CN110987617A - DIC (digital computer) measurement method for frequency conversion of tensile stress strain curve in necking stage by system - Google Patents
DIC (digital computer) measurement method for frequency conversion of tensile stress strain curve in necking stage by system Download PDFInfo
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- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
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Abstract
The invention discloses a DIC (digital computer) measuring method for frequency conversion of a tensile stress-strain curve in a necking stage, which is suitable for different types of plastic metal materials with necking stages. Firstly, according to the change trend of tensile stress recorded in real time in the process of stretching a standard tensile sample by an electronic universal testing machine, calculating and judging that a stretching stage where the tensile test is positioned reaches a necking stage by a DIC control center; and then the DIC control center sends a third sampling frequency to the CCD camera, and the CCD camera is controlled to acquire the image data of the standard tensile sample at a higher sampling frequency. Therefore, the system automatically judges and implements frequency conversion according to the collected stress-time curve, reduces the data collection amount, reduces the requirements on the performance of a computer, can improve the accuracy of DIC measurement, and obtains a more accurate stress-strain curve at the necking stage, thereby more accurately positioning a fracture point and providing more accurate performance indexes such as elongation and the like.
Description
Technical Field
The invention relates to the technical field of material mechanical property testing, in particular to a DIC (digital computer) measuring method for frequency conversion of a tensile stress strain curve in a necking stage by a system.
Background
The tensile test is one of the most commonly used mechanical property tests of materials, and is used for simultaneously measuring the strain and the stress of the materials under the action of tensile force and drawing a tensile stress-strain curve. The measured tensile property index is one of the basic mechanical properties of the material, and the size of the index represents the capability of the material for resisting the external force action and is an important basis for evaluating the material performance. Wherein, the performance indexes such as yield strength, tensile strength, elongation, Young modulus and the like can be reflected from a tensile stress strain curve. Currently, in the tensile test, the stress is acquired by an electronic universal tester, and the strain is mainly measured by using a traditional extensometer.
DIC technology (digital image correlation), which is a non-contact optical measurement testing technology, is an algorithm for comparing image correlation points. The measuring system of DIC technology generally comprises an illumination light source, an image acquisition card, a CCD camera and a computer, wherein the CCD camera acquires material images under illumination in real time, and the material images are automatically processed by the computer after being sent to the image acquisition card to acquire information such as required strain. The method has the advantages of simple light path, good environmental adaptability, wide measurement range, high automation degree and the like, and is widely applied to the field of engineering. At present, the method has been widely applied to many aspects of material research, especially mechanical property characterization. The most mature of these are DIC techniques instead of strain gauges based on strain gauges attached to the sample to measure the strain of the sample or conventional extensometers using contact sensors attached to the sample to measure the strain of the sample.
In a typical tensile test of a plastic material, the stress-strain curve of the stretching process can be generally divided into four stages, namely, an elastic stage, a yield stage, a strengthening stage and a necking stage in sequence according to time. In the first three stages, the strain changes relatively slowly, and the strain distribution is relatively uniform, so that generally, in the three stages, the strain rate set by the tensile test is small, the sampling frequency can be low, and the obtained sample amount is not too large. However, in the necking stage, because the strain is concentrated at the necking position, the local strain rate is high, the instantaneous strain at the fracture point is greatly changed, and the DIC measurement system needs to acquire strain values in real time at a high frequency, so that a large amount of data is generated, and heavy data processing is brought. Particularly, it is considered that since the whole stretching process is continuous, the start time of each stage cannot be accurately predicted, so that a higher frame rate is required to be used for image acquisition in the whole stretching test process to meet the strain change rate near the fracture point, which results in excessively large data volume, higher performance requirements on a computer, and higher time cost for processing data, i.e., the cost for post-processing is increased. However, if the image acquisition is performed on the whole process of the tensile test at a low frame rate, the measurement at the necking stage is not precise enough, so that the fracture moment is difficult to accurately position, and the stress-strain curve is not precise enough.
