Further Study On Dynamic Constitutive Models For Ductile Metals And Brittle Materials

Dynamic constitutive models for materials exert a significant influence on numerical simulations of engineering structures subjected to intense dynamic loadings.Metals,concrete-like materials and ceramics have been widely used in both civil and military engineering.Dynamic constitutive models are of great significance not only for the response and failure of engineering structures subjected to projectile impact or explosive loadings but also their applications in various engineering.In the past few decades,much progress has been made in dynamic constitutive models for these materials.However,area of further research is still needed due to the fact that dynamic mechanical properties of these materials are very complex.The objective of this thesis is to further study dynamic constitutive models for these materials.This thesis mainly consists of the following parts:1.An extended GTN model for metals has been developed.(1)An extended GTN model has been established to describe the dynamic behaviour of metallic materials with different initial void volume fractions under complex loading conditions(namely large deformations,high strain rates,high temperatures and high pressures).The strength model is constructed by modifying GTN model,in which the effects of Lode angle,strain rate and temperature are taken into account and void nucleation is related to initial void volume fractions.A new failure criterion is proposed which takes into account the effects of initial void volume fraction,stress state(Lode angle and stress triaxiality),stress state history,strain rate and temperature.(2)Void nucleation and ductile fracture for 2024-T351 aluminum alloy with different initial void volume fractions have been investigated.(3)To validate the present model comparisons have been made between the model predictions and the test data for 2024-T351 aluminum alloy in terms of true stress-true strain relationship,failure strain,force-displacement curve and fracture pattern of quasi-static uniaxial tension and upsetting tests,strain rate effect,temperature effect and residual velocity and ballistic limit in case of a flat-ended projectile.It is found that the present model predictions are in good agreement with the test data.(4)A numerical study has been conducted on the perforation of 2024-T351 aluminum plates with different initial void volume fractions struck by flat-ended projectiles.2.Dynamic increase factors for concrete-like materials in a wide range of strain rates(10-5s-1～106s-1)have been studied.(1)A method has been first proposed to determine dynamic increase factors for concrete-like materials in compression due to strain rate effect only at very high strain rates.(2)Various material tests(i.e.quasi-static uniaxial compression and tensile tests,SHPB and SHTB tests,plate impact test and dynamic compression-shear test)have been performed on a certain type of cement mortar and dynamic increase factors for the cement mortar with strain rate ranging from 10-5s-1 to 106s-1 have been obtained.(3)The plate impact test data for ultra high performance concrete(UHPC)available in the literature have been also analyzed and mechanical properties at strain rates up to 106s-1 have been acquired.(4)The test data for concrete-like materials have been compared with the previously proposed semi-empirical equations for dynamic increase factors.It is found that the semi-empirical equations can accurately describe the strain rate sensitive behavior of concrete-like materials in both tension and compression.3.A systematic study on the equation of state for concrete-like materials have been conducted.(1)A new method has been proposed to analyze plate impact test data by considering the effects of both wave structures and strength surface and the relationships between longitudinal stress,pressure,specific internal energy and volumetric strain for the cement mortar have been obtained by using the newly proposed method.(2)The accuracy of the P～α equation of state and the equation of state of HJC has been evaluated and the similarities and differences between them have been discussed.It is found that these two equations of state are not accurate enough at low and medium pressures and the equation of state of HJC can be regarded as an approximate case of the p～α equation of state when specific internal energy is ignored.(3)A new equation of state for concrete-like materials has been proposed based on the polynomial equation of state and that is in good agreement with the plate test data in term of pressure-volumetric strain relationship,particle velocity-time history,longitudinal wave velocity and HEL.(4)A modified p～α equation of state has been proposed and that is in good agreement with the plate test data for concrete-like materials and is advantageous over the p～α equation of state below fully compaction pressures.4.A new dynamic computational constitutive model for ceramic has been developed based on the previous work.(1)A new equation of state for ceramic has been proposed.(2)Plate impact tests have been conducted on AD995 ceramic and the relationships between longitudinal stress,pressure,specific internal energy and volumetric strain for AD995 ceramic have been obtained by using the newly proposed method.(3)To validate the present model comparisons between the model predictions and material test data have been made in term of strength surface,strain rate effect and pressure-volumetric strain relationship.It is found that the present model is in good agreement with the test data.(4)To further validate the present model comparisons between the numerical predictions and the experimental results have been made in term of residual velocity,crater morphology,crack pattern,particle velocity-time history,longitudinal wave velocity,HEL,target deflection.The type of these experiments is from simple to complex and the impact velocity from low velocity to hypervelocity.It is found that the numerical predictions are in good agreement with these experimental results.