Intermediate Strain Rate Tests
Most servohydraulic load frames have a functional upper strain rate limit of 1.0 to 5.0 s-1. Servohydraulic data above these strain rates are commonly plagued by oscillations that are superimposed on the load cell response. This is commonly referred to as "ringing" and can lead to questionable stress-strain data. These oscillations arise from the inertial response of the load frame. In other words, the specimen is not in equilibrium for the entirety of the experiment. At the other end of the spectrum, the typical split-Hopkinson bar apparatus has a lower strain rate limit of 400 to 500 s-1. Therefore, there is a strain rate range spanning roughly two orders of magnitude (5.0 to 400 s-1) where it is difficult to obtain high-quality data. Many common engineering applications, including automotive crash and low-velocity impact, lead to strain rates in this range. Because of this, the Dynamic Mechanics of Materials Laboratory designed and constructed an intermediate strain rate apparatus to bridge the gap between the serovohydraulic load frame and the split-Hopkinson bar.
The apparatus, see Figure 1 and Figure 2, consists of a servohydraulic actuator and a 130 ft long transmitter bar. The specimen is placed such that one end is adjacent to the transmitter bar and the other end is free. The specimen is loaded by the actuator as it impacts the specimen’s free end directly. Once loaded, the specimen deforms between the actuator and the transmitter bar. As the specimen is loaded and deformed, a compression wave propagates into the transmitter bar. The amplitude of this wave is measured with four 1000 Ohm strain gages in a full Wheatstone bridge circuit configured to measure axial load. The gages are placed on the transmitter bar at a distance roughly five bar diameters from the specimen. The wave in the transmitter bar propagates to the end of the bar and then reflects back toward the specimen. The experiment can continue until the reflected wave reaches the strain gages (roughly 16 milliseconds). "Ringing" in the measured load pulse is eliminated because the experiment is over before reflections reach the strain gages. Strain is measured using a high speed three-dimensional digital image correlation system. Strain can be measured directly on the surface of the specimen or by measuring the motion of both the actuator and transmitter bar. The apparatus can also be used for tensile testing by fixing the specimen to both the transmitter bar and actuator and reversing the direction of actuator motion.
Click here to view sample data from this apparatus.