title: Force sensor manufactured with laser-based powder bed fusion
Creators: name: Chadda, Romol
Publisher: name: UNSPECIFIED
Year: 2023

description: Structural health monitoring (SHM) and condition monitoring are gaining in importance due to the increasing digitalization in several fields such as industry automation, energy and aerospace industry as well as in plant construction. The main objective is to monitor the condition of a system or plant in order to facilitate predictive maintenance and prevent failures on one hand and to increase the productivity on the other hand. This requires high-quality data within relevant locations of the plant, which inevitably leads to the integration of sensors into existing structures to provide in-situ measurements. In this context, force sensors are of great importance, as they allow the detection of load peaks. However, since commercially available force sensors are designed as closed systems, it is not always possible to integrate them into existing structures, or only with high effort. Therefore, force sensors with a high degree of individualization in terms of geometry and measuring range are necessary. These requirements can be met with additive manufacturing processes such as laser-based powder bed fusion (LPBF), since they allow manufacturing of almost any geometry combined with their characteristic layer-by-layer build-up that enables the integration of a sensor at a specific layer. However, there is no systematic approach up to now for the realization of LPBF-manufactured force sensors based on strain gauges, which are the gold standard as sensing elements in conventional force sensors. In this thesis, a disruptive manufacturing approach is presented that breaks and re-arranges the well-known state of the art assembly sequence of force sensors, and, thus, enables a systematic integration of strain gauges into LPBF-manufactured parts. This approach is based on a published patent application and involves inserting a steel plate applied with strain gauges, which serves as a measuring element carrier, into an LPBFmanufactured part during a process interruption. The LPBF-manufactured force sensors (80 mm x 25.5 mm x 14 mm) are investigated regarding two key aspects that significantly influence their performance, which are: the strain transfer from the LPBF-manufactured part to the inserted measuring element carrier and the behavior of the strain gauge in terms of linearity and sensitivity after integration in the rough manufacturing environment. Investigating the first aspect has shown that the LPBF-manufactured force sensors using the disruptive manufacturing approach are reproducible and feature linearity and hysteresis errors of ±0.1 % and ±0.2 %, respectively. The achieved results are very similar to a conventionally manufactured reference force sensor showing the competitive potential of the new approach. At this stage of investigation the strain gauges are not fully encapsulated. In addition, the temperature and creep behavior of the LPBF-manufactured force sensors are also investigated. A comparison of the sensitivity of the LPBF-manufactured force sensors with a FEA model shows excellent agreement, which provides a reliable prediction of strain transmission, allowing for a targeted design of such force sensors in advance. The comparison of the strain gauge behavior before and after being exposed to the LPBF-process shows an irreversible change of the base resistance by up to −0.15 %. This irreversible change results from the temperature load during the LPBF-process as well as a plastic deformation of the measuring element carrier and is larger the closer the location of the strain gauge to the scanned surface. However, this does not negatively affect the linearity and sensitivity of the LPBF-manufactured force sensor due to the chosen geometry of the force sensor and the location of the strain gauges, which undergo a maximum temperature of 145 ◦C. However, a severe deformation of the measuring element carrier with a deflection of 0.5 mm in height direction is present due to large temperature gradients during the LPBF-process in combination with the chosen geometry of the force sensor. The onset of this thermally induced deformation of the measuring element carrier is visible in measurements of the resistance of the strain gauge during the LPBF process. It shows that a tensile stress acts during establishing a material connection between the measuring element carrier and the LPBF-manufactured base body, which promotes the deformation. This thermally induced deformation is reduced by up to 67 % by adapting the geometry with respect to the height of the material connection as well as some parameters of the LPBF-process, such as scanning strategy and inter layer time. Based on these findings, a geometry is developed for complete encapsulation of the strain gauges, which is not manufacturable by conventional subtractive or forming processes. The encapsulation increases the temperature experienced by the strain gauges, which reaches a maximum of 230 ◦C. It is shown that during the manufacturing time the strain gauges ar able to withstand these temperatures and maintain their characteristic values. The subsequent characterization of the LPBF-manufactured force sensor with complete encapsulated strain gauges shows linearity and hysteresis errors of ±0.1 % and ±0.2 %, respectively, which are in very good agreement with the LPBF-manufactured force sensors mentioned previously. Furthermore, the designed geometry offers possibilities for the realization of an overload protection as well as the adjustment of the measuring range. In the last part of this thesis, LPBF manufactured-threads are investigated. Until now, the high surface roughness of LPBF-manufactured parts has prevented the fabrication of usable threads by means of LPBF, causing screws to break off during tightening. Therefore, the flank angle and the clearance of threads of the sizes M3 to M8 are adapted such that usable threads can be fabricated by means of LPBF. For this purpose, a torque test bench is used to insert screws in a defined manner into the LPBF-manufactured threads while measuring the tightening torque. It is found that LPBF-manufactured threads with an additional clearance of 60 μm for flank angles from 80◦ to 100◦ do not require any significant torque when tightening the screws. The possibility of manufacturing threads during fabricating the part using LPBF eliminates the need for subsequent processing steps, and, thus, significantly reduces the production time while increasing the cost-effectiveness. This work proves that a defined integration of strain gauges into LPBF-manufactured parts is possible using the disruptive approach presented. Hence, custom additively manufactured parts can be fabricated that allow for in-situ measurements. In addition to the promising performance of the LPBF-manufactured force sensors introduced in this work, which is comparable with the conventionally manufactured force sensors, the design freedom allowed by the LPBF-pocess makes their integration in complex geometries a characteristic feature that can not be achieved otherwise. This work provide the basis for developing structurally integrated force sensors and shows the high potential for individualization.