This paper investigates the tear resistance of three cast and homogenized 6000-series alloys, namely AA6061, AA6063 and AA6110, all in temper T6, by means of Kahn tear tests. Of each alloy one commercial version and one tailor-made version were studied. The tailor-made alloys were designed to have approximately three times higher content of constituent particles by increasing the amount of Fe and Si in the chemical composition. The aim was to study in what way a higher constituent-particle content affects the tear resistance and properties of the alloys. The research showed that the unit initiation and propagation energies measured from the Kahn tear tests are markedly reduced when the constituent-particle content is increased, and that the tear resistance is reduced by a higher fraction than the failure strain of the smooth tensile tests. No major differences in the fracture mode and the fracture mechanisms between the alloys with normal and with high constituent-particle content were revealed by the use of computed tomography scanning or scanning electron microscopy imaging. It was concluded for the alloys studied that the increased content of constituent particles had a significant effect on the tear resistance, while the fracture mode and mechanisms remained the same.
simlab 3d crack in paper
Aluminium alloys contain particles of various sizes. As ductile fracture is governed by nucleation, growth and coalescence of voids, these particles and their interplay with the matrix become important for the fracture mechanisms. The toughness of an alloy defines its resistance to crack extension, and it follows that the ductile fracture mechanisms (and what affects these) are essential components for the toughness. For ideal toughness, a combination of high ductility and high strength is desired, but strength often comes at the cost of ductility, and normally higher yield strengths involve a net reduction in toughness (Hahn and Rosenfield 1975; Dumont et al. 2003; Petit et al. 2019).
The three particle groups that affect fracture in aluminium alloys are coarse constituent particles, dispersoids and precipitates. There is agreement that the coarse constituent particles have the strongest influence on toughness and ductility (Blind and Martin 1983; Liu et al. 2004). The constituent particles may crack at low strains, and thus initiate the fracture process (Broek 1973; Hahn and Rosenfield 1975). The ligaments between the voids nucleated at the constituent particles may then fail by void sheeting involving voids nucleated at dispersoids or precipitates (Broek 1973; Hahn and Rosenfield 1975; Sutton et al. 1997; Bron et al. 2004).
This experimental work is focused on the effect of the constituent particles on the mode I tearing behaviour of three cast and homogenized 6000-series aluminium alloys. By increasing the amounts of Fe and Si, we aim to achieve a second version of the alloys with higher constituent particle content. The goal is to get two versions of each alloy with different ductility, while retaining the strength level. By carefully controlling the chemical composition, we try to isolate the effect of increased constituent particle content on fracture and crack propagation in Kahn tear tests and relate these findings to previous observations on the effects on the tensile ductility. Detailed studies on the tensile ductility of these alloys were performed by Thomesen et al. (2020) and Tomstad et al. (2021), except for one of the alloys studied here. A summary of these results is presented in Sect. 2 of this paper, which is further organized as follows. The experimental program and data acquisition procedures are briefly presented in Sect. 3. Section 4 provides the results, which are discussed in Sect. 5. Concluding remarks are presented in Sect. 6.
Particle size distribution plots for the A and B versions of the three alloys: a 6061, b 6063 and c 6110. Some of the data have previously been presented in the papers by Thomesen et al. (2020) and Tomstad et al. (2021)
The initial notch radius was checked using edge tracing with the in-house Digital Image Correlation (DIC) software eCorr (Fagerholt 2017), and is presented for all the alloys in Table 3. It is noted that the observed deviations in notch radius may affect the initiation of the crack in the tear tests.
An element size of approximately 0.4 mm was used in the DIC analyses. Elements were eroded as they reached a critical element strain to avoid problems with the image correlation. The critical strain was chosen for the element erosion to best fit the crack propagation of the image series at hand. To obtain alternative displacement data to the crosshead displacement, a 6 mm virtual extensometer was placed just across the notch tip of the initial images, see Fig. 3. The virtual extensometer was used to measure the notch opening displacement (NOD).
