Microscopic fracture dynamic in-situ characterization method

Abstract

The invention discloses a microscopic crack dynamic in-situ characterization method, and relates to the technical field of crack in-situ detection. According to the method, a plurality of hardened cement paste bodies are soaked in a sulfate solution, the porosity development trend of the hardened cement paste bodies is obtained through a nuclear magnetic resonance test and a mercury injection test in the soaking process, a test sample and a test sample are prepared after soaking is finished, and the crack propagation length of the test sample is measured through a scanning electron microscope. According to the distance gradient between the sampling angle of the test sample and the exposed surface, space coordinates of crack cracking are converted into a time process along the center axis of the test sample, a delay microscope is established on the microscale, and the test sample is scanned by using X-ray computed tomography equipment in a matched mode; a crack propagation animation image is formed based on a crack form between adjacent X-ray tomography slices, and a crack development process is obtained by continuously capturing high-resolution images, so that omission of time information is avoided, and in-situ detection of microscopic cracks in a hardened cement paste sample is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of crack in-situ detection, and particularly relates to a micro-crack dynamic in-situ characterization method. BACKGROUND

[0002] Cracking, as a precursor of structural material failure, is a continuous dynamic process, which is of great significance to elucidate the origin and propagation mechanism of fracture.

[0003] At present, the microstructure characterization methods such as mercury injection, nitrogen adsorption and low-field nuclear magnetic resonance cannot describe the spatial information inside the rock in detail; the visualization methods such as X-CT and scanning electron microscope are limited by the sampling interval, and a large amount of time information is missed in the sampling results. Although the liquid cell-based in-situ transmission electron microscope can observe the atomic scale evolution in real time, this method cannot be applied to long-term (such as more than one month) continuous monitoring, and there is a disadvantage of nanoscale sample preparation leading to a large amount of inherent information loss.

[0004] Therefore, it is a great challenge to experimentally capture the dynamic development of material cracking at high resolution and continuously, and it is urgent to propose a micro-crack dynamic in-situ characterization method to realize the full capture of the material from uncracking to cracking. SUMMARY

[0005] The present application aims to solve the problems of the prior art, and proposes a micro-crack dynamic in-situ characterization method, which achieves the effect of time-lapse microscopy by containing the cracking information of hardened cement paste into the spatial information of micrometer size, continuously captures the high-resolution crack development, and realizes the in-situ detection of the dynamic development of cracks.

[0006] The present application adopts the following technical scheme:

[0007] A micro-crack dynamic in-situ characterization method comprises the following steps:

[0008] Step 1, a plurality of hardened cement paste is prepared, and the hardened cement paste is soaked in a sulfate solution for a preset soaking time, and the porosity curve of the hardened cement paste sample is obtained by nuclear magnetic resonance test and mercury injection test during the soaking process, and the development trend of the porosity of the hardened cement paste sample during the soaking process is determined;

[0009] Step 2, after soaking for a preset soaking time, a test sample and a test sample are prepared by using the hardened cement paste;

[0010] Step 3, the crack propagation length of the test sample is determined by using a scanning electron microscope, the gradient change of the crack cracking degree is determined according to the distance gradient between the sampling angle of the test sample and the exposed surface, and the space coordinates are converted into time progress along the central axis of the test sample in combination with the preset soaking time, and a time-lapse microscope is established on a microscale;

[0011] Step 4, scanning the test sample by using an X-ray computed tomography device, exploring the cracking behavior of the test sample, obtaining a plurality of X-ray tomographic slices of the test sample, and forming a crack propagation animation image based on the crack morphology between adjacent X-ray tomographic slices, thereby achieving in-situ detection of microcracks in the hardened cement paste sample.

[0012] Preferably, in step 1, a plurality of cement paste samples are prepared by mixing water and cement clinker according to a predetermined water-cement ratio, and each of the cement paste samples is cured under a predetermined humidity and a predetermined temperature for a predetermined duration, thereby obtaining a plurality of hardened cement paste samples.

[0013] After the side surface of each hardened cement paste sample is wrapped with epoxy resin, the hardened cement paste samples are divided into three groups, wherein the first group of hardened cement paste samples is used for nuclear magnetic resonance porosity measurement, the second group of hardened cement paste samples is used for mercury intrusion porosity measurement, and the third group of hardened cement paste samples is used for preparing test samples and test samples.

