Steel performance analysis: some basic issues in the tensile performance test of metal materials
Tensile is a simple mechanical performance test. In the test gauge length, the force is uniform, the measurement of stress, strain and its performance indicators is stable, reliable, and convenient for theoretical calculation. Through the tensile test, the most basic mechanical performance indicators in the process of elastic deformation, plastic deformation, and fracture of the material can be measured, such as positive elastic modulus E, yield strength σ0.2, yield point σs, tensile strength σb, elongation after fracture δ and reduction of area ψ etc. The mechanical properties obtained in the tensile test, such as E, σ0.2, σs, σb, δ, ψ, etc., are the inherent basic properties of materials and the main basis in engineering design.
Tensile test is the most common test in metal mechanical property test. The measurement results of the same material through different tensile test processes are not necessarily the same. What factors affect the tensile test?
Sampling location and method
Due to the uneven distribution of ingredients, organization, mechanism, defect processing and deformation in the material, different parts of the same batch or even the same product appear to be different. Therefore, when cutting samples, it should be strictly in accordance with the regulations in the GB/T-228 appendix.
3D drawing of tensile specimen
The test equipment directly affects the accuracy and authenticity of the result data, so it is necessary to ensure that the test machine is within the valid period of verification during the experiment. As shown in the figure, it is the WDW-50 universal testing machine. The equipment is regularly checked and inspected.
Computer controlled electronic universal testing machine
The impact of the test environment
The test environment mainly includes the influence of ambient temperature and the selection of clamping devices.
Spherical bearing chuck
Choice of test method
The test methods mainly include clamping method, stretching rate, stretching cross-sectional area and pattern size measurement method. When choosing the size of the measuring pattern, it is advisable to choose an outside micrometer, a vernier caliper or a vernier caliper for rectangular samples.
In addition, due to the different subjective factors and operating skills, it will also bring errors to the measurement results. Therefore, the inspector should pass strict training and carry out the test in accordance with the GB/T 228 standard method.
Some basic issues
For most metal materials, in the area of elastic deformation, the stress and the strain become proportional. When the stress or strain continues to increase, at a certain point, the strain will no longer be proportional to the applied stress.
At this point, the bond with the adjacent initial atom begins to break and is transformed with a new set of atoms. When this happens, the material will no longer return to its original state after the stress is removed, that is, the deformation is permanent and unrecoverable. At this time, the material enters the plastic deformation zone (Figure 1).
Figure 1 Schematic diagram of plastic deformation
In fact, it is difficult to determine the exact point where the material changes from the elastic zone to the plastic zone. As shown in Figure 2, a parallel line with a strain of 0.002 is drawn. Use this line to cut the stress-strain curve and determine the yielding stress as the yield strength. The yield strength is equal to the stress at which significant plastic deformation occurs. Most materials are not uniform, nor are they perfect ideal materials. Material yielding is a process, usually accompanied by work hardening, so it is not a specific point.
Figure 2 Stress-strain curve
For most metal materials, the stress-strain curve looks similar to the curve shown in Figure 3. When the loading starts, the stress increases from zero, and the strain increases linearly, until the material yields, the curve begins to deviate from the linearity.
Continue to increase the stress and the curve reaches its maximum value. The maximum value corresponds to the tensile strength, which is the maximum stress value of the curve, represented by M in the figure. The breaking point is the point where the material finally breaks, represented by F in the figure.
Figure 3 Schematic diagram of engineering stress-strain curve
The typical stress-strain test device and the geometry of the test sample are shown in Figure 4. During the tensile test, the sample is slowly pulled, and the changes in length and applied force are recorded at the same time, and the force-displacement curve is recorded. The stress-strain curve can be drawn using information such as the original length of the sample, the gauge length and the cross-sectional area.
Figure 4 Stress-strain test
For materials that can undergo tensile plastic deformation, two types of curves are most commonly used: engineering stress-engineering strain curves and true stress-true strain curves. The difference between them lies in the different areas used when calculating the stress. The former uses the initial area of the sample, and the latter uses the real-time cross-sectional area during the stretching process. Therefore, on the stress-strain curve, the true stress is generally higher than the engineering stress.
Figure 5 Schematic diagram of a typical stretch curve
Figure 6 True stress and true strain curves of a variety of real metal materials
There are two kinds of most common stretching curves: one is the stretching curve with an obvious yield point; the second is the stretching curve without an obvious yield point. The yield point represents the resistance of the metal to the initial plastic deformation. This is one of the most important mechanical properties in engineering technology.
Figure 7 Typical tensile curve with deformation hardening
How to define the plastic deformation of the actual metal in the project?
The amount of residual plastic deformation is an important basis. Usually, the resistance of the engineering metal at a certain amount of residual plastic deformation is artificially regarded as the yield strength, also known as the conditional yield strength. That is, if there is no obvious plastic yield point, there is no obvious yield strength. To know the yield strength of the actual metal, a judgment condition is needed, so there is a conditional yield strength.
For different metal components, the residual deformation corresponding to the conditional yield strength is different. For some harsh metal components, the residual deformation should be small, while the corresponding residual deformation when the ordinary metal components yield is larger. The commonly used residual deformation is 0.01%, 0.05%, 0.1%, 0.2%, 0.5% and 1.0%.
Figure 8 Conditional yield
The yield of metal is the result of dislocation movement, so the yield of metal is determined by the resistance of dislocation movement. For pure metals, it includes lattice resistance, dislocation interaction resistance, dislocation and other defects or structural interaction resistance.
Figure 9 Dislocations in actual metal aluminum
The straight line segment on the stretch curve, that is, the area corresponding to the elastic part is the elastic energy. From the beginning of elastic deformation to fracture, the total energy absorbed by the sample is called fracture work, and the energy absorbed by the metal before fracture is called fracture toughness.
The actual metal is usually accompanied by changes in mechanical properties during the stretching process. The most prominent phenomenon is work hardening. The work hardening of metal is beneficial to avoid the sudden fracture of actual engineering components when overloaded, which may cause catastrophic consequences.
Metal plastic deformation and deformation hardening are the prerequisites for ensuring uniform plastic deformation of metals. This means that in polycrystalline metals, where plastic deformation occurs, where it is strengthened, plastic deformation is suppressed, and the deformation is transferred to other more components.
From the actual tensile curve, after most metals yield at room temperature, the deformation will not continue under the action of the yield stress, and the resistance must be increased to continue the deformation. On the true stress-true strain curve, the flow stress continues to rise, and work hardening occurs. Such a curve is called a work hardening curve.
The work hardening index n is an important plastic index, which represents the ability of the material to resist continuous deformation.
Figure 10 Work hardening in metal plastic deformation
Finally, talk about strain rate. Generally, the tensile curves of metal materials tested are obtained by testing at a lower strain rate. Only some special metal components need to test their mechanical properties at higher strain rates, that is, components that undergo high-speed deformation. Under normal room temperature conditions, the strain rate is stretched, and the deformation of the material is mainly the slip or twinning of dislocations.
Figure 11 High-speed deformation curve of aluminum alloy
On the tensile curve, the maximum engineering stress on the engineering strain-engineering strain curve is called the ultimate tensile stress, which is the tensile strength.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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