Abstract |
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A recoverable plate impact testing technology
has been developed for studying fracture mechanisms of mode II
crack. With this technology, a single duration stress pulse with
submicrosecond duration and high loading rates, up to
108 MPa m1 ∕ 2s−1, can be
produced. Dynamic failure tests of Hard-C 60# steel were carried out under asymmetrical
impacting conditions with short stress-pulse loading.
Experimental results show that the nucleation and growth of
several microcracks ahead of the crack tip, and the interactions
between them, induce unsteady crack growth. Failure mode
transitions during crack growth, both from mode I crack to mode
II and from brittle to ductile fracture, were observed. Based on
experimental observations, a discontinuous crack growth model was
established. Analysis of the crack growth mechanisms using our
model shows that the shear crack extension is unsteady when the
extending speed is between the Rayleigh wave speed cR and the shear wave speed
cs. However, when the
crack advancing speed is beyond cs, the crack grows at a steady
intersonic speed approaching  cs. It
also shows that the transient mechanisms, such as nucleation,
growth, interaction and coalescence among microcracks, make the
main crack speed jump from subsonic to intersonic and the steady
growth of all the subcracks causes the main crack to grow at a
stable intersonic speed.
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Keywords
plate impact tests, dynamic fracture, failure mode transition, intersonic crack extension, discontinuous crack growth model
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Authors
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