High Strain Rate Compression Testing
Compression Split Hopkinson Bar specimens of the 1% sili- con alloy were tested at a strain rate of 3000 s-1
. Figure 10
shows the solution treated and 10 hour 530°C (986°F) aged specimens’ true stress vs. strain responses. MIL-A-1256021 rolled homogeneous armor compression results22
are includ-
ed for comparison. The solution treated specimen yielded at 950 MPa (137 ksi) and work hardened to 1,552 MP (225 ksi) and the fracture strain was 0.50. The 10 hour aged specimen
yielded at 1,334 MPa (193 ksi) and work hardened to 1,651 MPa (239 ksi) and the fracture strain was 0.45.
Fracture was observed to occur in locations of prior formed shear bands. Figure 11 shows fracture through a 10 hour aged specimen. Fracture occurred at maximum shear, 45° to the uniaxial loading direction, along the path of prior shear band formation. Specimen failure from through cracking was not observed to preferentially align with inclusions, mi- croporosity, dendrite orientation, or ferrite.
Inclusions
Figure 11. A 10 hour aged 1% silicon specimen was tested to fracture by loading at 448 kPa (65 psi). Crack formation through the thickness is 45° to the loading direction (LD); load direction is indicated in the lower left hand corner. The edges of the specimen and inclu- sions are indicated.
Figure 12. Crack formation is observed following a shear band pathway at a dendrite boundary. Planar slip is ob- served in the region next to the crack. Failure of this compression specimen originated at internal sites rather than at specimen edges. The loading direction (LD) is indicated in the upper left hand corner.
Figure 13. Adiabatic shear bands observed in a solu- tion treated compression specimen extend outward in a radial pattern from the lower right hand region of the porosity. Crack nucleation is seen in the encircled re- gion following the formation of adiabatic shear bands. The loading direction (LD) is marked in the upper left hand corner.
14
Figure 14. A high strain rate solution treated 1% silicon containing alloy compression sample was not loaded to failure in order to study the adiabatic shear band and crack formation. The encircled and highlighted central region shows the nucleation of an adiabatic shear band rotated 45° to the loading direction (LD). Material adja- cent to the nucleation site is deformed and the crystal structure is rotated to conform to the sheared material.
International Journal of Metalcasting/Winter 10
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