Meso-Mechanics Investigation Of Failure Process On Brittle-Matrix Composite

Composite material failure process has been a hotspot in these decades in mechanics investigations, materials and engineering investigations due to the difficulties and the significance related to it. The investigation on failure process and fracture mechanisms on brittle composite matrix is a great challenge to many researchers. However, such research will reveal the mechanism of fracture developing and toughness strengthening, and therefore, it is important and helpful to optimization design of brittle composite materials.Brittle composite materials are weak in their tensile resistance, the failure often occurs in a short time which will lead to a tragic consequence. How to improve their toughness is, therefore, a hot topic in recent composite material studies. However, due to cross coupling properties of various factors, which have great effect on the fracture developing of composite materials, there is lack of powerful means with analytical method to conduct theoretical research in this field. Currently, analytical works concentrate mostly on analyses of stress states and study of fractural criteria. The knowledge to fractural mechanism is still not adequate. Furthermore, the complicated and random behavior in fracture developing test, which can be generally owed to the randomicity and dispersancy of the composite material prosperities, limit the application of laboratory test in such research.It has been well known now that numerical study is one of the important approaches to analyze material failure process. It is almost now our final hope to provide a means to study the failure process of brittle composite in a full scope. However, although finite element method is a useful and effective tool among various numerical methods, the traditional one has its limit when failure problem is encountered. Normally, the traditional numerical methods can be applied to get a satisfactory initial stress, initial strain fields or the final stress state. But it cannot work well with collapsed cells, which means when failure occurs in the composite. New techniques and new methods should be adopted to meet the needs of mesomechanics. Now, with the improvement of computing environment and the practical demands in composite engineering, the failure process analysis is turning its steps to simulate the complete process of the whole structure. The appearance of high performance computing and simulation approaches aiming at large scale structures systematically is one of the main features of current computing mechanics.In the thesis, RFPA, a numerical code developed based on meso-damage mechanics, is adopted to simulate the whole failure process, i.e, crack initialization, coalescence, propagation and final rupture under uniaxial tensile load. In the simulation, heterogeneity in composite is modeled by using stochastic functions, such as Weibull, even and normal distribution function, and Monte-Carlo approach. A relatively simple constitutive law, strain-softening model, is applied to each elementary cell. The cell in the calculation will keep its remaining strength after it collapses.With these techniques, non-linear deformability of a quasi-brittle or ductile failure behavior of the composite can be simulated with RFPA. Furthermore, phenomena of strain-softening and some discontinuum mechanics problems can also be revealed by using a continuum mechanics model. The fracturing evolution in mesoscopic fragments, can help to understand mechanism of failure and principles of composite, which cannot be acquired by laboratory experiments and theoretical analysis.Fundamental theory of this research, numerical modeling method and verification of the modeling have been introduced in Chapters1-3.In Chapter4, whole failure process of brittle composite matrix reinforced by a single particle or particles under uniaxial tensile load has been studied. The properties of particles, the effect of particle size, the different failure mechanisms for particles with different flexibility, and the effect of particles alignment are consequently investigated.In Chapter5, the study revealed that mechanical properties of interface have significant effect on the composite and its final fracture mechanism. To understand such an effect, the failure process of particle reinforced brittle composite with interfaces in different strengths has been studied under uniaxial tensile load, while inhomogeneity in mesoscale is applied to the matrix, particles and interfaces, respectively.In the same Chapter, the failure process and mechanism of a brittle composite matrix, which is reinforced with long fibers with weak interface, is also studied. The fiber reinforcement with weak interface could bring out not only higher strength and stiffness, but also higher toughness of the composite. Some important phenomena, such as the interface debonding and sliding, crack deflexion, fiber bridging and pulling out, and etc. have been successfully reproduced and revealed. Finally, the accompanied result of acoustic emission is also presented.Failure mechanism and toughness enhancement of laminated composite material are studied in Chapter6. Such mechanism is totally different to traditional flaw-elimination method. It is actually a system of energy dissipation. Structurally, it is insensitive to the existing of micro flaws and could be considered as flaw-toleration. The simulation of fractural developing process for laminated material, carried out in this study, matches well with the experimental result. Consequently, the effect on the strength and toughness of the composite, made by the strength and Young’s Modulus of soft layers, thickness ratio of hard and soft layers, is investigated.Finally, A code RFPA3D, is be used to reveal the fractural process and mechanism of laminated composite in a real world scope. The parallel computing technique has been involved in to make the calculation in an acceptable time. The result of this three-dimensional simulation is presented in Chapter7.