Browsing by Author "Madadi, Hamid"

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Open Access
The high cycle fatigue testing of High‐Performance Concretes using high frequency excitation
(2023) Madadi, Hamid; Steeb, Holger
The effect of fatigue failure in brittle materials like (ultra) High Performance Concrete (UHPC) due to cyclic loading causes unexpected failure that consequently results in heavy costs in marine and civil structures. To characterize the effect of fatigue, cyclic loading tests are performed, and “the number of cycles to failure” are experimentally determined. One problem with these kinds of tests is that such experimental investigations are potentially expensive, i.e., time‐consuming process since the number of loading cycles could be extremely high. Further, within the different damage phases of the cycling tests, one has no access to the small‐scale, i.e., microscopical evolution of (micro‐)cracks. Additionally, a full characterization of the small‐strain stiffness evolution of the material is challenging. The goal of the research investigation is to combine a (large amplitude) High Cycle Fatigue experiment with a (low amplitude) Dynamic Mechanical Analysis (DMA). Using a setup based on the piezoelectric actuator, the (rate‐dependent) mechanical properties of the material in tangential space, and the failure modes of the material will be examined accurately. The excitation frequency is between 0.01 Hz to 1000 Hz which allows for reducing the experimental investigation time to failure. Further, it allows investigating the effect of frequency on the number of cycles to failure. Firstly, experimental results for HPC and berea sandstone samples will be presented. Harmonic experimental data include (direct) strain measurements in axial and circumferential directions as well as forces in axial directions. In addition, the resulting complex Young's modulus and evolving damage‐like “history” of HPC and berea sandstone specimens will be shown.
Open Access
High‐speed fatigue testing of high‐performance concretes and parallel frequency sweep characterization
(2023) Madadi, Hamid; Steeb, Holger
Cycling loading of brittle materials like ultra‐high‐performance concrete (UHPC), which is often used in marine and civil structures, results in unexpected failures. When a material is subjected to cyclic loading, its mechanical properties change due to the evolution of (micro‐)fractures often denoted as damage. To better understand the effective material's properties under such kind of fatigue load and to relate the material's properties to the specific time‐dependent loading characteristics, the mechanical response of the material shall be characterized at characteristic harmonic excitations. Therefore, cyclic loading experiments are conducted to determine how the evolution of microfractures, that is, fatigue, affects the material's effective mechanical properties and after how many cycles microfractures further evolve towards meso‐ and macrofractures leading finally to a critical number of cycles to material's failure. The problem with such cyclic fatigue tests is that they are potentially “expensive” to conduct as the number of loading cycles at failure can be extremely high. Moreover, it is not possible to observe and characterize further the evolution of (micro‐)fractures within the different damage phases of the cycling experiment. Further, it is challenging to characterize the material's small‐strain stiffness evolution. In this investigation, a combination of a (high‐amplitude) high‐frequency excitation and a high‐speed fatigue testing approach is used for the high cycle fatigue experiment along with a characterization approach of the material properties using a (low‐amplitude) dynamic mechanical analysis (DMA). The test setup applies harmonic excitations for high and low amplitudes using a high‐voltage piezoelectric actuators. Furthermore, the failure modes of the material will be examined. The excitation frequency 𝑓 for the fatigue test is significantly higher than in classical low- and high-cyclic fatigue approaches, that is, 10 < 𝑓 < 200 Hz, allowing to reduce the overall time of the experimental investigation time to failure. Further, the frequency-dependent number of cycles to failure is studied. Similar to standard DMA, effective complex mechanical properties of the material in tangential space are obtained in frequencies between 0.01 and 1000 Hz; while the observed mechanical properties of these materials change with increasing frequency. In the case of materials' behavior, by increasing the frequency, Young's modulus increases and Poisson's ratio decreases. Experimental fatigue results will be presented for UHPC samples. Harmonic experimental data include (direct) strain measurements in axial and circumferential directions as well as forces in axial directions. In addition, the resulting complex Young's modulus and evolving damage‐like “history” of UHPC will be shown.