CN111239253A - Detection method and application of aggregate elastic modulus - Google Patents
Detection method and application of aggregate elastic modulus Download PDFInfo
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- CN111239253A CN111239253A CN202010078681.9A CN202010078681A CN111239253A CN 111239253 A CN111239253 A CN 111239253A CN 202010078681 A CN202010078681 A CN 202010078681A CN 111239253 A CN111239253 A CN 111239253A
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Abstract
The invention discloses a detection method and application of elastic modulus of aggregate. The detection method comprises the following steps: after the volume of the aggregate is measured, pouring the aggregate and the cementing agent, and curing to obtain a test piece; measuring the propagation speed of the ultrasonic wave in the test piece by adopting a pair measuring method, and processing data to obtain the longitudinal wave speed of the ultrasonic wave in the test piece; calculating the elastic modulus, the volume modulus and the shear modulus of the test piece and the volume modulus and the shear modulus of the cementing agent according to the longitudinal wave velocity; calculating the volume modulus and the shear modulus of the aggregate according to a micromechanics formula; and calculating the elastic modulus and the Poisson ratio of the aggregate according to the volume modulus and the shear modulus of the aggregate. The detection method provided by the invention has the advantages of wide application range and high accuracy. The aggregate elastic modulus and the Poisson ratio obtained by detection can be used for calculating the wave velocity of ultrasonic waves in concrete made of the aggregate, predicting the performance of the concrete and improving the accuracy of safety evaluation and durability evaluation of a reinforced concrete structure.
Description
Technical Field
The invention relates to the field of building material detection, in particular to a method for detecting the elasticity modulus of aggregate in a concrete material.
Background
The reinforced concrete structure is the most common and important structure in the existing building structure, wherein the concrete material generally comprises water, coarse aggregate, fine aggregate and cementing material, and the performance of the poured concrete material is greatly different according to different component proportions. Meanwhile, the concrete member usually generates some defects and damages in the construction and use processes, including macroscopic defects such as holes or honeycombs of the concrete, and micro cracks of the concrete due to problems of early non-forced deformation, long-term service under corrosive conditions or freeze-thaw environments, and the like. These defects and damage can reduce the load bearing capacity of the concrete structure and thereby affect the performance properties of the building structure.
Therefore, the measurement of the concrete member performance has great significance. The aggregate is one of important components in the concrete material, the integral number of the aggregate body in the commercial concrete is usually more than 50%, and the performance of the aggregate plays a great control role in the performance of the concrete. However, in actual experiments and use, the design process still lacks accurate elasticity parameters of most aggregates, and the measurement of performance parameters of the aggregate part also lacks corresponding standard standards and accurate and effective methods.
The aggregate types of the concrete are various, the difference of the elastic parameters is large, artificial aggregates (such as lightweight aggregates and recycled aggregates) are added in addition to natural aggregates, and how to predict the concrete parameters through the aggregate parameters is the current hotspot. The elastic modulus of the aggregate influences the early mechanical property, the drying shrinkage property, the creep property and the like of the concrete, and the properties are not related to the elastic parameter of the aggregate in the engineering at present, but the academic community is well known. The aggregate has various types and large difference of elastic parameters, and besides natural aggregate, artificial aggregate (such as lightweight aggregate and recycled aggregate) is also provided, so that how to predict concrete parameters through the aggregate parameters is the current hot point, and the method has great significance for measuring the performance and the damage degree of the concrete under the normal condition.
The important performance parameters of the aggregate are similar to the concrete quality evaluation indexes, wherein the most important are the compressive strength and the elastic modulus. For the determination of strength parameters, the adopted compressive strength test method has clear principle and simple and convenient operation, and has been widely applied. However, the method for measuring the elastic modulus of concrete aggregate is still lacked, and particularly, a good method for measuring the elastic parameter of the aggregate which is already in small particles is not available.
With the development of ultrasonic nondestructive testing technology and the deepening of theoretical research, a method for detecting the strength and the elastic parameters of a concrete member and a constituent material thereof by using the ultrasonic nondestructive testing technology is developed and applied, and Chinese patent application with publication number CN110261485A discloses a method for ultrasonically measuring the elastic modulus and Poisson's ratio of each part in the material. Chinese patent application publication No. CN101163967A discloses an apparatus for determining dynamic elastic modulus of concrete and other materials using ultrasonic waves, a measuring device, and a method for using the apparatus for detection.