Therefore, in experiments based on DIC measurement of stress-strain curves applied in the engineering field, frequency modulation measurement is mainly performed in a mode of manually adjusting sampling frequency at different stages. That is, the trigger signal is given manually according to the approximate time of reaching different stages in the tensile test so as to adjust the image acquisition frame rates of different stages. However, it is difficult to relatively accurately locate characteristic points such as necking points and breaking points by a manual method, which may result in a large amount of redundant data generated in non-critical periods, increase the cost of post-processing, and may lose part of accurate data in critical stages, affecting the accuracy of acquisition. Meanwhile, the influence of human factors is large, the operation difficulty is also large, and the triggering signal of manual interference can increase the burden of the whole measuring system, so that the acquisition stability of the DIC measuring system is influenced.
Accordingly, further improvements and enhancements are needed in the art.
Disclosure of Invention
In view of the above-mentioned drawbacks of the prior art, the present invention provides a method for measuring a tensile stress-strain curve in a necking stage by means of a DIC with systematic frequency conversion, which is suitable for different types of plastic metal materials with necking stages and can obtain a more accurate stress-strain curve in a necking stage with less data processing.
In order to achieve the purpose, the invention discloses a DIC (digital computer) measuring method for frequency conversion of a tensile stress-strain curve in a necking stage, which is applied to the measurement of a plastic metal material in the necking stage, wherein an electronic universal testing machine is used for stretching a standard tensile sample made of the plastic metal material, and a DIC measuring system is used for measuring the real-time strain of the standard tensile sample; the DIC measuring system comprises a DIC control center and a CCD camera; the DIC control center controls the acquisition frequency of the CCD camera; the CCD camera collects the image data of the standard tensile sample under the collection frequency; the measuring method comprises the following steps:
a: taking a standard tensile sample, carrying out a tensile test by the electronic universal testing machine, and recording the tensile stress in real time by the electronic universal testing machine under a first acquisition frequency;
b: the CCD camera collects the image data at a second collection frequency;
c: the electronic universal testing machine synchronously transmits the tensile stress to the DIC control center;
d: the DIC control center analyzes the variation trend of the tensile stress and judges the tensile stage of the tensile test in real time; the stretching stage sequentially comprises an elastic stage, a yielding stage, a strengthening stage and a necking stage;
e: after the necking stage is judged to be reached, the control center controls the CCD camera to acquire the image data at a third acquisition frequency;
the second acquisition frequency is less than the third acquisition frequency.
Preferably, in the step D, the DIC control center analyzes a variation trend of the tensile stress, and specifically includes:
d1: the DIC control center performs median filtering processing on the tensile stress to obtain a real-time stress value;
d2: and the DIC control center calculates the stress value change between the adjacent real-time stress values in real time according to the real-time stress values at three continuous moments.
More preferably, in step D, the step of determining the stretching stage of the tensile test in real time includes determining the tensile test enters the necking stage in real time, specifically:
d3: the tensile stress is greater than a preset stress value;
d4: the change of the stress values at three consecutive moments is negative;
the preset stress value is set by the material performance of the standard tensile sample.
More preferably, the predetermined stress value is greater than the on-material yield strength or the proportional yield strength of the standard tensile specimen.
Preferably, the first acquisition frequency is an integer multiple of the second acquisition frequency and the third acquisition frequency.
Preferably, speckles convenient to identify are prepared on the surfaces of the parallel sections of the standard tensile sample, and the CCD is over against the speckles and shoots speckle images.
More preferably, the CCD camera is connected to an image acquisition card of a computer, the speckle images are transmitted to the computer through the image acquisition card, and the computer calculates a strain value in real time according to a position difference of the speckle images at adjacent moments to obtain a strain-time curve.
Further preferably, the electronic universal testing machine synchronously transmits the tensile stress to the computer, and the computer obtains a stress-time curve according to the first acquisition frequency.
Still further preferably, the stress-strain curve is obtained by combining the strain-time curve and the stress-time curve according to a time correspondence.
