Influence of the geometry of steel inserts on the fatigue behaviour of steel-composite bolted assemblies

This study concerns the behaviour of composite-to-steel bolted connections and tvas carried out as part of a programme to develop a composite materials transmission shaft for railway propulsion at high torque levels. The basis of the study was to embed various types of steel insert into flat composite plate specimens then perform tensile/compressive fatigue tests on the bolted assemblies. Inserts were incorporated during the hot-press manufacturing cycle of the specimens. Fatigue testing was carried out under constant load conditions at a frequency of 10 Hz. Three types of insert geometry were devised; a simple tube of length equal to the specimen thickness, a tube with a wide circumferential groove on the outer diameter and a tube with an outer circumferential ridge in the middle of the height. A fourth type of specimen, with no insert, was used as a reference basis. All specimens were gripped between nat steel plates for the tests. The results showed that under the chosen test conditions the simplest configuration (no insert) performed best, the load being entirely transmitted by friction. The simple tube geometry showed a reduction of about 16 % compared to the reference while the grooved and ridged specimens where reduced by 50 and 58 % respectively. These results have lead to the definition of a new insert assembly that uses friction for transmitting the working loads and can protect the composite against any over-load. This design is being currently evaluated.