Free-slit shielding of stress waves by the photoelasticity method

Abstract

The need to diminish the seismic effect of blasting on nearby structures frequently arises in performing blasting operations.One method of reducing dynamic stresses and strains due to the action of seismic-blast waves may be the use of slitlike shields so that the object being protected is in the zone of the slit's geometric shadow.To evaluate the expediency and effectiveness of these shields, it is necessary to investigate the laws governing the formation aid distribution of wave fields in a mass beyond the slit and the influence exerted by the geometric and acoustic parameters of the shield on the characteristics of the stress--strain state of the zone of the geometric shadow.The present study is devoted to an experimental model investigation of the distribution of dynamic stresses beyond a free shielding slit as a longitudinal wave from a blast source strikes the shield.The studies were conducted by the method of photoelasticity on a dynamic polarization apparatus in the stress-study laboratory at the Moscow V. V. Kuibyshev Civil Engineering Institute [i].The problem was solved in the plane elastic statement on models in the form of plates made of an optically sensitive epoxy-resin-base material.The pulse effect was created by the detonation of a cylindrical lead azide microcharge.The dynamic stressed state at internal points of the model was evaluated from the magnitudes and distribution o~ maximum tangential stresses, and on the free perimeter of the model from the magnitudes and distribution of the normal stress parallel to the perimeter.We investigated two basic cases: i) positioning of the slit in an infinite plate; and 2) the zone being shielded beyond the slit as a free surface along which variation in dynamic stresses is considered.The distance from the source to the shield 1 = (2-4.5)~I, where %~ is the length of the first phase of the incident pulse; the front of the incident wave is parallel to the plane of the shield.One half of the length of the slit was adopted as the characteristic dimension a of a slit in positioning the shield in the infinite medium, while its height from the surface to the head was adopted in the presence of a free surface beyond the shield.The dimension a was caried within the limits a = (0.2-0.5)~i.The incident pulse is characterized by the following parameters the ratio of the length of the segment of stress accumulated to the total length of the first phase %a/l~ ~ 0.6; the ratio of amplitudes in the compression and tension phases Tmax. Tmax-9 + = 2.5:1.When the cylindrical longitudinal wave interacts with a single free slit in an infinite medium, the stress field in the zone of the geometric shadow is determined by the diffracted longitudinal Pg and transverse Sg waves that propagate from the ends of the slit. The characteristic pattern of the wave field of stresses beyond the slit can be observed from the motion picture frames (Fig. i).Comparison of wave-field patterns in models beyond slits of different lengths and a similar pattern in a homogeneous model indicates tha...