Dynamic Mechanical Response Characteristics Of Brittle Materials Under Impact Loads

In this thesis, the true strain rate effect and the constitutive model of brittlematerials within a large strain rate range are studied. Specific work and theresearch content are as follows:1. The dynamic compressive strength enhancement is attributed to thecombined action of material strain-rate effect, lateral inertial effect and endfriction effect, and in actual split Hopkinson pressure bar (SHPB) tests they arecoupled together and could not be separated from each other. To determine thematerial strain-rate effect of the brittle materials in SHPB tests, the dynamiccompressive strength enhancement caused respectively by lateral inertial effectand end friction effect needs to be removed. The first method is to propose acorrection methodology to determine the real strain-rate effect for brittlematerials in actual SHPB tests, and a complete procedure to conduct thecorrection is recommended. The proposed method is verified using hybridnumerical simulations and experimental data. The second method is using thenumerical simulation method to determine the effect of lateral inertia and endfriction. Then removing the dynamic compressive strength enhancement causedrespectively by the lateral inertial effect and end friction effect from the dynamiccompressive strength data obtained from actual SHPB tests, the materialstrain-rate effect of the brittle materials in SHPB tests is determined. Finally,taking limestone specimens for example, two methods are verified.2. The classic Johnson-Cook constitutive model was analyzed. Problems havebeen found. The strain-rate effect expression in the Johnson-Cook constitutivemodel is modified to be suitable for different materials in a large range of strainrate. And this new model can better describe the phenomena of the weaksensitive area, strong sensitive area and saturated zone of strain rate dependence.Finally, the improved Johnson-Cook constitutive model was used in engineeringmaterials, such as brittle materials, metals and plastics. Results show that themodel can well describe the above materials.3. Taking granite specimens for example, to determine the parameters of Drucker-Prager model in numerical simulations, the uniaxial compressive,splitting, and triaxial compression tests were completed. The SHPB tests werecompleted using the large diameter (75mm) SHPB experiment device. Finally,the true strain rate effect of granite in the SHPB tests was determined. And it waswell described using the improved Johnson-Cook constitutive model.