Preferably, before the step a, the method further comprises the steps of:
a0: and taking a standard tensile sample, carrying out a tensile test by the electronic universal tester until the standard tensile sample is broken, obtaining a tensile stress-time curve, and calculating the yield strength or the proportional yield strength of the material.
Has the advantages that: the invention discloses a DIC (digital computer) measuring method for frequency conversion of a tensile stress-strain curve in a necking stage, which is suitable for different types of plastic metal materials with necking stages. Firstly, according to the change trend of tensile stress recorded in real time in the process of stretching a standard tensile sample by an electronic universal testing machine, calculating and judging that a stretching stage where the tensile test is positioned reaches a necking stage by a DIC control center; and then the DIC control center sends a third sampling frequency to the CCD camera, and the CCD camera is controlled to acquire the image data of the standard tensile sample at a higher sampling frequency. Therefore, the system automatically judges and implements frequency conversion according to the collected stress-time curve, reduces the data collection amount, reduces the requirements on the performance of a computer, can improve the accuracy of DIC measurement, and obtains a more accurate stress-strain curve at the necking stage, thereby more accurately positioning a fracture point and providing more accurate performance indexes such as elongation and the like.
The conception, the specific structure and the technical effects of the present invention will be further described with reference to the accompanying drawings to fully understand the objects, the features and the effects of the present invention.
Drawings
FIG. 1 is a flow chart of tensile stress strain curve measurement in a preferred embodiment of the present invention;
FIG. 2 is a flow chart of the DIC control center determining that the necking stage is reached;
FIG. 3 is a drawing of a standard tensile specimen layout in accordance with a preferred embodiment of the present invention;
FIG. 4 is a graph of stress versus time and stress change versus time measured in accordance with a preferred embodiment of the present invention;
FIG. 5 is a graph of stress-strain curves measured in accordance with a preferred embodiment of the present invention;
FIG. 6 is a graph of stress-strain curve comparisons taken at different acquisition frequencies during the later stages of the necking stage in a preferred embodiment of the present invention.
In the figure, 1 is a standard tensile sample, 2 is a speckle distribution area, 3 is an engineering stress curve, 4 is an engineering stress change curve, 5.1 Hz acquisition curve, 51.1 Hz breaking point, 6.7 Hz acquisition curve and 61.7 Hz breaking point.
Detailed Description
The technical contents of the preferred embodiments of the present invention will be more clearly and easily understood by referring to the drawings attached to the specification. The present invention may be embodied in many different forms of embodiments and the scope of the invention is not limited to the embodiments set forth herein.
In the drawings, structurally identical elements are represented by like reference numerals, and structurally or functionally similar elements are represented by like reference numerals throughout the several views. The size and thickness of each component shown in the drawings are arbitrarily illustrated, and the present invention is not limited to the size and thickness of each component. The thickness of the components may be exaggerated where appropriate in the figures to improve clarity.
The invention provides a DIC (digital computer) measuring method for frequency conversion of a tensile stress-strain curve in a necking stage, which is applied to the measurement of a plastic metal material with a necking stage, wherein an electronic universal testing machine is used for stretching a standard tensile sample 1 made of the plastic metal material, and a DIC measuring system is used for measuring the real-time strain of the standard tensile sample 1; the DIC measuring system comprises a DIC control center and a CCD camera; the DIC control center controls the acquisition frequency of the CCD camera; the CCD camera collects the image data of the standard tensile sample 1 under the collection frequency; the specific process of the measurement method is shown in fig. 1:
fig. 1 is a flow chart of the tensile stress-strain curve measurement in a preferred embodiment of the present invention, before the measurement, a standard tensile test specimen 1 meeting the requirements of the electronic universal testing machine is firstly prepared, and then one standard tensile test specimen 1 is taken and placed in a sample groove of the electronic universal testing machine, and the electronic universal testing machine performs the tensile test until the tensile test is broken. And reading and recording the tensile stress-time curve of the standard tensile sample 1 at the same time of the tensile test, and obtaining the material yield strength or the proportional yield strength of the standard tensile sample 1. For example, the material ratio yield strength of the standard tensile specimen 1 is obtained to be 900 MPa.
