Hail Impact Performance Assessment: Standards, Methods, and Evaluation Criteria

Hail impact performance assessment is a laboratory-based evaluation that verifies the ability of photovoltaic modules, automotive components, building materials, and other exposed products to withstand the kinetic energy of falling hailstones. Accredited third-party laboratories perform this testing in accordance with recognized standards such as IEC 61215-2, ASTM E1038, and UL 2218, providing objective evidence of mechanical durability, functional integrity after impact, and compliance with market access requirements. This article explains the sample types, test items, methodologies, evaluation criteria, and practical applications of hail impact performance assessment.

Technical Overview

Hail impact performance assessment is a controlled mechanical evaluation that measures how effectively a product resists damage from hailstone strikes. Natural hailstones range from approximately 10 mm to well over 100 mm in diameter and can strike surfaces at terminal velocities between 20 m/s and 40 m/s, concentrating impact energies from a few joules to several hundred joules onto a small contact area. Because a single severe hailstorm can destroy cover glass, fracture solar cells, delaminate coatings, puncture roofing membranes, and impair optical components, hail resistance has become a fundamental qualification requirement for products installed outdoors.

In the laboratory, hail events are simulated by launching calibrated ice balls from a pneumatic launcher or by dropping hardened steel balls onto the specimen, with impact parameters selected to reproduce the kinetic energy of natural hail. The principal governing standards include IEC 61215-2 (hail impact test MST 17 for terrestrial photovoltaic modules), IEC 61730-2 (PV module safety), ASTM E1038 (impact resistance of photovoltaic modules by hail balls), UL 2218 (impact resistance of roof covering materials), and the corresponding Chinese national adoption GB/T 61215. Typical failure modes investigated during the assessment include:

• Cover glass breakage, pitting, and through-cracks
• Silicon cell micro-cracks detectable by electroluminescence imaging
• Reduction of maximum output power in photovoltaic modules
• Delamination of encapsulants, coatings, and painted layers
• Puncture, tearing, or rupture of roofing and membrane materials
• Loss of optical clarity or structural integrity in glazing and lenses

Testing Samples

Photovoltaic Modules

Full-size crystalline silicon modules, thin-film modules, bifacial and glass-glass laminates, lightweight and flexible modules, building-integrated photovoltaic (BIPV) products, and solar tiles are the most frequently tested specimens. Because hail damage directly affects power output and service life, module manufacturers, project developers, and investors rely on hail impact data to support certification and bankability studies for installations in hail-prone regions.

Automotive Components

Windshields, side and rear glazing, panoramic roofs, headlamp and taillight lenses, grilles, mirror housings, radar and camera covers, and painted body panels are evaluated against hail and stone-impact conditions. Vehicle manufacturers typically specify ice ball diameters, velocities, and impact locations derived from field damage statistics for these components.

Building and Construction Materials

Asphalt shingles, clay and concrete tiles, metal roofing, waterproofing membranes, skylights, architectural glazing, curtain wall panels, polycarbonate and corrugated sheets, and facade systems are tested to establish impact resistance classifications. These results are widely used for insurance underwriting, code compliance, and product differentiation.

Aerospace and Outdoor Equipment

Radomes, antenna covers, composite panels, outdoor enclosures, HVAC condenser coils, and lighting covers are assessed to confirm survivability during ground and flight hail exposure. Custom test programs are developed when no product-specific standard exists.

Testing Items

A complete hail impact performance assessment typically encompasses the following evaluation items, tailored to the specimen type and the referenced standard:

• Hail impact resistance classification, such as Class 1 through Class 4 under UL 2218 or the hail class under IEC 61215-2
• Pre- and post-impact measurement of maximum power (Pmax) and determination of power degradation
• Visual inspection for major defects, including glass breakage, cracks, delamination, and surface damage
• Electroluminescence (EL) imaging to reveal hidden cell cracks and solder bond damage
• Insulation resistance and wet leakage current verification after impact
• Assessment of functional integrity, such as water tightness of roofing systems or optical quality of lenses
• Documentation of damage morphology, impact energy thresholds, and critical impact locations

Testing Methods

Ice Ball Impact Method

The ice ball method, specified in IEC 61215-2 (MST 17), GB/T 61215, and ASTM E1038, provides the most realistic simulation of natural hail. Ice balls are molded from deionized water, conditioned at subzero temperature to ensure uniform hardness, and launched from a compressed-air cannon at the specimen. Projectile velocity is measured immediately before impact using calibrated light gates, with tolerances of typically plus or minus 5 percent on both velocity and mass.

