Visual and Acoustic Emission Observations of Fracture in Barre GraniteBing Q. LiPhD Candidate,Department of Civil and Environmental EngineeringIn collaboration with Prof. Herbert H. Einstein

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting01 June, 2017

IntroductionMain research goal: Relate visual observations to acoustic emissions (laboratory analogue to microseismicity), given that fractures are not directly accessible in the field.

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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High speed video Microscopic images Specimen-scale1cm 250μm 5mm

Acoustic Emissions

Four Point Bending: Experimental Setup

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Front  (macro)  camera

Back  (micro)  camera

Loading  machine

Specimen

Loading  Geometry

8  AE  sensors  (150-­‐400  kHz  peak)5  MHz  acquisitionMagnitude  calibrated  with  ball  drop

Load-displacement Data

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Digital Image Correlation (DIC) Results

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Microstructural Changes

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Opening  Strain Oblique  Strain

AE Results

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Focal Mechanisms

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Double  CoupleNon-­‐Double  Couple

Microstructural Changes Compared to AE

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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General Visual Data Compared to AE

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Other Work on Hydraulic Fracture

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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Conclusions

MIT Earth Resources Laboratory2017 Annual Founding Members Meeting

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• At  a  specimen  scale,  changes  detected  by  visual  techniques  match  well  to  AE  and  load-­‐displacement  data

• On  a  microstructural  scale,  most  of  the  damage  occurs  prior  to  the  initiation  of  fracture  considered  at  a  specimen  scale• For  a  tensile  stress  field,  microstructural  damage  appears  to  be  mostly  shear

• For  the  beam  bending  stress  condition,  it  appears  that  non  double  couple  hypocenters  are  more  closely  related  in  space  to  the  fracture  than  double  couple  events

• The  process  zone  detected  by  the  visual  techniques  corresponds  well  to  the  majority  of  AE  hypocenter  locations  

• Ongoing  work  on  experiments  with  hydraulic  fracture  boundary  conditions  indicate  that  the  above  observations  also  apply