RESUMO
The fracture strength and crack propagation of monolayer molybdenum disulfide (MoS2) sheets with various pre-existing cracks are investigated using molecular dynamics simulation (MDS). The uniaxial tensions of pre-cracked monolayer MoS2 sheets with different crack tips, different locations of crack, different crack lengths and angled cracks are simulated and studied. The results show that the configuration of crack tip can influence significantly the fracture behaviors of monolayer MoS2 sheets while the location of crack does not influence the fracture strength. With the increase of crack length, the fracture strength of monolayer MoS2 sheets reduces almost linearly, and the fracture of monolayer MoS2 sheets is transformed from almost brittle to ductile. By making comparison between the MDS results and the predictions of continuum fracture mechanics theories, including Inglis' model, Griffith's model with and without finite size effect, it is found that MDS results agree well with the predictions of Griffith's model with finite size effect, differ from the predictions of Inglis' model and Griffith's model without finite size effect. Finally, the MDS results of monolayer MoS2 sheets with different angled crack are also analyzed based on the continuum fracture mechanics model.
RESUMO
<p><b>Objective</b>To analyze the high-frequency ultrasound image features of acute scrotum in children and explore the value of high-frequency ultrasonography in the diagnosis and differential diagnosis of the disease.</p><p><b>METHODS</b>This retrospective study included 256 children aged 2 days to 14 years undergoing color Doppler ultrasonography at 2 hours to 3 days after onset of acute scrotum. We analyzed the morphology, internal echo and blood supply of the testis in comparison with the clinical and pathological results.</p><p><b>RESULTS</b>Among the 256 cases, acute testicular torsion was found in 23, of which 16 were treated by complete resection the necrotic testis and the other 7 by surgical reduction of testicular torsion. Ultrasonographically, the involved testes presented different degrees of increase or decrease in volume, with uneven internal echoes, irregular hypoechoic flakes, and testicular hydrocele. Color Doppler flow imaging (CDFI) showed significant blood flow signals around the diseased testes but none within them. Acute testicular appendix torsion was found in 116 cases, in which ultrasonography manifested nodules with round or oval abnormal echoes between the upper pole of the testis and caput epididymidis, first hypoechoic and then gradually increased, heterogeneous internally. CDFI revealed enlarged epididymides and enriched testicular blood flow but no blood flow signals in the nodules. The 103 cases of acute epididymitis were ultrasonographically characterized by varied degrees of swelling of the involved epididymis with uneven internal echoes and rich blood flow signals on CDFI. Six of the cases were diagnosed as acute orchitis, with the ultrasonographic features of testicular swelling and low but uniform internal echoes, with rich blood flow signals on CDFI. Incarcerated inguinal hernia was confirmed in 15 cases, in which ultrasonography revealed intrusion of the hernia into the obviously enlarged scrotal sac with the mesentery and intestine in it, and blood flow visible on CDFI. Acute scrotal wall hematoma and edema was found in 8 cases, with the ultrasonographic characteristics of scrotal wall thickening, with visible blood flow signals on CDFI.</p><p><b>CONCLUSIONS</b>High-frequency ultrasonography has a high sensitivity and specificity for acute scrotum in children, which can be applied as the first-choice clinical imaging modality and provide reliable evidence for the diagnosis and differential diagnosis of the disease.</p>