The reference condition for standard module qualification is a 25 mm diameter ice ball traveling at 23.0 m/s, delivering approximately 2 joules of kinetic energy. For higher hail classes, larger projectiles are used at correspondingly higher velocities, for example 35 mm at approximately 27 m/s, 45 mm at approximately 31 m/s, and 75 mm at approximately 40 m/s, corresponding to impact energies of roughly 8 J, 21 J, and more than 150 J respectively. Specimens are struck at a prescribed pattern of impact points, generally eleven locations distributed across the center, corners, and edge zones of the sample, to cover the most mechanically vulnerable regions.

Steel Ball Drop Method

The steel ball method described in UL 2218 is used primarily for roofing materials. A hardened steel ball is dropped vertically, twice at each of several prescribed locations including joints, fastener lines, edges, and unsupported mid-span areas. Ball size and drop height increase with the resistance class, from a 1.25 in (31.8 mm) ball released from 12 ft for Class 1 up to a 2.00 in (50.8 mm) ball released from 20 ft for Class 4. A specimen passes a given class when the impacts produce no penetration, cracking, or fracture that exposes the substrate or permits water entry.

Pre- and Post-Impact Characterization

Before testing, specimens are conditioned and documented with baseline measurements, including visual records, electrical performance at standard test conditions, and where applicable EL images and insulation resistance values. After the impact sequence, all measurements are repeated and compared. For photovoltaic modules under IEC 61215-2, the principal pass criteria are the absence of major visual defects, a maximum power degradation of no more than 5 percent relative to the initial value, and continued compliance with insulation requirements. Test reports document ice ball dimensions, velocities, impact coordinates, high-speed imagery where available, and photographic evidence of every impact site, ensuring full traceability of results.

Applications

Hail impact performance assessment serves a wide range of engineering, commercial, and regulatory purposes across industries. Typical applications include:

• Type approval and market certification of photovoltaic modules under IEC 61215-2, IEC 61730-2, and GB/T 61215
• Bankability and site-suitability studies for solar projects located in high-risk hail regions
• Impact classification of roofing materials for insurance rating and building code compliance under UL 2218
• Research and development activities, such as glass thickness optimization, encapsulant selection, and frame or support structure design
• Incoming material qualification, supplier comparison, and routine quality control programs
• Failure analysis of field-damaged products and technical support for warranty evaluations

FAQ

What is the difference between ice ball and steel ball hail testing?

Ice ball testing launches frozen projectiles at controlled velocities and reproduces the mass, hardness, and energy transfer of genuine hailstones, making it the preferred method for photovoltaic and aerospace components. Steel ball drop testing, used mainly for roofing under UL 2218, applies repeatable and conservative mechanical loads and produces a standardized Class 1 to Class 4 rating, but it does not replicate the brittle fracture behavior of real ice.

Which ice ball size and velocity are required for PV module certification?

The default qualification condition in IEC 61215-2 is a 25 mm ice ball impacting at 23.0 m/s. Projects in severe hail zones frequently specify extended classes with larger diameters, commonly 35 mm, 45 mm, or 55 mm, tested at the velocities defined in the standard to confirm higher impact energy tolerance.

How is pass or failure determined after a hail impact test?

For photovoltaic modules, the specimen passes if no major visual defects are present, maximum power degradation does not exceed 5 percent, and insulation performance remains acceptable. For roofing materials, acceptance depends on the absence of penetration, cracking, or exposure of the underlying substrate at the specified impact class.

Can small cracks that do not affect power output still be significant?

Yes. Electroluminescence imaging frequently reveals cell micro-cracks that are invisible to the naked eye and produce little immediate power loss. Such cracks can propagate under thermal cycling and mechanical loading, causing progressive degradation over the service life of the module, so they are carefully documented in the assessment report.

How long does a typical hail impact assessment take?

The impact sequence itself is usually completed within one day, including ice ball preparation, velocity calibration, and the full set of strikes. Including baseline and post-test electrical measurements, EL imaging, and report preparation, accredited third-party laboratories generally deliver complete results within five to ten working days.