Gravel impact testing is a critical mechanical durability assessment for photovoltaic coated glass, evaluating the resistance of anti-reflective and other functional coatings to chipping, delamination, and substrate cracking when struck by projected particles. This article explains the sample requirements, test items, laboratory methods, and applicable international standards, including the ISO 20567 series, ASTM D3170, and the hail impact requirements of IEC 61215, helping module manufacturers and glass suppliers validate long-term field reliability.
Technical Overview
Photovoltaic coated glass, typically ultra-clear low-iron glass with an anti-reflective (AR), self-cleaning, or hydrophobic surface treatment, serves as the front cover of solar modules. During transportation, installation, cleaning, and operation, this glass is repeatedly exposed to hard particle impacts from wind-blown sand, roadside debris, and hail-like events. A gravel impact test, also called a chipping or shot impact test, projects standardized gravel or a defined impact body against the coated surface under controlled energy conditions to reproduce these stresses in a laboratory setting.
The test evaluates two failure levels simultaneously: damage to the functional coating, such as flaking, cratering, or adhesion loss that increases reflectance and reduces light transmission, and structural damage to the glass itself, including surface craters, radial cracks, or catastrophic fracture. Because AR coatings are often porous oxide layers only tens to a few hundred nanometers thick, their cohesion and adhesion after impact directly influence the annual energy yield of the module over its 25-to-30-year service life.
Relevant reference frameworks include the ISO 20567 series and ASTM D3170 for coating chipping resistance, IEC 61215-2 and IEC 61730-2 for module-level mechanical impact robustness, and GB/T 30984 series requirements for solar glass products. Accredited third-party laboratories routinely perform these tests to support product qualification, research and development, and quality control.
Testing Samples
Coated Cover Glass Coupons
Flat coupons cut from production-run coated glass, commonly with dimensions in the range of 100 mm × 200 mm to 100 mm × 300 mm, are the primary test specimens. Thicknesses of 2.0 mm to 4.0 mm and both single-sided and double-sided coated configurations can be evaluated. The coated face is clearly identified and oriented toward the impact source, and uncoated reference coupons are frequently tested in parallel to isolate the contribution of the coating.
Laminated Assemblies and Mini-Modules
To assess real-world failure modes, samples may be prepared as glass-encapsulant-glass or glass-encapsulant-backsheet laminates, including mini-modules with embedded cells. These samples allow engineers to measure crack propagation into the laminate, cell damage detectable by electroluminescence, and power degradation after impact, providing a direct link between surface damage and electrical performance.
Conditioning and Quantity
Unless otherwise specified, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours before testing. Edges are protected or masked when the standard requires, since edge chipping is excluded from coating evaluation. A typical test plan uses three to five specimens per condition to account for the statistical scatter inherent in particle impact testing.
Testing Items
The gravel impact examination of photovoltaic coated glass generally covers the following assessment items:
- Chipping resistance of the coating: chip count, crater diameter, and exposed substrate area per unit test zone.
- Coating adhesion after impact, verified by cross-cut or tape pull methods such as ASTM D3359 or ISO 2409.
- Glass structural integrity: presence of surface craters, star breaks, radial cracks, or through-thickness fracture.
- Change in luminous transmittance and haze caused by impact damage, measured per ISO 9050 or GB/T 2680.
- Coating thickness and uniformity before and after impact, using optical or profilometric techniques.
- Electroluminescence inspection of laminated samples for hidden cell cracks per IEC TS 60904-13.
- Maximum power degradation of mini-modules, referenced to STC power measurement procedures in the IEC 61215 series.
- Impact energy threshold determination: the lowest kinetic energy at which coating failure or glass fracture initiates.
Testing Methods
Multi-Impact Gravel Projection Test
Following the principles of ISO 20567-1 and ASTM D3170, a gravel projecting device accelerates a defined mass of standardized gravel, typically in the range of a few millimeters in particle size, through a nozzle using compressed air at the pressure and exposure duration specified by the method, with impact generally at or near normal incidence to the specimen surface. After exposure, loose debris is removed and a tape pull may be applied to detach poorly adhered coating fragments. Damage is then quantified by counting impact points, measuring crater dimensions, and assigning a chipping rating, and results are compared between coated and uncoated reference zones.
