G. Harish

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.

In-Situ Measurement of Near-Surface Fretting Contact Temperatures in an Aluminum Alloy

Fretting is the tribological phenomenon observed in nominally-clamped components which experience vibratory loads or oscillations. Associated with fretting contacts are regions of small-amplitude relative motion or microslip that occurs at the edges of contact. A newly-available infrared technology capable of resolving temperatures fields finely, both spatially and temporally, is used to characterize the near-surface conditions associated with fretting contact between an aluminum alloy cylinder and flat. Both frictional heating due to interfacial slip and the coupled-thermoelastic effect arising from strains in the material induce these temperatures. The experimental results provide insight into not only the magnitude and distribution of near-surface temperatures, but also the nature of the contact stress field and the mechanics of partial slip fretting contacts. Comparisons of the measured temperature fields are made with those predicted by considering both conduction of the frictional heat flux and coupled-thermoelastic theory.

An integrated approach for prediction of fretting crack nucleation in riveted lap joints

Fretting is a major cause of crack formation in aircraft structures like lap joints and dove tail joints. A tightly integrated mechanics-based method to relate betting crack nucleation to the various parameters that intluence it is presented. A tite element model of a rivet and the skin around it was analyzed to provide the stress state under various loads. The model incorporated residual stresses from riveting, plasticity and contact between the various surfaces. A multiaxial fatigue model was used to translate the results into a prediction of life to crack nucleation. Excellent comparisons with a set of controlled lap joint experiments provide the validation of this approach. Effects of the tite width of the experimental specimen were found to be small. The complexity of fretting crack nucleation prediction in a riveted aircraft structure can be attributed to the necessity of analyzing the manufacturing process conditions, the operating conditions and the fatigue properties of the material under consideration. This requires a thorough understanding of these fields as well as then relationships. The problem of fretting at the rivet/skin interface in aircraft riveted structures is one such example. Fretting is a contact phenomenon occurring at interfaces of nominally clamped bodies and is one of the causes for the initiation of multi-site damage.r Indeed the teardown inspections of aging aircraft have confirmed *Research Assistant, School of Aeronautics and Astronautics. AIAA Student Member. tProfessor and Head, AIAA Associate Fellow ‘Copyright 01999 by the Americaa Institute of Aeronautics and Astronautics, Inc. All rights reserved. fretting as a prime cause of crack nucleation at the rivet/skin interface.2 Fretting has long known to be a persistent cause of crack nucleation. The contact stresses at the interface are the primary contributors to the damaging effect of fretting. While some attempts at qualitative study of lap joints have been made, very little has been done to isolate the factors that influence the crack nucleation.3-6 Prior work by the authors indicate that the clamping produced during riveting might be a crucial factor.7 Also, the residual stresses generated during the riveting process, an aspect to which little detail has been paid in the past, is an important aspect of betting crack nucleation. Figure 1 shows a block diagram of the approach taken by the authors. The first step is the modeling of the riveting process using a Snite element approach. The end configuration of this model provides the i&mework to create a 2dimensional model of the plate and rivet with the correct initial strains and stresses. The loads are then applied to this model cyclically. The results of the 2-dimensional model are ported to a program that uses a multiaxial fatigue criterion to determine the location and the life to nucleate a crack. Controlled fatigue experiments of lap joints with rivets installed through force-controlled riveting provide the necessary experimental database to evaluate the predictions.

Full-field IR measurement of subsurface grinding temperatures

A multiple-element, forward looking IR system is used to measure the subsurface temperature field produced by dry grinding of steel with both aluminum oxide and cubic boron nitride (CBN) grinding wheels. The technique is base don imaging the IR radiation obtained from the side of the specimen. A recent theoretical analysis of heat partition and surface temperatures in grinding is reviewed. The analysis partitions heat on the two length scales pertinent to grinding between the workpiece, wheel, coolant and chips. Spectral analysis is combined with FFT techniques to calculate subsurface temperature contours from the predicted heat partition. The numerical predictions of the model are shown to agree wit the experimental results taken for a range of grinding conditions. It is found that heat partition varies over a wide range depending on grinding conditions. Also, heat partition is a strong function of position inside the grinding zone. The presence of the fluid inside the grinding zone can reduce the heat flux into the workpiece and the workpiece temperature significantly. For typical grinding of steel with CBN, or creep feed grinding of steel with aluminum oxide or CBN, it is possible to keep the fluid active and therefore to reduce thermal damage. However, the analysis suggests that the fluid may not be effective inside the grinding zone, in the conventional grinding of steel with aluminum oxide, due to boiling. It is also found that a moderate ratio of the workpiece velocity to wheel velocity gives high temperatures and therefore should be avoided.