CN111795894A - Non-contact type method and system for measuring large deformation of uniaxial stretching of high polymer plastic material - Google Patents
Non-contact type method and system for measuring large deformation of uniaxial stretching of high polymer plastic material Download PDFInfo
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Abstract
The invention provides a non-contact type method and a system for measuring uniaxial tension large deformation of a high polymer plastic material, wherein the method comprises the following steps: preparing a style: processing the polymer plastic material by stretching according to a stretching standard, cleaning the surface of the polymer plastic material, and manufacturing 5 uniform characteristic points in the middle of the surface of the polymer plastic material; data acquisition: stretching the polymer plastic material to enable a camera optical axis of the image acquisition equipment to be vertical to the surface of the polymer plastic material, and synchronously sampling data images according to the stretching rate; data image processing: and taking 2 feature points which are closest to the necking position in the 5 feature points selected from the data image as a marked region, and then performing feature extraction on the marked region to obtain the centroid positions of the 2 feature points of the marked region. The method is low in cost, simple, rapid and convenient, and can obtain the stress-strain curve and related tensile property parameters in the plastic large deformation process.
Description
Technical Field
The invention belongs to the technical field of automatic measurement of physical properties and instruments, and particularly relates to a non-contact type method and system for measuring uniaxial stretching large deformation of a high polymer plastic material.
Background
Currently, the common prior art in the industry for measuring large deformations is the following: the method comprises the following steps of (1) adopting two categories of contact type strain measurement and non-contact type optical digital image processing, wherein the contact type is a traditional measurement method, namely, a strain gauge formed by a sensitive grid is attached to the surface of a sample, and the resistance changes when the strain gauge deforms, so that the strain quantity is obtained; the other type is an extensometer, a mechanical clamping mode is adopted, the extensometer is mechanically fixed on a test piece, and the specified deformation is obtained through the extension of the extensometer. The non-contact optical digital image processing is to compare the images before and after deformation by using a video image or laser scanning to obtain the deformation of the sample.
In the existing common contact type tensile deformation measuring method, a traditional clamping type extensometer is mostly adopted, two knife edges of the extensometer are clamped on a test piece, deformation in two fixed gauge lengths is measured, when a uniaxial tensile test is carried out on the test piece in actual operation, the position of the test piece where necking occurs has various situations, when the necking position of the test piece does not appear in the middle part area of a clamping cutter, the clamping cutter can have various situations of slipping, dislocation and the like, the measurement is inaccurate, and the sensor can be damaged due to the breakage of the test piece; the measuring range of the extensometer cannot effectively measure the whole high-density polyethylene large deformation process; in other video non-contact methods, a plurality of marking points of digital speckles are adopted, but the number of strain gauges calculated by the strain gauges is huge due to excessive scattered points, so that the required calculation system has high requirement, resources and time are wasted, and meanwhile, the equipment is expensive and has high requirement on the use environment.
In summary, the problems of the prior art are as follows: the elongation rate of the polymer plastic material at tensile fracture is up to more than 300%, the traditional strain gauge can not meet the requirement due to the inherent measurement principle, the extensometer can only measure a certain amount of deformation, the standard strain is adopted in the GB/T1040.1-2018 plastic tensile property test standard, only the displacement of the upper and lower beams of the stretcher is recorded, but the displacement between the clamps is not measured, and the strain of large deformation in the stretching process is difficult to measure. In recent years, although the digital image-based measuring method can measure large strain materials, the digital image-based measuring method has high requirements on image acquisition equipment and experimental conditions, is expensive and high in professional pertinence, and cannot meet the requirements under special conditions, such as ultra-slow stretching of high polymer plastic materials and other practical conditions, the problem of unpredictable deformation strain data in the stretching process of acquired and processed data can occur.
In summary, it can be known that the measurement difficulty of the stretching of the polymer plastic material at present is as follows: the elongation at break of the polymer plastic material in the stretching process is extremely large, the polymer plastic material belongs to a large deformation range, and the necking breaking position is uncertain, so that the strain of a necking area is difficult to accurately measure; in the process of stretching the molecular plastic material at an ultra-slow speed, the change is small every second, and a large amount of data processing operation is needed for conventional digital images.
