Mechanisms Of The Interaction Between Blast Stress Waves And Cracks

The interaction between blast stress waves and cracks is a critical problem frequently occurring in blast engineering.In this dissertation,two kinds of photomechanical methods,i.e.photoelasticity and caustics methods,with theoretical analysis and numerical simulation were used to investigate mechanisms of the interaction between blast stress waves and cracks.The investigation was divided into three parts:mechanisms of the interaction between incident blast stress waves and cracks(including local-field and full-filed);mechanisms of the interaction between reflected blast stress waves and cracks;and an in situ simultaneous measurement system combining photoelasticity and caustics methods for blast-induced dynamic fracture.The local-field stress in the crack tip under incident blast stress waves was studied by caustics method,with an emphysis placed on effects of compression and tension stress of P waves on cracks.It is the first time to propose a theory of distorted caustics patterns to modify the classical caustics interpretation,which agrees well with experimental results and hence provides a precise method for stress intensity factors(SIF)measurements.Experimental results shown that the compressive phase of P waves decreased K? and crack velocity,while the tensile phase of P waves did opposite effects.Normally incident blast stress waves only cause mode I crack,while obliquely incident ones can produce mode I+II crack with K? and K? in the opposite variation.The full-field stress of the interaction between incident blast stress waves and cracks was studied by photoelasticity method.Impact-induced cracks,blast-induced static cracks and blast-induced dynamic cracks were subjected to blast stress waves.In these conditions,"Mach cone" phenomenon and Rayleigh waves were found and explained by the wave theory.Experimental results shown that "Mach cone"phenomenon was formed by two symmetrical head waves which were produced by the interaction between P waves and crack surfaces.These head waves belonged to shear waves and decreased SIF and crack velocity,while the following Rayleigh waves increased SIF and crack velocity.Both effects of "Mach cone" and Rayleigh waves on cracks explain unstable crack propagation under blast stress waves with the same propagation direction.Besides incident blast stress waves,reflected blast stress waves can also play an important role on cracks.Due to complicated isochromatic fringes of reflected blast stress waves in photoelasticity method,caustics method was utilized to study effects of single and double reflected waves on cracks.Similar to the case of incident blast stress waves,distorted caustics patterns occurred under reflected waves.The proposed theory of distorted caustics patterns was employed to precisely reproduce experimental results and provide precise SIF.The micro observations of crack surfaces were conducted after the explosion.Experimental results shown that reflected P waves caused mode I crack,while reflected S waves caused mixed mode crack.Reflected P waves made a large influence on cracks than reflected S waves,which violated K-dominated field in the crack tip and caused rough fracture surfaces,even crack branching.Effects of vertical and horizontal reflected blast stress waves on cracks were quantitatively compared in the same explosion.It was found that vertical reflected P waves decreased SIF and crack velocity,while horizontal ones did opposite effects.In the above experiments,photoelasticity and caustics method were utilized alone.In order to combine advantages of two photomechanical methods,an in situ simultaneous measurement system combining photoelasticity and caustics methods was built and first applied to blast-induced dynamic fracture.Two kinds of explosion experiments in which two boreholes were detonated at the same time with/without a pre-crack between two boreholes were conducted to verify the validity of the proposed measurement system.Experimental results shown that without a pre-crack between boreholes,two blast-induced cracks formed a lenticular shape after their interaction.The reason is that the loading of tension and shear occurred as two cracks propagated close to each other,which is indicated by the asymmetrical caustics pattern and asymmetrical isochromatic fringes in the crack tip.With a pre-crack between boreholes,direct coalescence appeared between two blast-induced cracks.The reason is that only tension loading existed as two cracks propagated close to each other,which is proven by the symmetrical caustics pattern and symmetrical isochromatic fringes in the crack tip.This system can provide simultaneous measurements of the local-field information of the crack tip and full-field information of blast stress waves,hence improving the understanding of the mechanisms of the interaction between blast stress waves and cracks.