However, the method for detecting the strength and elasticity parameters of the concrete member and the constituent materials thereof by using the ultrasonic nondestructive testing technology still has many defects, mainly including:
1. the parameters required by ultrasonic velocity measurement are still mainly determined according to the existing empirical formula at present;
2. the common ultrasonic instrument is difficult to measure granular aggregate to obtain the medium ultrasonic velocity of the aggregate;
3. the elastic parameters of the same aggregate have certain difference due to factors such as the shape, the grain diameter and the like.
Under the condition that the used aggregate elasticity parameters are inaccurate, the performance of the concrete cannot be accurately predicted, so that the method has great significance for obtaining the accurate concrete aggregate elasticity parameters.
Disclosure of Invention
The invention provides a method for detecting the elastic modulus of aggregate, which is characterized in that aggregate and a cementing agent are mixed to prepare a test piece, the longitudinal wave velocity of ultrasonic waves in the test piece is measured, and the elastic parameter of the aggregate is obtained through calculation of a calculation model without the limitation of the shape or the particle size of the aggregate.
The technical scheme provided by the invention for solving the technical problems is as follows:
a method for detecting elastic modulus of aggregate comprises the following steps:
(1) and (3) after the volume of the aggregate is measured, pouring the aggregate and the cementing agent, and curing to obtain the test piece.
The volume of the aggregate was measured by the drainage method.
The volume ratio of the aggregate to the cementing agent is 1: 0.42-0.75.
The volume fraction of the aggregate is the volume of the aggregate divided by the volume of the test piece. The volume fraction of the aggregate is controlled to be 60-70%, when the volume fraction of the aggregate is too large, the molding is difficult, a test piece is difficult to form, and pores are possibly caused by too many bubbles. When the volume fraction is too small, aggregate is difficult to ensure not to deposit at the bottom, the uniformity of aggregate distribution in the test piece is also difficult to ensure, and excessive errors may exist in measured values due to different quantities of aggregate in regions where sound waves pass when the longitudinal wave velocity of the test piece is measured.
The cementing agent is a mixture of resin and a curing agent.
The resin is epoxy resin.
Compared with cement paste, the epoxy resin has the advantages that the elastic modulus and the Poisson ratio of the cementing agent are easier to determine and more uniform.
The epoxy resin is preferably bisphenol A type epoxy resin, and more preferably E-44 type bisphenol A type epoxy resin.
The curing agent is 650 low molecular polyamide or T31 curing agent.
The cementing agent is a mixture of epoxy resin and 650 low-molecular-weight polyimide in a mass ratio of 1: 0.8-1, or a mixture of epoxy resin and T31 curing agent in a mass ratio of 1: 0.2-0.5.
The pouring method comprises the following steps: stirring the aggregate and the cementing agent, filling the aggregate and the cementing agent into a test mold in several times, tamping after filling each time, puncturing air bubbles in the cementing agent, and scraping the top surface.
And a release agent is coated in the test mold.
The release agent is vaseline or paraffin.
The curing conditions are as follows: curing at the indoor temperature of 18-22 ℃ for 20-24 h, demolding, and standing at the indoor temperature of 18-22 ℃ for 2-3 days.
(2) And measuring the propagation speed of the ultrasonic wave in the test piece by adopting a pair measuring method, and processing data to obtain the longitudinal wave speed of the ultrasonic wave in the test piece.
The propagation speed of the ultrasonic wave in the test piece is measured by an ultrasonic wave speed measuring instrument by adopting a pair measuring method.
The step of measuring by the butt-measuring method is to mark a butt-measuring point on the butt-measuring surface of the test piece; the ultrasonic wave speed measuring instrument uses vaseline as a coupling agent, a reflection probe and a receiving probe are arranged at a measuring point, and the propagation speed of ultrasonic waves in a test piece is measured by using a measuring method.
The measurement of the measurement method is carried out at a plurality of measurement points of the test piece, and each measurement point is measured for a plurality of times; and (4) eliminating abnormal values in the measured wave velocity, and taking the average value of the normal data as the longitudinal wave velocity of the ultrasonic wave in the test piece.
