title: Metal objects mapping after small charge explosions. A study on AISI 304Cu steel with two different grain sizes.
abstract: Evidence of exposure of a metal component to a small charge explosion can be detected by observing microstructural modifications; they may be present even if the piece does not show noticeable overall plastic deformations. Particularly, if an austenitic stainless steel (or another metal having a face-centered cubic structure and a low stacking fault energy) is exposed to an explosive shock wave, high-speed deformation induces primarily mechanical twinning, whereas, in nonexplosive events, a lower velocity plastic deformation first induces slip. The occurrence of mechanical twins can be detected even if the surface is damaged or oxidized in successive events. In the present research, optical metallography (OM) and scanning electron microscopy (SEM), and scanning tunneling microscopy (STM) were used to detect microstructural modifications caused on AISI 304Cu steel disks by small-charge explosions. Spherical charges of 54.5 or 109 g TNT equivalent mass were used at explosive-to-target distances from 6.5 to 81.5 cm, achieving peak pressures from 160 to 0.5 MPa. Explosions induced limited or no macro-deformation. Two alloy grain sizes were tested. Surface OM and SEM evidenced partial surface melting, zones with recrystallization phenomena, and intense mechanical twinning, which was also detected by STM and X-ray diffraction. In the samples' interior, only twins were seen, up to some distance from the explosion impinged surface and again, at the shortest charge-to-sample distances, in a thin layer around the reflecting surface. For forensic science locating purposes after explosions, the maximum charge-to-target distance at which the phenomena disappear was singled out for each charge or grain size and related to the critical resolved shear stress for twinning.
authors.affiliation: Dip. di Sc. dei Materiali ed Ing. Chimica, Politecnico di Torino, Torino, Italy. donato.firrao@polito.it
authors.forename: Donato
authors.initials: D
authors.lastname: Firrao
authors.forename: Paolo
authors.initials: P
authors.lastname: Matteis
authors.forename: Giorgio
authors.initials: G
authors.lastname: Scavino
authors.forename: Graziano
authors.initials: G
authors.lastname: Ubertalli
authors.forename: Maria G
authors.initials: MG
authors.lastname: Ienco
authors.forename: Gabriella
authors.initials: G
authors.lastname: Pellati
authors.forename: Paolo
authors.initials: P
authors.lastname: Piccardo
authors.forename: Maria R
authors.initials: MR
authors.lastname: Pinasco
authors.forename: Enrica
authors.initials: E
authors.lastname: Stagno
authors.forename: Girolamo
authors.initials: G
authors.lastname: Costanza
authors.forename: Roberto
authors.initials: R
authors.lastname: Montanari
authors.forename: Maria E
authors.initials: ME
authors.lastname: Tata
authors.forename: Giovanni
authors.initials: G
authors.lastname: Brandimarte
authors.forename: Santo
authors.initials: S
authors.lastname: Petralia