Material characterization

On the basis of tensile and compression tests on sheet specimens, cylindrical test bars, and upsetting specimens, forming-related material parameters and forming characteristics can be determined over a temperature range from −198 °C to 1000 °C and a strain rate range from 0.00001 s⁻¹ to 1000 s⁻¹:

• Elastic properties (Young’s modulus, Poisson’s ratio)
• Plastic properties (uniform elongation, strain hardening exponent n, fracture elongation)
• Strength parameters (yield strength, ultimate tensile strength)
• Stress-strain curves
• Anisotropy parameters (r-values)
• Flow curves

The strain rates during testing can be controlled by an optical, spatially resolved strain measurement. For the precise determination of the onset of yielding, the temperature change detectable without contact by a thermal camera (Joule-Thomson effect) is utilized.

Using specially developed test setups, additional mechanical-technological parameters can be determined:

• Forming Limit Curves (FLCs) (temperature-dependent)
• Coefficients of friction (temperature-dependent)
• Multiaxial material parameters

Objectives

• Provision of realistic material parameters for FE simulation
• Description of the formability of materials and material composites
• Consideration of phenomena (springback, Bauschinger effect, etc.)
• Validation of FE simulation results

At Fraunhofer IWU, a broad range of advanced testing facilities is available for the determination of thermophysical material properties. These measurements form the basis for calculating temperature-dependent thermal conductivity according to λ(T) = a(T) · cₚ(T) · ρ(T). Using state-of-the-art experimental setups, material properties can be determined under realistic, process-related conditions and made directly available for thermomechanically coupled simulations. This ensures a high level of accuracy and reliability in the numerical description of thermal processes.

Added value

• Provision of highly realistic material parameters for FE simulation
• Reliable validation of FE simulation results

X-ray diffraction is used to investigate residual stresses and to determine retained austenite. It is particularly well suited for ferritic steels and is generally applicable to all crystalline materials. A key advantage of X-ray diffraction is that it does not require a stress-free reference specimen for calibration; instead, it represents an absolute measurement method.

We offer the production and preparation of metallographic cross-section specimens for the assessment of macro- and microstructures, for hardness testing, and for the determination of cut surface characteristics.

Hardness testing of workpieces and components can be carried out using various static-load hardness testing methods. Depending on the material under investigation, hardness values according to Brinell, Vickers, Rockwell, or Knoop are determined in the micro- and low-load range. In addition, hardness profiles and hardness maps can be generated, and conversions between different hardness scales can be performed.

Our metallographic portfolio includes both the analysis of material microstructures with respect to microstructural constituents, phase transformations, and microstructural defects, as well as specific macrostructural investigations of fracture or etched surfaces. Typical applications include failure analysis of components and the visualization of fiber flow in formed workpieces. Sample preparation for a wide variety of materials is carried out using specially adapted cold-mounting materials tailored to different application requirements.

For the visualization of polished sections, modern stereo and reflected-light microscopes are employed. Image analysis and documentation are supported by the Olympus Stream Motion image processing and archiving database, which enables:

• Measurement of sheet thickness distributions and layer structures
• Determination of phase fractions in multiphase microstructures
• Particle size analysis
• Extended depth-of-field imaging with profile measurement and 3D visualization
• Panoramic image acquisition
• Grain size determination in accordance with ASTM / DIN EN ISO 643
• Wear assessment
• Additional geometric measurements