Design optimisation for optically tracked pointers

The use of mechanical pointers in optical tracking systems is needed to aid registration processes of unlocated rigid bodies. Error on the target point of a pointer can cause wrong positioning of vital objects and as such these errors have to be avoided. In this paper, the different errors that originate during this process are described, after which this error analysis is used for the optimisation of an improved pointer design. The final design contains six coplanar fiducials, favored by its robustness and low error. This configuration of fiducials is then analysed theoretically as well as practically to understand how it is performing. The error on tracking the target point of the pointer is found with simulation to be around 0.7 times the error of measuring one fiducial in space. However, practically this error is about equal to the fiducial tracking error, due to the non-normally distributed errors on each separate fiducial.

Experimental investigation of the weldability of tubular dissimilar materials using the electromagnetic welding process

This paper describes the magnetic pulse welding process (MPW) for tubes. Material combinations of aluminium to steel and copper to aluminium were experimentally evaluated. The first major goal of this work is to experimentally obtain the optimal input parameters like the discharge energy, the stand-off distance and the tool overlap for MPW of the material combinations. Welding windows with all possible input parameters are created for both material combinations. Furthermore, a comparison is done between three coil systems; a single turn coil with field shaper, a single turn coil with a field shaper and transformer and a multi-turn coil and field shaper. Metallographic investigation of the samples, hardness tests and leak tests were executed to determine the most suitable machine set-up and the optimal input parameters for each set-up. A second major goal is to determine the influence of the target tube wall thickness on the deformation of tube-tube welds when no internal support is used.

Cycle-by-cycle simulation of variable amplitude fatigue crack propagation

In variable amplitude fatigue of high strength low alloy (HSLA) steel components, overloads can severely retard subsequent crack propagation for a number of cycles. In order to be able to predict fatigue crack propagation with a reduced degree of conservatism, retardation has to be taken into account. Of all numerical models that have been developed over time, crack tip plasticity models are selected based on the need for a detailed and fast cycle-by-cycle simulation of high cycle. After introducing the load interaction zone concept, common to all crack tip plasticity models, the Wheeler and Willenborg models are discussed, implemented and compared to experimental data. It is concluded that the Modified Wheeler model provides the most promising results, whereas the main limitation of Willenborg models is the need for extensive experimental data.

Fatigue fracture assessment of high strength steel using thermographic analysis

Fatigue behaviour is most commonly evaluated in uni-axial cyclic stress tests using standardised dog-bone samples. When components are sharply bent into shape and subjected to cyclic loading, the fatigue damage will accumulate at the inside of the bend. This paper reports on an experimental investigation about the feasibility of infrared thermographic techniques to monitor fatigue damage initiation and accumulation. By monitoring spectral components of the thermal response, the fatigue limit, the onset of crack initiation and the ratio of initiation to propagation lifetime can all be determined. Also the effect of surface treatments on initiation properties is investigated. Most results are consistent with expected behaviour based on a previous study, indicating that thermographic techniques have a greater sensitivity and can be used to reduce the number of samples and time required for fatigue characterisation.

Feasibility study of the friction surfacing process

Friction surfacing is a solid state cladding process based on the plastic deformation of a translating and rotating metallic consumable rod pressed against a stationary substrate. It is mostly used on mild and stainless steel and on aluminium. Thanks to the solid state nature of the process, it allows to join dissimilar metal combinations, e.g. aluminium to steel or to ceramics or several combinations of non-ferrous metals. Moreover, a continuous and fine-grained deposition is formed. Most research has been focussed on the feasibility of certain material combinations and on correlating the deposited layer quality to input parameters. In this work, a methodical approach to evaluate clad layers and to assess their properties is discussed. This approach consists of a visual assessment, a macrographic examination and a performance analysis and has shown to be apt to compare the clad layer quality.

