Application of an edge detection algorithm for surface determination in industrial X-ray computed tomography

Surface determination is an essential step of the measurement process in industrial X-ray computed tomography (XCT). The starting point of the surface determination process step is a single grey value threshold within a voxel volume in conventional surface determination methods. However, this value is not always found in the reconstructed volume in the local environment of the surface of the measurement object due to various artefacts, so that none or incorrect surfaces are determined. In order to find surfaces independently of a single grey value, a three-dimensional approach of the initial contour determination based on a Prewitt edge detection algorithm is presented in this work. This method is applied to different test specimens and specimen compositions which, due to their material or material constellation, their geometric properties with regard to surfaces and interfaces as well as their calibrated size and length dimensions, embody relevant properties in the examination of joining connections. It is shown that by using the surface determination method in the measurement process, both a higher metrological structure resolution and interface structure resolution can be achieved. Surface artefacts can be reduced by the application and it is also an approach to improved surface finding for the multi-material components that are challenging for XCT.

Analysis of the interactions between joint and component properties during clinching

Clinching is a joining process that is becoming more and more important in industry due to the increasing use of multi-material designs. Despite the already widespread use of the process, there is still a need for research to understand the mechanisms and design of clinched joints. In contrast to the tool parameters, process and material disturbances have not yet been investigated to a relatively large extent. However, these also have a great influence on the properties and applicability of clinching. The effect of process disturbances on the clinched joint are investigated with numerical and experimental methods. The investigated process variations are the history of the sheets using the pre-hardening of the material, different sheet thicknesses, sheet arrangements and punch strokes. For the consideration of the material history, a specimen geometry for pre-stretching specimens in uniaxial tension is used, from which the pre-stretched secondary specimens are taken. A finite element model is set up for the numerical investigations. Suitable clinching tools are selected. With the simulation, selected process influences can be examined. The effort of the numerical investigations is considerably reduced with the help of a statistical experimental design according to Taguchi. To confirm the simulation results, experimental investigations of the clinch point geometry by using micrographs and the shear strength of the clinched joint are performed. The analysis of the influence of difference disturbance factors on the clinching process demonstrate the importance of the holistic view of the clinching process.

Numerical and experimental investigation of flat-clinch joint strength

The striving for energy savings by lightweight construction requires the combination of different materials with advantageous properties. For joining sheet metal components, clinching offers a good alternative to thermal joining processes. In contrast to thermal joining processes, the microstructure in the joining zone remains largely unaffected. Conventional clinch joints, however, have a protrusion on the underside of the joint, which restricts their use in functional and visible surfaces. Flat-clinching minimizes this disadvantage by using a flat anvil instead of a die. Due to the flatness on the underside, it can be used in visible and functional surfaces. This paper deals with the increase of joint strength by using an auxiliary joining element (AJE) in the second forming stage. To achieve optimum improvement in the joint strength of an aluminum Al99.5 H14 sheet metal joint and to save costs, the AJE was varied numerically in terms of volume, material and basic shape. The geometric parameters (e.g., interlocking f and neck thickness tn) do not allow direct derivation of the joint strength. For this reason, the 2D clinch model was extended for the first time to include 3D load models (cross tension, shear tension). To validate the numerical results, optimized flat-clinch joints with AJE and the associated load tests were implemented experimentally. The numerical models were used to improve the process development.

Investigation of the influence of varying tumbling strategies on a tumbling self-piercing riveting process

