Experimental design of three-point bending tests on circular magnesium alloy tubes with five shear–span ratios

Against the backdrop of research and innovation integration, magnesium alloys, with advantages such as light weight, high specific strength, excellent damping performance, and recyclability, have good applicability in lightweight building structures. However, research on their mechanical properties remains insufficient, which limits their engineering applications.

To fill this gap and promote these applications, five circular magnesium alloy tube specimens with different shear–span ratios were designed and fabricated and subjected to three-point bending tests. The failure modes, load–displacement responses, and strain distribution patterns of each specimen were systematically investigated. Combined with an analysis of the measured peak bearing capacity and theoretically calculated values, the control effect of the shear–span ratio on the mechanical behavior of magnesium alloy tubes was revealed.

The results show that 1) the transformation process of the failure mode was clarified. All specimens exhibited linear elastic deformation characteristics during the initial loading stage without obvious yield platforms. 2) The influence of the shear–span ratio was quantified. The shear–span ratio considerably affects the failure mode. When the shear–span ratio is less than 5.17, the specimens mainly undergo local buckling failure; as the shear–span ratio increases to 6.21–8.28, the failure mode transforms into typical mid-span overall bending failure. Meanwhile, as the shear–span ratio increases, the peak bearing capacity of the specimens decreases nonlinearly by up to 57.1%, while the mid-span displacement considerably increases by up to 69.9%. 3) The strain distribution and plastic deformation laws were revealed. During the bending process, the strains in the top compression zone and bottom tension zone at the mid-span are strictly symmetrical (with similar values but opposite signs), and the strain in the former zone is larger, verifying the mechanical attribute of a greater tendency for local buckling; moreover, it was clarified that when the load exceeds 0.6 times the peak load, the strain growth rate sharply increases, which is a key feature of entering the plastic deformation stage. 4) The applicability of the calculation method was verified. A calculation method was proposed for the ultimate bearing capacity of circular magnesium alloy tubes based on existing metal structure design codes. For specimens with a shear–span ratio greater than 6.21, the measured values closely match the calculated values, with all errors less than 7.0%, and the error of specimen MG3-1800 is only 0.2%, providing a reliable basis for engineering design.

1) The shear–span ratio is the core parameter for regulating the failure mode of circular magnesium alloy tubes under bending. Different shear–span ratios correspond to various failure modes, providing a basis for component selection and design. 2) As the shear–span ratio increases, the peak load shows a nonlinear attenuation trend, the mid-span displacement increases, and the ductility improves. For specimens with a low shear–span ratio, the load–displacement curve drops sharply after reaching the peak load, while for those with a high shear–span ratio, the curve declines slowly after reaching the peak load. This finding provides a reference for ductility control in disaster-resistant design. 3) The bending specimens have a symmetrical strain distribution, and local buckling tends to occur in the top compression zone. When the load exceeds 0.6 times the peak load, plastic deformation considerably increases. The calculation method based on existing standards applies to specimens with a high shear–span ratio and can be used in engineering design. 4) This application enables students to integrate the mechanical theories they have learned, understand the relationship between experimental design and loading mechanisms, master the evolution law of the force–displacement curve during the bending process, identify the failure characteristics and bearing capacity variation trends under different shear–span ratios, and thereby establish the correlation mechanism between component response and material constitutive behavior, providing a reliable basis for subsequent numerical simulation and theoretical analysis and effectively enhancing students’ practical ability and innovative thinking in the application of new metal structural materials.