Stress Wave Propagation And Spalling Performance Of Ultra High Toughness Cementitious Composites

Under the high-speed impact load,due to the propagation of the stress wave,a tensile stress zone will be formed in the structural member,resulting in dynamic tensile failure.This type of damage is called spalling,which usually occurs on the back of the loading surface.Since the tensile strength of cement-based materials is lower than its compressive strength,spaling is more likely to occur.UHTCC(Ultra High Toughness Cementitious Composites)exhibits excellent tensile properties under static loading,but its dynamic tensile properties under high-speed loading are not clear.Therefore,studying the spalling performance of UHTCC can improve the understanding of the dynamic tensile properties of UHTCC,and also provide experimental support for the application of UHTCC materials under high-speed impact loads.By using the modified ?80 mm Hopkinson bar,the propagation characteristics of stress waves and the spallation properties of UHTCC are studied.The microscopic mechanism of UHTCC performance under spalling is analyzed by scanning electron microscopy.The main contents and achievements are as follows:(1)The propagation law of stress wave in UHTCC was studied by modified Hopkinson bar,and the test method and data processing were discussed.The stress wave velocity in UHTCC was calculated using the measured waveform,which was 3.060 km/s.It is found that the peak value of stress wave in UHTCC is significantly attenuated,and the attenuation coefficient of stress wave in UHTCC is between 1.976 m-1-3.221 m-1 by exponential fitting.The method of testing Z-W-T constitutive parameters through experiments was discussed.(2)The spalling performance of UHTCC under 0.2,0.3,0.4,0.5 MPa impact pressure was tested.The fracture failure characteristics and failure process of UHTCC were analyzed.The dynamic elastic modulus of UHTCC is 16.8 GPa.The MATLAB program was written to calculate the spalling strength of UHTCC by the graphic method.The spalling strength of UHTCC is positively correlated with the impact pressure and has obvious strain rate effect.When the strain rate increases from 15s-1 to 35 s-1,the spalling strength of UHTCC increases from 8 MPa to 23 MPa.(3)The microscopic morphology of UHTCC spalling fracture surface was observed by scanning electron microscopy.The failure modes of matrix,PVA fiber and steel fiber were summarized.It is found that PVA fiber has two rare failure modes in spalling under high impact pressure:crushing failure and combined failure.Combined with the cracking process recorded by high-speed photography,the cause of these two failure types were analyzed.Based on the microscopic observation,the mechanism of the UHTCC spalling strength rate effect is analyzed.