And replacing another standard tensile sample 1, placing the standard tensile sample in a sample groove of the electronic universal testing machine, performing a tensile test, and recording the tensile stress of the electronic universal testing machine in real time under the first acquisition frequency. Meanwhile, the CCD camera is arranged right opposite to the standard tensile sample 1, and in the tensile process, the image data is collected and fed back to the DIC control center at a second collection frequency.
Meanwhile, the electronic universal testing machine is linked with the DIC control center, and the electronic universal testing machine synchronously transmits the tensile stress to the DIC control center. And the DIC control center analyzes the change trend of the tensile stress and judges the tensile stage of the tensile test in real time. Specifically, the stretching stage sequentially comprises an elastic stage, a yielding stage, a strengthening stage and a necking stage. The DIC control center mainly judges whether the stretching process reaches the necking stage. The judgment process is shown in fig. 2:
firstly, the loading data such as tensile stress are synchronously transmitted to the DIC control center from the electronic universal testing machine, and then the DIC control center performs median filtering processing on the tensile stress to obtain a real-time stress value. Real-time stress value sigma for a certain momentiFirst, the real-time stress value sigma is determinediWhether or not it is greater than a predetermined stress value sigma0. And only if said real-time stress value σ is presentiGreater than the predetermined stress value sigma0Then, it is determined that the stretch has already passed through the yield stage, because the stress value fluctuates slightly up and down in the yield stage, which may cause erroneous determination. That is, it is first determined that the following expression (1) holds:
σi>σ0(1)
wherein the predetermined stress value σ0Should be set according to the material of the standard tensile specimen 1, generally, the preset stress value σ0A material upper or proportional yield strength greater than that of the standard tensile specimen 1 is required and needs to exceed a certain magnitude, for example, over 10 MPa.
Secondly, when the real-time stress value sigmaiHas been determined to be greater than the preset stress value sigma0Then, the DIC control center again controls the post of three consecutive time instantsThe real time stress value is determined and only if it is determined to continue to decrease, the measurement is said to have entered the necking stage. Specifically, the DIC control center calculates the stress variation value delta sigma between adjacent real-time stress values in real time for the real-time stress values at three consecutive momentsiAnd, the following equation needs to be satisfied:
Δσi<0，Δσi-1<0，Δσi-2<0, where Δ σi＝σi-σi-1(2)
Wherein, Delta sigmaiAnd the stress change values between the moment i and the moment i-1 of the previous acquisition moment are represented, and the stress change values at three continuous moments are all negative numbers.
Referring to fig. 3, the geometric structure of the labeled tensile sample 1 may specifically be: a flat specimen having a thickness of 1.4mm and a total length of 200mm was made, and smoothly contracted parallel sections were provided in the middle portion, for example, the parallel sections were smoothly connected by a circular arc having a radius of 25mm in FIG. 2. The length of the parallel section is 60mm, and the width is 12 mm. In addition, in order to facilitate the identification of the positioning of the midpoint of the image data collected by the CCD camera, in a preferred embodiment, a speckle distribution area 2 is further disposed on the parallel section, a group of speckles for identification is regularly arranged in the speckle distribution area 2, and the lens of the CCD camera is aligned with the speckle distribution area 2 to shoot the speckle image, so as to facilitate the reading of the deformation of the image data at a later stage. In a better embodiment, in order to enhance the contrast and facilitate the collection of the CCD camera, the marking tensile sample 1 is first sprayed with a layer of white primer, and then black paint is uniformly sprayed with small black spots on the surface of the white primer of the speckle distribution area 2, so as to form speckles. Of course, the appearance of large black spots should be avoided when painting.
