Fragmentations Of Brittle Materials Under Quasi-static And Dynamic Compression

Brittle materials,such as ceramics,glass,rock,and some concretes,are usually characterized as having low fracture toughness and internal defects.Under dynamic loading,the failure often starts from multiple points,when many cracks initiate and propagate within the brittle materials,leading to a fragmentation process.Moreover,the tensile and compressive properties of the materials are usually asymmetric,with the compressive strength typically much higher than the tensile strength.Under compressive loading,once the failure occurs,the huge strain energy stored inside the material will be released in a very short time,the failure process will be accompanied by the simultaneous development of multiple cracks and the phenomenon of fragmentation.Comparing with the relatively mature research results on tensile fragmentation,the understanding of fragmentation properties under compressive loading for brittle materials is not enough,as the many controlling factors,the diverse failure mechanisms in the dynamic fragmentation process,are not clear.Also there exist great challenges to numerically simulate the failure and fragmentations of a brittle material under compressive loading.In this paper,the compressive fragmentations of brittle materials are studied from the perspectives of experiment,theoretical analysis,and numerical simulations.The major reseraches results were obtained as following:1.The alumina（Al₂O₃）and silicon carbide（SiC）ceramics were selected as the typical brittle materials,quasi-static and dynamic compression tests were conducted for investigating the dynamic fragmentation properties.The experimental results are:1)Under axial compressive loading,the failure of the cylindrical specimens began with the generation of many"longitudinal"cracks along the loading direction.As soon as the longitudinal cracks grow and cut through the specimens,the longitudinal（axial）splitting occurred;2)Under both quasi-static and dynamic compressive loading,the ceramic specimen all fail in an"explosive"way;3)Experimental results show that the compressive strength and the size of fragments exhibit the strain-rate effects for alumina and silicon carbide,namely,the apparent compressive strength of the materials increases with the increase of strain rate,while the size of fragments decreases with the increase of strain rate;4)Most of the fragments were collected during the test,the distribution of fragment size and average fragment size were quantitatively analyzed.5)After evaluating the different forms of energy in a compressive failure process,it was indicated that the fracture energy consumed only a small part of the total strain energy,while the kinetic energy carried by fragments consumed the most of strain energy during a dynamic fragmentation process.2.A model of theoretical analysis was established to describe the compressive failure and fragmentation process of a cylindrical specimens,the primary work and conclusion are as follows:1)Based on the experimental obsrevations of a static compressive failure process,an“axial splitting-transversal expansion-tensile fragmentation”model was proposed,which put forward that under compressive loading,once axial splitting occurs,the slender column is in a state of bending,therefore,it will expand outward to release the stored compressive elastic energy;2)The fragment ejection speed was estimated by the energy conservation relation,which is close to the experimental measurement;3)The research shows that the instantaneous"explosive"fragmentation of the specimen is dominated by tensile failure.The equivalent tensile strainrate was gained through the expansion velocity of the slender column,and then using the ZMR tensile fragment size formula,we provided the average fragment size for ceramic specimens,these theoretical estimates were close to the experimental data;4)Further,the“axial splitting-transversal expansion-tensile fragmentation”model was also applied in dynamic compression,the kinetic energy in dynamic loading process was considered during the calculation of equivalent expanding velocity,the theoretical size of fragments was calculated,which agreed well with the experiment data;5)The study indicated that the kinetic energy in the dynamic loading process contributes little to the fragment size.Under dynamic loading the fragments generated were smaller,which was mainly attributed to the increase of the compression strength under dynamic loading.With the compression strength increasing,more elastic strain energy was accumulated,resulting in the smaller fragments in the subsequent fragmentation process.3.The three-dimensional discrete element method was used to simulate the dynamic failure and fragmentation process of brittle materials.The simulation results show that:1)Both the compressive failure strength and the average flying velocity of the fragments increase with the strain rate;2)There exist obvious regional gradient feature in the compressive fragmentation process of cylindrical specimens,the flying velocity of fragments in the outer region of the specimen is the highest,and the velocity decreases monotonously along the radial directions to inner central region.3)The energy conversion in a“loading-failure-fragmentation”stage during the compressive failure process was analyzed,using the energy conservation equation,the ejection velocity was estimated.4.As a part of the research about the influence of brittle compressive failure and structural integrity on structure strength,quasi-static compression tests and drop hammer impact tests were conducted,to study the failure process and the energy absorption of the polyurea-coated and uncoated cylindrical concrete specimens.Experimental results confirmed that:1)Concrete samples show apparent strain rate effect,the dynamic failure strength is higher than the quasistatic strength,and the failure strength is hardly affected by the coating;2)The pressure bearing ability after failure show obvious difference:for the uncoated concrete,the bearing capacity is rapidly lost in a very small strain range,while the polyurea-coated concrete can sustain a certain load in a wide range;3)The polyurea coating increased the energy absorption ability of the concretes subjected to drop hammer impact.