A hail impact test evaluates the ability of products such as photovoltaic modules, automotive components, and building materials to withstand impacts from hailstones of specified size, mass, and velocity. This operating procedure describes the apparatus, sample preparation, calibration, test execution, acceptance criteria, and safety requirements for conducting hail impact testing in accordance with recognized standards such as IEC 61215-2 and ASTM E1038. It serves as a practical reference for engineers and technicians performing hail resistance verification in accredited third-party laboratories and in-house quality control facilities.
Technical Overview
The hail impact test is a destructive mechanical test that simulates the effect of natural hailstone bombardment on products exposed to outdoor service environments. A spherical ice ball of defined diameter and mass is propelled by a compressed-air launcher against the surface of a test specimen at a calibrated velocity. The kinetic energy delivered at the point of impact is calculated using the formula KE = 1/2 mv², where m is the mass of the ice ball and v is the measured impact velocity. After the impacts, the specimen is inspected for mechanical damage and re-tested for functional performance to determine whether it passes or fails.
The most widely applied reference standards include IEC 61215-2 (design qualification of terrestrial photovoltaic modules, hail test MQT 17), ASTM E1038 (impact resistance of solar collector covers by hail balls), UL 2218 (impact resistance of roofing materials using steel balls), and GB/T 9535 (the Chinese national standard corresponding to IEC 61215). Although test details vary between standards and product families, the fundamental apparatus, ice ball requirements, and procedure logic are consistent across most specifications.
Commonly specified ice ball conditions, derived from hailstone velocity-versus-diameter correlations used in the above standards, are summarized below:
- 25 mm diameter, approximately 7.5 g, at 23.0 m/s, delivering about 2 J of kinetic energy
- 35 mm diameter, approximately 20.7 g, at 27.2 m/s, delivering about 7.7 J
- 45 mm diameter, approximately 43.9 g, at 30.7 m/s, delivering about 20.7 J
- 55 mm diameter, approximately 80.2 g, at 33.9 m/s, delivering about 46 J
- 65 mm diameter, approximately 132 g, at 36.7 m/s, delivering about 89 J
- 75 mm diameter, approximately 203 g, at 39.5 m/s, delivering about 158 J
Testing Samples
Photovoltaic Modules and Solar Collectors
Crystalline silicon and thin-film photovoltaic modules are the most common test articles. The test verifies that the front glass, encapsulant, cells, and backsheet can withstand hail without cracking, cell breakage, or unacceptable power loss. Solar thermal collector covers are tested under a similar principle as specified in ASTM E1038.
Automotive Exterior Components
Body panels, mirror housings, headlamp and taillight lenses, radar and camera covers, sunroofs, and decorative trim are tested to confirm resistance to impact damage, coating adhesion loss, and functional impairment. Requirements are typically defined by vehicle manufacturers based on the intended market and mounting orientation.
Building Envelope and Construction Products
Roofing membranes and shingles, skylights, glazing units, polycarbonate sheets, composite cladding panels, and coatings are tested to support weather resistance classification, product certification, and insurance-related performance claims. Roofing materials are frequently classified using the steel-ball method of UL 2218, while ice-ball methods are used for other envelope components.
Aerospace, Rail, and Outdoor Equipment
Radomes, aircraft skin panels and transparencies, rail vehicle exterior components, telecommunication enclosure covers, and outdoor display windows are tested to verify survivability in severe weather conditions encountered in service.
Testing Items
Depending on the product type and governing specification, the hail impact test evaluates the following items:
- Visual integrity: surface cracking, glass fracture, chipping, denting, delamination, or permanent deformation at impact points
- Electrical performance: maximum power output or current-voltage characteristics measured before and after the test for photovoltaic modules
- Insulation and safety performance: insulation resistance or dielectric withstand verification after impact
- Coating and surface quality: paint adhesion, coating cracking, or optical property changes
- Functional performance: sealing integrity, water tightness, or optical transmittance where applicable
- Impact parameter verification: ice ball diameter, mass, conditioning temperature, and impact velocity records
Testing Methods
Apparatus Requirements
The test system consists of a pneumatic hail launcher (air cannon) with a compressed-air supply, pressure regulator, quick-release valve, and interchangeable barrels matched to the ice ball diameter; a velocity measurement device such as a ballistic chronograph or paired optical light gates positioned near the specimen; a freezer for ice ball conditioning; a rigid specimen mounting frame; and alignment aids such as a laser sight. A polycarbonate safety screen or enclosed test cell, remote firing controls, and interlocks are strongly recommended. Ice balls carry substantial kinetic energy at larger diameters and must be treated as high-energy projectiles.
Ice Ball Preparation and Verification
Ice balls are molded from distilled or deionized water to minimize inclusions and are frozen for a sufficient duration to ensure a uniform solid core. Each ball is inspected visually and rejected if it exhibits cracks, bubbles, chips, or out-of-round geometry. Diameter is measured with calipers in at least two axes, and mass is verified on a calibrated balance; both must fall within the tolerance bands stated by the applicable standard. Ice balls are conditioned in the freezer at the specified temperature, typically in the region of -10 °C, and each ball must be launched within approximately 60 seconds of removal from the freezer to prevent melting, which would alter mass and ballistic behavior.
