CN113533047B

CN113533047B - Method for obtaining dynamic tensile stress-strain curve of rock

Info

Publication number: CN113533047B
Application number: CN202110798907.7A
Authority: CN
Inventors: 夏开文; 李星; 陈柄杞; 徐颖
Original assignee: Tianjin University
Current assignee: Tianjin University
Priority date: 2021-07-15
Filing date: 2021-07-15
Publication date: 2022-09-30
Anticipated expiration: 2041-07-15
Also published as: CN113533047A

Abstract

The invention discloses a method for acquiring a dynamic tensile stress-strain curve of a rock, which adopts a method of combining a separated Hopkinson pressure bar system with a digital image processing technology to carry out a dynamic Brazilian disc test. The stress history at the center of the disc is obtained from a rock dynamic test method, and the strain history is estimated using digital image techniques. A rectangular area is selected to surround the center of the disk, and the average tensile strain of the area is used to reflect the deformation of the center of the disk perpendicular to the loading direction. The invention has the advantages that: the method has the advantages of non-contact measurement and high precision; in addition, an area with any size can be selected in the surface of the disc for deformation analysis, so that the method has strong flexibility and is suitable for different rock samples.

Description

Method for obtaining dynamic tensile stress-strain curve of rock

Technical Field

The invention relates to the technical field of dynamic mechanical properties of rocks, in particular to a method for acquiring a dynamic tensile stress-strain curve of the rocks in a dynamic Brazilian disc test.

Background

Dynamic tensile stress-strain behavior can provide rich information for predicting the instability and failure of rock materials in various rock engineering applications. The most intuitive approach is to perform a direct tensile test, but dynamic tensile tests are rarely used for rock materials due to sample preparation difficulties, cumbersome instrument setup, and inevitable errors caused by stress concentration and bending effects around the clamp. In addition, various indirect methods have been proposed to measure the tensile properties of brittle rock materials, with the dynamic Brazilian disc test being considered a fairly effective and convenient method. Unlike the direct tensile test, the dynamic Brazilian disc test does not provide a stress-strain curve directly, requiring the use of a conventional strain gage method to measure deformation. However, this method is limited in that the rock is a heterogeneous material on a microscopic or microscopic scale, and an accurate strain-strain curve requires averaging of the response under approximately constant stress-strain conditions over a suitably sized (neither too large nor too small) area, which is difficult to flexibly address with conventional strain gage methods.

The digital image processing (DIC) technology is used as a non-contact optical measurement test technology and has the advantages of high precision, good adaptability, wide measurement range, high automation degree and the like. The most mature application is the measurement of specimen strain by DIC techniques instead of strain gauges based on the attachment of strain gauges to the specimen. The DIC technology can measure the strain of any area on the surface of the disc, but the tensile strain is not the same near the center of the disc due to the heterogeneity of materials, and a local strain concentration phenomenon exists, so that the strain of the geometric center cannot truly reflect the strain characteristics of the rock. Based on a dynamic Brazilian disc test and combined with a DIC measurement technology, the invention aims to provide a method for obtaining a dynamic Brazilian disc stress-strain curve.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides a method for acquiring a dynamic tensile stress-strain curve of a rock, and solves the defects in the prior art.

In order to realize the purpose, the technical scheme adopted by the invention is as follows:

a method for obtaining a dynamic tensile stress-strain curve of a rock, comprising the following steps:

a, preparing a Brazilian disc sample, and attaching speckles to one side of the Brazilian disc sample; the brazilian disc sample was sandwiched between an incident rod and a transmission rod.

B, adjusting the position of the camera to enable the shooting direction to be vertical to the surface of the disc; the screen needs to exactly and completely display the speckle area of the disc; a transparent baffle is placed between the camera and the sample to protect the camera from damage.

And C, connecting the illumination light source and the high-speed camera with the trigger to ensure that the instrument is in a working state in the whole sample destruction process.

D, starting the Hopkinson bar device to perform a dynamic Brazilian disc test; meanwhile, the stress wave triggers the high-speed camera and the illumination light source to start working, and disk speckle pictures at different moments are obtained.

And E, transmitting the stress waves in the incident rod and the data shot by the high-speed camera to a computer terminal for data processing to obtain a strain cloud picture. And on the basis of the acquired data, the size of the rectangular sensing area RSR is further analyzed and determined, the center of the RSR rectangle needs to be overlapped with the circle center of the Brazilian disc sample, and the edge of the rectangle needs to be perpendicular or parallel to the loading direction.

