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  5. Mechanical Properties
[Translate to English:] Das fünfköpfige Team der Strukturmechanik am Leibniz-IWT steht in einem Labor vor Prüfmaschinen für mechanische Werkstoffversuche.

In the Department of Mechanical Properties, we investigate the mechanical properties of specimens and components and correlate them with other material and component properties. Our investigations cover material behavior under static and cyclic loading on undamaged specimens, fracture mechanics specimens, and components.

We conduct accompanying microstructural investigations, with a focus on elucidating failure mechanisms. We then derive models and quantitative descriptions of component failure that enable the prediction of strength and service life under mechanical loading. In collaboration with other research groups at Leibniz-IWT, we develop and investigate methods for increasing strength and improving mechanical properties.

 

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

Our department offers companies customized testing methods for characterizing metallic alloys under static and cyclic loads at a wide range of frequencies and temperatures. Our services include:

  • Determination of classic properties of metallic materials (e.g., material testing on steel, aluminum, copper, titanium, magnesium)
    • Tensile testing according to DIN EN ISO 6892-1 (specimen preparation according to DIN 50125)
  • Notched impact bending test according to DIN EN ISO 148-1 (ISO-U, ISO-V)
  • Mechanical testing under cyclic loads
    • Fatigue strength, endurance
    • Low-cycle fatigue (LCF), high-cycle fatigue (HCF), very high-cycle fatigue (VHCF)
    • Various specimen shapes and frequencies
    • Uniaxial and multiaxial loading
    • Temperature range: room temperature to 1000 °C
  • Determination of mechanical properties under cryogenic conditions down to 10 K
  • Heat treatment simulation using a thermomechanical testing system (Gleeble)
  • Crack propagation measurements on CT specimens under cyclic loading
  • Determination of fracture toughness
  • Digital image correlation (DIC) for 2D measurement of deformations

Contact us to learn more and collaborate with us on your projects.

Focus Areas

In the Department of Mechanical Properties, we experimentally investigate the relationships between microstructure, residual stresses, and the resulting mechanical behavior. To this end, we conduct mechanical tests on materials and components, both as part of research projects and on behalf of industrial companies. We translate the insights gained into models and apply them to component behavior through simulations. Our focus is on metallic materials.

We account for uniaxial and multiaxial stress and strain states as well as static and cyclic loads. Our investigations under cyclic loading cover the low-cycle, high-cycle, and very-high-cycle fatigue ranges with up to 10 billion load cycles. In addition, we conduct fracture mechanics investigations, such as crack propagation measurements and the determination of fracture toughness.

A particular focus is on thermo-mechanical materials testing. This includes both tests at elevated temperatures—such as on our Gleeble 3500 testing machine (up to 1100 °C) or the rotating bending machine (up to 1000 °C)—as well as mechanical tests in the cryogenic temperature range down to 10 K.

In the field of modeling, we focus on the following four areas:

Taking into account the existing microstructure, local hardness, and residual stresses, we develop a material model for stress resistance under cyclic loading, which can be integrated into the simulation of the process chain based on common constitutive models. In the future, this model will also be able to account for complex microstructures, such as those found in carbonitrided boundary layers.

Based on the already successfully developed defect modeling, which enables the calculation of fatigue strength, we develop a model that allows for discrete life prediction under loading in the time-dependent strength regime. Here, the locally present strengths and stresses serve as the basis for calculation.

In addition, the development of phenomenological material models for elasticity, plasticity, and fatigue is one of our central competencies. For simulation, we employ advanced numerical methods, including the finite element method (FEM), the scaled boundary finite element method (SBFEM), and the extended finite element method (XFEM). We use these methods for macroscopic simulations of components, microscopic modeling leveraging image-based analyses, and fracture mechanics analyses of crack propagation.

Furthermore, we have integrated the description of various types of defects into a single model. Different materials fail at different defect locations (pores, shrinkage cavities, precipitates, dispersoids, microstructural inhomogeneities, etc.) under cyclic loads. The model enables a differentiated description of these defects based on the multiple-flaw approach.

[Translate to English:] Ein Forscher untersucht eine Probe mit einer speziellen Vorrichtung unter Verwendung einer Beleuchtungseinheit. Er arbeitet an einer Testmaschine.
[Translate to English:] Ein Testgerät für die Metallbeständigkeitsprüfung zeigt ein Metallteil, das unter Druck gebogen wird. Die Vorrichtung wird verwendet, um die Widerstandsfähigkeit des Materials zu testen.

In the Department of Mechanical Properties, we have a wide range of testing machines that allow us to conduct tests under static and cyclic, uni- and multi-axial loads across a variety of temperature and frequency ranges. From testing in a 20 kHz ultrasonic pulser under alternating and pulsating loads to experiments at cryogenic temperatures, we offer a broad spectrum of testing capabilities. Our technical staff is experienced in conducting stress- and strain-controlled mechanical tests. The research staff assists with test planning and the evaluation of test results, with a focus on the assessment and interpretation of the findings. Investigations can be supported by digital image correlation, local temperature measurement, and a system for recording crack growth rates.

Using a well-preserved fracture surface, the history of a crack can be traced through meticulous work and analysis. Combined with microstructural examinations and residual stress measurements, this reveals the causes of failure and identifies corrective measures for practical applications. In addition to testing in-house specimens, the department has also specialized in assessing damage cases caused by cyclic loads. From a broken spring to a large gear wheel, various cases have already been resolved in collaboration with metallographic and physical analysis.

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