Modelling the Effect of Preload in a Lap-Joint by Altering Thin-Layer Material Properties 213 Fig. 2 Impact test of a free-free lap-joint Table 1 Measured natural frequencies for the first eight bending modes ModeNo. f1 f2 f3 f4 f5 f6 f7 f8 Exp. (Hz) 158.41 442.81 860.93 1383.5 2045.7 2869.4 3438.9 4279.7 (a) (b) Fig. 3 Conventional (a) and modified (b) thin-layer approach In addition, three miniature PCB accelerometers with 0.5 g mass, 10 kHz frequency range and 10 mg/s2 sensitivity were bonded to the middle and the two extremities of the beams were used to measure the response of the structure. The details of the experimental setup are presented in Fig. 2. The measured natural frequencies are tabulated in Table 1 and they are used in the next sections to identify the joint parameters. In this study, the bending behaviour of the lap-joint structure is of interest. Therefore, the first eight bending modes and their corresponding natural frequencies are considered and will be used in the identification of the joint structure in the rest of this study. 3 The Modified Thin-Layer Approach The force-displacement behaviour of a contact interface in the normal and tangential directions is characterized by the condition of the contact surface and the level of the bolt preload. For instance, by increasing the bolt preload, the linear stiffness of the contact interface in the tangential direction increases. The other parameter which has a direct effect on the force-displacement behaviour is the size of asperities or the contact surface roughness characteristics. Both bolt preload and contact surface roughness are variable over the contact interface: the former has a deterministic nature and the latter has a stochastic nature. In the conventional thin-layer approach, equivalent constant material properties are considered for the layer representing the contact interface, as shown in Fig. 3(a). The constant material properties are the result of this assumption that both the contact pressure and surface roughness have constant distributions. In an attempt to consider the variation of the contact pressure on the thin-layer characteristics, in this paper, the distribution of material properties shown in Fig. 3(b) is considered for the thin layer. The applicability of this approach is examined using 3D and 2D models of the lap-joint structure shown in Fig. 2.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==