Analysis of bulging process of aluminum alloy by overlapping sheet metal and its formability

The influences of strength coefficient K, work hardening exponent n and thickness t of the overlapping sheet on bulging process are analyzed based on hardening material model. Also, bulging experiments are carried out by taking the aluminum alloy LF21 as formed sheet metal, and selecting overlapping sheet with different thicknesses and material properties, by which accuracy of the above analysis result is verified in the aspects of geometric shape, thickness distribution and limit bulging height. The results show that higher strength coefficient K, larger work hardening exponent n and proper thickness of the overlapping sheet are helpful to improve the formability and forming uniformity of formed sheet metal.

Flow property of AA2B06 sheet using overlapping elliptical bulge test

A novel experimental approach was presented, namely the overlapping elliptical bulge test, which can load and research thickness normal stress. Theoretical analysis model of the overlapping elliptical bulging was described, the equivalent stress?strain curves of target sheets with different ellipticity ratios were determined experimentally, and influences of the material performance and thickness of overlapping sheets on the flow property of the target sheet were also researched. The results show that, in the overlapping hydraulic bulge test, the equivalent stress?strain curve can be determined up to larger strains before necking than in the no overlapping hydraulic bulge test. And as the die ellipticity ratio decreases, the flow stress curves tend to move away from the curve obtained by circular (b/a=1) bulging test. Meanwhile, the flow property of the target sheet can be improved by choosing higher strength coefficient K, larger work hardening exponent n and proper thickness of the overlapping sheet.

A graphene rheostat for highly durable and stretchable strain sensor

Strain sensors for human health monitoring are of paramount importance in wearable medical diagnostics and personal health monitoring.Despite extensive studies,strain sensors with both high durability and stretchability are still desired,particularly with the stability for different environmental conditions.Here,we report a series of strain sensors possessing the graphene network with a high density of intermittent physical interconnections,which produces the relative resistance change by varying the overlap area between the neighboring graphene sheets under stretching and releasing,analogous to the slide rheostat working in electronics.Our in-situ transmission electron microscope observation reveals the full recoverability of the structure from large deformation upon unloading for ensuring the exceptional cycle stability of our material on monitoring full-range body movements.The stable response is also demonstrated over wide temperature range and frequency range,because the peculiar dynamic structure can be maintained through the self-adjustment to the thermal expansion of the bulk material.Based on the working mechanism of graphene“slide rheostat,”the sensing properties of the strain sensor are tailored by tuning the graphene network structure with different mass densities using different concentrations of graphene oxide dispersion,while the stretchability and sensitivity can be separately optimized for different application requirements.