Single-Impact Methods
ISO 20567-2 describes a single-impact procedure using a free-flying, guidable impact body launched at controlled velocity, allowing precise calculation of kinetic energy using the relation E = ½mv². ISO 20567-3 provides a complementary approach with a free-falling ball. By stepping the impact energy or impact location across a specimen grid, laboratories can construct a damage threshold map for the coating-glass system, which is valuable for comparing coating formulations and tempering grades.
Hail Impact Corroboration
Where module-level robustness must be demonstrated, the hail impact test of IEC 61215-2 is applied. Frozen ice balls, commonly 25 mm in diameter launched at approximately 23 m/s, corresponding to roughly 2 joules of kinetic energy, and larger diameters for severe climate classifications, are directed at specified target points on the module. Post-test evaluation combines visual inspection, electroluminescence imaging, and power measurement to detect invisible cracks. While hail testing uses ice rather than gravel, it is often executed alongside gravel impact testing to cover the full spectrum of particle impact scenarios.
Post-Impact Characterization
After mechanical exposure, the specimen undergoes optical and microscopic characterization. Luminous transmittance and haze quantify the optical penalty of impact damage, cross-cut adhesion testing verifies remaining coating integrity, and optical microscopy or surface profilometry records crater morphology and depth. For laminates, electroluminescence imaging reveals micro-cracks in cells beneath apparently intact glass, and electrical performance testing quantifies any power loss against pre-impact baseline values.
Applications
Gravel impact testing of photovoltaic coated glass supports a wide range of engineering and commercial objectives:
- Qualification of new AR, self-cleaning, or hydrophobic coatings before adoption in module production lines.
- Incoming quality control and supplier comparison for coated cover glass and coated back glass in bifacial modules.
- Site suitability assessments for desert, coastal, high-altitude, and roadside installations with elevated particle impact risk.
- Development of vehicle-integrated and building-integrated photovoltaics, where debris impact exposure is significantly higher than in fixed ground-mount systems.
- Reliability research correlating impact damage with long-term transmittance loss, soiling behavior, and damp-heat or thermal-cycling durability in combined test sequences.
- Support for type approval and certification campaigns alongside the full IEC 61215 and IEC 61730 test sequences conducted at accredited third-party laboratories.
FAQ
Which standards apply to gravel impact testing of PV coated glass?
The ISO 20567 series and ASTM D3170 are the primary references for coating chipping resistance. For module-level impact robustness, the hail impact test in IEC 61215-2 and the mechanical requirements of IEC 61730-2 apply, while GB/T 30984 series standards define baseline requirements for solar glass products. Test programs are often tailored by combining these frameworks.
What are typical pass criteria?
Criteria are agreed between manufacturer and customer, but common requirements include no through-thickness glass fracture, no coating delamination exposing bare glass beyond an agreed chipping rating, adhesion classified at the highest levels of ASTM D3359 or ISO 2409 after impact, and post-impact transmittance loss limited to a small, defined fraction. For module samples, no new electroluminescence crack signatures and power degradation within the specified limit are typically required.
How is gravel impact testing different from hail impact testing?
Gravel impact testing uses many small hard particles or a single impact body at controlled energies to evaluate coating chipping and adhesion, whereas hail testing per IEC 61215-2 launches ice balls at defined velocities to verify structural survival of the complete module. The two tests are complementary: gravel testing characterizes the coating system, while hail testing validates the laminate.
Can gravel impact testing replace other mechanical tests?
No. It complements, rather than replaces, mechanical load testing, thermal cycling, and damp-heat exposure in the IEC 61215 qualification sequence. Combined sequences, for example gravel impact followed by damp heat, are used to study whether impact damage accelerates subsequent degradation.
Where can these tests be performed?
Accredited third-party laboratories, research institutes, and in-house reliability centers equipped with gravel projection devices, ballistic launchers, integrating-sphere spectrophotometers, and electroluminescence instrumentation can execute the full scope of testing described above and issue standardized test reports.