Figure 7 shows the effective strain field at the end of the test for one specimen of each material. For all materials, slant strained bands are observed, which are oriented between the loading direction and the direction of crack propagation. There are clearly higher intensity strains around the crack for 6063 than for 6061 and 6110, and for 6110 the deformation outside of the cracked region is limited. In all three cases, the strains in the specimens of the B-alloy are considerably reduced with respect to the corresponding A-alloy.
Studying the CT scans did not reveal any differences in the overall fracture mode between the alloys, as all tests failed by slant fracture. Figure 8 shows a render of the CT scan of all the specimens from one test series. The initial tunnelling results in a flat triangular region (Bron et al. 2004; Morgeneyer et al. 2010) on the fracture surface, with shear lips towards the specimen surface. This region can be seen at the notch tip of the tests in Fig. 8. As the crack progresses, the shear lips grow and merge. In most cases, the shear lips are in the same direction, and merge to form a slant fracture. This is commonly referred to as a flat-to-slant transition. However, in some tests, the shear lips form in opposite directions, merging to a V-shaped fracture mode after the flat triangular region. In all these tests, one of the halves of the crack eventually flips to the other shear band, continuing as a slant fracture. In some cases, this happens already in the initial stage, before the shear lips merge.
The slant crack that forms has an angle of approximately \(45^\circ \). The crack tends to have a slight S-shape, as shown by Fig. 9a for 6063B, but is sometimes more planar, as illustrated for 6110B in Fig. 9b. This shape change appears to have a slight correlation with the ductility of the alloy, but this connection is somewhat diffuse and the shape even varies within each test. Along the crack propagation, the fracture surface shows some roughness, particularly in the centre. An example is shown in Fig. 9c, but this is also clearly visible in Fig. 8. Some partial flipping to opposite shear bands also occurs from the surface of some specimens, as shown in Fig. 9d, but the crack never flips completely, and eventually it flips back. In some of these cases, cracking is visible in both shear bands simultaneously. One case is illustrated in Fig. 9e, where it may be spotted on the left side of the specimen.
Rendered CT scan of the test series of 6061B. From left to right: stopped at 20%, 40%, 60%, 80% and 98% (two repetitions) drop from peak force. Due to limited scanning resolution and unloading before scanning, the render is not capable of capturing the crack tip perfectly
Despite limited resolution of the scans, an indication of tunnelling of the crack can be observed by the crack being visibly open in the middle, while appearing closed towards the surface. This is shown in Fig. 9f, where edges on the surface can also be seen, indicating shearing towards the specimen surfaces. Also Fig. 8 illustrates the occurrence of tunnelling clearly. Again, due to the limited resolution, a very small distance between the two crack faces and elastic unloading, the crack tip must not be interpreted as perfectly represented, but the figure shows that the crack is more opened in the centre than towards the specimen surface close to the crack tip. It is noted that it has been shown in the literature that the tunnelling effect in slant cracks is certainly limited compared to the tunnelling effect in flat cracks or in the initial flat tunnelling (James and Newman Jr. 2003; Bron et al. 2004).
A selection of examples on miscellaneous crack behaviour from the CT scans: a slight S-shape of the slant fracture, b planar slant fracture, c non-smooth centre of the crack, d partial flipping of the crack on the specimen surface, e cracking in both shear bands, f edges formed on the surface at the location of the last faintly visible crack tip in a specimen from the series with largest thickness reduction, and g location of barely visible crack tip in a specimen from the series with lowest thickness reduction
Out-of-plane deformation close to the crack edge. a DIC strain field, b and c CT scan slices, d CT scan render and e SEM image. Arrows indicate the crack propagation direction
The microscopic dimple structure of the fracture surfaces from the Kahn tear tests was investigated by means of SEM images. From the CT study, it was found that a flat triangular zone occurs at the crack tip (i.e., in the initial tunnelling region) with a surface normal to the loading axis and shear lips on the sides towards the edges of the specimen. The flat triangular zone typically transitions into a slant fracture which is the preferred fracture mode in all of the tests.
SEM images of the fracture surfaces in the Kahn tear tests: a small and large dimples in 6110A, b intergranular fracture in 6063A, c dispersoids in 6110A, and d towards the edges of the slant crack in 6061A 2ff7e9595c
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