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Detailed introduction of electronic tensile testing machine (accurate calculation of yield point)

any material will deform under the action of external force. In the initial stage of the force, generally speaking, this deformation is basically in linear proportion to the external force. At this time, if the external force disappears, the deformation of the material will also disappear and return to its original state. This stage is usually called the elastic stage. Hooke's law in physics is the basic law describing this characteristic. However, when the external force increases to a certain extent, the deformation will no longer be in linear proportion to the external force. At this time, when the external force disappears, the deformation of the material will not disappear completely, and the overall size will not be completely restored to the original state. This stage is called the plastic deformation stage

all products and equipment are made up of materials with different properties. They will be affected by various external forces during use, and will naturally produce various deformations. However, such deformations must be limited within the elastic range, otherwise the shape of the product will change permanently, which will affect the continued use, and the shape of the equipment will also change. At least, the accuracy grade of the processed parts will be reduced, In serious cases, parts and components are scrapped, resulting in major quality accidents. So how to ensure that the deformation is within the elastic range? From the above analysis, the deformation of known materials can be divided into two stages: elastic deformation and plastic deformation. As long as the mechanical properties of known materials are found out for test and theoretical analysis, people have summarized the turning points of two stages: yield point and non proportional stress. Engineering designers can ensure the reliable operation of products and equipment

due to the wide variety of materials and great differences in properties, the transition between the elastic stage and the plastic stage is very complex. Indicators such as and residual stress are used as the turning point indicators of the elastic stage and the plastic stage to reflect the performance of the transition process of materials, among which yield point and non proportional stress are the most commonly used indicators. Although the yield point and the non proportional stress are the same indicators that reflect the "turning point" of the material in the elastic stage and the plastic stage, they reflect the characteristics of the material with different characteristics in the transition stage. Therefore, their definitions are different, their calculation methods are different, and the required equipment is not exactly the same. Therefore, the author will analyze these two indicators respectively. This paper first analyzes the situation of the yield point:

from the above description, it can be seen that the accurate calculation of the yield point is very important in the material mechanical property test. In many cases, its importance is even greater than the ultimate strength value of the material (ultimate strength is one of the indicators that must be calculated for all material mechanical properties). However, it is very accurate, In many cases, it is not easy. The actual temperature fluctuation of most of the test boxes produced by all environmental test equipment manufacturers can be controlled within the range of ± 0.3 ℃. It is restricted by many factors, which can be summarized as follows:

* the influence of fixture

* the influence of the measurement and control link of the testing machine

* influence of result processing software

* the influence of the experimenter's theoretical level, etc

each of these impacts has different aspects. The following analysis is carried out one by one

I. The Influence of fixture

this kind of influence has a high probability in the test, mainly manifested in the slipping of the sample clamping part or the large gap between some force value transmission links of the testing machine, which has a high probability in the old machine. After the machine is used for a period of time, the wear phenomenon will occur between the relative moving parts, which will significantly reduce the friction coefficient. The most intuitive performance is that the scale peak of the clamp block is ground flat, and the friction force is greatly reduced. When the stress on the specimen gradually increases to the maximum static friction, the specimen will slip, resulting in false yield. If the yield value of the previous test with this testing machine is normal, but the yield value of the current test is obviously low with high sensitivity, and the phenomenon is particularly obvious in the test of some hard or brittle materials, this reason should be considered first. At this time, the equipment shall be overhauled in time to eliminate the gap and replace the clamping block

II. Influence of testing machine measurement and control link

testing machine measurement and control link is the core of the whole testing machine. With the development of technology, at present, this link basically adopts various electronic circuits to realize automatic measurement and control. Due to the profound knowledge of automatic measurement and control, the complex structure and the opaque principle, once it is not considered carefully in the product design, it will have a serious impact on the results, and it is difficult to analyze the reasons. The following points are the most important to calculate the yield point of materials:

