Hail impact strength determination is a laboratory test that measures the ability of photovoltaic modules, glazing, roofing, and other exposed products to withstand the kinetic energy of real hailstones. Using calibrated ice balls fired from a pneumatic launcher under controlled conditions defined by IEC 61215-2, ASTM E822, and ASTM F330, the test verifies structural integrity and functional performance after severe hail impact. This article explains the sample types, test parameters, procedures, evaluation criteria, and typical applications of hail impact strength testing performed by accredited third-party laboratories.
Technical Overview
Hail impact strength determination, also known as simulated hail testing or ice ball impact testing, quantifies how well a product absorbs the kinetic energy of falling hail without suffering structural damage or loss of function. Natural hail events cannot be reproduced reliably in the field, so the test substitutes calibrated ice balls that are propelled at a specimen by a pneumatic launcher under strictly controlled conditions of mass, diameter, velocity, and temperature.
The method is a mandatory design-qualification test for terrestrial photovoltaic (PV) modules under IEC 61215-2 (test sequence MQT 17) and its Chinese national adoption GB/T 61215. Closely related methods cover solar collector covers (ASTM E822) and aerospace transparent enclosures such as aircraft windshields (ASTM F330). The impact energy follows the classical relationship E = ½mv²: a standard 25 mm ice ball of approximately 7.5 g striking at 23.0 m/s deposits roughly 2 joules, while the larger ice balls used for severe-hail qualification of roofing materials can deliver several tens of joules per impact.
Testing Samples
Photovoltaic Modules
Full-size crystalline-silicon and thin-film modules with glass-front laminates, glass-glass bifacial constructions, lightweight hybrid frontsheets, and flexible modules, as well as building-integrated PV (BIPV) panels and receiver assemblies. Modules are typically tested in the as-manufactured state, complete with frame and junction box.
Glazing and Transparent Components
Laminated safety glass, tempered glass, architectural glazing, skylights, automotive windscreens and lamp lenses, polycarbonate and PMMA windows, and protective covers for cameras, sensors, and display panels installed outdoors.
Roofing and Building Envelope Products
Concrete and clay tiles, asphalt shingles, metal roofing sheets and standing-seam profiles, waterproof membranes, sandwich and composite panels, façade cladding cassettes, and rainscreen systems.
Aerospace Transparencies
Aircraft windshield plies, cabin windows, and other transparent enclosures qualified to ASTM F330, where hail and bird-strike-type impact energies must be absorbed without loss of structural integrity or optical function.
Polymeric and Composite Products
Enclosures for outdoor electronics and EV charging equipment, leisure-vehicle body panels, marine glazing, and fiber-reinforced composite covers whose service life depends on impact tolerance.
Testing Items
Depending on the product type and the governing standard, a hail impact strength determination program typically includes:
- Verification of ice ball diameter, mass, temperature, and impact velocity, each normally held within ±5% of the specified value.
- Pre-test and post-test visual inspection for glass fracture, cell breakage, delamination, dents, coating loss, surface crazing, and frame or edge deformation.
- Maximum power (Pmax) measurement at standard test conditions before and after impact, to quantify power degradation.
- Electroluminescence (EL) imaging per IEC TS 60904-13 to reveal hidden micro-cracks in PV cells that visual inspection cannot detect.
- Insulation resistance, dielectric withstand, and wet leakage current tests to confirm electrical safety after impact.
- Assessment of through-cracking, penetration, and retention of the protective function for glazing, roofing, and enclosure products.
- Optional supplementary measurements such as dent depth, residual mechanical strength, and water-tightness after impact.
Testing Methods
Specimen Preparation and Mounting
Representative full-size articles or cut coupons are conditioned at laboratory ambient temperature. Modules are mounted in the same orientation and support configuration recommended by the manufacturer so that impact energy is absorbed by the product rather than by fixture movement. Building products and coupons are rigidly clamped to a heavy steel backing frame to prevent energy loss through specimen displacement.
Ice Ball Preparation
Ice balls are molded from distilled or deionized water to eliminate inclusions and bubbles, then frozen and conditioned at a controlled sub-zero temperature, commonly −4 °C ± 2 °C per IEC 61215-2. Immediately before firing, each ball is checked for diameter within ±5% of nominal and mass within ±5% of the calculated value, and is handled with insulated tools to prevent surface melting. Balls are launched within a short, defined window after removal from the conditioning cabinet.