[0014] The bottom surface of each hardened cement paste sample is immersed in a sulfate solution, so that the sulfate solution is immersed into the hardened cement paste sample from the bottom surface of the hardened cement paste sample and diffuses in one dimension in the hardened cement paste sample. During the immersion process, the hardened cement paste samples are sequentially taken out from the first group of hardened cement paste samples at predetermined time intervals for nuclear magnetic resonance testing to measure the porosity of the hardened cement paste samples, thereby obtaining a nuclear magnetic resonance testing porosity curve. Meanwhile, the hardened cement paste samples are sequentially taken out from the second group of hardened cement paste samples at predetermined time intervals for mercury intrusion testing to measure the porosity of the hardened cement paste samples, thereby obtaining a mercury intrusion testing porosity curve, and determining the development trend of the porosity of the hardened cement paste samples during the immersion process.

[0015] Preferably, the chemical composition of the cement clinker includes CaO, SiO2, Al2O3, and Fe2O3, wherein the proportion of CaO is 65%, the proportion of SiO2 is 19%, the proportion of Al2O3 is 5%, and the proportion of Fe2O3 is 5% by weight percentage.

[0016] The water and the cement clinker are mixed according to a water-cement ratio of 0.4 to prepare a cement paste sample with a size of 100mm×100mm×100mm, and the cement paste sample is cured in an environment with a relative humidity of 95% and a temperature of 20±3℃ for 28 days, thereby obtaining a hardened cement paste sample.

[0017] Preferably, according to the hardened cement paste sample porosity curves obtained by the nuclear magnetic resonance testing and the mercury intrusion testing, the development of the porosity of the hardened cement paste sample is divided into two stages, including a first stage and a second stage.

[0018] The first stage is an ion precipitation period, during which salt ions fill pores to cause cracks to initiate in the hardened cement paste sample, thereby forming a crack network.

[0019] The second stage is a cracking stage, at which time the cracks in the hardened cement paste expand.

[0020] Preferably, in the step 2, the test sample and the experimental sample are prepared by using the third group of hardened cement paste, two hardened cement paste samples are obtained by cutting the hardened cement paste from the bottom surface, the sampling positions of the two hardened cement paste samples are spaced apart by no less than 10 mm, the test sample is obtained by vertically sampling and polishing one of the hardened cement paste samples, and the experimental sample is obtained by obliquely sampling and polishing the other hardened cement paste sample.

[0021] The test sample and the experimental sample are cylindrical samples with the same size, the diameter of the bottom surface of the cylindrical sample is set according to the accuracy of the X-ray tomography, the preset included angle between the central axis of the experimental sample and the horizontal plane is θ, and the vertical distance between the experimental sample and the exposed surface is d.

[0022] Preferably, in the step 3, when the sampling angle of the experimental sample is greater than 45°, the distance gradient between the experimental sample and the exposed surface is determined in combination with the crack propagation length as follows:

[0023]

[0024] In the formula, d is the distance gradient between the experimental sample and the exposed surface, H is the crack propagation length of the test sample, L is the length of the experimental sample, and θ is the sampling angle of the experimental sample.

[0025] When the sampling angle of the experimental sample is not greater than 45°, the distance gradient between the experimental sample and the exposed surface is determined in combination with the crack propagation length as follows:

[0026]

[0027] According to the preset soaking time and the length of the experimental sample, the spatial coordinates are converted into a time process, and a time-lapse microscope is established at a microscale.

[0028] Preferably, in the step 4, the voltage of the X-ray computed tomography device is set to 40 kV, the current is set to 74 μA, the exposure time is set to 2 s, the device accuracy is 0.961 μm, and the distance between the experimental sample and the detector of the X-ray computed tomography device is 41.058 mm.

[0029] A plurality of X-ray tomography slices are obtained by continuously scanning the X-ray computed tomography device, the effective pixel size of the X-ray tomography slice is 0.961 x 0.961 μm 2 , and the black area in the X-ray tomography slice is a crack.

[0030] The microstructure information of hardened cement paste, including cement particles, internal hydrates and external hydrates, is obtained by X-ray tomography slices.

