How to test a universal test machine

Close the fractures of the broken specimen together, use a vernier caliper to measure the diameter at the fracture and calculate the area; according to the following formula, the cross-sectional shortening rate of low-carbon steel can be calculated. From the damaged low-carbon steel specimen, it can be seen that the residual elongation everywhere is not evenly distributed. The closer you get to the break, the bigger the deformation, and the farther you get from the break, the smaller the deformation. Therefore, the measured value is related to the location of the break. In order to calculate the consistent value, the value measured is based on the break in the section in the middle of the gauge length. If the break is not in the section, the method of breaking is required to be converted by the method of shifting the break. The method is as follows: Assume that there are grids engraved between the two punctuation points, and the distance between each grid before stretching is equal. On the right section of the longest fracture specimen, take the grid to the right from a line near the break, mark it as, which is equivalent to placing the fracture in the middle of the gauge distance, and then looking at the point to the point, there are many grids, so as to take the same grid number from the point to the left, mark it with marks, so that the length displayed by the leak is equal to the grid number in the gauge length. When the fracture is very close to both ends of the specimen and the distance from its head is equal to or less than twice the diameter, the all-round experimental machine generally believes that the execution result is invalid and requires re-execution.

Preparation of the experimental machine (recommended: universal testing machine): First, check the basic structure and operating methods of the material testing machine, and learn the operating procedures of the experimental machine. According to the strength limit of low carbon steel and the cross-sectional area of ​​the specimen, the load required for tensile specimen is initially estimated, the appropriate measurement dial is selected, and the corresponding pendulum is equipped with an echoing pendulum, the thought device is opened, the force measuring pointer is adjusted to "zero point", and then the position of the chuck under the experiment machine is adjusted, and the specimen clip is installed in the chuck.

Preparation of test pieces: The all-round experimental machine takes the gauge distance in the middle section of the test piece or uses a foot marking to mark both ends of the gauge distance as a marking, and uses a vernier caliper to measure the diameters at the middle and both ends of the test piece within the limit of the gauge distance to take a small value as the calculation of the cross-sectional area of ​​the test piece.

Perform execution: After the test piece is clamped, the test piece is loaded slowly and evenly. Use the automatic drawing device on the experimental machine to draw the relationship curve between external force and deformation. When the load is added to the point, the upper section of the stretching diagram is a straight line indicating the proportional relationship between the load and the specimen in this stage, that is, the elastic deformation limitations that conform to the Hook's law. When the load is added to the point, the dynamometer pointer stays motionless or suddenly drops to the point and then swings within a small limit. At this time, the deformation is added very quickly and the load is added very slowly. This shows that the stress that the material has developed in motion echoes the point is called the upper motion limit and the echoes the stress. Because the pressure of the light motion limit is relatively unchanged, the general rule of the material's movement limit is taken by the light motion limit. Dividing the load value corresponding to the point by the original cross-sectional area of ​​the specimen, the movement limit of the low carbon steel is obtained. After the movement stage, the specimen must bear greater external forces before it can continue to deform. To increase the plastic deformation, a load must be added, such as the point to point in the figure is the strengthening stage. When the load reaches the value point, the plastic deformation of the specimen is concentrated in a small section at a certain section. The cross-section shortens in this section, which means "necking" occurs. Block the machine, remove the broken specimen and tightly match the broken specimen together, use a vernier caliper to measure the length between the gauge spacing of the specimen after breaking, and calculate the elongation of the low carbon steel according to the following formula.