Modelling the mechanical properties of interface structured multi-layer composites under large deformations

• Applicant: Professorin Dr.-Ing. Jana Wilmers
• Subject Area: Mechanics ; Coating and Surface Technology
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449062206

• Final Report Year: 2023

Final Report Abstract

Stretchable electronics can be integrated directly into soft, curved or moving structures, thus helping to shape the technological progress of our time. Whether in robotics, consumer electronics, sports science or biomedicine -- new fields of application can be opened up with flexible electronic systems. In wound dressings, for example, stretchable electronic sensors can be used to monitor patients' bodily functions without restricting their freedom of movement. The challenge in developing such systems is that conventional electronic components and conductor materials would fail under the large strains in these applications. The necessary stretchability is only made possible in combination with soft polymers in multilayer composites and through sophisticated structure designs. This is based on a principle that is simple to formulate but difficult to implement: it is not the stiff material that is stretched, but the structure. In this project, the relationships between structure and stretchability in such multilayer systems with structured interfaces were investigated to elucidate the underlying mechanical principles and, as a consequence, to enable the development of structures with tailored properties. For this purpose, mechanical models of the materials and structures were developed and crack propagation and delamination in them were simulated numerically. In the structures under consideration, the mechanical properties of the polymer are specifically modified in areas ("islands") or controlled cracks are introduced into individual layers, relieving other areas of the composite. In this way, sensitive electronic components can be protected from cracks or delamination, while the structuring elements such as islands or cracks enable stretchability. How exactly this strain relief works and how reliable these mechanisms are was investigated numerically. To this end, algorithms were developed for the automated generation of representative volume elements, which can be used to efficiently create and model highly variable structures. In these structures, the influence of geometry and different material combinations on mechanical properties, stretchability, crack propagation and failure behavior were analyzed in a material- and thus resource-saving manner. The comprehensive understanding of the principles involved gained allows to maximize the fraction of "usable" functional material by optimizing the stress and strain fields and to improve the lifetime of the structures under a wide range of mechanical load cases. Moreover, since the knowledge gained is based on geometric-mechanical relationships, it is also transferable to other material classes and application areas.

Publications

• Highly Stretchable Multi-Layer Structures Utilizing Controlled Cracking, 9th GACM Colloquium on Computational Mechanics, Essen, Deutschland (2022).
  Philipp Kowol
  
• Strain relief by controlled cracking in highly stretchable multi-layer composites. Extreme Mechanics Letters, 54, 101724.
  Kowol, Philipp; Bargmann, Swantje; Görrn, Patrick & Wilmers, Jana
  
• Strain relief by controlled cracking of stretchable multi-layer polymer structures, 11th European Solid Mechanics Conference, Galway, Irland (2022).
  Philipp Kowol
  
• Controlled Cracking for Strain Relief in Highly Stretchable Multi-Layer Polymer Structures, 22nd GAMM Seminar on Microstructures, Wien, Österreich (2023).
  Philipp Kowol
  
• Delamination Behavior of Highly Stretchable Soft Islands Multi-Layer Materials. Applied Mechanics, 4(2), 514-527.
  Kowol, Philipp; Bargmann, Swantje; Görrn, Patrick & Wilmers, Jana
  
• Discontinuities in polymer-based composites: Interfaces and (controlled) cracking, 14th International Symposium on Continuum Models and Discrete Systems, Paris, Frankreich (2023).
  Jana Wilmers
  
• Python code for modeling multi-layer structures with controlled cracking and delamination. Software Impacts, 17, 100524.
  Kowol, Philipp; Bargmann, Swantje & Wilmers, Jana