Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718
- PDF / 713,624 Bytes
- 13 Pages / 606.24 x 786 pts Page_size
- 51 Downloads / 212 Views
2/7/04
2:53 PM
Page 771
Effect of Broaching on High-Temperature Fatigue Behavior in Notched Specimens of INCONEL 718 T. CONNOLLEY, M.J. STARINK, and P.A.S. REED Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.
I. INTRODUCTION
IN the broaching process, a cutting tool with multiple shaped teeth is drawn through a hole or over a surface to remove material by axial cutting.[1] Figure 1 shows a schematic diagram of a broaching tool and its application in the machining of a U-notch specimen of the type used in this study (Figure 2). With appropriate tool design, complex profiles can be machined with the single pass of a tool. In gas turbines, the blades are usually attached to the rims of turbine discs using a fir-tree root fixing and the complex notch geometry in the rim is commonly produced by broaching. The stress concentration effect of the notch means that fatigue initiation in this region is more likely. One type of in-service damage to turbine discs that must therefore be considered is low-cycle fatigue (LCF) cracking in the fir tree root fixtures. For the present work, the INCONEL* 718 (IN718) nickel*INCONEL is a trademark of INCO Alloys International, Huntington, WV.
based superalloy was chosen due to its current and projected widespread use for gas turbine discs. The majority of published LCF studies on this alloy have concentrated on the effects of temperature and environment on long crack propagation rates. This has included studies of mechanisms of grain-boundary oxidation and its influence on fatigue crack propagation.[2,3,4] Dwell times at maximum or minimum load were generally found to increase crack propagation rates, with the dwell permitting the time-dependent oxide embrittlement of grain boundaries.[5–11] Most of these studies were on long cracks using compact tension (CT) specimens, but the hold time effect has also been observed for preinitiated cracks in U-notch specimens tested in bending.[12] Compared to the wealth of knowledge on IN718 obtained from laboratory long crack fatigue testing, relatively little is T. CONNOLLEY, Research Engineer, is with the National Centre for Biomedical Engin
Data Loading...
