Mechanisms for fatigue crack growth in meso/micro and nano specimens - crack initiation and short crack growth under geometrical and mechanical constraints

• Applicants: Professor Dr. Christian Motz; Dr.-Ing. Florian Schäfer
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521371248

Wider research context / theoretical framework: Fracture mechanical concepts are known since many decades and become more and more important in many fields of applications to optimise the efficiency of materials in parts and components, which is essential for e.g. lightweight constructions to save resources and energy. Size effects are an intrinsic and commonly known problem in fracture mechanics even at the macroscopic scale which leads to size requirements even for the samples defined in the according test-standards. The situation becomes even more complicated when complex specimen geometries are used e.g. for testing of MEMS and/or comparably large plastic and process zones are present in front of the crack tip which is typically the case in small-scale fracture testing. In addition, a complex microstructure, found e.g. in multilayers, can strongly influence crack initiation and propagation through mechanical and microstructural constraints. These effects and challenges have not been addressed in past in detail and will be the focus of the current proposal. Hypotheses/research questions /objectives: How must specimen size, boundary conditions (constraints), material behaviour and inhomogeneities be considered regarding the applicability of the fracture mechanical concepts for small-scale specimens and are there necessary pre-sets for testing of micron-sized specimens (e.g. fatigue pre-cracks)? Approach/methods: To analyse short crack growth under mechanical constraints we will not only vary the specimen size, but also design tailored multilayer structures with varying grain sizes via electrodeposition. We propose a scale-bridging investigation. Meso to micron-sized samples are tested at the Saarland University, Montanuniversität Leoben provides testing of nano and micro samples. We will vary the sample size, the layer system and the grain size regarding quasi-static and LCF physically short cracks. Combining our experience in micro and nano-mechanics we will achieve a characterization of the crack via the displacements and strain fields in the plastic zone. Level of originality / innovation: Up to now, work has mostly focussed on understanding the origin of static size effects, but there is no detailed study of size dependent evolution of cyclic plasticity and fracture. Furthermore, knowledge of the underlying mechanisms arising from microstructural and mechanical constraints is crucially missing. The aim of this project is to gain an improved understanding of the driving force of physically short cracks, both quasi-statically and in the low cycle fatigue regime (LCF), under geometrical, microstructural and mechanical boundary conditions and constraints to develop a comprehensive model of fatigue and fracture in inhomogeneous materials in micron-sized dimensions.

• DFG Programme: Research Grants
• International Connection: Austria
• Partner Organisation: Fonds zur Förderung der wissenschaftlichen Forschung (FWF)
• Cooperation Partner: Professor Dr. Daniel Kiener