CN104034601A - Method for accurately determining high-temperature mechanical property parameters of heat preventing material based on digital image related technology - Google Patents

Abstract

The invention discloses a method for accurately determining high-temperature mechanical performance parameters of heat-resistant materials by using digital image correlation technology, which belongs to the technical field of test-determined material parameters. The invention obtains the deformation of the material during the test process based on the digital image correlation technology, and combines the load and temperature data provided by the high-temperature mechanical testing machine to determine the mechanical performance parameters of the material in vacuum (inflated) and at a specific temperature. The invention uses blue light to supplement light, adds a blue light filter in front of the DIC optical measurement lens, and bakes the speckle splashed on the sample to meet the correct use of DIC at high temperature. The invention is easy to operate and has strong repeatability. It can not only simulate the thermal shock of the material, but also more accurately and comprehensively reflect the mechanical response of the heat-resistant material under high temperature conditions, which can be directly reflected in the test that could not be observed and measured before The overall deformation and lateral deformation coefficient of the material can make the characterization of material performance parameters more realistic and effective.

Description

A Method for Accurately Determining High Temperature Mechanical Properties Parameters of Heat-resistant Materials Using Digital Image Correlation Technology

technical field

The invention belongs to the technical field of test and measurement of material parameters, and relates to a test method for the test parameters of material mechanical properties of heat-resistant materials under high temperature conditions.

Background technique

Heat-resistant materials such as ultra-high temperature ceramics and C/C composite materials are widely used in national defense, aerospace, modern space technology and other fields to meet their needs for heat insulation systems under high temperature conditions. Under high temperature conditions, not only certain thermal properties need to be met, but also higher requirements are placed on the mechanical properties of materials. This is due to the short time when the material reaches high temperature from normal temperature, which will cause a strong thermal shock to the material and generate thermal stress. Therefore, effectively simulating this thermal shock and characterizing the mechanical properties of materials at high temperatures has become a key step in the application of heat-resistant materials.

The high-temperature mechanical property testing system adopts rapid power-on heating technology to simulate the material from normal temperature to high temperature under vacuum (inflated) conditions, and uses an electronic universal testing machine for mechanical property testing to characterize the mechanical properties of the material under high-temperature conditions. The demand for data on mechanical properties of materials in the design and research of supersonic vehicles and thermal protection systems. In the past, the mechanical performance data of materials were obtained by performing high-temperature tensile, compression and bending tests in high-temperature, vacuum or air-filled environments, and using high-temperature extensometers (tension and compression) and high-temperature differential transformers (bending) to obtain deformation data to complete. However, the installation of the high-temperature extensometer and the high-temperature-resistant differential transformer is more complicated, and the accuracy is poor. During the measurement process, the indenter needs to press the sample tightly to meet the needs of deformation together with the material, which undoubtedly has a certain impact on the deformation of the material, and the lateral deformation coefficient (Poisson's ratio) of the material cannot be obtained. The combined effects of factors make it difficult for existing technologies to characterize the mechanical properties of materials.

The optical non-contact deformation measurement system uses digital image correlation (DIC) technology to track the speckle image on the surface of the object, and then compare the deformed speckle with the reference speckle before deformation, which is equivalent to pasting a A number of virtual strain gauges are installed to realize the measurement of the three-dimensional coordinates, displacement and strain of the object surface during the deformation process. It has the characteristics of portability, high speed, high precision and easy operation. However, under high temperature conditions, the material itself will heat up, glow, and produce red light, which will have a great impact on speckle observation, and may even fail to observe speckle. More importantly, speckles will gradually fall off the surface of the sample under high temperature conditions, making it impossible to obtain relevant data.

Contents of the invention

Aiming at the test and characterization of the mechanical properties of heat-resistant materials under high-temperature and vacuum (inflated) conditions, the present invention proposes a method for accurately measuring parameters, that is, using digital image correlation technology to accurately determine the high-temperature mechanical performance parameters of heat-resistant materials Methods.

The purpose of the present invention is achieved through the following technical solutions:

A method for accurately determining high-temperature mechanical performance parameters of a heat-resistant material by using digital image correlation technology, comprising the following steps:

1. The white powder with strong adhesion at high temperature and easy to disperse at room temperature, such as aluminum-based ceramic powder, can be mixed with water in a mass ratio of 2.5~4.5:1, and the two are mixed and fully stirred to form a suspension , and then use a brush to evenly spray it on the surface within the gauge length section of the sample, while ensuring that the white powder spots on the surface are clearly visible and roughly evenly distributed, so as to form speckles with different characteristics such as shapes and sizes;

2. Bake the sample with speckles in a constant temperature box at 80-120°C for 18-36 hours to volatilize the moisture in the powder, and at the same time, the adhesion between the speckle and the sample will change after the constant temperature baking treatment. Stronger, the speckle is not easy to fall off in the test;

