Investigation On The Stability Of Aluminium Alloy 6082-T6 Members In Axial Compression

Aluminum alloy has a number of advantages over conventional carbon steel, such as high strength-to-weight ratio, satisfactory corrosion resistance, aesthetics and so on. Aluminum alloy is getting more applications in building structures, while various studies on the performance of aluminum alloy structures and structural members are booming. Among 6000 series alloys, 6082 alloy is a relatively new aluminum alloy. Compared with the widely used 6061-T6 alloy, 6082 alloy provides a better combination of properties such as strength, satisfactory corrosion resistance, and good weld-ability. 6082-T6 is popular in Europe and experiencing much in use in America. In recent years, it has been being gradually introduced to Chinese market, especially in transportation engineering, and has many prospective applications in building structures. Axial compression members are the most basic structural components. To provide a reference for design and application of aluminum alloy structures, this dissertation conducts a systematical study with experimental tests, numerical simulations, and theoretical investigation of 6082-T6 aluminum alloy axial compression members. The works of this dissertation are summarized as follows.The mechanical parameters of domestic aluminium alloy 6082-T6 are obtained from tensile tests of 117 coupons, including elastic modulus E, 0.2% proof stress f0.2, tensile strength fu, percentage total extension at fracture At, and Poisson’s ratio. Comparison of experimental stress-strain curves of 6082-T6 aluminium alloy and the Ramberg-Osood constitutive model demonstrates that the Ramberg-Osood model can accurately represent the stress-strain curve of domestic aluminium alloy 6082-T6.Several methods are examined to calculate hardening exponent n of the Ramberg-Osgood equation. Fast simulated annealing(FSA) algorithm is used for parameter optimization and to get the best n value, which has the best overall fit. The results indicate that the n values obtained using FSA have a smaller scatter than those using the traditional two-point method. When the significance level is lower than 0.05, n does not refuse to obey a normal distribution.The initial geometric imperfection and loading eccentricity significantly affects the stability of compression members. This study measures the initial geometric imperfection of 45 domestic 6082-T6 aluminium alloy extrudes with circular and angle sections, and then obtains the maximum initial imperfection and the imperfection at midspan. A formula is derived to calculate the initial eccentricities at midspan of the 73 specimens. The results indicate that: 1) the initial eccentricities at midspan of approximately 10% of the specimens are greater than 1.8L/1000(L is the distance between the rotational centers of the upper and lower bearings), and 2) referring to the allowable value of initial crookedness for ordinary level precision extrudes according to "Aluminium Building Material Part 1: Basic Material", we recommend to appropriately increase the representative initial crookedness at midspan, such as 1.8L/1000.The experiments cover a total of 73 specimens, including 17 with an H-section, 15 with rectangular-section, 15 with circular-section, and 30 with angle-section. The norminalized slenderness ratio varies from 0.45 to 3.54. Knife-edged bearings are used to simulate hinges. Experimental results indicate that: 1) all specimens with H-, rectangular-, and circularsections failed by flexural buckling about the minor axis, 2) the angle-section specimens with equal legs failed by torsional-flexural buckling when the slenderness ratio is small, whereas failed by flexural buckling otherwise, and 3) all angle-section specimens with unequal legs failed by torsional-flexural buckling. Comparison of the test results from this study and the aluminium column curve from Chinese code indicates that the experimental stability coefficients have a relatively even distribution on both sides of the column curve from the current Chinese code.An accurate finite element(FE) model is created to simulate the axial compression members using the finite element software ABAQUS. Comparison of the numerical and experimental results validates the reliability of the FE model to predict the mechanical performance of aluminium members under axial compression. The validated FE model is used to analyze the effects of some factors, such as initial crookedness, initial eccentricities, material properties, and section type and size, on the stability coefficient of aluminium member under axial compression. The results indicate: 1) the initial crookedness, initial eccentricities, material properties, and section type and size all affect the stability coefficient, 2) the effect of the Ramberg-Osgood exponent n on the stability coefficient is negligible when n is larger than 20, and 3) the effect of width-to-thickness and height-to-width ratios of all four section types of specimens on the stability coefficient is negligible.The experimental stability coefficients of the specimens under axial compression are compared with those from major domestic and foreign aluminium design specifications. The FE model is employed to obtain the stability coefficients of a total of 300 aluminium alloy columns with four cross-section types. The Perry-Roberson equation is used to fit the numerical results to get a fitting column curve. Comparison of the fitting column curve obtained by this study with the column curves from various specifications and the test results indicates that the fitting curve is safer than the column curve from the current Chinese code.