Fraunhofer Institute for Mechanics of Materials IWMAdditively manufactured turbine components made of nickel alloys such as IN718 are considered lightweight and efficient. The HT-Threshold-II project is investigating just how reliable they actually are in operation. It applies a proven concept for fracture mechanics evaluation to real-world conditions such as high temperatures, varying frequencies, and dwell times. The project will provide verified and validated materials data, an advanced software tool for evaluating materials defects, and practical recommendations that can be applied without specialized knowledge of fracture mechanics.
Advanced computational methods and testing techniques for evaluating the threshold and propagation behavior of mechanically short cracks in various temperature and frequency ranges
Ongoing research project
Project description
Market needs and challenges
Additive manufacturing (AM) is finding its way into safety-critical high-temperature applications such as gas turbines and aircraft engines. Components made of nickel alloys such as IN718, produced using the PBF-LB/M process, exhibit manufacturing-related microdefects such as pores and bonding defects. To date, there has been no reliable assessment of their structural durability, particularly under high-cycle fatigue (HCF) at high temperatures. For SMEs in the engineering services sector, this knowledge gap represents a significant barrier to market entry.
Project approach
The project builds on a fracture mechanics evaluation concept developed in a previous project, which describes short cracks in the threshold range. This concept is now being systematically extended to include additional temperatures relevant to practical applications (RT, 600 °C, 650 °C), test frequencies (40 Hz and 300 Hz), and dwell times. Tests conducted in air and within a vacuum allow for the separate evaluation of internal and external defects. Modeling is performed using the advanced ΔCTOD concept; results are incorporated into an enhanced, user-friendly software tool.
Contribution to addressing challenges
The project provides SMEs with a validated, expanded database and a user-friendly calculation tool that can be used without in-depth knowledge of fracture mechanics. Case studies from the aviation, energy, and automotive sectors facilitate the application of these findings to specific tasks. In addition, “best practice” recommendations with a guideline character are being developed, which are intended to be incorporated into international standards (DIN, ASTM).
Fraunhofer IWM’s work packages in the project:
Fraunhofer IWM prepares all specimens for testing and conducts threshold and crack growth tests (up to 40 Hz) in air and in a vacuum at room temperature (RT), 600 °C, and 650 °C, including dwell time tests. In addition, validation tests in the fatigue strength range are conducted in a vacuum. Concrete result: a comprehensive, validated test dataset for forged and additively manufactured IN718.
The existing ΔCTOD-based short-crack model is extended to cover all new test conditions. Component evaluation is demonstrated using industry-relevant examples. The concrete result is an expanded, operational Python software tool with a graphical user interface, as well as a recommendation for action in the form of guidelines.
Based on the project results, Fraunhofer IWM can offer industrial companies the following research and development services:
Fracture mechanics-based defect evaluation for AM components
Evaluation of pores and bonding defects in additively manufactured high-temperature components regarding their growth potential.
Determination of cyclic R-curves at high temperatures
Efficient determination of crack-length-dependent threshold behavior at temperatures up to 650 °C in air and in a vacuum for forged and additively manufactured materials.
HCF fatigue strength assessment using a software tool
Application of the validated calculation tool for evaluating the fatigue strength of complex component geometries under defined load cases. The tool can be used even without specialized knowledge of fracture mechanics.
Comparative materials evaluation: AM vs. conventional
Systematic comparison of additively manufactured and forged variants of IN718 in terms of threshold value, crack growth, and fatigue strength as a basis for informed make-or-buy decisions.
High-temperature crack propagation testing in a vacuum
Characterization of fatigue crack behavior under simulated internal component conditions (vacuum). This allows environmental influences and internal defects to be evaluated separately.
Validation of component designs using FE-coupled fracture mechanics
Combination of FE stress analyses with the short-crack evaluation model to identify critical defect sizes in real component geometries, e.g., for components used in turbomachinery, automotive, and plant engineering.
