Investigation On Breakage And Diffusion Characteristics Of Brittle Materials Under Low Speed Impact

Brittle materials are widely used in impact protection,aerospace and other fields.In the process of application,they will be damaged and broken by shock wave,collision,explosion,and other dynamic loads.Among them,the generation and development of failure fronts is a special phenomenon in the dynamic breakage process of brittle materials,and the generation of failure fronts is usually caused by local non-uniform deformation of materials.At present,the research on the evolution of failure front is mainly focused on the plate impact experiment,and the relationship between the formation and propagation of the front and the non-uniform deformation is relatively few.At the same time,the driving force and local diffusion resistance in the propagation process of the failure front are important parameters to accurately describe the failure front,and the related research is relatively scarce.A series of low-speed impact experiments were carried out on three typical transparent brittle materials,i.e.quartz glass,polymethyl methacrylate(PMMA),and transparent ceramics(AlON),to analyze the dynamic breakage performance of the three brittle materials.The influences of stress state,contact condition,nonuniform deformation and other factors on the generation of failure front and diffusion process were studied,and the influences of diffusion path on the evolution of driving force and local resistance in the diffusion process were revealed.In this paper,the dynamic compression tests of cuboid transparent ceramic samples,cuboid quartz glass samples,and trapezoidal quartz glass samples was carried out based on the traditional Hopkinson pressure bar device.And the dynamic mechanical properties of transparent ceramic and quartz glass were investigated.The breakage process was analyzed by high-speed photography and DIC method.The results show that the quartz glass samples have strain rate effect,but the transparent ceramics have no strain rate effect.Quartz glass breakage is mainly caused by the rapid diffusion of the failure front of the contact end.The breakage of transparent ceramics is caused by the combined effect of the failure fronts along the impact direction and parallel to the impact direction.Based on the stress-strain curve,the deformation energy absorption of samples is discussed and the relationship between maximum specific energy absorption and strain rate is established theoretically.In order to investigate the energy release characteristics in the breakage process of brittle materials,the impact crushing experiments of single quartz glass sphere and double quartz glass sphere were carried out based on the Hopkinson pressure bar combined with infrared temperature measurement experiment system(ITMS).The breakage process and elevated temperature were discussed based on the ITMS and DIC methods.The results show that the infrared elevated temperature is caused by the plastic elevated temperature produced by the micro-fracture and the fracture energy released by the macro crushing.The macro crushing temperature rise is the main temperature rise during breakage.The relationship between the peak elevated temperature and ε·2/3 is linear.By introducing rigid contact mode and elastic contact mode,the local strain energy release during the glass sphere breaking process is analyzed,and an effective method for evaluating the strain energy release mechanism is proposed.In order to investigate the influence of stress state on the process of dynamic breakage,the change of stress state of glass sphere sample was changed by changing the material of transmission bar and double glass sphere chain sample.Larger critical strain and lower critical strain rate could be obtained when the transmission bar is changed into PMMA bar.The dynamic breakage of glass sphere is divided into two stages.First,the failure front at the contact end of the specimen is generated by the local shear failure,and then the specimen is broken by the penetrating crack caused by the lateral tensile failure.When the transmission bar is a steel bar,the failure front would be generated at both ends of the sample.When the transmission bar is changed into aluminum bar and PMMA bar,the failure front is generated only at the incident end.In order to further obtain the evolution of shear failure mechanism and tensile failure mechanism,double Weibull distribution function was used to analyze the broken products.The results show that when the transmission bar is changed into aluminum bar and PMMA bar,the proportion of tensile failure mechanism increases.In the double sphere samples,the shear failure mechanism is the main mechanism for the first broken samples at low speed,while the tensile failure mechanism is the main mechanism for the samples broken later.With the increase of impact velocity,the breakage mechanism of glass sphere changes from tensile failure mechanism to shear failure mechanism.Based on the shear activated diffusion equation,the diffusion process of the failure front is described,and the influence of local strain,local strain gradient and local strain rate on the failure law of sphere is preliminarily discussed.In order to further investigate the relationship between formation of the failure front and non-uniform deformation,a series of low-speed impact experiments were carried out on cuboid,trapezoid and plate PMMA specimens.Based on high-speed photography and DIC method,the generation and propagation of failure fronts in samples were tracked and analyzed.The results show that when the sample changed into trapezoid,the failure front is generated only at the incident end.The non-uniform deformation distribution of the sample increases with the increase of impact velocity,sample size and decrease of the wave impedance of transmission bar,which results the formation of failure front at the incident end.Based on the shear activated diffusion equation,the calculation method of the generalized driving force was proposed.Combined with the strain distribution,the generalized driving force was calculated in the compression process and the front propagation process,and the evolution law of the driving force in the compression process and the front propagation process is obtained.Based on the stress diffusion equation,the stress distribution of the sample is discussed preliminarily.The results show that the stress distribution of the sample is basically consistent with the strain distribution.The diffusion process of the failure front was further analyzed and the influence of the diffusion path on the diffusion process of the failure front was discussed.Based on the shear activation diffusion equation combined with the deformation distribution,the evolution law of the generalized driving force was obtained.The calculation method of local shear resistance is proposed,and the evolution law of local shear resistance was revealed by combining transmission load boundary and strain distribution.The results show that the development of failure front slows down or even stops when the sample changes from cuboid to trapezoid or spherical,and the diffusion path changes from constant to linear and quadratic.The driving force amplitude was related to the speed of the failure front,and the speed of the failure front increases exponentially with the increase of the driving force.As the diffusion path increases,the generalized driving force increases.The local shear resistance in the intact region is larger,the shear resistance near the front is the smallest,and the shear resistance in the broken region shows a violent oscillation.With the increase of the failure front velocity,the local shear resistance decreases logarithmically.As the diffusion path increases,the local shear resistance increases.The generalized driving force and local shear resistance of quartz glass are higher than PMMA.