Fracture analysis of Pre-cracked 8YSZ TBCs with edge and internal cracks under Thermo-mechanical load: A numerical approach

Abstract

The thermal barrier coatings (TBCs) are used for hot component sections of gas turbines and aero-engines. The paper presents the effect of thermo-mechanical on the pre-cracked TBCs to analyze fracture parameters using the FEM method. The TBC model is considered with edge and internal cracks with different crack configurations such as crack length and cracks position between ceramic top coat and bond coat to find the strain energy release rate (SERR) and stress intensity factor (SIF). The TBC mixed-mode I/II fracture criterion is established. The numerical results showed that the crack tip field is shear dominant under thermo-mechanical loading irrespective of different crack configurations. The edge crack's SERR and SIF mode I/II are more significant than the internal crack, resulting in delamination. The long edge crack showed crack driving force of 2.13 J/m2 at P1 position, whereas the shift in crack position till P7 reduces the SERR to 1.48 J/m2. The internal crack at P1 showed the SERR of 0.91 J/m2 which reduces to 0.85 J/m2 at position P7. The edge crack resulted in a high amount of shear stress which is caused due to large opening of crack faces. In all crack configurations, the increase in crack length showed an increase in crack driving force and large opening of crack faces due to high shear stress. Also, the crack positioned in the ceramic topcoat showed a high value of crack driving force, resulting in more probability of surface rupture followed by crack positioned at the interface of the topcoat/bond coat and crack positioned in the bond coat. The interface crack showed a high crack driving force compared to crack located near the interface due to bi-material and geometry discontinuity, resulting in stress intensification. The edge crack showed a high value of SIF mode I in the range of 0.1 to 0.35 MPa.m1/2 compared to internal crack which shows in the range of 0.05 to 0.18 MPa.m1/2. The edge crack shows high value of SIF mode II in the range of −0.05 to −0.22 MPa.m1/2 compared to internal cracks showing upto −0.1 MPa.m1/2. The crack length and position play an important role in crack initiation and propagation. The TBC structure with edge crack appears unstable and more likely to damage than internal crack, leading to an early failure mechanism.