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The phased array laser ultrasonic technology is suitable for non-destructive testing (NDT) of coating/substrate structure due to its excellent ability to generate ultrasonic waves with a high signal-to-noise ratio and flexible regulation. This research focuses on the propagation characteristics of ultrasonic waves generated by phased array laser in the coating/substrate (Ni/Al) structure. Considering the spatiotemporal distribution of phased array laser energy and the boundary conditions at the interface, the finite element model of phased array laser-generated ultrasonic waves in the coating/substrate structure was established. Furthermore, the propagation characteristics of bulk wave and Rayleigh wave were investigated, and the influences of element spacing and time delay of phased array laser on the steering characteristics of bulk wave and tuning characteristics of Rayleigh wave were analyzed. Finally, the accuracy of the simulation was verified through experiments based on the laser ultrasonic detection system. The simulation and experimental results indicate that the phased array laser can enhance the bulk wave at the steering direction, with the best enhancement effect occurring when the steering direction coincides with the maximum radiation direction in the directivity pattern of the laser-generated bulk wave. Besides, phased array laser can enhance certain frequency components of tuned Rayleigh waves by adjusting element spacing or time delay. The enhancement effect on low-frequency components is more significant with a short time delay or large element spacing. In contrast, the enhancement effect on high-frequency components is more significant with a long time delay or small element spacing. The research work in this paper has clarified the propagation characteristics for different modes of ultrasonic waves generated by phased array laser in the coating/substrate structure, which can provide a theoretical basis for subsequent defect detection. [ABSTRACT FROM AUTHOR]

Variations in mitochondrial DNA (mtDNA) have been fundamental for understanding human evolution and are causative for a plethora of inherited mitochondrial diseases, but the mutation signatures of germline mtDNA and their value in understanding mitochondrial pathogenicity remain unknown. Here, we carried out a systematic analysis of mutation patterns in germline mtDNA based on 97,566 mtDNA variants from 45,494 full-length sequences and revealed a highly non-stochastic and replication-coupled mutation signature characterized by nucleotide-specific mutation pressure (G > T>A > C) and position-specific selection pressure, suggesting the existence of an intensive mutation–selection interplay in germline mtDNA. We provide evidence that this mutation–selection interplay has strongly shaped the mtDNA sequence during evolution, which not only manifests as an oriented alteration of amino acid compositions of mitochondrial encoded proteins, but also explains the long-lasting mystery of CpG depletion in mitochondrial genome. Finally, we demonstrated that these insights may be integrated to better understand the pathogenicity of disease-implicated mitochondrial variants. [ABSTRACT FROM AUTHOR]

The ability of activation-induced cytidine deaminase (AID) to efficiently mediate class-switch recombination (CSR) is dependent on its phosphorylation at Ser38; however, the trigger that induces AID phosphorylation and the mechanism by which phosphorylated AID drives CSR have not been elucidated. Here we found that phosphorylation of AID at Ser38 was induced by DNA breaks. Conversely, in the absence of AID phosphorylation, DNA breaks were not efficiently generated at switch (S) regions in the immunoglobulin heavy-chain locus (Igh), consistent with a failure of AID to interact with the endonuclease APE1. Additionally, deficiency in the DNA-damage sensor ATM impaired the phosphorylation of AID at Ser38 and the interaction of AID with APE1. Our results identify a positive feedback loop for the amplification of DNA breaks at S regions through the phosphorylation- and ATM-dependent interaction of AID with APE1. [ABSTRACT FROM AUTHOR]