Experimental research on the influence of palliatives in fretting fatigue
There is an increasing demand of the industry to design and manufacture machines and constructions that are more powerful and at the same time smaller than previous ones. This leads to increased power density of the construction and the individual parts. Also the connection techniques that connect all the parts is become increasingly loaded. In all sectors of the industry connection techniques are seen in different shapes and dimensions. In the automotive industry engines, electro engine and combustion engines are mostly connected to the gearbox by a splined connection. In the aerospace industry the skin of the fuselage is connected to the main structure of an airplane by riveted connections. The wheels of a train are connected to the shaft by means of interference fit connections. In the energy industry turbines are used to convert fluid energy to mechanical energy. The blades of a turbine are connected to the rotor by dovetail connections. In engineering bolted connections are frequently used as a connection technique which is dismountable, simple and cheap. Examples are tower cranes that are assembled and disassembled on construction sites. Parts of the crane are connected to each other by prestressed bolted lap joints. Connection techniques that are smaller and transfer more force are more efficient but also more sensitive to premature failure. Connection techniques transfer forces from one or more parts toward others. The force is typically transferred by one or more contacts. Since the power density of machines increases this results in higher stress states on the contact areas. The likelihood of damage on the contact areas also increases. In a contact, damage can range from minimal topographic changes to substantial crack initiation. Crack initiation in combination with dynamic forces leads to crack propagation. Dynamic forces can originate from the purpose of the connection technique i.e. transferring dynamic forces, or can come from external sources such as vibrations. The combination of crack initiation at a contact with crack propagation caused by dynamic loading leads to the fretting fatigue mechanisms and results in much lower lifetimes compared to only the fatigue mechanisms. The goal of this work is to provide a better insight in the fretting fatigue mechanisms by experimental research. The research focusses on surface treatments that can be applied on connection techniques and result in higher fretting fatigue lifetimes. The second goal of this work is to demonstrate the potential of several field measurement techniques during fretting fatigue research. The first focus of this work is the quantification of the efficiency of some surface treatments based on the fretting fatigue lifetime. Three surface treatments are studied, all based on two palliative mechanisms. The first palliative mechanism is the application of a residual stress field that has a compressive stress state at the surface. Such a stress field is applied by deep rolling. The second palliative mechanism is the evacuation of debris particles from the surface by applying surface cavities in which particles can disappear. Cavities are applied at the surface by a laser ablation process. The combination of both palliative mechanisms is also studied. The technique used to apply both is a deep rolling process with textured rollers. For this research two fretting fatigue test rigs were designed and constructed. The first test rig is a coupon scale test rig where most important parameters can be controlled and measured. The second test rig is a full scale test rig that is very similar to real application in terms of geometry and loading conditions. The second test rig is a bolted lap joint connection that uses a prestressed bolt to connect three plates symmetrically. The combination of the test rigs on different test scales is extraordinary and allows drawing conclusions independent of test scale. More than 74 experiments are performed during this research. The samples are examined in several ways prior and after an experiment. Prior to an experiment, geometry, topography and characteristics of the palliative mechanism(s) are measured. Post mortem, the contact area and the fracture surface are microscopically and macroscopically examined. The results give more insight in the fretting fatigue mechanisms and the influence of the palliative mechanisms. The results of the surface treatments are significant in terms of lifetime and are witnessed on both types of test rigs. The deep rolling process gives a lifetime increase of a factor 2.7 at a lifetime of one million cycles. The laser ablation process does not give significant changes in lifetime. The deep rolling process with textures rolls, combining both palliative mechanisms, gives results which are in line with the ordinary deep rolling process. It can be concluded that the first palliative mechanism, applying a residual compressive stress field at the surface, influences the fretting fatigue mechanism. The second palliative mechanism, introducing surface cavities to evacuate debris particles from the contact, is not effective enough to influence the fretting fatigue mechanism. The second focus of this work is the implementation of field measurement techniques during coupon scale fretting fatigue research. By means of a camera contactless images are captured from the specimens. The measurement technique does not influence the experiment. Also, it is possible to measure closer to the contact compared to the traditional measurement techniques. Two measurement techniques have been studied: thermography and digital image correlation. Through a thermographic camera a field of temperatures is measured during an experiment. In the measured temperature field several mechanisms can be observed. The variation of the temperature of the samples can be related to the variation of the sum of the principal stresses. Thermographic measurements are successfully used to measure the fatigue load and the tangential load. In addition, the plastic zone at a crack tip can be visualised by thermography from which crack growth is calculated. Further research on thermography will give more detailed information of the stress state around the contact. Also crack propagation of smaller cracks will be measurable as well as crack initiation. Digital image correlation measures the position of every point of the samples in the three dimensional space. Prior to an experiment, the alignment of the samples is inspected. During an experiment the relative displacement of both contacts is independently measurable. From these measurements friction loops and tangential contact stiffness's are calculated. Further optimisation of the measurement technique will allow online measuring of the slip distribution in the contacts.