(3) And calculating the elastic modulus of the test piece, the volume modulus of the test piece, the shear modulus of the test piece, the volume modulus of the cementing agent and the shear modulus of the cementing agent according to the longitudinal wave velocity of the ultrasonic wave in the test piece.
The elastic modulus of the test piece is obtained by calculating the longitudinal wave velocity, the density and the Poisson ratio of the ultrasonic waves in the test piece, and the calculation formula is as follows:
wherein the content of the first and second substances,
e is the elastic modulus of the test piece;
v is the longitudinal wave velocity of the ultrasonic wave in the test piece;
rho is the density of the test piece;
ν is the poisson ratio of the test piece.
The density of the test piece is the mass of the test piece divided by the volume of the test piece.
When the cementing agent is a mixture of epoxy resin and 650 low-molecular-weight polyamide in a mass ratio of 1: 0.8-1, the Poisson ratio v of the test piece is 0.2-0.4.
When the cementing agent is a mixture of epoxy resin and 650 low-molecular-weight polyamide in a mass ratio of 1: 0.8-1, the change of the material to the Poisson ratio is extremely small, and the change range of the Poisson ratio value of the obtained test piece is not large and is within the range of 0.2-0.4. And in the calculation process, the influence of the Poisson ratio on the accuracy of the calculation result is extremely small.
The Poisson's ratio V of the test piece can be obtained by ultrasonic velocity test or pressure test measurement, and the transverse wave velocity V of the test piece is measured by a counter-measuring method by using an ultrasonic instrumentsThe Poisson ratio V of the test piece is determined by the longitudinal wave velocity V and the transverse wave velocity V of the test piecesAnd calculating according to the following formula:
wherein the content of the first and second substances,
v is the Poisson's ratio of the test piece;
v is the longitudinal wave velocity of the ultrasonic wave in the test piece;
Vsis the transverse wave velocity of the ultrasonic wave in the test piece.
The volume modulus and the shear modulus of the test piece are obtained by calculating the elastic modulus and the Poisson ratio of the test piece, and the calculation formula is as follows:
wherein the content of the first and second substances,
k is the volume modulus of the test piece;
g is the shear modulus of the test piece;
e is the elastic modulus of the test piece;
ν is the poisson ratio of the test piece.
The volume modulus and the shear modulus of the cementing agent are obtained by calculating the elastic modulus and the Poisson ratio of the cementing agent, and the calculation formula is as follows:
wherein:
Kris the bulk modulus of the cement；
GrIs the shear modulus of the cement;
Eris the modulus of elasticity of the cement;
νris the poisson's ratio of the cement.
The elastic modulus and the Poisson ratio of the cementing agent are standard parameters of the material or obtained by an ultrasonic velocity test or a pressure test of the cementing agent.
The ultrasonic velocity test comprises the following steps: the transverse wave velocity V of the solidified and agglomerated cementing agent is obtained by measuring through an ultrasonic instrumentrsVelocity V of sum longitudinal waverThe formula for the calculation of the modulus of elasticity of the cement is as follows:
wherein the content of the first and second substances,
Eris the modulus of elasticity of the cement;
ρrthe density of the cementing agent after solidification;
Vrthe longitudinal wave velocity of the ultrasonic wave in the cementing agent;
Vrsis the transverse wave velocity of the ultrasonic wave in the cementing agent.
The poisson's ratio of the cement is calculated as follows:
wherein the content of the first and second substances,
νris the poisson's ratio of the cementing agent;
Vrthe longitudinal wave velocity of the ultrasonic wave in the cementing agent;
Vrsis the transverse wave velocity of the ultrasonic wave in the cementing agent.
When the cementing agent is a mixture of epoxy resin and 650 low-molecular-weight polyamide with the mass ratio of 1: 0.8-1, according to the standard parameters of the material, E isr22-35 GPa; v isr0.38 to 0.44.
(4) Calculating the bulk modulus and the shear modulus of the aggregate according to the bulk modulus and the shear modulus of the test piece determined in the step (3) and the bulk modulus, the shear modulus and the volume fraction of the cementing agent according to a micromechanics formula;
in the step (4), the calculation formula of the micromechanics formula is as follows:
wherein:
in the formula Fr＝(Gr/6)[(9Kr+8Gr)/(Kr+2Gr)]；
KaIs the bulk modulus of the aggregate;
Gashear modulus for aggregate;
Kris the bulk modulus of the cement;
Gris the shear modulus of the cement;
k is the volume modulus of the test piece;
g is the shear modulus of the test piece;
fais the volume fraction of the aggregate in the test piece;
fris the volume fraction of cement in the test piece.
