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The transient behavior of a cantilever beam, driven by periodic force and repeated impacting against a rod-like stop, is the subject of this investigation. As impact and separation phase take place alternately, the transient waves induced either by impacts or by separations will travel in more complicated ways. Thus the transient responses of both the beam and the rod during repeated impact become an important issue. In both impact phase and separation phase, the transient wave propagations are solved by the expansion of transient wave functions in a series of Eigenfunctions (wave modes). From the solutions, the answer of impact force is derived directly, so that the divergence problem, encountered in solving impact force numerically by a strongly non-linear equation coupled the unknown impact force with motions, has been avoided. Numerical results show the convergence of the time-step size and truncation number of wave modes in the calculations of impact force by the present method. As the transient wave effect is considered, the numerical results can show several transient phenomena involving the propagation of transient impact-induced waves, sub-impact phases, long-term impact motion, chatter, sticking motion, synchronous impact, non-synchronous impact (including asynchronous impact) and impact loss

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Objective Endoscopic submucosal excavation (ESE) has been established as an effective method for removal of gastric submucosal tumors (SMTs). The aim of the present study was to explore risk factors for technical difficulties in ESE. Methods In this retrospective study, we collected clinical data from patients who underwent ESE for gastric SMTs. Difficult ESE was defined as a procedure time ≥90 minutes, piecemeal resection, and/or occurrence of major adverse events. Univariate and multivariate analyses were performed to explore the risk factors for a difficult ESE. Results ESE was successfully performed in 96.5% (195/202) of patients from April 2011 to December 2019. The average tumor size was 17.41 mm, and en bloc resection was achieved in 97.4% of patients (190/195). Five patients (2.56%, 5/195) had complications, including two with delayed bleeding, two with fever, and one with chest pain accompanying ST-T changes in an electrocardiogram. Twenty-four patients (11.88%, 24/202) had a difficult ESE. Logistic analysis showed that outgrowth behavior and an inexperienced surgeon were risk factors for a difficult ESE. Conclusion ESE may be safe and effective to treat patients with gastric SMTs. Outgrowth behavior and an inexperienced surgeon were risk factors for a difficult ESE.