The author thought it to be important that the following test hardware spalling observations be reported:
The 304L stainless steel expended test hardware was cross sectioned to examine the barrier damage by shock wave spalling. The observations of the cross sections lead to the following postulation:
The degree of deformation is determined by the strength of the shock wave and the area of free surface as the shock wave exits the media into a media of porous EM. The deformation occurs in the form of delamination along grain direction boundaries.
Figure 13: 304L SS Grain Direction Spalling Influence
Figure 14: Redundant Donor Cavity With Parabolic Cornered Flat Bottom EM Cavity
A parabolic bottom cavity was tested using a single donor in the redundant donor cavity hardware, Figure 13c. The result was a failure to initiate TiH0.65-KClO4. A review of the design using Huygens’ graphic analysis indicated a flat bottom cavity with a corner radius, Figure 14, may be more efficient in a redundant donor cavity design.
A flat bottom cavity with a machined 0.030″ corner radius rotated 360° around the bottom was manufactured and tested with successful results. The redesign was limited to a single test.
Although the material failure is in tension, the inherent shear stress in the 304L SS is the cause of failure. The material actually fails in tension along the shear stress planes created during the plate rolling or bar extrusion manufacture process.
Grain directions in 304L SS are created during manufacture process of plate rolling and bar extrusion. The direction of the grain is set by the direction of the plate rolling or bar extrusion.
During the plate rolling or bar extrusion manufacture process ( assuming the force is applied toward the center of material), the 304L SS is stretched in the direction of roll/extrusion. This causes a gradient motion of the material. First motion is at the surface where the force is applied and then gradates inward toward the center. The grading tensile loads creates shear stress planes parallel to the direction of roll/extrusion. A method to help relieve some of the stress is to heat treat under a double vacuum melt process.
Knowing the grain direction (grain boundaries) of the test hardware, it has been observed that the spalling pockets within the bulkhead were consistently elongated in the direction of the material’s grain direction regardless of the direction of the passing shock wave. This phenomenon should be the subject of a future study. See Figure 13 a, b, c, d, and e.
The pocket (spalling) is caused by failure in tension (Figure 13) of the 304L SS material. The separation of the 304L SS grain boundaries can be explained by the shock wave front compressing (increasing the density) and displacing the material particles in the direction the shock wave is traveling. The particles behind the shock front are traveling at varying velocities as a result of the boundary conditions (surroundings), which tend to dissipate the shock compression and return the material to its normal density. This dissipative rarefaction wave follows in behind the shock front, so that the density is decreased. When two such rarefaction waves, traveling in different directions intersect, the material is accelerated (pulled) in two opposing directions, and this produces a state of tension in the material.
It was also observed that the spalling was least in the test sample having the curved parabolic and quasi-parabolic EM cavities. These observations could lead to designs that would mitigate the spalling ejecta of the medium surface as a shock wave leaves the surface of the one medium into a medium of a much lower impedance (air).