Welding: insights from high-temperature analogue experiments

The rheological behavior of pyroclastic deposits during welding is incompletely understood and is based on a surprisingly small number of experimental studies. Here we present results from a new experimental apparatus comprising an automated uniaxial compression load frame that can run constant load (up to 1150 kg) or constant displacement rate (10−6 to 0.25 cm/s) tests at elevated temperatures (≤1100 °C). Deformation experiments were performed on pre-fabricated cylinders (4.5 cm diameter, ∼6 cm length) of soda lime silica glass beads (N=32), sintered rhyolite ash (N=7) and cores of pumiceous rhyodacite (N=6). Experimental runs used strain rates from 10−5 to 10−3 s−1 and stresses of ∼0 to 5.24 MPa. Temperatures varied from 535 to 650 °C for experiments on soda lime silica glass beads and 825 to 950 °C for natural materials. In all cases experimental cores showed a strain-dependent rheology that is more strongly affected by temperature than by load or strain rate. Results from these experiments are used to develop a relationship in which the effective viscosity (ηe) of the experimental cores is predicted by: η e = η o exp − 5.3 ( ϕ f 1 − ϕ f ) where η0 is melt viscosity and ϕf is sample porosity. This rheological model provides a means for exploring the relative roles of emplacement temperature, load and glass transition temperature in welding of pyroclastic deposits.