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Systematic investigation of the influence of T-stress and LCF overloads in biaxially loaded cruciform specimen on plasticity-induced crack closure using digital image correlation

The T-stress as a measure for the multiaxiality or constraint is well-known in linear-elastic fracture mechanics. It is also used as a measure for the size and shape of the plastic zone in front of the crack tip and therefore has potentially a direct influence on plasticity-induced crack closure (PICC). At the same time, individual overload cycles also influence the size of this plastic zone both during the current LCF cycle and subsequent HCF cycles.

The focus of the investigation is to experimentally study the Influence of multiaxial far field loading on the crack opening displacement (COD) and hence crack closure und opening in two materials commonly used in gas turbine engines. For this, crack propagation tests were performed on the titanium alloy Ti6246 and tempered steel…

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The T-stress as a measure for the multiaxiality or constraint is well-known in linear-elastic fracture mechanics. It is also used as a measure for the size and shape of the plastic zone in front of the crack tip and therefore has potentially a direct influence on plasticity-induced crack closure (PICC). At the same time, individual overload cycles also influence the size of this plastic zone both during the current LCF cycle and subsequent HCF cycles.

The focus of the investigation is to experimentally study the Influence of multiaxial far field loading on the crack opening displacement (COD) and hence crack closure und opening in two materials commonly used in gas turbine engines. For this, crack propagation tests were performed on the titanium alloy Ti6246 and tempered steel 26NiCrMoV14-5 at room temperature. The tests were utilizing cruciform specimens and a biaxial tests-rig. This experimental set-up allows to systematically adjust and compare different T-stress levels as well as combined LCF-HCF-cycles on the plastic zone of the crack tip area. The tests are monitored and analysed using a newly developed Digital Image Correlation (DIC) camera. This makes it possible to analyse crack growth and crack flank opening and to measure crack closure forces for different crack flank positions. Ultimately, this approach allows PICC to be studied systematically.

In conclusion, biaxial tests on Ti6246 and 26NiCrMoV14-5 are evaluated and compared using cruciform specimen. These were subjected to different multiaxial loads on one hand and to combined LCF-HCF cycles on the other. A workflow is developed to automatically calculate COD, crack closing forces, compliance of both closed and opened cracks and finally the compliance offset out of the DIC data. The findings reveal that despite the influence of both individual LCF cycles and T-Stress on the plastic zone, the crack-closing forces of the biaxially loaded cruciform specimens remain largely unaffected.

Reference
ICMFF14-2025-27

Title
Systematic investigation of the influence of T-stress and LCF overloads in biaxially loaded cruciform specimen on plasticity-induced crack closure using digital image correlation
Author(s)
A. Zaiat, A. Blug, G. Laskin, A. Bertz, F. Conrad, C. Kontermann, M. Oechsner
DOI
10.48447/ICMFF14-2025-27
Event
14. International Conference on Multiaxial Fatigue and Fracture 2025
Year of publication
2025
Publication type
conference paper (PDF)
Language
English
Keywords
PICC,Biaxial-Testing,Cruciform Specimens,T-Stress