Resume：

AA7475 Aluminum alloy is widely used as structural material in aerospace and automotive industries, such as aircraft wing beams, fuselage skins, fuselage frames, and automotive components. However, these components are subjected to complex loading conditions during operation, requiring reliable experimental data on biaxial stress-strain and yield behavior to ensure their safety and optimized design.

The September 2024 issue of the Journal of Alloys and Compounds published a study by the Department of Mechanical Engineering at the Indian Institute of Technology (IIT) on the yield trajectory and texture evolution of AA7475-T761 aluminum alloy under plane biaxial loading. The research aimed to investigate how the initial crystal texture influences the biaxial deformation response of AA7475-T761 aluminum alloy under varying load ratios.

Based on the cross-shaped specimen, the work carried out the test under five loading ratios of 2:1, 4:3, 1:1, 3:4 and 1:2, and compared with the deformation under uniaxial load. The yield behavior of AA7475-T761 aluminum alloy under large strain under different loading conditions was summarized. The microstructure and texture evolution of the fracture surface were analyzed by electron backscatter diffraction (EBSD) and X-ray diffraction.

Experiment design：

The work utilized AA 7475 aluminum alloy and T-761 tempered condition plates with a thickness of 2.5 mm. Three slits, each 2 mm wide and 40 mm long, were designed on each arm, with the gauge area accounting for 40% of the total plate thickness to achieve uniform and substantial strain.

Figure 1 Size of the cross-shaped specimen used

The dual-axis testing machine employed in this study is the MTS 100 kN servo hydraulic planar dual-axis fatigue tester, with all tests conducted under load control at specified loading ratios. Among the five loading ratios, the lower loading rate was consistently maintained at 1 kN/min. Displacement and strain measurements were performed using two 1280 x 1024 pixel CCD cameras at 2000 frames per second, with DIC image acquisition conducted at 2 frames per second. The data was analyzed using VIC-3D software.

Figure 2 Biaxial Testing Machine and Test Platform

Test result ：

Due to material plane anisotropy, AA7475 exhibits distinct responses in the roll direction (RD) and transverse direction (TD). The total strain in TD is lower than in RD, indicating that TD samples demonstrate reduced ductility compared to RD samples—a finding consistent with uniaxial tensile test results. The maximum strain observed under biaxial loading reaches 7.8% in the RD direction, closely approximating uniaxial RD strain. As shown in Figure 3, biaxial yield strength exceeds uniaxial yield strength in both RD and TD directions.

Figure 3: Biaxial stress-strain curves of AA7475-T761 under different load ratios: (a) RD and (b) TD

Under different loading ratios, failure consistently occurs at the center of the cross-shaped specimen. When the load ratio is 1:2 and 3:4, failure happens in the direction opposite to the higher load, indicating that the TD (Tensile Direction) experiences greater force. Conversely, at 2:1 and 4:3 ratios, failure occurs in the opposite direction, suggesting higher loads are applied to the RD (Compression Direction). At the 1:1 ratio, failure in the RD direction results from the TD load's higher strength and lower ductility.

Figure 4 Biaxial specimens after biaxial tensile tests under different load ratios: (a) 1:2, (b) 3:4, (c) 1:1, (d) 4:3, and (e) 2:1

Microstructure and Textural Analysis：

The test results show that AA7475-T761 exhibits plane anisotropy, and the elongation varies with the loading direction. Therefore, the deformation behavior of the material under multi-axial loading is studied through comprehensive fracture morphology, microstructure and crystal texture analysis.

In the microstructure, the fracture mechanism is predominantly transgranular, exhibiting alternating pit regions perpendicular to the principal stress axis and shear zones parallel to it. Transcrystalline cracks propagate through individual metal particles. This fracture mode typically arises from the combined effects of high stress and material defects (e.g., voids within grains or inclusions). Larger voids result from fracture within the particles themselves, while smaller voids are caused by delamination at the particle/matrix interface.

Figure 5 Fracture images of uniaxial tensile tests performed under (a) RD, (b) TD, and (c) DD conditions; and biaxial tests conducted at (d) 1:1, (e) 2:1, (f) 4:3, (g) 1:2, and (h) 3:4 ratios

The samples deformed under biaxial tensile loading showed no significant changes in grain shape and size, as illustrated in Figure 6. This may be attributed to the relatively low strain achieved in each direction. However, the development of orientation differences was observed due to higher effective strain. The biaxial specimen in Figure 6(c) exhibited a pronounced non-indexed point, resulting from the increased effective strain during deformation (Figure 7). As deformation progressed, the dislocation density increased while the index decreased.

Figure 6: Histogram of random grain distribution (a) at receiving stage (b) uniaxial (c) isotropic biaxial (d) 4:3 (e) 2:1 (f) 3:4 (g) 1:2 and (h) under load conditions, showing average grain diameter.

Figure 7 Average grain orientation difference under different load ratios at effective strain

Full text summary:

1. The double axis yield and ultimate strength are higher than the single axis yield and ultimate strength in both RD and TD directions.

2. The maximum plastic strain achieved in the standard gauge area was 7.8%, and a Yield locus was constructed for AA7475-T761 up to a higher plastic strain of 0.07. The yield locus was compared with the Hill's 48, Hill's 93, and Yld2000-2d yield standards. The Yld2000–2d yield standard provided an accurate prediction yield locus with minimal deviation (~1% error).

3. Microstructural analysis of AA7475-T761 single-axis tensile test specimens indicates that void nucleation and growth constitute the primary failure mechanisms, with voids coalescing due to plastic deformation. The fracture surfaces exhibit transgranular ductile fracture with pit clusters, whereas biaxial tensile tests demonstrate a combination of ductile and brittle fracture, including cleavage planes and pits.

4. The texture analysis shows that the uniaxial deformation is favorable to the cube and inverse copper texture, while the biaxial deformation is obviously inverse Gaussian texture and its splitting.

Article source：

Amir Hamza Siddiqui, Priya Tiwari, Jeet P. Patil, Asim Tewari, Sushil Mishra,

Yield locus and texture evolution of AA7475-T761 aluminum alloy under planar biaxial loading: An experimental and analytical study,

Journal of Alloys and Compounds,

Volume 1000,

2024,

175115,