[0031] The present application has the following beneficial effects:

[0032] The microcrack dynamic in-situ characterization method provided by the present application has the following advantages: the bottom surface of the hardened cement paste sample is soaked in the sulfate solution, so that the sulfate solution is transported upward along the pores of the hardened cement paste sample, and after the gradient change of cracking degree is generated at different spatial positions in the interior of the hardened cement paste sample, the gradient change of the internal cracks of the hardened cement paste sample is obtained by using the X-ray computed tomography equipment, the cracking information in the material interior over a long period of time is converted into the spatial information of micron size, the whole process from uncracking to cracking of the material is captured, the effect of a delay microscope is achieved, a large amount of time information is avoided to be missed in the long-time continuous monitoring process, and the method has important significance for studying the origin and expansion mechanism of cracking. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The figure is the porosity development trend curve of the hardened cement paste.

[0034] Figure 2 The figure is a schematic diagram for preparing the sample; in the figure, (a) is a schematic diagram for extracting the test sample and the experimental sample in the hardened cement paste, and (b) is a sectional view of the test sample and the experimental sample.

[0035] Figure 3 The figure is a schematic diagram when the sampling angle of the experimental sample is greater than 45°.

[0036] Figure 4 The figure is a schematic diagram when the sampling angle of the experimental sample is not greater than 45°.

[0037] Figure 5 The figure is an X-ray tomography slice of the experimental sample; in the figure, (a) is a microstructure cracking image of the experimental sample soaked for 30 days, (b) is a microstructure cracking image of the experimental sample soaked for 33 days, (c) is a microstructure cracking image of the experimental sample soaked for 36 days, (d) is a microstructure cracking image of the experimental sample soaked for 39 days, (e) is a microstructure cracking image of the experimental sample soaked for 42 days, and (f) is a microstructure cracking image of the experimental sample soaked for 43 days.

[0038] In the figure, cs is the test sample, es is the experimental sample, H is the crack propagation length of the test sample, L is the length of the experimental sample, θ is the sampling angle of the experimental sample, and d is the distance gradient between the experimental sample and the exposed surface. DETAILED DESCRIPTION

[0039] The present application will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] A micro-crack dynamic in-situ characterization method is proposed in the embodiment to in-situ detect the dynamic development of cracks, specifically including the following steps:

[0041] Step 1, multiple hardened cement pastes are prepared, and the hardened cement pastes are soaked in a sulfate solution for a preset soaking time. During the soaking process, the porosity curve of the hardened cement paste sample is obtained through nuclear magnetic resonance test and mercury injection test to determine the development trend of the porosity of the hardened cement paste sample during the soaking process.

[0042] In the embodiment, cement clinker with chemical components including CaO, SiO2, Al2O3 and Fe2O3 is mixed with water at a water-cement ratio of 0.4 to prepare multiple cement pastes with a size of 100mmx100mmx100mm, wherein the proportion of CaO is 65%, the proportion of SiO2 is 19%, the proportion of Al2O3 is 5%, and the proportion of Fe2O3 is 5% by weight percentage. Each cement paste is cured for 28 days in an environment with a relative humidity of 95% and a temperature of 20±3℃ to obtain multiple hardened cement pastes.

[0043] After the side surface of each hardened cement paste is wrapped with epoxy resin, the hardened cement pastes are divided into three groups. The multiple hardened cement pastes in the first group are used for nuclear magnetic resonance porosity measurement, the multiple hardened cement pastes in the second group are used for mercury injection test porosity measurement, and the hardened cement pastes in the third group are used for preparing test samples and test samples.

[0044] The bottom surface of each hardened cement paste is soaked in a sulfate solution with a concentration of 1mol / L for 43 days, so that the sulfate solution is immersed into the hardened cement paste from the bottom surface of the hardened cement paste and diffuses in one dimension in the hardened cement paste. During the soaking process, a hardened cement paste is taken out from the first group of hardened cement pastes every 5 days for nuclear magnetic resonance test. In the embodiment, the constant magnetic field is set to 0.3T, the coil is 25mm, the working frequency is 12MHz, the porosity of the hardened cement paste is measured, and the nuclear magnetic resonance test porosity curve is obtained.

