It is important to elucidate interfacial fracture and evaluate an adhesion strength of dissimilar metals, which is widely used for various industries. In actual use, cyclic loading is sometimes applied to the interface, resulting in interfacial fatigue fracture of dissimilar metals. One possible method of adhesion testing is Laser Shock Adhesion Test (LaSAT), in which laser ablation induces strong ultrasonic waves, resulting in interfacial fracture. Numerical computation with finite element method (FEM) is also carried out to estimate interfacial stress, which corresponds to the adhesion strength. The advantage of this method involves quick measurement, non-contact technique and “no interfacial edge” for adhesion testing. It is well known that interfacial edge usually becomes stress concentration, which is a significant trigger of interfacial fracture. Thus, adhesion strength is affected by the interfacial edge as well as materials combination. To avoid such a geometrical factor of interfacial edge, LaSAT may become a useful technique, since there is no edge for interfacial fracture when LaSAT experiment is conducted. In other words, LaSAT is a good candidate to purely evaluate adhesion strength of dissimilar materials. By using this method, we evaluate adhesion strength at the interface between Cu and Fe. First is the evaluation of adhesion strength and interfacial fracture toughness. Next is the adhesion durability of Cu/Fe, by using repeated LaSAT. In this method, repeated laser irradiation is conducted to apply cyclic loading to the interface. This enables us to evaluate fatigue interfacial strength, indicating the diagram of applied maximum stress vs. the number of cycles to delamination (so-called interfacial life). Subsequently, in order to elucidate the mechanism of interfacial fracture induced by LaSAT, molecular dynamics (MD) simulation is carried out using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). We first created a single crystal model of dissimilar Cu/Fe metals, which is similar to the above LaSAT experiment. The interatomic potential used in this study is an Embedded Atom Method (EAM). Pre-crack or edge crack (atomic vacancy) is introduced in this model to investigate fracture toughness at the interface. It is found that, at the critical tensile loading, the crack starts to grow, and the model reaches the complete fracture. This study focuses on crack tip opening displacement to evaluate interfacial fracture toughness. The obtained fracture toughness is a good agreement with the estimation by using LaSAT. We also investigate the effect of crystalline orientation, defects, and grain boundary on the fracture toughness, since the LaSAT experiments address polycrystalline dissimilar metals (Cu/Fe). The estimated toughness by MD simulation is ranged in the results of the experiment. Next, cyclic loading is applied to the MD model to elucidate the mechanism of the interfacial fatigue fracture. For this MD model, a polycrystalline structure is created, and cyclic tensile loading whose maximum stress is constant is applied. Noted that this model is a cubic shape with macroscopically smooth (there is no pre-crack). During cyclic loading, it is found that crack initiates at grain boundary around the interface. It is appeared that the initiation site of a fatigue crack is found to be near the grain which is plastically deformable. In other words, the grain boundary near the grain with a large Schmid factor is critical for fatigue crack initiation. It is suggested that the grain undergoes plastic deformation gradually during cyclic loading, and then the grain boundary near the grain becomes stress concentration. Thus, it is predicted that fatigue crack initiates at the grain boundary and propagates as fatigue damages. This means that an interfacial edge is not necessary for fatigue crack initiation. Thus, this finding may support our LaSAT experimental results, i.e. fatigue damage occurs without interfacial edge. As a result, our comprehensive study using LaSAT experiment and MD simulation evaluates adhesion durability (i.e. interfacial fatigue strength) of dissimilar materials and elucidates the mechanism of crack initiation. This finding may be useful for atomistic design toward the development of a tough interface under static and cyclic loadings.


Interfacial Fatigue Fracture of Dissimilar Metals Studied by Molecular Dynamics Simulation and Experiment

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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