Launch System and Velocity Calibration
A compressed-air or compressed-gas launcher accelerates each ice ball along a horizontal trajectory toward the specimen. Impact velocity is measured close to the target using optical or laser ballistic sensors and must fall within ±5% of the specified speed, with the trajectory normal to the specimen surface within approximately ±5°. The system is verified before and during the test series with calibration shots, and high-speed video may be used to confirm flight stability and contact geometry.
Impact Locations and Severity Levels
IEC 61215-2 specifies eleven impact points distributed over the front surface of the module, including module corners, cell corners, cell centers, inter-cell regions, points near the frame, and any location judged structurally most vulnerable, such as the junction box area. Other product standards define their own impact grids, typically covering mid-span, corner, edge, and fastener positions. The standard severity pairings between ice ball size and velocity for PV testing are approximately:
- 12.5 mm diameter at 40.0 m/s
- 15 mm diameter at 34.5 m/s
- 20 mm diameter at 27.3 m/s
- 25 mm diameter at 23.0 m/s (standard qualification level)
- 35 mm diameter at 18.6 m/s
- 45 mm diameter at 15.5 m/s (severe-hail level)
Because the mass of an ice sphere grows with the cube of its diameter while the specified velocity decreases, larger balls deliver substantially higher kinetic energy, allowing the same apparatus to simulate everything from light to extreme hail.
Post-Impact Evaluation and Pass Criteria
For PV modules, the specimen passes when no major visual defects are found, the degradation of maximum power does not exceed 5% relative to the initial measurement, and the insulation resistance and wet leakage tests remain within the limits of IEC 61215-2. For glazing and aerospace transparencies, typical criteria are the absence of through-cracks, penetration, or loss of optical function. For roofing and cladding products, criteria address rupture of the weatherproofing layer, exposure of the substrate, coating breach, and permanent deformation beyond the product specification.
Test Report
A compliant report issued by an ISO/IEC 17025-accredited third-party laboratory identifies the specimen and standard applied, lists the ice ball diameter, mass, temperature, and measured velocity for every shot, documents the impact point map, records all pre- and post-test inspection and electrical results, and states an explicit pass or fail conclusion.
Applications
Hail impact strength determination supports product development, certification, and risk management across multiple industries:
- Design qualification and type certification of PV modules for global market access under IEC 61215 and GB/T 61215.
- Research and development of front glasses, encapsulants, frames, and mounting concepts, including comparative benchmarking of hail resilience between designs.
- Severe-hail qualification of roofing, cladding, and skylight products requested by insurers, developers, and building codes.
- Qualification of aerospace transparencies and ground-equipment enclosures exposed to extreme weather.
- Incoming quality control, supplier verification, and failure analysis after field hail damage, including technical support for warranty assessments.
FAQ
Which standards govern hail impact strength testing?
The most widely applied standards are IEC 61215-2 (MQT 17) and GB/T 61215 for PV modules, ASTM E822 for solar collector covers, and ASTM F330 for aerospace transparent enclosures. Each standard defines ice ball preparation, velocity tolerances, impact locations, and acceptance criteria specific to the product family.
What are the pass criteria for a PV module?
The module must show no major visual defects, suffer no more than 5% degradation of maximum power measured at standard test conditions, and continue to pass insulation resistance and wet leakage current requirements after the full sequence of eleven impacts.
What ice ball size and speed are used by default?
For standard PV qualification, a 25 mm ice ball weighing approximately 7.5 g is launched at 23.0 m/s, delivering about 2 joules of kinetic energy. Higher severity levels use 35 mm or 45 mm balls at correspondingly lower velocities, while smaller balls are fired at higher speeds for thinner or more fragile components.
How many samples and impacts are required?
IEC 61215-2 requires eleven impacts on one representative module at specified locations. Development programs and roofing or aerospace specifications frequently add multiple specimens and extended impact grids to map the weakest zones of the product.
Why are real ice balls used instead of steel or glass projectiles?
Natural hail has a specific density, brittleness, and crush behavior that strongly influence how energy is transferred to the target. Ice balls conditioned at sub-zero temperature reproduce this behavior faithfully, whereas steel or glass projectiles produce a different contact stiffness and can either overstate or distort the damage mode.
Can the test be performed at low temperature or combined with other tests?
Yes. Specimens may be conditioned at sub-zero ambient temperatures to simulate winter hail events, and hail impact testing is commonly sequenced with thermal cycling, damp heat, and mechanical load testing as part of a full design qualification program.
What information must appear in the test report?
The report should document the standard applied, the identification of the specimen, every ice ball's diameter, mass, temperature, and measured velocity, the impact point layout, all inspection and electrical measurement results before and after impact, and a clear pass or fail verdict.