JEC COMPOSITES MAGAZINE - Issue #33 - June 2007 - 34

andwich face sheets are getting thinner and thinner; and the core material is getting less dense in order to optimise sandwich structures. This is a result of industrial demands to be more cost efficient and environment friendly. More effective production methods, less expensive materials, and better load-introduction solutions are important issues when developing more economical lightweight sandwich structures that fully compete with conventional construction techniques. Considering the amount of load introduction in a sandwich construction today, there is much to gain by increasing the performance and lowering the weight of inserts. Increasing the strength of inserts makes it possible to reduce the amount of load introduction and, thereby, to save on manufacturing cost significantly. JEC Composites Magazine / No33 June 2007 S The optimised non-destructive postfitted foam sandwich insert The load introduction issue HÅKAN JOHANSSON, M.SC., CEO COMPOSULT COMPOSITE CONSULTING AB, SWEDEN PETTER SJÖDIN, M.SC. One of the biggest difficulties in the sandwich industries today is load introduction. It is very important that the insert has a well-designed load-carrying geometry to ensure the strength of the sandwich structure. A new non-destructive, post-fitted insert system with optimised geometry for load introduction into sandwich structures makes it possible to cut manufacturing costs, minimise weight, and simplify design. resulted in an optimised insert geometry. The aim of the optimisation was to obtain a more even stress distribution in the core and to minimise the weight of the IMD for a transverse tension loading. The initial design was a standard axisymmetric cylindrical IMD. Shipsha, Söderlund and Zenkert showed that, by changing the insert geometry, a more evenly distributed stress could be obtained along the IMD surface and, by that, a higher predicted failure load. Figure 1b shows a standard cylindrical IMD and Figure 1a, the optimised IMD geometry with the same weight, but with reduced number of stress concentrations. Fig. 1: Models of sandwich panels with IMD. Singular points are marked with circles [1]. (a) Optimised design. (b) Cylindrical design. FEATURE “Construction” When applying high local loads through an insert in a sandwich structure, the loads have to be spread over a large surface because of the thin face sheets and weak sandwich core. Such an insert should preferably have a decreasing rigidity in order to avoid stress concentrations. Local stress concentrations in bimaterial edges often initiate failures. Well-designed optimised lightweight sandwich structures are often penalised with added weight when the loads introduced require higher density core/thicker face sheets or larger inserts. Shipsha, Söderlund and Zenkert presented a paper on an optimised internal metal doubler (IMD) for load introduction in sandwich panels at the 4th International Conference on Sandwich Construction (UK) [1]. They performed finite element analyses and shape optimisation of a pre-fitted IMD, which Shipsha, Söderlund and Zenkert studied a 60-mm PVC/foam (Divinycell H100) sandwich panel with glass-fibre-reinforced face sheets, 4 mm thick. The initial and optimised design was also experimentally tested in transverse tension to obtain the fracture loads. The results are summarised in Table 1. The predicted failure load for the optimised IMD was three times higher than for the cylindrically shaped IMD and the

JEC COMPOSITES MAGAZINE - Issue #33 - June 2007

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