High strain-rate experiments using high-speed photography

Sammanfattning: In many technical processes, material is deformed under conditions that contain high strain rates. Examples of such processes are collisions, impact, penetration, metal forming and crack propagation. Constitutive models including viscoplasticity have been proposed for these kinds of situations. Common for all models is that they contain material parameters, which are not well known. Experimental techniques like Taylor impact test and split Hopkinson pressure bar have been designed for the investigation of materials subjected to high strain rates. In these methods high strain rates are achieved by subjecting short specimens to rapid loading. For a simple interpretation of experimental results, a homogeneous state of stress and strain is desirable. Short specimens imply high strain rate but if the length and width are similar a nonhomogeneous state of stress and strain will result and the reliability in the evaluated quantities decreases. With the testing techniques mentioned above, it is difficult to use specimens, which are short but slender. In this thesis an experimental method to study material behaviour at high strain rates is developed. In contrast to the classical techniques, this method does not require a homogeneous state of stress and strain. A very small specimen (sub mm) and several larger specimens (up to 5 mm) have been used in the experiments. They are subjected to rapid tensional loading in devices similar to the Hopkinson bar arrangement. For the larger specimens a complete split Hopkinson bar is used, while one of the bars, namely the incident bar, has been omitted when the shorter specimen is tested. The deformation of the specimens is captured with a high-speed camera of image converter type. For the small specimen of sub mm size, the extension of its entire length is evaluated. The estimated strain rate reached well over 10^4 1/s. The larger specimens are evaluated using digital speckle photography (DSP) to give in-plane strain fields. Strains in the domain, 0.01-0.25, are evaluated and strain rates up to 3000 1/s are achieved.