Research On Dynamic Diagnosis Of Metal Spallation Under Extremely High Strain Rate Load

Spallation is a kind of dynamic tensile fracture of metal materials under shock wave loading,which is one of the research focuses of scholars from all over the world.Before the wide application of laser technology,shock waves were mostly produced by light-gas gun or chemical explosion.The strain rate of these two methods is below 10⁵s^-1,while the strain rate under laser shock wave can reach 10⁶s^-11 to10⁹s^-1.The study of material spalling characteristics under such extremely high strain rate loads is helpful to understand the dynamic damage phenomenon of materials under extreme conditions.Based on this,this paper takes TC4 alloy and pure titanium foil as examples and studies the spalling characteristics of metal materials under nanosecond and femtosecond laser shock wave loading.After modeling shock wave and material with ABAQUS finite element analysis software,the dynamic process of spalling is simulated,the velocity curve and tensile stress distribution in thickness direction of target free surface are obtained,spall strength under different strain rates,the relationship between the size of spall scab and laser parameters is analyzed.The main work and research contents of this paper are as follows:?1?This paper introduces the loading methods of various impact loads and the widely used spalling diagnosis technology at this stage,analyzes the spalling theoretical model proposed by different scholars,and introduces the laser shock wave and spallation damage mechanism,which is the theoretical basis for this study.?2?Using ABAQUS finite element analysis software to simulate the spalling process,the simulation results show that the strain rate and spalling intensity of TC4increase with the increasing of the peak pressure of laser shock wave under the nanosecond laser loading,and increase with the decreasing of the loading time of shock wave.The maximum strain rate and spall strength of TC4 target is 3.34×10⁶s^-1and 5.26GPa,when laser energy is 10J and pulse width is 5ns and spot diameter is3mm.?3?The spalling behavior can only occur in the center area of the laser shock wave.The area of the spalling area is smaller than the area of the laser spot,and the spalling location is close to the free surface of material;Increasing the peak pressure of the laser shock wave will increases the tensile stress inside the material,but will not change the location where the spalling occurs;The dynamic process of the spall scab flying away from the matrix is simulated,the radial size of the spall scab increases with the increase of the peak pressure of the laser shock wave,but it will not exceed the laser spot diameter.When TC4 thickness is 1mm,the thickness of the spall scab basically remains about 0.03mm.?4?The spalling characteristics of 1um thick titanium foil under femtosecond laser loading are studied.The results show that increasing the peak pressure of shock wave can increase the peak value of free surface velocity,and the strain rate and increase the spalling strength of titanium foil,but the oscillation period of free surface velocity remains unchanged,and the thickness of spall scab will not change.When the peak pressure of femtosecond laser shock wave is 1000GPa,the pulse width is200fs,and the spot diameter is 0.5mm,the maximum strain rate and spall strength of titanium foil are 4.59×10⁸s^-11 and 15.33GPa.?5?Under the impact of femtosecond laser,the spallation behavior of titanium foil is more sensitive to the thickness.When the thickness of titanium foil is changed,the free surface velocity curve will change greatly,such as the decrease of peak velocity,the delay of velocity rebound time,the increase of oscillation period,etc.;the spallation deformation area of titanium foil is mainly concentrated in the area of femtosecond laser loading,and the deformation increases with the increasing of laser energy and pulse width.