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High-strength bolts are widely used in outdoor steel structures such as transmission towers and bridges, where they not only endure cyclic wind loads and vehicle loads but also frequently operate in low-temperature environments. However, there is limited research on the axial fatigue performance of high-strength bolts, particularly regarding their mechanical behavior at low temperatures. Therefore, this study conducted a series of fatigue tests on high-strength bolts at 20 °C and 0 °C, both with and without pretension. We established S-N curves and fatigue limits for the three scenarios, revealing that pretension significantly enhances the fatigue life of the bolts, with a 10% increase in fatigue limit at 0 °C compared to 20 °C. However, due to the influence of pretension, the external load has a minimal effect on the actual stress experienced by the bolts, resulting in S-N curves for bolts with pretension being very similar to those for bolts without pretension during cyclic loading. Additionally, we obtained the load–displacement curves and corresponding stiffness degradation patterns of the bolts at both temperatures, finding that all bolts exhibited significant stiffness degradation after reaching 0.8 times their fatigue life. The high-strength bolts at 0 °C demonstrated greater stiffness and faster crack propagation rates, with increases of approximately 6% and 8%, respectively. Furthermore, electron microscope scans were used to clarify the fatigue crack initiation and the evolution of fatigue striations at both temperatures. Finally, by combining refined numerical simulations with the local stress–strain method, the effectiveness of the local stress–strain method for evaluating the fatigue life of bolts without pretension was validated. Building on this, we extended the method to bolts at 0 °C and those subjected to pretension, recommending notch sizes of 0.4 mm and 1.1 mm for fatigue life assessment of bolts with pretension at 0 °C and 20 °C, respectively. [ABSTRACT FROM AUTHOR]

In order to eliminate the influence of acoustic velocity variable on the estimation accuracy when the signal propagates under water, this paper proposes a velocity‐independent two‐dimensional (2‐D) direction‐of‐arrival (DOA) estimation algorithm with three parallel uniform linear arrays. Based on the double parallel linear arrays (DPLA), this algorithm adds another parallel linear array that is not in the same plane as DPLA, which is called three parallel linear arrays. By employing the matrix signal processing among those three arrays, the acoustic velocity variable can be removed from the expressions of elevation and azimuth. And the azimuth and elevation angles of the target are obtained by linear partitive operation, which does not need spectrum peak search or additional angle matching procedure. The simulation results demonstrate that the proposed algorithm performs better than the traditional 2‐D DOA algorithms in the underwater environment of unknown acoustic velocity. Compared with the velocity‐independent 2‐D DOA algorithm, it has lower computational complexity. [ABSTRACT FROM AUTHOR]

Among energetic materials, there is a significant challenge facing researchers: to seek an optimal balance between detonation performance and molecular stability. By introducing an azo1,2,4‐oxadiazole backbone and low‐sensitivity C‐NO2 moieties, a promising energetic molecule, 1,2‐bis(3‐nitro‐1,2,4‐oxadiazol‐5‐yl)diazene (4) was prepared through three‐step reaction and well‐characterized by IR and multinuclear NMR spectroscopy, DSC measurement, elemental analysis, and single‐crystal X‐ray diffraction. It shows high thermal stability (Td=260 °C), good mechanical sensitivity (IS: 21 J, FS: 220 N), and superior detonation performance (D: 9336 m s−1, P: 37.7 GPa). The combination of advanced performance and desirable safety as well as the simple reaction process make it highly competitive in the field of energetic materials. [ABSTRACT FROM AUTHOR]