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A Measuring Method for Crack Initiation Stress of Brittle Materials

A stress measurement, brittle material technology, applied in the direction of measuring device, analyzing material, using stable tension/pressure test material strength, etc., can solve the problem of small number of acoustic emission events, large influence of micro-cracks, and cracks that are not easy to be monitored. and other problems, to achieve the effect of reliable results, convenient operation, and improved authenticity and reliability

Active Publication Date: 2019-10-08
CENT SOUTH UNIV
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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Problems solved by technology

[0004] Under the action of load, the path of crack propagation is greatly random due to the influence of material heterogeneity, and the path and time of crack propagation are difficult to determine, while the direct observation method based on CT, SEM and other observation equipment strongly Because of the subjective judgment of the experimenter, the crack initiation stress of brittle materials is generally not easy to measure; acoustic emission is a low-energy seismic wave caused by inelastic deformation caused by particle dislocation or crack initiation inside the rock, but in soft rock The number of acoustic emission events that can be monitored is so small that it is impossible to determine the crack initiation stress based on acoustic emission, and the acoustic emission activity is greatly affected by the noise during the test process, and sometimes the crack initiation moment is not easy to be monitored; C.D.Martin and N.A.Chandler proposed to use the crack volume strain method to determine the crack initiation stress of the rock, and the formula ε cv =ε v -σ 1 (1-2v) / E can obtain the crack volume strain of the rock cylindrical standard sample under uniaxial compression conditions, and the crack initiation stress of the rock can be determined by drawing the crack volume strain-axial stress curve
This method can accurately determine the crack initiation stress of rock cracks. However, it can be seen from the above formula that the crack volume strain is greatly affected by elastic parameters (elastic modulus E and Poisson's ratio ν), so the accuracy of this method is largely depends on the exact determination of the elastic parameters
E. Eberhardt found that Poisson's ratio ν is greatly affected by microcracks inside the rock sample, so when the rock sample contains a large number of microcracks before the test, this method is no longer applicable

Method used

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  • A Measuring Method for Crack Initiation Stress of Brittle Materials

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Embodiment 1

[0051] See attached figure 1 , 2 , 3, 5, a kind of brittle material initiation stress measuring system of the present invention, described measuring system comprises load-time signal receiving device 6, digital image signal receiving device 7, lighting equipment 5, CCD camera 4, synchronous trigger 3, Loading device 2, sample 1; a circular through hole is prefabricated in the center of the sample 1, and an artificial speckle field is produced by spraying a matte white primer on the surface of the sample 1, and spraying a matte black paint on the surface of the primer; The sample 1 is placed between the loading devices 2 as required; the loading device 2 and the CCD camera 4 are controlled by the synchronous trigger 3 to realize synchronous triggering and real-time measurement; the CCD camera 4 is installed on the axis of the circular hole of the sample, And the camera lens plane is perpendicular to the axis of the through hole, and the lens plane is approximately parallel to ...

Embodiment 2

[0053] Embodiment 2, sample is marble cuboid sample, and size is:

[0054] h×w×t=100mm×60mm×20mm, circular hole radius r=7.5mm;

[0055] A method for measuring crack initiation stress of brittle materials, comprising the steps of:

[0056] Firstly, the marble rock sample is processed into a cuboid sample whose size is h×w×t=100mm×60mm×20mm, and then a circular through hole with radius r=7.5mm is prefabricated in the geometric center of the sample; The surface is made of artificial speckle field with spray paint. First, the surface of the sample is cleaned and then sprayed with white paint evenly. After it dries, black paint is randomly sprayed to form random speckle ( Figure 5 ); then place the speckle sample between the loading devices as required; adjust the lens of the CCD camera on the axis of the circular through hole, and the camera target surface is approximately parallel to the surface of the sample, and the object-to-image ratio is 0.0588mm / pixel; set the shooting...

Embodiment 3

[0063] Using the same marble sample as in Example 2, the radius of the central circular hole is reduced to r=5.0mm, the loading rate is 0.01mm / s, and the sample size and test implementation steps are the same as in Example 2. The measured crack initiation time T=24.90s; the crack initiation stress corresponding to this time is σ ci =55.72MPa, the peak stress of the sample is σ c =85.46MPa; Substituting the data into it, it can be obtained that the crack initiation stress level of this sample is K=65.2%.

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Abstract

The invention discloses a brittle material crack initiation stress measurement system and a brittle material crack initiation stress measurement method. The brittle material crack initiation stress measurement system comprises a loading device, a CCD camera, a synchronizer trigger, a load-time signal receiving device and a digital image signal receiving device. The brittle material crack initiation stress measurement method includes synchronously triggering the loading device and the CCD camera by the synchronizer trigger, loading a sample and outputting a load-time signal; shooting sample surface images by the CCD camera in real time; not stopping shooting until the sample is destroyed; selecting a first frame of images shot by the CCD camera, marking 2 measurement points on the circumference of a through hole, continuously measuring tensile strain-time curves of the 2 measurement points in a whole loading process, and determining stress values, namely crack initiation stress sigmaci,on the stress-time curves corresponding to tensile crack initiation time T. The brittle material crack initiation stress measurement system and the brittle material crack initiation stress measurement method have the advantages that the position and the expansion direction of brittle material crack initiation can be controlled, the crack initiation time and the crack initiation stress can be determined objectively, precisely and quantitatively, authenticity and reliability of brittle material crack initiation stress measurement are improved effectively, and the method simple to operate, reliable in result and objective in quantization is provided for measuring crack initiation stress of rock-like brittle materials with prefabricated holes under uniaxial compression.

Description

[0001] Technical field [0002] The invention relates to a measurement system and method for crack initiation stress of brittle materials, in particular to a DIC technology-based measurement system and method for crack initiation stress of rock-like brittle materials containing prefabricated holes; it belongs to the technical field of fracture damage analysis of brittle materials. Background technique [0003] Crack initiation stress is a basic parameter in fracture mechanics and one of the important indicators for judging the failure symptoms of rock-like brittle materials. Determining the crack initiation stress of brittle materials is of great significance for judging the fracture mechanism of brittle materials under external loads, for reducing the occurrence of safety accidents and geological environmental disasters in mining and tunnel engineering, and improving the effect of rock hydraulic fracturing in oil drilling engineering etc. also have important engineering pract...

Claims

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Application Information

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Patent Type & Authority Patents(China)
IPC IPC(8): G01N3/10G01N3/06
Inventor 李地元朱泉企周亚楠韩震宇邱加冬李夕兵
Owner CENT SOUTH UNIV
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