JEC COMPOSITES MAGAZINE - Issue #33 - June 2007 - 35

Development Cylindrical design Optimised design experimental failure load was two times higher. The failure initiated at the predicted singular point 3 for the cylindrical IMD (Figure 1b) and at the singular point 2 for the optimised IMD (Figure 1a). Table 1: Critical loads from predictions and experiments [1]. Equipment Predicted failure load [kN] 7.1 20.0 Experimental failure load (average) [kN] 10.0 20.6 Load introductions shaped like Shipsha, Söderlund and Zenkert’s IMD can be both pre- and post-fitted. A disadvantage of pre-fitted load introductions is that the location of all fastenings has to be predetermined before the sandwich panel is assembled, which is very time-consuming during design and manufacturing. Nor are the intended locations of fastenings always known before the sandwich structures are used. A post-fitted insert is implemented in the core after assembling the sandwich. The core cavity is normally made with an ordinary milling tool. However, such a tool cannot create an optimised shaped cavity in a sandwich structure with an entry hole in the face sheet that is smaller than the cavity. A normal milling tool, therefore, needs to be used before the sandwich structure is assembled. Using such a tool on an assembled sandwich panel requires replacing or repairing the face sheet. This procedure is labour intensive, and may result in a weaker sandwich structure and an uneven surface. To simplify manufacturing and to make post-fitted inserts stronger, a development project was started in 2001. The objective of the project was to develop an economical nondestructive system to post-fit an insert with optimised geometry. To be able to enlarge the portfolio, support different sizes of optimised inserts and reach a bigger market, the project will also continue seeking a partner. Shipsha, Söderlund and Zenkert’s optimised geometry was the starting point for developing a system for optimised post-fitted inserts. According to their experimental testing of the optimised IMD, the failure initiated at point 2 (Figure 1a). This indicates that if the singular point can be eliminated by a smoother transition to singular point 1, the failure load will be further increased. This modification can be achieved with a small weight increase. Figure 2 shows the resulting geometry based on Shipsha, Söderlund and Zenkert’s result with an additional smoother transition to point 1. Experimental testing of the final geometry is scheduled. This new insert system includes a milling tool that creates a cavity with this specific geometry for post-fitted inserts without removing the face sheet. The geometry is also well suited for through-sandwich inserts. Fig. 2: Optimised cavity created by the milling tool. (a) Optimised geometry. (b) Optimised through-sandwich geometry. (c) The cavity created by the milling tool in a Divinycell H80 core. Fig. 3: The patented milling tool. (a) CAD model with closed up cutting sheath. (b) CAD model with partially expanded cutting sheath. c) CAD model with fully expanded cutting sheath. (d) Prototype. No33 June 2007 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #33 - June 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #33 - June 2007