Matthias Nestler

Multi-layer compounds with integrated actor-sensor-functionality

A method is presented to integrate the cost and time consuming afterwards-joining technologies of piezo actors and sensors direct in the forming processes for metal blank structures. Possible applications for such parts are vibration/ noise damping, deformable shape control, energy harvesting or several sensor tasks. Different forming processes are experimentally investigated and the limits according to deformation of the brittle piezo components discussed. In the numerical research the piezomodule components (piezo fibre, electrodes and plastics embeddings) are homogenized to create a computation-time reducing simplified material model. In a back-transfer of global loads in the forming simulation a representative volume element (RVE) with cyclic boundary conditions is used to evaluate the loading of the piezoceramic material to describe the function degradation due to forming operation. The comparison of numerically and experimentally determined results in a linear manner lead to the necessarity of further numerical research. The location of maximum piezo-patch loading corresponds well with the numerical investigation. The numerical integral model for function degradation shows a large difference in comparison to the integral experimentally determined values. Therefore extensive experimental research direct on the piezomodule outside the forming compound is planned to fit the degradation model in a nonlinear manner.

Smart Semi-finished Parts for the Application in Sheet-Metal Structures

In many fields of industry, e.g. automotive industry, light-weight concepts are gaining in importance. The background is an increasing shortage of natural resources combined with increasing requirements for comfort and safety. Through the use of light-weight concepts the efficiency of automotive vehicles and the driving dynamics are improved. Because the car body has amounts to 30% of total weight, it offers high potential for weight reduction. An increasing strength of sheet metal materials allows the reduction of thickness of auto body parts. Using the same materials this reduction leads to a decrease of structural stiffness. Adaptronic devices can be used to influence the stiffness locally. Sensors and actuators, e.g. based on piezoceramics, allow the detection and the reduction of unwanted vibrations. The state of the art application of piezomodules like Macro-Fibre-Composites (MFC) is performed with significant manual work. For high-volume production a new process chain has to be used. The paper presents a method which allows the application of piezomodules on 3D-formed parts. The application takes place before the forming operation. The encapsulation of the piezomodule is performed in different ways. In addition to a double layered sheet metal-composite a local applied sheet metal or plastic film can be used. The presented method offers good protection of the piezomodule against mechanical and chemical influences like splash water and dirt on the automotive parts. The local application method leads to a significant weight reduction.

Evaluation of Actuator, Sensor, and Fatigue Performance of Piezo-Metal-Composites

Fabrication technologies for multilayer-composites with sensor and actuator functionality were proposed in a previous study. Inside the compounds, macro-fiber-composites (MFCs) are embedded in a layer of epoxy adhesive between two outer aluminum sheets. Forming takes place while the adhesive is still in an uncured state. Relative displacements between the layers and the MFC is possible, which reduces friction loads for the brittle piezoceramic fibers of the MFC. This paper deals with the experimental and numerical characterization of the actuator and sensor functionality dependent on different bending radii and additionally the fatigue behavior of the compounds. The sensor functionality is tested with a shaker, which initiates a defined deflection. The electric response of the integrated MFC is measured and simulated by use of an analytical sensor function model based on strain components. The actuator performance, measured with a distance laser sensor, is modeled with a voltage-temperature-analogy and compared with values from the experiment. The fatigue behavior and the performance reduction of embedded MFC are investigated with cyclic four-point-bending loading. Loads near and above the elastic limit of the sheet metals cause a higher delamination tendency. Specimens that are loaded at a reasonable distance from the elastic limit reach the high cycle fatigue limit of 2.0E + 06 cycles. Thus, the production method for multilayer-composites with embedded piezoceramic fiber modules discussed in this paper is suitable for the manufacturing of structural parts that undergo a high number of load cycles.

Actuator and sensor performance of piezo-metal-composites

New technologies for the fabrication of Multi-Layer-Composites with sensor and actuator functionality were used. The compounds consist of Macro-Fiber-Composites (MFC) embedded in a layer of epoxy adhesive between two aluminum sheets. Forming takes place in an uncured state of the adhesive, so that a relative displacement between the layers is possible. This paper deals with the experimental and numerical characterization of the actuator and sensor functionality dependent on different bending radii. A shaker initiates a defined deflection and the electrical answer of the integrated MFC is measured and simulated by use of an analytical sensor function model based on strain components. The actuator performance, measured with a distance laser sensor, was modeled with a voltage-temperature-analogy and compared with values from the experiment.