The increasing demands for the reduction of carbon dioxide emission require intensified efforts to increase resource efficiency. Especially in the mobility sector with large moving masses, resource savings can contribute enormously to the reduction of emissions. One possibility is to reduce the weight of the vehicles by using lightweight technologies. A frequently used method is the implementation of multi-material systems. These consist of dissimilar materials such as steel, aluminium or plastics. In the production of these systems, the joining of the different materials and geometries is a central challenge. Due to the increasing demands on the joints, the challenges for the joining processes itself are also increasing. Since conventional joining processes are rather rigid and can only react to a limited extent to disturbance variables or changing process variables, new methods and technologies are required. A widely used conventional joining method with these properties is self-piercing riveting. Because of the rigid tool combination and the fact that the rivet geometry that can be used is related to the tools, the joining of multi-material systems requires tool and rivet changes during the process. In order to extend the process window of joining with self-piercing rivet elements, the process is enhanced with a tumbling kinematic of the punch. The integration of tumbling results in a significant increase in the adjustable process parameters. This enables a higher material flow control in the joining process through a specific tumbling strategy. The materials investigated are a steel and an aluminium alloy, which differ significantly in their mechanical properties and have many applications in automotive engineering, especially for structural car body components. The steel material is a galvanized HCT590X+Z dual-phase steel, which is characterised by a low yield strength, combined with high tensile strength and a good hardening behaviour. The aluminium alloy is an EN AW-6014. The precipitation-hardening alloy consists of aluminium, magnesium and silicon with a high strength and energy absorption capability. The objective of this work is to obtain a fundamental knowledge of the new tumbling self-piercing riveting process. With different mechanical properties and different sheet thicknesses of the joining partners, the influences of these parameters on the tumbling strategy of the riveting process are analysed. Such a tumbling strategy is based on the tumbling angle, the tumbling onset and the tumbling kinematics. These parameters are investigated in the context of the work for selected combinations of multi-material systems consisting of HCT590X+Z and EN AW-6014. With the variation of the parameters, the versatility of the process can be investigated and influences of the tumbling on the self-piercing riveting process can be identified. To illustrate the results, force–displacement curves from the joining process of the individual joints are compared and the geometry of the rivet undercut and rivet heads are geometrically measured. Furthermore, micrographs allow the analysis of the characteristic joint parameters interlock, residual sheet thickness and end position of the rivet head.

Joining of CFRT-steel hybrid parts via hole-forming and subsequent pin caulking

In times of increasing global warming, the awareness of the necessity for significant CO2 reduction is growing. Especially in the transport and aerospace sector, lightweight construction has potential to achieve emission reduction goals by reducing the overall vehicle weight. Thereby, adding lightweight fibre-reinforced composites to materials such as steel and aluminium is used to achieve weight savings. Furthermore, continuous-fibre-reinforced thermoplastics (CFRTs) begin to replace more traditional thermoset thermoplastics due to their easier bulk production and uncomplicated storage. Hybrid parts often consist of a CFRT and a higher strength metal component. Here, the joining process poses the main challenge, due to different chemical and physical properties of the components. In the current state of the art, riveted and bolted joints are commonly used, leading to increased weight due to auxiliary elements and requiring precise bolt holes often destroying load-bearing fibres. Joining with cold formed pin structures is an innovative and versatile joining process, which avoids the need for auxiliary elements. These pins are subsequently inserted in warm formed holes in the CFRT component and then caulked to create a form-fitting hybrid joint. To obtain a fundamental understanding of this joining process, hole-forming and pin-caulking, are investigated in this study. First, the hole-forming with IR-radiation is investigated with regard to suitable process parameters and resulting fibre morphology. The formed holes are consequently mechanically characterized. Second, the caulking-process is investigated by iteratively upsetting a pin and subsequently measuring the geometry. Based on these findings two different suitable caulking degrees are defined and samples for mechanical as well as microscopic investigations are manufactured. The created joints are first investigated via micro-sections and reflected light microscopy to identify possible damage in the CFRT component, which can result from the pin caulking process. Second, a mechanical characterisation under shear load as well as pin extraction loads normal to the sample surface is conducted and the normal load tests are compared with the bearing strength of CFRT samples.

A contribution on versatile process chains: joining with adaptive joining elements, formed by friction spinning

Nowadays, manufacturing of multi-material structures requires a variety of mechanical joining techniques. Mechanical joining processes and joining elements are used to meet a wide range of requirements, especially on versatile process chains. Most of these are explicitly adapted to only one, specific application. This leads to a less flexibility process chain due to many different variants and high costs. Changes in the boundary conditions like sheet thickness, or layers, lead to a need of re-design over the process and thus to a loss of time. To overcome this drawback, an innovative approach can be the use of individually manufactured and application-adapted joining elements (JE), the so-called Friction Spun Joint Connectors (FSJC). This new approach is based on defined, friction-induced heat input during the manufacturing and joining of the FSJC. This effect increases the formability of the initial material locally and permits them to be explicitly adapted to its application area. To gain a more detailed insight into the new process design, this paper presents a detailed characterization of the new joining technique with adaptive joining elements. The effects and interactions of relevant process variables onto the course and joining result is presented and described. The joining process comprises two stages: the manufacturing of FSJC from uniform initial material and the adaptive joining process itself. The following contribution presents the results of ongoing research work and includes the process concept, process properties and the results of experimental investigations. New promising concepts are presented and further specified. These approaches utilize the current knowledge and expand it systematically to open new fields of application.

