This paper reviews the mechanical properties of interface adhesion between the fiber and matrix mainly by interpreting the authors’ past articles. First, the methods for evaluating interface mechanical properties are described considering micromechanical testing using single-fiber composite specimens. In particular, the test-type dependence of the obtained interfacial strength is discussed, and this paper suggests that the cruciform specimen technique is an appropriate testing method. Then, this paper presents another way to obtain interface properties that is extracted from a bulk composite test. Moreover, the time and temperature dependence of the interface strength and the interface failure envelope under a combined stress state are described, respectively. Interface toughness is also discussed at the end of this paper, and it is implied that the value might be much lower than those presented in many conventional articles.

Herein, we evaluate the mechanical properties of carbon fiber/polymer interfaces using three types of specimens: carbon fiber/vinyl ester resin, carbon fiber/epoxy resin, and carbon fiber/polyimide resin. Microbonding tests were performed and the fiber load at interfacial debonding was obtained. By performing finite element analysis, the true interfacial strengths were determined. The strength values followed the order: polyimide > epoxy > vinyl ester for the specimens tested. Molecular modeling was also performed for these specimens. Three stacked graphene layers were used to represent the carbon fiber surface and the interfacial energy was determined. The interfacial energy of the three systems followed the same order as the strength values. The molecular simulations allowed for a qualitative discussion of the material properties. In addition, an interfacial debonding simulation was performed, however the resin part failed instead of the interface, indicating that we evaluated the resin strength but not the interfacial strength. For the quantitative evaluation of interfacial debonding strength, further studies are necessary.

Evaluation of interface strength is important in composite material design such as carbon fiber reinforced plastic. Molecular simulation, which considers aspects such as chemical structure, can be used to evaluate the composite interface strength. Therefore, in this study, the interface energies between graphene, considering the electric charge state, and resin (TriA-X polyimide, DGEBA, triethylenetetramine, vinyl ester, and PA6) were evaluated via molecular dynamics simulation. First, the interface energy and experimentally obtained interface strength were compared. Subsequently, the coefficient of determination R² was calculated using linear approximations from the interface energy and interface strength. In addition, the correlation coefficient was calculated and showed a high correlation. Based on this data, it was conjectured that a relationship exists between the interface strength and interface energy. Furthermore, the validity of the relationship magnitude between the experimentally obtained interface strength and the interface energy obtained via simulation was evaluated. Moreover, considering graphene oxidation, the interface energies between the resin and three forms of graphene (functionalized with OH, COOH, and O groups) were obtained, and the effect of various oxidation surface treatments of graphene on their corresponding interface strengths was investigated.

In this study, the atomistic-scale mechanisms affecting the interfacial stability of a thermoplastic polymer/graphene oxide interface are investigated using molecular dynamics simulations. Different combinations of thermoplastic polymers (polyethersulfone (PES) and polyetherimide (PEI)) and graphene oxides modified with –O–, –OH, and –COOH are prepared. PES is found to be more strongly stabilized with modified/functionalized graphene oxide in the order of –COOH, –OH, –O–, which is opposite to the stability order of PEI. Our results suggest that these orders of stability are governed by a balance between the following two factors resulting from electrostatic interactions: (1) atoms with a strong charge bias attract each other, thereby stabilizing the interface; (2) the excluded-volume effect of the functional groups on graphene oxide destabilizes the interface by preventing $\mathsf{\pi }$-$\mathsf{\pi }$ stacking of aromatic rings.

Herein, the stress-state dependence of fracture entropy for a polyamide 6 material is investigated through molecular dynamics simulations. Although previous research suggests that a constant entropy increase can be universally applied for the definition of material fracture, the dependence of stress triaxiality has not yet been discussed. In this study, entropy values are evaluated by molecular dynamics simulations with varied combined stress states. The calculation is implemented using the 570,000 all-atom model. Similar entropy values are obtained independently of stress triaxiality. This study also reveals the relationship between material damage, which is correlated with void size, and the entropy value.