Issue 32

N. Golinelli et alii, Frattura ed Integrità Strutturale, 32 (2015) 13-23; DOI: 10.3221/IGF-ESIS.32.02 15 The design of the MR damper consisted in two main parts: the hydraulic-mechanical design [12, 13] and the magnetic circuit design. The specifications of the damper we developed are listed in Tab. 2. Maximum force, (N) 2000 Maximum cylinder diameter, (mm) 40 Maximum working current, (A) 2 Maximum pressure, (bar) 40 Stroke, (mm) 50 Maximum velocity, (mm/s) 100 Table 2 : Damper specifications. In order to keep the manufacturing of the damper as simple as possible, a commercial hydraulic cylinder and the associated cylinder head were chosen [14]. Hence, knowing the outer diameter of the cylinder (50 mm) and the wall thickness (5 mm), even the inner diameter of the cylinder was also fixed (40 mm) (Fig. 3c, d). The axial length of the hydraulic cylinder is 192 mm. The commercial cylinder head is arranged for a piston rod diameter of 20 mm (Fig. 3a) and it has its own system of seals (Fig. 3b). The minimum axial length of the piston head was also fixed and had to be at least L = 90 mm. That is because we decided to compensate for the piston volume using the piston head, so it has to host a compensating bottom rod (50 mm), as presented in Fig. 2. (a) (b) (c) (d) Figure 3 : Commercial components. Cylinder head (a) and sealing system (b) . Commercial hydraulic cylinder (c) with welded boss. Bottom part of the cylinder (d) . Optimal design of magnetorheological devices requires the knowledge and the characterization of the properties of the materials involved. Firstly, the knowledge of the yield shear stress of the fluid as function of the magnetic field is necessary. The yield stress τ B ሺH mrf ሻ of MRF 140-CG [15] is given by the experimentally-derived equation from [16, 17] and depends on the magnetic field intensity and the particle volume fraction φ :        1.5239 6 271700 tanh(6.33 10 ) B mrf mrf H C H (1)