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Whipple shield, a dual-wall system, as well as its improved structures, is widely applied to defend the hypervelocity impact of space debris (projectile). This paper reviews the studies about the mechanism and process of protection against hypervelocity impacts using Whipple shield. Ground-based experiment and numerical simulation for hypervelocity impact and protection are introduced briefly. Three steps of the Whipple shield protection are discussed in order, including the interaction between the projectile and bumper, the movement and diffusion of the debris cloud, and the interaction between the debris cloud and rear plate. Potential improvements of the protection performance focusing on these three steps are presented. Representative works in the last decade are mentioned specifically. Some prospects and suggestions for future studies are put forward.

Shock wave is emitted into the plate and sphere when a sphere hypervelocity impacts onto a thin plate. The fragmentation and phase change of the material caused by the propagation and unloading of shock wave could result in the formation of debris cloud eventually. Propagation models are deduced based on one-dimensional shock wave theory and the geometry of sphere, which uses elliptic equations (corresponding to ellipsoid equations in physical space) to describe the propagation of shock wave and the rarefaction wave. The “Effective thickness” is defined as the critical plate thickness that ensures the rarefaction wave overtake the shock wave at the back of the sphere. The “Effective thickness” is directly related to the form of the debris cloud. The relation of the “Effective thickness” and the “Optimum thickness” is also discussed. The impacts of Al spheres onto Al plates are simulated within SPH to verify the propagation models and associated theories. The results show that the wave fronts predicted by the propagation models are closer to the simulation result at higher impact velocity. The curvatures of the wave fronts decrease with the increase of impact velocities. The predicted “Effective thickness” is consistent with the simulation results. The analysis about the shock wave propagation and unloading in this paper can provide a new sight and inspiration for the quantitative study of hypervelocity impact and space debris protection.

In this paper, the gauge points setting is introduced in the SPH simulation to analyze the debris cloud structure generated by the hypervelocity impact of disk projectile on thin plate. Compared with the experiments, more detailed information of the debris cloud structure can be classified from the numerical simulation. However, due to the solitary dispersion and overlap display of the particles in the SPH simulation, accurate comparison between numerical and experimental results is difficult to be performed. To track the velocity and spatial distribution of the particles in the debris cloud induced from disk and plate, gauge points are locally set in the single-layer profile in the SPH model. By analyzing the gauge points’ spatial coordinate and velocity, the location and velocity of characteristic points in the debris cloud are determined. The boundary of debris cloud is achieved, as well as the fragments distribution outside the main structure of debris cloud.

In hypervelocity impacts of projectiles into thin flat targets, shock initiation and interaction dominate the responses of projectiles and targets, and especially dominate the features of the debris cloud. To estimate the geometric features of the wave front during the first complete propagation in the spherical-projectile, the Geometric Propagation Model (GPM) is built in this paper to describe the geometry of the shock wave front, which proposes an ellipse contour as a function of time and equivalent speed. The GPM identifies the geometric features of the wave front as a function of time and impact velocity successfully. Combined with the GPM and SPH simulation, the shock pressure distribution and attenuation in the spherical-projectile have been obtained. Meanwhile, the attenuation of shock pressure and speed are presented as a function of impact velocity, respectively, and a method for obtaining the equivalent speed of the shock wave is proposed by the GPM. The GPM may be applicable to hypervelocity events involving any monolithic materials as long as the equivalent speed could be supplied from numerical simulation. The GPM proposed in this paper and the corresponding shock wave analysis provide a new insight into the processes of the quantitative analysis of the initiation of the debris cloud. Keywords: Hypervelocity impact, Shock wave, Numerical simulation, Debris cloud formation

The protective performance of a Whipple shield to hypervelocity impact could be improved by using an impedance-graded plate. Transmission and reflection of the shock wave on the impedance interface affect the pressure state in the projectile and its fragmentation, and further change the characteristics of the debris cloud. Models of the shock waves and reflection waves in the materials were built in this paper, which describes the geometric characteristics and the pressure states of wave fronts in the impedance-gradient-plate impact. The models predicted theoretically that a smaller central fragment with a series of larger circumferential fragments would be produced by high/low impedance-graded plate. Based on the models, various front plate combinations of aluminum, magnesium and steel (Al/Mg, Al, Mg/Al, Al/steel) were analyzed experimentally and numerically. The pulse attenuation, material spalling, debris cloud characteristics and witness plate damage were analyzed. The results show that the impedance-gradient structure would eventually change the debris cloud characteristics and witness plate damage, and affect the performance of the shield. The plate of Al/Mg (high/low impedance) has a better performance than that of Al, Mg/Al (low/high impedance) and Al/steel (low/high impedance) with the same areal density. The discussion and analysis are also suitable to impedance-gradient plates made up with other materials.

Hypervelocity impact (HVI) of Al spheres upon thin Al plates arouses high amplitude shock waves. Along with the wave propagation, the sphere and plate undergo massive failure, breakup and phase changes, and end up with diffusing debris clouds. Combined with the theories of wave interaction and tensile failure, this paper reveals the failure evolution during HVI of Al spheres onto thin Al plates through Smoothed Particle Hydrodynamics (SPH) method. The relation between the failure evolution and the formation process of debris cloud is analyzed. The influence of impact conditions (impact velocity and projectile-diameter-to-plate-thickness ratio) on the failure evolution, the debris cloud formation, and the largest fragment is also discussed. The analysis of the failure evolution and debris cloud formation in this paper can provide new insight for the quantitative study of HVI and space debris protection.

The damage pattern of rear wall/witness plate could express the fragmentation features of projectile relatively, in the ground-based tests of Whipple shield and its improved structure. In this paper, the damages of rear wall for 20 Whipple shield tests are statistically analyzed, and the linkage between the damages with impact conditions (impact velocity and plate-thickness-to-sphere-diameter ratio) are obtained. The damage pattern of rear wall, or the fragmentation pattern of sphere is defined as Intact, Ruptured and Smashed. The perforation hole on rear wall is described quantitatively. The fragmentation patterns under different impact conditions are analyzed based on the propagation and evolution of shock waves. A series of SPH simulations prove and extend the analytical and experimental works, and clearly reveal the normalized relationship between fragmentation pattern and impact condition.