Mechanical Properties and Microstructure Evolution of Aluminum Alloy Tubes with Normal Gradient Grain Under Biaxial Stress

Grain size gradient materials are a type of new structural material with the advantages of both coarse and fine grains. To study the effect of normal gradient grain on the mechanical properties and microstructure of aluminum alloy tube during hydroforming, the normal gradient grain distribution of the outer fine and inner coarse grains was obtained using spinning and annealing methods, and the biaxial stress was acquired using hydraulic bulging experiments. The thickness of the outer refined area was 105, 470, and 570 μm, respectively, where the grain size was refined to within 50 μm. Under biaxial stress, the tensile strength of the tube was 79, 89, and 106 MPa, the maximum expansion rates were 18%, 17%, and 10%, and the work-hardening indexes were 0.19, 0.20, and 0.17, respectively. The gradient grain tube with a refined thickness of 470 μm exhibited both strength and plasticity and was suitable for the hydroforming of aluminum alloy tubular parts. With increasing refined grain area, the density of the low angular grain boundary increased and make the chance of stitching dislocation increased in the process of intracranular deformation. However, the increase in the refined region weakened the deformation coordination, leading to a decrease in plasticity.

Numerical Investigation of the Composite Action of Axially Compressed Concrete-Filled Circular Aluminum Alloy Tubular Stub Columns

This paper studied the composite action of concrete-filled circular aluminum alloy tubular (CFCAT) stub columns under axial compression. A fine-meshed finite three-dimensional (3D) solid element model making use of a tri-axial plastic-damage constitutive model of concrete and elastoplastic constitutive model of aluminum alloy was established. A parametric study utilizing the verified finite element (FE) model was carried out and the analytical results were exploited to investigate the composite actions of concrete-filled circular aluminum alloy tubular stub columns subjected axial compression. Compared with the concrete-filled steel tube (CFCST) stub columns, the aluminum alloy tube exerted a weaker constraint effect on the infilled concrete due to its lower elastic modulus. Based on the FE analytical results and regression method, the composite action model of concrete-filled circular aluminum alloy tubular stub columns was proposed. By generalizing the stress nephogram of the concrete-filled circular aluminum alloy tubular stub column at the limit state, a design formula was proposed to estimate the ultimate bearing capacity the columns using the superposition method. The predicted results of the proposed formula show a good agreement with both the experimental and FE analytical results. The comparison between the proposed formula and current design methods indicates that the proposed formula is more accurate and convenient to use.

Experimental Studies on Cracking and Local Strain Behaviors of Rock-Like Materials with a Single Hole before and after Reinforcement under Biaxial Compression

In deep underground engineering, circular roadways are widely used; many rock engineering problems can usually be simplified as mechanical analysis of rock structures with holes. To reveal the influence of intrahole reinforcement on the mechanical properties of rock with a single hole, this paper takes the single-hole rock-like material specimens with different reinforcement conditions as the research object. The RYL600 rock shear rheometer was used to conduct biaxial compression tests and, combined with HD industrial cameras and high-precision strain gauges, to study the effects of different reinforcement thicknesses and different lateral pressure on the mechanical properties of single-hole rock-like materials during the total stress and strain process. The thickness of the reinforced aluminum alloy pipe in the whole test is divided into four types: 0, 1, 1.5, and 2 mm. Under different reinforcement conditions, it is divided into 4 series of 0, 0.5, 1, and 1.5 MPa according to the different lateral pressure. Research shows the following: (1) Under the same lateral pressure, as the reinforcement thickness of the aluminum alloy tube increases, the reinforcement effect of the aluminum alloy tube on the specimen increases, and the strength of the reinforced specimen is increased by 1.42%~33.04% compared with the strength of the unreinforced specimen; under the same reinforced thickness of the aluminum tube, the peak strength of the specimen increases with the increase of lateral pressure, and the peak strength of the specimen with lateral pressure is 3.34%~50.26% higher than that of the specimen without lateral pressure. (2) Increasing the lateral pressure can significantly reduce the primary tensile cracks of the specimen. As the reinforcement thickness increases, the primary tensile cracks and remote cracks of the specimen are significantly reduced, and the failure surface of the specimen gradually tends to the middle of the sample. (3) The failure modes of specimens with holes can be divided into five types: single bevel type I, single bevel type II, single bevel type III, bevel T type, and single part shear type. All of these five failure modes are shear cracks that develop into fracture surfaces, while remote cracks and primary tensile cracks do not develop into fracture surfaces.