The dynamic response of ni-6Al-4V alloy at high strain rate is investigated with tensile split Hopkinson bar test using plate type of specimens. High strain rate tensile tests are then performed with the above said material in order to construct their appropriate constitutive models for use in aircraft structures under dynamic conditions.

Aircraft structures are generally constructed from sheet metals of deep-drawing quality. The dynamic behavior of the materials is different from the static one because of inertia effect and the propagation of stress waves. An adequate experimental technique has to be developed for the corresponding strain rate level. A high strain rate testing apparatus was devised by Kolsky (1963) in 1949, which is known as split Hopkinson pressure bar (Follansbee, 1978). The stress-strain curves for the high strain rate ranging from 1000 to 10,000/s can be acquired from the stress waves propagating through the incident and the transmission bars in the apparatus. The split Hopkinson pressure bar apparatus can be modified for high strain rate tensile tests. Even though there are some difficulties in the design of grips, these grips are not considered for simplicity. For anvil effect, successful high strain rate tensile tests need control of state variables such that the stress, strain and strain rate in the specimen must be homogeneous (Johnson et al., 1986). Hence, the geometry of a specimen used in high strain rate tensile test is important for acquiring uniform deformation.

Nicholas (1981) used threaded bar type tensile specimens to obtain high strain rate stress-strain curves for about 15 to 20 different materials. Lindholm and Yeakley (1968) performed high strain rate tensile tests with hat type specimens. The above said tests were easy to perform but the design of hat specimens was complicated and expensive. In these experiments, wave distortion occurs at the clearance of the threaded region of the specimen. Staab and Gilat (1991) investigated the effect of the bar type specimen geometry in direct tension split Hopkinson bar tests. When the length to diameter ratio of the specimen was greater than 1.50, the experimental results showed that the dynamic tensile strength was consistent. Zhao and Gary (1996) performed compression tests for Ti-6Al-4V alloy plates for the aircraft structure using compression split Hopkinson pressure bar apparatus. The above-said methods give results for different material models which are used in numerical analysis of crashes. The material behavior cannot be described in a general way, hence, it is necessary to describe the various types of constitutive relations to describe the dynamic behavior of materials. Johnson and Cook (1983) proposed a constitutive model and found five material constants (obtained from Hopkinson pressure bar apparatus) in the constitutive relation for materials subjected to large strains, high strain rates and high temperatures.

In this paper, the high strain rate tensile tests have been carried out with a split Hopkinson pressure bar apparatus, designed specifically for sheet metals. Tensile tests are performed for several sheet metals of deep-drawing quality. Experimental results from both the quasi-static and dynamic tests are interpolated to construct a constitutive relation, which can be applied to the crash analysis of aircraft structures made up of sheet metals.

Mechanical Engineering Journal, Aircraft Structures, Mechanical Behavior, Sheet Metals, Numerical Analysis, Tension Split Hopkinson Bar, Elastic Wave, Young’s Modulus, Homogeneous Temperature, Solid Mechanics, Finite Element.