| Experimental methods used to study the dynamic mechanical behavior of materials can be generally classified into categories: the split Hopkinson pressure bar(SHPB)or Kolsky bar technique and the wave propagation inverse analysis(WPIA)or Lagrangian analysis(LA)technique.The traditional method to record the strain in the bars is using strain gauges.Reliable measurements require good adhesion of the gauges to the measured surface.Later,other testing methods were developed,such as quartz piezo gauge or PVDF piezoelectric thin film to directly measure the stress profiles of the specimen.The WPIA experiment needs to measure the mechanical parameters(such as stress,strain,particle velocity),and still limited the stress time history and the strain time history measurement,which greatly restricts the more extensive application of this method in the field of experimental mechanics.The stress and strain are measured with contact measurement method popularly.In recent years,the non-contact optical measurement method has been developed rapidly.For example,based on the technology of digital image correlation,the video extensometer whose application is mostly confined to the field of quasi-static mechanical properties testing has gradually replaced the traditional mechanical extensometer.In this paper,the advanced non-contact optical measurement technology is extended to the field of material dynamic mechanical property testing with the aim of studying the application of multi-probe all fiber laser doppler velocimeter and the digital image correlation full field strain measurement system based on ultra-high speed camera in the experiment of split Hopkinson pressure bar and the wave propagation inverse analysis experiment.The main research results of this paper are summarized as follows:the particle velocity monitored by the double-probe all fiber laser doppler velocimeter which based on Doppler shift principle is converted into the strain and stress of the specimen by the stress wave propagation theory.In the non-contact optical testing system of the SHPB experiment,Laser upright incidence and laser oblique incidence testing techniques are put forward for ductile materials and brittle materials respectively.Taking aluminum alloy and PZT ceramics as an example,the validity of the two testing techniques is verified by comparison with the traditional strain gauge testing results and the results of DIC measurement.Compared with the traditional strain gauge testing technology,the new laser interference testing technology which is helpful to realize the standardization of the SHPB test system has many advantages,such as calibration-free,anti-interference,high reliability,and so on.Based on the multi-probe all fiber laser interferometric velocity measurement technology,a long-bar testing device is set up in the split Hopkinson pressure bar system to study the dynamic constitutive relation of brittle materials.The axial Euler particle velocity of long specimen bar is monitored with laser oblique incidence method.Moreover,the conversion relationship between the Euler particle velocity and the Lagrange particle velocity is established based on the onedimensional stress wave propagation theory.By using the measured several particle velocity-time curves,the whole velocity field is connected with the path-lines method.The numerical solution of the Lagrangian inverse analysis method is realized by combining the zero-initial condition on the path line.Finally,the dynamic stress-strain curves of the brittle material(Polymethyl methacrylate,PMMA)are obtained with this method,and the results are compared with the results of the Hopkinson pressure bar experiment and the quasi-static compression experiment.Based on the technique of ultra-high speed camera combined with digital image correlation full field strain analysis,the traditional split Hopkinson pressure bar experimental system is improved,and the strain distribution in the specimen is ananlysed.The experimental results are shown as follows: The strain uniformity in the ductile aluminum alloy specimen is good,and the strain uniformity in the brittle ceramic specimen is poor.For the aluminum alloy with strong deformability,the local strain distribution inhomogeneity has little effect on the dynamic stressstrain curves of the measured material.It is proved that the traditional SHPB testing technique is suitable for the ductile metal material.For the PZT ceramics with poor deformability,the inhomogeneity of strain distribution is more obvious because of the stress concentration at the boundary of the specimen.It is necessary to correct strain measurement results to get more accurate dynamic stress-strain relationship curves of PZT ceramics.Based on the method of digital image correlation analysis combined with ultra-high speed camera,a long-bar testing technique is developed to study the dynamic constitutive properties of brittle materials under small deformation conditions in the split Hopkinson pressure bar experimental system.The speckle images of the long-bar specimens are photographed in real-time by the ultra-high speed camera,and the digital image correlation(digital image correlation,DIC)of the speckle images is analyzed to obtain the velocity field and strain field on the surface of the long-bar specimen.Taking the brittle material(PMMA)as an example,the partical velocity profiles at different Lagrange positions are extracted from the velocity field obtained by DIC analysis,and the whole velocity field is connected with the path-lines method combined with the zero-initial condition.The real-time curves of strain and stress in the specimen can be obtained by numerical solution,and the dynamic stress-strain curves of the PMMA are obtained after the elimination of the time parameters.The results of the split Hopkinson pressure bar experiment and quasi-static compression experiment are compared with the stress-strain relationship curves solved by the Lagrangian method to reveal the viscoelastic constitutive characteristics of PMMA material under small strain condition. |
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