1. The frequency band of the sensor amplifier is too narrow.

because the force value detection elements used in the current testing machine are basically load sensors or pressure sensors, and these two types of sensors are analog small signal output types, signal amplification must be carried out in use. As we all know, in our environment, there are various electromagnetic interference signals, which will be coupled to the measurement signals through many different channels and amplified together, resulting in the useful signals being submerged by the interference signals. In order to extract useful signals from interference signals, a low-pass filter is usually installed in the amplifier according to the characteristics of the material testing machine. Setting the cut-off frequency of the low-pass filter reasonably and limiting the frequency band of the amplifier to an appropriate range can greatly improve the measurement and control performance of the testing machine. However, in reality, people often regard the stable display of data as very important, ignoring the authenticity of the data, and set the cut-off frequency of the filter very low. In this way, while fully filtering out interference signals, useful signals are often filtered out together. In daily life, the data of our common electronic scales are very stable. One of the reasons is that its frequency band is very narrow, and the interference signal can hardly pass through. The reason for this design is that the electronic scale weighs the steady-state signal, and the transition process of symmetrical quantity is not concerned. The material testing machine measures the dynamic signal, and its frequency spectrum is very wide. If the frequency band is too narrow, the signal with higher frequency will be attenuated or filtered, resulting in distortion. This distortion is not allowed when the yield is shown as the force value fluctuates up and down for many times. As far as the universal material testing machine is concerned, the author believes that the minimum frequency band should also be greater than 10Hz, preferably 30Hz. In practice, although the frequency band of the amplifier sometimes reaches this range, people often ignore the frequency band width of the a/d converter, so that the actual frequency band width is less than the set frequency band width. Take the AD7705, AD7703, ad7701, etc. selected by many test machine data acquisition systems as examples. When the a/d converter operates at the "maximum output data rate of 4kHz", its analog input processing circuit reaches the maximum bandwidth of 10Hz. When working at the output data rate of 100Hz, which is the most commonly used test machine, the actual bandwidth of its analog input processing circuit is only 0.25hz, which will lose a lot of useful signals, such as the fluctuation of the force value at the yield point. Of course, correct test results cannot be obtained with such a circuit

2. The data acquisition rate is too low

at present, the data acquisition of analog signals is realized through a/d converter. There are many kinds of a/d converters, but the sigma delta type a/d converter is the most used in the testing machine. This kind of converter is flexible to use, and the conversion rate can be dynamically adjusted. It can not only realize high-speed and low-precision conversion, but also realize low-speed and high-precision conversion. On the testing machine, the data acquisition rate is not required to be too high. Generally, it can meet the demand by tens to hundreds of times per second. Therefore, generally, lower conversion rate is used to achieve higher measurement accuracy. However, on some testing machines manufactured by some manufacturers, in order to pursue high sampling resolution and high data display stability, it is not advisable to reduce the sampling speed very low. Because when the sampling speed is very low, the high-speed signal can not be accurately collected in real time. For example, what will the golden airplane look like? In the material performance test, when the material yields and the stress value fluctuates up and down, the signal changes so that the upper and lower yield points can not be accurately calculated, resulting in the failure of the test. As a result, the watermelon is lost

how to judge the bandwidth and sampling rate of a system

strictly speaking, this requires many special testing instruments and professionals to complete. However, through the simple method introduced below, a qualitative understanding can be made. When the sampling resolution of a system reaches more than one in tens of thousands, but the display data still does not fluctuate or the display data has an obvious sense of lag, it can be basically determined that its passband is very narrow or the sampling rate is very low. Except for special occasions (such as high-precision calibrator for verifying the force value accuracy of the testing machine), it is not allowed to be used on the testing machine

3. Improper use of control methods

for the relationship between stress and strain when the material yields (when yielding occurs, the stress does not change or fluctuates up and down, but the strain continues to increase), the control mode recommended in the national standard is constant strain control, while the control mode of the elastic stage before yielding occurs is constant stress control, which is difficult to complete in most testing machines and some tests. Because it requires changing the control mode when the yield phenomenon just occurs, and the purpose of the test itself is to require taking the yield point. How can the control switch be carried out under the condition of unknown results? Therefore, in reality, the same control mode is generally used to complete the whole test (even if different control modes are used, it is difficult to switch at the upper yield point, and generally a little ahead will be selected). For the testing machine with constant displacement control (speed control), since the stress rate of the material in the elastic stage is proportional to the strain rate, as long as the appropriate test speed is selected, the speed control throughout the whole process can be compatible with the control characteristics of the two stages. However, for the testing machine with only one mode of force control, if the response of the testing machine is particularly fast (which is the purpose of automatic control efforts), the process time of yielding will be very short. If the speed of data acquisition is not high enough, the yield value will be lost (as explained in point 2), and the excellent control performance will become the cause of error. Therefore, it is better not to select a single load control mode when selecting the testing machine and control method

III. impact of result processing software

most of the current production testing machines are equipped with different types of computers (such as PC, single chip microcomputer, etc.) to complete various data tests defined by standards or users. Compared with the graphic method widely used in the past, it has made great progress. However, due to the lag of the standard, some of the original definitions are not clear enough. For example, the definition of yield point has only qualitative explanation, but no quantitative explanation, which is not suitable for the needs of computer automatic processing. This leads to:

1. Respective setting of judgment conditions

in terms of yield point (taking metal tensile gb/t as an example), the standard is defined as follows:

"yield strength: when the metal material shows yield phenomenon, it reaches the stress point where plastic deformation occurs but the force does not increase during the test, and the upper yield strength and lower yield strength shall be distinguished.

upper yield strength: the specimen yields

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