Disclosure of Invention
In view of the above, an object of the present invention is to provide a non-contact method for measuring large deformation of a polymer plastic material by uniaxial stretching, wherein the method can measure large deformation of the polymer plastic material.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a non-contact type method for measuring large deformation of a high polymer plastic material by uniaxial stretching comprises the following steps:
preparing a style: processing a high polymer plastic material by stretching according to a stretching standard, cleaning the surface of the material, and manufacturing 5 uniform characteristic points in the middle of the surface of the material; the 5 characteristic points are obviously contrasted with the surface color of the material;
data acquisition: stretching the polymer plastic material to enable a camera optical axis of the image acquisition equipment to be vertical to the surface of the material, and synchronously sampling data images according to the stretching rate;
data image processing: and taking 2 feature points which are closest to the necking position in the 5 feature points selected from the data image as a marked region, and then performing feature extraction on the marked region to obtain the centroid positions of the 2 feature points of the marked region.
Further, performing the synchronous data image sampling according to the stretching rate by adopting a high-pixel mobile device; the high-pixel mobile equipment comprises a mobile phone, a tablet personal computer and a camera.
Further, the step of sampling the data image according to the stretching rate specifically includes:
when the stretching speed is slow and the stretching time is long, acquiring a digital image in a preset period;
when the stretching speed is high and the stretching time is short, the video with high frame number is shot, and then the video is processed to obtain the digital image of continuous frames.
Further, the feature extraction method comprises the following steps:
and performing numerical processing on the digital image, adjusting a gray threshold parameter according to the information characteristics of the digital image to enable the image to only reserve 2 pieces of characteristic point information in the marked area, and calculating the centroid positions of two characteristic points in the marked area through the pixel positions of the characteristic points of the marked area after the image is binarized.
Further, the method also comprises the following steps:
processing the continuous data images to obtain the strain of the marked area;
and according to the strain of the marking region, increasing the identification of the characteristic points of the next frame data image, and dynamically identifying the marking region of the next frame data image.
Further, the strain is obtained by the following formula:
wherein,
is the strain of the nth digital image,
is the distance between the centroids of the 2 feature points of the marker region of the nth digital image,
is the distance between the centroids of the 2 feature points of the marker region of the (n-1) th digital image,
、
is the centroid position of 2 feature points of the marker region of the nth digital image,
、
is the centroid position of the 2 feature points of the marker region of the n-1 th digital image.
The second purpose of the invention is to provide a non-contact type system for measuring the large uniaxial tension deformation of the high polymer plastic material, which can be used for measuring the large uniaxial tension deformation of the high polymer plastic material.
In order to achieve the purpose, the technical scheme of the invention is as follows: a non-contact type high polymer plastic material uniaxial stretching large deformation measuring system comprises a high polymer plastic material, a stretcher, image acquisition equipment and a server; wherein,
the polymer plastic material is used for making a uniaxial tension measurement pattern, and 5 uniform characteristic points are made in the middle position of the surface of the polymer plastic material;
the stretching machine is used for stretching the high polymer plastic material and can control the stretching speed;
the camera optical axis of the image acquisition equipment is vertical to the surface of the polymer plastic material and is used for synchronously sampling data images according to the stretching rate set by the stretcher;
the server is in communication connection with the image acquisition equipment and is used for receiving the data image acquired by the image acquisition equipment, taking 2 feature points which are closest to the necking position and are selected from the 5 feature points in the data image as a marked area, then performing feature extraction on the marked area, and calculating the centroid positions of the 2 feature points in the marked area.
Further, the image acquisition device is a high-pixel mobile device, and the high-pixel mobile device comprises a mobile phone, a tablet computer and a camera.
Further, when the stretching speed set by the stretching machine is high, the image acquisition equipment acquires digital images in a preset period;
when the stretching speed set by the stretcher is low, the image acquisition equipment shoots a video with a high frame number, and then the video is processed to obtain digital images of continuous frames.