The volume fraction of the aggregate in the test piece is the volume of the aggregate divided by the volume of the test piece.
The volume fraction of the cementing agent in the test piece is 1-fa。
No air bubbles are needed to control the pouring in the test process, so the volume fraction of the pores is ignored.
(5) And calculating the elastic modulus and the Poisson ratio of the aggregate according to the bulk modulus and the shear modulus of the aggregate.
The calculation formulas of the elastic modulus and the Poisson ratio of the aggregate are as follows:
wherein the content of the first and second substances,
Eais the modulus of elasticity of the aggregate;
νais the poisson's ratio of the aggregate;
Kais the bulk modulus of the aggregate;
Gais the shear modulus of the aggregate.
The Poisson ratio of aggregate used in common concrete is not changed greatly and is 0.1-0.3; the calculated Poisson's ratio of the aggregate is in an empirical value range, so that the method can be used for calculating the elastic modulus of the aggregate more accurately.
After the final measurement is carried out to calculate the poisson ratio of the aggregate, the assumed poisson ratio can be verified or corrected, and the cycle calculation can be carried out.
The poisson ratio of the whole test piece, the elastic modulus of the cementing agent and the poisson ratio of the cementing agent are calculated through an ultrasonic velocity test or a pressure test, and the elastic modulus and the poisson ratio of the aggregate which are calculated at the moment are accurate values, so that the cyclic convergence process is not needed.
The detection method is used for calculating the wave velocity of ultrasonic waves in concrete made of aggregate, can better predict the internal composition and durability of the concrete, and is used for calculating the early mechanical property, the drying shrinkage property and the creep property of the concrete.
The invention has the following beneficial effects:
1. according to the invention, the epoxy resin is used as a cementing material for pouring the test piece, so that the material performance is more stable, and the elastic modulus and the Poisson ratio can be accurately obtained; meanwhile, the pouring forming time is shorter, and the test time is saved.
2. The method can measure the elasticity parameters of the same kind of aggregates with different grades and particle sizes, and can more accurately measure the actual elasticity modulus of the aggregates with specific particle sizes used in the concrete material compared with the prior method.
3. The calculation model used in the elastic parameter calculation is deduced according to the micromechanics theory, the theory is mature, the calculation parameters are clear, the calculation process is clear, and the aggregate elastic parameters obtained through calculation are accurate.
4. By the method for measuring the ultrasonic velocity, the elastic parameter of the aggregate in the concrete material is obtained through calculation of the calculation model, so that the performance of the aggregate is judged more accurately, more accurate parameters are provided for the prediction of the performance of the concrete material, and the accuracy of the evaluation of the safety and the durability of the reinforced concrete structure is improved.
Drawings
FIG. 1 is a flow chart of the method for detecting the elastic modulus of the aggregate according to the invention.
FIG. 2 is a schematic diagram of ultrasonic velocity detection of a test piece; 1 is an epoxy resin and aggregate test piece, and 2 is an ultrasonic sensor.
Detailed Description
The present invention will now be described in detail by way of examples, which are given for the purpose of illustration only and are not intended to limit the scope of the invention. The compounds of the present invention may be prepared by a variety of synthetic methods well known to those skilled in the art, including the specific embodiments listed below, embodiments formed by combinations thereof with other methods of compound synthesis, and equivalents thereof known to those skilled in the art, and may also be commercially available. Preferred embodiments include, but are not limited to, examples of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made in the specific embodiments of the invention without departing from the spirit and scope of the invention.
Example one, the modulus of elasticity of a coarse aggregate and a crushed stone concrete aggregate was measured.
Step (1): and (5) manufacturing a test piece.
The aggregate density required for measuring the modulus of elasticity is about 2620kg/m33 cubic test blocks of 100mm × 100mm × 100mm are expected to be cast, and the volume fraction of the aggregate in the test blocks is controlled to be about 60%. Therefore, part of the aggregate was uniformly taken out from the aggregate pile, 4716g of the aggregate was weighed by an electronic scale, and the mass of the coarse aggregate in each test piece was 1572 g.
