A finite element fretting investigation of a plane-strain cylindrical contact of INCONEL 617/INCOLOY 800H at room and high temperatures

Huaidong Yang and Itzhak Green

School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA

INTRODUCTION: Fretting occurs when two contacting surfaces experience small amplitude of oscillatory relative motion under normal load. Depending on whether a stick area exists or not, the steady-state fretting regimes can be divided into partial slip and gross slip. The partial slip condition is mainly responsible for the mechanical failure of surface crack initiation and propagation, while the gross slip conditions introduce wear¹. Fretting damage can significantly reduce the operational lifetime of components in nuclear reactors, including valve stems and seats, control rod drive mechanisms, fuel handling mechanisms, and helium circulators. In particular, fretting damage of steam generator tube materials in the nuclear plants can lead to the leakage of the primary cooling water².

Alloy 800H and Inconel 617 are promising structural materials for high temperature and very high temperature gas cooled reactors (HTGRs/VHTRs); they possess excellent high temperature thermo-mechanical properties and also offer environmental protection. These two materials are the subjects for the analysis in this work.

In this work, the finite element approach, using ANSYS17.1, is applied to analyze the fretting damage for a 2-D plain strain contact between a half cylinder and a flat block of materials Inconel 617 and Alloy 800H in room temperature and 800 °C. The development of the von-Mises stresses and plastic strains near the contacting surfaces, the junction growths, the evolution of the tangential forces in the initial few cycles of oscillatory tangential loads, and the scars on the surface of the block are obtained. The results of this work agree well with the former related experimental data. The objective is to form an in depth understanding of the mechanisms of fretting wear, and the propensity for crack initiation and propagation.

METHODS:  The model is a 2D plane-strain contact between a half cylinder and a flat block. It is a displacement-controlled analysis with the quasi-static inputs of vertical interference and horizontal oscillatory fretting motion applied on the top of the half cylinder. The material of the block is the weaker one, INCOLOY 800H, while the cylinder is INCONEL 617. The elastic-perfectly plastic behavior is used. The mesh of the model is refined within the contact region so as to yield accurate results compared with the theoretical values^3,4.

  

Figure 1 – A) Schematic of a half-cylinder in contact with a flat block with displacement controlled inputs and outputs. B) The converged finite element model in ANSYS 17.1

RESULTS:  Figure 1 and 2 show the distribution of von-Mises stress and plastic strain after three cycles of fretting motion. The large von-Mises stresses and plastic strains stay at the edges of the contact.

Figure 3 shows the junction growth during the fretting motion. The more pronounced growth on the two edges is in the same direction

of the tangential force experienced by the surface of the deformed body (not shown).

Figure 4 shows the evolution of the tangential force. It is typically the fretting loop for the initial cycles as indicated by Courtney-Pratt and Eisner⁵.

Figure 2 - A) The distribution of von-Mises stress after three cycles of fretting motion  B) The distribution of plastic strain after three cycles of fretting motion  C)The junction growth during three cycles of fretting motion D)The evolution of tangential load

DISCUSSION: During the oscillatory tangential loading, the two contact edges tend to experience the largest von-Mises stress. It is, therefore, postulated that cracks and fatigue are most likely to initiate and propagate at the contact edges. During the fretting between INCOLOY 800H and INCONEL 617, INCONEL 617 is hard to yield. The explanation could be that as the INCOLOY starts to yield, its resistance to the increasing load does not change much, which leads to the elastic behavior of INCONEL.

The junction growth is found during the fretting motion. The more pronounced growth on the two edges is in the same direction of the tangential force experienced by the surface of the deformed body. The conclusion agrees with the results of Brizmer⁶ whose model is the contact between a deformable sphere and a rigid flat under full stick condition. It is indicated that the junction growth is caused by the plastic deformation on the contacting surfaces.

There are abrupt changes of the curvature at the edges of the indentation on the surface after the fretting motion (not shown). When the COF is large enough to reach the fully stick condition, pileup will appear at the position of the abrupt change. In the case with the appearance of pileup, the plastic strain at the edges of the contact is correspondingly larger.

The temperature elevation (from 20C to 800C) causes the increase of the plastic strain and the decrease of the tangential force, normal force, and the maximum von-Mises in INCONEL 800H. Therefore, the fretting between the alloys will bear more damages in the elevated temperature under the same load input.

REFERENCES:  1.Slack, Tribology Transaction (2013), 2.Mei, Proc. Advanced Materials Research (2013), 3.Johnson, Contact Mechanics (1987), 4.Green, Applied Mechanics and Engineering (2005), 5. Courtney-Pratt and Eisner, Proceedings of the royal society of London A: Mathematical, Physical and engineering Sciences (1959), 6.Brizmer, Journal of Tribology (2007).