Hail Impact Test Methods: Standards, Procedures and Evaluation Criteria

Hail impact testing is a destructive mechanical test that evaluates the ability of photovoltaic modules, automotive and aerospace components, and building materials to withstand high-velocity hailstone strikes under controlled laboratory conditions. Accredited third-party laboratories perform this test in accordance with international standards such as IEC 61215, ASTM E1038, ISO 9806, and GB/T 9535, launching calibrated ice balls or equivalent impactors at specified velocities to verify product durability, safety, and long-term reliability in hail-prone environments.

Technical Overview

Hail impact testing simulates the mechanical loading imposed by natural hailstones on outdoor products. A hailstone striking a surface transfers kinetic energy in an extremely short contact time, which can cause glass breakage, brittle fracture, delamination, cell micro-cracking, and sealant failure. Because natural hail events are random and unrepeatable, laboratory methods standardize the projectile (an ice sphere or a steel ball of equivalent energy), the launch velocity, the temperature conditioning, and the impact locations so that results are reproducible and comparable between laboratories.

The core physical principle is straightforward: the kinetic energy of a hailstone is defined by E = ½mv², where m is the projectile mass and v is its velocity at impact. Test specifications therefore pair each ice ball diameter with a calibrated velocity. For example, the default hail test for terrestrial photovoltaic (PV) modules in IEC 61215-2 (test MQT 17) uses a 25 mm ice ball at 23.0 m/s, delivering approximately 2.2 J of impact energy, while severe-climate specifications extend to 75 mm balls at 39.5 m/s, delivering approximately 170 J.

Depending on the product family, hail impact resistance is governed by different standardization frameworks, including IEC 61215 for terrestrial PV modules (with corresponding national adoptions such as GB/T 9535 for crystalline silicon and GB/T 18911 for thin-film modules), UL 1703 for flat-plate PV modules, ASTM E1038 for hailball impact on PV modules, ASTM F320 for transparent aerospace enclosures, and ISO 9806 / EN 12975 for solar thermal collectors.

Testing Samples

Photovoltaic Modules

Crystalline silicon and thin-film PV modules are the most common hail impact test samples. Modules are typically tested as complete assemblies with frame, glass front, encapsulant, cells, backsheet, and junction box, because hail resistance depends on the full mechanical system rather than on the glass alone. Building-integrated PV (BIPV) laminates and dual-glass modules are also evaluated, often with additional impact points at mounting and edge locations.

Automotive and Aerospace Components

Windshields, panoramic roofs, headlamp lenses, radar and lidar covers, and aircraft transparencies such as radomes and cockpit windows are tested for hail resistance. Aerospace transparent enclosures are commonly evaluated using the compressed-air ice ball launch method described in ASTM F320, with test severity matched to the flight envelope of the vehicle.

Building and Construction Materials

Roof tiles, skylights, architectural glazing, conservatory panels, and solar thermal collector covers are tested to demonstrate fitness for use in hail-prone regions. Solar thermal collectors are addressed by ISO 9806, which permits either an ice ball launch or a standardized steel ball drop depending on the product configuration and market requirement.

Testing Items

Hail impact testing is not a single measurement but a structured evaluation package. Typical items verified before, during, and after impact include:

• Structural impact resistance at the specified kinetic energy for each ice ball diameter and velocity class
• Visual integrity after impact, including glass cracks, shattering, delamination, bubbles, and surface defects
• Maximum power degradation of PV modules, typically limited to no more than 5% relative to the pre-test value
• Insulation resistance and dielectric withstand after impact to confirm electrical safety
• Hidden cell cracking detected by electroluminescence (EL) imaging
• Water-tightness or seal integrity for glazing, laminates, and collector covers where required by the product standard

Testing Methods

Test Equipment

The reference method uses a pneumatic hail cannon, a purpose-built compressed-air launcher that propels frozen ice balls against a rigidly clamped specimen. The essential equipment set includes:

• A compressed-air launcher with pressure regulator and interchangeable barrels matched to each ice ball diameter
• Ice ball molds producing smooth spheres from deionized or distilled water
• A freezer for conditioning ice balls at approximately −10 °C, together with a low-temperature storage container for transport to the launcher
• Photoelectric velocity gates placed immediately before the specimen to verify that impact speed is within tolerance of the specified value
• Rigid sample fixturing that supports the specimen in the same orientation as its intended installation
• Ancillary instrumentation such as EL imaging systems and I-V flash testers for pre- and post-test electrical characterization of PV modules

Ice Ball Launch Method

The ice ball launch method is the internationally recognized reference procedure. Ice balls are molded, inspected for freedom from cracks and air bubbles, and conditioned at low temperature so they remain fully frozen at the moment of impact. Each ball is loaded into the launcher and fired within seconds of removal from the freezer. The air pressure is adjusted until the measured velocity, recorded by the light gates, is within the permitted tolerance of the nominal value, and the ball then strikes the specimen at the designated point. The specimen surface is normally at standard room temperature, reflecting typical service conditions in which a warm product is struck by cold hail.

