F. Streicher

Effects of interfacial debonding on the transverse loading behaviour of continuous fibre-reinforced metal matrix composites

Abstract The current study aims at investigating two issues seriously affecting the transverse loading behaviour of unidirectionally fibre-reinforced metal matrix composites: (1) the effect of thermally induced residual stresses due to a cooling down from processing temperature and (2) the interaction between inelastic matrix deformation and interfacial debonding at the fibre–matrix interface. The unit cell method in combination with the nonlinear finite element technique enhanced by a sophisticated viscoplastic material model and a cohesive zone model for the interface is employed to investigate these influences.

Inelastic behavior of metal-matrix composites reinforced with fibres of silicon carbide, alumina or carbon: a finite-element analysis

The thermomechanical behavior of an aluminum alloy bidirectionally reinforced by different types of fibers is computed by the finite-element method. After having derived the geometrical model of the composite, a sophisticated viscoplastic material model for the aluminum matrix is presented since the macroscopic behavior of the composite is decisively affected by the inelastic deformation of the matrix. Damage processes in the matrix are also considered. Several different fiber types described by isotropic or transversely isotropic elastic material laws are specified. Computations of various monotonic and cyclic loadings show the strong effect of the fiber material and the volume fraction of reinforcement on the deformation behavior of the matrix and therefore also on the macroscopic response of the whole composite.

Influence of the fiber volume fraction and the fiber Weibull modul on the behavior of 2D woven SiC/SiC - a finite element simulation

SummaryThe behavior of two-dimensional woven SiC/SiC ceramic matrix composite (CMC) is studied by numerical simulations based on the finite element method (FEM). Starting point of the investigations is a micromechanical model regarding a three-dimensional unit cell, which takes damage and fracture of the single components—fiber bundles and inter yarn matrix—into account. The scattering of the strength values which is characteristic for ceramic material is involved using Weibull distribution. In a first step the unit cell regarded within the simulations is cooled down to consider the residual thermal stresses resulting from the fabrication process. In a second step the unit cell is subjected to tensile loading and its behavior—especially the influence of the scattering of the strength values—is studied. To be able to estimate the influence of important parameters on the behavior of the composite a macrostructure is built up using the results obtained for a large number of unit cell. Thus an averaging effect is reached and the behavior obtained for the macrostructure should be characteristic for the composite. Doing so, the influence of the fiber volume fractionvf and the fiber Weibull modulMf on the composite behavior can be studied.

Finite element analysis of damage evolution in Al/SiC composite laminates under cyclic thermomechanical loadings

Abstract In the current study, the nonlinear finite element method is used to formulate a micromechanical model of an aluminium alloy reinforced bidirectionally with high-modulus SiC fibres. Therewith, comprehensive numerical investigations of the thermomechanical cyclic behaviour of the laminate are possible. Primarily, the geometrical model of the laminate is presented from which a three-dimensional unit cell can be deduced which is the starting point of further investigations. Subsequently, special emphasis is placed on the inelastic deformation behaviour of the metal matrix because of its significant influence on the composite behaviour. Therefore, a model which not only takes into account the coupling between viscoplasticity and damage but also allows an improved material description of the elastic–inelastic transition range by using the transition flow potential is recommended. Furthermore, thermal residual stresses induced by the manufacturing process have to be considered in the model. At first, the cyclic mechanical behaviour of the laminate at a constant temperature is investigated at various strain amplitudes by closely analysing the evolution of the macroscopic hysteresis loops and the microscopic damage growth in the course of the cycles. Superimposing a cyclic thermal load to the mechanical load strongly influences the behaviour of the composite, whereby the phasing between the mechanical load and the temperature has a significant impact.

Numerical modeling of the damage evolution in an aluminum alloy reinforced bidirectionally by ceramic fibers

Abstract It is expected that the application range of metal matrix composites (MMCs) will strongly increase within the following years because of their usability in the lightweight construction. Therefore the complex deformation and damaging behavior of this class of material have to be clarified more closely. The current study intends to contribute to this by numerically investigating the mechanical cyclic behavior of an aluminum alloy reinforced bidirectionally by continuous SiC-fibers. In the first instance a three-dimensional unit cell can be derived by assuming a geometrical idealization of the fiber arrangement. This unit cell can be analyzed representatively for the whole composite by using the finite element method (FEM). The metallic matrix in the composite is modeled by a comprehensive material model which includes a coupled viscoplastic-damaging law. Special emphasis is placed on the material behavior under cyclic loads. Therefore the material law was complemented by a transition flow potential (TFP) which permits an improved description in the elastic–inelastic transition range. In order to approximate the real behavior of the composite as realistically as possible, cooling processes during manufacture have to be considered because they induce an inhomogeneous residual stress state in the composite and therefore have a significant influence on the stress–strain response of the composite. Various low cycle fatigue loads with different strain amplitudes show a continuous shift and a narrowing of the stress–strain hysteresis. The extent of inelastic deformation and damage rises significantly with increasing external loading amplitude. These damage phenomena are concentrated in the transversely loaded ply of the composite. Also the fiber volume fraction has a great influence on the deformation behavior of the composite. Here a strong incline of damage can be observed with increasing fiber volume fraction.

Einfluß der Werkstoffmodellierung auf die Güte der Prozeßsimulation am Beispiel des Blechbiegens

ZusammenfassungProzeßsimulationen sollen Fertigungsabläufe optimieren. Die Güte der Simulation hängt u.a. wesentlich von der Qualität der Stoffbeschreibung ab. Im Fall des Blechumformens ist das Biegeumformen zur Klärung grundsätzlicher Fragen sehr gut geeignet. Daher wird an diesem Verfahren exemplarisch der Einfluß der Stoffmodellierung auf die Güte der Prozeßsimulation gezeigt.

Einfluß verschiedener thermomechanischer Belastungen auf das Schädigungsverhalten faserverstärkten Aluminiums

Under cyclic thernomechanical loading fatigue of metal matrix composites is strongly affected by the phasing between the mechanical and the thermal load. Therefore the current study focusing on an Al/SiC-composite aims at numerically analysing the influence of different thermomechanical loadings on the inelastic matrix deformation and damage. Thereby the non linear finite element method (FEM) which is applied here allows a good access to the problem.

Numerical investigation of low cycle fatigue of aluminium reinforced by SiC–fibres

Due to complexity the analysis of low cycle fatigue behaviour of long fibre reinforced metal matrix composites needs to take into consideration viscoplastic deformation and damage of the matrix, damage deactivation processes and manufacture residual stresses. Therefore, a comprehensive material model especially feasible for the description of cyclic material behaviour is presented for the metallic matrix. This model not only takes into account damage treated by means of the continuum damage mechanics. In addition, a final failure criterion allows the consideration of the crack path in the matrix of the composite. As multidirectional laminates show a much more complex damaging behaviour than unidirectional composites do, a bi-directional fibre arrangement is investigated. Therefore a three-dimensional finite element analysis is necessary to examine such types of composites. Premising the presented material model not only the development of continuous damage in the composite can be computed for various external loadings. Cracks following the continuous damage process grow parallel and circumferentially to the fibre in the off–axis layer. Therefore, it can be stated that the fatigue behaviour is strongly influenced by plies perpendicularly loaded. The damage and crack process shows also strong effects on the macroscopic mechanical behaviour of the composite such as a decrease of the absolute values of maximum and minimum stresses and a loss of unloading modulus.