Cratering and Perforation
Just like those impacts on the moon, hypervelocity impacts often leave craters on the surface of the impacted object. Deep craters in a thin plate can often cause perforations. A hypervelocity impact usually manifests itself through cratering and perforation. A typical crater will have a frozen raised lip around its perimeter.
The extreme energies generated during the hypervelocity impact event cause the material to melt momentarily. This melted material is ejected out the front surface by the force of the impact, where it quickly cools to solid and is "frozen" in time. If the impacted material is thin enough, the crater will perforate the rear surface.
Reflected shock waves can cause internal cracking, or they can propel detached material from the back of an impacted object at potentially lethal speeds to any astronaut in harm's way. When a target is impacted by a projectile traveling at hypervelocity, a compressive (pushing) shock wave is generated. When it reaches the free unsupported surface at the back of the plate, it is reflected as a tensile (pulling) wave. The tensile wave is often strong enough to pull some of the material away from the back of the plate, and produce internal cracking.
In some cases, material is thrown off the back of the plate (at very high velocity) without generating a complete perforation of the plate. This is referred to as detached spall, and can be just as lethal as if the projectile had passed all the way through the plate. While there is no perforation or hole on the back surface of the aluminum slab, material has been ejected out the rear by the shockwaves induced.
Impacts at hypervelocities between metal objects usually cause the metals to behave like fluids for a short period of time. This phenomena is called hydrodynamic flow. Hydrodynamic flow, is not a result of temperature melting the metal, but rather is caused by extremely high stress concentrations generated within the metal during impact.
Impacts with sufficient velocity and energy can induce shock waves in the impacted metals which are accompanied by extremely high stresses (millions of pounds per square inch). The stresses are so high, in fact, that they cause the metal to flow-like a fluid.
The result of hydrodynamic flow is seen in the photograph of the crater in the thick aluminum target. Note the lip surrounding the crater. Impact into this thick, semi-infinite target is very much like a drop of fluid hitting a liquid surface.
Shockwaves in Materials
One reason metals behave like fluids is that stress waves move through the metal at supersonic speeds. Hypervelocity impacts occur with enough energy to generate shock waves within metals. This means that the stress waves generated during impact are traveling faster than the speed of sound in that metal.
The generation of shock waves in a spacecraft hull can be very damaging, because spall can be formed. But, shock waves can also be used in designing shields that destroy an impacting projectile before it reaches the spacecraft.
The Debris Cloud
When a projectile impacts and perforates a thin plate, a debris cloud is propelled out the rear of the plate, while an ejecta cloud is propelled back out the front surface. Both clouds contain fragments of both projectile and plate material.
When a projectile traveling at hypervelocity (greater than 3 km/s) impacts a thin plate, like those found in many orbital debris shields, it fragments into smaller particles. As previously explained, shock waves from the impact will cause fragmentation, leading to possible melting and vaporization. If the plate is too thin to completely stop the projectile, a cloud of debris will be expelled out the rear of the thin plate.
Likewise, an ejecta cloud may be ejected out the front surface of the plate. Both clouds are composed of material from both the thin plate and the projectile. Clouds consist of various combinations of solid, liquid, and gaseous materials, depending on impact parameters like projectile density, shape, impact angle, and impact velocity.
The resulting debris cloud is less dense than the original projectile, and the force of the debris cloud impact on any structure downstream is spread out over a larger area. More insight on debris clouds can be found in the high-speed camera section.
Meteoroid and debris shields are rated based on the size projectile they can stop at a given velocity. Ballistic limit curves functionally describe the shield efficiency based on its ability to withstand the physical phenomena described above.
The ballistic limit describes the diameter-velocity threshold at which point a specific meteoroid and orbital shield defeats a given projectile. Any slight damage increase beyond this threshold would result in shield failure. Pass and failure of a particular shield is generally a predefined spacecraft requirement and is dependent on the criticality of the component it is protecting. Generally, shield failure is deemed to occur when the shield rear wall is visibly perforated or spalled. The ballistic limit for a shield is a function of many parameters including projectile diameter, velocity, impact angle, density, target areal and volume density, and more.
The ballistic limit may not always behave as one may expect. For example, you would expect that shield damage would consistently increase with projectile velocity (the bigger the impact, the greater the damage). In fact, low velocity projectiles (~3 km/s) can cause more damage than faster moving projectile at 7 km/s. This is because low velocity projectiles do not break up and fragment as higher energy projectiles do. Thus, lower velocity projectiles are often capable of penetrating deeper into the shield.
The graphs provided illustrate a generic ballistic limit curve for a single-wall monolithic shield and for a multi-wall shield. The ballistic limit curves show projectile diameter on the vertical axis and impact velocity on the horizontal axis. Ballistic limit equations are important because they functionally describe a shield's performance, and they are combined with the space meteoroid and orbital debris environment model to produce an overall risk assessment for actual spacecraft.