Direct metal-resin bonding plays a critical role in jointing of dissimilar materials and is widely used for various mechanical parts and structural components. Especially, epoxy resin adhesive is a typical resin that can strongly bonds metal parts and resin parts. In actual use, impact or dynamic force is repeatedly applied, resulting in interfacial delamination. Thus, it is essential to quantitatively evaluate the adhesion strength and the adhesion durability at the interface and elucidate strength mechanisms. In this study, we first evaluated adhesion strength and adhesion durability against repetitive loading by using a non-contact testing method which uses pulsed laser and induces strong elastic waves to generate interfacial fracture. This method is called Laser Shock Adhesion Test (LaSAT), which enables us to investigate impact interfacial fracture. As a result of repetitive tests (cyclic loading tests), it was found that the adhesive strength of thermally cured sample at a high temperature of 100℃ increases compared to the cured sample at room temperature of 20℃. It was also found that the adhesion strength shows the same tendency as the fatigue strength of the bulk metallic materials. Therefore, we additionally investigated the molecular mechanism of the interfacial fracture using molecular dynamics simulations.

By using J-OCTA, which is a commercial software of molecular dynamics simulation, we first created Al~2~O~3~ model which is naturally oxidized on the surface of aluminum alloy. On the surface of Al~2~O~3~ model, main agent molecules (DGEBA) and curing agent molecules (44DDS) are cross-linked and cured. For the computational condition of cross-linking reaction, when the epoxy group at both ends of DGEBA and the amino group at both ends of 44DDS approached within a certain distance, a new bond was formed with a probability of 10%. In addition, as in the experiment, to investigate how the adhesion strength changes by curing at high temperature, a cross-linking reaction was performed under NPT ensemble and its temperature of 300K and 400K in the molecular dynamics simulation. These are designated as 300K model and 400K model in this study. Similar to the LaSAT experiments, normal stress was repeatedly applied to this interface model by giving velocity to the above part of epoxy model under NVT ensemble. This process enables us to apply cyclic stress range which is from zero to the maximum tensile stress. In other words, this is deformation control under the constant of cyclic stress range. In order to investigate the relationship between the maximum tensile stress and the number of cycles to delamination, these cyclic deformations were performed until interfacial delamination occurred.

As a result of the repetitive deformation, the 400K model showed higher tensile stress for delamination than that of the 300K model. In addition, both models showed that the number of deformation cycles to delamination was increased when lower maximum tensile stress. This trend is very similar to the results of LaSAT experiments. In the molecular structure point of view, it was found that the atomistic density of the epoxy resin near the interface was higher than the density of bulk epoxy resin. It is considered that during the cross-linking (curing) and relaxation simulation of the epoxy resin, the epoxy atoms near the interface were agglomerated due to strong attractive force toward the interface due to strong interaction between Al~2~O~3~ atoms. Indeed, the number of atoms near the interface in the 400K model was larger than that in the 300K model. It is also found that during cyclic deformation, the number of atoms at the interface decreased as the number of deformation (loading) cycles increased. This is the mechanism of adhesion strength and durability.

This comprehensive study with LaSAT experiment and molecular dynamics simulation reveals adhesive strength of Al/epoxy interface and its fracture mechanism from the molecular structure point of view. This will be useful to newly design molecular structure so that it improves impact adhesion strength and durability.


Molecular Dynamics Study on Interfacial Fracture of Aluminum Alloy and Epoxy Resin Adhesive Subjected to Cyclic Loading

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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