Fatigue induced cracks in welds may result in the failure of welded structures and therefore it is important to detect the fatigue crack in the early stage. In this paper, we proposed a coda wave interferometry (CWI)-enabled fatigue cracks detection method using piezoceramic transducers. Because of its high sensitivity to the tiny change in the medium, the CWI method employs the multiple scattered coda waves to detect the slight change of welding fatigue cracks. Firstly, a theoretical model is developed by combining the CWI theory with the acoustoelastic effect. The derivation shows that there is a linear relationship between the crack width change and the coda wave relative velocity variation. To verify the correctness of the derived result, the CWI technique is applied to the detection of micro fatigue cracks of three butt welded specimens under fatigue loading. Experimental results show that the relative velocity variation of coda wave increases linearly with the increase of the width of welding fatigue cracks. In addition, energy-based active sensing (EAS) approach was also used as a comparison. Compared with the EAS method, the CWI technique is more sensitive to slight changes of welding fatigue cracks and therefore holds a great potential for the monitoring of micro fatigue crack in the early stage.

After the "One Belt One Road" strategy was put forward, Shandong Province's export trade has a good momentum of development. This paper analyzes the degree of trade dependence between Shandong Province and the countries along the "Belt and Road" and selects the export trade between Shandong Province and the countries along the "Belt and Road" between 2005 and 2017. The data analyzes the trade efficiency between Shandong Province and the countries along the "Belt and Road" from the horizontal and vertical dimensions, and further analyzes the trade inefficiency factors affecting the two places. The study found that the gross domestic product of the countries along the line and the GDP of Shandong Province have an important impact on the efficiency of Shandong's export trade, whether to sign a free trade agreement is an important factor affecting trade inefficiency.

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