And F, after the RSR is determined, calculating to obtain the average tensile strain of the RSR, and obtaining a tensile stress-strain curve according to the stress history and the average strain history.

Further, in step E, the width L of RSR _C Determining:

firstly, the central strain of the disc measured by DSG with different lengths in the vertical loading direction is obtained, then a strain-measured length relation image is made, and the length corresponding to the second strain peak point is selected as L _C 。

Length W of RSR _C Determining: firstly, the selected RSR must comprise a crack initiation point generating the maximum strain; the normalized compressive stress varies little only near the central region of the brazilian disc sample, so the length cannot be too large; and the strain of the circle center of the Brazilian disc sample slightly fluctuates, so that the calculation area is properly increased, and the measurement precision is effectively improved.

Further, the maximum strain fluctuation amplitude in the length selected in the step E is lower than 5% of the strain at the circle center; and thirdly, the maximum amplitude of the strain of the circle center of the Zhongbaxi disc sample is 2%, and the maximum value of the selected length is selected under the condition that the length satisfies the second value.

Compared with the prior art, the invention has the advantages that:

when the heterogeneous rock sample is tested, if the theoretical strain value near the central point is not changed greatly, the strain measurement problem of the point is converted into the strain measurement of a region near the point, and the strain of the central point is estimated by using the strain average value of the region, so that the reliability and the accuracy of the test result can be greatly improved, and the method has the advantages of non-contact measurement and high precision; the deformation analysis can be carried out by selecting an area with any size in the surface of the disc, and the method has strong flexibility and is suitable for different rock samples.

Drawings

FIG. 1 is a layout diagram of a virtual DSG according to an embodiment of the present invention;

FIG. 2 is a graph of strain versus DSG length for an embodiment of the present invention;

FIG. 3 is a schematic diagram of the distribution of the stress field of the Brazilian disc according to an embodiment of the present invention;

FIG. 4 is a strain profile for an embodiment of the present invention in the horizontal direction;

FIG. 5 is a schematic diagram illustrating the difference between the measurement result of the strain gage and the measurement result of the DIC under different loading rates according to the embodiment of the present invention.

The labels in the figure are: 1. incident rod 2, transmission rod 3, Digital Strain Gauge (DSG) 4, brazilian disc sample.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings by way of examples.

A method for acquiring a rock dynamic tensile stress-strain curve comprises the following steps:

a: speckles are attached to the surface of the disc, which is an important component for calculating strain by using a DIC measurement technology; it should be noted that the speckles must be flatly attached to the surface of the disc, and air bubbles cannot enter the speckle during the attaching process, otherwise, the small bumps also affect the accuracy of the measurement. Vacuum grease was then applied to both sides of the sample to better adhere it to the middle of the two rods, as shown in figure 1.

B, debugging the position of the camera before the test is carried out, ensuring that the mirror surface is parallel to the plane of the disc, and just clearly observing the whole area of the disc, so as to improve the precision of the test result. The Brazilian disc is in the destruction in-process, can inevitably splash many little rubbles, probably strikes the camera mirror surface, consequently needs to place a transparent baffle between camera and rock sample, and the baffle surface needs to be guaranteed cleanly to avoid influencing the shooting effect.

And C, the high-speed camera and the illumination light source need to be connected with a trigger, and when an experiment is started, the two devices also need to start working, so that the simultaneity is ensured. Since a high-speed camera can continuously take a large number of images in a short time and the sample destruction process is completed in a short time, the equipment and the test must be performed simultaneously to coordinate the whole process for more efficient use of the equipment. The equipment triggering is that stress waves in the incident rod enable the strain gauge on the incident rod to generate current signals, and the current signals are converted through the trigger to trigger the two pieces of equipment, so that the two pieces of equipment can work simultaneously.

And D, pressing a switch, triggering the high-speed camera and the illumination light source to start working when the stress wave hits the position of the incident rod 1 attached with the strain gauge after the bullet flies out, so that the whole process of sample damage can be clearly shot, a plurality of speckle images are shot at the same time interval, and the method is an important data source for calculating the disc surface strain by using the DIC technology.

E: after strain fields at different moments in the disc damage process are obtained, a proper rectangular area needs to be found out for analysis. Because of the desired tensile strain at the center of the disk, the selected area should encompass the center of the disk and then be sized for that purpose. The idea of determining the size is to find the most suitable area so that the most suitable area can effectively reflect the strain of the center; supposing that if the area is too small, the size of rock particles is quite obvious to the test result, and the problem of insufficient sensing area occurs; conversely, if the selected area is too large, the measurement results will be smaller due to the larger difference between the central strain and the distant strain. Based on this, it is important to find a suitable size.

