Cracks, corrosion, metal fatigue, and other surface and sub-surface defects can be responsible for wind turbine failures in the field. Various NDT methods can provide more insight than the eye can see.
Jesse Herrin, Contributor
Wind turbine component manufacturers conduct a battery of quality and performance tests to ensure that their materials and designs can withstand the conditions they’ll face after installation. By and large, OEMs conduct these tests in controlled environments — big spaces where individual pieces and joints can be methodically subjected to nondestructive testing and teardown over an extended period of time.
It’s a different situation for technicians in the field. Severe rotational forces, variable loads, impacts, lightning strikes, and other hazards can create cracks, corrosion, and irregularities that are too small to see with the naked eye but nonetheless can compromise the integrity of gears, bearings, blades, and structural pieces.
Non-destructive testing (NDT) is a broad category of inspection methods that technicians use to highlight these flaws and defects. If you contract NDT services as part of an operations and maintenance (O&M) program, or your company handles its own O&M, it’s critical to understand the pros and cons of the various NDT technologies used on gearing, bearings, turbine blades, and tower structures.
Liquid penetrant testing (PT) is a simple, time-tested method for identifying surface-breaking defects and discontinuities in metal and other nonporous materials. PT involves applying a colored liquid to the surface and allowing it to be drawn into minute openings by capillary action. Defects become visible under ultraviolet light or by the contrasting color of the dye being used.
Magnetic particle testing (MT) uses magnetic fields to locate surface and near-surface discontinuities in ferromagnetic materials. Very fine ferromagnetic particles are applied to the metal and are drawn into discontinuities on the surface, which indicate the presence of defects to the technician.
PT and MT are common because they’re affordable (and they’re affordable because they are common). But each has its limitations.
PT testing can only detect surface cracks and requires the technician to properly handle and dispose of chemicals, while MT is effective only on ferromagnetic materials. Both techniques — including surface prep and cleanup — are time consuming and the results will vary depending on the skill and patience of the inspector, especially when the work environment is hazardous, uncomfortable, or hard to reach.
Eddy Current Testing
Eddy current testing (ECT) is a nondestructive technique that’s capable of detecting surface and sub-surface defects including cracks, corrosion, and heat damage in conductive materials.
In simple terms, ECT involves placing a probe or coil to a metal surface. The probe generates a changing electromagnetic field that induces electrons to flow in the material. Any cracks or changes in metallurgical structure will distort the flow like eddies in a river; these distortions are captured and analyzed by an instrument and displayed for the technician to review. The results are precise, and the digital record allows technicians to share, store, and review inspection data.
Under comparable conditions and with a skilled technician, single-coil eddy current testing and PT will produce comparable pass/fail-type results.
However, a handheld eddy current tool with a C-scan display can present a digital “big picture” that helps inspectors find more defects in less time. More advanced instruments can conduct dual-frequency testing, digital conductivity testing, and nonconductive coating thickness measurement.
Today the market is shifting toward portable tools with powerful software, touchscreen interfaces, and multi-coil probes. This combination makes ECT feasible for anyone who wants faster, more accurate inspections.
Take, for example, a simple weld inspection on a tower.
Say the average PT inspection on a 1-foot section of weld takes 30 minutes, not including cleanup. The same inspection with a single-coil eddy current probe would take less time but still require the technician to scan the weld multiple times with a probe in order to complete the job. With a multi-coil probe, the technician could potentially scan the body of the weld, toes, and heat-affected zone in a single pass. The scan would likely take less than three minutes, and the probability of detection would be improved.
Composites and Phased Array UT
NDT inspections of large carbon fiber-reinforced polymer (CFRP) turbine blades and rotor tips typically happen during production. But blade and rotor tips face their biggest tests after they leave the factory, where these components are susceptible to lightning strikes, impact damage, and fatigue.
CFRP and other composite materials tend to be more complex than those in metal. Cracks can occur on the surface or beneath the surface — and in different layers or plies — with no predictable orientation. De-lamination defects can occur and propagate quickly especially in components where a laminate is loaded through the thickness, like at spar caps. Furthermore, failure can be sudden and catastrophic with no warning or obvious signs of fatigue. A report from 2014 indicated that there are around 3,800 blade failures per year, accounting for almost 41 percent of insurance claims.
Phased-array ultrasonic testing (UT) is the one method of NDT inspection for composite materials. A phased-array UT instrument generates pulses of high-voltage electricity, which are converted to high-frequency ultrasonic energy by a transducer. The transducer emits pulsed sound waves into the material at precise intervals and set angles. When these ultrasonic waves encounter a defect or discontinuity, some of that energy is reflected back from the flaw surface like an echo.
A dedicated probe configuration, a high-performance phased-array UT instrument, and advanced software can deliver accurate, detailed results about the depth and size of flaws in composite materials quickly without special skills or robotics.
Because some sort of coupling agent (typically water or a gel) is required between the probe and surface of the part to guarantee a high-quality reading, phased-array UT is more common for quality and performance testing at the OEM. However, given the increasing use of composites and the effectiveness of phased-array UT on these materials, O&M providers and NDT technicians need to be familiar with this technique. Its maintenance and repair applications in renewable energy are bound to grow.
Although materials and components evolve, fatigue cracks and corrosion will continue to threaten reliability and uptime for wind turbines. High-quality inspection and NDT testing can serve as an important part of an O&M program to guard against unplanned downtime. The right solution can make a big difference in terms of inspection quality, speed, and cost.
Jesse Herrin is a mechanical design engineer and software developer who has spent 10 years working in product management and engineering roles at Zetec.