A statistically-designed experimental program engendered to validate an analytical approach for the prediction of fretting crack nucleation in 2024-T351 aluminum alloy has been completed. The test results indicate that the near-surface cyclic contact stress and strain field can be juxtaposed with a multiaxial fatigue life parameter relying on uniaxial strain-life constants to predict crack nucleation for a wide range of load intensities and conditions representative of those experienced in riveted joints. With this approach validated, efforts have been initiated to predict fretting-induced fatigue failures in riveted single lap joint structures. Research was targeted at characterizing the conditions at and around the rivet/hole interface, including finite element modeling of both the mechanics of load transfer in riveted joints and the residual stress field introduced during the rivet installation process. Model results and an ancillary set of fatigue tests of single lap joint test articles have identified a strong correlation among riveting process parameters, the mechanics of load transfer, and the subsequent tribological and fatigue degradation of the joints. Final comments are offered regarding the ability of this integrated approach to predict the fatigue performance of riveted lap joint structures.

Fretting, the deleterious and synergistic combination of wear, corrosion, and fatigue phenomena driven by the partial slip of nominally clamped surfaces, has been linked to severe reductions in service lifetimes of a myriad of contacting components, including bearings, turbine blades and mechanically fastened joints-both structural and biological. This paper serves to frame the aggregate of economic, operational, and technical developments responsible for engendering renewed interest in fretting fatigue of critical structural elements of both commercial and military aerospace systems, including riveted primary structure and the blade/disk pair in jet propulsion plants. A collection of both empirical evidence of fretting-induced componential degradation and an overview of results from recent investigations conducted by the authors serves to motivate the need for design-oriented metrics that can be used to ensure the structural integrity and safe operation of both current and future aerospace systems.

Two bodies are said to be in fretting contact when they are clamped together under the action of a normal force and see an oscillatory motion of small amplitudes at the contact interface due to the effect of shear force and bulk stress. The contact stresses that drive crack nucleation are very sensitive to the shape of the contacting surfaces and the coefficient of friction. To have an understanding of fretting at the contacts in turbine engine components, it is important to simulate similar temperature, load, and contact conditions in the laboratory and develop tools to analyze the contact conditions. Efforts made to simulate the temperature and load conditions typical of engine hardware in a controlled laboratory setting, similar to that developed previously at room temperature, are described. It is shown that the temperature in the contact region can be held constant at nominally 600°C. Preliminary results are given in terms of loads and measured total lives along with thoughts on development of a total life model for single crystal materials.

The fretting problem is of particular interest to the damage tolerant design of turbine blades in today's gas turbine engines. The exotic environment, high-frequency, and variable amplitude load history associated with the dovetail blade/disk connection create a critical location for fretting induced crack nucleation. With little work having been done on investigating fretting contact behavior at high-frequencies and variable amplitude load spectra, sufficient impetus has been generated to better characterize these two currently ambiguous fretting factors. The threat of early crack nucleation and propagation due to these fretting conditions has led to several major research efforts aimed at explicating the high cycle fatigue (HCF) and low cycle fatigue (LCF) interaction and behavior of advanced materials used in modern aircraft turbomachinery. As a part of this effort, a well-characterized experimental setup has been constructed to aid the observation and analysis of the aforementioned frequency and loading factors in fretting. A detailed description of the designed high-frequency fretting rig is presented. Significant vibration and bending results observed during high frequency operation suggest further design modification for improved specimen and pad alignment. Preliminary experimental observations illustrate crack nucleation and failure in specimens subjected to operational frequencies between 100 and 350 Hz. A stress invariant equivalent stress life model is employed for comparison of predicted and observed experimental crack nucleations. The paper concludes with suggested future work aimed at experimentally explicating the frequency and variable amplitude factor effects in fretting fatigue.

The near-surface stress field in fretting has long escaped experimental characterization due in large part to the fact that the friction coefficient in the slip zones associated with the partial slip contacts cannot be evaluated from measured forces. Attempts at circumventing this through measurements of microslip or extent of the slip zones have been inconclusive. However, newly available infrared detector technology is capable of resolving finely, both spatially and temporally, subsurface temperatures near the fretting contact. These temperature changes are induced by both frictional heating at the surface due to microslip as well as the coupled thermoelastic effect arising from the strains in the material. A finite element model has been developed for fretting that includes the heat generation due to sliding and partial slip; and the coupled thermoelastic effect. The model also incorporates heat conduction, thermal deformation, and contact. The correlation between the temperature changes measured by the infrared camera and those predicted by the finite elements is remarkable. During gross sliding, a patch of heating throughout the contact length, attributed to frictional heating, is observed. As the friction coefficient rises and the contact transitions to a partial slip regime, the temperature changes are more clearly associated with strain through the coupled thermoelastic effect. The excellent agreement of the finite element results with the experiments demonstrates the ability of the model to provide validated values for fretting-induced stresses and microslip.