This study fabricated nanocomposites with aligned graphene platelets and examined their mechanical properties. Both reduced graphene oxide (RGO) and functionalized graphene (FG) were used in this study. Graphene platelets were incorporated into epoxy matrix through mechanical mixing, followed by sonication for homogeneous dispersion. To align the graphene platelets, before curing, an electric field was applied to the epoxy-graphene compound. Real-time microscopic observation revealed that under an electric field, the graphene platelets rotated, translated, and eventually formed a chain-like microstructure along the electric field. Tensile tests were conducted on the nanocomposites with aligned graphene, and failure mechanisms were identified using scanning electrical microscopy of the fracture surfaces. For comparison, nanocomposites with randomly oriented graphene were prepared and then subjected to tensile loading.Moreover, the moduli of the graphene nanocomposites were characterized using the modified Mori-Tanaka micromechanical model. The experimental results indicated that the nanocomposites with aligned graphene exhibited higher tensile moduli and tensile strength than those with randomly oriented graphene. Moreover, the nanocomposites with FG had superior interfacial properties and thus superior mechanical properties compared with those with RGO.

The peridynamic is a kind of unique theory which can deal with continuous models, defect of crack and particles in a single framework. It does this by formulating problems in terms of integro-differential equation rather than partial differential equations. This theory can be used to investigate the dynamic mechanical behavior of the discontinuous material, such as the propagation, bifurcation, unstability process of crack. However, there is no reasonable force function for the interface of bimaterial. In this paper, we utilize a weight function method to establish the force function of the bimaterial interface. We discussed crack propagation along the interface in a prenotch bimaterial model by peridynamics, and analyzed the influence of different mesh shapes on the deflection angle of crack path. By comparing the peridynamic solution for these problems with the results published in the literature, we concluded that peridynamics with the proposed method is a reliable formulation for modeling the interfacial dynamic fracture of bimaterial.

The effect of strength mismatch (ratio between the yield stress of weld metal and base metal, M y ) on the ductile crack growth resistance of welding pipe was numerically analyzed. The ductile fracture behavior of welding pipe was determined while using the single edge notched bending (SENB) and single edge notched tension (SENT) specimens, as well as axisymmetric models of circumferentially cracked pipes for comparison. Crack growth resistance curves (as denoted by crack tip opening displacement-resistance (CTOD-R curve) have been computed using the complete Gurson model. A so-called CTOD-Q-M formulation was proposed to calculate the weld mismatch constraint M. It has been shown that the fracture resistance curves significantly increase with the increase of the mismatch ratio. As for SENT and pipe, the larger M y causes the lower mismatch constraint M, which leads to the higher fracture toughness and crack growth resistance curves. When compared with the standard SENB, the SENT specimen and the cracked pipe have a more similar fracture resistance behavior. The results present grounds for justification of usage of SENT specimens in fracture assessment of welding cracked pipes as an alternative to the traditional conservative SENB specimens.

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Plastic predeformation (prestrain) in pipe reeling process not only influences the steel yield and flow properties but also reduces fracture toughness. In this work, a combined effect of prestrain history and weld strength mismatch on the crack tip constraint was investigated. Three prestrain histories and three mismatch levels were considered. A four-parameter approach (crack tip opening displacement [CTOD], Q, P, and M) was employed to characterize the crack tip constraint levels. The study addressed an evaluation procedure for the fracture resistance in mismatched welded joints under different prestrain levels using CTOD values. Furthermore, it illustrates the interaction including prestrain histories and strength mismatch effect. The findings indicated that the constraints of prestrain history and overmatch degree are positive for the same crack tip opening displacement level. The crack tip stress field was more significantly influenced by the strength mismatch than the prestrain history in examined cases.