JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia - 39

Application with a woven-roving complex that would provide good mechanical properties with the low-technology moulding technique, while the mat would naturally provide a good wear surface. Finite element analysis iterations were performed in the following order: solid plate of uniform thickness, solid plate with variable thickness, sandwich plate with constant thickness skins and, finally, sandwich plate with variable thickness skins. As expected, the last configuration gave the most promising results and was used for the next step. Isophthalic polyester resin was chosen for the matrix (no gelcoat) and a typical woven roving/mat complex was used as reinforcement (300 g/m² mat + 800 g/m² WR). A first plate was successfully produced using a PU foam core. Although a carefully calculated quantity of resin was used and very little resin was pulled out of the composite during fabrication, the final thickness was found to be lower than the theoretical value (about 13 mm instead of 15 mm). The plate was then placed over a shallow 800 x 800 mm hole located at the entrance to a SRBG warehouse where mixed traffic would be passing throughout the day. It was held in place by 8 bolts screw-hammered into the asphalt and passing through coneshaped bushings fitted into holes moulded in the plate. A fully laden lorry with a wheel positioned in the centre of the plate was used to define the initial deflection basis by measuring the height of the upper surface with respect to the surrounding ground; then the plate was left to survive as best it could. After a month, new readings were taken, showing increased deflection under load. On investigation, this was caused by a deterioration of the foam core material and partial subsidence of the sides of the hole. It was reassuring to note that, despite the considerable relative movement between the plate and the road surface, there was virtually no wear on the underside. The upper side was also undamaged and had taken on a slight polish due to the traffic. The main conclusion at this stage was that the application needed a better-performing core material as the foam was slowly reduced to powder by the repeated load cycles. It was decided to replace the PU foam core with balsa, which offers high transverse compression strength. Balsa would also better transfer shear stresses between skins. Additionally, two slots were moulded in two opposite sides of the plate to provide handgrips, with no impact on performance. Two plates were produced using the new core material, one for mechanical testing and the other one to replace the first plate and continue the on-site evaluation process. Fig. 1: Computation result with MSC Nastran. Prototype testing Fig.2: Sandwich plate detail. Prototype production A simple mould was built using a plywood base 18 mm thick surrounded by a tubular steel frame. The resulting 1,200 x 1,200 mm working area eliminated the need to machine the plates after moulding. Regularly spaced holes were bored on the inside of the frame to enable uniform vacuum application all around the plate from a single vacuum source. The hollow frame also provided a convenient resin trap. Vacuum bagging consumables were used in the usual way, but an absorption layer was placed on both sides of the plate to ensure that the core material does not impair resin distribution throughout the plate thickness. Testing was carried out on a 50-tonne load frame equipped with an hydraulic actuator and appropriate load cells. A square base was produced using rectangular tubes, with an 800 x 800 mm passage to simulate a hole in the road. Rubber buffer strips were placed between the road plate and the steel base to avoid local high compression spots that might have initiated a failure crack. Similarly, rubber sheeting was piled under a central loading plate (200 x 300 mm) to simulate the behaviour of tyre action. The loading schedule consisted in a series of ratcheting load increases with pauses up to rupture. The whole test lasted only 20 min. Throughout the test, the main strains were measured using a series of strain gauges (X). The results are presented in Figure 4, showing the failure mode of the plate. No44 October 2008 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #44 - October 2008 - Special Asia