Effects of notch radius and thickness on the tensile and fatigue strength of Al6061‑T6 monolithic and GFRP/CFRP coated aluminum

Abstract

Tensile and fatigue behaviors of monolithic aluminum alloys (6061-T6) and GFRP/CFRP coated aluminum are important in safety design for aircraft lightweight structures. It is well-known that notch radius and specimen thickness are important variables that affect stress states in the mechanical structures. Recently the issue of stress triaxiality defined as the ratio of the hydrostatic tensile stress to the von Mises equivalent stress has been more important than the stress concentration factor for predicting the plastic deformation and fracture mechanisms of ductile materials. The maximum triaxial tensile stress state would promote the nucleation, growth and coalescence of micro-voids in the mechanical structures. The maximum triaxial tensile stress would also vary with the notch circumstances. The position and size of the maximum triaxial tensile stress would dominate the fracture initiation phenomena. The notch effect would cause even a notch strengthening or notch insensitivity for ductile materials. However previous research results still remain uncertainty which should be clarified to apply to design and operation of actual mechanical structures. In this research, experimental and finite element analysis were performed for a systematic analysis of notch and thickness effects on tensile and fatigue behaviors for double edge U-type notched Al6061-T6 alloy and GFRP/CFRP+3plies/Al specimens. For the tensile test of the monolithic Al alloy, the triaxial tensile stress state in notch specimens caused notch strengthening. It is shown that the position of the maximum stress triaxiality varied with varying notch radius, and it was located from the notch tip towards the center with an increase in notch radius. The thickness effect on the tensile behavior was evaluated by comparing stress triaxiality behaviors at the notch tip, skin, center and the maximum stress triaxiality sites. 8mm-thick S-notched specimens showed a more notch strengthening than that of 2mm and 4mm-thick S-notched specimens. The largest stress triaxiality was also observed in 8mm-thick S-notched specimens. Fracture observation revealed that the shear fracture mode transformed into a tensile-shear mixed fracture mode with an increase in specimen thickness or decrease in notch radius. Comparing with notch strengthening effect under static tensile loading, the high cycle fatigue strength of 2mm-thick S-notched and 8mm-thick all kind of specimens were lower than that of 2mm-thick un-notched specimen. Subsequently, fatigue behaviors of fiber-metal laminates (FMLs) were analyzed with varying stress states in aluminum substrates. In high cycle fatigue range, fatigue strength of all kind of GFRP +3plies/Al decreased and S-notched CFRP +3plies/2mm-thick Al increased in comparison with that of monolithic aluminum. In low and high cycle fatigue ranges, fatigue life of S-notched GFRP +3plies/2mm-thick Al increased after experiencing initial high stress amplitude cycles, while fatigue life of S-notched CFRP +3plies/2mm-thick Al decreased with an increased in initial LCF cycles. In conclusion, this research confirmed that both notch radius and specimen thickness played important roles in tensile and fatigue behaviors: The maximum stress triaxiality increased with a decreased in notch radius and an increase in thickness, which caused notch strengthening effect in aluminum monolithic. The stress triaxiality behaviors at the skin and in the interior varied with varying notch radius and thickness, which lead to shear fracture mode transformed to a tensile-shear mixed mode. Furthermore, notch and thickness gave a different influence on fatigue behaviors in high cycle fatigue range in that the fatigue strength decreased in notched and thick specimens. The fatigue strength of S-notched CFRP+3plies/2mm-thick Al was much higher than that of S-notched Al monolithic. However, the FRP skin had no strengthening effect on fatigue strength of S-notched FRP+3plies/ 8mm-thick Al specimens. It is believed that these research results could be used as important information for safety design of aircraft lightweight structures.