Material Brief Analysis of Metal Material Tensile Experiment Influence Factors
As a common means to characterize the strength, plasticity and stiffness of metal materials, tensile testing has the advantages of simple operation and low cost, and is widely used in industry and scientific research. As a new advanced manufacturing technology, metal additive manufacturing is more and more widely used in aerospace, medical, automotive and other industries, but there are also problems of large performance fluctuations and poor batch stability. Therefore, the characterization and analysis of mechanical properties such as strength, plasticity and elastic modulus of metal additive manufacturing components is an important work of additive manufacturing material engineers, and also an important means to improve and optimize process technology. In view of this, this paper analyzes and discusses the influence factors of tensile testing, especially the influence factors of elastic modulus, in order to throw bricks and jade.
Effect of strain rate
The research shows that for 2024 aluminum alloy (10 sets of data mean, Table 1), with the increase of strain rate, the tensile strength decreases slightly, the yield strength increases, the area shrinkage and elongation decrease, and the elastic modulus increases.
Table 1 Tensile properties of 2024 aluminum alloys at different strain rates
Effect of coaxiality
In the tensile test, the coaxiality error of the system comes from three aspects: the coaxiality of the sample, the coaxiality of the equipment and the coaxiality of the tooling, as shown in Table 2. The data show that the coaxiality of the system has a great influence on the elastic modulus. When there is a large error in the coaxiality, the specimen in the tensile process is not only subjected to tension, but also to a bending moment (Figure 1). Taking fig. 2 as an example, the specimen is bent under the action of bending moment. obviously, the deformation of the left and right sides of the specimen is different, and the elongation of the left side is> the right side. since the extensometer knife edge is pressed on one side of the specimen, when the knife edge is pressed on the right side, the strain becomes smaller, the measured elastic modulus is larger, and when the knife edge is pressed on the left side, the strain becomes larger and the elastic modulus is smaller. Due to the randomness of the sample clamping position, the elastic modulus is greatly dispersed. It should be noted that the national standard GB/T 228.1 does not make clear requirements for the coaxiality of the test equipment and the coaxiality of the fixture, and it is difficult to control the coaxiality accuracy of the two. ISO 6892-1 and GB/T 22315 make detailed requirements for the tensile test elastic modulus, as shown in Table 3 for comparison. Therefore, the clamping method of the extensometer has become an important means to improve the elastic modulus, and the main methods to improve the elastic modulus are: 1) the double extensometer method. That is, one extensometer is installed symmetrically on the axis of the tensile specimen to test the deformation on both sides, and then the average value is taken in the circuit, thus offsetting the influence of bending deformation and obtaining pure tensile deformation. Table 4 to Table 6 are the relevant test results, it can be seen that this method is effective, ISO6892-1 is also so required. However, the instrument is more expensive and the installation is more complicated; 2) Test multiple data in different directions of the same sample and take the average value. During the test, first clamp the extensometer in the 0 ℃ direction of the sample, apply force to a certain elastic deformation range, then unload, perform the same operation in the 90, 180, and 270 ° directions of the same sample, analyze and test the elastic modulus in all directions, and take the average value.
Therefore, for the problem of large dispersion of elastic modulus in tensile test, the possible reasons are: 1) poor coaxiality of "equipment tooling test rod"; 2) The loading method of the extensometer is not standardized, and the measurement is carried out with a single extensometer; 3) The original data processing is not standardized.
Table 2 System coaxiality error classification
Table 3 Comparison of test requirements for elastic modulus of three standards
Fig. 1 Expressive form of sample positioning error
Fig. 2 Schematic Diagram of Force Deformation
Table 4 Comparison of elastic modulus test results of 2024 aluminum alloys by tensile method
Table 5 Test results of elastic modulus of low carbon steel
Table 6 Test results of elastic modulus of duralumin
Effect of specimen size
The literature shows that there is a difference between the test results of the rod-shaped tensile test bar in the machined state and the plate-shaped tensile test bar in the rolled state (Table 7). The elongation of the rod-shaped test rod is higher than that of the plate-shaped test rod, but the strength has no significant rule. There are two reasons for this difference: 1) the rod-shaped sample has a smooth surface and fewer defects after machining, while the plate-shaped sample is a rolled surface with relatively more surface defects; 2) The stress and strain states of the two are different during stretching, and the surrounding shrinkage of the rod-shaped sample becomes uniform during stretching, while the plate-shaped sample is not.
Table 7 Effect of specimen shape on tensile properties
For cold-rolled plate, when the gauge length is constant, with the increase of the sample width, the tensile strength and yield strength decrease, and the elongation increases. As shown in Table 8, the reasons are: with the increase of the sample width, the derived stress increases, and the sample gradually shifts from unidirectional tension to plane stress state, thus increasing the number of materials participating in rheology.
Table 8 Effect of Specimen Width on Tensile Properties at a Certain Gauge Distance
The literature shows that parallel length has no significant effect on tensile resistance and yield, but has a significant effect on elongation, as shown in Table 9. The influence mechanism is: the strength index only depends on the stress on the minimum section of the sample. When the section is the same, the strength index should be the same. The average elongation increases with the increase of parallel length. When the parallel length reaches a certain value, the elongation tends to be stable and decreases slightly. In this case, the test fixture and transition arc no longer affect the deformation of the gauge part, the instability point can appear both inside and outside the gauge length. The longer the parallel length is, the smaller the probability of fracture in the middle of the sample is, and the dispersion of elongation becomes larger. Therefore, the parallel length should have an optimal range.
Table 9 Influence of parallel length on tensile properties when gauge length is constant
Methods and suggestions for improving the accuracy of the elastic modulus of the tensile method (static method)
As mentioned above, GB/T228.1 specifies the test of tensile strength, yield strength, elongation and reduction of area, and does not specify the test of elastic modulus. The specification documents for testing elastic modulus based on unidirectional tensile method are static methods in ISO6892-1 and GB/T 22315. Obviously, the test conditions and data processing in these two documents are relatively strict and standardized, among which ISO 6892-1 is the most stringent. Combined with the current situation of processing and testing equipment and the research of scholars, it is difficult to use 2 groups of extensometers in the stretching process, and the operability is poor. Therefore, the economical, simple and feasible method is: when testing the elastic modulus in 4 directions, take its mean value, and the specific operation is:
1) Marked sample gauge length
2) Mark 0 °, 90 °, 180 ° and 270 ° azimuths circumferentially along the gauge length line.
3) Sample clamping
4) Install the extensometer with the knife edge corresponding to the 0 ° position
5) Load force to 50% of material yield strength
6) Unloading force
7) Change the position of the blade of the extensometer, press it at 90 °, 180 ° and 270 ° respectively, and repeat the above steps 2)~ 6)
8) Intercept the data points between 10% and 40% of the yield strength, fit, obtain the elastic modulus, and take the mean as the elastic modulus of the material.
9) Test strength and plasticity. According to the normal tensile process, break the test bar that has been tested for elastic modulus, and test the strength and plasticity (the test bar for elastic modulus has no effect on the subsequent tensile test results, and relevant research has been done)
If a worker wants to do a good job, he must first sharper his tools. With the development of additive manufacturing technology, the service environment of products has become increasingly complex and harsh, and the corresponding performance requirements for materials will become more and more stringent. Therefore, optimizing detection methods and improving test accuracy have important guiding significance for improving the performance of additive manufacturing products, and at the same time will promote the standardization of additive manufacturing product characterization technology.
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