An Evaluation of Stiffener Attachment Methods

by Paul Deffenbaugh | 2 August 2021 12:00 am

Some of the most common methods include flexible silicone sealants, acrylic foam tapes, rigid structural adhesives and welding. This article summarizes an independent third-party analysis conducted by Intertek in its Fridley, Minn., lab of six different stiffener attachment methods in industry-accepted ASTM test methods for evaluating the performance of architectural panels.

Three aluminum stiffeners were attached to 5-foot by 8-foot (1.5-m x 2.4-m) aluminum panels with the below attachment methods. (Figures depict each specific attachment.)

1. Stitch-Welding
2. Structural Adhesive: 3M Scotch-Weld Metal Bonder Acrylic Adhesive DP8407NS (liquid-applied rigid structural adhesive)
3. Silicone Sealant: DOWSIL 795 Building Sealant (liquid-applied flexible adhesive/sealant)
4. Acrylic Foam Tape: 3M VHB Architectural Panel Tape G16F
5. Acrylic Foam Tape: 3M VHB Tape GPH-160GF
6. Acrylic Foam Tape: 3M VHB Architectural Panel Tape B90F

The ASTM test methods used in this study are trusted to determine suitability for high-risk attachment of stiffeners and panels to building exteriors and are relevant to other markets as they test structural integrity across varying stress loading, pressure cycling and environmental conditions. Center panel deflection was measured during wind load structural testing to determine the ability of each stiffener attachment method to meet the L/60 criterion for aluminum wall panels according to the International Building Code (IBC) Section 1604 Deflection Limits. “L” is the longest panel dimension.

Test Results:

All panels passed the rigorous testing sequence and can provide acceptable performance in stiffener attachment applications. The panels passed both positive and negative wind load structural tests up to 120 pounds per square foot (psf), which corresponds to a sustained wind speed of 220 mph (350 kph). For the hurricane impact tests, 96-inch (2.4-m) missiles were launched at panels between 15-16 meters per second velocity, and the stiffeners attached with acrylic foam tape, structural silicone sealant and stitch welds remained bonded. Hurricane pressure cycling tests at a design pressure up to 80 psf (3.8 kPa) were also passed. The table below summarizes test results of each attachment method:

Conclusion

After four different ASTM tests run sequentially, each set of panels and attachment likely saw greater stresses in a condensed time period than many real applications will see in their lifetime. None of the stiffeners failed when subjected to pressures equivalent of 220 mph (355 km/h) sustained wind speeds, and there was only one failure during missile impact testing.

The entire application process, labor, cost and aesthetics should be assessed alongside the stiffener performance to determine an appropriate stiffener attachment method. Acrylic foam tapes offer the greatest process flexibility through ease of use and immediate handling strength, and have the most aesthetically pleasing result, whereas welding and rigid structural adhesives have more complicated or less efficient processes but provide the greatest shear strength performance when needed for an application.

From a panel deflection viewpoint, the two thinnest acrylic foam tapes (3M VHB Tape G16F and 3M VHB Tape GPH-160GF) and the rigid structural adhesive (3M Scotch-Weld Metal Bonder Acrylic Adhesive DP8407NS) outperformed all the other attachment methods by passing the L/60 criterion up to 80 psf (3.8 kPa), while the other attachment methods only passed up to 40 psf (1.9 kPa).

Attachment Method Overview

Welding:

• 1000-plus pounds per square inch (psi) in overlap shear strength.
• Retained stiffener after impact.
• Requires trained, skilled labor to implement with most employers preferring certified technicians.
• Limits potential materials that can be used for joining. Aluminum and steel welding require different equipment, and dissimilar metals cannot be welded due to galvanic corrosion.
• Intense localized heat causes thermal distortion in final product, requiring extra time and labor to refinish with abrasives.

Rigid Structural Adhesive – 3M Scotch-Weld Metal Bonder Acrylic Adhesive DP8407NS:

• 1000+ psi in overlap shear strength when fully cured
• Two-part chemistry requires mixing prior to application, and clamping or fixturing while adhesive cures. Typically reach handling strength in under an hour.
• Acrylic adhesives usually require the least amount of surface preparation on metals, and can bond through thin layers of surface oils.
• Decent aesthetic with no refinishing necessary. However, during the curing process the adhesive can shrink, which may “telegraph” through thinner panels and be visible to the end user.
• Impact stress caused the stiffener to fail.

Silicone Sealant – DOWSIL 795 Building Sealant

• 45-60 psi ultimate strength.
• One-part chemistry requires moisture to cure. Seven to 14 days at 77 F (25 C) and 50% relative humidity (RH) to cure and 14 to 21 days before reaching full adhesion.
• Spacer tape also needed to maintain consistent thickness of 1/4 inch.
• Greater flexibility than rigid structural adhesives—retained stiffener after impact.
• Similar aesthetic to structural adhesives.
• Requires cleaning and may need adhesion promoters on some substrates.

Acrylic Foam Tapes – 3M VHB Tapes

• 65-plus psi ultimate strength.
• Fully-cured construction gives immediate handling strength once pressure has been applied, allowing for extra manufacturing flexibility.
• Simple application instructions allow for unskilled labor to be used.
• Smoothest aesthetic of the attachment methods tested.
• Allows for 300% shear strain movement, allowing for a thinner bonding solution compared to silicone sealant—retained stiffener after impact.
• Requires cleaning and may need adhesion promoters on some substrates.

A simplified comparison of results is shown below in Table 2:

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Steve Austin is an application engineer at 3M[1], St. Paul, Minn., and has worked in this role for 17 years in support of customers using adhesives and tapes for bonding applications in the construction market. Prior to this, he was a product development engineer at 3M for 14 years.

Reese Weber is a chemical engineer who has worked with many adhesives at 3M[1] since 2012. He is currently a senior application engineer for 3M VHB Tapes, working with customers on their design, technical testing, product selection and more.

Source URL: https://www.metalconstructionnews.com/articles/an-evaluation-of-stiffener-attachment-methods/

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