Drying the coarse aggregate, measuring the volume of the aggregate by using a drainage method, and calculating to obtain the actual volume of the aggregate in the test piece to be 1.86 multiplied by 10-3m3。
The adhesive is prepared by mixing about 1.8L of epoxy resin (type E-44 of phoenix brand epoxy resin) and 1.8L of curing agent 650 low molecular weight polyamide in a ratio of 1: 1.
Pouring a test piece: fully stirring the prepared cementing agent and the aggregate, and filling the mixture into a test mold for 2 times; each loading is tamped with a tamper and a long needle is used to pierce the air bubbles in the cement and to scrape the top surface. Vaseline is coated in the test mould to be used as a release agent.
And (3) maintaining the test piece: and (3) curing the test piece at the indoor temperature of 20 +/-2 ℃, demolding and taking out after curing for 24 hours, and continuously curing for 3 days in the room to finish the manufacture of the test piece.
Step (2): the wave velocity of the ultrasonic wave in the test piece is measured.
(1) Wave velocity measurement: and taking out the test piece, cleaning surface impurities, wiping water, selecting a measuring surface, marking measuring points on the measuring surface, and uniformly selecting 3 measuring points on the diagonal line of the measuring surface. An ultrasonic wave speed measuring instrument is installed, and vaseline is used as a coupling agent. As shown in the attached figure 2, the ultrasonic velocity of each measuring point of the test piece is measured by the pair measuring method, and each measuring point carries out 5 times of longitudinal wave velocity measurement with 3 measuring points.
(2) And (3) data processing: except the abnormal value in the measured longitudinal wave velocity, the average value of other data is the longitudinal wave velocity measured by the test, and V is 5968.3 m/s.
And (3): and (4) calculating the elastic parameters of the test piece and the cementing agent.
Calculating the elastic modulus: and according to a correlation formula, calculating the integral elastic modulus of the test piece according to the ultrasonic longitudinal wave velocity, the density and the Poisson ratio of the known test piece. The poisson's ratio of the test piece is set to 0.35.
The bulk modulus and shear modulus of the test pieces were:
the bulk and shear moduli of the cement are:
(4) and (3) calculating the elastic parameters of the aggregates: and calculating the bulk modulus and the shear modulus of the aggregate according to a related micromechanics formula and by using the bulk modulus and the shear modulus of the known test piece and the bulk modulus, the shear modulus and the volume fraction of the cementing agent.
Wherein the content of the first and second substances,
(5) calculating the elastic modulus of the aggregate: and calculating the elastic modulus and the Poisson ratio of the aggregate according to the bulk modulus and the shear modulus of the aggregate.
By the method, the elastic modulus E of the measured concrete coarse aggregate is obtained through measurement and calculationa65.8GPa, Poisson's ratio va＝0.30。
According to the method, the invention can obtain more accurate concrete aggregate elastic modulus.
From the aggregate elastic modulus and poisson's ratio obtained as described above, the calculation methods in references 1 to 4 calculate the wave velocity of ultrasonic waves in concrete made of aggregate and calculate and predict the early mechanical properties, drying shrinkage properties, creep properties, and the like of concrete.
Document 1: the Effect of the modulus of elasticity of the Aggregate on the dry Shrinkage of the Concrete was studied in Effect of Aggregate on Drying Shrinkage of Concrete, Journal of advanced Concrete Technology,6(2008) 31-44.
Document 2: a multiscale micromechanics-hydration model for the early-age properties of Cement-based materials, center and Concrete Research,33(2003)1293-1309 proposes a high-precision model for predicting the aging elasticity of Cement-based materials by using aggregate elasticity parameters.
Document 3: a multiscale cross model as a basis for correlation of early Concrete crack behavior, Computers and Concrete,5(2008)295-328 further applied the aggregate elasticity parameter to creep prediction of early Concrete.
Document 4: from micron-sized, near-sized, substrates to meter-sized, near-sized, wet-shaped shells, Acta Geotechnica,3(2008)273 and 294 propose related methods for reasonably predicting the elasticity parameters and strength of shotcrete through the elasticity parameters of aggregates.
