Failure Process Of Composites Research Articles

複合材料対称斜交積層板の界面効果を考慮した最適化設計に関する研究

Unlike homogeneous materials, composites have heterogeneous structure with distinct interfaces. These interfaces may be classified as microscopic surfaces within a lamina and macroscopic surfaces between laminae. An accurate evaluation of the stress distribution in the boundary layer should bring about a better understanding in the failure process of composites. Furthermore, in designing composite laminates, knowledge of the macroscopic interface stress is particularly important, since delamination of macroscopic adhesive surface results in a severe drop in strength.In this paper, an optimization procedure for the design of angle-ply laminate is offered, considering not only macroscopic behaviour as an in-plane problem but also non-uniformity of interlamina stress.At the beginning of the design process, in-plane modulus and interlamina stress were approximated as a constraint function and an objective function, respectively. And the ply orientation was optimized by non-linear programming (SUMT method). Consequently, the following results were obtained.A bidirectional laminate was optimized so as to minimize interlamina stress. As the design variables, the ply angle and the rigid-body rotation of a laminate from the principal axis were taken and they were determined by the optimization procedure. It is concluded that this procedure has wide applicability to various optimum design problems and is quite valuable in engineering.

一方向ハイブリッド繊維強化材の引張破壊過程のシミュレーション

The failure process of fiber reinforced composites is a complicated accumulation process of damage due to random failure of fibers, matrix and interfaces, which leads to a catastrophic fracture. A Monte Carlo simulation is one of the most effective methods to analyze such a complicated probabilistic phenomenon as a failure process of composites, and several investigations have been carried out in the past. In most of the past investigations, a simple failure model has been applied in which only random fiber break and stress concentration in the nearest fiber to the broken fiber are taken into consideration. This formulation, however, leads to no more than a flat cleavage plane of a specimen, which does not apparently agree with an actual observed phenomenon.The present paper proposes a new failure model considering interfacial debonding between fibers and matrix as well as fiber breakage. A tensile failure process simulation was carried out for unidirectional carbon fiber (C)/glass fiber (G) hybrid composites with two different kinds of hybrid constitution (concentrated versus dispersed) based on the proposed failure model. The simulation was compared with the observed results of hybrid model experiments based on an acoustic emission (AE) method.The present simulation showed a complicated zigzag cleavage plane in the specimen and also the clear difference in failure process depending upon hybrid constitutions. Furthermore, the simulated results agreed well in tendency with the model experiments in regards to the characteristic shape of stress-strain diagrams and AE properties.

Fracture behaviour of a single-fibre graphite/epoxy model composite containing a broken fibre or cracked matrix

A micromechanical analysis of crack initiation and propagation from a broken fibre end, or in the region of a matrix crack, in a graphite/epoxy composite model is considered. The model consists of a single fibre embedded in an annular sheath of matrix material subjected to axial tension. An elastoplastic, axisymmetric finite element analysis has been used. Curing residual stresses, and hygrothermal effects induced due to changes in service temperature and humidity, are included. The influence of the interface between the fibre and matrix material on the behaviour of propagating cracks is also studied. The concept of crack-growth resistance curves (KR -curves) has been used to determine the point of crack instability. Results demonstrate the usefulness of the analytical model in understanding the role of the matrix material in the failure process of composites.