Full-field Ultrahigh-speed Quantification of Dynamic Shear Ruptures Using Digital Image Correlation

V Rubino,N Lapusta,A J Rosakis

doi:10.1007/s11340-019-00501-7

Abstract

Producing dynamic ruptures in the laboratory allows us to study fundamental characteristics of interface dynamics. Our laboratory earthquake experimental setup has been successfully used to reproduce a number of dynamic rupture phenomena, including supershear transition, bimaterial effect, and pulse-like rupture propagation. However, previous diagnostics, based on photoelasticity and laser velocimeters, were not able to quantify the full-field behavior of dynamic ruptures and, as a consequence, many key rupture features remained obscure. Here we report on our dynamic full-field measurements of displacement, velocities, strains and strain rates associated with the spontaneous propagation of shear ruptures in the laboratory earthquake setup. These measurements are obtained by combining ultrahigh-speed photography with the digital image correlation (DIC) method, enhanced to capture displacement discontinuities. Images of dynamic shear ruptures are taken at 1-2 million frames/s over several sizes of the field of view and analyzed with DIC to produce a sequence of evolving full-field maps. The imaging area size is selected to either capture the rupture features in the far field or to focus on near-field structures, at an enhanced spatial resolution. Simultaneous velocimeter measurements on selected experiments verify the accuracy of the DIC measurements. Owing to the increased ability of our measurements to resolve the characteristic field structures of shear ruptures, we have recently been able to observe rupture dynamics at an unprecedented level of detail, including the formation of pressure and shear shock fronts in viscoelastic materials and the evolution of dynamic friction.

Highlights

Earthquakes occur as dynamic frictional ruptures along preexisting interfaces in the Earth’s crust
We present correlation results obtained using different subset sizes to show their effect on spatial resolution and measurement accuracy of dynamic shear ruptures
Recently the advances in highspeed camera technology have made it possible to record a large number of images at a high enough frame rate with a low enough noise level, making them suitable for ultrahigh-speed digital image correlation applications

Summary

Introduction

Earthquakes occur as dynamic frictional ruptures along preexisting interfaces (or faults) in the Earth’s crust. An example of the displacement fields associated with the dynamic ruptures produced in our setup, and obtained through processing the raw images with Vic-2D, are shown in Fig. 5(a) and (b) These images were acquired with a gated-intensified camera at 1 million frames/s, 50 ns exposure time and MCP gain setting minimized to reduce noise. The actual spatial resolution of a displacement field obtained with DIC depends on a combination of the physical dimension of the subset size (see section BDigital image correlation of dynamic shear ruptures^) and the camera. The correlation performed with the smallest subset size

C Velocimeter measurements

Conclusions