Modeling mechanical response of AISI 4340 steel under various compressive strain rates

Date

2023-12

Author

Aygüzer, Cansu

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In defense industry, materials that could endure the extreme conditions of bullet impacts and explosions are requested. For that purpose, high strength steels and alloys as in quenched and tempered state are frequently preferred as protective structures. The subject steel in use is dynamically deformed due to high strain rates and elevated temperatures. The underlying mechanism in this kind of applications is quite complex and difficult to resolve analytically. Since protective structures are of great importance, the performance of these materials during the exposure has always been investigated and new remedies are tried. Therefore, models derived to express the mechanical response of materials has gained attraction. Modeling and simulating the flow behavior of materials facilitates the examination of appropriateness of improvements tried in materials by eliminating the requirement of experimentation. In this study, the plastic behavior of AISI 4340 armor steel is investigated through experiments conducted by Split Hopkinson Pressure Bar and physical-based modeling. Specimens manufactured from AISI 4340 steel are tempered in temperatures in between 450°C and 650°C. In the experimental part, strain rates around 500 s-1 and 1000 s-1 are tested. Here, having a generic approach to express the mechanical response of materials strengthened by quenching and tempering is aimed. For that purpose, yield strength is modeled by benefiting from the equations developed for each strengthening mechanism separately. For the flow stress at high strain rates, models of Kocks-Mecking and Bergström are combined and evolution of dislocation density under the effect of strain hardening and dynamic recovery is covered. The total yield strength and flow stress values are summed up with three different methods. The linear summation method and method proposed by Galindo and Nava are capable of giving a closer insight about the yield point of AISI 4340 steel while the RMS method fails to result in approximate values. For the flow stress, for samples tempered at relatively lower temperatures the linear summation and Galindo-Nava method yield a great agreement with the experimental data while the RMS method underestimates the stress values for all the samples. From both of the strain rates, it is concluded that this model is not capable of representing the dynamic recovery effect accurately although the slopes of the curves are similarly obtained by RMS method. Therefore, it is declared that this model is not able to cover the plasticity of quenched and tempered samples for all loading conditions.

Subject Keywords

AISI 4340 steel, Split Hopkinson pressure bar, Constitutive modeling, Kocks-Mecking model, High strain rate

URI

https://hdl.handle.net/11511/107732

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Graduate School of Natural and Applied Sciences, Thesis