Shock wave characteristics of a hydraulic damper for shock test machine

Abstract

A hydraulic damper is developed to generate the shock wave in this paper. The working principle of the damper is explained and the corresponding mathematical model is established. The shock wave characteristics under different shock velocities are obtained by using the numerical computation and experiment. The results show that the shock wave characteristics directly relate to the sectional area of the exhausted passages. The computational results agree well with the experimental data, which means that the proposed mathematical model can be used for the engineering design.

Introduction

Shock and vibration cannot only reduce the equipments’ lifetime and working precision, but also lead to structural degradation and component failure, which have been discussed by Wang and Hua [1], Scavuzzo and Pusey [2], Rittweger et al. [3], Yang et al. [4], and Richard et al. [5].

In order to anticipate the anti-shock ability of the equipment or the isolators, various shock test machines are developed. The general configuration of the shock machine is the drop machine, which consists of several vertical guide rods on which the table carrying the test item drops from certain height freely (Fig. 1). When the table strikes the machine base, the impact occurs. In order to obtain an impact characteristic with a given shape, the shock programmer is usually set between the test table and the machine base. The conventional drop machine usually uses the elastic material as the shock programmer, which has three disadvantages. First, the tested mass or impact severity is limited due to the strength of the programmer, and it cannot satisfy the requirements of modern aero and marine industries. Fig. 2 shows three kinds of damaged conventional programmers during our experiments, in which the white one, the red one and the blue one are made of a kind of density felt, polyurethane, and strengthened polyurethane, respectively. Second, the repetition depends on the characteristic of the programmer. Because the permanent deformation usually occurs after several impacts, the shock wave form cannot repeat accurately. Third, the conventional drop machine can only generate single shock wave which is usually true in the air, but in the underwater explosion, there exist dual shock waves. One is the positive shock, the other is the negative shock, which is discussed by many researchers including Wadley and Dharmasena [6], Moyer [7], Li [8], O’Daniel et al. [9], and Chen et al. [10]. In order to simulate this kind of dual shock waves, the machine which can generate dual shock waves should be developed, the first shock is positive shock which accelerates the tested item, the second shock is negative shock which makes the input velocity go to zero and the negative shock wave must be controllable. Elder and Arden [11] and Hunziker [12] used the servo valve to control the shock wave, which can only test lighter mass and supply mild impact.

The hydraulic damper is often used as an absorber to alleviate the violent impacts and attenuate the vibrations. Smooth force characteristic is emphasized in these applications, which are studied by Wang et al. [13], Hou [14], Eyres et al. [15], Jia et al. [16], Hu et al. [17], Lee and Moon [18] and Swevers et al. [19]. However, the hydraulic damper can also be developed to be a programmer, and because the kinetic energy can be absorbed simultaneously, the input velocity can go to zero in the end. By adjusting the parallel passage, the shock wave characteristics can be regulated. The shock wave characteristics of the hydraulic damper are discussed in this paper. This kind of hydraulic damper can be used in the drop machine to generate the serious impact, and it can also be used in the dual chock machine to generate the second shock wave soon after the first wave finishes. Because the shock wave is generated by decreasing or cutting off the fluid passage, the shock velocity and shock severity as well as the tested mass can be increased.