Incipient Plastic Deformation And Birefringence Of Crystal CaF₂ Under Shock Loading

Transparent window materials are widely used in shock experiments for optical measure-ments while maintaining a high pressure state.During shock compression,the window will be compressed to high pressure state.So the dynamic response along with optical properties of the window is important to signal diagnosis.Besides,the window is transparent to electromagnetic wave of some bands,the inner physical processes can be monitored and measured in situ by general photoelectric detection techniques.It is of great convenience for study with respect to opaque sample.So transparent matters are very important research objects,and the obtained dynamic knowledge is helpful for understanding the dynamic response of the opaque objects.Dynamic response and optical properties of transparent materials under shock loading have long been of interest.Many research have been reported on elastic precursor decay,Hugoniot equation of state and shock induced phase transition.Many optical properties like refractive index are reported on elastic compression zone as it can be described by photoelastic theory.There is still no clear theoretical description on incipient plasticity,incubation time,nucleation and evolution.In addition,when laser displacement interferometer(DPS)is applied to measure the particle velocity of the interface,the amplitude of the signal is usually be modulated and this is often be attributed to the interface's reflectivity change during shock compression.However,the modulation may also originate from the plasticity or birefringence and this is not clear yet.Based on one dimensional experiment facts and combining with theoretical analysis,this dissertation investigated the dynamic response,incipient plasticity and optical properties of crystal CaF2 under elastic shocking and shock slightly above the HEL.The main contents are summarized as below:1.The shock response and optical properties of elastically shocked[100],[110]and[111]CaF2 are measured by Doppler Pins system(DPS)with Short Time Fourier Transfer(STFT)analysis.Results clearly show single wave structure and their Hugoniot Equations of State are acquired for three crystallographic orientations.When shock wave propagat-ing in the[100]and[111]CaF2 sample,the amplitude of the fringe patterns is stable and so is the relevant particle velocity history.While shock wave propagating in the[110]CaF2,the amplitude of the fringe patterns is modulated and the relevant particle velocity is blurred with ripples.The ripples on the velocity history are similar to the result of a-cut sapphire measured with VISAR by Jones.Fit the modulated signal with Fourier function,two frequencies are obtained(also two apparent velocities and two re-fractive indices obtained).So the modulated fringe patterns are proved to be the result of birefringence.2.Based on Neumann principle,the symmetry is analysed for CaF2 under elastic shock compression.CaF2 changes to be tetragonal lattice when compressed from[100]di-rection and changes to be trigonal lattice when compressed from[111]direction.The resulted tetragonal lattice and trigonal lattice are uniaxial crystal with optical axis par-allel to[100]and[111]respectively.When wave vector of the probe light is parallel to the optical axis,birefringence will not appear.When compressed from[110],the lattice changes to be orthorhombic(crystal axis is perpendicular to each other but the lattice constant is not equal to each other).The optical axis is not parallel to[110],so the probe light travelling in[110]will experience birefringence.By symmetry analysis and exper-imental design,the affection from birefringence to optical measurement is eliminated successfully.According to strain optical theory,combining compression ratio and the measured refractive index,some piezo-optical coefficients at 1550 nm are obtained for CaF2.3.The[100]CaF2 single crystals are shock-compressed slightly above HEL via symmetric planar impact,and the flyer plate-target interface velocity histories are measured with a laser displacement interferometry.When shock wave travelling in the sample,the beat patterns can roughly be divided into three parts:the amplitude is stable;the amplitude is modulated like a dumbbell;the amplitude is modulated rapidly and irregularly.For the first part,the relation of relevant apparent particle velocity(ua)versus true particle velocity(up)is on the extension of elastic ua-up relation.For the second part,it can be well fitted by a Fourier function with two frequency components.This fit yields two frequencies and also two apparent particle velocities,ua1 and Ua2.ua1 is approximately equal to ua of the first part and ua2 shows some deviation from ua.The deviation is randomly changed when the stress and location change.For the third part,the fringe patterns changes rapidly.Through STFT analysis,this part can be classified into two categories:the relevant apparent particle velocity rises or drops.The rise or drop is attributed to the perturbation from plasticity to the impact face.The amplitude of rise or drop is several times the difference between ua and Ua2.Both birefringence and the plasticity induced apparent velocity rise and drop will severely affect the performance of high pressue window.