Abstract
Understanding the composite damage formation process and its impact on mechanical properties is a key step towards further improvement of material and higher use. For its accelerated application, furthermore, practice-related modeling strategies are to be established. In this collaborative study, the damage behavior of carbon fiber-reinforced composites under cyclic loading with load reversals is analyzed experimentally and numerically. The differences of crack density evolution during constant amplitude and tension-compression block-loading is characterized with the help of fatigue tests on cross-ply laminates. For clarifying the evolving stress-strain behavior of the matrix during static and fatigue long-term loading, creep, and fatigue experiments with subsequent fracture tests on neat resin samples are applied. The local stress redistribution in the composite material is later evaluated numerically using composite representative volume element (RVE) and matrix models under consideration of viscoelasticity. The experimental and numerical work reveals the strong influence of residual stresses and the range of cyclic tension stresses to the damage behavior. On the microscopic level, stress redistribution dependent on the mean stress takes place and a tendency of the matrix towards embrittlement was found. Therefore, it is mandatory to consider stress amplitude and means stress as inseparable load characteristic for fatigue assessment, which additionally is influenced by production-related and time-dependent residual stresses. The phenomenological findings are incorporated to a numerical simulation framework on the layer level to provide an improved engineering tool for designing composite structures.
Highlights
During mechanical loading of fiber-reinforced plastics (FRP), damage phenomena occur with increasing stress levels and duration, which can lead to material degradation
Cross-ply laminates under constant amplitude and block-loading conditions, mathematical methods for describing the mesoscopic degradation as well as the mesoscopic damage behavior and micromechanical explanations for the observed behavior based on neat resin experiments and representative volume element (RVE) simulations for constant and fatigue loading
Accompanied resin creep and fatigue experiments in tension revealed a tendency for increasing stiffness and strength of the matrix over time with the potential of influencing the damage behavior at different stress ratios
Summary
During mechanical loading of fiber-reinforced plastics (FRP), damage phenomena occur with increasing stress levels and duration, which can lead to material degradation. The damage is determined by the inhomogeneity of the material across different scales, which is characterized microscopically by the characteristic lengths of filament and matrix, mesoscopically by the thickness of the diverse oriented layers, and macroscopically by the spatial laminate expansion. Analogous to this geometrically motivated scale definition, the damage phenomena can be classified into micro, meso, and macro damage. Composite material damage can be observed as stiffness and strength degradation through the formation of damage on the meso, or layer, level The damage phenomena, such as delamination, fiber-breaking and matrix cracks, are formed statistically distributed in the laminate and are initiated by microscopic phenomena in the matrix and fiber-matrix interphase. Additional factors influencing the crack initiation and growth are the load frequency and load sequence, environmental conditions (temperature, humidity, etc.), manufacturing process, and material configuration
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