H. Murthy

Efficient Modeling of Fretting of Blade/Disk Contacts Including Load History Effects

Fretting is a frictional contact phenomenon that leads to damage at the contact region between two nominally-clamped surfaces subjected to oscillatory motion of small.amplitudes. The region of contact between the blade and the disk at the dovetail joint is one of the critical locations for fretting damage. The nominally flat geometry of contacting surfaces in the dovetail causes high contact stress levels near the edges of contact. A numerical approach based on the solution to singular integral equations that characterize the two-dimensional plane strain elastic contact of two similar isotropic surfaces presents itself as an efficient technique to obtain the sharp near-surface stress gradients associated with the geometric transitions. Due to its ability to analyze contacts of any two arbitrary smooth surfaces and its computational efficiency, it can be used as a powerful design tool to analyze the effects of various factors like shape of the contact surface and load histories on fretting. The calculations made using the stresses obtained from the above technique are consistent with the results of the experiments conducted in the laboratory.

Contact problem with partial slip for the inclined punch with rounded edges

Abstract The 2-D contact problem for the inclined punch having a flat base and rounded edges and an elastic half-space is considered. It is assumed that the applied forces provide for conditions of partial slip within the contact region and that the contacting bodies have similar elastic properties. The analytical expressions for distribution of contact pressure and shear stresses, the σx stress component, slip displacement, etc. are reduced based on Muskhelishvili’s method. It is shown that only one stick zone exists within the contact region. The location of the contact region and the stick and slip zones are analyzed for various external conditions. The model is used to evaluate the dependence of the stresses within and near the contact zone, on the normal and tangential forces and the moment applied to the indenter and the particular indenter geometry.

Load History Effects on Fretting Contacts of Isotropic Materials

The load history that blade/disk contacts in jet engine attachment hardware are subject to can be very complex. Using finite element method (FEM) to track changes in the contact tractions due to changing loads can be computationally very expensive. For two-dimensional plane-strain contact problems with friction involving similar/dissimilar isotropic materials, the contact tractions can be related to the initial gap function and the slip function using coupled Cauchy singular integral equations (SIEs). The effect of load history on the contact tractions is illustrated by presenting results for an example fretting mission. For the case of dissimilar isotropic materials the mission results show the effect of the coupling between the shear traction and the contact pressure.

APPROXIMATE SOLUTION FOR SHEAR IN NOMINALLY FLAT CONTACTS UNDER PARTIAL REVERSE SLIP

Nominally flat contacts are representative of various components like blade/disk contacts in aircraft engines. In literature, there are only numerical solutions for this class of plane contact problems under partial reverse slip conditions, when bulk stress is applied. A general shear traction distribution based on boundary conditions and the direction of slip in slip zone has been proposed to obtain closed form solution for this problem. To obtain the closed form solution for specific problem of flat base in contact with a nominally flat punch, pressure distribution had to be approximated to evaluate some integrals. An appropriate approximation valid for all ratios of flat length to edge radii has been proposed. Deviations in shear traction distribution due to this approximation was found to be very small. For sake of brevity, loading case of only bulk stress applied is presented.

A Structured Lifing Approach to Fretting Fatigue of Ti -6Al -4V with Variable Initiation Length

An attempt has been made to use a structured approach to predict fretting fatigue lives of Ti -6Al -4V specimens used in laboratory experiments. Knowing the contact tractions, differe nt damage parameters have been obtained, from which initiation life has been calculated using uniaxial fatigue curve after applying stressed area correction to account for the length scale effects. Propagation lives have been calculated using both fixed in itiation length and variable initiation length. Short crack propagation has been included using the El Haddad approach. Total life has been calculated as a sum of initiation and propagation lives. The distribution of damage parameter and the nature of its variation as a result of pad profile machining have been studied.