Specimen Preparation and Mounting
Before testing, the specimen is assigned a unique identification, photographed, and subjected to the initial inspections required by the standard, including visual inspection and, for photovoltaic modules, baseline electrical performance measurement. The specimen is then rigidly clamped to the mounting frame in the orientation specified by the procedure, typically with the exposed face perpendicular to the barrel axis. Unless otherwise specified, the test is performed at ambient laboratory temperature, and ambient temperature and humidity are recorded. Impact locations are marked on the specimen surface; for photovoltaic modules tested to IEC 61215-2, impacts are distributed over defined points, commonly including the module center, locations near each corner within approximately 50 mm of the edges, and additional points selected as most vulnerable, such as areas above cell interconnects or near the junction box, for a total of eleven impacts.
Velocity Calibration
Prior to testing the specimen, calibration shots are fired at a target dummy or sacrificial panel. Air pressure is adjusted incrementally until the measured velocity at the impact plane falls within the permitted tolerance of the target value, typically ±5% for the specified diameter. The chronograph is zeroed and verified before each test session, and the distance between the velocity sensors and the impact point is minimized in accordance with the standard so that the recorded velocity represents the true impact velocity. Calibration records, including pressure settings and measured velocities, are documented.
Impact Execution
For each impact, a conditioned ice ball is loaded into the breech, the launcher is aimed at the marked location, all personnel are cleared from the downrange zone, and the shot is fired remotely. The measured velocity is recorded, and any shot falling outside the tolerance band is voided and repeated. The procedure continues until all specified impact locations have been tested. After each impact, the specimen is examined and observations such as crack length, dent depth, or surface damage are recorded with photographs.
Post-Test Inspection and Acceptance Criteria
After the final impact, the specimen undergoes the post-test evaluations defined by the governing standard. Typical acceptance criteria include:
- No major visual defects, such as through-cracks, delamination, or fragments of sharp-edged detached material
- Maximum power degradation within the specified limit, typically no more than 5% for photovoltaic modules
- Maintained insulation resistance or dielectric withstand capability for electrical products
- No functional failure of the component in its intended application
Any specimen exhibiting a defect defined as major by the standard is classified as failed regardless of residual performance.
Test Report
The report shall identify the test standard, specimen description and identification, ice ball diameter, mass, and conditioning temperature, measured impact velocities, impact location map, environmental conditions, pre- and post-test inspection and measurement results, photographs of damage, and an explicit pass or fail conclusion.
Applications
Hail impact testing supports product certification, design verification, and quality control across multiple industries. Typical applications include:
- Design qualification and type approval of photovoltaic modules for certification purposes
- Comparative evaluation of cover materials, laminates, and coatings during product development
- Weather resistance classification of roofing, glazing, and cladding products
- Verification of automotive and aerospace exterior components for severe weather markets
- Failure analysis and validation of design changes after field hail damage claims
- Supplier qualification and incoming material audits in manufacturing quality systems
FAQ
What is the default hail test condition for photovoltaic modules?
Under IEC 61215-2, the default condition is a 25 mm ice ball of approximately 7.5 g launched at 23.0 m/s, applied at eleven defined impact locations. Larger ball diameters with correspondingly higher velocities may be agreed between customer and laboratory for sites with more severe hail exposure.
How is the impact velocity controlled and verified?
Velocity is set by adjusting the launcher air pressure during calibration shots and is verified for every impact using a chronograph or optical light gates positioned close to the specimen. Each measured velocity must fall within the tolerance, typically ±5%, of the specified value for the shot to be valid.
Why must ice balls be used so quickly after removal from the freezer?
Once removed from the freezer, an ice ball begins to melt and accumulate surface water, which changes its mass distribution, aerodynamics, and effective hardness. Standards therefore require launching within a short window, commonly about 60 seconds, to ensure the impact energy remains representative of a solid hailstone.
Can steel balls be used instead of ice balls?
Steel-ball impact methods exist, notably UL 2218 for roofing material classification, but they are distinct procedures with different energy transfer characteristics. Ice-ball methods must be used when the governing specification, such as IEC 61215-2 or ASTM E1038, requires true hailstone simulation; the two methods are not interchangeable.
What safety precautions are required?
The launcher must be treated as a projectile weapon. Required precautions include an enclosed test cell or protective screen, remote firing, a cleared downrange zone with warning signage, safety glasses and hearing protection for operators, secure specimen clamping, and verification that the barrel is clear before pressurization.
How many specimens are required for a test program?
This depends on the governing standard and test purpose. Certification programs such as module design qualification specify the number of dedicated test specimens in the test sequence, while development or comparative testing may use one specimen per condition agreed between the customer and the laboratory.