3. Set up two DIC digital optical cameras near the external observation window of the vacuum furnace of the high-temperature mechanical testing machine. Add a blue light filter in front of the optical camera lens. After the test preparation is completed, use a 3cm×3cm white board with black spots randomly distributed to debug and initialize the digital optical measurement system of the DIC digital optical camera, adjust the aperture and exposure of the lens, so that The black dot image on the whiteboard captured by the camera can still be clearly identified under the highest exposure frequency of the DIC digital optical camera;

4. Conduct high-temperature tensile, compression or three-point bending tests on heat-resistant materials at a temperature between 1000°C and below 2000°C on a high-temperature mechanical testing machine. The spot is photographed and collected in real time until the sample is destroyed;

5. DIC will obtain the full-field coordinates, displacement, and strain data in the gauge length section of the material, and combine them with the load, temperature and other data obtained by the mechanical testing machine. High temperature mechanical properties parameters.

The invention obtains the deformation of the material during the test process based on the digital image correlation technology, and combines the load and temperature data provided by the high-temperature mechanical testing machine to determine the mechanical performance parameters of the material in vacuum (inflated) and at a specific temperature. Compared with the prior art, the present invention has the following advantages:

1. The present invention uses blue light for supplementary light, adds a blue light filter in front of the DIC optical measurement lens, and bakes the speckle splashed on the sample to meet the correct use of DIC at high temperature.

2. The invention is easy to operate and highly repeatable. It can not only simulate the thermal shock of the material, but also more accurately and comprehensively reflect the mechanical response of the heat-resistant material under high temperature conditions. The observed and measured overall deformation and lateral deformation coefficient of the material can make the characterization of material performance parameters more realistic and effective.

Description of drawings

Figure 1 is a speckle pattern sputtered onto the sample surface;

Figure 2 is a schematic diagram of the measurement principle of DIC technology;

Figure 3 is the cloud diagram of the displacement of the sample at a certain time during the stretching process;

Fig. 4 is the strain nephogram of the sample surface at a certain time during the stretching process;

Fig. 5 is a cloud diagram of Poisson's ratio on the surface of the sample at a certain time during the stretching process.

Detailed ways

The technical solution of the present invention will be further described below in conjunction with the accompanying drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered by the present invention. within the scope of protection.

Take the high-temperature tensile test of graphite materials in vacuum and at 1200°C as an example to illustrate:

1. Set up two DIC optical lenses near the observation window outside the vacuum furnace of the high-temperature mechanical testing machine, respectively install blue filters in front of the two lenses, and install two blue supplementary light sources at the windows. The purpose is because at high temperature, the sample will be red and shiny, but the ceramic powder on the surface of the sample will not shine, so the powder spots will change from white spots at normal temperature to black spots at high temperature, in order to make the surface of the sample It can better distinguish feature points, perform blue light supplement, and install a blue filter in front of the lens;

2. After processing the heat-resistant material to the size and size requirements required for the high-temperature mechanical performance test, prepare a mixture of aluminum-based ceramic powder and water according to the ratio of 3:1 according to the temperature requirements required for the experiment, and fully stir it to Form a suspension, and then use a brush to spray it on the surface of the sample to ensure that the mixture is evenly distributed within the gauge length of the sample, so that the mixture is discretely distributed on the surface of the sample, and the spots can be clearly distinguished. Then put the sample sputtered with the mixture into an incubator for 24 hours of baking at 200F (93.3°C) to ensure that the moisture in the mixture is fully volatilized and the spots are fully attached on the surface of the sample. The sample after constant temperature treatment is shown in Figure 1;

3. Prepare the ultra-high temperature mechanical property test system and DIC equipment to the test state, and use the standard board to calibrate the DIC equipment. Install the sample on the chuck of the testing machine, and install a high-temperature extensometer in the gauge length section of the sample for comparison with the contact deformation system;

4. Close the front and rear doors of the vacuum furnace, draw a vacuum in the vacuum furnace to make the vacuum in the furnace about 20Pa, then set the heating program, such as the final temperature, heating rate, etc., turn on the main electrode power supply, and energize the sample heating;

5. After the temperature of the sample reaches the set value and stabilizes, the tensile test is carried out on the sample at a rate of 0.5mm/min. At the same time, two high-speed cameras are used to monitor the speckle on the surface of the sample in real time at a frequency of 1Hz. Take photos and collect until the end of tensile failure of the sample;

6. Turn off the main electrode power supply and the mechanical pump power supply, take out the sample, and observe the damage form of the sample. The ideal state is that the sample is damaged at a certain position in the gauge length section. Then use digital image correlation method, such as Zhang calibration method (Zhang Zhengyou. A flexible new technology for camera calibration. IEEE Transactions on Pattern Analysis and Machine Intelligence. 2000.22 (11): 1330-1334) to realize the data collected by DIC equipment Match the speckle with the deformation point on the surface of the sample, reconstruct the object surface according to the parallax data of each point and the camera parameters obtained in advance to calculate the three-dimensional coordinates of the points, and compare the changes of the three-dimensional coordinates of each point in the measurement area by comparing each deformation state The displacement field of the object surface is obtained, and the strain field of the object surface is obtained through further calculation, the schematic diagram of which is shown in Figure 2;

