Permeability and Surface Hardness Surveying of Stone Damaged by Ballistic Impact

Abstract

Recent instances of the destruction of cultural assets in conflict zones have demonstrated the need to develop methods which will allow for the assessment of damage to heritage stone in the field. In particular, non-destructive methods would be invaluable when working on sites damaged by contemporary ballistics. Permeability (TinyPerm 3) and surface hardness (Equotip) surveys of stone damaged by 7.62 × 39 mm (AK-47) projectiles were undertaken to determine the ability of these methods to identify the spatial distribution of damage patterns such as shear faces and surface fractures. Results demonstrate the ability of surface hardness surveys to distinguish between non-impacted surfaces of the target stone and surfaces which shattered/sheared upon impact. Whilst spatial distribution analysis (“heat mapping”) of Equotip data did not correlate directly with surface fractures, permeability data heat maps were found to be indicative of surface fracture distribution. The data suggests that compaction of the stone matrix at the impact crater results in a lesser reduction of hardness in this area relative to the wider damaged surface. Surveys of impacted stone using the methods outlined here can identify damage patterns that are not visible to the naked eye, thus aiding in damage identification on fragile sites.

Surface and Subsurface Damage Caused by Bullet Impacts into Sandstone

“…Surface observations within the impact crater and spall zone show a shift of fracturing towards circumgranular pathways with increased distance from the impact, which is also seen throughout the subsurface sections and reflected in the fracture intensity plots. The micro-fractures provide evidence to support Gilbert et al's [7] suggestion that increased permeability and decreased surface hardness associated with the impact crater is related to micro-fracturing, as well as mirroring observations of grain fracture proximal to impact by Mol et al [42]. Gilbert et al [7] observed a light powdery appearance on the crater floor and a smaller surface hardness reduction relative to other areas of the impact damage.…”

“…This study extends the work of Gilbert et al [7] by characterising the surface morphology of impact damage and quantifying macro-scale fracture networks using 3D models generated by photogrammetry. It describes the microscale surface damage within the crater using electron microscopy and highlights the link to subsurface damage observed through thin section microscopy and fracture intensity analysis.…”

“…Impacts cause compaction and grain size reduction near the point of contact, causing relatively less surface hardness reduction than surrounding regions. Surrounding regions also exhibit increased surface permeability, suggesting greater susceptibility to the ingress of weathering agents such as moisture and salt [5][6][7]. Moisture can act to dissolve matrix minerals and cement in the stone, loosening grains, increasing porosity, and reducing overall strength [8][9][10].…”

“…In situ measurement of stone properties is therefore highly desirable for heritage conservation efforts but is generally restricted to non-destructive testing [13][14][15][16]. Field instruments such as surface hardness probes, permeameters, ultra-pulse velocity meters, moisture probes, and infra-red scanners can provide valuable information on stone conditions, but they cannot be used safely in current conflict areas [5,7,17]. The non-destructive nature of these methods precludes direct observation of subsurface damage, for which alternative methods are required.…”

“…Beyond engineering focussed studies of ceramic and metal plate targets, few experiments exist which can provide insights into the effects of projectiles fired by small arms [21,22]. Gilbert et al [7] studied the effects of bullet impact on the surface hardness and surface permeability of sandstone. Non-destructive testing on the stone surface highlights areas of increased permeability and decreased hardness, with the greatest permeability increases associated with large radial fractures.…”