Furthermore, the server is further configured to process the continuous data images to obtain a strain of the marker region, increase identification of the feature point of the next frame data image according to the strain of the marker region, and dynamically identify the marker region of the next frame data image.
Compared with the prior art, the invention has the following advantages:
in the stretching experiment process, the fracture necking position is always within the mark point;
the video image acquisition equipment has small limitation, can use the existing mobile equipment such as a mobile phone and the like, and has low cost and convenience;
when the video image is collected and processed, various parameters can be adjusted according to requirements, and data storage and processing pressure can be reduced;
in the process of large stretching deformation, the positions of the marking points are dynamically acquired according to the change condition of the previous step, so that the deviation caused by large deformation of the marking points in the stretching process is avoided.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It is obvious that the drawings in the following description are some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive exercise.
FIG. 1 is a schematic structural diagram of an embodiment of a non-contact uniaxial tension large deformation measurement system for polymer plastic materials according to the present invention;
FIG. 2 is a schematic flow chart of a non-contact measurement method for large uniaxial tension deformation of a polymer plastic material according to the present invention;
FIG. 3 is a schematic view of a marking area according to the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. It is to be understood that the embodiments described are only a few embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The examples are given for the purpose of better illustration of the invention, but the invention is not limited to the examples. Therefore, those skilled in the art should make insubstantial modifications and adaptations to the embodiments of the present invention in light of the above teachings and remain within the scope of the invention.
Example 1
Referring to fig. 1, a schematic structural diagram of an embodiment of a non-contact type polymer plastic material uniaxial tension large deformation measurement system of the present invention is shown, specifically, a non-contact type polymer plastic material uniaxial tension large deformation measurement system includes:
polymer plastic material 1: the sample for uniaxial tension measurement is manufactured, and 5 uniform characteristic points are manufactured in the middle position of the surface of the polymer plastic material in the embodiment;
in order to facilitate experiments, the measurement system in this embodiment can process the polymer plastic material 1 into a dumbbell shape, and the two slightly wider ends can be fixed on a stretcher, wherein 5 uniform characteristic points are manufactured in the middle of the polymer plastic material 1 in this embodiment, and the 5 characteristic points are on the same vertical line when the polymer plastic material is vertically placed and on a horizontal straight horizontal line when the polymer plastic material is horizontally placed (refer to fig. 3);
the stretching machine is connected with two ends of the polymer plastic material 1, is used for stretching the polymer plastic material and can control the stretching speed;
referring to an embodiment in fig. 1, a stretcher stretches two ends of a polymer plastic material 1 in a uniaxial manner, the stretcher of this embodiment may be a commonly used stretcher for stretching materials, and before the stretcher is used, the stretcher is set on a tensile testing machine, and the tensile testing machine includes various testing parameters such as a stretching rate; carrying out zero setting calibration on the stretcher; ensuring the clamping state of the polymer plastic material 1;
the image acquisition equipment 2 is used for synchronously sampling data images according to the stretching rate set by the stretching machine, in the embodiment, the image acquisition equipment 2 can be mobile equipment with high pixels such as a mobile phone, a tablet personal computer and the like, and can also be a professional camera, a DDC camera and the like, a camera optical axis of the image acquisition equipment 2 is set to be vertical to the surface of a polymer plastic material, so that the surface of the polymer plastic material 1 is ensured to be straight, the optical axis of the camera is kept in a parallel state with the ground, the proper working distance between a test piece and the image acquisition equipment is determined, a lamp source is turned on, objects with bright contrast around the stretching machine are properly covered, and the image acquisition equipment is adjusted to form clear data images with reasonable brightness;
preferably, the image capturing device 2 in this embodiment may design different ways of capturing data images according to the stretching speed of the stretching machine: when the stretching speed is high, the video with higher frame number can be shot, and then the digital image recognition processing is carried out to obtain a data image; when the stretching speed is low, a certain period can be set for collecting digital images, and the sampling time is ready to be recorded;
the server 3 is in communication connection with the image acquisition device 2 and is used for receiving the data image acquired by the image acquisition device 2, taking 2 feature points which are closest to the necking position and are selected from 5 feature points in the data image as a marked area, then performing feature extraction on the marked area, and calculating the centroid positions of the 2 feature points in the marked area;
preferably, the server 3 in this embodiment may further process the continuous data images to obtain the strain of the marker region, and increase the identification of the feature point of the next frame data image according to the strain of the marker region, so as to dynamically identify the marker region of the next frame data image.
