Propagation Properties of Backward Lamb Waves in Plate Investigated by Dynamic Photoelastic Technique

The dynamic photoelastic technique is employed to visualize and quantify the propagation properties of backward Lamb waves in a plate. Higher energy leakage of second-order symmetric backward wave mode S2b in contrast to third-order anti-symmetric backward mode A3b is shown by the dispersion curve of a plate immersed in water, and then verified by experiments. To avoid the considerable high leakage, the plate is placed in air, both group and phase velocities of modes S2b and A3b in the glass plate are experimentally measured. In comparison with the theoretical values, less than 5% errors are found in experiments.

Influence of Negative Bias Voltage on the Mechanical and Tribological Properties of MoS_2/Zr Composite Films

MoS2/Zr composite films were deposited on the cemented carbide YT14 （WC＋14%TiC＋6%Co） by medium-frequency magnetron sputtered and coupled with multi-arc ion plated techniques.The influence of negative bias voltage on the composite film properties,including adhesion strength,micro-hardness,thickness and tribological properties were investigated.The results showed that proper negative bias voltage could significantly improve the mechanical and tribological properties of composite films.The effects of negative bias voltage on film properties were also put forward.The optimal negative bias voltage was -200 V under this experiment conditions.The obtained composite films were dense,the adhesion strength was about 60 N,the thickness was about 2.4 μm,and the micro-hardness was about 9.0 GPa.The friction coefficient and wear rate was 0.12 and 2.1×10-7 cm3/N·m respectively after 60 m sliding operation against hardened steel under a load of 20 N and a sliding speed of 200 rev·min-1.

Plate reinforced square hollow section X-joints subjected to in-plane moment

The static test of 13 square hollow section(SHS) X-joints with different β and different types of plate reinforcement under in-plane moment in brace was carried out. Experimental test schemes, failure modes of specimens, moment-vertical displacement curves, moment-deformation of the chord, and strain strength distribution curves were presented. The effect of β and plate reinforcement types on in-plane flexural property of SHS X-joints was studied. Results show that punching shear of chord face disappears, brace material fracture appears and concave and convex deformation of chord decrease when either collar plates or doubler plates were welded on chord face. Moment-vertical displacement curves of all specimens have obvious elastic, elastic-plastic and plastic stages. As β increases, the in-plane flexural ultimate capacity and initial stiffness of joints of the same plate reinforcement type increase, but ductility of joints decreases. With the same β, the in-plane flexural initial stiffness and ultimate capacity of doubler plate reinforced joints, collar plate reinforced joints, and unreinforced joints decrease progressively. Thickness of reinforcement plate has no obvious effect on in-plane flexural initial stiffness and ultimate capacity of joints. As thickness of reinforcement plate increases, the ductility of reinforced X-joints decreases. The concave and convex deformation of every specimen has good symmetry;as β increases, the yield and ultimate deformation of chord decrease.

Inhomogeneity of microstructure and mechanical properties in the nugget of friction stir welded thick 7075 aluminum alloy joints

In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively.