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# Experimental Technique for Elevated Temperature Mode I Fatigue Crack Growth Testing of Ni-Base Metal Foils

[+] Author and Article Information
L. Liu

Materials and Aerospace Structures Laboratory, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0150

J. W. Holmes1

Materials and Aerospace Structures Laboratory, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0150john.holmes@ae.gatech.edu

1

Corresponding author.

J. Eng. Mater. Technol 129(4), 594-602 (Oct 27, 2006) (9 pages) doi:10.1115/1.2772324 History: Received October 03, 2005; Revised October 27, 2006

## Abstract

Details are provided for an experimental approach to study the tensile fatigue crack growth behavior of very thin metallic foils. The technique utilizes a center-notched specimen and a hemispherical bearing alignment system to minimize bending strains. To illustrate the technique, the constant amplitude fatigue crack growth behavior of a Ni-base superalloy foil was studied at temperatures from 20°C to 760°C. The constant amplitude fatigue tests were performed at a frequency of $2Hz$ and stress ratio of 0.2. The crack growth rate versus stress intensity range data followed a Paris relation with a stress intensity range exponent $m$ between 5 and 6; this exponent is significantly higher than what is commonly observed for thicker materials and indicates very rapid fatigue crack propagation rates can occur in thin metallic foils.

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## Figures

Figure 11

Crack length versus fatigue cycles for 76μm thick Inconel 617 foil subjected to tension-tension fatigue at RT, 538°C, and 760°C. The machined notch for the specimens that are shown had a total length that ranged from 1.73mm to 2.14mm. For all specimens, the fatigue cracks grew uniformly from both sides of the center notch.

Figure 12

Experimentally measured relationship between the applied stress intensity range ΔK and the cyclic fatigue crack growth rate da∕dN during tension-tension fatigue loading of Inconel 617 foil at room temperature

Figure 13

Experimentally measured relationship between the applied stress intensity range ΔK and the cyclic fatigue crack growth rate da∕dN during tension-tension fatigue loading of Inconel 617 foil specimens at 538°C

Figure 14

Experimentally measured relationship between the applied stress intensity range ΔK and the cyclic fatigue crack growth rate da∕dN during tension-tension fatigue loading of Inconel 617 foil specimens at 760°C

Figure 15

Comparison between the experimentally measured relationships between the applied stress intensity range ΔK and the cyclic fatigue crack growth rate da∕dN during tension-tension fatigue loading at RT, 538°C, and 760°C

Figure 16

Schematic representation of the thickness dependence of fracture toughness. For very thin materials, shear fracture and plastic thinning can occur. For cyclic loading, the low fracture toughness of thin foils and surface roughening at the tip of a growing crack will have an important influence on crack growth behavior.

Figure 9

Experimental arrangement used study fatigue crack growth in foil-thickness specimens. The self-centering hemispherical bearing (Fig. 2) and mechanical grips were installed on a servohydraulic load frame. Detailed drawings for the face-loading grips are provided in Fig. 1.

Figure 10

Experimental setup used to study the elevated temperature fatigue crack growth behavior of metal foils. The furnace (shown with outer half removed) had a heated cavity height of 75mm and a width of 40mm. Electrically heated SiC elements were located behind a 2mm thick layer of Al2O3 that acted as a radiation shield (this arrangement provided uniform heating of the specimens).

Figure 1

Detailed drawings of face-loaded grips used for fatigue testing of metal foils

Figure 7

Comparison of transverse displacement component u1 versus position along specimen length for the double-edge-notched and center-notched specimen geometries shown in Figs.  46, respectively. The left-edge, centerline, and right-edge displacements are plotted. These results illustrate the significantly larger in-plane displacements that would occur for unequal notch lengths in the double-edge-notched specimen geometry.

Figure 8

Center-notched specimen geometry used to study the fatigue crack growth behavior of metal foils. The center notch was machined by electrodischarge machining (EDM). Gripping for axial loading was achieved by mechanically clamping the specimen ends.

Figure 2

Self-centering bearing arrangement used to provide uniform axial loading of thin metal foils. The upper specimen grip was attached to the bearing stem (detailed drawings of the mechanical face-loaded specimen grips are provided in Fig. 1). The self-centering bearing, which rotates 360deg about the two-axis, can pivot about any plane perpendicular to the 1–3 plane and, in addition, can translate parallel to the 1–3 plane.

Figure 3

(a) Finite element model of single-edge-notched specimen geometry. A 10mm long edge crack is simulated in the analysis using the one-forth node technique (14). (b) Displaced mesh after application of a uniform traction to the lower edge of the model. In an actual specimen, the nonuniform deformation would promote out-of-plane buckling of the foil.

Figure 4

(a) Finite element model of a double-edge-notch specimen with nonsymmetric crack lengths. (b) Displaced mesh showing nonuniform deformation of the specimen under axial tensile loading. Significant bending deformation occurs, which further increases the stress intensity for the longer crack (note that the stress intensity factor has contributions from axial loading and from bending; the bending further increases the stress intensity factor on the right side of the specimen).

Figure 5

(a) Axial stress (S22) distribution during tensile loading of a double-edge-notched thin-foil specimen. An axial stress of 344MPa was used in the analysis. For unequal crack lengths, a nonuniform stress distribution develops. (b) Transverse displacement (u1) profile during tensile loading. The nonuniform displacement is caused by bending deformation along the length of the specimen (the bending is due to a shift in the centerline of loading). It is important to note that nonuniform in-plane displacement can promote out-of-plane buckling in a thin foil.

Figure 6

(a) Finite element model of a center-edge-notched specimen. The effect of nonuniform crack growth on the mode I stress intensity factor and specimen deformation was examined by translating the notch (along the horizontal axis) 2.5mm from the geometric center. (b) Displaced mesh showing deformation of the specimen under axial tensile loading. Compared to the double-edge-notched geometry (Figs.  45), there is minimal bending deformation. (c) Axial stress (S22) distribution, which is much more uniform compared to the double-edge-notch geometry with unequal crack lengths.

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