The determination of the width Lc of the rectangular area is to select DSGs with different lengths to measure the deformation at different moments, and then draw a strain-DSG length image (as shown in fig. 2), so that it can be observed that when the DSG is short, the measurement result fluctuates obviously, and there are two peaks, which are caused by rock particles, heterogeneity and other factors, and when the DSG exceeds the second peak, the measurement result of the strain decreases sharply, which is because the brazilian disc test itself has characteristics, which have no relationship with the rock itself, and this can be seen from the theoretical solution of fig. 3. The length corresponding to the second peak point is the best choice.

Determination of the rectangular zone length Wc, again trying to see if Wc is too long or too short, would make the measurement result unable to represent the strain at the center of the disc because its tensile strain (in the vertical loading direction) decreases from the center to the edge; the crack initiation point of the disc which is possibly too short is not in the region, so that the research data has no credibility; from the stress distribution diagram of the Brazilian disc, the stress is approximately equal near the center of the disc along the x-axis direction, and the measurement accuracy is higher the larger Wc is, under the condition that the stress distribution diagram is satisfied. Strain value relation images (as shown in fig. 4) of rectangular areas with different lengths Wc at different time under the condition of determining the width Lc can be made, and an optimal length value is selected according to the strain value relation images, so that the measured values of the central strains of the circular discs are approximately the same.

F: after the rectangular area is determined, the strain history (vertical loading direction Y) of the disc center can be obtained through DIC measurement technology, and a tensile stress-strain image can be made by combining dynamic stress history data.

As shown in fig. 5, it can be seen that the DIC measurements are significantly higher, nearly 2-3 times higher, than the strain gage.

It will be appreciated by those of ordinary skill in the art that the examples described herein are intended to assist the reader in understanding the manner in which the invention is practiced, and it is to be understood that the scope of the invention is not limited to such specifically recited statements and examples. Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.

Claims

1. A method for obtaining a rock dynamic tensile stress-strain curve is characterized by comprising the following steps:

a, preparing a Brazilian disc sample, and attaching speckles to one side of the Brazilian disc sample; clamping a Brazilian disc sample between an incident rod and a transmission rod;

b, adjusting the position of the high-speed camera to enable the sample and the high-speed camera to be at the same horizontal position, wherein the shooting direction is vertical to the surface of the disc; the screen needs to exactly and completely display the speckle area of the disc; a transparent baffle is arranged between the high-speed camera and the sample to protect the high-speed camera from being damaged;

connecting the lighting source and the high-speed camera with a signal processor to ensure that the instrument is triggered and in a working state in the whole process of sample destruction;

d, starting the separated Hopkinson bar device to perform a dynamic Brazilian disc tensile test; simultaneously triggering the high-speed camera and the illumination light source to start working by the stress wave to obtain a plurality of high-definition disc speckle pictures at different moments;

e, transmitting the stress wave in the incident rod and the data shot by the high-speed camera to a computer end for data processing to obtain a strain cloud picture; on the basis of the acquired data, the size of a rectangular sensing area RSR is further analyzed and determined, the center of the RSR rectangle needs to be overlapped with the circle center of the Brazilian disc sample, and the edge of the rectangle needs to be perpendicular or parallel to the loading direction;

width L of the RSR _C Determining:

firstly, the central strain of the disc measured by the digital strain gauges DSG with different lengths in the vertical loading direction is obtained, then a strain-measurement length relation image is made, and the length corresponding to a second strain peak point is selected as L _C ；

Length W of RSR _C Determining: the selected RSR must include a crack initiation point that produces the maximum tensile strain; the normalized compressive stress varies little only in the vicinity of the central region of the brazilian disc sample, so the length cannot be too large; the maximum strain fluctuation amplitude in the selected length is lower than 5% of the strain at the circle center; the maximum amplitude of the strain of the circle center of the Brazilian disc sample is 2%, the maximum value of the selected length is selected under the condition that the length meets the length II, the calculation area is properly increased, and the measurement precision is effectively improved;

and F, after the RSR is determined, calculating the average tensile strain of the RSR to represent the tensile strain of the center of the disc, and obtaining a tensile stress-strain curve according to the stress history and the average strain history.