Holistic investigation chain for the experimental determination of fracture mechanical material parameters with special specimens

In addition to the classical strength calculation, it is important to design components with regard to fracture mechanics because defects and cracks in a component can drastically influence its strength or fatigue behavior. Cracks can propagate due to operational loads and consequently lead to component failure. The fracture mechanical analysis provides information on stable or unstable crack growth as well as about the direction and the growth rate of a crack. For this purpose, sufficient information has to be available about the crack location, the crack length, the component geometry, the component loading and the fracture mechanical material parameters. The fracture mechanical properties are determined experimentally with standardized specimens as defined by the guidelines of the American Society for Testing and Materials. In practice, however, especially in the context with damage cases or formed material fracture mechanical parameters directly for a component are of interest. However, standard specimens often cannot be extracted at all due to the complexity of the component geometry. Therefore, the development of special specimens is required whereby certain arrangements have to be made in advance. These arrangements are presented in the present paper in order to contribute to a holistic investigation chain for the experimental determination of fracture mechanical material parameters with special specimens.

Application of reinforcement learning for the optimization of clinch joint characteristics

Due to increasing challenges in the area of lightweight design, the demand for time- and cost-effective joining technologies is steadily rising. For this, cold-forming processes provide a fast and environmentally friendly alternative to common joining methods, such as welding. However, to ensure a sufficient applicability in combination with a high reliability of the joint connection, not only the selection of a best-fitting process, but also the suitable dimensioning of the individual joint is crucial. Therefore, few studies already investigated the systematic analysis of clinched joints usually focusing on the optimization of particular tool geometries against shear and tensile loading. This mainly involved the application of a meta-model assisted genetic algorithm to define a solution space including Pareto optima with all efficient allocations. However, if the investigation of new process configurations (e. g. changing materials) is necessary, the earlier generated meta-models often reach their limits which can lead to a significantly loss of estimation quality. Thus, it is mainly required to repeat the time-consuming and resource-intensive data sampling process in combination with the following identification of best-fitting meta-modeling algorithms. As a solution to this problem, the combination of Deep and Reinforcement Learning provides high potentials for the determination of optimal solutions without taking labeled input data into consideration. Therefore, the training of an Agent aims not only to predict quality-relevant joint characteristics, but also at learning a policy of how to obtain them. As a result, the parameters of the deep neural networks are adapted to represent the effects of varying tool configurations on the target variables. This provides the definition of a novel approach to analyze and optimize clinch joint characteristics for certain use-case scenarios.

Hybrid manufacturing of topology optimized machine tool parts through a layer laminated manufacturing method

Load-oriented lightweight structures are commonly designed based on topology optimization. For machine tool parts, they enable the reduction of moving masses and therefore increase the resource and energy efficiency of production systems. However, this usually results in complex part structures that are difficult or impossible to produce using conventional manufacturing methods. In this paper, a hybrid layer laminated manufacturing (LLM) method is proposed enabling manufacturing of topology-optimized machine tool parts. The method is referred to as hybrid, as the subtractive structuring of metal sheets is combined with the additive joining of the sheets by adhesive bonding. This enables enclosed inner cavities without support structures, which are used to approximate the optimal density distribution from a topology optimization via manufacturing. The proposed LLM method is validated on the basis of a bearing block of a ball screw feed drive. A experimental study in the time and frequency domain on a test rig confirms the principle suitability of the LLM method for the production of industrial applicable lightweight components.

Computed tomography investigation of the material structure in clinch joints in aluminium fibre-reinforced thermoplastic sheets

Recent developments in automotive and aircraft industry towards a multi-material design pose challenges for modern joining technologies due to different mechanical properties and material compositions of various materials such as composites and metals. Therefore, mechanical joining technologies like clinching are in the focus of current research activities. For multi-material joints of metals and thermoplastic composites thermally assisted clinching processes with advanced tool concepts are well developed. The material-specific properties of fibre-reinforced thermoplastics have a significant influence on the joining process and the resulting material structure in the joining zone. For this reason, it is important to investigate these influences in detail and to understand the phenomena occurring during the joining process. Additionally, this provides the basis for a validation of a numerical simulation of such joining processes. In this paper, the material structure in a joint resulting from a thermally assisted clinching process is investigated. The joining partners are an aluminium sheet and a thermoplastic composite (organo sheet). Using computed tomography enables a three-dimensional investigation that allows a detailed analysis of the phenomena in different joining stages and in the material structure of the finished joint. Consequently, this study provides a more detailed understanding of the material behavior of thermoplastic composites during thermally assisted clinching.