Testing Military-Grade Adhesive in Extreme Loading Conditions

Military performance requirements for adhesives have been traditionally derived to fulfill niche defense needs in harsh operational environments with little consideration for dual-use commercial potential.

August 1, 2024

U.S. Army Research Laboratory, Aberdeen, MD

Figure 1. Bonded tensile butt joints clamped in alignment fixtures undergoing oven cure at 140°C. A thermocouple is placed at the bond line and a spring is used compensate for mismatches in thermal expansion.

The term “military-grade” can have a variety of meanings that are perspective dependent. In 2014, Ford Motor Company emphasized the term heavily in advertising campaigns to garner consumer acceptance for the transition from steel to aluminum in the body of their flagship F150 model. As cited by Ford, “Engineers selected these high-strength, military-grade aluminum alloys because of the metals’ unique ability to withstand tough customer demands.” From this point-of-view, military-grade implies superior performance. However, the bureaucratic and logistical barriers required for certification to military-grade acceptance levels per DoD performance requirements can also be perceived as impediments to innovation and the transition of fundamental science into tangible product. This is in-part due to the legacy age of many DoD performance standards dating to the 1950s and 1960s when the US military peaked in technology market share and was responsible for approximately two-thirds of domestic research and development (R&D) and one-third of global R&D.

In 2023 the commercial private sector provides the overriding funding stream for technology development for primarily non-military applications. Since the “golden age” of DoD-derived performance specifications the interactive roles between requirements and innovation are now understood to be dependent on their timings to product life cycle, which is typically ignored universally across the materials domain. Traditional DoD adhesive specifications are measures of late life cycle quality assurance for low-risk bonding applications with long-term historical usage and well-understood design allowables.

Military-grade certification for commercial industry can also imply a high degree of qualification effort for a limited defense market with marginal dual-use consumer return on investment. This is an unfavorable position for DoD, as commercial industry requires a sustainable business model to produce defense-relevant materials.

MIL-PRF-3266210 represents a forward-looking and “anticipatory” specification to deliberately coincide with the timing of emergent, and potentially lucrative, adhesive technologies by encouraging, high-risk/high-payoff commercial product innovation. The hypothesis of this approach is that defining DoD technology drivers in terms of industrially relevant performance metrics, with high non-DoD profit potential, will decrease transition time and increase DoD access to sustainable commercial products with leading state-of-the-art properties. Prior to the release of MIL-PRF-32662 in 2020, no adhesive, with bonded structural armor as the primary focus, had bridged the “valley of death” from basic research to qualified product.

The technical challenge posited by Army ground vehicle applications is the need for concurrent high-strength and high-fracture toughness, which is a difficult material property trade-space for adhesives. PPG Industries (PPG) was the first commercial adhesive manufacturer to meet MIL-PRF-3266210 Group 1 requirements with the development of their PR-2930 one-component epoxy adhesive. This adhesive targets the Army ground vehicle requirements put forth in MIL-PRF-32662 but is also formulated for compatibility with commercial automotive assembly line bonding processes. The next question to be answered is can the performance value of military-grade be quantified using a benchmark of an extreme testing condition referenced by the commercial sector?

The adhesive bonding joint configuration used for this study was a tensile butt joint with a 40 cm² surface area for bonding, which is specified by Guinness World Records. The most recent record was set in 2019 to suspend a 17,200 kg (37,919 lb.) construction vehicle at a height of 1 m for 1 h. In this work, the world record hoist and static hold conditions were mimicked using a laboratory loading frame. Results show that an adhesive meeting MIL-PRF-32662 Group 1 requirements readily sustains 22,680 kg (50,000 lb.) of static loading for 1 hour, for a 32 percent increase against the current world record benchmark. The results offer a potential quantitative measure for military-grade and show the utility of leveraging anticipatory military performance standards to lead material advancements.

This work was performed by Daniel Deschepper and a team of researchers from the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) below. ARLTR-9915