金屬基地 複合材료의 界面特性變化에 따른 彈塑性擧動 評價에 關한 硏究

Abstract

So many innovations in ship, transportation and aerospace industries have depended on high-performance structural materials. The advantages of metal matrix composite(MMC) are having an outstanding performance through the combination of the light weight, high stiffness, strength and corrosion resistance of their fiber reinforcement with toughness of a metal matrix. MMC offers the potentials of having an outstanding mechanical properties unobtainable in conventional metals because it makes full use of the high strength fibers under longitudinal loading. The properties of continuous fiber reinforced MMC are significantly influenced by the properties of the fiber/matrix interface. The quality of the interface, which is crucial for failure initiation, is commonly characterized in terms of interface strength or fracture toughness. Therefore, the low transverse property of unidirectional composites can be regarded as one of the major limitations in the application of composite materials. The interfacial region between fiber and matrix in a fiber reinforced MMC and its properties play a great role in the stiffness and strength of the composite. The current work aim is to evaluate the interfacial characteristic and the interfacial perpendicular crack behavior focused on fiber arrangement and interfacial characteristic when the MMC is loaded transversely. The interface of fiber and matrix is modeled as thin multi layers with properties linearly gradient to distinguish the interface from the fiber and matrix. And, the influence of different regular fiber arrangement as like square and hexagon arrangement on the strength of transversely loaded fiber reinforced MMC is analyzed. And fiber volume fraction is changed from 5% to 60%. The numerical analyses are based on a 2-D generalized plain strain model of a cross-section of a unidirectional fiber reinforced MMC. The fiber and matrix were described using 2-D quadratic eight-node the element. Finite element analysis of composites was carried out using the ANSYS code. In the region of the interface, high stress gradients are expected, what is more the bonding capability is considered. Thus, we suggested multi thin layer interface model. The interface boundary consists of the multi thin layers with other mechanical properties linearly. One quarter of fiber entire model was considered for FEA with a symmetry boundary condition. The following conclusions could be drawn from this study. 1) The maximum normalized stresses for increasing fiber volume fractions tend to decrease along the x-axis in the interface. In the case of the layer model, the maximum normalized stresses are lower than those of a single interface model at the interface. In the elastic fiber region, von Mises stress curves along the x-axis increase with higher value according to decreasing fiber volume fraction. 2) The stresses along the y-axis are analogous to the x-axis case. In the case of square model, normalized stress distributions are gradually increased from fiber center to the interface. However, those of hexagon models are uniformly distributed from fiber center to the interface, and then unsteady stress distribution is shown in matrix. 3) The normalized von Mises stress along the single interface is shown by rapid variation, while the normalized von Mises stress along the multi thin layer interface is shown by gentle variation, because it has multi thin layer with different properties. In the case of hexagon model, the maximum normalized stresses position of hexagon model gradually increased from 45° to 56° according to increasing fiber volume fraction. In the case of square model, the variations of the normalized von Mises stresses at the interface are analogous to the hexagon models case. The maximum normalized stresses position of square model for different fiber volume fraction from 5% to 60% occurred in the interface about 45°, have not so difference in a value. 4) In the case of single interface model, plastic strain occurred in the overall interface, but those of layer model occurred in a part of the interface. In case of square model, maximum plastic strain in single interface model is about 2.4～4 times as large as a layer model under the same loading. In case of hexagon model, maximum plastic strain in single interface model is about 2.1～3 times as large as a layer model under the same loading. 5) KI in a single interface model is somewhat as large as a multi layer interface model under the same loading and volume fraction. If it has the same crack length, KI in a single interface model is somewhat as large as a multi layer interface model. KI tend to decrease according to the increasing fiber volume fraction. 6) Interfacial stress state have nothing to do with the changes of layer thickness. However, the maximum normalized stress values in round notch tip tend to gradually decreased according to increasing layer thickness.