Effect of environmental thermal cycling on the lap shear and fatigue strength of adhesively bonded hybrid composite-metallic joints
Laine, Jere (2026)
Diplomityö
2026
School of Energy Systems, Konetekniikka
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2026060261560
https://urn.fi/URN:NBN:fi-fe2026060261560
Tiivistelmä
Adhesively bonded carbon fibre reinforced polymer (CFRP)–aluminium joints are increasingly used in lightweight hybrid structures. The mismatch in coefficients of thermal expansion between the substrates generates cyclic stresses within the adhesive bondline during service-related temperature fluctuations, potentially affecting joint durability. Comparative studies on the thermal-cycling resistance of commercial structural epoxies remain limited.
This study investigates the influence of thermal cycling on the static and fatigue behaviour of single-lap joints between EN AW-6061-T6 aluminium and a quasi-isotropic CFRP laminate. Three toughened two-component structural epoxy adhesives were evaluated: Loctite EA 9466, Permabond ET5429, and 3M DP490. Specimens were manufactured according to ISO 4587 and tested in the as-manufactured condition and after 200 thermal cycles between −20 °C and +80 °C. Quasi-static lap shear tests were performed for all adhesive systems, while fatigue tests were conducted on 3M DP490 joints at 50% and 65% of the ultimate lap shear strength (ULS).
Thermal cycling reduced the mean ULS of joints with 3M DP490 and Permabond ET5429 by 14.0% and 4.6%, respectively. Loctite EA 9466 joints showed a 19.7% increase attributed to interfacial failure in the reference specimens rather than genuine strengthening. Thermal cycling shifted the failure mode towards the CFRP substrate across all systems, with fibre tearing indicating progressive surface ply degradation. Thermally cycled fatigue specimens performed comparably at 65% ULS and significantly better at 50% ULS. Fibre tearing appeared in every thermally cycled fatigue specimen and none of the reference specimens, confirming a fundamental shift in failure mechanism. Thermal cycling response is strongly adhesive-dependent and modifies failure pathways even when strength changes are limited.
This study investigates the influence of thermal cycling on the static and fatigue behaviour of single-lap joints between EN AW-6061-T6 aluminium and a quasi-isotropic CFRP laminate. Three toughened two-component structural epoxy adhesives were evaluated: Loctite EA 9466, Permabond ET5429, and 3M DP490. Specimens were manufactured according to ISO 4587 and tested in the as-manufactured condition and after 200 thermal cycles between −20 °C and +80 °C. Quasi-static lap shear tests were performed for all adhesive systems, while fatigue tests were conducted on 3M DP490 joints at 50% and 65% of the ultimate lap shear strength (ULS).
Thermal cycling reduced the mean ULS of joints with 3M DP490 and Permabond ET5429 by 14.0% and 4.6%, respectively. Loctite EA 9466 joints showed a 19.7% increase attributed to interfacial failure in the reference specimens rather than genuine strengthening. Thermal cycling shifted the failure mode towards the CFRP substrate across all systems, with fibre tearing indicating progressive surface ply degradation. Thermally cycled fatigue specimens performed comparably at 65% ULS and significantly better at 50% ULS. Fibre tearing appeared in every thermally cycled fatigue specimen and none of the reference specimens, confirming a fundamental shift in failure mechanism. Thermal cycling response is strongly adhesive-dependent and modifies failure pathways even when strength changes are limited.