Claims (10)
1. The method for detecting the elastic modulus of the aggregate is characterized by comprising the following steps of:
(1) after the volume of the aggregate is measured, pouring and maintaining the aggregate and a cementing agent to obtain a test piece, wherein the cementing agent comprises resin and a curing agent;
(2) measuring the propagation speed of the ultrasonic wave in the test piece by adopting a pair measuring method, and obtaining the longitudinal wave speed of the ultrasonic wave in the test piece after data processing;
(3) calculating the elastic modulus, the volume modulus and the shear modulus of the test piece according to the longitudinal wave velocity obtained in the step (2), and calculating the volume modulus and the shear modulus of the cementing agent;
(4) according to a micromechanics formula, calculating the volume modulus and the shear modulus of the aggregate according to the volume modulus and the shear modulus of the test piece, the volume modulus and the shear modulus of the cementing agent, the volume fraction of the aggregate in the test piece and the volume fraction of the cementing agent in the test piece;
(5) and calculating the elastic modulus and the Poisson ratio of the aggregate according to the bulk modulus and the shear modulus of the aggregate.
2. The detection method according to claim 1, wherein the volume ratio of the aggregate to the cementing agent is 1: 0.42-0.75.
3. The detection method according to claim 1, wherein the cementing agent is a mixture of epoxy resin and 650 low molecular weight polyamide in a mass ratio of 1: 0.8-1, or a mixture of epoxy resin and T31 curing agent in a mass ratio of 1: 0.2-0.5.
4. The method of claim 1, wherein the modulus of elasticity of the test piece is calculated by the formula:
wherein the content of the first and second substances,
e is the elastic modulus of the test piece;
v is the longitudinal wave velocity of the ultrasonic wave in the test piece;
rho is the density of the test piece;
ν is the poisson ratio of the test piece.
5. The detection method according to claim 1, wherein the calculation formula of the bulk modulus and the shear modulus of the test piece is as follows:
wherein the content of the first and second substances,
k is the volume modulus of the test piece;
g is the shear modulus of the test piece;
e is the elastic modulus of the test piece;
ν is the poisson ratio of the test piece.
6. The method of claim 1 wherein the bulk modulus and shear modulus of the cement are calculated as follows:
wherein:
Kris the bulk modulus of the cement;
Gris the shear modulus of the cement;
Eris the modulus of elasticity of the cement;
νris the poisson's ratio of the cement material.
7. The detection method according to any one of claims 1 and 4 to 6, wherein the cementing agent is a mixture of epoxy resin and 650 low-molecular polyamide in a mass ratio of 1: 0.8-1, v is 0.2-0.4, and E isr22-35 GPa; v isr0.38 to 0.44.
8. The detection method according to claim 1, wherein the micromechanics formula is
Wherein:
in the formula Fr＝(Gr/6)[(9Kr+8Gr)/(Kr+2Gr)]；
KaIs the bulk modulus of the aggregate;
Gashear modulus for aggregate;
Kris the bulk modulus of the cement;
Gris the shear modulus of the cement;
k is the volume modulus of the test piece;
g is the shear modulus of the test piece;
fathe volume fraction of the aggregate in the test piece is shown;
fris the volume fraction of cement in the test piece.
9. The detection method according to claim 1, wherein the calculation formulas of the aggregate elastic modulus and the poisson's ratio are as follows:
wherein the content of the first and second substances,
Eais the modulus of elasticity of the aggregate;
νais the poisson's ratio of the aggregate;
Kais the bulk modulus of the aggregate;
Gais the shear modulus of the aggregate.
10. The method of claim 1, wherein the method is used to calculate the wave velocity of ultrasonic waves in concrete made of the aggregate, the early mechanical properties of the concrete, the drying shrinkage properties, or the creep properties.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN111724869A (en) * | 2020-06-17 | 2020-09-29 | 中国铁道科学研究院集团有限公司铁道建筑研究所 | Shrinkage control-based machine-made sandstone aggregate cast-in-place concrete mix proportion design method |
| CN111948295A (en) * | 2020-08-19 | 2020-11-17 | 深圳职业技术学院 | Fault detection system based on sound waves and motor fault detection method |
| CN112697585A (en) * | 2020-12-11 | 2021-04-23 | 山东大学 | Creep test method for cement stone microscopic sample |
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| CN111948295A (en) * | 2020-08-19 | 2020-11-17 | 深圳职业技术学院 | Fault detection system based on sound waves and motor fault detection method |
| CN112697585A (en) * | 2020-12-11 | 2021-04-23 | 山东大学 | Creep test method for cement stone microscopic sample |
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