Procedure for Tensile Testing of Metal deep drawing
1. Relevant indicators of metal tensile properties The tensile properties of metals at room temperature usually include four indicators: tensile strength, yield strength, also known as yield point or specified yield strength, elongation and area shrinkage. The former two are called strength indexes, and the latter two are called plasticity indexes.
The so-called strength refers to the ability of the metal to resist deformation or breaking at any moment during the axial tensile load process of the sample, which is generally expressed as the force on the original unit cross-sectional area. The plasticity is the elongation of the gauge length expressed in percent and the reduction rate of the original cross-sectional area at the fracture after the sample is stretched to break.
2. Tensile test steps
- Prepare the test piece. Tensile tests were performed on plain carbon steel and aluminum alloy samples of the same size, shape, and shape. Use a grading machine to engrave a circular line within the original gauge range. Divide the gauge length into 10 equal-length squares. The original diameter was measured to be 10mm, and the original gauge length was 100mm.
- Adjust the testing machine. Manually control the upper collet to the proper clamping position. Select the appropriate force gauge. Start the testing machine and raise the worktable by about 10mm to eliminate the influence of the self-weight of the worktable system. Adjust the active pointer to align with the zero point, the driven pointer and the active pointer are close to each other, and adjust the automatic drawing device.
- Clamp the specimen. First clamp the specimen in the upper chuck, then move the lower chuck to a suitable clamping position, and finally clamp the lower end of the specimen. (The aluminum alloy material has no obvious yield phenomenon, and the electronic extensometer needs to be reproduced)
- Inspection and test drive. Check the completion of the above steps. Start the testing machine, preload a small amount of load (the stress corresponding to the load cannot exceed the proportional limit of the material), and then unload to zero to check whether the testing machine works normally.
- Carry out the test. Start the testing machine, load it slowly and evenly, and carefully observe the rotation of the force measuring pointer and the drawing of the graph by the drawing device. Note that the yield load value is captured and recorded for the calculation of the yield point stress value. Pay attention to the slip phenomenon in the yield stage. After the yield phase, the loading speed can be faster. When it is about to reach the maximum value, pay attention to observe the phenomenon of “neck”. Stop immediately after the specimen is broken, and record the maximum load value. (The aluminum alloy sample has no obvious yield phenomenon)
- Remove the test piece and recording paper.
- Use a vernier caliper to measure the gauge length after the break.
- Use a vernier caliper to measure the minimum diameter of the neck down
3. Four stages in the deep drawing process of metal materials
The tensile curve finally obtained by the test is actually a load-elongation curve. There are four stages in this curve: elastic stage, yield stage, strengthening stage, and necking stage.
- Elastic stage: As the load increases, the strain increases proportionally with the stress. If the load is removed, the specimen will return to its original state, showing elastic deformation, and the elastic modulus E of the material can be measured at this stage.
- Yield stage: plain carbon steel: After exceeding the elastic stage, the load is almost unchanged, but fluctuates up and down within a small range, and the elongation of the sample increases sharply, which is called yielding. If this small fluctuation in the load reading is ignored, this stage can be represented by a horizontal line segment on the tensile diagram. The plastic deformation starts suddenly and the load number will drop suddenly. If all the loads are removed, the specimen will not return to its original length and manifest as deformation. For aluminum alloys, the end point of the elastic region is not accompanied by a sudden drop in load or other obvious change, and it is a smooth gradual curve from the elastic stage to the plastic stage.
- Strengthening stage: After the sample passes through the yield stage, the curve shows an upward trend. Due to the continuous strengthening of the material in the process of plastic deformation, the deformation resistance of the material is enhanced. This phenomenon is called strain hardening. If the deformation is not completely disappeared when the load is unloaded to zero at this stage, the residual strain when the stress is reduced to zero is called plastic strain or residual strain.
- In the necking stage and the fracture stage, after the sample is stretched to a certain extent, the load reading gradually decreases.
4. Relevant calculations
For materials with obvious yielding phenomenon:
- Upper yield strength ReH=FeH/S0 (S0 represents the original cross-sectional area, FeH represents the axial force corresponding to the upper yield point)
- Lower yield strength ReL=FeL/S0 (S0 represents the original cross-sectional area, FeL represents the axial force corresponding to the lower yield point)
- Tensile strength Rm=Fmax/S0 (Fmax refers to the maximum axial force)
For materials with no obvious yield phenomenon, it is stipulated that the stress value that produces 0.2% residual deformation is its yield limit, which is called conditional yield limit or yield strength. External forces greater than this limit will cause the parts to fail and cannot be recovered.