Example 2
Referring to fig. 2, a flow of a non-contact type method for measuring large uniaxial tension deformation of a polymer plastic material according to the present invention is shown, and specifically, the method for measuring large uniaxial tension deformation of a polymer plastic material includes the following steps:
s400: preparing a style: processing a high polymer plastic material by stretching according to a stretching standard, cleaning the surface of the material, and manufacturing 5 uniform characteristic points in the middle of the surface of the material; then, step S500 is executed;
in this embodiment, the polymer plastic material is measured for large tensile deformation, so that the polymer plastic material is first processed into a dumbbell shape, and then the surface of the polymer plastic material is cleaned, and the obtained polymer plastic material for measurement should satisfy the following requirements: the surface of the material has no cracks, scratches or other sample surface damages, if the surface of the material has burrs, the burrs need to be removed by corresponding means such as manual and chemical methods, but the surface of the material cannot be damaged;
then, manufacturing five uniform characteristic points with distinct contrast on the surface of the material subjected to cleaning treatment, namely selecting a color system with fresh and distinct contrast with the surface color of the test piece, and manufacturing characteristic (mark) points, so that the mark can be conveniently positioned before and after the test piece deforms; the positions of the 5 marking points are uniformly distributed in the area where the material may be necked and broken, in this embodiment, 5 uniform characteristic points are manufactured in the middle of the polymer plastic material, the 5 characteristic points are on the same vertical line when the polymer plastic material is vertically placed, and are on a horizontal straight horizontal line when the polymer plastic material is horizontally placed (refer to fig. 3); measuring and recording the positions between the mark points; in the whole stretching process, the mark points change along with deformation, the front and the back of the size of the mark points can be clearly distinguished and identified through image gray level correlation, clear and regular mark characteristics are kept during the extension period, and characteristic extraction is facilitated;
preferably, the 5 feature points in this embodiment are appropriate in size and are uniformly distributed on the pattern, and the size of the feature points needs to be selected reasonably, and the feature points cannot be too large or too small.
S500: data acquisition: stretching the polymer plastic material to enable a camera optical axis of the image acquisition equipment to be vertical to the surface of the polymer plastic material, and synchronously sampling data images according to the stretching rate; then, step S600 is performed;
in this embodiment, the slightly wider two ends of the polymer plastic material cleaned in step S400 are fixed on a stretching machine, the flatness of the material surface is ensured as much as possible, and then preparation is carried out by debugging, and the stretching machine is provided with various test parameters including the stretching rate and the like; carrying out zero setting calibration on the stretcher; then selecting proper image acquisition equipment, ensuring that the optical axis of a camera of the equipment is vertical to the surface of the material and the optical axis of the camera is parallel to the ground, determining the proper working distance between the material and the image acquisition equipment, turning on a light source, properly covering objects with sharp contrast around the stretcher, and adjusting to enable the image acquisition equipment to form clear images with reasonable brightness; the image acquisition equipment can be mobile equipment with high pixels such as a mobile phone, a tablet personal computer and the like, and also can be a professional camera, a DDC camera and the like;
in this embodiment, when stretching starts, it is necessary to ensure that image acquisition and a stretching tester are performed synchronously; selecting proper sampling according to the stretching rate, and setting a certain period for collecting digital images aiming at long-term slow stretching to prepare for recording the sampling time; for short-time rapid stretching, a video with a higher frame number can be shot, and then digital image recognition processing is carried out to obtain continuous multi-frame digital images;
preferably, in this embodiment, the mechanical data loaded by the tensile testing machine changing with time and the digital image data collected changing with time need to be saved; meanwhile, the sampling frequency needs to be set properly, so that the smooth and continuous strain curve obtained subsequently is ensured, the sampling points cannot be too dense, the processing difficulty is increased, and the strain change is small;
s600: data image processing: and taking 2 feature points which are closest to the necking position in the 5 feature points selected from the data image as a marked region, and then performing feature extraction on the marked region to obtain the centroid positions of the 2 feature points of the marked region.
