Design of strain gauge tester

1 Introduction

The various dynamic parameters of artillery in the bore, in flight, and when hitting the target are necessary means to obtain important data in the development and improvement of artillery systems. The lack of these dynamic data in the artillery independently developed by China has brought many difficulties to the development and improvement work. The artillery chamber pressure test requires that the volume of the test instrument must be less than 2.5% of the chamber volume, and the current test instruments cannot meet the small-caliber chamber pressure test. In order to reduce the volume of the test instrument, a strain gauge tester was designed using storage technology. This tester can meet the requirements of small and medium-caliber artillery chamber pressure testing, and provide an effective testing method for the development of various new artillery, ammunition, and fuze systems. It has a wide range of civilian promotion value.

2 System working principle

Figure 1 shows the structure of the strain gauge tester. It consists of a high-strength elastic body, a resistance strain gauge, a circuit module, etc. When the elastic body senses the measured value, it is deformed and its surface is strained. The resistance strain gauge attached to the elastic body and the surface constitutes the sensor of the tester, which will produce strain with the deformation of the elastic body, that is, it will change accordingly with the resistance value of the resistance strain gauge. In this way, the measured value is determined by measuring the change in the resistance value of the resistance strain gauge. After the measured value is determined, the circuit module completes data acquisition and storage.

3. Tester structure design

3.1 Elastomer structure design

The strain tester measures the magnitude of the measured force by measuring the strain at the patch part on the elastomer through the resistance strain gauge. To ensure that the stress at the patch part and the measured force maintain a strict correspondence, it is actually to ensure that the strain at the patch part on the elastomer is distributed according to a certain rule when the tester is subjected to force. In practical applications, the major factor affecting the strain distribution at the patch part of the elastomer is the change in the stress conditions of the elastomer. For the tester, under the condition of sufficient bearing strength, if the unpatched part around the patch part of the elastomer is hollowed out. The stress can only be distributed in the unhollowed part, or the local stress concentration is caused by locally weakening the elastomer. Both operations can make the stress at the place where the resistance strain gauge is pasted higher than the stress level of other parts. Figure 2 shows the designed elastomer structure.

Since the elastic body structure is a thick plate with a variable cross-section, it can be seen from the thick plate deformation theory that at r=0, the X-direction stress is equal to the Y-direction stress, the tangential strain is basically equal to the radial strain, and has a positive maximum value. The strain under the external pressure of 500 MPa is analyzed using ANSYS software, as shown in Figure 3. As can be seen from the figure, the strain of the elastic body at r=0 is the most obvious. Therefore, the resistance strain gauge should be pasted in the center.

When different pressures are applied to the elastic body using ANSYS software, the strain of the elastic body patch under different external pressures is simulated. The relationship between the strain and external pressure at the patch part shown in Figure 4 is plotted using the simulated strain data. It can be seen from Figure 4 that the strain at the elastic body patch is distributed according to a linear law.

3.2 Simplified Design Analysis

Figure 5 shows the compression diagram of the cylinder.

When the cylinder is subjected to external pressure, its elastic deformation stress is:

Where: ri is the inner radius of the cylinder; re is the outer radius of the cylinder; r is the radial coordinate of the stress calculation point; pe is the external pressure.
The yield condition of Mises is:

According to the Mises yield condition, when the maximum stress reaches the yield stress, the inner wall enters the plastic state. Assuming the maximum external pressure is pEL, we have:

Based on the relationship between ri and re in formula (6), we can design the values ​​of ri and re that keep the cylinder in elastic deformation and minimize its volume under the maximum external pressure.

Through the structural design analysis of the strain gauge tester, a design scheme is proposed to minimize the tester volume when the volume of the circuit module is determined. The advantage of this design is that the tester shell is used as the sensitive device of the strain gauge sensor. This greatly reduces the space for installing the piezoelectric sensor in the tester and provides a simplified body with the smallest volume. The experimental results show that this design scheme has good practicality and reduces the volume of the tester by at least 5 to 10 m3.

The most important components in the structural design of strain gauge testers are the sensitive elastic body that senses external pressure and the high-strength cylinder. The design of the elastic body must meet the following requirements: first, the stress at the patch part must maintain a strict correspondence with the measured force; second, the patch part should have a higher stress. In addition to considering the connection with the elastic body, the cylinder design must also analyze its stress conditions. While ensuring its strength, the wall thickness of the cylinder should be the thinnest and the volume should be the smallest, so that the volume of the tester can be minimized.