Hail resistance testing evaluates the ability of plastic films, including greenhouse cover films, photovoltaic encapsulant films, and architectural membrane films, to withstand ice impact without puncture, tearing, or loss of mechanical integrity. This article explains the test principle, applicable standards from ISO, IEC, ASTM, EN, GB, and UL, typical sample types, test items, procedures, and acceptance criteria, helping manufacturers and buyers verify film durability in hail-prone regions through accredited third-party laboratories.
Technical Overview
Plastic films are widely used as outdoor protective and functional layers in greenhouse agriculture, photovoltaic (PV) modules, architectural structures, and industrial covering systems. Because these applications expose thin polymer films directly to severe weather, hail impact is one of the most destructive mechanical loads they encounter. A single hailstorm can cause punctures, through-tears, crack initiation, delamination of multilayer structures, and catastrophic loss of protective function.
Hail resistance testing of plastic films simulates the kinetic energy of ice stones striking a film surface under controlled laboratory conditions. Depending on the product and specification, testing is performed either as a direct simulated hail impact test, in which calibrated ice balls are launched at defined velocities against the film or a film-based laminate, or as standardized mechanical impact testing, such as free-falling dart and puncture methods, which quantify the fundamental impact toughness of the film material.
- Simulated hail tests are mainly specified in IEC standards for PV module qualification, together with their GB and EN national adoptions.
- Fundamental impact tests for films are specified in ISO, ASTM, and GB standards.
- UL standards for PV module safety also include impact evaluation relevant to film-based laminates.
- Equivalent JIS film testing methods are available for the Japanese market.
Accredited third-party laboratories combine these methods to provide manufacturers, distributors, and end users with an objective assessment of whether a plastic film product can survive realistic hail events in service.
Testing Samples
Greenhouse and Agricultural Cover Films
Low-density polyethylene (PE), ethylene-vinyl acetate (EVA), polyolefin (PO) multilayer co-extruded, and soft PVC films used as greenhouse covering materials, typically ranging from approximately 0.05 mm to 0.30 mm in thickness. Films may be tested in the as-received state or after artificial weathering to represent aged material.
Photovoltaic Encapsulant Films and Backsheets
EVA, POE, and EPE encapsulant films, as well as PET-based fluoropolymer backsheet films, are usually tested after lamination into glass-film-film or glass-film-backsheet coupons and mini-modules, because hail performance depends on the complete laminate rather than the film alone. Testing follows the hail test philosophy of IEC 61215 and GB/T 9535.
Architectural and Structural Membrane Films
ETFE, FEP, and PVDF films used in skylights, façade systems, and inflated cushion structures for stadiums, airports, and botanical gardens. These films are evaluated for resistance to puncture and tear propagation under ice ball impact.
Industrial and Protective Film Laminates
Laminated woven films, tarpaulin composites, anti-hail covering laminates, and heavy-duty protective films for outdoor equipment and storage, where hail impact resistance is a key purchasing specification.
Testing Items
The scope of hail resistance evaluation for plastic films typically includes the following test items:
- Simulated hail impact test using launched ice balls of specified diameter and velocity.
- Free-falling dart impact resistance (ASTM D1709, ISO 7765-1, GB/T 9639), reporting the impact mass causing 50% failure (F50).
- Pendulum impact resistance of plastic film (GB/T 8809).
- Instrumented puncture impact (ISO 6603-2, ISO 7765-2), reporting peak force and puncture energy with full force-displacement data.
- Tensile strength and elongation at break (ASTM D882, ISO 527-3, GB/T 1040.3) before and after hail or weathering exposure.
- Tear resistance (ASTM D1922, ASTM D1004, ISO 6383-2).
- Combined aging and hail sequence: artificial weathering (for example ISO 4892-2 xenon-arc exposure) followed by hail impact, to evaluate retained performance.
- Post-impact integrity assessment, including visual inspection, leak or pressure-hold testing for greenhouse films, and, for PV laminates, dielectric withstand and electroluminescence imaging to reveal hidden damage.
