Abstract
A new method based on time-resolved X-ray diffraction is proposed in order to measure the elastic strain and stress during ultrasonic fatigue loading experiments. Pure Cu was chosen as an example material for the experiments using a 20 kHz ultrasonic fatigue machine mounted on the six-circle diffractometer available at the DiffAbs beamline on the SOLEIL synchrotron facility in France. A two-dimensional hybrid pixel X-ray detector (XPAD3.2) was triggered by the strain gage signal in a synchronous data acquisition scheme (pump-probe-like). The method enables studying loading cycles with a period of 50 µs, achieving a temporal resolution of 1 µs. This allows a precise reconstruction of the diffraction patterns during the loading cycles. From the diffraction patterns, the position of the peaks, their shifts and their respective broadening can be deduced. The diffraction peak shift allows the elastic lattice strain to be estimated with a resolution of ∼10-5. Stress is calculated by the self-consistent scale-transition model through which the elastic response of the material is estimated. The amplitudes of the cyclic stresses range from 40 to 120 MPa and vary linearly with respect to the displacement applied by the ultrasonic machine. Moreover, the experimental results highlight an increase of the diffraction peak broadening with the number of applied cycles.
Highlights
Many mechanical structures are submitted to repeated loadings during their service and can break under stresses lower than their ultimate tensile stress, especially if deformation is repeated for a very large number of cycles
To reduce the testing time, new approaches using ultrasonic fatigue machines have been developed during the last decades, which are based on a severe increase of the loading frequency in order to characterize the fatigue behavior within a few hours
The purpose of this paper is to present a time-resolved X-ray diffraction (XRD) approach which enables the evolution of the mechanical state of the material to be followed during a single cycle and throughout high-frequency fatigue tests
Summary
Many mechanical structures are submitted to repeated loadings during their service and can break under stresses lower than their ultimate tensile stress, especially if deformation is repeated for a very large number of cycles. This phenomenon, called the fatigue of materials, can be encountered in many industrial sectors such as transport and energy. Fatigue design is a crucial step in mechanical engineering and it requires a precise characterization of material behavior under repeated loadings to ensure the safety and reliability of the structures throughout their life. The characterization of the fatigue behavior of materials requires fatigue tests, during which the specimen is loaded cyclically, to be conducted at different stress amplitudes until fracture. To reduce the testing time, new approaches using ultrasonic fatigue machines have been developed during the last decades, which are based on a severe increase of the loading frequency in order to characterize the fatigue behavior within a few hours
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