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  5. Mechanical Properties
[Translate to English:] Gruppenbild der Strukturmechanik

In the Department of Mechanical Properties, we investigate the mechanical properties of specimens and components and their relation to other material and component properties. This includes studying material behavior under monotonic and cyclic loading on undamaged specimens, fracture mechanics specimens, and components. 

Accompanying microstructural investigations are conducted on both the initial state and the loaded state, with a primary focus on clarifying failure mechanisms. Based on these studies, we derive models and quantitative descriptions for component failure, enabling us to predict strength and durability under mechanical loading. We also collaborate with other working groups within the institute to develop and investigate methods for increasing strength.

 

[Translate to English:] Ein Mitarbeitender bedient eine Maschine.

Focus Areas

The Structural Mechanics Department conducts mechanical testing of materials and components as part of research projects and on behalf of industrial clients. The main focus lies on cyclic loading. In addition, the department’s researchers develop models based on the results of these tests. These models are used to predict material behavior. The department concentrates its efforts in this regard on three main areas:

Taking into account the existing microstructure, local hardness, and residual stresses, the department is developing a material model for load-bearing capacity under cyclic loading. This model can be integrated into process chain simulations using common equivalent stress hypotheses. In the future, the aim is to also incorporate complex microstructural features, such as those found in carbonitrided surface layers.

Building upon the already successful flaw modeling that allows for the calculation of fatigue strength, we are developing a model for discrete lifetime prediction at stress levels within the fatigue strength range. This model is based on locally existing strengths and stresses.

Furthermore, the description of various types of defects will be integrated into a unified model in the future. Different materials fail under cyclic loading due to different types of defects (pores, shrinkage cavities, precipitates, dispersoids, microstructural inhomogeneities, etc.). The goal of the model is to enable a detailed characterization of these defects based on the multiple-flaw approach.

The common goal of these focus areas is to evaluate the generalizability of the models with respect to material properties, loading conditions, and geometry. This will help to reduce future testing efforts and enable the integration of the finalized models as modules in simulation tools.

In addition, the department is involved in further research projects focusing on the investigation of load interactions during component machining, the development of testing methods for unconventional specimen geometries, and the creation of meaningful data structures for the structured storage of research data.

 

The Structural Mechanics Department operates 21 fatigue testing machines covering a wide range of load and frequency capacities, as well as 5 rolling contact fatigue test rigs. Our capabilities range from testing large specimens in a horizontal pulser with ±100 kN at 30 Hz, to ultrasonic pulsers operating at 20 kHz under alternating and pulsating loads. Our technical staff is highly experienced in performing stress- and strain-controlled fatigue tests. Scientific staff members support test planning and data evaluation, with a strong focus on interpreting and contextualizing the results. Tests can be complemented by digital image analysis, local temperature measurements, and systems for monitoring crack growth rates.

With a well-preserved fracture surface, the history of a crack can be reconstructed through detailed forensic work. When combined with microstructural examinations and residual stress measurements, the root causes of failure and potential corrective actions can be identified for practical applications. In addition to analyzing specimens tested in-house, the department specializes in evaluating damage cases caused by cyclic loading. From broken springs to large gearbox gears, numerous cases have been solved in cooperation with metallographic and physical analysis teams.

Projects of the Mechanical Properties Department

Bearings Fatigue Life Testing and Calculation (BALTIC)

In the BALTIC joint research project, raceway and structural fatigue of rotor blade bearings in wind turbines are investigated both experimentally and through simulation. The goal is to determine the service life of the bearings with high precision.

At the Leibniz-IWT, the subproject titled "Materials Engineering Investigation of Pre-Damaged Blade Bearings Evaluation of Models for Estimating Crack Initiation Probability and Simulating Crack Growth Based on Fatigue Tests" is being carried out within the BALTIC consortium. A special focus lies on the interaction of typical damage mechanisms such as raceway fatigue, wear, ring fatigue, and corrosion.

Initial analyses of field-returned bearings revealed cracks propagating both from the bore towards the raceway and vice versa. Chemical investigations of the fracture surfaces identified corrosion in the boreholes as a root cause of cracking.

To further investigate these mechanisms, comprehensive tensile-compression fatigue tests and crack propagation experiments at varying ambient temperatures are planned. For this purpose, specimens will be extracted from a bearing ring. One group of specimens will represent the ductile core material of the bearing ring, while another will be specifically hardened to replicate the microstructure of the induction-hardened raceway.

In parallel, simulation work is being conducted to model the stress state within the bearing. For this, the depth profile of residual stresses at relevant positions is critical. Initial surface stress measurements using mobile X-ray diffraction have already been performed. Based on the results, segments have been selected for determining the residual stress distribution in depth. These investigations are essential to understanding the complex loading conditions in rotor blade bearings.