In this step, the data image obtained in the stretching process is processed, and the processing can be completed in a server, such as a computer, or a script program can be compiled by MATLAB software for processing, or other languages can be compiled;
specifically, the positions of 5 feature points in the data image are found, and 5 features in the image are selected, wherein two feature points with the nearest necking positions in the feature points are marked regions, and other marked regions can also be selected for research and analysis, and because any two points of the material have difference and change in the stretching process, other two feature points can also be selected in other researches, in the embodiment, the two points closest to the necking positions are selected for calculating the deformation measurement of the material, so that the deformation of the high polymer plastic material can be more accurately and obviously reflected; then, extracting the positions of the mark points according to the features of a certain frame of picture, performing numerical processing on the picture during extraction, adjusting parameters such as a gray threshold value and the like according to the features of the picture information to ensure that only the feature point information in the mark area is reserved in the picture as much as possible, and calculating the centroid positions of the mark points according to the feature point pixel positions of the mark area after the binarization of the picture;
referring to fig. 3, in a specific embodiment, for the nth digital image, a rectangular coordinate system is established for a polymer plastic material region in the image, two characteristic points AB are selected as a mark region, coordinates of the two points AB are obtained, and according to the positions of the characteristic points, when the coordinates in the coordinate system as shown in fig. 3 are the same, the ordinate of the two points AB is the same, so that the centroid between the two points AB can be obtained only by obtaining the abscissa of the two points AB, and thus the strains of the two characteristic points are calculated:
wherein,
is the strain of the nth digital image,
is the distance between the centroids of the 2 feature points of the marker region of the nth digital image,
is the distance between the centroids of the 2 feature points of the marker region of the (n-1) th digital image,
、
is the centroid position of two characteristic points AB of the marker region of the nth digital image,
and the centroid positions of two characteristic points AB of the marked region of the n-1 th digital image respectively.
Preferably, in the embodiment, during continuous time image processing, when selecting the mark area of the data image of the next frame, the identification of the feature point may be increased according to the strain information, the position, and the like of the data image of the previous frame, the mark area may be dynamically identified, the centroid position of the mark point may be acquired, and the deviation of the position change caused by large deformation may be reduced.
The invention adopts a low-cost, simple, quick and convenient non-contact method to measure the large deformation strain of the polymer plastic material: the method adopts five-point marking mode, 5 characteristic points are uniformly marked in the middle of a pattern as marking positions, the necking positions can be ensured to be within 5 marking points, two characteristic points with the nearest necking are selected in the later period to calculate the uniaxial stretching deformation strain measurement of the high polymer plastic material, and the data analysis pressure is reduced;
the 5-point mark adopted in the invention can measure the sample with uncertain shrinkage position no matter whether the necking deviates from the middle position of the sample, and the fracture necking position is within two points of the 5 marked points.
Low-cost video acquisition: the invention provides a mobile device which can be used as a video acquisition device, and the pixels of the current mainstream mobile phones reach 1 hundred million pixels and can be used as the acquisition device. The image acquisition device of the present invention may be: the mobile equipment with high pixels, such as a mobile phone, a tablet personal computer and the like, can also be a professional camera, a DDC camera and the like, and has low use cost, convenience and quickness;
and (3) data processing according to needs: optionally, selecting a proper frame number image from the collected video image according to the stretching speed, and stretching the collected video image at a slow speed for a long time so as to intermittently and directly collect the image; for short-time quick stretching, a video with a higher frame number can be adopted; optionally, mathematical image processing such as contrast and gray scale is adjusted, and data processing is optimized;
dynamically acquiring a mark: and converting the image data into a gray matrix, determining the centroid position of the characteristic point, dynamically marking a deformation area by taking the centroid position of the mark point as a strain measurement datum point and taking the change of the displacement of the previous step as a reference area, and obtaining the centroid position of the mark point to reduce the deviation of position change caused by large deformation.