[0045] Meanwhile, a hardened cement paste is taken out from the second group of hardened cement pastes every 5 days during the soaking process for mercury injection test. The porosity of the hardened cement paste is measured by the mercury injection test to obtain the mercury injection test porosity curve and determine the development trend of the porosity of the hardened cement paste during the soaking process, as shown in Figure 1 .

[0046] Based on the development trend of the porosity of the hardened cement paste during the 43-day soaking process, the development of the porosity of the hardened cement paste sample is divided into two stages, including a first stage and a second stage, wherein the first stage is an ion precipitation period, at which time salt ions fill pores to cause crack initiation in the hardened cement paste, and finally to a crack network; the second stage is a cracking period, at which time the cracks in the hardened cement paste expand.

[0047] Step 2, after soaking to a preset soaking duration, test samples cs and experimental samples es are prepared from the hardened cement paste.

[0048] In this embodiment, test samples and experimental samples are prepared from the hardened cement paste in the third group, and two hardened cement paste samples are obtained by cutting the hardened cement paste from the bottom surface. In order to reduce the interference of the cutting and sampling process on the microstructure of the hardened cement paste, the sampling position interval of the two hardened cement paste samples is ensured to be not less than 10 mm. Further, in order to avoid the influence of cutting high temperature, the cross-sectional side length of the hardened cement paste sample is ensured to be greater than 1 mm and rectangular during sampling and cutting, so as to avoid the influence of cutting high temperature.

[0049] For the two hardened cement paste samples, one of the hardened cement paste samples is vertically sampled and polished into a cylindrical test sample, and the other hardened cement paste sample is obliquely sampled and polished into a cylindrical experimental sample.

[0050] The test sample and the experimental sample are cylindrical samples with the same size, as shown in Figure 2 As shown, the bottom surface diameter of the cylindrical sample is set according to the accuracy of the X-ray tomography, and in this embodiment, the bottom surface diameter of the cylindrical sample is 1 mm. The preset included angle between the central axis of the experimental sample and the horizontal plane is θ, and the vertical distance between the experimental sample and the bottom surface of the hardened cement paste is d.

[0051] Step 3, the crack propagation length of the test sample is determined by using a scanning electron microscope.

[0052] Based on the backscattered electron image of the scanning electron microscope, the crack propagation length of the test sample is determined by using the scanning electron microscope. Since the backscattered electron image is based on the elastic scattering of high-energy electron beams with sample atomic nuclei, the signal strength is positively correlated with the atomic number of the sample, and the element distribution in the test sample can be visualized and analyzed. In the visual image obtained by the scanning electron microscope, the crack area appears black, so the crack propagation length in the test sample can be determined.

[0053] According to the distance gradient between the sampling angle of the experimental sample and the exposed surface, the gradient change of the crack opening degree is determined, so as to convert the spatial coordinates along the central axis of the experimental sample into a 43-day time process, thereby establishing a time-lapse microscope on a microscale.

[0054] In this embodiment, when the sampling angle of the test sample is greater than 45°, as shown in Figure 3 According to the total immersion time of 43 days, combined with the observation time x, c is determined, and further according to the relationship in the right triangle, θ is obtained According to the relationship in the right triangle, θ and b are determined, combined with the crack propagation length, the distance gradient between the test sample and the exposed surface is Combined with the crack propagation length, the distance gradient between the test sample and the exposed surface is

[0055]

[0056] In the formula, d is the distance gradient between the test sample and the exposed surface; H is the crack propagation length of the test sample; L is the length of the test sample; and θ is the sampling angle of the test sample.

[0057] When the sampling angle of the test sample is not greater than 45°, as shown in Figure 4 According to the total immersion time of 43 days, combined with the observation time x, c is determined, and further according to the relationship in the right triangle, θ and b are obtained, combined with the crack propagation length, the distance gradient between the test sample and the exposed surface is

[0058]

[0059] In the formula, d is the distance gradient between the test sample and the exposed surface; H is the crack propagation length of the test sample; L is the length of the test sample; and θ is the sampling angle of the test sample.

[0060] According to the preset immersion time and the length of the test sample, the space coordinates are converted into time progress along the equal scale interval of the central axis of the test sample, and the time-lapse microscope is established on the microscale.

