Hail Impact Strength Testing: Standards, Methods, and Applications for Product Durability Evaluation

Hail impact strength testing is a laboratory-based durability evaluation that simulates hailstone strikes by propelling calibrated ice balls or steel balls onto products at controlled velocities. This test is essential for manufacturers of photovoltaic modules, automotive components, roofing systems, and aerospace transparencies who need to verify impact resistance, meet international standards such as IEC 61215-2, UL 2218, and ASTM F330, and qualify their products for insurance and market access. This article explains the sample types, testing items, methodologies, and typical applications of hail impact strength testing.

Technical Overview

Hail impact strength testing, also known as hail resistance testing or ice ball impact testing, is a controlled laboratory procedure used to evaluate how materials and finished products withstand the kinetic energy delivered by falling hailstones. During the test, spherical ice projectiles of specified diameter, mass, and temperature are launched against a specimen at calibrated velocities, reproducing the mechanical loading that occurs during severe hailstorms. The test reveals failure modes such as glass fracture, polymer cracking, cell micro-cracking, delamination, coating damage, and structural deformation.

The physical basis of the test is straightforward: the kinetic energy of a hailstone scales with both its mass and the square of its velocity. Because hailstone diameters in natural storms range from less than 10 mm to over 70 mm, laboratory procedures define matched combinations of projectile size and launch velocity so that the delivered energy realistically represents the desired severity level. Testing is typically performed by accredited third-party laboratories using pneumatic hail cannons equipped with velocity measurement systems, and results are assessed against internationally recognized standards published by organizations such as IEC, ISO, ASTM, UL, EN, GB, and JIS.

• IEC 61215-2 – Hail impact test (MQT 17) for terrestrial photovoltaic modules
• ISO 9806 – Solar thermal collector test methods, including hail resistance
• UL 2218 – Impact resistance of roof covering materials using steel ball impact
• ASTM F330 – Hail impact resistance of aerospace transparent enclosures
• ASTM E822 – Impact resistance of solar collector cover materials
• EN 13583 – Determination of hail resistance for flexible waterproofing sheets
• GB/T 61215 – Chinese national standard aligned with the IEC 61215 series for PV module qualification
• JIS C 8990 – Japanese design qualification standard for crystalline silicon PV modules

Testing Samples

Photovoltaic (PV) Modules

Solar modules are among the most frequently tested products because front glass, encapsulant, and solar cells are directly exposed to weather. Both crystalline silicon and thin-film modules, including glass-glass and glass-backsheet constructions as well as flexible lightweight designs, are submitted for hail impact verification as part of design qualification and type approval.

Automotive Components

Vehicle manufacturers evaluate laminated and tempered glazing, panoramic roofs, exterior trim, radar and camera covers, lighting lenses, painted body panels, and EV charging enclosure materials. Testing supports durability validation against the hail loads specified in OEM engineering specifications.

Building Envelope and Roofing Materials

Asphalt shingles, metal roofing, clay and concrete tiles, membrane waterproofing sheets, skylights, polycarbonate and safety glazing, curtain wall panels, and exterior insulation systems are tested to determine impact classification levels that influence insurance ratings and building code acceptance.

Aerospace Transparencies and Advanced Composites

Aircraft windshields, cabin windows, radomes, and composite skin panels are evaluated for hail impact resistance during ground and flight operations. Fiber-reinforced polymer structures used in wind turbine blades and rail vehicles are also increasingly subjected to equivalent impact evaluations.

Outdoor Equipment and Enclosures

Telecommunication enclosures, outdoor display panels, greenhouse glazing, HVAC condenser fins, and protective covers for industrial electronics may be tested to demonstrate weather robustness in hail-prone regions.

Testing Items

A hail impact strength test program is defined by a set of projectile parameters, impact conditions, and post-impact evaluation items. Depending on the product and applicable standard, the following items are typically recorded and assessed.

Projectile and Impact Parameters

• Ice ball diameter and mass (e.g., 7.53 g for a 25 mm sphere)
• Launch velocity and measured impact velocity, verified with chronographic instrumentation
• Delivered kinetic energy at each impact point
• Ice ball temperature conditioning, commonly −4 °C ± 2 °C per the IEC procedure
• Number and coordinates of impact locations on the specimen
• Specimen temperature and mounting orientation during exposure

Post-Impact Performance Evaluation

• Visual inspection for cracks, breakage, surface damage, and permanent deformation
• Maximum power output measurement before and after impact for PV modules
• Electroluminescence (EL) imaging to detect hidden cell micro-cracks
• Insulation resistance and wet leakage current verification
• Dimensional checks, coating adhesion, and functional operation of the tested component

Environmental Conditioning

Specimens may be conditioned at standard laboratory temperature prior to impact, while ice projectiles are stored in a controlled freezer to ensure uniform density and consistent fracture behavior upon impact. Documentation of conditioning parameters is a mandatory part of the test report.