In one embodiment, for example, the predetermined stress value σ0The setting is 910MPa, namely the proportional yield strength is 900MPa + the excess value is 10 MPa. Namely, the requirements are as follows:
σi>910MPa (1.1)
and when the DIC control center detects that the real-time stress values at three consecutive moments i, i-1 and i-2 forward from the moment i are continuously reduced, the tensile stage can be judged to enter a necking stage, at the moment, the DIC control center changes the acquisition frequency of the CCD camera, and the image data is acquired at a third acquisition frequency.
And the third acquisition frequency should be greater than the second acquisition frequency, i.e. after the stretching is in the necking stage, the acquisition frequency should be increased to cope with the accelerated change in strain. For example, in one embodiment, the second acquisition frequency is set to a low frequency, 1-2Hz, while the third acquisition frequency is set to a high frequency, e.g., above 7 Hz.
In order to correspond to the stress-time curve acquired by the electronic universal tester, that is, to ensure that the acquired stress value corresponds to each acquisition time of the image data, the first acquisition frequency needs to be an integral multiple of the second acquisition frequency and the third acquisition frequency.
In order to conveniently process data, the CCD camera is connected to an image acquisition card of a computer, the speckle images are sent to the computer through the image acquisition card, and the computer calculates strain values in real time according to the position difference of the speckle images at adjacent moments to obtain a strain-time curve.
And the electronic universal testing machine also synchronously transmits the tensile stress to the computer, and the computer obtains a stress-time curve according to the first acquisition frequency.
In a preferred embodiment, the stress-time curve collected by the electronic universal tester is shown in fig. 4, and a stress variation-time curve is plotted in fig. 4 by computer processing. It can be seen that in the initial phase of stretching, the stress increases rapidly and substantially linearly with time, until after about 30 seconds, the yield phase is entered, but the stress still increases with time, only very slowly, the stress variation value Δ σiClose to 0. At about 80 seconds, the necking stage is entered, the stress starts to continuously decrease after reaching the peak top, and the stress variation value delta sigmaiThe transition is to a negative value,and the magnitude of the decrease also begins to increase, transitioning to a sharp decrease in the stress value, indicating that the standard tensile specimen 1 is immediately snapped off.
By combining the stress-time curve and the strain-time curve of fig. 4, according to the time correspondence, a stress-strain curve of the standard tensile specimen 1 during the tensile process can be obtained, as shown in fig. 5. It can be seen that in the final stage of stretching, i.e. after entering the necking stage, the stress drops sharply with increasing strain, so that the acquisition frequency of the strain needs to be increased, i.e. the third acquisition frequency needs to be higher than the second acquisition frequency.
In the final stage of stretching, the necking stage, the stress-strain diagram is shown in fig. 6. The graph shows two curves obtained after acquiring the image data at two acquisition frequencies and calculating the strain: 1Hz acquisition curve 5 and 7Hz acquisition curve 6. It can be seen that in the beginning of the necking stage, the data collected at different collection frequencies do not change much and the two curves remain substantially coincident. However, if the second acquisition frequency is continued, i.e. at 1Hz, the stress shows a substantially smooth drop, whereas the 1Hz breaking point 51 occurs immediately after 6.7 seconds. But when changing to the third acquisition frequency, i.e. at 7Hz, in the final phase the stress does not drop smoothly, its drop still has a small amplitude of oscillation, and the 7Hz breaking point 61 obtained is later, postponed until close to 6.8 seconds.
It can be seen that with the third acquisition frequency, a more detailed change in the stress-strain curve can be revealed.
In summary, the present invention provides a DIC measurement method for frequency conversion of a tensile stress-strain curve in a necking stage, which is suitable for different types of plastic metal materials with necking stages. Firstly, according to the change trend of tensile stress recorded in real time in the process of stretching a standard tensile sample by an electronic universal testing machine, calculating and judging that a stretching stage where the tensile test is positioned reaches a necking stage by a DIC control center; and then the DIC control center sends a third sampling frequency to the CCD camera, and the CCD camera is controlled to acquire the image data of the standard tensile sample at a higher sampling frequency. Therefore, the system automatically judges and implements frequency conversion according to the collected stress-time curve, reduces the data collection amount, reduces the requirements on the performance of a computer, can improve the accuracy of DIC measurement, and obtains a more accurate stress-strain curve at the necking stage, thereby more accurately positioning a fracture point and providing more accurate performance indexes such as elongation and the like.