Piezo-Metal-Composites as Smart Structures

In many applications, vibration and noise are unwanted, but often inevitable. For instance, for passengers in an automobile, a reduction of comfort for the passenger is a result. For products like high precision machine tooling, the appearance of unwanted vibrations can cause negative influences on the quality of produced parts. Adaptronic devices can help to improve the vibration and noise behaviour of structures. Today the production of adaptronic structures consists of two process chains: Firstly, the fabrication of structural parts which occurs in a very efficient process with a low production time. Secondly, there is the functional integration step which normally takes place under laboratory conditions and needs a lot of production time. For creating a smart structure, the assembly of the structural part includes the use of a piezo-composite-module. The authors propose a new process chain for this with the material fabrication and functional integration taking place in one process. This can occur through a laminar piezo-module being inserted between two metal sheets and fixed by using a slow curing adhesive. After assembling the sandwich, the semi-cured adhesive allows forming of the sandwich with a reduced generation of tensile loads due to friction between the metal sheets and the piezo-module. As a last step in the process chain, the adhesive fully cures and giving a high stiffness coupling . Experimental tests have been performed to characterize the functionality and to examine the process limits. Numerical studies have evaluated the stresses and strains in the piezo-module during the forming.

Requirements for a Process Chain for Manufacturing of Piezoceramic-Aluminum Sandwiches

Lightweight solutions and functional integration become more and more important in different fields of industry. In order to achieve a sensor and actuator functionality of shaped sheet metal parts, today a generally manual application step of the piezomodule is necessary. This subsequent process is time consuming and leads to high costs. In earlier studies a method was presented allowing the fabrication of a formable compound with an integrated sensor and actuator functionality. The formability of the compound is achieved using a viscous adhesive, surrounding the piezomodule during the forming operation. The low viscosity of the adhesive allows a relative movement between the piezomodule and the sheet metals and drastically reduces the transfer of critical strains to the piezomodule. Curing of adhesive takes place after the forming operation. To improve the efficiency of the process chain an advanced adhesive system with robust application properties has to be used. Furthermore, the productivity of several fabrication steps and their sequence in the process chain have to be verified and improved. The paper presents the process chain designed for automated production of formable sandwich sheets with integrated piezomodules, including the production steps for the fabrication of the semi-finished part as well as the forming operation. Aiming on a good formability during the shaping operation and on a stiff connection between the piezomodule and the sheet metal in the finished sandwich part, one focus is set on the adhesive properties required during the different process steps.

Piezo-Module-Compounds in Metal Forming: Experimental and Numerical Studies

The integration of piezoceramics in metal-structures offers the possibility to achieve different functionalities like active control of dynamic behaviour, health monitoring or energy harvesting in structural parts. To avoid the time-consuming additional application of piezo-modules on a formed semi-finished part, piezoceramic-modules were embedded in a double-layer-sheet. The presented method of integration allows the forming of the piezo-metal-compounds after the integration process. The use of a semi-cured adhesive offers the possibility to form the compound avoiding tensile stresses due to shear loads in the piezoceramic-module. The process limits and the level of operability of integrated piezo-ceramic-modules after forming were determined experimentally by examining different functionalities. In numerical studies using homogenization-localization theories the stresses and strains of the piezoceramic-module during different forming processes are evaluated. A homogenization of the local periodical substructure with a unit-cell-model is performed. Global loads of the piezo-module due to forming are transferred in a submodel to obtain local loads of the piezo-ceramic.

Aktive Halbzeuge – Blechverbunde mit Piezo-Kern

Kurzfassung Piezoelektrische Wandler, wie z.B. Macro-Fibre-Composites (MFC), können bei der Reduktion von Bauteilschwingungen, Health Monitoring, Crashfallerkennung und Energiegewinnung eingesetzt werden. Die Ausweitung der Einsatzfelder aktiver Komponenten auf hocheffiziente Wirtschaftszweige (z.B. Automobilbau) lässt sich nur mit einer Optimierung der bisher zeit- und kostenintensiven Applikation der Wandler nach der Umformung realisieren. Eine Möglichkeit, um dieses Ziel zu erreichen, besteht darin, die Applikation direkt in die Bauteilfertigung zu integrieren. Bei der vorgestellten Methode wird ein in fluidem Klebstoff eingebetteter MFC mit hochspröden piezoelektrischen Fasern zwischen zwei Blechen platziert. Hierdurch werden bei der sich anschließenden Umformung des Blech-MFC-Verbunds die infolge der Reibung in den MFC eingebrachten Zugbeanspruchungen vermindert. Nach der Umformoperation erfolgt die Aushärtung des Klebstoffs, wodurch eine Anbindung des MFC an die umgebenden Bleche entsteht.