NCAL: Intermediate Strain Rate Testing

Summary

The speed at which a material deforms (strain rate) often has a substantial effect on the force (stress) it can carry. The majority of mechanical testing occurs at slow (quasi-static) strain rates (10^-4 s^-1 to 1 s^-1) or at high dynamic strain rates (> 500 s^-1).  The goal of this project is to develop intermediate strain rate (1 s^-1 to 400 s^-1) testing methodologies using a high-rate servohydraulic testing machine. A complete set of experimental data from quasi-static, intermediate, and high strain rates will be used to accurately quantify the evolution of strain rate dependent mechanical properties of advanced and novel materials for automotive, aerospace, and military industries. 

Description

This project seeks to improve servohydraulic testing methods at intermediate strain rates by addressing the well-known problems associated with excessive stress oscillations (ringing) that currently limit our understanding of the mechanical behavior of engineering materials for loading conditions critical to automotive crashworthiness, many dynamic manufacturing processes (e.g. sheet metal forming), and dynamic failure of engineering structures. 

Material Response to Strain Rate

While the vast majority of our knowledge regarding the mechanical behavior of materials has been obtained from test data at quasi-static strain rates (low deformation velocities), in real-world applications materials experience a wide range of dynamic loading conditions during part production and in their service life where their strength and ductility can differ markedly from typical testing conditions. Measuring material behavior at different loading rates is essential for designing safe automobiles, optimizing dynamic manufacturing processes such as high-speed metal forming, or making structures more resilient under dynamic loads. During high-speed automobile crashes, the structure protecting the occupants is subjected to dynamic loads and with high levels of plastic deformation (strain). The safety of the vehicle structure is intimately tied to the mechanical behavior of its constituent materials over a wide range of strain rates. Unfortunately, very limited experimental data exists at intermediate strain rates that are typical for vehicle crash events (i.e. 1 s^-1 to 400 s^-1). 

Force Signal Ringing

A major challenge of intermediate strain rate testing is that these data are often riddled with experimental uncertainty, especially when high-rate servo-hydraulic load frames are employed. The force measurements on such machines exhibit large stress oscillations (ringing) especially at the highest attainable strain rates, which result in unwanted fluctuations in the applied forces during testing. The ringing arises from the actuator response, and inertia effects of the load frame, grips, and piezo electric load washers, which worsen with increasing actuator velocity and vary from test machine to test machine. While various methods have been proposed to reduce these stress oscillations, such as applying strain gauges to measure forces directly on each specimen, reducing inertial effects in the load path, and employing advanced signal processing techniques (e.g., smoothing), opportunities remain to provide methods for more reliable and consistent mechanical data at these critical intermediate strain rates.

Active Work

This project explores several approaches to minimize the stress oscillations at intermediate strain rates to accurately measure the strain rate dependent mechanical properties of materials. We employ an array of advanced measurement tools, such as high-speed stereo Digital Image Correlation (DIC) to measure dynamically-evolving strain distributions on test specimens and high-speed data acquisition (DAQ) instrumentation to fully characterize the dynamic loading behavior. Improved tests will be cross-compared with dynamic tests (strain rate > 500 s^-1) using a Kolsky bar (split Hopkinson pressure bar). Our end-goal is to provide accurate material data from quasi-static to dynamic strain rates to form a comprehensive understanding of how materials behave over a full range of loading conditions. 

Major Accomplishments

• Benzing, J. T., W. E. Luecke, S. P. Mates, D. Ponge, D. Raabe, and J. E. Wittig. “Intercritical Annealing to Achieve a Positive Strain-Rate Sensitivity of Mechanical Properties and Suppression of Macroscopic Plastic Instabilities in Multi-Phase Medium-Mn Steels.” MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 803 (January 28, 2021). https://doi.org/10.1016/j.msea.2020.140469.

• Benzing, J. T., Y. Liu, X. Zhang, W. E. Luecke, D. Ponge, A. Dutta, C. Oskay, D. Raabe, and J. E. Wittig. “Experimental and Numerical Study of Mechanical Properties of Multi-Phase Medium-Mn TWIP-TRIP Steel: Influences of Strain Rate and Phase Constituents.” ACTA MATERIALIA 177 (September 15, 2019): 250–65. https://doi.org/10.1016/j.actamat.2019.07.036.