7. According to the principle of step 6, obtain the displacement nephogram of the sample at a certain time during the stretching process, as shown in Figure 3, since the lower end of the sample and the lower chuck remain in place, the upper end of the sample and the upper chuck are kept together. Movement, the material is uniformly deformed, so the displacement changes linearly from the lower end to the upper end along the stretching direction, and the color on the cloud map gradually becomes darker from the lower end to the upper end along the stretching direction. The actual displacements are also in good agreement, showing that the DIC technique is quite reliable. Then obtain the strain nephogram on the surface of the sample, as shown in Figure 4, and at the same time obtain the distribution of the transverse deformation coefficient (Poisson's ratio) that cannot be observed by previous techniques during the stretching process, as shown in Figure 5;

8. Repeat the operation of step 7 to obtain the displacement, strain, and Poisson's ratio distribution and data of the material at intervals during the entire stretching process, and then combine the load data of the mechanical testing machine to divide the load by the transverse distance of the gauge length section The stress of the cross-sectional area is obtained, and the stress-strain relationship curve is obtained, and the slope of the linear section is taken as the modulus of the material at a certain ultra-high temperature. If the final damage of the material occurs in the gauge section, the maximum load is divided by the gauge length The strength of the material at a certain ultra-high temperature can be obtained from the cross-sectional area of the section, and the Poisson's ratio can be obtained directly from the Poisson's ratio cloud map;

9. Finally, organize the data and pictures obtained in the above 8 steps to obtain the mechanical performance parameters such as modulus, strength, and Poisson's ratio of the heat-resistant material at a certain temperature, and at the same time obtain the displacement of the material during the test, Variation laws of strain, Poisson, etc.

Claims (7)

1. A method for accurately determining the high-temperature mechanical performance parameters of heat-resistant materials using digital image correlation technology, characterized in that the method steps are as follows: 1. Mix the white powder, which has strong adhesion at high temperature and is easy to disperse at room temperature, with water and fully stir to form a suspension, and then evenly spray it on the surface within the gauge length section of the sample to form speckles; 2. Bake the sample with speckle to volatilize the moisture in the powder; 3. Set up two DIC digital optical cameras connected to a computer for digital image acquisition and processing near the external observation window of the vacuum furnace of the high-temperature mechanical testing machine, and use a blue light source to supplement the light inside the vacuum furnace. Add a blue light filter in front of the camera lens. After the test preparation is completed, debug and initialize the digital optical measurement system of the DIC digital optical camera, adjust the aperture and exposure of the lens, so that the black dot image on the whiteboard captured by the camera It can still be clearly distinguished under the highest exposure frequency of the optical camera; 4. Conduct high-temperature tensile, compression or three-point bending tests on heat-resistant materials at a temperature between 1000°C and below 2000°C on a high-temperature mechanical testing machine. The spot is photographed and collected in real time until the sample is destroyed; 5. DIC will obtain the full-field coordinates, displacement, and strain data in the gauge length section of the material, combine them with the data obtained by the mechanical testing machine, and obtain the high-temperature mechanical properties of the heat-resistant material under tension, compression, or three-point bending conditions after processing. performance parameters. 2. The method for accurately determining high-temperature mechanical performance parameters of heat-resistant materials using digital image correlation technology according to claim 1, characterized in that the white powder with strong adhesion at high temperature and easy dispersion at room temperature is aluminum-based ceramic powder. 3. The method for accurately determining high-temperature mechanical performance parameters of heat-resistant materials using digital image correlation technology according to claim 1 or 2, characterized in that the mass ratio of the white powder to water is 2.5-4.5:1. 4. The method for accurately determining high-temperature mechanical performance parameters of heat-resistant materials by using digital image correlation technology according to claim 3, characterized in that the mass ratio of the white powder to water is 3:1. 5. The method for accurately determining high-temperature mechanical performance parameters of heat-resistant materials using digital image correlation technology according to claim 1, characterized in that the baking temperature is 80-120°C, and the baking time is 18-36 hours. 6. The method for accurately determining high-temperature mechanical performance parameters of heat-resistant materials using digital image correlation technology according to claim 5, characterized in that the baking temperature is 93.3°C and the baking time is 24 hours. 7. The method for accurately determining the high-temperature mechanical performance parameters of heat-resistant materials using digital image correlation technology according to claim 1, characterized in that a 3cm × 3cm whiteboard with black spots randomly distributed is used for digital optical measurement of DIC digital optical cameras The system is debugged and initialized.