[0061] Step 4, the test sample is scanned by using the X-ray computed tomography equipment, the cracking behavior of the test sample is explored, 1000 X-ray tomography slices are obtained, the black area in the X-ray tomography slice is a crack, and the effective pixel size of the X-ray tomography slice is 0.961*0.961 μm 2 , which is small enough for the micro information of hardened cement paste (cement particles, internal hydration products, and external hydration products). At this time, based on the crack morphology between adjacent X-ray tomography slices, a crack propagation animation image is formed, which is equivalent to in-situ detection of micro cracks in the hardened cement paste sample. That is, the method reduces the distance between adjacent slices in X-ray tomography to a very small range, forms a continuous crack propagation animation image between adjacent 2D X-ray tomography slices, and realizes in-situ detection of micro cracks in the hardened cement paste sample.

[0062] In this embodiment, the voltage of the X-ray computed tomography device is set to 40 kV, the current is set to 74 μA, the exposure time is set to 2 s, the device accuracy is 0.961 μm, and the distance between the test sample and the detector of the X-ray computed tomography device is 41.058 mm.

[0063] The length L of the test sample is 0.5 cm, the distance gradient d between the test sample and the exposed surface is 0.5 mm, the sampling angle θ of the test sample is 45°, and the cracks generated by the test sample are converted into a time process of 43 days along the central axis of the test sample, i.e., the crack development process is converted into a time display. 2 The effective pixel size of the X-ray tomographic slice is 0.961 x 0.961 μm

[0064] In this embodiment, the length L of the test sample is 0.5 cm, the distance gradient d between the test sample and the exposed surface is 0.5 mm, the sampling angle θ of the test sample is 45°, and the cracks generated by the test sample are converted into a time process of 43 days along the central axis of the test sample, i.e., the crack development process is converted into a time display. Figure 5 The X-ray tomographic slices of the test sample are shown in Figs. (a) and (f), Figure 5 There are 1000 X-ray tomographic slices between Figs. (a) and (f), and the distance between each X-ray tomographic slice is 0.961 μm, which is controlled by the accuracy of the X-CT device, and the distance between each X-ray tomographic slice is the same, which is 192.2 μm. Figure 5 (a), Figure 5 (f) are closer to the surface of the sulfate solution, and by observing the X-ray tomographic slices at different times, the dynamic process of crack propagation can be understood, and in-situ detection of microcracks in the hardened cement paste sample can be achieved.

[0065] Of course, the above description is not a limitation on the present application, and the present application is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present application should also be within the scope of the present application.

Claims

1. A method for dynamic in-situ characterization of microcracks, characterized in that, The method comprises the following steps: Step 1, preparing hardened cement paste blocks, and immersing the hardened cement paste blocks in a sulfate solution for a preset immersion time, and obtaining the porosity curve of the hardened cement paste block sample during the immersion process through nuclear magnetic resonance test and mercury injection test, and determining the development trend of the porosity of the hardened cement paste block sample during the immersion process; Step 2, after the immersion for the preset immersion time, preparing a test sample and a test sample from the hardened cement paste; Step 3, determining the crack propagation length of the test sample by using a scanning electron microscope, determining the gradient change of the crack cracking degree according to the distance gradient between the sampling angle of the test sample and the exposed surface, and converting the spatial coordinates into a time process along the central axis of the test sample in combination with the preset immersion time, and establishing a time-lapse microscope on a micro scale; Step 4, scanning the test sample by using an X-ray computed tomography device, exploring the cracking behavior of the test sample, obtaining a plurality of X-ray tomographic slices of the test sample, and forming a crack propagation animation image based on the crack morphology between adjacent X-ray tomographic slices, and realizing in-situ detection of the micro cracks in the hardened cement paste sample.