The foregoing detailed description of the preferred embodiments of the invention has been presented. It should be understood that numerous modifications and variations could be devised by those skilled in the art in light of the present teachings without departing from the inventive concepts. Therefore, the technical solutions available to those skilled in the art through logic analysis, reasoning and limited experiments based on the prior art according to the concept of the present invention should be within the scope of protection defined by the claims.
Claims (10)
1. A DIC measuring method of frequency conversion of a tensile stress-strain curve in a necking stage is applied to the measurement of a plastic metal material with a necking stage and is characterized in that a standard tensile sample made of the plastic metal material is stretched by an electronic universal testing machine, and the real-time strain of the standard tensile sample is measured by a DIC measuring system; the DIC measuring system comprises a DIC control center and a CCD camera; the DIC control center controls the acquisition frequency of the CCD camera; the CCD camera collects the image data of the standard tensile sample under the collection frequency; the measuring method comprises the following steps:
a: taking a standard tensile sample, carrying out a tensile test by the electronic universal testing machine, and recording the tensile stress in real time by the electronic universal testing machine under a first acquisition frequency;
b: the CCD camera collects the image data at a second collection frequency;
c: the electronic universal testing machine synchronously transmits the tensile stress to the DIC control center;
d: the DIC control center analyzes the variation trend of the tensile stress and judges the tensile stage of the tensile test in real time; the stretching stage sequentially comprises an elastic stage, a yielding stage, a strengthening stage and a necking stage;
e: after the necking stage is judged to be reached, the control center controls the CCD camera to acquire the image data at a third acquisition frequency;
the second acquisition frequency is less than the third acquisition frequency.
2. The method for DIC measurement with frequency conversion according to claim 1, wherein in step D, the DIC control center analyzes the trend of the tensile stress, and specifically comprises:
d1: the DIC control center performs median filtering processing on the tensile stress to obtain a real-time stress value;
d2: and the DIC control center calculates the stress value change between the adjacent real-time stress values in real time according to the real-time stress values at three continuous moments.
3. The method for DIC measurement with system frequency conversion according to claim 2, wherein in step D, the step of determining the stretching stage of the stretching test in real time comprises the step of determining the stretching test entering the necking stage in real time, specifically:
d3: the tensile stress is greater than a preset stress value;
d4: the change of the stress values at three consecutive moments is negative;
the magnitude of the preset stress value is set by the material performance of the standard tensile sample.
4. The method of claim 3, wherein the predetermined stress value is greater than the on-material yield strength or the proportional yield strength of the standard tensile specimen.
5. The method of claim 1, wherein the first acquisition frequency is an integer multiple of the second acquisition frequency and the third acquisition frequency.
6. The method for DIC measurement with system frequency conversion according to claim 1 wherein the parallel segments of the standard tensile specimen are prepared with speckles on their surfaces for easy identification and the CCD is facing the speckles and captures images of the speckles.
7. The method as claimed in claim 6, wherein the CCD camera is connected to an image capture card of a computer, the speckle images are transmitted to the computer through the image capture card, and the computer calculates strain values in real time according to the position difference between the speckle images at adjacent times to obtain a strain-time curve.
8. The method as claimed in claim 7, wherein the electronic universal tester transmits the tensile stress to the computer synchronously, and the computer obtains a stress-time curve according to the first collection frequency.
9. The method as claimed in claim 8, wherein the strain-time curve and the stress-time curve are combined to obtain a stress-strain curve according to a time correspondence.
10. The method for measuring DIC of claim 1, further comprising, before the step A, the steps of:
a0: and taking a standard tensile sample, carrying out a tensile test by the electronic universal tester until the standard tensile sample is broken, obtaining a tensile stress-time curve, and calculating the yield strength or the proportional yield strength of the material.
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