Tension is a simple mechanical property test. Within the test gauge distance, the stress is uniform, the measurement of stress, strain and performance indicators is stable, reliable and convenient for theoretical calculation. Through tensile test, the most basic mechanical property indexes in the process of elastic deformation, plastic deformation and fracture can be measured, such as positive elastic modulus E and yield strength σ 0.2. Yield point σ s. Tensile strength σ b. Extension rate after fracture δ And reduction of area ψ Etc. The mechanical property indexes obtained in the tensile test, such as e σ 0.2、 σ s、 σ b、 δ、ψ Etc. are the inherent basic properties of materials and the main basis in engineering design.
Relationship between metal plastic deformation and tensile strength
For most metal materials, in the elastic deformation region, the stress and 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 atoms begins to break and is modified with a new group of atoms. When this happens, the material will not return to the original state after the stress is removed, that is, the deformation is permanent and irrecoverable, and then the material enters the plastic deformation zone (Fig. 1).
In fact, it is difficult to determine the exact point at which the material changes from the elastic zone to the plastic zone. As shown in Fig. 2, a parallel line with a strain of 0.002 is drawn. The stress-strain curve is truncated by this line, and the yield stress is determined 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. The yield of materials is a process, usually accompanied by work hardening, so it is not a specific point.
For most metal materials, the stress-strain curve looks similar to that shown in Fig. 3. When the loading begins, 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 the 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 at which the material finally breaks and is indicated by F in the figure.
The typical stress-strain test device and test sample geometry are shown in Fig. 4. During the tensile test, the sample is pulled slowly, and the changes of length and applied force are recorded. The force displacement curve is recorded. The stress-strain curve can be drawn by using the original length, gauge length and cross-sectional area of the sample.
Figure 4 stress strain test
For materials that can undergo tensile plastic deformation, two kinds of curves are most commonly used: Engineering stress-engineering strain curve and true stress-strain curve. The difference between them is that the area used in calculating the stress is different. The former uses the initial area of the sample, and the latter uses the real-time cross-sectional area during the tensile process. Therefore, on the stress-strain curve, the true stress is generally higher than the engineering stress.
Fig. 6 true stress and true strain curves of various real metal materials
There are two kinds of most common tensile curves: one is the tensile curve with obvious yield point; Second, the tensile curve without obvious yield point. The yield point represents the resistance of the metal to initial plastic deformation. This is one of the most important mechanical properties in engineering technology.
How to measure tensile strength from metal plastic deformation?
Residual plastic deformation is an important basis. Generally, the resistance corresponding to the engineering metal when a certain residual plastic deformation is artificially taken as the yield strength, also known as the conditional yield strength. That is, there is no obvious plastic yield point, and there is no obvious yield strength. If you want to know the yield strength of the actual metal, you need a judgment condition, so there is the 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 of ordinary metal components is large when they yield under conditions. The commonly used residual deformation is 0.01%, 0.05%, 0.1%, 0.2%, 0.5% and 1.0%.
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 and dislocation interaction resistance with other defects or structures.
The area corresponding to the straight section on the tensile curve, that is, the elastic part, is the elastic property. From the beginning of elastic deformation to the process of 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 mechanical properties of real metals usually change during the tensile process, and the most prominent phenomenon is work hardening. The work hardening of metal is helpful to avoid the sudden fracture of practical engineering components when overloaded, resulting in disastrous consequences.
Metal plastic deformation and deformation hardening are prerequisites for ensuring uniform plastic deformation of metal. That is to say, in polycrystalline metal, where plastic deformation occurs, it is strengthened, and then the plastic deformation is suppressed, so that the deformation can be transferred to other places more easily.
According to the actual tensile curve, after most metals yield at room temperature, the deformation will not continue under the action of yield stress, and the resistance must be increased to continue the deformation. On the true stress-strain curve, the rheological stress is rising and work hardening phenomenon appears. Such a curve is called a work hardening curve. Work hardening index n is an important plasticity index, which represents the ability of materials to resist continuous deformation.
Finally, let’s talk about the strain rate. Generally, the tensile curves of metal materials are obtained by testing at a lower strain rate. Only some special metal components need to test their mechanical properties under high strain rate, that is, the components with high-speed deformation. Under normal room temperature, the deformation of the material is mainly dislocation slip or twinning.
The maximum engineering stress on the tensile curve, that is, the engineering strain curve, is called the ultimate tensile stress, that is, the tensile strength.