2. The method for dynamic in-situ characterization of microcracks according to claim 1, characterized in that, In the step 1, the water and the cement clinker are mixed according to a preset water-cement ratio to prepare a plurality of cement paste blocks, and each cement paste block is cured at a preset humidity and a preset temperature for a preset time to obtain a plurality of hardened cement paste blocks; Each hardened cement paste block is wrapped with an epoxy resin on the side surface, and then divided into three groups, wherein the hardened cement paste blocks in the first group are used for nuclear magnetic resonance porosity measurement, the hardened cement paste blocks in the second group are used for mercury injection test porosity measurement, and the hardened cement paste blocks in the third group are used for preparing a test sample and a test sample; The bottom surface of each hardened cement paste block is immersed in a sulfate solution, so that the sulfate solution is immersed into the hardened cement paste block from the bottom surface of the hardened cement paste block and diffuses in one dimension in the hardened cement paste block; during the immersion process, the hardened cement paste blocks are taken out from the first group of hardened cement paste blocks at preset time intervals for nuclear magnetic resonance test, the porosity of the hardened cement paste block is measured, and the nuclear magnetic resonance test porosity curve is obtained; at the same time, the hardened cement paste blocks are taken out from the second group of hardened cement paste blocks at preset time intervals for mercury injection test, the porosity of the hardened cement paste block is measured, and the mercury injection test porosity curve is obtained, so as to determine the development trend of the porosity of the hardened cement paste block during the immersion process.

3. The method for dynamic in-situ characterization of microcracks according to claim 2, characterized in that, The chemical composition of the cement clinker comprises CaO, SiO2, Al2O3 and Fe2O3, and the proportion of CaO is 65%, the proportion of SiO2 is 19%, the proportion of Al2O3 is 5%, and the proportion of Fe2O3 is 5% by weight percentage; The water and the cement clinker are mixed according to a water-cement ratio of 0.4 to prepare a cement paste block with a size of 100mm*100mm*100mm, and the cement paste block is cured in an environment with a relative humidity of 95% and a temperature of 20±3℃ for 28 days to prepare a hardened cement paste block.

4. The method of dynamic in-situ characterization of microcracks according to claim 2, characterized in that, According to the porosity curve of the hardened cement paste block sample obtained by the nuclear magnetic resonance test and the mercury injection test, the porosity development of the hardened cement paste block sample is divided into two stages, including a first stage and a second stage; The first stage is an ion precipitation stage, at which salt ions fill pores to cause cracks to initiate in the hardened cement paste, forming a crack network; The second stage is a cracking stage, at which cracks in the hardened cement paste expand.

5. The method of dynamic in-situ characterization of microcracks according to claim 1, characterized in that, In the step 2, the third group of hardened cement paste is used to prepare a test sample and a test sample, two hardened cement paste samples are obtained by cutting the hardened cement paste from the bottom surface, the sampling positions of the two hardened cement paste samples are not less than 10 mm apart, one of the two hardened cement paste samples is vertically sampled and polished to obtain the test sample, and the other hardened cement paste sample is obliquely sampled and polished to obtain the test sample; The test sample and the test sample are cylindrical samples with the same size, the diameter of the bottom surface of the cylindrical sample is set according to the accuracy of the X-ray tomography, the preset angle between the central axis of the test sample and the horizontal plane is θ, and the vertical distance between the test sample and the bottom surface of the hardened cement paste is d.

6. The method of dynamic in-situ characterization of microcracks according to claim 1, wherein, In the step 3, when the sampling angle of the test sample is greater than 45°, the distance gradient between the test sample and the exposed surface is determined in combination with the crack propagation length as follows: In the formula, d is the distance gradient between the test sample and the exposed surface; H is the crack propagation length of the test sample; L is the length of the test sample; and θ is the sampling angle of the test sample. When the sampling angle of the test sample is not greater than 45°, the distance gradient between the test sample and the exposed surface is determined in combination with the crack propagation length as follows: According to the preset soaking time and the length of the test sample, the space coordinates are converted into time processes, and a time-lapse microscope is established on a microscale.

7. The method of dynamic in-situ characterization of microcracks of claim 1, wherein, In the step 4, the voltage of the X-ray computed tomography equipment is set to 40 kV, the current is set to 74 μA, the exposure time is set to 2 s, the equipment accuracy is 0.961 μm, and the distance between the test sample and the detector of the X-ray computed tomography equipment is 41.058 mm. A plurality of X-ray tomography slices are obtained by continuous scanning using an X-ray computed tomography device, an effective pixel size of the X-ray tomography slices being 0.961*0.961 μm 2 , and a black region in the X-ray tomography slice being a crack. The X-ray tomography slice is used to obtain the micro information of the hardened cement paste, including cement particles, internal hydration products and external hydration products.

Images