Correspondence between adhesive joint strength, surface treatment and surface quality of CFRP
Blanco Garde, Daniel (2023)
2023
Master's Programme in Materials Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
This publication is copyrighted. You may download, display and print it for Your own personal use. Commercial use is prohibited.
Hyväksymispäivämäärä
2023-06-26
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202306226895
https://urn.fi/URN:NBN:fi:tuni-202306226895
Tiivistelmä
The aircraft industry uses a high amount of composite materials and employs adhesive bonds extensively to join the different structures. Bonded joints do not requiring any mechanical drilling and are able to distribute the loads better than mechanical joints. The main challenge of adhesive bonds is that it is not possible to check their structural integrity once they are formed. Additionally, adhesive joints are very dependent on the surface treatments applied to the adherend before bonding. This is a problem, especially in such a safety-oriented industry like the aircraft one. Some methods have been developed to assure that the surface quality has the right characteristics for bonding, such as measuring the contact angle that a drop of water forms once it is deposited on the bonding surface.
This master’s thesis aimed to investigate the correlation between the bond surface contact angle and the strength of adhesive bonds in carbon/epoxy composite laminates. This correlation could be useful in the aircraft industry for preventing weak adhesive bond joints by simply measuring the contact angle (CA) of the bond surface after the surface treatment and before even producing the adhesive bond joint.
To study the correlation, different surface treatments were applied to carbon/epoxy laminates and their CA was measured using a top-view and hand-held device. Single lap shear (SLS) and double strap joints (DSJ) were manufactured using epoxy adhesive and carbon/epoxy laminates. Finally, both types of adhesive joints were mechanically tested, recording their maximum load. After testing, their failure surfaces were studied. Additionally, the effect of exposing adhesive joints with different surface treatments to an elevated temperature wet (ETW) environment was also evaluated. The applied surface treatments varied in the surface roughness level, in the cleaning process and in the substances used to contaminate the bond surface. This variety in surface treatments achieved very different CA values.
Based on these results, CA measuring was able to distinguish clearly different surface roughness as well as different contaminants on the surface of carbon/epoxy laminates. DSJ highest strength results were obtained applying a baseline sanding, whereas cleaning the surface with a solvent reduced the bond strength as much as contaminating it. SLS testing strength results indicated an opposite trend, which could be influenced by strong peel forces in SLS samples and delamination as main failure mode. A not-unequivocal correlation was observed for the minimum CA of each DSJ sample and its corresponding testing strength, with low CA surface treatments leading to the highest bond strength. Samples exposed to ETW environment had lower strength and followed a CA-strength correlation like DSJ.
This master’s thesis aimed to investigate the correlation between the bond surface contact angle and the strength of adhesive bonds in carbon/epoxy composite laminates. This correlation could be useful in the aircraft industry for preventing weak adhesive bond joints by simply measuring the contact angle (CA) of the bond surface after the surface treatment and before even producing the adhesive bond joint.
To study the correlation, different surface treatments were applied to carbon/epoxy laminates and their CA was measured using a top-view and hand-held device. Single lap shear (SLS) and double strap joints (DSJ) were manufactured using epoxy adhesive and carbon/epoxy laminates. Finally, both types of adhesive joints were mechanically tested, recording their maximum load. After testing, their failure surfaces were studied. Additionally, the effect of exposing adhesive joints with different surface treatments to an elevated temperature wet (ETW) environment was also evaluated. The applied surface treatments varied in the surface roughness level, in the cleaning process and in the substances used to contaminate the bond surface. This variety in surface treatments achieved very different CA values.
Based on these results, CA measuring was able to distinguish clearly different surface roughness as well as different contaminants on the surface of carbon/epoxy laminates. DSJ highest strength results were obtained applying a baseline sanding, whereas cleaning the surface with a solvent reduced the bond strength as much as contaminating it. SLS testing strength results indicated an opposite trend, which could be influenced by strong peel forces in SLS samples and delamination as main failure mode. A not-unequivocal correlation was observed for the minimum CA of each DSJ sample and its corresponding testing strength, with low CA surface treatments leading to the highest bond strength. Samples exposed to ETW environment had lower strength and followed a CA-strength correlation like DSJ.