For PV modules under IEC 61215-2, the standard severity pairs are:

• 12.5 mm diameter at 16.0 m/s
• 15 mm diameter at 17.8 m/s
• 20 mm diameter at 21.0 m/s
• 25 mm diameter at 23.0 m/s (default severity)
• 35 mm diameter at 27.2 m/s
• 45 mm diameter at 30.7 m/s
• 55 mm diameter at 33.9 m/s
• 65 mm diameter at 36.7 m/s
• 75 mm diameter at 39.5 m/s

A minimum of 11 impacts are applied at defined locations, including each corner region near the frame, the geometric center, the center of selected cells, one point on an edge, and one point near the junction box. These locations concentrate on the structurally most vulnerable zones identified by engineering judgment and field failure statistics.

Steel Ball Drop Method

For products whose standards permit an equivalent-energy approach, such as solar thermal collectors under ISO 9806, a steel ball of specified mass is dropped in free fall from a calculated height onto the sample. The drop height is derived from the required impact energy using the same kinetic energy relationship. The steel ball method offers excellent repeatability and simple equipment, but because a hard steel sphere behaves differently from a fracturing ice sphere, it is accepted only where the governing standard explicitly allows it and should not be substituted for ice ball testing in certification of PV modules.

Test Procedure Summary

• Condition the specimen and complete pre-test characterization, including visual inspection, EL imaging, and power measurement for PV modules
• Manufacture and inspect ice balls, then condition them at the specified low temperature
• Mount the specimen rigidly in the launcher line of fire at the required orientation
• Calibrate air pressure with practice shots until the measured velocity falls within tolerance
• Perform the specified number of impacts at the required locations and severities, replacing each ice ball with a freshly conditioned one
• Conduct post-test visual inspection, EL imaging, and electrical measurements, and compare results with acceptance criteria
• Issue a test report documenting projectile size, velocity, impact points, and all measured results

Evaluation Criteria

A specimen passes the hail impact test when it exhibits no visible damage that would compromise its function or safety, such as through-glass cracks, shattering, exposed live parts, or separations in laminated construction. For PV modules, the additional requirements are maximum power degradation not exceeding the limit defined by the standard, no major visual defects, and continued compliance with insulation requirements. Micro-cracks revealed only by EL imaging are recorded in the report and assessed according to the applicable acceptance rules of the certification scheme.

Applications

Hail impact test data supports a wide range of engineering and commercial decisions across multiple industries:

• Design qualification and type approval of PV modules for market access in hail-prone regions
• Comparative evaluation of glass thickness, encapsulant, and frame designs during product development
• Verification of automotive glazing, sensor covers, and aircraft transparencies against specified hail threat levels
• Compliance demonstration for building codes and insurance requirements covering roofing, skylights, and glazing products
• Quality control audits of incoming materials such as tempered glass and polymer front sheets
• Root-cause analysis and warranty assessment after field hail damage claims

FAQ

What is the purpose of a hail impact test?

The test verifies that a product installed outdoors can survive hailstone strikes of defined size and velocity without functional failure, safety hazards, or unacceptable performance loss, providing evidence for certification, design validation, and warranty support.

Which standard applies to PV module hail testing?

IEC 61215-2 (test MQT 17) is the international reference for terrestrial PV modules, with corresponding national adoptions such as GB/T 9535 and GB/T 18911. UL 1703 and ASTM E1038 address hail resistance from the North American regulatory and methodological perspective, and accredited third-party laboratories can test to any of these frameworks.

What are the default ice ball size and velocity?

The default severity for the standard PV module test is a 25 mm ice ball striking at 23.0 m/s, corresponding to roughly 2.2 J of kinetic energy. Higher severities up to 75 mm at 39.5 m/s are specified for products intended for severe hail climates.

Why must ice balls be conditioned at low temperature?

A partially melted or warm ice ball behaves softly and dissipates energy by deformation rather than transferring it sharply to the specimen, which understates real hail damage. Cold conditioning ensures the ball fractures like natural hail and produces a conservative, reproducible impact.

Is a steel ball equivalent to an ice ball?

No. A steel ball delivers the same nominal energy but with a harder contact and different load distribution, so results are not directly interchangeable. Steel ball drop methods are used only where the governing product standard, such as ISO 9806 for solar collectors, explicitly defines them.

How is failure determined after the test?

Failure is judged by post-test visual inspection supported by instrumental checks: any through-crack or shatter of the front glass, major visual defects, insulation failure, or power degradation beyond the standard limit, commonly 5% for PV modules, constitutes a fail. EL imaging is used to document internal cell cracking as part of the complete evaluation.