Testing Methods

Ice Ball Preparation

Ice balls are molded from deionized water to eliminate bubbles and inclusions that would alter mass distribution. Each sphere is weighed and inspected for sphericity and transparency, then tempered at the standard-specified temperature until use. Only projectiles within the permitted mass tolerance are accepted for launching.

Pneumatic Launch Procedure

A compressed-air hail cannon accelerates the ice ball through a barrel toward the clamped specimen. Two light gates or an equivalent ballistic velocity measurement system record the actual velocity immediately before impact, and the launch pressure is calibrated in advance so that every strike falls within the velocity tolerance of the standard, typically on the order of ±5% of the nominal value. The specimen is struck at defined locations, and each impact is documented with high-speed imaging where required.

IEC 61215-2 Hail Test for PV Modules

The IEC 61215-2 procedure (MQT 17) prescribes a 25 mm ice ball launched at 23.0 m/s as the default condition, delivering approximately 2 J of kinetic energy, while larger projectile sizes may be selected to represent site-specific hail exposure. Impacts are applied at a prescribed pattern of locations covering corners, edges, and the central region of the module. The standard defines matched size–mass–velocity combinations, for example:

• 20 mm diameter – 3.86 g – 21.0 m/s – approximately 0.85 J
• 25 mm diameter – 7.53 g – 23.0 m/s – approximately 2.0 J
• 35 mm diameter – 20.7 g – 27.2 m/s – approximately 7.7 J
• 45 mm diameter – 43.9 g – 29.9 m/s – approximately 19.6 J
• 55 mm diameter – 80.2 g – 32.6 m/s – approximately 42.6 J

Steel Ball Impact Methods

For roofing products, UL 2218 specifies repeated drops of solid steel balls of graded diameters from defined heights onto the specimen, with severity classes from Class 1 through Class 4. The steel ball approach provides a repeatable, conservative surrogate for hail loading and is widely used for impact classification of roof coverings in North America. Equivalent regional procedures are referenced in EN, GB, and JIS documents where applicable.

Pass and Fail Criteria

A specimen is considered to have passed when it exhibits no major visual defects such as glass breakage, through-cracks, or delamination; when electrical performance remains within the degradation limits of the applicable standard; and when insulation and safety requirements are maintained after impact. Failures are documented with photographs, velocity records, and measured performance data to support root-cause analysis and design improvement.

Applications

Hail impact strength testing supports product development, certification, and market entry across multiple industries. Typical applications include:

• Qualification and type approval of PV modules for utility-scale, commercial, and residential solar projects, including extended hail severity programs for installations in high-risk climatic zones
• Impact classification of roofing and waterproofing products to obtain insurance recognition and code compliance
• Durability validation of automotive glazing and exterior components under OEM and fleet operating requirements
• Certification of aerospace transparencies and composite structures for hail exposure on the ground and in flight
• Comparative material studies, supplier benchmarking, and failure analysis to improve laminates, coatings, and polymer formulations
• Due-diligence and bankability documentation for lenders, insurers, and project developers requiring verified hail resistance data

FAQ

What hailstone size does a standard PV module hail test simulate?

The default IEC 61215-2 condition uses a 25 mm ice ball traveling at 23.0 m/s, which corresponds to a moderate natural hailstone. Larger projectiles up to 75 mm can be used when the installation site experiences more severe hail, and the selected severity level is stated in the test report.

Why are ice balls used instead of steel balls in PV testing?

Ice reproduces the density, brittleness, and contact behavior of genuine hailstones, giving a realistic energy transfer and fracture pattern. Steel ball methods remain valuable for roofing classification because they offer high repeatability and conservative loading, but they are not representative of ice contact mechanics on brittle front glass.

How is the impact velocity controlled and verified?

Launch pressure is calibrated against velocity before testing, and every shot is measured with light-gate chronographs positioned near the specimen. A shot is only counted if the measured velocity falls within the tolerance band defined by the standard.

Can hidden damage be detected after a hail test?

Yes. Electroluminescence imaging, power measurement, and insulation tests are commonly added after visual inspection to reveal internal cell micro-cracks that are invisible to the naked eye but can cause long-term power loss in PV modules.

How long does a hail impact strength test take?

The impact phase itself usually requires only a few hours, including ice ball preparation and velocity calibration. When pre-test and post-test performance measurements, EL imaging, and report preparation are included, a complete test campaign typically takes one to two weeks depending on the scope.

Which standard should be selected for a specific product?

PV modules are normally tested to IEC 61215-2 or its GB and JIS national adoptions, solar thermal collectors to ISO 9806, roofing products to UL 2218 or EN 13583, and aerospace transparencies to ASTM F330. Accredited third-party laboratories can advise on the correct combination of standards and severity levels for the intended market and service environment.