Testing Methods
Simulated Hail (Ice Ball) Impact Test
Test specimens or laminated coupons are mounted on a rigid support frame that reproduces the actual installation condition as closely as possible. Ice balls are frozen from purified water, and their diameter, mass, and temperature are controlled before launching. A pneumatic launcher propels the ice ball perpendicular to the specimen surface, and the velocity is measured immediately before impact using photoelectric gates.
Impact severity is defined by the relationship between ice ball size and velocity. Following the calibration approach used in IEC 61215-type hail qualification, a 25 mm ice ball is typically launched at approximately 23 m/s, while larger stones are fired at higher velocities, for example a 75 mm ball at approximately 39.5 m/s, reflecting the higher terminal velocity of large hailstones in nature. The test is performed at a defined array of impact points covering corners, edges, and the central area of the specimen. Depending on the specification, specimens may be conditioned at low temperature before impact to represent hailstorm conditions realistically.
Free-Falling Dart Test
Following ASTM D1709 or ISO 7765-1, film specimens are clamped over an annular opening, and a hemispherical-headed dart with a defined drop height is released onto the film. Using the staircase (statistical bracketing) method, the mass at which 50% of specimens fail is determined as the F50 impact strength value. This provides a reproducible, quantitative index of film impact toughness that can be compared across materials, formulations, and production batches.
Supporting Mechanical Tests
Tensile, tear, and instrumented puncture tests are performed on the same material to build a complete mechanical profile. For weathering-related studies, specimens are exposed in xenon-arc or fluorescent UV apparatus before impact testing, allowing the hail resistance of new and artificially aged film to be compared.
Acceptance Criteria
Typical pass criteria, which may be adapted to customer or product specifications, include:
- No through-puncture or through-crack at the specified hail diameter and velocity.
- Tear length from the impact point not exceeding the specified limit.
- No delamination between film layers visible after impact.
- Retention of tensile strength and elongation at or above the specified percentage.
- For PV laminates, no cracks detected by electroluminescence imaging and successful dielectric withstand after impact.
Applications
Hail resistance test data for plastic films support a wide range of engineering, commercial, and regulatory purposes:
- Qualification of greenhouse cover films for hail-prone agricultural regions and high-altitude climates.
- Design verification and incoming material control of PV encapsulant and backsheet films within module development programs.
- Product certification and market access documentation supported by accredited third-party laboratories.
- Insurance risk assessment and loss-prevention engineering for protected agriculture and building envelope projects.
- Comparative evaluation of competing film formulations, thicknesses, and multilayer structures.
- Warranty substantiation and quality control for continuous film production lines.
FAQ
What hail sizes and impact velocities are used in the test?
Ice ball diameters commonly range from 12.5 mm to 75 mm. Velocity increases with stone size; for example, a 25 mm ball is launched at approximately 23 m/s, while a 75 mm ball is launched at approximately 39.5 m/s. The specific diameter and velocity are selected according to the applicable standard or the hail climate class agreed with the client.
How does the ice ball test differ from the free-falling dart test?
The ice ball test reproduces an actual hailstone strike on the installed product or laminate and is used for pass/fail qualification. The free-falling dart test measures the intrinsic impact toughness of the film material and reports a statistical F50 value. The two methods are complementary: the dart test characterizes the film itself, while the ice ball test validates the complete application.
Can hail testing be performed on aged film?
Yes. Specimens are first exposed to artificial weathering, for example xenon-arc exposure according to ISO 4892-2, and then subjected to hail impact. Comparing the results with unaged material quantifies how UV, heat, and moisture degradation affect long-term hail resistance.
What samples and quantities are required?
For dart and puncture tests, standard film specimens are cut from roll material, typically requiring one to two square meters of film. For simulated hail tests on laminates or mounted films, full-size coupons prepared according to the product construction are needed. Exact dimensions and quantities are confirmed based on the selected standards.
What is included in the test report?
The report documents film identification and thickness, conditioning conditions, ice ball diameters and measured velocities, impact locations, dart test F50 values, tensile and tear results, post-impact inspection findings, and a clear statement of conformity against the specified acceptance criteria.
How long does testing take?
Basic mechanical impact testing is usually completed within one to two weeks. Test programs that include artificial weathering followed by hail impact, or evaluation of multiple hail classes, require additional exposure time and are quoted on a case-by-case basis by the laboratory.