To obtain lightweight structures, the adhesive bonding technique has been established for many years, especially in the engineering and automotive industry. Even though there had been investigations on the mechanical properties of the adhesively bonded joints, the studies on the fatigue strength of this class of joints are limited. This research work is a continuation of the previous research works from our group (1; 2) and aims to study the effect of hygrothermal aging conditions as per PV 1200:2004 standard on fatigue characteristics of carbon-fiber-reinforced polymer (CFRP)/ aluminium alloy (Al) single-lap joints (SLJ) subjected to mechanical tensile-tensile cyclic loading. The single-lap joints are constructed as per the DIN EN 1465:2009 standard with the adhesive TEROSON^® EP 5065 from Henkel. Two surface preparation methods – that have prooved to be suitable in static tests (1) – acetone cleaned (AC) and laser-treated surface in near-infrared wavelength (NIR) with energy density, ED = 37.5 J/cm², are chosen to study the behavior of the joints.

The results of the fatigue experiments showed in comparison to the quasi-static results (1) that acetone cleaned aged specimens showed lower strength and life cycles to failure compared to their unaged counterparts. This could be related to the fact that hygrothermal aging has a detrimental influence on the load-bearing capability of the adhesive. Since the laser-treated specimens improved the wettability and surface roughness of the bonding region a better adhesion could be achieved between the adhesive and the substrates. This effect was reflected by higher tensile strengths and life cycles to failure. The microscopic analysis of the failed specimens subjected to the fatigue test showed that the predominant failure pattern in the bonding region of the SLJs is a combination of adhesive (AF) and cohesive (CF) failure. In the case of AC specimens, the failure pattern shifted from CF to AF with decreasing maximum load (F_max). While in AC aged specimens, even though CF decreased with F_max, more than 75% of failure occurred through the adhesive. Laser-pretreated specimens showed two different types of failure behavior. Failure pattern shifted from AF to CF until a F_max = 5 kN and then from a F_max = 4 kN, i.e. 55 % of static tensile load, a crack propagated through the cross-section of the Al alloy substrate, and the joint failed predominantly by AF. A probability of failure curve was plotted using Rossow’s method to provide information on the safe application region of the single-lap joints. For the same F_max, the laser-treated joints withstand more life cycles and higher endurance limit compared to the acetone-cleaned joints. The endurance limit of the acetone cleaned unaged and aged joints tend to be the same at lower F_max. This provides us the information that aging doesn’t significantly affect the fatigue property of the joints at lower F_max.

Keywords: Mechanical properties, fatigue, cyclic loading, hygrothermal aging, SLJ, Al, CFRP, EP 5065, surface treatment, laser pretreatment, optical microscopy, failure pattern

References

1. Comprehensive study on the influence of different pretreatment methods and structural adhesives on the shear strength of hybrid CFRP/aluminum joints. Schanz, J., Meinhard, D., Dostal, I., Riegel, H., De Silva, A. K., Harrison, D. K., & Knoblauch, V. 2021, The Journal of Adhesion, pp. 1-29.

2. Effect of drilling-induced damage on the open hole flexural fatigue of carbon/epoxy composites. Haeger, A., Grudenik, M., Hoffmann, M. J., & Knoblauch, V. 2019, Composite Structures, pp. 238-248.