USA - Areas in which adhesives can outperform conventional fasteners in wind turbine blade manufacture

The wind industry uses adhesives most often in the construction of turbine blades. That application produces long and relatively light structures that are cyclically loaded, work at high and low temperatures, and are exposed to salt spray and UV light for long periods. What could be more abusive?

In addition to primary blade assembly, there are other areas in which adhesive can outperform conventional fasteners. The results of several tests and successful applications may spark the imagination of turbine designers. For instance, do these jobs sound right for mechanical fasteners?

• Attach heavy glass and metal architectural panels to skyscrapers.

• Assemble wings and fuselage of passenger airplanes.

• Construct cargo trailers, elevators, and farm equipment.

• Assemble of golf clubs, bicycle frames, and baseball bats.

Perhaps, but for such diverse and demanding structural applications and many others, liquid adhesives and acrylic foam tapes provide a better overall solution for the manufacturer and deliver the necessary performance and durability in the assembly of high-quality products.

Adhesives 101:
The most important attribute of adhesives is that they provide uniform stress distribution over a larger bond area than typical mechanical fasteners, so they provide a bond with higher total strength. At the same time, adhesives eliminate stress concentrations and localized failure points common with mechanical attachments.

Despite the common perception that bolts, screws, and rivets are stronger, adhesives can outperform the mechanical attachments in many common applications, especially under more real-life conditions of cyclic loading at lower stresses. Adhesives also keep the integrity of bonded materials intact, along with decorative or protective coatings that contribute to the product’s appearance or corrosion resistance.

Mechanical attachments, on the other hand, require drilling holes or introducing internal component damage during insertion. Unlike mechanical fasteners that attach through the material, adhesives bond to a part’s surface, allowing use of thinner gauge materials that could result in significant cost savings and weight reduction.

Adhesives are also particularly useful when bonding dissimilar materials. They easily bond metals to plastics, wood, or composite materials, and they can provide the stress relaxation needed to compensate for the differences in these materials’ thermal expansion. Most bonding products, whether structural adhesives or acrylic foam tapes, fill gaps well to simultaneously bond and seal, often providing a water-tight seal against weather intrusion. Some adhesives provide vibration damping for applications in which noise control becomes important. Finally, adhesives and tapes often give engineers increased freedom to shape unique designs with improved appearance.

Of course, adhesives and tapes are not perfect. On price alone, adhesives may cost more than bolts, screws, or rivets. However, other benefits provided by adhesives often result in an overall more cost effective solution for the manufacturer. Mechanical fasteners also have the attribute of providing immediate bond strength. While a pressure sensitive adhesive on acrylic foam tapes allow immediate handling of a bonded assembly, even the fastest high-performance liquid adhesives can take several minutes to reach handling strength. Also, the performance of mechanical attachments remains generally unchanged over wide ranges of temperature and humidity, whereas the mechanical properties of adhesives and tapes can change somewhat with these conditions. These factors do not exclude adhesives from consideration, but must be taken into account when designing with adhesive products. Finally, because adhesives and tapes bond to the outer surface of a material, preparation of that surface is critical to adhesive performance and durability. For example, typical surface preparation for liquid adhesives involves first removing oil and other contaminants, then sandblasting or light abrasion using fine grit sandpaper, followed by a solvent wipe to remove the loose debris. On the other hand, through-fasteners don’t care much about the surface condition of the attached parts.

Liquid adhesives and acrylic-foam tapes have been around for more than 30 years, with a long history of successful real-life structural applications in markets such as military, aerospace, construction, automotive, and sporting goods. The base materials used to manufacture these adhesive products (primarily epoxy, acrylic, and urethane chemistries) consist of polymers that inherently resist degradation over time. The ability of these adhesives to withstand cold and hot temperatures, UV light exposure, humidity, and other environmental conditions has been documented through both actual applications and accelerated aging studies.

Performance tests:
The most common procedure used to determine bond strength of structural adhesives is the single lap shear test. It involves bonding two rigid panels together and then pulling them apart to measure the peak force required to cause failure. Lap-shear tests were performed to measure the pull force required to separate two aluminum panels bonded with a high-performance, toughened, epoxy adhesive (3M Scotch-Weld DP920) along with welding, spot welding, and two different types of mechanical fasteners. The adhesive provided the highest overall bond strength, illustrating the main adhesive benefits of uniform stress distribution and increased bond area. On the other hand, spot welded, riveted, and bolted panels show deformation after attachment, and panel damage during the lap-shear tests.