Project Partners: Fraunhofer Institute for Wind Energy Systems (IWES), Eolotec GmbH, WRD Research and Development GmbH, Liebherr-Components Biberach GmbH
Funding: Federal Ministry for Economic Affairs and Climate Action (BMWK), Germany
 

 

Contact:
Dr.-Ing. Johanna Eisenträger
Tel.: +49 421 218 51320
E-Mail: eisentraeger@iwt-bremen.de

AI-Assisted Development of Diatom-Inspired Structures for Additively Manufactured Endoprostheses Made of Ti-6Al-4V

Approximately 20% of all hip replacement surgeries worldwide are revision procedures, indicating a significant need for optimization. Diatoms exhibit complex open-porous structures that combine lightweight properties with high strength and crack-resistant behavior, making them highly attractive for applications in endoprosthetics. The research project “KIKI” aims to develop an AI-based method for the bioinspired structuring of test specimens, laying the foundation for the successful structural design of endoprostheses in future projects.

An effective AI tool requires a comprehensive dataset. For this purpose, specimens made of Ti-6Al-4V are produced using powder bed-based selective laser melting and subjected to quasi-static compression and bending tests as well as cyclic crack growth experiments. The mechanical testing and analysis of damaged specimens serve as the data foundation for developing the AI tool and validating the bioinspired structures.

Collaboration: Alfred Wegener Institute (AWI) Bremerhaven
Funding: University of Bremen – UBRA / AI Center for Health Care

 

 

 

Contact:
Dr.-Ing. Johanna Eisenträger
Tel.: +49 421 218 51320
E-Mail: eisentraeger@iwt-bremen.de
 

 

Transregional CRC 136 “Process Signatures” – Transfer project T06: Data-based lifetime prediction for a function-oriented induction hardening process

The project aims to predict the process parameters of induction hardening in such a way that required 
component properties, such as the fatigue strength of shafts, are fulfilled in the context of function-orientated 
production. This is achieved by inverting the process signature and calculating local fatigue strengths.

 

 

Cooperation: eldec Induction GmbH
Funding: DFG (Transferbereich SFB TRR136) – Projectnumber 223500200

 

 

Contact:
M.Sc. Tobias Heinrich
Tel.: +49 421 218 51338
E-Mail: heinrich@iwt-bremen.de

Cross-Scale Correlation between Defects and Fatigue Strength of Additively Manufactured Aluminum Alloys Samples (AM Scaling)

The overarching goal of this collaborative research project is to significantly improve the reliability of additively manufactured (3D-printed) components made from aluminum alloys. The central challenge lies in the formation of component-weakening defects such as pores during additive manufacturing, which strongly affect the fatigue strength and thus the service life of the final product.

The overall project is divided into two complementary research pillars:

  • Department of Lightweight Materials (Collaboration):  Focuses on the precise characterization of porosity and defect distribution within the additively manufactured component. The relationship between the process parameters used in 3D printing and the resulting defects is analyzed using machine-learning methods. Through extensive printing trials, an AI model based on artificial neural networks can be trained to reliably predict component porosity.

  • Department of Mechanical Properties (Our Department): Focuses on the mechanical assessment and the development of the final, cross-scale fatigue model that links defect information with material properties. To rapidly characterize material behavior, we perform micro compression tests on additively manufactured cylinders directly on the building plate. These results are then interpreted using the finite element method (FEM).

Core Methodological Competence

The central method is micro compression testing (MCT) on tiny cylinders. This enables rapid characterization of the elastic-plastic material properties—the so-called matrix strength—directly on the building plate. To obtain a robust data foundation, extensive cyclic MCTs as well as conventional fatigue tests on larger specimens (small-scale and standardized samples) under alternating tensile loading are additionally carried out.

Interpretating Experimental Data via Simulation

Using the finite element method, we interpret the experimental results. This includes understanding the influence of testing conditions and creating a direct connection between MCT results and conventional standardized material properties. This correlation is essential for transferring the rapid MCT measurements to real-world component assessment.

The Fatigue Model

At the heart of the research is the development of a comprehensive model based on fracture mechanics. Here, voids and defects in the component are treated as critical cracks whose growth under cyclic loading is described. The model links the sensitive MCT data (as a measure of the intrinsic material strength) with the defect-size distribution provided by the partner research group.

Objective and Validation

The objective is an accurate, cross-scale prediction of the fatigue strength of components made from the alloy AlSi10Mg. Finally, the method is evaluated using a second high-strength aluminum alloy (Scalmalloy®) to verify the robustness and alloy-independence of the developed model.

Funded by Deutsche Forschungsgemeinschaft (DFG) – Projectnumber 553531445

 

 

 

 

Contact:
Dr.-Ing. Johanna Eisenträger
Tel.: +49 421 218 51320
E-Mail: eisentraeger@iwt-bremen.de