CRACKED BRAZILIAN TESTS OF LAMELLAR TIAL

Transcription

1 CRACKED BRAZILIAN TESTS OF LAMELLAR TIAL Gunes Uzer1,a, Fu-pen Chiang2,a, Andrew H. Rosenberger3,b 1 SUNY Graduate Student, 2 SUNY Distinguished Professor& Chair 3 Materials and Manufacturing Directorate, AFRL/MLLN a Dept. of Mechanical Engineering Stony Brook University Stony Brook, NY b Weight-Patterson Air Force Base, Ohio guzer@ic.sunysb.edu ABSTRACT: Brazilian tests are performed for the purpose of investigating the mixed mode crack propagation characteristics in lamellar TiAl with the grain size of ~450μm. Circular disks of about 1mm in thickness and 3.5 mm to 9 mm in diameter are manufactured. Slits of 0.15mm in width are cut into the central part of the disks with length being 1/3 of the diameter. A multi-scale speckle method is employed to measure the full field strain distributions at both macro and micro scales. Specimens with different slot orientations are tested in order to control the crack initiation. Micro scale tests are performed inside the chamber of Hitachi S2460-N scanning electron microscope. Results indicate that the highest strain occurs at the point where the surface of the straight line edge of the slot meets the curved edge, regardless of the slot orientation. Crack propagation is always in a mixed mode. The crack propagation speed slows down when the crack approaches a grain boundary. It then either stops or jumps to the next grain depending on the grain orientation. A technique is proposed to evaluate the mode mixity from two representative displacement vectors on either side of the crack. INTRODUCTION: A disc specimen containing a center-crack and subjected to a compressive was load first introduced by Awaji and Sato in 1978 [1]. Since then, the cracked Brazilian disk (CBD) specimen has been employed frequently to investigate mixed mode brittle fracture for different materials e.g. [2-8]. Fracture test is conducted by applying a diametral compressive load. We employed a multi-scale speckle method [9] to measure the full field deformation. Different combinations of mode I and mode II can be provided by changing the angle between the crack line and the loading direction. When angle is zero, the specimen is subjected to pure mode I. Depending on the crack length, orientation and the disc radius of various mixity can be achived [10]. Experiments have shown that fracture toughness of TiAl compounds is a function of the grain size [11]. Grain orientation with respect to approaching crack during fracture has also been shown to influence the behavior of the crack.[12]. The aim of this paper is an effort to understand the crack propagation behavior of TiAl under different loading modes. EXPERIMENTS: The composition of lamellar TiAl was determined by EDAX energy dispersive spectroscopy. Average grain size was 450μm Cracked Brazilian Disk (CBD) specimens were manufactured. Geometry as shown in Fig.1. In order to see the grains and lamellar structure in more detail, specimens were mechanically polished by 0.05μm alumina particles and chemically etched using nitric acid. Micro Brazilian tests were performed in a loading stage which is inside the chamber of Hitachi S2460N electron microscope. Tests were performed in constant displacement rate of the loading grips and at a vacuum of 7x10-4Pa.

2 Specimen A* B** C** D** E** F** G** H** K** L** D (mm) A (mm) 2a/D β O 0O 32O 32O 57O 7O 34O 25O 20O O * Specimens tested with Instron Machine ** Specimen tested with SEM Figure 1. Geometry of the Cracked Brazilian Disc Test Specimen As shown in Fig. 2, specimen A was loaded to 430 lbs. Further loading led to failure without obtaining any additional information. Examination of the v-field displacement contour indicates the strain concentration at the tips of the central slot. Figure 2. Displacement Fields of a 9 mm Brazilian Disk Under Compression Prior to Crack Initiation. In order to reveal more information at the crack tip, several CBD specimens were tested inside SEM and viewed at different magnifications. To control the deformation mode we used the results given in Fett. et al [10]. Specimen B ( β =149o) was tested in mixed mode. Fig. 3 shows the displacement field under a load of 287 lbs. Examination of the v-field displacement contour reveals that the highest fringe density (thus the highest εyy ) occurs at the point where the straight line edge of the slot meets the curved surface.

3 Figure 3. Displacement Field of Specimen B Prior to Crack Initiation. (a) U-field (b) V-field An additional loading to 290 lbs resulted in the simultaneous appearance of two cracks at the ends of the slot and the load dropped to 287 lb. The locations are indeed as predicted by the strain field shown at the points where the straight and curved edges meet as shown in Fig.4. Figure 4. Two Cracks Appear at the Ends of the Central Slot of the 7mm Brazilian Disc. Further loading with P = 287 lb increased to P = lb did not advance the crack much (only a few microns). This is the region where the crack was arrested at the triple junction of grains. The displacement contours of an incremental load of P = 282.3lb to P = lb are shown in Fig. 5. Also shown in the figure is the crack tip position relative to the grains. Additional load caused the crack to propagate unstably and led to dynamic fracture of the specimen. Figure 5. Crack Tip Deformation Fields and Location

