NIMS fatigue data sheet on gigacycle fatigue properties of A6061-T6 (Al-1.0Mg-0.6Si) aluminium alloy at high stress ratios

ABSTRACT The new fatigue data sheet, No. 132, discloses gigacycle fatigue properties of the A6061-T6 aluminium alloys at high stress ratios. The fatigue tests were conducted mainly by the ultrasonic fatigue testing at 20 kHz, while conventional fatigue tests at 100 Hz were also conducted for comparison. The fatigue test results indicated that fatigue limits were obscure in the A6061-T6 alloys. Many specimens failed at over 107 cycles, developing internal fractures as well as surface fractures. On the other hand, fatigue failures at over 109 cycles were very rare, suggesting the presence of new fatigue limits in the gigacycle region. The differences in fatigue test results were negligible between the 20 kHz and 100 Hz tests, demonstrating that the 20 kHz tests were comparable to the conventional fatigue tests on this material. The fatigue strengths evaluated by the stress amplitude were decreased according to increase in the stress ratios, while the degradation of the fatigue strengths was not so serious. The fatigue strengths were higher than the modified Goodman lines, meaning that the stress ratio effects could be estimated by conventional ways. GRAPHICAL ABSTRACT


Introduction
NIMS fatigue data sheets comprise a huge database of fatigue properties of structural materials [1]. The total number of fatigue data sheets is 131 (Nos. 0-130) to date and is still increasing. This paper introduces a new fatigue data sheet designated as No. 132.
This fatigue data sheet discloses gigacycle fatigue properties of A6061-T6 aluminium alloy at high stress ratios. A series of previous fatigue data sheets had disclosed gigacycle fatigue properties of A5083P-O and A7075-T6 alloys [2][3][4][5][6][7]. The former data sheets were comprised of three types. The first one was low-and high-cycle fatigue properties tested under strain-and load-control conditions, respectively. The second one was gigacycle fatigue properties tested by rotating-bending and ultrasonic fatigue testings. The third one was gigacycle fatigue properties at high stress ratios. This was also the case of the A6061-T6 alloy, and the low-and high-cycle and the gigacycle versions had already been published [8,9]. This fatigue data sheet is thus the third one of the A6061-T6 series.
The fatigue tests were conducted mainly by the ultrasonic fatigue testing at 20 kHz, while conventional fatigue tests at 100 Hz were also conducted for comparison. The previous fatigue data sheet had demonstrated that the results were comparable between the ultrasonic and conventional fatigue testings [9]. Details of the new fatigue data sheet are as follows. Tables 1 and 2 show processing details and chemical compositions of the tested alloys. The tested alloys were a hot-rolled plate and extruded round bars sampled in 2017. Table 3 shows the mechanical properties. The tensile strengths of the tested alloys were around 300 MPa, which were close to those of A5083P-O. The mechanical properties of Heat A were disclosed both in longitudinal and in transverse directions, while the anisotropy was very small. Figures 1 and 2 show the microstructures of the tested alloys. Figure 1 is polarized light images which reveal grain sizes and shapes. Figure 2 is forward light images which reveal distributions of precipitates. The grain sizes of Heat B are finer than those of others. The precipitates are not so dense and the differences between heats are not remarkable. Table 4 shows the fatigue test conditions. Two types of fatigue testing machines were used. One was an electromagnetic resonance type at 100 Hz. The other was the ultrasonic type at 20 kHz. The cutoff cycle numbers were 10 8 at 100 Hz and 10 10 at 20 kHz. The stress ratios were R = −1, 0 and 0.3. In addition to these stress ratio conditions, σ max = σ y tests were applied. In the σ max = σ y tests, maximum stresses were fixed at 0.2% proof stresses instead of fixing the stress ratio. The σ max = σ y test condition corresponds to the highest stress ratio condition, assuming that the materials are used in an elastic region.