A significant real-life example of a structural application performed with adhesives is the attachment of glass and metal architectural panels to a curtain-wall framework used in the construction of large buildings and skyscrapers. To prove their performance and durability, an insulated glass unit bonded to an aluminum curtainwall frame with acrylic foam tape (3M VHB Tape G23F) was subjected to simulated hurricane conditions and wind loads corresponding to wind speeds up to 250 kph at temperatures ranging from -25 to 71°C, with no failure of the adhesive bond or water leakage through the perimeter of the bonded panel. Acrylic foam tapes were used to attach the architectural metal panels on the Burj al Arab hotel in Dubai and many others around the world.

Structural adhesives are frequently used to assemble of sporting goods such as golf clubs and bicycle frames. These applications require an ability to bond to a wide variety of materials (metals, plastics, composites, rubber) with a infinite design geometries, requirements liquid adhesives handled well.

A key performance requirement for these applications is to withstand extremely high impact. Bonds to golf club drivers are tested by shooting golf balls at over 160 kph at club heads to ensure no failure after thousands of impacts.

As a measure of the cyclic-fatigue resistance of acrylic foam tapes versus mechanical fasteners, two large cargo trailers were loaded to 120% of their design-carrying capacity and then driven over a variety of road conditions. One trailer was assembled using acrylic foam tape (3M VHB Tape CV62F) to attach the aluminum side panels to the steel trailer frame and driven an equivalent of 160,000 km. Inspection upon completion showed no adhesive-bond failures. In addition, a high-pressure washer was used to spray water over the seams. No water leaked into the enclosed compartment. The other trailer, assembled with self-tapping screws to industry-standard spacing, was driven only 58,000 km, at which point an inspection found 33% of the mechanical fasteners loose.

Exposure tests for the wind industry:
Because a real life 30-year exposure test would take that long, accelerated exposure tests are used instead to provide confidence in an adhesive’s ability to survive the expected environmental conditions for a specific application. While these tests subject a bonded assembly to severe, often unrealistic conditions to speed the process, excellent information can be obtained in weeks or months instead of many years. In one series of accelerated exposure tests, representative structural adhesives of the three most common chemistries (epoxy, acrylic, and urethane) were subjected to 1,000 hours of continuous immersion in a variety of solvents and at high temperature and humidity conditions. Results show excellent shear strength retention compared to the unexposed adhesive bond, with structural epoxy adhesive showing a best combined resistance. Also, any decrease in the measured lap-shear strength as a result of these severe exposure tests was most likely due to a reversible drop in modulus, not a contact area loss or adhesive degradation.

In a broader study, the wind industry will find of interest, a high-performance toughened structural adhesive (3M Wind Epoxy Structural Adhesive W1115) was subjected to 1,000 hours of exposure to a variety of conditions, including salt water, acids, bases, and additional solvents. The adhesive provided more than 80% retention of original strength in every case, corresponding to over 18 MPa of lap shear strength even after continuous exposure in these extreme conditions. Keep in mind that the adhesive bond in many applications does not contact the solvents and other materials, because the bonded parts conceal the adhesive bond line, or the entire bonded assembly receives a coating that prevents direct adhesive exposure.

Accelerated weathering tests (UV light with cycling temperature and humidity) were also conducted to demonstrate the ability of acrylic foam tapes to withstand a service lifetime of outdoor weather conditions. High-intensity UV light from a xenon arc source was directed through clear glass panels that were attached to black anodized aluminum frames with three different acrylic foam tapes. The retention of tensile strength was checked periodically up to 10,000 hours of total exposure, which predictability models equate to about 30 years of service life under typical real-life exposure conditions. The results showed no loss in performance over the length of the test, indicating no decrease in bond strength or adhesive deterioration. Some acrylic foam tapes show a noticeable increase in measured tensile strength.

Most people in the wind industry are familiar with the use of structural adhesives to bond turbine blades. But there are many other applications where structural adhesives and tapes could provide improved performance and processing advantages. This includes additional applications in blade assembly as well as the construction of wind turbine nacelles and towers. While not suitable for all attachments, structural adhesives and acrylic foam tapes are high-performance, durable products that are ideally suited for many wind energy applications. Hundreds of different permanent bonding products are available to meet the wide variety of applications in the wind industry. The challenge for design engineers and manufacturing personnel is to keep their eyes open for applications where structural adhesives and tapes can improve the overall performance of their product.

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