Assessment of digital image correlation method in determining large scale cemented rockfill strains

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جزئیات بیشتر

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۲۰۲۱

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scopus – master journals – JCR

ایمپکت فاکتور

۴٫۲۷۶ در سال ۲۰۲۰

شاخص H_index

۲۶ در سال ۲۰۲۱

شاخص SJR

۰٫۹۰۱ در سال ۲۰۲۰

شاخص Quartile (چارک)

Q1 در سال ۲۰۲۰

مدل مفهومی

ندارد

پرسشنامه

ندارد

متغیر

ندارد

رفرنس

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Assessment of digital image correlation method in determining large scale cemented rockfill strains


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Assessment of digital image correlation method in determining large scale cemented rockfill strains

 

Abstract

A conventional contact method (using linear transducers) and a  non-contact method are  deployed to measure the  axial and lateral deformations of  large scale cylindrical cemented  rockfill specimens. Experimental works incorporating two pinhole cameras to create one stereovision by  digital image cor- relation shows that the non-contact method is as reliable for testing large cylindrical specimens as mea- surements done by using linear variable displacement transformer and string potentiometer. Considering this particular large specimen, the experiment resulted in the acceptable mean difference between lateral strain using both methods is 5.1  percent, and 14.5 percent for  the axial strain. This  occurrence is inevi- table due to the heterogeneity of the concrete system and the placement of the monitoring point in dig- ital  image correlation method, although the comparison of stress-strain relationship in both methods still indicates a conformity. Based on the results of the present experiments, the authors recommend the non- contact method for  a  detailed investigation of  the material behavior during the uniaxial compressive strength tests. Full  field strain measurement enables this digital method to examine local strains near cracks at any point, a very useful tool for  studying material deformation behavior.

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  1. Introduction

Consolidated  backfilling is  one   type of  underground  mining activity where the crushed rock,  e.g. waste rock  is placed back  to the goaf  with the addition of a binding agent. The purpose is obvi- ous   that the binder such as  cement can   improve the product’s strength  and modify its  deformation behavior, resulting  in  the better underground stability. Cemented Rock  Fill (CRF) is type of consolidated backfill with higher compressive strength compared to other types of backfill. While the underground stability is insep- arable with compression stresses, the accurate measurement  of CRF’s strains caused by  the corresponding stress are  of interest of researchers investigating the CRF properties.

In order to  produce meaningful results during the unconfined compressive strength (UCS) testing of  the CRF material the pre- pared samples have to  be  large enough to  maintain high ratio between the diameter of  the sample and the aggregate size  [۱]. However, measuring the sample deformations using the traditional strain monitoring techniques such as  the linear variable differen- tial   transformer (LVDT) is  usually challenging due  to   the  size restrictions  of  the  most  monitoring equipment. It  is  also   not

recommended to place strain gages on the surface of a CRF sample to measure its overall deformations. As such measurements would give  erroneous results that highly depend on the placement of the strain gauges due to the CRF material’s composition consisting of a stiff  aggregate material and a soft  binding agent.

The   strain measurement methods that  use   strain  gages or LVDTs are  also  restricted due to  internal limitations and external factors, such as gage’s  length limitation or compressometer-extens ometer’s limited ring  size  and temperature. In addition, mounting misalignment is another cause of frequent errors during the defor- mation measurements. The recent sustainable and flexible measur- ing  device working based on  a  digital image correlation (DIC), a non-contact technique for strain measurements, promises to over- come the flaws of the former methods. In this study, the DIC sys- tem from Correlated Solutions’ instruments has  been utilized [2].

DIC generates the displacement data by  comparing the differ- ence in  the position of a point of interest based on  two different images taken from the same position in a global coordinate system. Summarized by  Chu  et al. in  ۱۹۸۵ [۳],  the displacement function and finite strain equations in  digital-correlation work after object projection onto a plane using a linear Taylor’s  expansion:

where exx  is the  lateral strain; eyy  the  axial  strain; u and  v are  the components of an  arbitrary point displacement in  the  lateral and axial  direction respectively.

Since  the object essentially needed to be projected onto a plane, the DIC method was  then mainly focused on how to obtain the true coordinate of  any  kind of  object surface by  using optical instru- ment. In the early 1990s [4,5],  DIC concepts finally advanced into stereovision systems  that  enabled obtaining the  true  three- dimensional (3D)  position of  each point on  a  non-planar object.

۳D-DIC  designates two or  more pinhole cameras to  collect and then compare object region images from two or more viewpoints.

For  in-plane surface deformations, 3D-DIC  is  able   to  extract each component accurately, despite undergoing rigid body transla- tion and rotation [6].  This  final  experiment conducted by  Sutton et al. proved that one  set  of a stereovision system, which consisted of two standard lenses interpreting matching strains (slope of lin-ear  best fit of eyy  is   ۱٫۳۱    ۱۰   ۶ which basically approaches 0) on the out  of plane translation experiment without any  load  applica- tion or deformation.

In 3D-DIC, monitoring is not  limited to tests on flat surfaces but it  has  been used when testing cylindrical specimens. These tests were described by  several researchers who were able   to  obtain good  results [7–۹]. However, it is still  difficult to find  literature on testing large CRF specimens and investigating strain along develop- ing fractures during the UCS tests. The monitoring of fracture devel- opment is another advantage of non-contact DIC method.

In  this paper, conventional fixed   mounted LVDTs and string potentiometers are  reassessed by  the 3D-DIC  method during CRF specimens testing.

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