Detection and Location of Surface Damage Using Third-Order Combined Harmonic Waves Generated by Non-Collinear Ultrasonic Waves Mixing.

Metals which are widely used in many types of industries are usually subjected to fatigue and surface corrosion. There is a demand to detect the surface damage caused by fatigue and corrosion at an early stage to ensure the structural integrity. In this paper, a novel nonlinear ultrasonic technique based on the measure of third-order combined harmonic generation, is proposed to detect and locate the surface damage in 6061 aluminum alloy. Third-order combined harmonic generation caused by non-collinear mixing of one longitudinal wave and one transverse wave at different frequencies, is firstly analyzed and experimentally observed. An experimental procedure of nonlinear scanning is proposed for the damage detection and location by checking the variation of frequency nonlinear response. The correlations of nonlinear frequency mixing responses and surface damage in the specimens are obtained. Results show that the nonlinear response caused by fatigue damage and surface corrosion can be identified and located by this method. In addition, this approach can exclude the nonlinearity induced by the instruments and simplify the signal processing.

Loss of CRY2 promotes regenerative myogenesis by enhancing PAX7 expression and satellite cell proliferation.

The regenerative capacity of skeletal muscle is dependent on satellite cells. The circadian clock regulates the maintenance and function of satellite cells. Cryptochrome 2 (CRY2) is a critical component of the circadian clock, and its role in skeletal muscle regeneration remains controversial. Using the skeletal muscle lineage and satellite cell-specific CRY2 knockout mice (CRY2scko), we show that the deletion of CRY2 enhances muscle regeneration. Single myofiber analysis revealed that deletion of CRY2 stimulates the proliferation of myoblasts. The differentiation potential of myoblasts was enhanced by the loss of CRY2 evidenced by increased expression of myosin heavy chain (MyHC) and myotube formation in CRY2-/- cells versus CRY2+/+ cells. Immunostaining revealed that the number of mononucleated paired box protein 7 (PAX7+) cells associated with myotubes formed by CRY2-/- cells was increased compared with CRY2+/+ cells, suggesting that more reserve cells were produced in the absence of CRY2. Loss of CRY2 leads to the activation of the ERK1/2 signaling pathway and ETS1, which binds to the promoter of PAX7 to induce its transcription. CRY2 deficient myoblasts survived better in ischemic muscle. Therefore, CRY2 is essential in regulating skeletal muscle repair.