4 Specimen C was tested in mode I ( β =0o). Fig. 6 (a) and Fig. 6 (b) depicts the deformation pattern prior to crack initiation. Fig. 6 (c) shows a crack initiated at the bottom tip of the central crack at P=190 lb. With further loading the crack started to propagate. At the load P= 263 lb crack tip approached to a grain boundary and two the cracks on the bottom and top of the central slot initiated. When loading continued the first crack entered a blunting process, with larger crack opening displacement as the load increased. Figure 6. U and V Displacement Fields of Specimen C Prior to Crack Initiation. (c) Crack after Initiation. As illustrated in Fig. 7 as the second crack on the bottom started to link itself with the first crack. After the two cracks met in the same grain and they linked themselves with a ligament. Further loading to P=344 lb broke the disk into two pieces Figure 7. Linkage between Two Cracks via Mode II. Specimen D was tested with β =32o as shown in Fig.8. Cracks above and below the central slot were initiated at P=166 lb and P=183 lb respectively. The upper crack approached to a grain boundary whose orientation was nearly perpendicular to crack. The crack was arrested in that region and the blunting process started. With further loading another crack initiated near the upper crack in the same direction with the grain. The lower crack did not encounter a grain with normal orientation. Thus it continued to propagate as further loading was applied and failure resulted.

5 1mm 0.5 mm 0.5 mm Figure 8. Displacement and strain field in specimen D. During the tests we observed microcracks in front of the main crack tips. These microcracks were initiated with the orientation almost parallel to the loading direction and they propagated through the grain by extending the crack length in both directions. DISCUSSION: Experimental results demonstrate exclusively that grain boundaries act as crack retarders. Whenever a crack enters the junction of a cluster of grains, it slows down and seeks the path of least resistance; it tends not to propagate along the direction perpendicular to the lamellar layers. When the crack tip reaches the grain boundary, the direction of its further extension can be predicted by the strain field surrounding the crack tip region. Fig. 9 shows such an example. It shows the displacement and strain fields surrounding the upper edge of the notch in specimen C prior to crack initiation. Strain field indicates the area of high strain concentration. Further loading resulted in new mode I crack initiated at a distance away from the main crack in grain with orientation similar to the loading direction. Within the area indicated by of high strain concentration. Figure9. Displacement Fields of Specimen C Prior to Crack Initiation. (a) U-field (b) V-Field.(c) xx Strain Field.(d) Crack Initiated in the Site Indicated by Strain Field.

6 In Brazilian tests we observed microcracks in front of the crack tip as depicted in Fig. 7., these microcracks initiated in the same direction with grains (and also loading direction) and again main crack connected itself with these microcracks with shear band as shown in Fig. 7. The microcracks initiated in the same direction with the grains and they propagated in both directions through the grain. It could be depicted that crack propagation of lamellar TiAl is always in a mixed mode (I + II) unless the crack is perpendicular to the lamellar orientation. When the crack reaches a grain boundary normal to the crack, it blunts first and then penetrates the grain with a zigzagging path with a deformation where mode I is dominant. Thus crack assumes different mode mixity as it meanders through different grains with different lamellar orientations. However at the end of the journey it always resorts to mode I propagation to failure. Fig 10 shows various examples. C E β= 0 O D = 3.2 mm = 32O β = 4 mm D F D O β= 32 β = 57O D= 4 D = 3.2 mm Figure10. Mode I Failure is the Dominant Failure Mode Irrespective of the Initial Slot Orientation. Here we propose a technique that can evaluate the mode mixity from two representative displacement vectors on either side of the crack as illustrated in Fig. 11 Figure11. Evaluation of Mode Mixity on CBD Specimen It can be concluded that electron speckle technique is an effective tool to study crack propagation characteristics. In TiAl the grain boundary retards the crack advance by providing large amount of resistance. The weakest path of the γ α2 lamellar TiAl is the interface of the γ α2 layers. When a crack runs into the junction of a cluster of grains with different orientation the crack seeks the γ α2 interface which changes its direction from place to place. This act tends

7 to retard the crack propagation. This phenomenon may explain why smaller grain TiAl compounds tend to have high fracture toughness. Smaller grains have many more grain boundaries for the crack to cross. References 1. Awaji, H., Sato, S., J. Engng. Mater. Technol., vol. 100, , Atkinson, C., Smelser, R.E., Sanchez, Int. J. Fract., vol. 18, , Shetty, D.K., Rosen, A.R., Duckworth, W.H., Engng. Fract. Mech., vol. 26(6), , Liu C., Huang Y., Stout, M.G., Acta. Mater., vol. 46(16), , Krishnan, GR et al., Int. J. Rock Mech. Min. Sci., vol. 35(6), , Khan, K., Al-Shayea N.A., Mech. Rock Engng., vol. 33(3), , Scherre, S.S. et al, J. Biomed. Mater. Res. (Appl Biomater), vol. 53, , Chang, S.H., Lee, C.I., Jeon, S., Engng. Geology vol. 66, 79 97, Chiang, F.P., Optical Engineering, vol. 42(5), , Fett, T., Engng Fract Mech vol. 68, , F. Appel, R Wagner, Materials Science and Engineering, R22 (1998) F.P Chiang, G.Uzer, Y. Ding Crack Tip Behavior in TiAl When Approaching Grain Boundary, Proceedings of16th European Conference of Fracture, Alexandroupolis, Greece, July 3-7, (2006)