Lamb waves have a relatively slow geometrical decay of amplitude with propagation distance, which makes them well suited for wide-area inspection and SHM （structural health monitoring）. This paper presents an extens...Lamb waves have a relatively slow geometrical decay of amplitude with propagation distance, which makes them well suited for wide-area inspection and SHM （structural health monitoring）. This paper presents an extensive computational study of the interaction of the fundamental symmetrical Lamb wave mode （SO） with a through-thickness edge crack in an isotropic plate. The incident wave is generated by a point source that is symmetrical with respect to the plate＇s midplane, and whose time dependence is a 10-cycle Harmed windowed toneburst of centre frequency below the cut-off for the first order shear-horizontal mode （SH 1）, so that the only propagating modes are SO and SH0. Results are presented showing the angular dependence of the SO and SH0 scattered fields for various angles of incidence. The dependence of scattering amplitude on crack size is also studied, with a view to facilitating the inverse problem of determining crack size from measurements of the scattered field. It is shown that the scattered field due to a small crack can be considered to be equivalent to a point source consisting of a combination of force doublets that depend on the angle of incidence. The implications of these results for SHM are briefly discussed.展开更多
It is known that there is a lag time for smoke plume induced by fires transporting from a fire origin to the location of interest underneath an unconfined and flat ceiling.This lag behavior of smoke plume also exists ...It is known that there is a lag time for smoke plume induced by fires transporting from a fire origin to the location of interest underneath an unconfined and flat ceiling.This lag behavior of smoke plume also exists for a fire under a sloped ceiling,and is fundamental to estimate the activation time of a fire detector or other fire extinguishing system.This study focuses on the lag time of smoke plume under a sloped ceiling.Based on the weak-plume theory at early-fire phase and previous studies concerning the fire plume characteristics under a sloped ceiling,a calculation method on lag time of fire plume transporting is presented in theory.Meanwhile,two dimensionless equations predicting the lag time of fire plume for steady fire and unsteady fire are proposed respectively.Furthermore,the critical time calculation equation is also proposed to determine the applicability of quasi-steady assumption for a time-dependent fire.展开更多
文摘Lamb waves have a relatively slow geometrical decay of amplitude with propagation distance, which makes them well suited for wide-area inspection and SHM （structural health monitoring）. This paper presents an extensive computational study of the interaction of the fundamental symmetrical Lamb wave mode （SO） with a through-thickness edge crack in an isotropic plate. The incident wave is generated by a point source that is symmetrical with respect to the plate＇s midplane, and whose time dependence is a 10-cycle Harmed windowed toneburst of centre frequency below the cut-off for the first order shear-horizontal mode （SH 1）, so that the only propagating modes are SO and SH0. Results are presented showing the angular dependence of the SO and SH0 scattered fields for various angles of incidence. The dependence of scattering amplitude on crack size is also studied, with a view to facilitating the inverse problem of determining crack size from measurements of the scattered field. It is shown that the scattered field due to a small crack can be considered to be equivalent to a point source consisting of a combination of force doublets that depend on the angle of incidence. The implications of these results for SHM are briefly discussed.
基金supported by the National Natural Science Foundation of China(Grant No.50909058)"Chen Guang" Project of Shanghai Municipal Education Commission and Shanghai Education Development Foundation Science&Technology(Grant No.10CG51)
文摘It is known that there is a lag time for smoke plume induced by fires transporting from a fire origin to the location of interest underneath an unconfined and flat ceiling.This lag behavior of smoke plume also exists for a fire under a sloped ceiling,and is fundamental to estimate the activation time of a fire detector or other fire extinguishing system.This study focuses on the lag time of smoke plume under a sloped ceiling.Based on the weak-plume theory at early-fire phase and previous studies concerning the fire plume characteristics under a sloped ceiling,a calculation method on lag time of fire plume transporting is presented in theory.Meanwhile,two dimensionless equations predicting the lag time of fire plume for steady fire and unsteady fire are proposed respectively.Furthermore,the critical time calculation equation is also proposed to determine the applicability of quasi-steady assumption for a time-dependent fire.
