Integrable Sensors on Foil for Smart Systems and Bondline Surveillance

Foil-based strain sensors, such as strain gauges, are usually structured using photolithography in industrial production. In this process, a metal foil laminated onto a carrier substrate or a metallic thin film produced by plasma-assisted deposition is coated with a light-sensitive polymer. After exposure and development of the light-sensitive polymer the underlying metallic layer is structured by wet chemical etching. In this way, very precise structures can be created within geometrically and electrically low tolerances. Appropriately designed batch production, usually with hundreds of systems manufactured in parallel, enables cost-effectiveness. Corresponding sensors can be used universally and cover a very wide range of applications thanks to a large number of different variants and designs. In this thesis, new processes and materials for strain sensors are described in two different approaches, enabling novel production and application areas for foil-based piezoresistive sensors. Extremely high energy densities can be achieved using ultrashort laser pulses in the femtosecond range, enabling the local removal of a coating without a large thermal influence zone in the underlying substrate foil. In the first part of this work, a method was developed for the laser-assisted structuring of thin-film strain sensors that enables the systematic identification of suitable laser process parameters for piezoresistive materials and makes "try and error" experiments obsolete. The focus was on achieving selective ablation, i.e. removing the sensor-active coating without (or with only minimal) damage to the underlying substrate layer. The methodology developed is based on the determination of well-defined ablation thresholds in "single spot" experiments, in which different numbers of laser pulses are emitted onto identical spots on the substrate. As the ablation behavior changes due to the cumulative energy input of the pulses, quantitative criteria for selective ablation were derived for both line and area ablation. Laser-structured strain sensors were manufactured in compliance with these criteria using a special contour cutting process. This special type of structuring allowed the smallest possible overall size of the finished measuring grid in relation to the laser spot size. The apparatus used enabled a measuring grid dimension of less than one millimeter, so that the finished sensor could be used for highly localized measurements. Such a measuring grid size is not available for conventional strain gauges, as it is almost impossible to position the active measuring grid precisely at the desired point on a workpiece. In contrast, laser micromachining makes it possible to roughly pre-position and fix sensor material coated foil blanks without a measuring grid before writing the sensor at the desired position with an accuracy of a few micrometers. Structuring on three-dimensionally shaped surfaces also appears to be possible with this process. In addition, this chemical-free structuring method is also suitable for sensitive coating and substrate materials. To demonstrate the advantages, laser direct writing was used to create strain sensors on the side surfaces of miniaturized blister structures. A foil with several such blisters can be applied as a tactile sensor skin to any surface, e.g. of surgical instruments, which could improve the haptic feedback for the operator and thus the safety of laparoscopic interventions. In the second part of this work, measuring grids were structured on a special foil material in order to enable a new, innovative system solution for monitoring structure-critical adhesive layers between composite components. Structurally critical bonds are those whose sudden failure can potentially lead to a catastrophic loss of the overall structure. In general bonding is an ideal joining technology for composite materials, such as those used in the lightweight construction of aircraft components. Due to the layered construction and the anisotropic, multiphase structure of fibers and matrix material, composites are susceptible to local stress increases due to imperfections. By force transmission over the entire surface of the joint through the adhesive layer, such bonding eliminates the material damage required for other joining technologies such as screws or rivets, e.g. through drilling. However, the complex prediction of the long-term adhesive strength achieved by bonding, which is dependent on environmental influences, is a significant disadvantage. For a "secure" bonded joint, a costly and often uneconomical quality assurance process therefore appears to be essential. An elegant solution promises the possibility of permanently monitoring the integrity of the bondline over the entire life cycle of the bonded structure. Such an early warning system should be able to indicate structural weakening before it reaches a critical level, regardless of whether this damage is caused by manufacturing errors, overloading during operation or other external influences. Strain sensors were lithographically applied to a special thermoplastic foil material in order to achieve reliable sensor-monitored structural bonding of fiber composites using flexible foil inlays. The sensor-based crack detection principle of the developed smart inlays is based on the measurement of a strain distribution within the joining partners that changes as a result of damage within the bondline. Thanks to the special foil material, the inlays can be integrated during component production, resulting in an excellent bond and making the technology suitable for industrial use. The inlay was supplemented by a cover layer made of a particularly stretchable and ductile foil material. Plastic deformation within the foil absorbs stress peaks such as those that occur at crack fronts, which inhibits their progression for a certain period of time and limits crack spreading. The smart inlays therefore increase the applicability of adhesive bonds for structural joints, allowing the lightweight construction potential of composite structures to be better utilized.