oriented polyethylene terephthalate films are the most widely used insulation films in distribution transformers, providing excellent dielectric strength, good mechanical toughness and cost efficiency. They are generally assigned thermal class E or B (120 to 130 °C) and are used for layer insulation, interturn insulation, wrap insulation and lead exit taping. Typical thicknesses range from 20 to 350 micrometers in roll or sheet form.
Polyimide (PI) Films
Polyimide films offer outstanding thermal stability with a continuous service capability of up to 220 °C or higher (thermal class H/C), exceptional dielectric performance across a wide temperature range and inherent flame resistance. They are specified for traction transformers, furnace transformers, dry-type transformers and other high-temperature applications. Corona-resistant polyimide film variants are used in equipment exposed to high-frequency voltage waveforms from power electronics converters.
Polyethylene Naphthalate (PEN) Films
PEN films combine mechanical properties similar to polyester films with higher thermal endurance, typically rated at thermal class F (155 °C). They are increasingly adopted in compact transformers and rotating machines where an improved thermal margin over standard PET film is required without the cost of polyimide.
Flexible Laminates and Composite Films
Composite structures such as polyester-film-based laminates (for example, a nonwoven polyester fabric bonded to both sides of a PET film core) and aramid-polyimide laminates (aramid paper bonded to both sides of a polyimide film core) combine the dielectric strength of the film with the mechanical resilience and impregnation-friendly surface of the fibrous layers. These laminates are widely used as slot insulation, phase insulation and interlayer barriers in dry-type transformers, motors and generators. Samples are usually submitted as rolls, sheets or finished laminate strips.
Testing Items
The core parameters evaluated in a tensile strength test program for transformer insulation films include:
- Tensile strength in the machine direction and transverse direction, expressed in MPa
- Elongation at break in both directions, expressed as a percentage
- Tensile stress at break and, where required, tensile stress at specified strain
- Tensile modulus (Young's modulus) derived from the initial linear region of the stress-strain curve
- Statistical evaluation of results, including mean values, minimum values and standard deviation
- Comparison of measured values against the specification limits defined in the applicable material standards or customer drawings
- Optional supplementary evaluations such as tensile properties after thermal aging or heat treatment, where required by the insulation system qualification program
Testing Methods
Standards Framework
The principal test methods referenced by transformer manufacturers and accredited third-party laboratories include IEC 60674-2 (methods of test for electrical insulating plastic films, part of the IEC 60674 series), ASTM D882 (tensile properties of thin plastic sheeting), ISO 527-1 and ISO 527-3 (determination of tensile properties of plastics, including films), GB/T 13541 (test methods for electrical insulating plastic films) and JIS K 7127 (tensile properties of plastic film). While equipment requirements are broadly similar, specimen dimensions, gauge length and crosshead speed differ between standards, so the governing method must be agreed before testing.
Specimen Preparation and Conditioning
Rectangular strip specimens, commonly 10 mm or 15 mm in width with a length sufficient for the grip separation, are cut cleanly from the film in both the machine direction and the transverse direction. Edges must be smooth and free of nicks, since any defect acts as a stress concentrator and reduces measured elongation. Film thickness is measured at several points along each specimen using a calibrated low-pressure thickness gauge in accordance with ISO 4593 or equivalent practice, because thickness measurement is the dominant source of uncertainty in the calculated stress. Specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum period defined by the governing standard, typically 40 hours or more, before testing.
Test Procedure
The test is performed on a universal testing machine with a calibrated force transducer of appropriate capacity, typically in the range of 100 N to 1 kN for thin films. A gauge length of 100 mm is widely used, and the crosshead speed is selected according to the standard and the expected elongation of the material, commonly 5 to 100 mm/min with 50 mm/min frequently applied to rigid electrical-grade films. Each specimen is clamped in pneumatic or mechanical grips faced with rubber or emery cloth to prevent slippage without causing premature fracture, a small preload is applied, and the specimen is extended to break at constant speed while force and extension are continuously recorded. At least five valid specimens are tested in each direction, and specimens that fail within or at the grips are discarded and replaced, as such failures underestimate the true material strength.
Calculation and Evaluation
Tensile strength is calculated by dividing the maximum force recorded during the test by the original cross-sectional area (specimen width multiplied by measured thickness). Elongation at break is calculated as the extension at break divided by the initial gauge length, expressed as a percentage. The tensile modulus is determined from the slope of the initial linear portion of the stress-strain curve using the chord or regression method specified in the standard. The test report identifies the material, thickness, direction, conditioning, standard used, individual and mean results, and the location and type of fracture for each specimen.
As an indicative reference, biaxially oriented polyester films typically exhibit tensile strength in the range of 160 to 230 MPa with elongation at break of 60 to 120 %, polyimide films commonly reach 170 to 250 MPa with elongation of 40 to 80 %, and flexible laminate composites generally fall between 30 and 90 MPa depending on the ratio of film to fibrous layers. These values are orientation- and thickness-dependent and must always be verified against the relevant product specification and datasheet.
Applications
Tensile strength testing of transformer insulation films supports quality assurance and engineering activities across the electrical equipment industry, including:
- Incoming inspection of insulation film rolls and laminate sheets by transformer and rotating machine manufacturers
- Material qualification and supplier approval for new film grades, corona-resistant films and composite laminates
- Layer, interturn, barrier and wrap insulation verification in oil-immersed power and distribution transformers
- Slot liner, phase insulation and end-winding insulation evaluation in dry-type transformers, motors and generators
- Insulation system qualification programs involving aged and unaged comparative testing for thermal class assessment
- Failure analysis of insulation breakdown events, where degraded tensile properties often reveal thermal overstress or moisture exposure
- Compliance demonstration against material specification standards within the IEC 60674-3 series and equivalent national standards
FAQ
Why must insulation films be tested in both the machine direction and the transverse direction?
Electrical insulation films are biaxially oriented during manufacture, which produces deliberate anisotropy in mechanical properties. Machine direction and transverse direction values can differ substantially in both strength and elongation, and winding operations apply stress in different directions depending on the geometry. Testing both directions guarantees that the weaker axis still satisfies the specification.
Which test standard should be selected?
The choice depends on the target market, the customer specification and the material specification sheet. IEC 60674-2 is widely referenced in international tenders, ASTM D882 and ISO 527-3 dominate in North America and general plastics applications, while GB/T 13541 and JIS K 7127 apply in the Chinese and Japanese markets respectively. When specimen geometry, gauge length and speed are aligned, results from different standards are generally comparable.
What are typical acceptance criteria?
Acceptance limits are defined by the material specification rather than the test method. Common specification minima are approximately 150 MPa for polyester films and 135 to 165 MPa for polyimide films, with additional requirements on elongation at break. The purchase specification, drawing or relevant part of the IEC 60674-3 series always takes precedence over generic reference values.
How does specimen preparation affect the results?
Thin films are highly sensitive to edge quality. Nicks, burrs or ragged cut edges concentrate stress and can reduce measured elongation at break by a significant margin while barely affecting tensile strength. Sharp die-cutting or blade cutting, careful handling with gloves and clean conditioning are essential for reproducible results.
What should be done when specimens break inside the grips?
Grip-end fractures are invalid because they reflect grip pressure rather than true material strength. The test is repeated with adjusted grip pressure, compliant grip faces or a longer grip separation, and all grip-break results are excluded from the statistical evaluation in accordance with the governing standard.