L. Van Schepdael

Large-scale evaluation of constrained bearing elements made of thermosetting polyester resin and polyester fabric reinforcement

Polymer composites are increasingly used as sliding materials for high-loaded bearings, however, their tribological characteristics are most commonly determined from small-scale laboratory tests. The static strength and dynamic coefficients of friction for polyester/polyester composite elements are presently studied on large-scale test equipment for determination of its bearing capacity and failure mechanisms under overload conditions. Original test samples have a diameter of 250 mm and thickness of 40 mm, . corresponding to the practical implementation in the sliding surfaces of a ball-joint, and are tested at various scales for simulation of edge effects and repeatability of test results. Static tests reveal complete elastic recovery after loading to 120 MPa, plastic deformation after loading at 150 MPa and overload at 200 MPa. This makes present composite favorable for use under high loads, compared to, e.g., glass-fibre reinforced materials. Sliding tests indicate stick-slip for pure bulk composites and more stable sliding when PTFE lubricants are added. Dynamic overload occurs above 120 MPa due to an expansion of the nonconstrained top surface. A molybdenum-disulphide coating on the steel counterface is an effective lubricant for lower dynamic friction, as it favorably impregnates the composite sliding surface, while it is not effective at high loads as the coating is removed after sliding and high initial static friction is observed. Also a zinc phosphate thermoplastic coating cannot be applied to the counterface as it adheres strongly to the composite surface with consequently high initial friction and coating wear. Most stable sliding is observed against steel counterfaces, with progressive formation of a lubricating transfer film at higher loads due to exposure of PTFE lubricant. Composite wear mechanisms are mainly governed by thermal degradation of the thermosetting matrix (max. 162°C) with shear and particle detachment by the brittle nature of polyester rather than plastic deformation. The formation of a sliding film protects against fiber failure up to 150 MPa, while overload results in interlaminar shear, debonding, and ductile fiber pull-out.

Short-beam-shear testing of composite rings with variable cross section as representative for full-scale delamination

Carbon fiber reinforced/epoxy (CFR-E) rings are used as radial reinforcement for polymer bearing elements with diameter 249 mm, functioning under 150-MPa contact pressure. Full-scale loading revealed shear failure caused by a critical radial-axial shear stress. This stress, acting in a section of the CFR-E ring under nonhydrostatic conditions, should be representatively simulated on small-scale short-beam-shear tests. Finite element analysis will be used for verification of a small-scale stress failure criterion. A standard sample geometry could not be applied as the reinforcing ring contains machined edges leading to stress concentrations. A nonhydrostatic stress condition similar to full-scale tests should be simulated in the center of the small-scale beam. Therefore, the effects of various loading conditions and testing parameters such as cross-sectional geometry, beam support and span, beam curvature, and convex/concave loading are investigated. It is concluded that curved beams are preferred over flat geometries and convex loading is preferred above concave loading since it induces a nonhydrostatic stress situation identical to full-scale tests. During small-scale testing of nonrectangular ring sections, an asymmetric stress distribution near the machined edge is representatively simulated. Experimental short-beam-shear tests confirm the influence of the test geometry and a 27-kN equivalent normal load is required for avoiding fulls-cale fracture.

Characterization of Composites for the Maeslant Storm Surge Barrier

The south-west part of the Netherlands is located in a low-lying delta, where the Rhine and IJssel rivers run into the North Sea. In 1953, the fatal combination of a north-western storm and spring tide resulted in the inundation of large parts of the provinces of Zeeland and South Holland. In order to prevent a repetition of the disaster, a set of measures were laid down in the Delta Act. As the seaports of Rotterdam and Antwerp had to remain accessible, no dams could be built in the New Waterway and the Western Scheldt. The elevation of the dikes was not an option as well. Finally, in the 1980s, a movable storm surge barrier appeared to be the most attractive in terms of cost, environmental effects and safety [1].