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| Titel |
Brittleness of fracture in flowing magma |
| VerfasserIn |
Mie Ichihara, Masaharu Kameda, M. B. Rubin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045006
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| Zusammenfassung |
| Understanding the transition from fluid-like response to solid-like response of flowing
magma is essential for estimating the explosiveness of a volcanic eruption. Intuitively, this
explosiveness is related to the brittleness of fracture. However, two essential problems
remain for quantifying fracture of a flowing material like magma, which are: how to
define a criterion for the initiation of fracture and how to measure the brittleness of
that fracture process. Although currently there is great interest in exploring the
mechanisms causing the non-Newtonian response of magma at high strain rates and their
potential influence on fracture, quotations on this subject in the literature present a
somewhat confused picture of the relationship between shear thinning and brittle
fracture and the competing effects during heating of decreased brittleness due to
viscosity reduction and increased brittleness due to increased strain rate. For a Maxwell
viscoplastic model it is clear that the material response becomes more elastic as the
total strain rate is increased. However, since steady-state response is characterized
by no change in elasticity it is unclear whether the brittleness of fracture can be
inferred by shear thinning measured at steady-state. The objective of this study is to
present a quantitative measure β of brittleness of fracture and to eexamine these
issues.
The parameter β represents the ratio of the rate of change of the elastic strain
energy to the mechanical power. From the perspective of β, dependence of the brittle
fracture of magma on stress, decompression rate, strain rate, and shear-thinning
effects are reviewed. The experimental data for rapid decompression of analogous
materials to bubbly magma have also been reexamined. In a previous study [Kameda et
al.,2008], the observed transition from ductile expansion to brittle fragmentation
was related to the ratio between the relaxation time and the decompression time,
and the brittleness of fragmentation was quantified by the magnitude of this ratio
after sufficient pressure reduction occurred. However, this quantification depends
explicitly on the temporal profile of the decompression in the experiments and is not
necessarily applicable to general cases. Here, it is shown that β exhibits a strong
transition which correlates well with the observed transition of the fragmentation
behaviors.
Moreover, β is shown to be useful in considering the effects of shear thinning and
steady-flow conditions on brittle fracture. Statements in the literature which suggest that
materials behave in a brittle manner at sufficiently high strain rate are correct. However, since
these statements have not precisely quantified the notion of high strain rate, they are easy to
misinterpret when considering the effect of shear thinning. Although shear thinning tends to
increase the absolute magnitude of the total strain rate, it does not necessarily increase
brittleness. Also, in principle, it is unlikely for brittle fracture to occur at steady-state
flow. Cracking observed in laboratory experiments during steady flow of magma
[Lavallee et al.,2007; Lavallee et al.,2008] may be attributed to small fluctuations with
sufficiently high frequencies, which are required to satisfy the conditions for brittle
fracture. |
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