Experimental and numerical analysis of deformation patterns in notched heterogeneous welds

Standardized weld flaw assessment techniques assume the weld region to be homogeneous which is a strong idealisation of reality. Characterising the effects of heterogeneous properties of welds through the analysis of deformation patterns and slip lines is the major concern of this research. It is the goal to investigate which effects these variations in properties within the weld material have on the propagation of cracks within the weld material. Performed experiments are SENT tests on strongly heterogeneous welded connections. The same material is also simulated with a weld heterogenisation model in ABAQUS®. Results from both experiments and simulations are discussed and compared. It is shown that slip lines tend to avoid zones of high hardness in a way that a path of least resistance is found. Related to this, it is seen that the slip line angles deviate from the theoretical 45° for homogeneous material. Obtained results validate the numerical model used.

Investigation of the weldability of copper to steel tubes using the electromagnetic welding process

Magnetic pulse welding is an innovative joining method which allows joining of dissimilar metal combinations. However, much remains unknown about the process and its parameters. In this paper, the weldability of copper tubes to steel rods and tubes is discussed, with the goal of examining the influence of the wall thickness of the supporting steel tube on the weld and the deformation of the components. Large deformations were observed, causing an undesirable decrease in diameter of the tubes. The quality of the obtained welds was shown to decrease with decreasing inner tube thickness as well, most likely due to the deformation of the workpieces in radial direction. Because of this, it is advisable to use an internal support to prevent deformation of the support tubes. To gain more insight in the precise mechanisms of weld formation and failure, numerical simulations are advised.

A comparison of fatigue lifetime prediction models applied to variable amplitude loading

The loads imposed on, amongst others, offshore structures can vary considerably with time. Lifetime prediction methodologies need to consider possible acceleration and retardation of the crack growth rate due to load sequences. Models based on a linear accumulation of damage will have a limited accuracy and are not considered as a sufficiently valuable asset in lifetime prediction of structures subjected to variable amplitude loading. This necessitates more complex nonlinear damage evolution models that can be applied in a so-called cycle-by-cycle analysis. In this paper, a comparison is made between three cumulative damage models (Miner, modified Miner and weighted average) and two yield zone models (Wheeler and Willenborg). Experimental data of fatigue crack growth in offshore steel specimens subjected to sequential loading is used as basis of the comparison. The modified Miner model is the most promising of the cumulative damage models but the determination of the parameter α requires laboratory tests. Evaluation of the effects of variation in the model input parameters on estimated lifetime reveals a large influence for the Miner and weighted average approaches.

3D finite element modeling of edge and width drop behavior in hot rolling mill

Hot rough rolling is a conventional forming process in modern steelmaking practice in which high deformations are applied to a steel slab at high temperatures. Due to the sequence of edge rolling followed by rough rolling, so-called edge and width drop phenomena are observed at the head and tail of the slab. These unwanted effects govern a yield loss and need to be minimized as much as possible. By means of a finite element study this research aims to discover the main influencing parameters on the observed edge and width drop behavior. An overview and comparison of the relative contributions of several edge rolling settings are presented. The net edger roll opening is the most important influencing parameter on edge and width drop behavior. The effect of width and thickness of the slab on the edge drop is less strongly pronounced; only the thickness influences the width drop behavior.

Metallographic evaluation of the weldability of high strength aluminium alloys using friction spot welding

Friction spot welding is a recent solid-state welding technique well suited for spot-joining lightweight materials in overlap condition. Aerospace and transport industries show great interest in this technique to join high-strength aluminium alloys, but published research is still limited. In this project, the link between process parameters and weld quality is investigated for EN AW-7075-T6 material. Techniques used are metallographic qualification, measurement of hardness reduction and lap shear strength. This paper focusses on the metallographic investigation of the weld region and its imperfections. Increasing joining time and heat input creates an easier material flow resulting in fewer imperfections. Limited plunge depths lead to typical interface imperfections. Variation in the rotational speed shows distinctive stir zone shapes as a consequence of severe stirring and frictional heat.