PSI - Issue 68

J.C. Gomez-Mancilla et al. / Procedia Structural Integrity 68 (2025) 1208–1215 Gomez-Mancilla J / Structural Integrity Procedia 00 (2025) 000–000 St

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The present work was carried out during the author's sabbatical year as a Guest Professor at the Laboratory of Mechanical Engineering Processes and Materials, PIMM, Ècole Arts et Metiers ENSAM , Paris. The project is about Fast Determination of Fatigue Behavior of Materials Beyond One Billion Cycles, FastMat . The objectives are to characterize materials and manufacturing processes against damage by Giga-fatigue in the transportation, aeronautics, and energy industries and define it as the application of repeated loads at very low-stress intensity and with very high cycles, 10^9 and more of a vibratory type. FastMat characterizes materials and manufacturing processes. The graphs and diagrams of numerical calculations and experimental results of this work illustrate the enormous advantage of also measuring at least one flexural response along with the axial one, which, except for this test, axially is the single direction that is currently monitored and used. Vibration detects the beginning of structural damage. The PIMM Laboratory uses a Branson piezoelectric ultrasound machine, Emerson brand, where in this work, the tested specimen is made of stainless steel AISI 302 mounted and screwed to the ultrasonic exciter. Said exciter moves only in the axial direction of the test piece, so, perhaps for greater convenience, international researchers have only considered monitoring axially. Using new ultrasound equipment and tools, Giga-fatigue researchers spend much time determining the state of applied stresses, i.e., Jacquemain (2021). In this regard, the author proposes alternatives to resort to vibration models and simulations based on FEM codes against which to compare and refine the model: the Boundary Conditions, the appropriate excitation validating the previous simulations vs. experimental measurements, and do not limit oneself to axial displacements but also include orthogonal bending displacements is recommended. The above complements measurements with laser beam and strain gauges. Relevant information is that the experimentally monitored signals, axial (Ch1) and bending (Ch2), are divided and analyzed into 145-time segments. Each segment equals a time window of approximately 0.2625 seconds, and both The present study successfully applies to the analysis for techniques that accelerate the detection of initial damage in Giga-fatigue tests, identifying the elusive onset of damage and fractures. The references contain a work submitted for review relevant to this research, Gomez-Mancilla (2024). Here, the effectiveness of adding, recording, and using the orthogonal measurement to the axial axis of the specimen under test, as information that identifies the beginning of structural damage, is demonstrated. This test positively validated the recommendations that the author proposed and implemented. Furthermore, analyzing the vibrations of the responses using the three criteria proposed here makes it possible to detect the beginning of structural damage in VHCF tests. The duration of the experiment is divided into 145 segments or time windows, i.e., 9,502,720 (145x 2 ^ 16) is the amount of acquired data from each channel (Ch1 and Ch2) where the cyclic load is applied with a total duration of 38.273 seconds. Each channel's sampling frequency is 250,000 samples per second, equivalent to data at every 4 µs. Each of the time window segments and its corresponding frequency spectrum is obtained by applying the Discrete Fourier Transform DFT. A table is created with the values of the axial and flexural frequencies from the first, second, third, and fourth harmonics of the excitation, i.e., Ax1, Ax2, Ax3, Ax4, Fx1, Fx2, Fx3, Fx4 and rigid body displacement (amplitude at zero frequency). Likewise, the amplitudes corresponding to the first four harmonics are recorded. Also, to save time, the Short-Time Fourier Transform, STFT is applied to the Flex flexural signal, which allows for identifying the time segment in which a flexural frequency is split into two. their spectral and amplitude content are studied as the test evolves. 2. VHCF Analysis Procedure & Test Set-up Modification