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The paper presents the results of a series of combined mechanical and in-situ morphological investigations on highstrength strain-hardening cement-based Composites (SHCC). Tension and compression experiments were performed in a CT scanner employing a dedicated mechanical testing rig. The in-situ microtomographic scans enabled correlating the measured specimen response with relevant microstructural features and fracture processes. The microstructural segmentation of SHCC was performed in the framework of Deep Learning and it targeted an accurate segmentation of pores, fibers and aggregates. Besides their accurate volumetric representation, these phases were quantified in terms of content, size and orientation. The fracture processes were monitored at different loading stages and Digital Volume Correlation (DVC) was employed to spatially map the strains and cracks in the specimens loaded in compression. The DVC analysis highlighted the effect of loading conditions, specimen geometry and material heterogeneity at the mesolevel on the strain distribution and fracture localization.
The nonlinear behaviour of epoxy resins is studied on standard tensile tests. A strain field measurement system is applied (Aramis) in order to monitor local strains. The residual strain is measured by recovering the specimens for up to 68 hours after unloading. The time span the specimen is exposed to load has a large influence on the creeping process and the residual strain after recovering. This is studied by comparison of instantaneous unloading with keeping the specimen under permanent load for thirty minutes. It is shown that moderate differences in the initial strain can lead to large differences in the creep behaviour as well as in the residual strain.
Failure of fiber reinforced composites – role of the matrix behaviour and the interface strength
(2016)
Failure of fiber reinforced composites initiates at microdefects in the matrix or at the interface. The interface strength is calculated from the stress field directly before crack initiation by nonlinear finite element simulations. Large deformations and plastic material behaviour are taken into account. Microscopical inspections of the fibers after failure show only small regions with adhering matrix material. This indicates that the interface strength is below the matrix strength.
In situ thermography of crack growth in sandwich shell segments with manufactured imperfections
(2016)
A new test rig for the efficient material testing of curved GFRP composite specimens is developed. The presented intermediate scale test facility allows the fatigue testing of representative shell segments of rotor blades for the wind power industry. The non-destructive condition monitoring is performed by a combination of thermography and optical 3 D deformation analysis, automated in situ throughout the fatigue test. In many cases cracks in the shells of wind turbine rotor blades are detected long time before the calculated lifetime of 20 years. As a consequence of the harsh weather conditions damage progress occurs, that causes costly in-service repairs. Approximately 70% to 80% of defects in rotor blades are the consequence of imperfections through the manufacturing process or the rotor blade design. Production-related imperfections are supposed to be one possible reason for crack formation and crack growth. Therefore, a test rig for shell structures is constructed and launched into operation. In the test bench sandwich shells and full laminate shells of different sizes with artificially created defects (e.g. wrinkles, laminate change, etc.) are tested under cyclic tension and compression load. The characterization of the damage state and evolution during the fatigue test by simultaneous passive thermography and optical 3D deformation analysis is performed. The in- and out-off-plane deformations of sandwich shells under cyclic tension and compression load are determined by optical 3 D deformation analysis (with ARAMIS). For the non-destructive detection of developing material defects (e.g. cracks) in the shells due to cyclic loading thermography is used. The passive thermography is well suited for the characterization of the crack formation and crack growth of composites during fatigue tests. The cyclic strain energy introduces the damage and the damage progress can be recorded and documented in the material. By thermography the damage is recognized in the laminate long time, before the surface of the painted specimen is damaged. The experiments show a correlation between the hotspots, observed by passive thermography and the location of the final failure.
In the paper the design of the test bench and the results of the experiments will be presented.
For the design of laminates the knowledge of the failure behaviour of plies under multiaxial stresses is a necessary precondition. The strength of plies under multiaxial stresses commonly is determined by standard off-axis tests using fixed clamps. By varying the off axis angle the ratio of shear to normal stresses can be prescribed. However, by preventing the rotation a complex stress field develops which strongly varies with the off axis angle as well. While these effects are not crucial when determining the elastic material parameters since the stress state in the center of the specimen is not far from the ideal uniaxial stress state they have a great influence on the failure behaviour.