While the present invention has been described with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, which are illustrative and not restrictive, and it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (10)
1. A non-contact type method for measuring uniaxial stretching large deformation of a high polymer plastic material is characterized by comprising the following steps:
preparing a style: processing the polymer plastic material by stretching according to a stretching standard, cleaning the surface of the polymer plastic material, and manufacturing 5 uniform characteristic points in the middle of the polymer plastic material; the 5 characteristic points are obviously contrasted with the surface color of the high polymer plastic material;
data acquisition: stretching the polymer plastic material to enable a camera optical axis of the image acquisition equipment to be vertical to the surface of the polymer plastic material, and synchronously sampling data images according to the stretching rate;
data image processing: and taking 2 feature points which are closest to the necking position in the 5 feature points selected from the data image as a marked region, and then performing feature extraction on the marked region to obtain the centroid positions of the 2 feature points of the marked region.
2. The method of claim 1, wherein said synchronizing with a high pixel motion device samples the data image according to a stretching rate; the high-pixel mobile equipment comprises a mobile phone, a tablet personal computer and a camera.
3. The method according to claim 1, wherein said step of synchronizing the sampling of the data images according to the stretching rate comprises in particular:
when the stretching speed is slow and the stretching time is long, acquiring a digital image in a preset period;
when the stretching speed is high and the stretching time is short, the video with high frame number is shot, and then the video is processed to obtain the digital image of continuous frames.
4. The method of claim 1, wherein the feature extraction method is as follows:
and performing numerical processing on the digital image, adjusting a gray threshold parameter according to the information characteristics of the digital image to enable the image to only reserve 2 pieces of characteristic point information in the marked area, and calculating the centroid positions of two characteristic points in the marked area through the pixel positions of the characteristic points of the marked area after the image is binarized.
5. The method according to any one of claims 1-4, further comprising the step of:
processing the continuous data images to obtain the strain of the marked area;
and according to the strain of the marking region, increasing the identification of the characteristic points of the next frame data image, and dynamically identifying the marking region of the next frame data image.
6. The method of claim 5, wherein the strain is obtained by the following equation:
wherein,
is the strain of the nth digital image,
is the distance between the centroids of the 2 feature points of the marker region of the nth digital image,
is the distance between the centroids of the 2 feature points of the marker region of the (n-1) th digital image,
、
is the centroid position of 2 feature points of the marker region of the nth digital image,
is the centroid position of the 2 feature points of the marker region of the n-1 th digital image.
7. A non-contact type high polymer plastic material uniaxial stretching large deformation measuring system is characterized by comprising a high polymer plastic material, a stretcher, image acquisition equipment and a server; wherein,
the polymer plastic material is used for making a uniaxial tension measurement pattern, and 5 uniform characteristic points are made in the middle position of the surface of the polymer plastic material;
the stretching machine is used for stretching the high polymer plastic material and can control the stretching speed;
the camera optical axis of the image acquisition equipment is vertical to the surface of the polymer plastic material and is used for synchronously sampling data images according to the stretching rate set by the stretcher;
the server is in communication connection with the image acquisition equipment and is used for receiving the data image acquired by the image acquisition equipment, taking 2 feature points which are closest to the necking position and are selected from the 5 feature points in the data image as a marked area, then performing feature extraction on the marked area, and calculating the centroid positions of the 2 feature points in the marked area.
8. The system of claim 7, wherein the image capture device is a high pixel mobile device; the high-pixel mobile equipment comprises a mobile phone, a tablet personal computer and a camera.
9. The system according to claim 7, wherein when the stretching speed set by the stretching machine is fast, the image acquisition device acquires digital images in a preset period;
when the stretching speed set by the stretcher is low, the image acquisition equipment shoots a video with a high frame number, and then the video is processed to obtain digital images of continuous frames.
10. The system of claim 7, wherein the server is further configured to process the consecutive data images to obtain a strain of the mark region, and increase the identification of the feature point of the next frame data image according to the strain of the mark region, so as to dynamically identify the mark region of the next frame data image.
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