Study On Laser-induced Thermal Explosion Mechanism Of Confined Explosives

Laser technology has been wildly used in both civilian and military applications.Under laser irradiation,solid explosives confined by structures can be heated until an explosion occurs.This laser-induced initiation technology has important value in landmines excavation with contactless method.On the other hand,laser ignition provides a new experimental method for thermal explosion of high-energy explosives under fast heating condition,due to that the times to explosion for laser ignition are usually measured in terms of seconds.The heat flux is usually relatively large in comparison to fast cookoff.In this paper,experimental,theoretical and numerical simulations are performed to study the laser-induced thermal explosion behavior of confined explosives.The goal of this thesis is to investigate the thermal explosion mechanism of confined explosives induced by laser irradiation,and to incorporate theoretical and numerical models for prediction thermal-chemical-mechanical response of confined explosives under laser loading.The main research results are as follows:1.Explosive device is simplified as shell/explosive multilayer structure.The ignition theoretical model has been developed with consideration of contact thermal resistance at the interface of shell and explosive.Transient temperature distributions of shell and explosives are obtained with integral method,and the influences of laser heat flux,contact thermal resistance,and surface airflow on laser heating effect are illuminated.With the Frank-Kamenetskii approximation of chemical reaction source term,an explicit expression for the time to explosion of laser-induced ignition is obtained based on thermal explosion theory.2.Laser-induced thermal explosion experiments are carried out.The experiments include three parts:laser ablation of shell material,high temperature mechanical test of shell material,and laser-induced explosion.Firstly laser ablation behaviors of shell are investigated under fiber laser loading.The ablation morphology and transient temperature of shell are obtained.Then high temperature mechanical behaviors of metal shell are studied with high temperature material test.The fracture strengths under high temperature are measured.Finally thermal explosion experiments of confined explosive under laser irradiation are performed.Failure modes of shell structure,the temperature variation and ignition time are all obtained.The experiment results show that the failure mode of small-scale explosive device is deflagration,but not detonation.As the explosive is confined,high pressurization rates will result,with accompanying high-strain-rate deformation of metal shell,and the possibility of compaction and shock wave formation that may lead to a transition from deflagration to more violent explosion.3.A coupled thermal-chemical-hydro model for laser-induced ignition of RDX is established,and a 1D finite difference program is developed.The ignition and combustion process of RDX under laser irradiation are simulated.The simulated results of flame temperature,gas species concentration and ignition time are in good agreement with experimental data in literature.Based on numerical simulation,laser ignition tests for HMX and RDX crystal are performed.A CO2 laser is used to ignite energetic material,and high-velocity camera is used to obtain the images of the flame.Numerical model is validated by experiment data.4.Laser-induced ignition model is extended to simulate the thermal explosion process.1D reaction model for laser-induced thermal explosion is built.The reaction model of explosives has been validated by laser-induce ignition experiment,which include condensed-phase reaction mechanism and gas-phase reaction mechanism.Considering the laser interaction with material,melting behavior of shell,fluid motion of gas in the shell/explosive gap and explosive chemical reaction,laser-induced thermal explosion is simulated.The species-concentration profiles of gas-phase and condensed-phase are obtained.The model provides an understanding of the details in the laser-induced explosion phenomenon.The ignition condition for thermal explosion is also investigated.5.Thermal initiation of confined explosive is investigated base on global chemical reaction kinetics.The objective is to provide a guideline for engineering application.Chemical reaction of explosive is simplified as global multistep chemical kinetic model.Melting and ablation effect,shell/explosive interface thermal resistance,and laser irradiance distributions are all considered.The finite element and finite volume codes are developed to simulate the problem.Calculated results are compared with experiment data to validate the model.6.The post-ignition behaviors of confined explosives are investigated.The deflagration models are used to simulate the pressure increasing of solid explosive.2D and 3D explicit finite element codes are developed,and the thermal explosions of explosive device under laser irradiation are simulated.