Figures 3 and 4
show the fatigue test results. Many specimens were fractured at over 10 7 cycles, so the fatigue limits were obscure. Internal fractures occurred in several specimens. The internal fractures were more frequent under high stress ratio conditions. Slight differences were observed in the fatigue strength between the heats. Heats B and C,    In the 20 kHz tests, specimens were air cooled using a heat exchanger and 5.5 kW compressor (60ℓ/min), therefore, the temperature rises of the specimen were less than 10℃.
b Under the condition fixing the maximum stress at the 0.2% proof stress.
c Surface finishing was performed by longitudinal polishing with 600 grade silicon carbide paper.    which were extruded round bars, revealed slightly higher fatigue strength than the hot-rolled plate of Heat A. Figure 5 shows S-N diagrams to compare the results between frequencies and stress ratios. The differences between the 100 Hz and 20 kHz tests were negligible, meaning that the 20 kHz tests were comparable to the 100 Hz tests. The fatigue strengths evaluated by the stress amplitudes were lower under high stress ratio conditions, while the degradations were not so remarkable. The fatigue failures at over 10 9 cycles were very rare. This suggested the presence of new fatigue limits [10] in the gigacycle region. Table 5 shows estimated mean fatigue strengths at 10 7 , 10 8 and 10 10 cycles. The mean fatigue strengths are average values between the maximum stress amplitude at which no specimen is fractured and that just above it. The fatigue strengths decrease according to the increase in the cycle numbers. The decreases are larger between 10 7 and 10 8 cycles than between 10 8 and 10 10 cycles. Figure 6 shows comparison of fatigue strength at 10 7 and 10 10 cycles among A6061-T6 and other materials. In general, there are linear relationships between the fatigue strength σ W and the tensile strength σ B . The relationships under R = > −1 are σ W = 0.53σ B for the quenched and tempered (QT) steels and σ W = 0.39σ B for the austenitic stainless steels and the normalized (N) steels. The fatigue strength at 10 10 cycles of A6061-T6 is lower than σ W = 0.39σ B as in the case of A7075-T6. On the other hand, the fatigue strength at 10 7 cycles is almost equal to σ W = 0.39σ B . These indicate that the conventional high-cycle fatigue properties of A6061-T6 are close to those of the austenitic stainless steels and the normalized steels, while the fatigue strength is reduced in the gigacycle region. This is also the case of R = 0. Figure 7 shows endurance limit diagrams. The endurance limit diagrams compare the fatigue strengths of A6061-T6 with modified Goodman lines. The fatigue strengths of A6061-T6 are higher than those of the modified Goodman lines. Figure 8, Figure 9, Figure 10, Figure 11 show typical fracture surfaces. Both the surface and internal fractures are observed, while fish-eye patterns are not clear even in the case of the internal fractures. In several internal-fractured specimens, precipitates are observed at around the fracture origins. The precipitates are Al-Mg-Si or Al-Fe-Si types. It is, however, unknown whether these precipitates affect the internal crack initiation or not. The differences of fracture surfaces are negligible between frequencies and between stress ratios.

Discussion
The fatigue test results at 20 kHz were comparable to those at 100 Hz. The 20-kHz fatigue testing is thus applicable to this material. This is the first point to be noted in these fatigue test results.
The second point is that the fatigue limits are obscure in A6061-T6. The former fatigue data sheets [3,4,6,7] demonstrated that the fatigue limits were clear in A5083P-O but not in A7075-T6. The tensile strengths of A6061-T6 are close to those of A5083P-O, while the heat treatment, T6, is close to that of A7075-T6. This means that the heat treatment had more effects on the fatigue limits than the tensile strength. In other words, the fatigue limits are obscure in the "T6" materials, while they are clear in the 'O' materials.
The third point is that the gigacycle fatigue strengths of A6061-T6 are higher than the modified Goodman lines. This means that the stress ratio effects are not so serious on A6061-T6, unlike Ti-6Al-4 V alloys. In the case of the Ti-6Al-4 V alloys, the stress ratio effects were so large that the gigacycle fatigue strengths were lower than the modified Goodman lines at around R = 0 [11][12][13]. These serious stress ratio effects are not observed in A6061-T6. This is also the case of A7075-T6 [7].
In the case of A5083P-O, the fatigue strengths were lower than the modified Goodman lines, while in those cases, the maximal stress exceeded the 0.2% proof stresses. Namely, the fatigue strengths of A5083P-O were close to the yield limits in the   endurance limit diagram [4]. In conclusion, the stress ratio effects on A6061-T6 can be estimated by conventional ways, such as modified Goodman lines, as on A7075-T6. In other words, the stress ratio effects on A6061-T6 are normal, while those on Ti-6Al-4 V alloys are abnormal.

Summary
The NIMS fatigue data sheet of No. 132 discloses gigacycle fatigue test results on the A6061-T6 aluminium alloys at high stress ratios. The fatigue tests were conducted both by ultrasonic fatigue testing at 20 kHz and by conventional fatigue testing at 100 Hz. Many specimens were fractured at over 10 7 cycles, indicating that the fatigue limits were obscure in the A6061-T6 alloys. The differences between the 100 Hz and 20 kHz tests were negligible, so the 20 kHz tests were comparable to the 100 Hz tests. The fatigue strengths evaluated by the stress amplitudes were decreased according to increase in the stress ratios, while the stress ratio effects could be estimated by conventional ways such as modified Goodman lines.