Title: Next Generation of Liquid Shims for Structural and High Temperature Aerospace Applications

Authors: Adam Meng

DOI:

Abstract: The global aerospace industry's increasing emphases on lightweight design, fuel efficiency, and cost optimization has accelerated the adoption of advanced composite materials. Assembly gaps from manufacturing tolerances have proven to be a challenge in the use of composite structures. Curable liquid shims eliminate labor intensive, slow placement of solid shims. This paper discusses the next generation liquid shim (NGLS). This shim cures more quickly and incorporates extremely hard, low-coefficient of thermal expansion (CTE) fillers to achieve highperformance. Cure kinetics were evaluated with Differential Scanning Calorimetry (DSC) and Shore D hardness testing throughout the cure. Handleability was characterized by slump and application time. Mechanical properties were evaluated by Tensile Lap Shear (TLS), dogbone tensile strength, and compression testing. To evaluate crack resistance, the liquid shim was thermocycled between −55°C (−67°F) and 177°C (350°F). The NGLS demonstrates significant performance compared to the state-of-the-art liquid shim product, LOCTITE EA 9377. Key improvements include a reduced cure time (from > 9 hours to approximately 6.25 hours), superior thermocycling crack resistance from 121°C to 177°C (250°F to 350°F), and a higher compressive strength from 121 MPa to 184 MPa (17.6 ksi to 26.7 ksi). These results confirm that the NGLS successfully meets the demands for high-manufacturing throughput, improved thermocycling crack resistance, and superior mechanical performance for advanced aerospace structures.

References: [1]. E. Petrie, Handbook of adhesives and sealants. McGraw Hill Professional, 2005 [2] C.Y. Attahu and L. An, " Influence of assembly gap and shims on the strain and stress of bolted composite-aluminum structures," ARPN Journal of Engineering and Applied Sciences, vol. 12, pp. 1593–1619,2017. [Online]. https://www.arpnjournals.org/jeas/research_papers/rp_2017/jeas_0317_5800.pdf [3] J. X. Dhôte, A. J. Comer, W. F. Stanley, and T. M. Young, “Study of the effect of liquid shim on single-lap joint using 3D Digital Image Correlation,” Composite Structures, vol. 96, pp. 216225, Feb. 2013, [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0263822312004242 Available: [4] 3M, "Shims: Filling Gaps for Aircraft Assembly," 3M, [Online]. Available: https://www.3m.com/3M/en_US/aerospace-us/resources/insights-perspectives/fullstory/?storyid=44e5b9c2-79ec-409a-99bf-fdcf5cf35b1c [5] “LOCTITE® EA 9377 AERO,” Henkel-adhesives.com, 2024. [Online]. Available: https://next.henkeladhesives.com/us/en/products/industrial-adhesives/central-pdp.html/loctite-ea9377- aero/SAP_IB-75695.html. [6] Archive Market Research, "Aerospace liquid shims strategic roadmap: Analysis and forecasts 2026-2033," Dec 2025. [Online]. Available: https://www.archivemarketresearch.com/reports/aerospace-liquid-shims-376815?tab=summary [7] A. J. Comer, J. X. Dhôte, W. F. Stanley, and T. M. Young, "Thermo-mechanical fatigue analysis of liquid shim in mechanically fastened hybrid joints for aerospace applications," Composite Structures, vol. 94, no. 7, pp. 2181–2187, Jun. 2012. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0263822312000207 [8] ASTM D638 – Standard Test Method for Tensile Properties of Plastics, http://dx.doi.org/10.1520/D0638-22 [9] ASTM D3418 – Standard Test Method for Transition Temperatures and Enthalpies of Fusion and Crystallization of Polymers by Differential Scanning Calorimetry, http://dx.doi.org/10.1520/D3418-21 [10] ASTM D2240 – Standard Test Method for Rubber Property—Durometer Hardness, http://dx.doi.org/10.1520/D2240-15R21 [11] ASTM D2202 – Standard Test Method for Slump of Sealants, http://dx.doi.org/10.1520/D2202-00R23 [12] ASTM D1002 – Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal), http://dx.doi.org/10.1520/D1002-10R19 [13] ASTM D695 – Standard Test Method for Compressive Properties of Rigid Plastics, http://dx.doi.org/10.1520/D0695-23 [14] ASTM D638 – Standard Test Method for Tensile Properties of Plastics, http://dx.doi.org/10.1520/D0638-22

Conference: SAMPE 2026

Publication Date: 2026/04/27

SKU: 238

Pages: 18

Price: $36.00