Figure 1 shows the small Hopkinson pressure bar equipment used in this experiment. Figure 2 is a schematic diagram of the separate Hawkinson pressure bar experimental device. Hopkinson pressure bar equipment, simple structure. Among them, the most demanding parts are bullet, incident rod and transmission rod, which have the same diameter and material, and the flatness of the contact face between them determines the trend of the stress-strain curve. Their bearing strength determines the maximum strain rate that can be achieved, so bullets, incident rods and transmission rods must be heat treated, then straightened, and finally polished with a grinder.
The high sensitivity strain gauge affixed to the incident rod and the transmission rod is used to collect the stress wave flowing through the rod, and then the waveform collector receives the stress wave, which is filtered and output to the computer. The sticking position of the strain gauge on the incident rod and the transmission rod should be symmetrical, otherwise, the output incident wave and transmission wave will be misplaced, and the calculated flow stress will be deviated. The schematic diagram of the flow direction of the stress wave in the equipment is shown in figure 3, the contact surface between the incident rod and the sample is recorded as the contact surface 1, and the contact surface between the sample and the transmission rod is recorded as the contact surface 2.
Before the experiment, the air pump is first inflated, and then the bullet is fired from the gun barrel under the action of the air pump, and the bullet hits the incident rod, and the incident wave is generated in the incident rod. The strain gauge on the incident rod will collect the incident wave, and the collected incident wave can directly reflect the strain rate that can be achieved. Then the incident wave is transmitted to the contact surface 1, one part of the stress wave is transmitted to the sample, and the other part reflects back to the incident rod. The stress wave transmitted to the sample is transmitted to the transmission rod through the contact surface 2 to form a transmission wave. The strain gauge on the transmission bar collects the transmission wave, and the quality of the transmission wave directly affects the flow stress under the current experimental conditions. According to the one-dimensional stress wave theory, the relations of strain, strain rate and stress with time are calculated.
The displacements of contact surface 1 and contact surface 2 are respectively.
In the formula, G is the wave velocity of the stress wave in the incident rod and the transmission rod, and Q is the strain produced by the incident wave in the rod, the strain caused by the reflected wave in the rod, and coincidentally the strain produced by the transmitted wave in the rod.
The average strain of the sample can be obtained from the formula.
Where L is the length of the sample. The loads at contact surface 1 and contact surface 2 are respectively.
Where An is the cross-sectional area of the incident rod and the transmission rod. The average stress of the specimen is.
Where As is the cross-sectional area of the sample. According to the assumption of uniformity.
Substitute the formula into the formula to get.
Where E is the elastic modulus of the incident rod and the transmission rod.
Different strain rates are achieved by controlling the pressure of the air pump, because the air pump itself is not an accurate device, so the speed of bullets fired at the same pump pressure is not the same, but it will be near a certain value, which does not affect the progress of the experiment. When doing dynamic impact under the condition of high temperature, the sample is heated to a preset temperature by using a high temperature heating furnace alone, and the synchronous device enables the incident rod and the transmission rod to contact the sample quickly, while the operator controls the firing of the bullet. in this process, heating the incident rod and the transmission rod should be avoided.