Project Objective

The major objective for this project is to experimentally determine the high-strain-rate behavior of metallic materials of interest to NASA MSFC for structural design and analysis purposes, particularly the stress versus strain and fracture behavior.

Project Description

The NASA materials of interest, consisting of AL7075, Inconel 718, and A286 steel, are experimentally tested at the Experimental Mechanics and Multiphysics Modeling (E3M) laboratory at the University of Alabama in Huntsville (UAH). This consists of high-rate tension tests using the lab’s specialized split-Hopkinson bar (SHB) high-strain-rate testing apparatus, as well as low-rate tension tests using a hydraulic load frame. Both low- and high-rate tests are performed using the same specimen geometry and NASA-supplied material feedstock to eliminate any potential discrepancies between geometry difference or material lot-to-lot variation between this study and prior past studies. The base material plate stock is supplied by NASA, and the machining of the test specimens from the base stock is a joint effort between UAH and NASA MSFC. The tests are performed primarily by a graduate research assistant (GRA) under the supervision of the UAH PI, Dr. Nathan Spulak. The experimental data will be used to determine the Johnson-Cook material model parameters associated with this high-strain-rate behavior. These material models can then be used by NASA for design and analysis of structural components made by each of the various different material types.

Project Results and Conclusions

Fabrication of the test specimens for all materials has been completed, and testing is ongoing. The low-rate testing and majority of the high-rate testing has been completed for the AL7075 aluminum alloy. Low-rate testing is currently ongoing for the Inconel 718 and A268 steel alloys, and the material strength data gained from the low-rate test data will be used to design the specific test parameters (i.e., striker bar velocity) needed to generate the desired strain rates for the high-rate SHB tests for each material. Testing is performed at strain rates ranging from quasi-static rates (0.001 s^-1) to dynamic rates on the order of 10³ s^-1, and in both the rolling and transverse directions of the plate stock material. Testing at similar strain rates and for both rolling and transverse directions will be performed for the Inconel 718 and A286 steel alloys.

Preliminary stress versus strain data from the AL7075 material indicate that, after the initial transient period and inertial effects/accelerations, the flow stress of Al7075 is relatively insensitive to the loading rate. However the fracture strain shows a marked decrease at elevated strain rates compared to loading at low rates. Once testing has been completed for all three materials, the changes to the flow stress and the fracture strain at different strain rates will be used to construct a Johnson-Cook strain rate sensitive material model that can be used by NASA for structural design purposes.

One unanticipated problem was that initially testing was planned for an AL2219 aluminum alloy. However during the initial low-rate testing, it was discovered that the AL2219 material had an unknown and incorrect temper, a discrepancy from the supplier documentation for this material of interest. It was concluded that the tested batch of AL2219 has clearly been subjected to a different (and unknown) heat treatment than what is documented. This unknown processing history and lack of traceability, coupled with the fact that the actual tested material properties make it undesirable for NASA’s use as a structural material, mean that the experimental data obtained is not of use for actual design and analysis purposes by NASA. Therefore, the A286 steel and Inconel 718 materials were identified as substitute materials of interest and added to the project. Because of this unanticipated problem, a one-year no-cost extension was requested and granted for this project, which now has a new end date of 02/20/2026.

The material property data generated by this project will be used to create more accurate, reliable, and safe designs of aerospace structures by NASA. The efforts of this proposed project are particularly in-line with TX 13.2.1 (Mechanical/Structural Integrity Testing), which calls for characterizing material properties to ensure reliable and safe structural components, as well as verifying the component performance under dynamic conditions. This project is also in-line with TX 12.2.4 (Tests, Tools, and Methods), which calls for a better understanding of vehicle response to flight environments (such as impacts and dynamic loading), in order to advance the reliability of aerospace structures. Specific components of interest include separation flanges and debris impact shielding, which are subject to high strain rate loading and therefore it is critical that high strain material strength and fracture data are used when designing such components.