Estimate residual strength of degraded bonded joints by combining analytical models with non-destructive evaluations

ABSTRACT

Durability issues associated with adhesive degradation have hindered widespread use of bonding in aircraft applications and contributed to several bond-related in-service accidents, fostering extensive related research. While the non-destructive evaluation (NDE) of bondline quality remains elusive, some authors suggested that NDE could establish bond strength thresholds, supporting the theoretical concept of a residual effective bond overlap (REBO). These authors related strength thresholds to NDE of specimens with artificial weak bonds only phenomenologically. This work proposes two physical-based approaches, combining NDE with analytical models for stress analysis, to estimate the residual strength of environmentally degraded metallic bonded joints considering an approximated REBO. These approaches incorporate fundamental characteristics of two physically distinct NDE techniques and a water-related aging mechanism. Statistically treated experimental data for verification was obtained from aluminum-epoxy bonded joints typical of aeronautic application. Two sets of these joints were exposed in a hot/wet controlled environment – to induce representative degradations – for different periods, non-destructively inspected, and mechanically tested. Each set of specimens differed only in the surface preparation to prompt dissimilar failure modes – adhesion and cohesion failures. Estimated residual strengths for different levels of degradation and failure modes were in agreement with quasi-static loads in a conservative fashion.

KEYWORDS:

• Structural bonding
• non-destructive evaluation
• analytical model
• weak bonds
• residual strength
• adhesive degradation

Acknowledgements

The authors are very thankful to the National Civil Aviation Agency (ANAC) in Brazil for the financial support throughout this work, Mr. John van den Berg from the Australian Department of Defence for providing the specimens, and Mr. Steven Lee from the National Institute for Aviation Research (NIAR) for performing all NDE and mechanical tests. We also gratefully acknowledge the invaluable in-kind technical support provided by Mrs. Cindy Ashforth and Dr. Larry Ilcewicz from the Federal Aviation Administration (FAA), Dr. Max Davis (retired) from the Australian Department of Defence, and Dr. Dennis Roach (retired) from the Sandia National Laboratory.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The experimental data (e.g., NDE, mechanical test, and microscopy outputs) and codes supporting this study’s findings are available in figshare at doi.org/10.6084/m9.figshare.19193096.v1

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by ANAC.