1 Development and Significance of Hot Forming Technology of FMLs

1.1 Introduction of FMLs

Fiber metal laminate (FML)^[1] is a kind of interlaminar hybrid composite, also described as super hybrid laminate, which is made of metal sheet and fiber composite in alternate layers and solidified under certain temperature and pressure.

FLMs combine the characteristics of traditional fiber composites and metallic materials. It has many advantages including high strength, excellent fatigue resistance, impact resistance, corrosion resistance and fire resistance.

For example, the Al-Li alloy not only has the characteristics of high specific strength , such as Al-Li alloy in T8 state^[2] ,whose tensile strength and yield strength of can reach 551.12 MPA and 509.10 MPA respectively , it also has the advantage of low density. Replacing traditional aluminum alloy with Al-Li alloy can reduce the weight of existing aircraft by 10% , what’s are more, the new type of aircraft can be reduced by 15% to 20% respectively. However, there are still many drawbacks, the failure mechanism of FMLs materials is complex and the forming is difficult. There are also many scientific problems which need to be resolved in terms of interface action, damage theory and mechanical properties.

1.2 Research and development of FLMs

1.2.1 Background of FMLs

Since the 1950s, transportation industry in China has seen a rapid development, especially for the transportation system industries, such as aviation, aerospace and rail, as well as special vehicles. In order to meet the design requirements of these transportation systems, FLMs emerged as the need of the moment. It is , gradually, edging out the traditional single metal or composite metal and is being used in some vital parts to meet the intended requirements of strength and stiffness.

Until now, four generations of fiber metal laminates have been successfully developed.

The first generation of these FMLs was aramid fiber-aluminum alloy laminates, which combined the advantages of aluminum alloy materials and fiber composites, and had the advantages of low density, high strength and easy processing, etc. Moreover, because of the "bridging" effect of fiber over the crack of the laminate, the laminate had high damage tolerance and fatigue strength. The first-generation fiber-metal laminate was first applied to the fuselage and wing of C-17 military transport aircraft in the United States.